Category:460 General Requirements for Bituminous Construction

460.1 Description

The General Requirements for Bituminous Construction is to guide the bituminous plant and roadway inspectors in the inspection of construction of various types of hot mix asphalt pavements. This section contains information common to all bituminous specification sections. However, this is not a contract document and is to be used solely as a commentary on the Standard Specifications.

It is the inspector’s responsibility to assure that all contract requirements are met. The inspectors will be familiar with the Standard Specifications and all contract documents. It is the contractor’s responsibility to produce an acceptable product that meets the requirements of the contract. Each should recognize and respect the position of the other. Maintain, within the framework of these instructions, an attitude of cooperation and professionalism toward the contractor. Do not delay operations unnecessarily. Continuous paving operations are desirable for both the quality of the product and the contractor’s production. The plant and roadway inspectors are directly responsible to the Resident Engineer (RE). Any misunderstandings that the inspector cannot readily resolve should be promptly brought to the RE’s attention. The plant inspector and roadway inspector should keep in close contact with each other.

460.2 Quality Control/Quality Assurance (QC/QA)

Under QC/QA, the contractor does the quality control (QC) testing while MoDOT does the inspection and quality assurance (QA) testing to verify the QC results. MoDOT is also responsible for independent assurance sampling (IAS) to verify that both QC and QA are performing the testing correctly. The contractor is paid based on the results of the QC tests. In addition, the contractor controls the materials, mix, and the plant and paving operations. The approved quality control plan (QC Plan) for Superpave mixes shall include the contact information of the contractor’s QC representative, lot and sublot sizes and how they will be designated, the test method for determining asphalt binder content, the number of cores to be cut for density determination, and the independent third party for dispute resolution. A QC Plan is not required for bituminous base (BB) and pavement (BP) mixes.

The inspector will do less sampling and testing, allowing more time for actual inspection. The inspector also has the authority to require the contractor to conform to the approved QC Plan as a part of the contract. The contractor's technicians and the MoDOT inspectors will be certified technicians, having demonstrated proficiency in sampling and testing the mix for the properties required in the QC/QA specifications.

Occasionally, the QC test may not represent the population or, due to individual differences in sampling and testing procedures, the QA and QC test results may not compare within the required limits. One of the first steps in resolution could be an immediate computation check, comparison of sampling and testing procedures, or split sample retests. Testing of the QC split sample or joint sampling and testing could determine if variations in the mix production or sampling and testing procedures are responsible for the differences. If resolution is not possible at the project level, either party may request that the approved independent third party test the mix. Third party test results will be binding upon both parties. The party whose results were incorrect will pay for the third party testing. Third party testing is costly and time consuming and should only be used when project level reconciliation is not possible.

It is not intended that any mix be produced outside the specification limits. Operating out of the specifications may reduce the contractor's pay and/or the pavement service life. When QC tests are out of specification tolerances, the contractor should adjust the production to bring the mix back in. When QA tests are out of specification tolerances, the contractor should be notified immediately. The contractor is responsible for deciding when adjustments are made to control the mix. Some test properties may be allowed to deviate beyond specification limits occasionally, provided that adjustments are made and the following tests show that production is back within limits.

Production may be required to cease if the QC or QA test results are either out of specifications far enough to indicate that the mix may be subject to failure or beyond the specification removal limits. Production should cease until the problem is corrected. An order record should be written describing the deficiency and the location and amount of mix affected. The contractor may elect to continue production in order to run more tests. If so, the order record should state that any mix produced after the order record was issued is at the contractor’s risk. Final disposition of the mix can then be made based on all tests and observations and may consist of acceptance at a reduced price or removal and replacement.

Superpave mixes contain a quality level analysis with statistically based incentive/disincentive pay factors. The QC tests are analyzed on each lot of mix and the pay is based on the percentage of the mix that statistically falls within the limits of the specifications. It is possible to have a given lot of mix with all tests falling within the specifications, but because of a large amount of deviation between test results, the percent within limits would be very low, giving the contractor a correspondingly low pay factor. Very consistent test results, with a correspondingly low standard deviation and high percent within limits, will produce high pay factors. Test results that are consistent may produce bonus pay factors while those that fluctuate within the specification limits will produce low pay factors.

460.3 Pre-paving Meeting

A pre-paving meeting should be held as close to actual production time as possible. MoDOT representatives should discuss the scope of the project and the contract documents. Any aspects of plant and roadway operation that may be controversial or unusual should be discussed to help avoid confusion in the field. The contractor should provide a progress schedule with enough detail to illustrate the methods that will be used to complete the project on schedule. Lists of contractor and MoDOT personnel assigned to the project and their roles and responsibilities should be provided. The agenda should include materials sources, plant production rates, haul distances and routes, paving widths and speed, type and operation of compaction equipment, traffic control, and safety. Also, the sampling and testing methods and frequencies and acceptance and rejection criteria should be discussed. Finally, the QC Plan should be reviewed and clarified, if needed, and a dispute resolution process should be established.

460.4 Plant Inspection

The plant inspector’s primary responsibility is to assure the acceptable quality of bituminous mixes. This involves constant testing for quality and proportions of the various materials used in the mixes. The plant inspector also performs tests to control the uniformity of materials used in the mixes and the uniformity of the final mix. Although special conditions may make other tests necessary, job control testing will consist of sieve analyses of aggregates, binder and moisture contents of the mix, volumetric testing, and mat density determinations. Sometimes, these tests will control production. It is, therefore, necessary that they always be performed as promptly and accurately as possible. The plant inspector should also become familiar with the operation of the plant. Bituminous Mixing Plants guidance goes into more detail about plant equipment.

460.4.1 Plants

The purpose of the plant is to blend the aggregate, binder, and any additives that are used together at an elevated temperature to produce a homogeneous mix.

460.4.1.1 Batch Plants

The major components of a batch plant are the cold feed system, asphalt binder supply system, drier, mixing tower, and emission control system. The mixing tower consists of the hot elevator, screen deck, hot bins, weigh hopper, asphalt binder weigh bucket, and pugmill.

The aggregate used in the mix is removed from the stockpiles and placed in individual cold feed bins. Aggregates are proportioned from the bins by a combination of the size of the opening of the gate at the bottom of each bin and the speed of the feeder belt under each bin. The feeder belt then deposits the aggregate on a gathering conveyor located under all of the cold feed bins. The gathering conveyor transfers the aggregate to a charging conveyor where it is carried to the drier.

The drier operates on a counter-flow basis. The aggregate is introduced into the drier at the upper end and moves downward by the rotation (gravity flow) and the flight configuration of the drum. The burner is located at the lower end of the drier, and the exhaust gases from the combustion and drying process move toward the upper end, against the flow of the aggregate. As the aggregate tumbles through the exhaust gases, it is heated and dried. Moisture is removed and carried out of the drier as part of the exhaust gas stream. The hot, dry aggregate is then discharged from the drier at the lower end.

The hot aggregate is transported to the top of the mixing tower by a hot elevator. After the aggregate is discharged from the elevator, it passes through a set of vibrating screens and into one of four hot bins. The finest aggregate goes into the No. 1 bin and the coarser aggregate particles are separated by the different sized screens and deposited into one of the other bins. The separation of aggregate into the hot bins depends on the size of the screens in the screen deck and the gradation of the aggregate in the cold feed bins.

The heated, dried, and resized aggregate is held in the hot bins until being discharged from a gate at the bottom of each bin into the weigh hopper. The correct proportion of each aggregate is determined by weight. At the same time that the aggregate is proportioned and weighed, the binder is pumped from the storage tank into a separate weigh bucket located just above the pugmill. The required amount of binder is weighed and held until being discharged into the pugmill.

The aggregate in the weigh hopper is emptied into the pugmill and mixed dry for 10 to 15 seconds, depending on the type of mix being produced. Dry mixing begins when all of the aggregate has been transferred to the pugmill and ends with the introduction of the binder. At this time, wet mixing begins. The maximum amount of time allowed to discharge the binder into the pugmill is 15 seconds. The wet mixing time shall continue for a minimum of 30 seconds. It is important that the aggregate is completely and uniformly coated with binder and that there is a thorough distribution of binder throughout the aggregate. The wet mixing time ends when the discharge gates on the bottom of the pugmill are opened. When mixing has been completed, the discharge gates are opened, and the mix is discharged into the haul truck or a conveying device that carries the mix to a silo.

The plant is equipped with emission control devices, consisting of primary and secondary collection systems. A dry collector is normally used as the primary collector. A baghouse is normally used as the secondary collection system to remove particulate matter (very small, fine particles) from the exhaust gases that flow out of the drier. Clean air is sent into the atmosphere through the stack.

460.4.1.2 Drum-mix Plants

The major components of a drum-mix plant are the cold-feed system, binder supply system, drum mixer, surge or storage silos, and emission control system.

The cold feed bins are used to proportion the aggregate that is delivered to the drum. The amount of aggregate drawn from each bin is controlled by the size of the gate opening and the speed of the feeder belt under the bin. The aggregate on each feeder belt is deposited onto a gathering conveyor that runs underneath the cold feed bins. The combined aggregate normally passes through a scalping screen to remove any over-sized material. Then, the combined aggregate is transferred to a charging conveyor and carried to the drum mixer.

The charging conveyor is equipped with a belt scale to measure the weight of the aggregate passing over it and a sensor that determines the speed of the belt. These two values are used to calculate the wet weight of the aggregate, in tons per hour, entering the drum. The wet weight is converted to dry weight, using the moisture content of the aggregate fractions, so that the amount of binder needed in the mix can be determined. For this reason, it is critical that the aggregate moisture contents are determined frequently. If the plant setting is lower than the actual moisture content of the combined aggregate passing over the belt scale, the plant will add too much binder. If the plant setting is higher than the actual moisture content, the plant will not add enough binder.

QC must determine the moisture contents of the virgin aggregate fractions in the stockpiles, in accordance with AASHTO T255, at a minimum of once a day before production begins. Moistures should again be performed at any time that conditions in the stockpiles have changed, such as after a rain or delivery of new aggregates. Also, the moisture content may vary throughout the stockpile. The moisture content of the combined aggregate is calculated by the weighted average.

The moisture content of each aggregate fraction is multiplied by the corresponding bin percentage, in decimal form, and the results are added together. As a check, the moisture content of the combined aggregate can be determined on the gradation samples. If adjustments to the plant settings are necessary, moistures must be performed on each aggregate fraction. The plant settings cannot be adjusted based on the combined moisture alone because the proportioning of the aggregate from the cold feed bins will not be correct.

In a parallel-flow drum mixer, the exhaust gases and the aggregate move in the same direction. The burner is located at the upper end (aggregate inlet end) of the drum. The aggregate enters the drum either from an inclined chute above the burner or on a conveyor under the burner. The aggregate moves down the drum by a combination of gravity and the configuration of the flights inside the drum. As the aggregate moves down the drum, it is heated and the moisture removed. A dense veil of aggregate is built up near the midpoint of the drum length to assist in the heat-transfer process. The aggregate then moves to the rear of the drum.

The binder is pulled from the storage tank by a pump and fed through a meter, where the proper volume is determined. The binder is then delivered into the rear of the drum with the aggregate. Coating of the aggregate occurs as the binder and aggregate are mixed together and moved to the discharge end of the drum. Mineral filler and/or baghouse fines are also added into the back of the drum, either just before or in conjunction with the addition of the binder.

The mix is deposited into a conveying device and transported to a storage silo. The silo converts the continuous flow of mix into a batch flow for discharge into the haul trucks.

In general, the same type of emission-control equipment is used on the drum-mix plant as on the batch plant. A primary dry collector and either a wet scrubber system or a baghouse secondary collector can be used. If a wet scrubber system is used, the collected fines cannot be recycled back into the mix and are wasted. If a baghouse is used, the fines can be returned in whole or in part to the mixing drum, or they can be wasted.

In the counter-flow drum-mix plant, the heating and drying of the aggregate are accomplished in a manner similar to that of a conventional batch plant dryer. Two basic types of counter-flow drum-mix plants are in use. One has the mixing unit extended on the end of the aggregate drier portion of the drum. The other has the mixing unit folded back around the aggregate drier portion of the drum. With both designs, the aggregate enters the drum from the upper end, similar to a batch plant drier. The aggregate moves down the drum against the flow of the exhaust gases in a counter-flow manner. The mixing of the binder with the heated and dried aggregate is accomplished behind or underneath the burner, outside of the exhaust gas stream.

In the counter-flow drum mixer with the mixing unit extended, the hot aggregate passes the burner into a mixing zone. At the upper end of the mixing zone, the baghouse fines and/or mineral filler are added to the aggregate. A short distance later, the binder material is introduced into the drum. The mixing of the aggregate and binder takes place behind (downstream of) the drier in a separate mixing zone, out of contact with the exhaust gases from the burner.

In the counter-flow drum mixer with the mixing unit folded around the aggregate drier, the inner drum acts as an aggregate drier and the outer drum serves as the mixing unit. The binder is introduced to the aggregate after the aggregate has been discharged into the outer drum. The blending of the two materials occurs as the aggregate and binder are conveyed back uphill in the outer drum by a set of mixing paddles attached to the inner drum. The inner drum rotates and the outer drum is stationary. This type of drum-mix plant is known as a double-barrel plant. Any mineral filler or baghouse fines, as well as RAP material, enters the drum between the inside and outside drums. Therefore, the materials are kept away from the exhaust gases from the burner.

460.4.2 Plant Inspector’s Daily Responsibilities

The plant inspector’s daily responsibilities are listed below. More detail is given to some items in following sections.

(1) Make sure that the necessary sampling and testing equipment is on hand and in good condition.

(2) Visually inspect all components of the plant.

(3) Visually inspect the aggregate stockpiles for segregation and/or contamination and make sure that the stockpiles are kept separate.

(4) Occasionally, verify the QC moisture contents (drum-mix plants only).

(5) Verify that the plant has been calibrated and is up to date.

(6) Verify that the QC testing equipment has been calibrated and is up to date, according to Standard Specification Section 403.17.3.

(7) Make sure that the cold feed percentages and the binder percentage compare with the job mix formula (JMF). However, the binder percentage may differ slightly from the JMF if dust is removed from the aggregate by the dust collection system. Also, check the plant hot bin percentages.

(8) Record the plant percentages on the first day of production and each change thereafter.

(9) Record the number of trucks on the job in the plant diary.

(10) Inspect the truck beds before loading.

(11) Throughout the production period, observe the trucks for correct tarping procedures.

(12) Check all scales for zero balance and sensitivity (Figure 400.1), if applicable. The truck scales must be kept free of foreign material and spillage.

(13) Take the mix temperature on the first truck.

(14) Sign the first ticket and record the mix temperature on the ticket. (15) The truck tickets must contain the following information:

(a) Unique ticket number.

(b) Truck number.

(c) Date and time the truck was loaded.

(d) Job number(s).

(e) Mix number.

(f) Route.

(g) County.

(h) Gross weight of the load (not required for batch plants).

(i) Tare weight of the truck (not required for batch plants).

(j) Net weight of the load.

(k) Binder content.

(16) Verify that all materials delivered to the plant and their producers/suppliers correspond with those listed on the JMF.

(17) Obtain the daily binder sample for the Central Laboratory in accordance with GCM Section 400.4.13.

(18) Perform sieve analyses at the required frequency.

(19) Sample and test for the binder content of BB and BP mixes at the required frequency.

(20) Sample and test for the moisture content of all mixes as needed.

(21) Sample for Superpave volumetrics.

(22) Occasionally, verify all QC sampling and testing procedures.

(23) Test the QC split sample at a minimum of once every 5 production days.

(24) Assign a SiteManager ID number to all samples.

(25) Perform density tests and report the results to the represented production.

(26) Record all QA test data and results for each production period in the plant diary and the Asphalt Plant Inspector’s Worksheet (APIW).

(27) Assign SiteManager ID numbers to any additional samples taken for the Central Laboratory and record them in the plant diary.

(28) Periodically check all scales for accuracy.

(29) Check the temperature of the binder and mix frequently.

(30) Check the ambient air temperature in accordance with MoDOT Test Method TM-20 periodically. Record the daily maximum and minimum temperatures in the APIW.

(31) Visually inspect the mix in the trucks for uniformity and complete mixing frequently.

(32) Develop a mental picture of the appearance of proper mix.

(33) Keep accurate records of all amounts of binder, mineral filler, hydrated lime, etc. received and used up to date.

(34) Record the daily quantities produced.

(35) If producing dedicated project specific mixes, record the start and finish totalizer readings.

(36) If recording totalizer readings, calculate aggregate, additive, and binder percentages for the tonnage produced.

(37) Keep the MoDOT field laboratory organized and clean.

(38) Keep personal diary up to date.

(39) Enter all sampling and testing information into SiteManager in a timely manner.

460.4.3 Supplies

Inspection personnel will have a copy of the approved JMF and QC Plan, the contract and all applicable special provisions, Standard Specifications, and enough forms and field books to record all tests, materials received, and mix produced. Upon completion of a project, all records are maintained by the District Office. The quantity of the following items to be furnished by the District will depend on the size of the project:

(1) Safety equipment.

(2) 1-pint friction top cans.

(3) Thermometer(s) ranging from 500F to 4000F for mix temperatures.

(4) MAX-MIN thermometer for ambient air temperature.

(5) Sample containers and insulated cooler.

(6) Shovel.

(7) Cloth sample sacks and tying twine.

(8) Cardboard boxes and tape for cores and/or loose mix samples.

(9) Material Identification Sheet, Form T-617R.

(10) Laptop computer with the APIW and Pay Factor spreadsheets.

460.4.4 Asphalt Plant Inspector’s Worksheet (APIW) and Pay Factor Worksheet

The Asphalt Plant Inspector’s Worksheet serves as a daily report and will be prepared each day the plant produces mix for MoDOT projects. A separate report will be prepared for each mix produced. A detailed description of the APIW and Pay Factor Worksheets can be found in the SiteManager Quick Reference Guide in Lotus Notes. Reports will be downloaded in a timely manner.

400.4.5 Plant Diaries

The following information must be recorded in the plant diary each day:

(1) Date.

(2) The time the inspector arrived at the plant and left the plant.

(3) Contract and job numbers, mix number(s), route, county.

(4) Weather conditions and temperature.

(5) Record that the zero balances and sensitivity of the scales were checked, if applicable.

(6) Record that the conditions of the stockpiles were checked.

(7) Record that the conditions of truck beds and tarps were checked.

(8) Record the hot bin and/or cold feed percentages and the binder percentage.

(9) Record the moisture contents of each fraction and the combined (drum-mix plants only).

(10) Record the time and mix temperature of the first load.

(11) Record the mix temperatures and the time checked.

(12) Record the binder temperatures and the time checked.

(13) Record the time of sampling for all tests.

(14) Record the lot, sublot, and the tonnage of the QC and QA volumetric samples.

(15) Record the date of the last gyratory compactor verification and calibration.

(16) Record all QA test results:

a. Passing or failing gradation.

b. Binder and moisture contents of the mix.

c. Air voids.

d. VMA.

e. Density.

(17) Record all SiteManager sample ID numbers.

(18) Record when visited by an IAS inspector.

(19) Record any events out of the ordinary or other pertinent information.

(20) Record the totalizer readings if running dedicated mixes.

(21) Record the total quantities for each mix produced.

(22) Inspector’s signature.

460.4.6 Testing Equipment

The contractor shall provide all testing equipment as required by the Standard Specifications. Testing equipment must be of a type and in such condition that inspection personnel may accurately perform the required job control tests in accordance with the approved test methods. If the specialized testing equipment (gyratory compactor, etc.) does not function properly, production should cease. Otherwise, production is continued at the contractor’s risk. The District may provide independent testing equipment if they so choose.

460.4.7 Field Laboratory

The contractor shall provide a Type 3 Field Laboratory in accordance with Standard Specification Section 601. By definition, the type 3 lab contains all of the components of the type 1 and 2 labs. In order to comply with the requirements of the type 3 lab, the contractor may elect to provide two separate labs. As long as the plant operations can be observed from a type 2 lab, the type 3 lab may be located anywhere that is convenient to the project.

460.4.8 Temperature Control

Checking the temperatures of the binder, aggregates, and mix is an important part of inspection because the durability of the mix could be seriously affected. Variations in temperature can cause improper mixing and/or non-uniform workability of the mix. The temperatures of the binder and mix should be checked at a minimum of four times during the production period. Record the temperatures and the time checked in the plant diary and the APIW.

In all plants, a device to measure the temperature of the binder should be located near the feed line discharge into the mixing unit. In drum-mix plants, another device should be located near the discharge chute. In batch plants, another device should be located in at least one hot bin. These devices should be in such a position that the material will flow around the device so that the temperature of the material can be accurately determined. The temperature readings shall be displayed in a location accessible to the inspector.

460.4.9 Mineral Filler, Hydrated Lime, and Baghouse Fines

The purpose of mineral filler is to provide stability to the mix by increasing the dust-to binder ratio (-200/AC). Mineral filler may be hydrated lime, Portland cement, fly ash, limestone dust, steel slag dust, or cement kiln dust. However, Portland cement and cement kiln dust are not to be used as mineral filler in Stone Matrix Asphalt (SMA) mixes. Hydrated lime may be used to reduce the effects of moisture damage (stripping, aging, etc.) to the mix.

Mineral filler and hydrated lime shall be stored dry and added to the mix separate from the aggregate. If used, these materials must be added uniformly and continuously to the mixer in the percentage indicated on the JMF at the point of introduction of the binder without loss to the dust collection system (i.e., baghouse).

Record the gradation from the JMF and the percentage of each material used in the APIW. The Mineral Filler and/or Hydrated Lime Records (Figures 400.2 and 400.3, respectively) may be used to document shipments. Periodically, an approximate determination of the mineral filler and/or hydrated lime usage should be made. This is accomplished by checking the quantity used in the project against the quantity delivered to the plant for a predetermined period of time, accounting for the quantity on hand.

Baghouse fines are essentially a mineral filler and serve the same purpose. Sometimes, fines are removed from the combined aggregate by the baghouse before being incorporated into the mix. Any portion of the fines that are returned and used in the mix must be returned uniformly and continuously. If all of the fines are returned, the combined gradation of the aggregate is essentially the same as it was before the fines were removed. If none of the fines are returned, the gradation may be considerably different.

In batch plants, the percentage of baghouse fines used in the mix is shown on the JMF and accounted for in the combined gradation. This is an attempt to compensate for the breakdown of aggregate that occurs from intense heat and mixing. In drum-mix plants, the baghouse fines percentage is not shown on the JMF. This is because the aggregate gradation is taken from the combined cold feed before being subjected to the effects of mixing.

460.4.10 Liquid Anti-Strip Additives

Liquid anti-strip additives are typically used to improve the adhesion of the binder to the surface of the aggregate and to increase resistance to moisture damage (stripping). Type I and II liquid anti-strip additives shall be in accordance with Standard Specification Section 1071.5. Type I additives are blended into the binder using an in-line mixer and Type II additives are sprayed onto the combined cold feed aggregate. Both must be added uniformly and continuously at the dosage rate indicated on the JMF. The brand name and manufacturer will be shown on the JMF and must be included on the qualified list.

Liquid anti-strip additives can be used in all mixes. However, they will most likely only be used in Superpave mixes because of the cost.

400.4.11 Reclaimed Asphalt Pavement (RAP). A maximum of 15% RAP may be substituted for aggregate in BB, BP, and SL mixes. At this time, RAP is not used in Superpave mixes but may be in the future.

RAP material may or may not come from the MoDOT system. If the material does not come from the MoDOT system, the contractor shall perform Micro-Deval testing, in accordance with AASHTO TP58, at a frequency of once every 1500 tons. If the material has a loss greater than 20%, it cannot be used. At a minimum of once per project, a sample of the RAP material will be sent to the Central Laboratory for testing. Micro-Deval testing will be waived if the material comes from the MoDOT system. The material is subject to the deleterious requirements of Standard Specification Section 1002.2.1.

For the purposes of gradation control, the material will be sampled from the RAP feeding system on the plant. The contractor shall determine the gradation of the RAP at a minimum of once per day, removing the asphalt coating by either extraction or binder ignition. The results of the sieve analysis are used to determine compliance of the combined gradation with the specifications.

The Standard Specifications do not require the plant inspector to perform a sieve analysis on the RAP material. However, in order to determine the combined gradation, the gradation of the RAP is needed. Therefore, the plant inspector should either perform a sieve analysis or use the contractor’s results. The contractor is required to retain a split of all samples, including RAP, for a minimum of 7 days. Perform a sieve analysis on the contractor’s RAP split sample at least once every 5 production days.

For the purposes of binder content control, the total binder content of the mix will be evaluated. The JMF will show the virgin binder content and the total binder content of the mix, which includes the binder content of the RAP. If AASHTO T308 is used to determine the binder content of the mix, the ignition oven is calibrated in accordance with MoDOT Test Method TM-77.

If RAP is incorporated into the mix in a batch plant, it is placed in a separate cold-feed bin and delivered to the plant by a separate gathering/charging conveyor system. The RAP can be added to the new aggregate in one of three locations: the bottom of the hot elevator, the hot bins, or the weigh hopper. The weigh hopper is the recommended location. Heat transfer between the superheated new aggregate and the RAP begins as soon as the two materials come in contact with each other and continues during the mixing process in the pugmill. The binder is added to the blended materials in the pugmill.

If RAP is added to the mix in a drum-mix plant, it is introduced by its own cold-feed bin and gathering/charging conveyor system into an inlet located in the drum. The RAP must be added at a location in the drum where it is protected from the flame of the burner. The RAP is heated when it comes in contact with, and blended with, the superheated new aggregate. The binder is then added to the blended materials.

460.4.12 Asphalt Binder

It is important that the binder is circulated through the feed lines and the storage tank(s) so that the temperature of the binder is kept uniform and within the mixing ranges in [http://www.modot.state.mo.us/business/standards_and_specs/Sec1015.pdf Standard Specification Section 1015.6] during production. All pumps and feed lines shall be properly insulated to keep the binder at the required temperature.

The plant inspector will accept truck shipments of binder on the basis of the supplier’s certification of specification compliance (see Standard Specification Section 1015.2.2.1). The certifications will serve as inspection reports. These materials may be used immediately in the project. All accepted shipments will be reported in the APIW. The Asphalt Binder Record may also be used. If the binder is not properly certified or contains incorrect or incomplete information, the shipment will not be accepted and shall not be mixed in the storage tank(s).

The amount of binder in the storage tank(s) shall be determined before and after each production period so that the amount of material on hand and the exact amount used during the production period can be determined. This can be done with a meter or other methods. Periodically, an approximate determination of binder usage should be made. This is accomplished by checking the quantity used in the project against the quantity delivered to the plant for a predetermined period of time, accounting for the quantity on hand.

460.4.13 Asphalt Binder Sampling

Take a daily sample, consisting of a one-pint friction top can of material, from the binder line supplying the mixer to submit to the Central Laboratory. If the plant produces two or more mixes with the same PG binder grade, only one sample is required to represent that day’s production. If the plant produces more than one mix in a day and each mix requires different PG binder grades, one sample is required for each binder used that day. Retain all of the daily binder samples on the project until one of the following occurs:

• Fifteen (15) binder samples have been accumulated.
• The last binder sample for the project has been obtained.
• A possible binder related problem is observed.

When any of the above occurs, immediately ship the accumulated samples to the Central Laboratory. If a possible binder related problem is observed, notify the Central Laboratory immediately so that the samples can be given priority when they arrive.

Enter a SiteManager record only for the first binder sample taken for each mix number used on a project. All binder samples for that mix number on that project will be reported to that SiteManager record. All testing performed on the binder samples taken for that mix number from that project will be reported by the Central Laboratory in that SiteManager record by adding additional test templates for each succeeding sample. The Central Laboratory will select random samples from each shipment for testing. If a test fails, all samples from the shipment will be tested todetermine the extent of the problem. The project will be notified of failures as soon as possible.

When sampling from the valve on a truck, a storage tank, or the plants’ binder supply line, discard enough material to drain all lines of any material already present. This is important to ensure that the sample is representative of the material being sampled and does not include any material from previous shipments. All PG binders must meet specifications after the introduction of any liquid anti-strip additives. Therefore, ensure that the samples are taken after the introduction of any liquid anti-strip additives. The inspector should either perform the sampling or observe the plant personnel conducting the sampling. Before sampling, make sure the can is clean and dry. Samples containing even small amounts of water usually cannot be tested and may also pose a hazard for the Central Laboratory personnel during the heating process.

Leave approximately ½ in. of free space in the can to allow for proper sealing. However, do not substantially under fill the can. As soon as the sample has been taken, the lid should be secured tightly. Do not allow the can to sit in the rain and do not clean the can by dipping it in solvent. Allowing a hot can of binder to sit in the rain or submerging the can in solvent can create a vacuum inside the can that may draw the binder past the friction seal as the binder cools. When necessary, the outside of the can should be cleaned with a rag dipped in solvent, taking care not to let the solvent accumulate near the friction seal.

Label the can with the following information, keeping in mind that many samples will be identified only by the information on the sample container:

(1) SiteManager ID number that has been assigned to that mix number for that project.

(2) Inspector’s SiteManager UserID.

(3) Date sampled.

(4) PG binder grade.

(5) Producer/supplier.

(6) Mix number.

For locations performing rotational viscosity testing in the field (Brookfield Viscosity), enter a test template in the SiteManager record that has been created for that mix. Be certain to date each template for the day the sample was taken.

All SiteManager records for a mix will be authorized by the Central Laboratory with the status “Informational” unless a sample fails to meet specifications. If a sample fails, that SiteManager record will be authorized with the status “Rejected”. The Central Laboratory will retain all samples for approximately 1 year for additional testing, if necessary.

460.4.14 Asphalt Binder Content

Asphalt binder is the glue that holds the aggregate particles together. Binder also provides lubrication at high temperatures and cohesion at in-service temperatures. The binder content affects the mix properties and the pavement performance. An excessively high or low binder content may cause the mix to have a low stability. A high binder content results in low air voids and may promote flushing and cause plastic rutting of the pavement. A low binder content results in high air voids and may promote stripping and cause consolidation rutting.

MoDOT employees who operate nuclear gauges must have received Radiation Safety Training. MoDOT’s license with the U.S. Nuclear Regulatory Commission (NRC) requires the storage of its nuclear gauges to be under a three-lock system; the door of the asphalt gauge, the shipping case the gauge is stored in, and the door of the building where the gauge is being stored or used. For added security, a fourth lock and chain should be used to secure the shipping case to a fixed object in the laboratory. Any untrained person will not have access to this area without the presence of the inspector. The NRC requires storage areas to have a radiation placard affixed to the door and an NRC Form 3 placed near the placard.

When a nuclear gauge is transported in a car, it should be stored in the trunk. If the gauge is transported in the bed of a pickup truck, it should be near the tailgate. The gauge must be secured either in a lockable box bolted to the bed or chained through the handles of the shipping case and locked to the bed of the truck. There should not be any slack in the chain, if used.

Contractor owned nuclear gauges may or may not be regulated by the NRC, depending upon which radioactive isotope the gauge uses. It is the contractor’s responsibility to comply with all state and federal laws. However, the inspector should report all unsafe practices to their supervisor.

460.4.14.1 Loose Mix Sampling

Sampling behind the paver is the recommended method of obtaining a sample. Samples should be taken in one increment behind the paver prior to breakdown rolling. The sample should extend the full depth of the lift and include all of the mix from the sample location. Use a clean sample container and do not contaminate the sample with underlying material. The inspector should always be consistent in the sampling procedure to reduce, and possibly eliminate, testing errors.

For BB, BP, and SL mixes, the binder content sample may be taken at the plant or on the roadway. This is because the binder content of the mix is the same regardless of where the sample is taken.

460.4.14.2 Nuclear Gauge Methods

The binder content of a mix using a nuclear gauge will be determined in accordance with MoDOT Test Method TM-54. A background count must be performed daily before the binder content can be determined in order to account for the environmental conditions. If the conditions change, another background count should be performed. The gauge should be operated at least 15 ft. away from any hydrogen source (water, people, etc.) and any other binder source (loaded trucks, etc.). Do not exert force on the mix while placing it in the pan.

Record the background count, the number of counts, and the percent binder content for each test in the gauge diary and the APIW. Report the binder content to the nearest 0.1%. If a plant is producing the same mix for more than one project during the production period, the test results are reported to each project receiving that mix. It is recommended that a binder content test be performed at any time that the visual appearance of the mix changes dramatically. The statistical stability (stat) and drift tests are required to be performed every 30 days or at the frequency recommended by the manufacturer. Record the results and the date performed in the gauge diary.

460.4.14.3 Use of the Contractor’s Nuclear Gauge

To eliminate any possible problems that may occur as a result of having two nuclear gauges in the field laboratory, the inspector may use the contractor’s gauge if the contractor approves. The same safety principles should be employed when using contractor owned nuclear gauges. Specification compliance will be verified by independent QA samples until a favorable comparison is obtained. At that time, the inspector may begin using the contractor’s gauge and the QC test results will be used to determine the Pay Factor. A favorable comparison is obtained when the QA test results are within two standard deviations of the QC test results. An unfavorable comparison should be investigated and resolved immediately. The contractor shall be responsible for calibration (including stat and drift tests) of their gauge and all information regarding the calibration shall be available to the inspector.

460.4.14.3 Moisture Content

A mix with a high moisture content results in an improper coating of the aggregate with binder, which may also lead to stripping. If the mix contains too much moisture because the aggregate was not thoroughly dried at the plant, the moisture will over lubricate the mix at high temperatures and prevent the binder from entering the aggregate pores. As a result, excess binder will be present between the aggregate particles instead of partially absorbed by the aggregate.

The moisture content must be subtracted from the binder content test results. The moisture content sample will be obtained at the same time as the binder content sample and performed in accordance with MoDOT Test Method TM-53. The moisture content for all mixes shall not be greater than 0.50%. Record the moisture content results to the nearest 0.01% in the APIW.

Samples should be taken daily at the beginning of a project but may be reduced to onceper week if the results are not changing significantly. If the weather changes, such as after a rain, the moisture content should again be determined on a daily basis until the moisture content of the mix stabilizes. When time is limited, the inspector may use the previous moisture content as an estimate if the recent test results show that the moisture content and the binder content have been stable. However, this is only an estimate.

If the moisture content of the mix is high, there are two methods that the contractor may employ to reduce the moisture. Both methods increase the retention time of the aggregate in drier so that the aggregate is actually dried longer. One method is to slow down production rates. The other method is to reduce the incline of the drier or to rearrange the configuration of the flightsinside the drier. These methods are more effective than simply increasing the temperature of themix.

460.4.14.4 Deviations in Asphalt Binder Content

It is strongly recommended that the sampling and testing frequency be increased whenever the mix appears questionable and/or the binder content test results show that the mix is approaching the specification limits. It is also recommended to test several QC split samples at the beginning of a project to ensure that both QC and QA are performing the test properly.

See the applicable GCM Sections for the procedure on failing test results. If there is reason to believe that the gauge is producing questionable test results or not operating properly, perform a stat and drift test to determine if the gauge is counting correctly. If a stat and drift test is not needed at this time, perform check samples with QC. If a problem is confirmed, send the gauge in for repairs as soon as possible. Technical assistance is available from the Construction and Materials Field Office.

460.4.15 Sieve Analysis

The gradation of the aggregate in the mix is controlled from sieve analyses of the hot, dry aggregate fractions produced at a batch plant or the combined aggregate at a drum-mix plant. When sampling aggregates from the hot bins or the combined cold feed, it is important that the plant is operating at the normal production rate. The sample should be drawn across the full discharge stream rapidly enough to prevent overflow of the sampling device. Each sample should consist of at least three equal increments taken over a period of several minutes. The required sample size is based on the nominal maximum size of the aggregate and is given in Materials Manual Section 1001.3.1.5. The increments should be combined and remixed. Then, the sample will be reduced to the necessary testing size in accordance with AASHTO T 328 and a sieve analysis performed in accordance with AASHTO T11 and T27. Record the results of all sieve analyses in the APIW.

The inspector should always be consistent in the sampling procedure to reduce, and possibly eliminate, testing errors. It is recommended that the remaining portion of the sample be retained for testing if necessary.

460.4.15.1 Deviations in Gradation

It is strongly recommended that the sampling and testing frequency be increased whenever material appears questionable and/or sieve analyses show that the material is approaching the specification limits. It is also recommended to test several QC split samples at the beginning of a project to ensure that both QC and QA are performing the test properly.

See Failing Test and “Gradation Control” on page 48 of the Construction Manual Section 400 for the procedure on failing test results. Minor deviations from the specification tolerances may be acceptable if the overall quality and performance of the mix is satisfactory. Production will not be allowed to be in and out of tolerance without an attempt being made to correct the problem. It is intended that a uniform, acceptable mix be produced. If excessive deviations persist, or if they are considered detrimental to the pavement performance, the material will be rejected. The following checklist can be used when trouble shooting gradation control problems:

(1) Check to see if the testing equipment is in satisfactory condition.

(2) Increasing the number of increments that the sample represents will compensate for variations in the stockpiles by providing a more representative sample.

(3) For drum-mix plants, check the uniformity of the combined aggregate. Variabilitymay occur by the clumping of fine aggregate or screenings as they fall onto the feedbelt. A larger composite sample will compensate for variations in the stockpile gradations and the amount of fine aggregate at the point of sampling.

(4) It may help to perform sieve analyses on the stockpiles. Sample the stockpiles in accordance with AASHTO T2. These gradations should be compared to the gradations of the individual fractions on the job mix formula and the aggregate inspection reports. Some variation is expected. However, by working the entire stockpile, the loader operator can usually blend in minor variations in segregation or moisture.

(5) New aggregates may need to be fed separately, or alternately, with older aggregates to even out gradation and moisture variations that may be present.

(6) Observe the bin charging procedure for spillage between the cold feed bins.

(7) Check the cold feed and/or hot bin settings to make sure that they have not been disturbed.

(8) Use a sample splitting procedure that thoroughly mixes and evenly splits the aggregate. Run both halves of the split occasionally as a check.

460.4.15.1 Bin Percentages

The JMF shows the virgin aggregate gradations of the individual fractions, the combined gradation of the mix, and the percentage of each aggregate fraction that will be used to achieve the combined gradation. The gradations of the virgin aggregate in the stockpiles at the plant should be similar to those on the JMF so that the combined gradation can be produced. The plant percentages will be recorded in the Bin Percentages and Scale Settings Worksheet on the first day of production and each time they are changed thereafter.

In drum-mix plants, each cold feed bin is charged with a different aggregate fraction. In some cases, depending on the amount of an aggregate fraction used, two cold feed bins may contain the same fraction. The cold feed percentage for each bin should correspond with the percentage of each aggregate fraction shown on the JMF. Since the JMF percentages are based on 100% aggregate and the plant operates on the basis of 100% mix, the plant percentages will need to be calculated. Multiply the cold feed percentages by the percent mineral aggregate (in decimal form) shown in the lower right-hand corner of the JMF. The plant percentages, including the binder percentage, should add up to 100%. Due to rounding, the percentages may need to be adjusted slightly.

In batch plants, each cold feed bin is charged with a different aggregate fraction. Again, two cold feed bins may contain the same aggregate fraction. Aggregate is drawn from each cold feed bin with the corresponding percentages indicated on the JMF and combined onto one conveyor. The aggregate is then carried through the drier and to the top of the plant where it is rescreened on the screen deck, separated into a maximum of four different sizes, and stored in the hot bins. Generally, the No. 1 bin contains the finest aggregate and the No. 4 bin contains the coarsest aggregate. Obviously, the gradations of the aggregate in the hot bins will not be the same as the virgin aggregates. For this reason different hot bin percentages must be used.

The contractor is responsible for determining the percentages of aggregate drawn from each hot bin in order to produce the combined gradation, within tolerances, shown on the JMF. However, the contractor may use the following process to determine a starting point for the hot bin percentages. First, the plant is emptied and the cold feed bins, which are set according to the percentages on the JMF, are charged. Then the plant is run long enough to charge the hot bins with a sufficient amount of aggregate. The plant is stopped and the aggregate from one hot bin is weighed and recorded. The same procedure is done for the remaining hot bins. All of the individual weights are added together to obtain a total. Finally, each individual weight is divided by the total weight to determine the percentage for each hot bin. These are the initial hot bin percentages and may need to be adjusted slightly. Using this procedure will help assure that the plant is producing in a balanced manner. Since the JMF percentages are based on 100% aggregate and the plant operates on the basis of 100% mix, the plant percentages will need to be calculated. Multiply the hot bin percentages by the percent mineral aggregate (in decimal form) shown in the lower right-hand corner of the JMF.

The plant percentages, including the binder percentage, should add up to 100%. Due to rounding, the percentages may need to be adjusted slightly. To determine the exact amount of aggregate from each hot bin needed in a batch, the weight of the batch is multiplied by the corresponding plant percentage. To determine the exact amount of binder needed in a batch, the weight of the batch is multiplied by the percent binder content from the JMF.

460.4.16 Density

The unit weight, or density, of a material is the weight of the material that occupies a certain volume. Density is an indication of the degree of compaction of the mix. AASHTO T166, Method A is used to determine the specific gravity of the core. If a lift is placed thicker than 6 times the nominal maximum aggregate size, the cores will be cut in half and the specific gravity of each half determined separately. This must be done because of differential compaction. On thick lifts, the compactive effort applied to the surface of the lift is much greater than it is near the bottom. Each half of the core will be evaluated independently.

The density is calculated by dividing the specific gravity of the core (G_mc) taken from the roadway by the theoretical maximum specific gravity (G_mm). For BB and BP mixes, the G_mm of the mix is shown in the lower left-hand corner of the JMF. For Superpave mixes, the G_mm of the mix is determined by AASHTO T209. The test result from the corresponding production period is then used to calculate the density of the core.

All test data will be recorded in the APIW for the represented production. Also, the applicable portions of the corresponding Asphalt Roadway Report (Figure 400.6) should be completed and returned to the roadway inspector.

One or more of the following factors may affect the test results. The cores should be free of tack and all other foreign material. The cores may need to be separated from previous lifts. This should be done in a manner that will not harm the core. The cores should be cooled to room temperature. The scale should be tared before each weight is obtained. When weighing in water, the suspension apparatus should be centered on the scale and hang freely in the container, the core should be completely submerged, and the water in the container should be maintained within the proper temperature range and at the level of the overflow outlet. The water should be potable. The cores should not be stacked in the basket because this may trap air bubbles. Also, all air bubbles should be removed from the basket before performing the test.

Using a dry towel to dry the cores will absorb water from the voids and decrease the surface dry weight. This will give a test result that is higher than the actual density of the lift. Violent shaking or bumping of the cores will not be permitted. Doing so forces more water into the voids of the core and increases the surface dry weight. This will give a test result that is lower than the actual density of the lift. However, the cores may be rotated slowly underwater to remove any air bubbles.

460.4.17 Visual Inspection of Mix

The plant inspector should observe the mix frequently. The quality of the mix can be quickly evaluated by visual inspection. Visual inspection does not take the place of the job control tests (gradation, binder content, volumetrics, density, etc.), but it does have a place in the control of the mix. The inspector should also become familiar with the appearance of the mix in the correct temperature range. Some unsatisfactory conditions that may be easily recognized are described in Hand Spreading. If any of these conditions are observed, the inspector should immediately notify the contractor.

460.4.17.1 Segregation

Segregation results in the non-uniform distribution of aggregate in the mix and is detrimental to the long-term performance of the pavement. Also see Mat Problems. If segregation is noticed at the plant, it may be caused by:

(1) The material is not being thoroughly mixed.

(2) Improper delivery of mix from the plant to the silo(s), if used.

(3) The discharge gate is too high above the truck bed.

(4) The discharge gate is not opening and closing fast enough or to the full extent.

(5) Loading trucks in a manner that the material segregates. To prevent this, trucks should be loaded in three or five drops depending on the size of the truck. If a typical haul truck is used, the first and second drops should be towards the front and back of the truck bed, respectively. The third drop should be in between the first and second drops. If a longer haul truck is used, the first and second drops should be towards the front and back of the truck bed, respectively. The third drop should be in the middle of the bed. The fourth and fifth drops should be between the second and third and the first and third drops, respectively.

460.5 Paving Equipment

The following sections give important and useful information about the paving equipment. The inspector should become familiar with all of the equipment used on the project.

460.5.1 Distributor

This information applies to tack coats, prime coats, seal coats, scrub seals, and fog seals. Each of these will be discussed in more detail in articles on Tack Coat, Prime Coat, Seal Coat, and Surface Treatments. The purpose of a distributor is to apply the bituminous material in a uniform and continuous spread and at the rate required by the applicable Standard Specifications. The bituminous material may be an asphalt emulsion, a cutback asphalt, or a PG binder depending on the type of coat or seal being applied. Proper application is dependent upon interference and proper pump speed, spray bar height, and nozzle operation.

The distributor will be calibrated in accordance with Materials Manual General Section 10.3.6. After calibration, the distributor and the dipstick will be issued a calibration number.

460.5.1.1 Interference

The nozzles on the spray bar must be adjusted so that the spray patterns do not interfere with, or distort, each other. Normally, each nozzle opening is adjusted to an angle of approximately 30 degrees with the centerline of the spray bar. A 30 degree angle will allow the passage of air currents without undue distortion of the fan pattern.

460.5.1.2 Pump Speed

The highest possible pump speed, which does not cause distortion of the spray pattern, should be used. A pump speed that is too low will result in streaking and non-uniform discharge of material from the nozzles. A pump speed that is too high will result in excessive distortion of the spray pattern. The ideal spray pattern should be a solid, triangular sheet of bituminous material from each nozzle onto the surface.

460.5.1.3 Spray Bar Height

The spray bar height should be set at a height that provides for uniform, sufficient application of the bituminous material on the surface. Overlapping of the bituminous material varies with the type of coat or seal used and is governed by the applicable Standard Specifications. It should not be set so low that blotches and/or voids of material occur because of air turbulence. Some method must be in place to ensure that the spray bar height will remain constant as the amount of material in the distributor decreases.

460.5.1.4 Nozzle Operation

The nozzles on the spray bar should be fully open, functioning, and clean. Most clogged nozzles are a result of allowing the spray bar to cool between applications. This allows the material remaining in the nozzle to harden and act like a plug. Before each application, hot material should be circulated through the spray bar until the temperature of the bar reaches the temperature of the material. This procedure will usually soften the hardened material. The material must be heated within the spraying temperature ranges in Standard Specification 1015.6.

460.5.2 Haul Trucks

The truck beds must be clean and free of foreign material. Also, the bed should be reasonably smooth without major indentations or depressions where release agent or mix can accumulate. A minimum amount of lime solution or an approved release agent, without runoff, should be used. The idea is to coat the inside of the bed to prevent mix from sticking to it. If a release agent is used, it should be diluted no less than the minimum rate specified and applied with equipment recommended by the manufacturer. Also, the release agent shall be in accordance with Standard Specification Section 1071.3. Diesel fuel, fuel oil, or other detrimental products will not be allowed for cleaning truck beds, as a release agent, or to dilute the release agent. The use of these products is considered as contamination of the mix.

If necessary, the truck beds should be insulated in order to keep the mix within the desired temperature range for proper placement. The purpose of a tarp is to insulate the mix and protect it from the weather during transport from the plant to the jobsite. All trucks must be equipped with a tarp that completely covers the top of the load and extends over the sides and tailgate of the truck. The tarp must be water repellent, of sufficient weight and strength to resist tearing, and in good condition with no holes or tears. There should be enough tie-down points to secure the tarp and prevent it from flapping in the wind during delivery.

If the tarp does not completely cover the load or is not properly secured, the rate of cooling of the mix will increase because of airflow under the tarp. The rate of cooling depends on the mix temperature, ambient air temperature, the efficiency of the tarp, and any insulation in the bed. Cooling results in the formation of a crust on top of the mix. The extent of crusting depends on whether or not the load was tarped and how long the mix has aged. In a sense, crusting is beneficial because it helps insulate and reduces the rate of cooling of the mix underneath. However, the crust decreases the workability of the mix and may have adverse effects on the mat.

400.5.3 Material Transfer Vehicle (MTV)

The most common type of MTV is essentially a surge bin on wheels. The trucks dump the mix into the hopper on the front of the MTV. The mix is carried by the augers in the hopper to a conveyor, which delivers the mix into the storage chamber where it is reblended. From here, the mix is carried by another conveyor and dropped into the paver hopper. This second conveyor can rotate so that the MTV can operate directly in front of the paver or off to one side. This allows for greater mobility in confined areas. Also, the MTV eliminates the problem of trucks bumping into the paver, which causes bumps in the mat.

One advantage of using this type of MTV is that it reblends the mix reducing segregation and temperature variations allowing for proper compaction and uniform density. The second advantage is that it allows for continuous paving because of its storage capacity, as long as mix is continuously supplied from the plant. In turn, a constant head of material can be kept in front of the screed, producing a smoother mat.

There is another type of MTV that transfers mix from the trucks to the paver. The mix is dumped from the trucks into the hopper, carried up a conveyor, and dropped into the paver hopper. The storage capacity of this type of MTV is significantly less than the type discussed above. Since this type of MTV does not reblend the mix, any possible segregation in the mix may or may not be affected. This type of MTV does, however, provide a smoother mat by allowing for the possibility of maintaining a constant head of material in front of the screed and by eliminating the problem of trucks bumping into the paver. A material pickup machine may sometimes be used as an MTV.

460.5.4 Pavers

The purpose of the paver is to place the mix to the desired width, grade, cross slope, and thickness and to produce a uniform mat texture. The paver should also be able to place the mix in a manner that results in improved ridability and smoothness of the roadway. There are two types of pavers, track and rubber-tire. The track paver is generally used when paving over a soft or yielding base. The rubber-tire paver is generally used when paving over well-compacted bases or existing pavements. The paver consists of two main units, the tractor and the screed.

400.5.4.1 Tractor Unit

The tractor unit provides motive power to the paver and pushes the haul trucks if a material transfer vehicle (MTV) is not used. The tractor also tows the screed unit. On the track pavers it is important that both tracks are properly adjusted so that they are snug but not too tight. If they are loose, the tractor may have a rhythmic bumping movement. This movement is reflected in the screed and causes a ripple effect in the mat. This rippling effect frequently cannot be removed by rolling and remains in the finished pavement. On rubber-tire tractors, it is important that the tires are uniformly and properly inflated to provide maximum traction and smooth screed operation. The tractor unit includes the push rollers and the material feed system consisting of the receiving hopper, slat conveyors, material flow gates, and distribution augers.

460.5.4.1 Push Rollers. Push rollers are located on the front of the paver hopper and are used to maintain contact with the haul truck. The push rollers must rotate freely. If they do not, the truck tires will slide on the rollers increasing the load on the paver making it more difficult for the paver to push the truck. If one roller rotates freely and the other does not, the paver may be more difficult to steer.

460.5.4.2 Receiving Hopper. The receiving hopper is used for temporary storage of mix. An insert with a larger capacity is available for use with an MTV. If mix is delivered directly to the paver, the hopper must be wide enough to allow the body of the haul truck to fit inside it. Also, the hopper must be low enough to allow the truck bed to be raised without putting weight on the hopper. The front of the hopper is designed to minimize spillage of mix.

460.5.4.3 Slat Conveyors. The purpose of the slat conveyors is to carry the mix through the paver from the hopper to the augers. The slat conveyors operate independently from each other so that more or less mix can be fed to one side of the paver without affecting the other. In some cases, the slat conveyors operate at speeds independent from the speed of the paver and augers.

460.5.4.4 Material Flow Gates. There is usually a material flow gate over each slat conveyor that regulates the amount of mix delivered through the tunnels and back to the corresponding auger. The flow gates can be adjusted vertically to allow more or less mix to be carried back to the augers. Flow gates are not used on pavers with conveyors and augers that operate independently.

460.5.4.5 Distribution Augers. The mix deposited into the auger chamber by the slat conveyors is distributed across the full width of the paver by the augers.

The augers on either side of the paver operate independently from each other. However, the auger on one side of the paver usually operates in conjunction with the slat conveyor on the same side. The auger gearbox is located in the center of the paver. Typically, a reverse auger or paddle is adjacent to the gearbox on either side. These are used to place mix under the gearbox so that sufficient mix is present at the screed directly behind the gearbox.

On some pavers, the height of the augers can be changed. Generally, the augers are set according to the thickness of the lift and as low as possible to minimize the amount of mix carried in the auger chamber.

460.5.4.2 Screed Unit

The screed unit’s function is to establish the thickness and provide the texture and initial compaction of the mat. The screed unit is attached to the tractor unit at the tow (or pull) points, one on either side of the tractor. The tow points allow the screed leveling arms (also called side arms or pull arms) of the screed to rotate or pivot around these points. Therefore, the screed “floats” over the mix, averaging out any changes in the grade or elevation experienced by the wheelbase of the tractor unit. This allows for more mix to be placed in low spots and less mix to be placed in high spots. In order for the floating-screed principle to work properly, both tow points must be set at the same level.

Screed Strike-offs. Sometimes the screed is equipped with a strike-off. The purpose of a strike-off is to control the amount of mix passing under the nose of the screed and to reduce the wear on the leading edge of the screed. The exact location of the strike-off depends on the make and model of the paver being used and on the thickness of the mat. No compaction of the mix occurs under the strike-off.

Screed Heaters. Screed heaters are used to heat the plates on the bottom of the screed to the temperature of the mix in order to prevent the mix from sticking to the screed. If mix does stick to the screed, the mat will tear, causing a rough texture. If the screed is properly heated, a more uniform mat texture and consistent mat thickness will be achieved. If the screed is overheated, permanent warping of the screed plates may occur.

Screed Crown Control. The screed can be angled at its center to provide for positive or negative crown. The crown can also be introduced into the screed extensions, rigid and hydraulic, at the points where the extensions are attached to the main screed. The lead crown (front of the screed) should be set slightly above the tail crown (back of the screed) so that the mix will flow easily under the screed and provide a more uniform mat texture.

Screed Vibrators. The screed vibrators may or may not be used. The amount of compaction imparted to the mix by the screed is a function of the properties of the mix, the amount of bearing pressure applied to the mix by the screed, the amount of mix passing under the screed, and the paver speed. The frequency (number of vibrations per minute) and the amplitude (amount of force) of vibration within the screed itself also affect the degree of compaction.

The frequency of vibration is controlled by the speed of rotation of the vibrator shaft and can be adjusted. Changing the frequency changes the compactive effort. In general, the highest frequency setting should be used to obtain the greatest compactive effort possible. The amplitude is determined by the location of the eccentric weights on the shaft, which can be altered to change the compactive effort. Low amplitude should be used for thinner lifts and high amplitude should be used for thicker lifts.

Rigid Screed Extensions. Rigid screed extensions are used to increase the width of the paver. The width of the rigid extensions added to the screed should be equal on both sides, if possible, in order to keep the paver in balance. Extensions should be securely attached to the main screed at the same elevation and angle of attack as the main screed. Auger and auger chamber extensions of the same length as the screed extensions should also be used. If a strike-off is used on the main screed, a strike-off must be added to the extensions at the same elevation.

Hydraulically Extendable Screeds. Hydraulically extendable screeds may be in front of or behind the main screed. It is important that the elevation and angle of attack of the extendable screeds are properly set. If they are not properly aligned with the main screed, a difference in thickness, surface texture, and compaction will result. For front-mounted extendable screeds, the elevation is usually set slightly below the main screed. For rear-mounted extendable screeds, the elevation is normally set the same as the main screed with a greater angle of attack. The forces that act on the extendable screeds and the main screed are similar.

If a strike-off is used on the main screed, strike-offs should be used on the extendable screeds. If a front-mounted extendable screed is to be used in a fixed position for a long period of time, the paver should be equipped with auger and auger chamber extensions to distribute mix across the width of the mat and help maintain a constant head of material. If a rear-mounted extendable screed is used, auger extensions should not be used because an excess of mix will occur in front of the extendable screed resulting in non-uniform texture and variable mat thickness. The mix will flow out to the end of the extendable screed without assistance from the augers. With either front- or rear-mounted extendable screeds, the flow control sensors should be mounted on the end plate so that enough, but not too much, mix is delivered to the ends of extendable screeds.

Automatic Screed Control. Automatic screed controls, consisting of grade references and slope controls, are used to keep the elevation of the tow points on the paver constant. Keeping the elevation of the tow points constant allows the screed to maintain a consistent angle of attack, which helps to produce a smooth mat. As the tractor unit moves over the existing grade, the tow points move up and down less than they would if a reference was not used. Deviations in the underlying surface are averaged out over the length of the reference.

There are three basic types of grade references: erected stringline, mobile reference, and joint-matching shoe. Each has a sensor that adjusts the elevation of the tow points as the reference reacts to changes in the grade. The location of the sensor, in relation to the reference, does make a difference in the reaction of the tow points and screed to the grade of the underlying surface and is different for the different types and manufacturers of the references. The purpose of the stringline and the mobile reference is to average out the deviations in the underlying surface over a distance that is greater than the wheelbase of the tractor unit. The purpose of the joint matching shoe is to match the grade of the adjacent lane.

The grade references can be used in any combination on either side of the paver. Using a reference on both sides of the paver will average out deviations in the grade on both sides of the mat. However, using grade references on both sides of the paver generally will not produce a uniform cross slope.

The grade reference is typically located on the centerline side of the paver and the slope control is typically located on the shoulder side. The slope controls may cause the paver to place mix at variable thicknesses on its side, depending on the condition of the underlying surface. More mix is placed in low spots and less mix is placed in high spots. However, a constant cross slope will be maintained.

460.5.5 Rollers

The size and type of compaction equipment varies with the type of mix being placed.

Static Steel Wheel Rollers. Steel wheel rollers should be equipped with scrapers or a wetting system to prevent mix from sticking to the drums. However, excessive amounts of liquids will not be permitted. Diesel fuel, fuel oil, and other detrimental products shall not be used.

The compactive effort applied to the mat by static steel wheel rollers depends on the weight of the roller and the contact area of the drums with the mix. Adding ballast can change the weight of the roller. The effective contact pressure over the contact area is the key variable and is dependent on the depth of penetration of the drums into the mix. On the first pass of the roller, the depth of penetration and the contact area are the greatest, so the compactive effort on the mix is less. On subsequent passes the mix becomes denser so that the drums penetrate less causing the weight of the roller to be applied over a smaller area and, therefore, the compactive effort to increase. Rollers with large-diameter drums have lower rolling resistance than smaller drums. Therefore, rollers with large-diameter drums are more efficient because they do not penetrate as far into the mix to develop the same contact area.

Pneumatic Tire Rollers. The compactive effort applied to the mix is dependent on the wheel load of the roller, the tire design and pressure, the contact area, and the depth of penetration of the tires into the mix. All of the tires on the roller should be the same size, ply, and pressure. Tire pressure should be adjusted depending on the type of mix and its position in the roller train. The greater the contact pressure between the tire and the mix, the greater the compactive effort applied by the roller. The wheels must oscillate (up and down movement), however wobble (side to side movement) wheel rollers are not to be used. The most effective practice in preventing roller pickup is to keep the temperature of the tires as close as possible to the temperature of the mix.

Vibratory Rollers. The vibratory roller can be operated in three modes: static (without vibration), with only one drum vibrating, and with both drums vibrating. Vibratory rollers should be equipped with a wetting system to prevent mix from sticking to the drums. However, excessive amounts of liquids will not be permitted. Diesel fuel, fuel oil, and other detrimental products shall not be used. The vibratory roller compacts by weight and impact, not by vibration. The impact is caused by a rotating eccentric weight inside the drum(s).

Vibratory rollers are equipped with controls to adjust both the amplitude and the frequency. The amplitude controls the amount of force applied to the mix by the movement of the eccentric weight. The amplitude setting is dependent on the thickness of the lift. However, it is recommended that the roller be operated in static mode for thin lifts. The amplitude should not be increased if density is not being achieved. Increasing the amplitude increases the applied force, which may cause the aggregate in the mix to fracture and the mat density to decrease. Instead, the number of passes made over the mat should be increased.

The frequency is the number of complete rotations of the eccentric weight per minute. The roller should generally be operated at high frequency in order to increase the number of impacts in a given distance at a given roller speed. The spacing of the impacts depends on the frequency and the roller speed. As frequency decreases and roller speed increases, the distance between impacts increases. As frequency increases and roller speed decreases, the distance between impacts decreases. Smaller impact spacing is preferred because the number of impacts in a given distance is increased which also increases the compactive effort.

400.6 Roadway Inspection

The roadway inspector’s primary responsibility is to assure the placement of mixes to produce a smooth surface meeting the requirements of thickness, density, and cross section established by the contract. To do so, the inspector should understand and be familiar with all paving operations and equipment. Inspectors assigned to the roadway receive material tickets, check for correct truck tarping procedures, check temperatures, observe the placement and compaction of mix, check yield, straightedge and profilograph the surface, and procure density samples. It is important for the inspector to respond to inspection requests promptly and to be consistent in those inspection procedures. The contractor should be notified immediately of any problems that are observed by the inspector. Also, the inspector must maintain open lines of communication with all of the parties involved in the paving operation. Finally, the inspector will maintain complete and accurate records. 400.6.1 Supplies and Equipment. Inspection personnel will have a copy of the plans, the contract and applicable special provisions, Standard Specifications, the approved job mix formula, and enough forms and field books to document all pertinent information and all work and materials to be paid for. Upon completion of a project, all records are maintained by the District Office. (1) Safety equipment. (2) Material receipt forms. (3) Asphalt Roadway Report. (4) A method of determining random numbers. (5) Thermometer(s) ranging from 50° F to 400° F. (6) Shovels. (7) Sample containers. (8) Marking paint (white and orange). Division of Construction and Materials 400-24 © Missouri Department of Transportation 2006 (9) Paint sticks. (10) Measuring devices (wheel, stick rule, tape, etc.). (11) 10 ft. straightedge. (12) 4 ft. level. 400.7 Paving Operations. See the applicable Standard Specifications and GCM Sections for more information. 400.7.1 Traffic Control. Warning signs of the type shown on the plans are to be furnished and erected by the contractor. Due to the relatively fast movement of paving operations, most traffic hazards will be concentrated around the placement area. Traffic should be directed through the construction zone by warning signs and flaggers in a way that provides maximum safety for the workers and traffic and that provides the least interruption of work. Another concern that should be addressed is how the haul trucks will enter and leave the work zone. Signing must be placed only when construction is active or when a hazard exists and promptly removed after work moves on. Special attention should be given to the use of the proper warning devices during night operations. 400.7.2 Surface Preparation. The performance of a pavement under traffic is directly related to the condition of the underlying surface. The extent of the pavement repair will be determined before the project begins and may be included as part of the contract. In some cases, District Maintenance may perform minor pavement repairs, such as the removal of crack and joint sealant and the placement of a leveling course or spot wedging before the project begins. Otherwise, repair of the existing asphalt or PCC pavement shall be completed by the contractor as required by the contract and in accordance with Standard Specification Section 613. Subgrade. Any surface that mix is to be placed on is referred to as the subgrade. The subgrade may be soil, an aggregate base, an existing pavement, or the previously placed lift. Typically, bituminous mix is not placed directly on a soil subgrade. Instead, the soil subgrade is prepared in accordance with Standard Specification Section 209 and some type of base course constructed in accordance with the applicable Standard Specifications. The base course is then primed. If the subgrade is the existing pavement, any necessary repairs should be made and the surface should be cleaned and tacked. If the subgrade is the previously placed lift, the surface should be cleaned and tacked. The subgrade must be able to support the haul trucks without rutting. If rutting occurs during the paving operation, the subgrade should be reworked and stabilized. Soft spots must be removed. Tack and prime coats are discussed in more detail in GCM Sections 407 and 408, respectively. Existing Asphalt Pavement. The condition of the existing asphalt pavement will determine the degree of preparation needed. All existing pavement areas where severe load-related distress has occurred shall be removed and replaced, including the subgrade and/or base course if necessary. Subsurface drainage should be installed as needed. Localized failures (potholes) shall be patched properly. Badly cracked sections, especially those with pattern cracking, must be patched or removed and replaced. Any asphalt pavement that is removed must be replaced with asphalt. Large cracks General Construction Manual © Missouri Department of Transportation 2006 400-25 shall be cleaned and sealed to a level just below the pavement surface. Excessive amounts of crack sealant could create a bump in the new pavement during the compaction process. In small cracks, the sealant may not penetrate into the crack and instead pool at the surface. Surface Treatments. If extensive cracking has occurred, a surface treatment may be more practical than filling individual cracks. The cracks should be cleaned, if possible, and the surface treatment applied when the pavement surface is clean and dry. Surface treatments are discussed in more detail GCM Section 413. Leveling Course and Spot Wedging. A leveling course, also known as a wedge or scratch course, may be required by the contract. The purpose of a leveling course is to eliminate any irregularities in the existing pavement surface by placing a layer of mix with variable thickness prior to resurfacing. The mix is placed thicker in low areas and thinner in high areas. However, due to differential compaction, the thicker areas tend to compact to a higher degree than the thinner areas. As a result, the low areas still exist to some extent and multiple leveling courses may need to be placed in order to obtain a smooth surface. The mix is to be uniformly spread and compacted to the desired grade and cross section. Rigid thickness control is required and only minor segregation can be accepted. Leveling courses are generally placed very thin (minimum of 1 in.) Therefore, the reaction of the mat to compaction must be watched very closely. Spot wedging may be necessary prior to placing the leveling course to compensate for small areas, such as dips, that are in extremely bad condition. Spot wedging may require feather edging. Typically, a BP or SP125 mix is used for the leveling course and spot wedging. A pneumatic tire roller shall be used as the initial (breakdown) or intermediate roller on leveling courses. Milling. Milling is the preferred method of removing irregularities in the existing pavement surface, as opposed to placing a leveling course. If the milling machine is equipped with automatic grade and slope controls, a level surface can be produced in one pass over the existing surface. In addition, if the milled surface is properly cleaned, its texture can enhance the bond and provide friction between the new and old pavements. Additional tack may be required to ensure an adequate bond because of the increased surface area of the milled pavement (from the grooves left by the cutting teeth on the milling machine). Existing Portland Cement Concrete Pavement (PCCP). Any severely distressed areas in the PCCP shall be removed and replaced with either Portland cement concrete (PCC) or asphalt using full-depth slab replacement techniques. If necessary, the subgrade and/or base course should also be repaired. Any severely spalled areas at the joints shall be repaired with PCC using partial-depth slab repair methods. Rocking slabs should be stabilized. Joint sealant and filler material should be removed to a level below the surface of the existing PCCP. In the case of badly damaged joints, the sealant and filler material should be removed and the joints cleaned. Then, the joint should be sealed to a level below the surface of the existing PCCP. A bump may occur in the asphalt overlay due to excess sealant. A leveling course, if necessary, may be placed over existing PCCP. 400.7.3 Weather Conditions. The Standard Specifications put certain weather restrictions on the placement of bituminous mixes. Wet or Frozen Surface. Division of Construction and Materials 400-26 © Missouri Department of Transportation 2006 Mix must not be placed on a wet or frozen surface. Mix that is placed on a wet or frozen surface will cool very quickly because of rapid heat transfer from the mix to the underlying surface. The required density will be difficult to achieve because of the low mix temperature. As a result of the lack of compaction and corresponding high air void content, the pavement will perform poorly. Sometimes, the pavement “sweats” as a result of the dew point exceeding the pavement temperature. This generally occurs on a warm day immediately following a cold day. It is the contractor’s responsibility to start and stop the plant. However, production should stop when rain starts. Loaded trucks may be placed if standing water is not visible on the existing surface. Tickets issued for loads that were not placed are void and an appropriate notation will be made on the ticket. Placement of mix under these conditions does not relieve the contractor of the responsibility for all mix to be in satisfactory condition. If the contractor insists on laying mix contrary to these provisions, the contractor is doing so at their own risk. An order record will be issued immediately, informing the contractor that the mix placed is rejected. Temperature Requirements. There are no temperature requirements for BB, BP, or Superpave mixes. However, the air, surface, and mix temperatures will be checked for quality assurance purposes in accordance with MoDOT Test Method TM-20. 400.7.4 Spreading. The subgrade must be dry, clean, and free of all foreign material before the tack or prime coat can be applied. Generally, a prime coat is applied to the base course and a tack coat is applied to the existing pavement or the preceding lift. A tack coat must be properly applied between each lift. Just before application of the tack or prime coat, the amount of material in the distributor tank should be determined by using the calibrated dipstick. The distributor should be level and the temperature of the material checked and recorded to ensure that it is within the desired application range. The temperature is also needed so that the quantity of material that has been used can be adjusted for payment. After the material has been applied to the surface, the amount remaining in the tank should be determined. Once the tack or prime coat has been applied, the mix should be spread and struck off so that the surface is smooth and true to cross section, free from tearing and all irregularities, and of uniform density. The alignment of the outside edge should also be uniform. The mix should be handled in such a manner that segregation is avoided at all points during the paving operation. Unloading of Mix. Just before unloading the truck, the truck bed should be raised with the tailgate still secured. This will allow the mix to shift and slide back to the tailgate causing any segregated coarse aggregate to be incorporated back into the mix. Also, when the tailgate is released, the load of mix will flood the MTV or paver hopper in a mass, further reducing segregation. When delivering mix directly to the paver, the truck should never bump into the paver because this will result in a corresponding bump in the mat. Instead, the truck should stop and the paver will initiate contact. Also, the truck bed should never be raised to the extent that weight is transferred to the front of the paver. Doing so may also cause a bump in the mat. Finally, both sets of duals should be in firm contact with the push rollers and the truck should be in line with the paver, not skewed. If the truck is skewed, the paver tends to skew, requiring continual correction by the paver operator and leading to irregular lines. The truck driver should only brake when General Construction Manual © Missouri Department of Transportation 2006 400-27 absolutely necessary. Many pavers are also equipped with a truck hitch that has additional rollers to engage the truck’s wheel rims. The purpose of the hitch is to ensure and maintain contact with the truck and, in turn, eliminate mix being dumped in front of the paver. Once the truck is unloaded, the hitch is released and the truck is able to pull away from the paver. If the mix under production is Superpave, the load should be rejected if a crust has formed on the load or if lumps have formed. If the mix is BB, BP, or SL, the crust and lumps must be broken down completely prior to being incorporated into the project. Operation of the Material Feed System (Receiving Hopper, Slat Conveyors, Material Flow Gates, and Distribution Augers). The key to placing a smooth mat is the proper use of the material feed system as a whole to maintain a constant head of material in front of the screed by the continuous operation of the conveyors and augers and the proper positioning of the flow gates (if any). This is dependent upon the type and speed of the paver, the width of the mat, and the thickness of the lift. A change to any component of the system will result in a corresponding change to the mat. The amount of mix in the hopper should be kept at a level above the top of the tunnel openings or flow gates and as constant as possible. This will keep the conveyors on the paver full and maintain a constant head of material in front of the screed. Mix that is allowed to stand in the sides of the hopper for a long period of time will cool. The wings of the hopper can be folded to move mix from the sides to the middle in order to keep the mix hot enough for proper placement and compaction. However, folding of the hopper wings may cause segregation and increased mat problems and, therefore, should be held to an absolute minimum. If an insert is used with an MTV, the hopper wings cannot be folded. It is recommended that the flow gates be adjusted so that the depth of mix carried in the auger chamber is level with the center of the auger shaft and kept as consistent as possible to maintain a constant head of material in front of the screed. If the augers are buried or overexposed, variations in the texture of the mat and surface shadows may develop. If enough mix is not placed under the gearbox, a longitudinal streak may occur in the center of the mat behind the gearbox. This streak will be darker in color and more open in texture than the surrounding mix and is the result of a lack of mix being placed under the gearbox and passing under the screed at that point, not segregation. The use, or adjustment, of a reverse auger or paddle system should eliminate this streak. There are three types of material feed systems: constant speed, variable speed, and sonic control. However, in all cases the amount of mix delivered to the screed is still essentially regulated by the position of the flow gates, with or without the use of flow control sensors. If a flow control sensor is used, it should be set near the paver end plate to maintain a constant head of material in front of the screed by keeping the augers running continuously. The location of the sensor is important in preventing too much or too little mix from being carried at the outside edge of the screed. In the manual operation of pavers with constant speed material feed systems, the conveyors and augers operate at a constant speed. Therefore, the only way to maintain a constant head of material in front of the screed is by the proper adjustment of the flow gates. A flow control sensor may be used to monitor the amount of mix. These sensors are limit switches that float on top of the mix and rotate as the amount of mix changes. When too little or too much mix is present on either side of the screed, the sensor turns the corresponding conveyor and auger on or off. Intermittent operation of the conveyor and auger systems may cause roughness in the mat behind the Division of Construction and Materials 400-28 © Missouri Department of Transportation 2006 screed, as well as shadows and ripples. In the manual operation of pavers with variable speed material feed systems, the speed of the conveyors can be selected and the speed of the augers is set accordingly. A constant amount of mix is maintained by adjusting the speed of the conveyors and augers. A flow control sensor can also be used with these systems. As the amount of mix increases and decreases, the speed of the material feed system decreases and increases, respectively, to maintain a constant head of material in front of the screed. Since the conveyors and augers are operating continuously, the roughness in the mat and the shadows and ripples are avoided. Most pavers are equipped with a sonic feed system that uses reflected sound waves to sense the amount of mix. The system sends out pulses several times per second. A timing circuit is started when the pulse is sent out and stopped when the first echo is received. The length of time is used to calculate the distance to the material. The system then varies the speed of the conveyors and augers on each side of the machine proportionally to maintain a constant head of material in front of the screed. Operation of the Screed. Two primary forces constantly act on the screed as the paver places mix. The first is the towing force of the tractor, which varies as the speed of the paver varies. The second is the head of material pushing against the screed. The forces must be in equilibrium in order for the screed to maintain a constant angle of attack. When a change in any one force occurs, the angle of attack of the screed changes causing the screed to rise or fall, and the thickness and texture of the mat to change accordingly. The screed will react to the change in the force against it until equilibrium is reestablished. The line of pull refers to the angle at which the screed is pulled forward by the tractor. The line of pull will influence the angle of attack of the screed. The smoothest mat will be placed when the towing force is applied parallel to the grade. For this reason, the tow points should be set according to the thickness of the mat. If the tow points are set too high or too low, the towing forces will be angled upward or downward, respectively. Therefore, to maintain thickness, the angle of attack of the screed must be decreased (nose down) or increased (nose up), respectively, and only the front or rear portion of the screed will compact and finish the mat. Either way, this causes poor mat texture and extreme wear on that portion of the screed. The head of material is the volume of mix carried in the auger chamber in front of the screed. One of the major factors affecting the thickness and texture of the mat is the consistency of the head of material. If the head of material is increased, the force on the screed also increases, causing the angle of attack to increase (nose up) and the screed to rise until equilibrium is reached, resulting in a thicker mat. If the head of material is decreased, the force on the screed also decreases, causing the angle of attack to decrease (nose down) and the screed to fall until equilibrium is reached, resulting in a thinner mat. The head of material is directly affected by the operation of the conveyors and augers on each side of the paver. When the conveyors and augers are operating, the mix is pulled from the hopper and distributed across the front of the screed by the augers. As long as the flow of mix is constant, the head of material pushing against the screed remains constant as well, and the mat is smooth with a consistent thickness and texture. If the head of material is allowed to vary, the screed moves up and down (commonly referred to as “hunting”) because of the change in the forces acting on it. If the amount of mix carried in the auger chamber is decreased because the slat conveyors and augers are off, the screed falls, reducing the thickness of the mat. When the slat conveyors and augers turn on, more mix is General Construction Manual © Missouri Department of Transportation 2006 400-29 carried back to the auger chamber. This increases the force on the screed and causes it to rise, resulting in a thicker mat. The head of material is affected each time the conveyors and augers are turned off and on. Therefore, the position of the flow gates is very important in regulating the amount of mix that the conveyors and augers distribute in front of the screed. Varying thickness also results in varying smoothness. Another factor that affects the consistency of the head of material in front of the screed is the temperature of the mix. A load of cold, stiff mix increases the force acting on the screed and causes the screed to rise, increasing the thickness of the mat. A load of hot, less stiff mix decreases the force acting on the screed and causes the screed to fall, decreasing the thickness of the mat. Adjusting the elevation of the tow points or the screed pivot point (by turning the thickness control cranks) will change the thickness of the mat by changing the angle of attack of the screed. However, it takes approximately five times the length of the leveling arms for the screed to complete the thickness change. Approximately 65% of the change will occur in the first leveling arm length and the remaining 35% will occur in the last four lengths. This is the case regardless of whether the thickness is increased or decreased and regardless of the amount of thickness change. In manual control, the screed operator must be aware of the reaction time of the screed. If a second thickness change is made before the first change has been completed, the first change will never be completed and it will take five leveling arm lengths for the second thickness change to be carried out. For this reason, continually adjusting the thickness controls will not produce a smooth mat. Manual changes in the angle of attack of the screed by turning the thickness control cranks should not be made when the paver is operating with automatic controls (grade and slope controls). Otherwise, the automatic controls will change the elevation of the tow points as a result of the manual change and any attempt to adjust the thickness of the mat will not occur. Paving Speed. When paving operations start, the speed of the paver should be adjusted to the plant production rate or the rate that mix is delivered to the paver. In order to obtain the smoothest mat possible, the paver operation should be continuous and at a constant speed. The head of material in front of the screed and the angle of attack of the screed should also be constant. As soon as the first load of mix has been placed, the texture of the surface should be checked for uniformity. Any necessary adjustments shall be made to ensure uniform placement of the mix to the proper line and grade. A straightedge should be used to determine if a smooth surface is being obtained. Yield should be checked occasionally by comparing the amount of mix placed in a given area to the amount required by the plans. Frequent checks will prevent unanticipated overruns or underruns. It is not good practice to operate the paver at a high speed and then stop and wait for another load. If a gap occurs in the delivery of mix, the MTV, if used, should be stopped without being completely emptied. This will retain a sufficient amount of mix on the augers to blend with the new, possibly segregated, mix in the next load. In addition, the paver should be stopped before the level of mix in the hopper is below the top of the tunnels or flow gates so that the head of material in front of the paver screed remains constant and the proper smoothness of the mat is achieved when resuming placement. Mat thickness is a function of the angle of attack of the screed and the paver speed. If the paver speed is changed during placement, there will be a change in the mat thickness and texture, Division of Construction and Materials 400-30 © Missouri Department of Transportation 2006 provided no other changes to the system are made. Adjustment of either without corresponding adjustment of the other will affect the mat. If the paver speed is increased, the angle of attack of the screed and, therefore, the thickness of the mat will decrease. If the paver speed is decreased, the angle of attack of the screed and, therefore, the thickness of the mat will increase. If the paver must be stopped because mix is temporarily unavailable, it should be stopped as quickly and smoothly as possible before the level of mix in the hopper is below the top of the tunnels or flow gates. This will keep the head of material in front of the screed constant and minimize the effect of the rapid speed change on the angle of attack of the screed. When more mix is deposited into the hopper, the paver should be brought back to speed as quickly as possible, again minimizing the effect of the change in speed on the angle of attack. Since the head of material has been kept constant, a smooth mat will be constructed. The length of a delay so that it is still possible to place and compact the mix to obtain smoothness and the required density will depend on the environmental conditions (air temperature, surface temperature, and wind), the temperature of the mix in the hopper, the uncompacted thickness of the lift, and other factors. If the paving operation is put on hold until the next load arrives, the paver should be stopped with the hopper as full of mix as possible. The mix will remain in a mass in the hopper, reducing its rate of cooling. In addition, the paver should remain in one position until more mix is available. If the paver is moved forward periodically, the amount of mix in the hopper is decreased and the remaining mix will cool faster. The mix retained in the auger chamber and underneath the screed will cool faster because of the small amount of mix in these areas and because some mix is contact with the existing surface. Also, if the paver is moved, the thickness and texture of the mat will be affected since the head of material in front of the screed will decrease. There will be portions of the mat that the rollers cannot compact because of the curvature of the drums and the overhang of the screed. Density may be very difficult to achieve in these areas once the paver is moved because the mix has cooled. For all of these reasons, the paver should be moved as little as possible during long stops. Before compaction of the mix, the surface should be checked and any irregularities corrected by adding or removing mix. 400.7.5 Hand Spreading. There may be areas where the paver may not be able to place mix. In these areas, hand placement is permitted. The mix should be thoroughly loosened and distributed uniformly so that segregation does not occur. When mix is dumped in piles, it should be placed as close as possible to its final location to avoid overworking the mix and possibly causing segregation. The mix is then spread by hand using shovels, rakes, or lutes. Any mix that has formed into lumps and does not break down easily should be discarded. Before compaction of the mix, the surface should be checked and any irregularities corrected. 400.7.6 Material Acceptance. Roadway inspectors are responsible for collecting tickets for each load of mix accepted. The inspector will keep one copy of the ticket in order to keep a running total of the tonnage received during the production period. If mix that does not comply with specifications arrives at the paver it should be rejected. When a load is rejected, a notation should be made on the ticket stating the reason for rejection. The rejection should also be recorded in the inspector’s diary. General Construction Manual © Missouri Department of Transportation 2006 400-31 Reasons for rejecting mix are: (1) Too hot. Blue smoke rising from the mix indicates overheating. The temperature should be checked immediately. (2) Too cold. A generally stiff appearance or an improper coating of aggregates indicates insufficient heating. The temperature should be checked immediately. (3) Excess moisture. Steam rising from the mix and/or stripping of binder from the aggregate may indicate that the aggregate was not sufficiently dried before mixing. (4) A lean mix is the result of too much fine aggregate, not enough binder, or both. A dull brown color and/or a peaking of the load usually indicate a lean mix. (5) A fat mix is the result of too much coarse aggregate, too much binder, or both. A slick or greasy appearance and/or a slumping of the load usually indicate a fat mix. (6) Segregation. Segregation of the mix may occur because of improper handling at any point during production or placement and may be serious enough to warrant rejection. See below and Section 400.8 “Mat Problems.” on page 35. (7) Contamination. Mix that has been heavily contaminated with any material not included in the JMF will be rejected. Minor contamination such as trash or other foreign material, must be removed from the mix before it is placed. (8) Defective. Mix that is considered defective contains excess binder, is loose or broken, is mixed with dirt, etc. Defective mix must be rejected or removed and replaced. Segregation must not be permitted. If segregation occurs, adjustments should be made to the plant and/or paving operations immediately. If segregation continues, production shall cease and will not resume until the cause has been determined and corrected. However, if random segregation occurs intermittently, production may continue and the segregated areas will be properly addressed within a reasonable time frame. All segregated areas will be removed and replaced with satisfactory mix, at the contractor’s expense, to the limits determined by the engineer. The density gauge (nuclear or non-nuclear) may be used in accordance with MoDOT Test Method TM-75 to solve disputes with the contractor over the existence of segregation. The Standard Specifications do not put restrictions on who performs the testing. However, if QC personnel perform the testing, QA should be present to observe. Any mix that is removed because it is segregated, contaminated, or defective must be replaced with satisfactory mix at the contractor’s expense. The mix shall then be immediately compacted to the required density. 400.7.7 Compaction. Compaction is the final stage in the paving process and is the most important factor in the performance of the pavement. Through compaction, the strength of the pavement is developed and the texture and smoothness are established. Compaction is the process of compressing the mix and reducing its volume by applying external forces. By reducing the volume, the density of the mat is increased. The aggregates in the mix are forced closer together, which increases the friction between the aggregates and reduces the air void content of the mix. Adequate compaction of the mix improves the pavement’s resistance to fatigue and permanent deformation (rutting). Compaction also decreases the pavement’s low-temperature cracking potential and reduces air and moisture penetration into the pavement to help prevent stripping. The factors that affect compaction are the mix properties and temperature, the environmental conditions, the lift thickness, and the condition of the underlying surface. Division of Construction and Materials 400-32 © Missouri Department of Transportation 2006 The ability to obtain the required density is influenced by a number of factors. One factor is the aggregate properties (surface texture, particle shape, number of fractured faces, and dust content) and the combined gradation of the mix. In general, aggregates with properties that improve the pavement’s resistance to fatigue and rutting require increased compactive effort. Another factor is the asphalt binder grade and the binder content of the mix. A binder grade with a high viscosity or low penetration will generally produce a stiff mix. In general, a mix with insufficient binder content may be stiff, whereas a mix with excess binder content will compact easily and may shove under the rollers. A stiff mix usually requires greater compactive effort. Finally, the production temperature of the mix also affects the compactive effort needed. A mix produced at a low temperature will be stiff, whereas a mix produced at a high temperature will be tender and very easy to compact. The contractor shall determine the rolling zone and pattern. However, rolling shall be performed within the proper time and temperature intervals for each lift and shall continue until no further consolidation of the mat occurs and all roller marks are removed from the surface of the mat. The rolling zone is the area of the mat that is at the correct temperature to be efficiently compacted. The roller pattern depends on the type of compaction equipment, paving width, roller width, the number of passes, and the roller speed. The time available for compaction is defined as the time it takes for a mix to cool from the temperature behind the screed to a minimum compaction temperature. This is highly dependent on the rate of cooling of the mix. Once the mat has cooled to a certain temperature, which is different for all mixes, additional compaction results in very little density gain. Rolling below this temperature may result in aggregate fracture and a decrease in density. In general, rolling should occur at as high a temperature as possible, without causing shoving, in order to achieve the required density with minimum compactive effort. The lift thickness, mix temperature, and environmental conditions (the ambient and underlying surface temperatures, wind, rain, etc.) also affect the time available for compaction. The rollers should move at a slow, uniform speed. The faster a roller passes over a point in the mat, the less time the weight of the roller is applied to that point. Therefore, less compactive effort is applied to that point. As the roller speed increases, the amount of density gained with each pass decreases. As the roller speed decreases, the amount of density gained with each pass increases. Roller speed is also governed by the displacement and tenderness of the mix. To obtain uniform density, each point in the mat must be rolled a certain number of times. The width of the mat divided by the width of the roller drums can be used to determine the number of passes needed to cover every transverse point in the mat. However, the necessary number of passes over each point depends on the type of roller and its position in the roller train, the lift thickness, the mix temperature and properties (binder grade and content and aggregate properties), and the environmental conditions. Roller passes must be distributed uniformly over the width and length of the mat. The rollers should be moving most of the time. If the line of rolling is suddenly changed or if the direction of rolling is suddenly reversed, displacement of the mix may occur. Therefore, any pronounced change in the direction of the roller should be made on stable material. The reversal points of the rollers may need to be staggered to prevent shoving of the mix. The roller should not sit on the hot mat. Otherwise, an indentation will occur that may not be removed by additional rolling. After the mix has been spread, struck off, and any irregularities in the surface corrected, the mix shall be compacted thoroughly and uniformly to obtain the required compaction while the General Construction Manual © Missouri Department of Transportation 2006 400-33 mix is in a workable condition. Excessive rolling, to the extent of aggregate degradation, will not be permitted. Breakdown rolling shall begin as soon as the mat will support the roller without shoving. The breakdown roller shall also be kept as close to the paver as possible to ensure maximum compaction while the mix is hot. Again, it should not be kept so close that shoving or checking of the mat occurs. Breakdown rolling is usually accomplished with a vibratory roller, but a pneumatic tire roller may also be used. When intermediate rolling is used, it should follow breakdown rolling as closely as possible while the mix is still hot. Intermediate rolling is usually accomplished with a pneumatic tire roller. Finish rolling should be done while the mix is still workable enough to remove all roller marks and is usually accomplished with a steel wheel roller. If a vibratory roller is used as the finish roller, it must be operated in static mode. Otherwise, the aggregate at the surface of the mat may fracture. Finish rolling is defined as the final pass of the last roller. 400.7.8 Transverse Joints. The type of transverse joint constructed depends on whether or not traffic will be allowed on the mat before paving is resumed. If traffic will be allowed, a temporary depth transition (also called a tapered joint) is necessary to provide a smooth and safe transition. The temporary depth transition must be constructed to the satisfaction of the roadway inspector. If traffic will not be allowed, a vertical header can be constructed. If a temporary depth transition is constructed, treated release paper or a similar material may be used. The mix will not stick to these materials (and they do not stick to the underlying surface) allowing for easy removal of the transition. Before paving is resumed, a vertical edge must be constructed by cutting back the previously laid mat to a location with the full thickness of the lift and the required density. Other methods of constructing a vertical edge may be used if satisfactory results are obtained. Any excess mix should be removed and the area adjacent to the vertical edge cleaned and tacked. The vertical edge may be lightly tacked, if directed by the RE, just before paving is resumed to help seal the joint. Starting blocks may be used when paving operations are resumed. The purpose of starting blocks is to ensure that the uncompacted thickness of the lift will provide for the required thickness after compaction. Also, the dip that occurs near the joint as a result of insufficient mix is avoided if starting blocks are used properly. As a rule of thumb, mix will compact 20% to 25% under proper compaction procedures, depending on its properties. This means that the uncompacted mix must be placed between 120% and 125% of the compacted thickness of the lift. For example, if the compacted lift thickness is to be 2 in., the uncompacted thickness should be roughly 2-½ in. and the starting blocks, if used, should be ½ in. thick. If the compacted lift thickness is to be 3 in., the uncompacted thickness should be roughly 3-¾ in. and the starting blocks, if used, should be ¾ in. thick. Hand manipulation of the mix should be minimal so that unsightly surface texture is avoided. Breakdown rolling should be accomplished as quickly as possible. The roller should pass slowly and completely over the joint before reversing. The joint should be straightedged to ensure that the joint has been matched. The completed transverse joint should be well sealed and maintain a consistent grade, line, and surface texture between the adjoining mats. 400.7.9 Longitudinal Joints. Longitudinal joints in the final surface shall be placed at the locations specified on the plans, generally at or outside of the lane lines of the roadway. However, pavement markings shall Division of Construction and Materials 400-34 © Missouri Department of Transportation 2006 not be placed on a longitudinal joint. The longitudinal joints in the underlying lifts shall be offset by a minimum of 6 in. so that the joints do not fall in the same location throughout the full depth of the pavement. In construction of the first pass, the paver should run as straight as possible so that the joint can be matched on the next pass. Also, the unconfined edge must be properly compacted so that the joint will not deteriorate under traffic. The roller should make as many passes over the unconfined edge as it does over the rest of the mat. Also, the roller should extend over the unconfined edge by approximately 6 in. to ensure that the compactive effort of the roller is applied in a vertical direction and to reduce lateral displacement of the mix during compaction. No edge differential shall be left in place for more than 7 days, unless approval is granted from the RE. In construction of the second pass, the exposed edge may be lightly tacked, if directed by the RE, to help seal the joint. The adjacent lane should be overlapped by approximately 1 in. or more to ensure that sufficient mix is available to properly seal the joint. If excessive overlapping occurs, the joint should be “bumped” or the excess mix should be removed. Hand manipulation of the mix should be minimal so that unsightly surface texture is avoided. Again, the uncompacted mix should be placed between 120% and 125% of the compacted lift thickness to account for roll down into the joint when the mix is compacted. If too much or too little mix is placed at the joint, the elevation of the adjacent lane will not be matched and the required density may not be achieved. The longitudinal joint should be compacted on the hot side of the mat with the roller extending approximately 6 in. over the joint. This does not necessarily have to be accomplished with the first pass. The joint should not be rolled to the extent that degradation of the aggregate occurs. Other methods of constructing a longitudinal joint may be used if satisfactory results are obtained. However, the completed longitudinal joint should be well sealed, flush, and along true lines. 400.7.10 Density Samples. All mixes having density requirements will be sampled and tested in accordance with the applicable Standard Specifications. Samples are cut from the roadway within 24 hours of mix placement and compaction. Cores, with a minimum diameter of 4 in. and a thickness equal to the full depth of the lift, are taken at locations determined by random numbers. The contractor is responsible for cutting the cores and repairing the roadway. The holes shall be filled with the mix under production, or an approved cold patch, and thoroughly compacted immediately to the satisfaction of the roadway inspector. It is preferred that the APIW or the Random Locations spreadsheets are used to determine core locations. These spreadsheets will automatically generate the random numbers and calculate the stationing and offset for each core. Exhibit A in MoDOT Test Method TM-41 is a table of random numbers that may also be used to determine core locations. The ‘A’ value is used to calculate the distance, in stations, from the beginning of the mat or sublot. The ‘B’ value is used to calculate the offset from the edge of the mat. Start at any point in the table and move either vertically or horizontally, using corresponding ‘A’ and ‘B’ values. Skipping pairs of numbers or reversing the direction is not permitted because this does not result in true random sampling. The cores should be examined and any in questionable condition should be discarded. The cores are to be delivered to the plant inspector free from broken edges, deformations, and distortions. Each core should be clearly marked with an identification number. Additional cores should be taken only if the inspector suspects that the first core was damaged. Coring often leads General Construction Manual © Missouri Department of Transportation 2006 400-35 to a pothole in the pavement at that location. Therefore, excessive coring of the mat will not be permitted. The roadway inspector is responsible for completing an Asphalt Roadway Report (Figure 400.6) for each production period. One copy is provided to the plant inspector along with the density sample(s). The plant inspector will complete the applicable portions and return it to the roadway inspector. The inspector responsible for acceptance of the mix will sign the Asphalt Roadway Report and give a completed copy to the contractor immediately. 400.8 Mat Problems. The following sections describe some important mat problems, possible causes and solutions, and their effects on the long-term performance of the pavement. However, the solutions may vary with the characteristics of the mix and may not work in all cases. The intent is to give the inspector an idea of what may be causing the problem and the possible solutions that the contractor may employ. Non-uniform Texture. Non-uniform texture is a difference in the appearance of the mat, both transversely and longitudinally, as it is placed and compacted. Normally, minor differences in surface texture will be apparent because of differences in the alignment of the coarse aggregate particles. In addition, a mix with a high percentage of fine aggregate will have a more uniform surface texture than a mix containing a high percentage of coarse aggregate. A non-uniform surface texture may be caused at nearly any point in the paving operation. The easiest way to obtain a uniform surface texture is to maintain a constant head of material in front of the screed. Also, the mix should be handled properly at all times. The paver and screed should be well maintained and in good operating condition. Non-uniform surface texture is usually associated with non-uniform density. As the density decreases, the air void content increases, and the durability of the pavement decreases significantly. Surface (Auger) Shadows. Surface shadows are dark areas that appear in the surface of the mat. The shadows may extend across the full width of the mat or only partially. In most cases, the shadows cannot be seen until after the pavement has been subjected to traffic and some of the binder has worn off the aggregate near the surface. In severe cases, surface shadows may be visible immediately behind the paver. Surface shadows are caused primarily by overloading the augers. Whenever the amount of mix in the auger chamber buries the augers, shadows will be formed. The spacing of the shadows normally corresponds to the points where the augers were turned on after being shut off. The amount of mix carried in the auger chamber should be maintained at a level near the center of the auger shaft. This means the flow gates and the speed of the conveyors and augers should be set so that the conveyors and augers operate as close to 100% of the time as possible and stopping and starting of the conveyors and augers is minimized. In no case should the top of the augers be completely covered with mix. Surface shadows are not detrimental to the long-term pavement performance of the mix. The main concern with surface shadows is the visual appearance of the pavement. Division of Construction and Materials 400-36 © Missouri Department of Transportation 2006 Bleeding and Fat Spots. Bleeding is the result of asphalt binder flowing to the surface of the pavement. Bleeding usually occurs shortly after traffic is allowed on a new pavement during hot weather and while some moisture is still present in the mix. Traffic essentially continues to compact the pavement, which decreases the air void content and squeezes some of the binder out of the mix. Bleeding may be caused by an excess of binder and/or moisture in the mix, or by a lack of adequate space in the mix for the binder. Another possible cause of bleeding is excessive crack and joint sealant or tack coat on the underlying surface, which may be drawn up through the mat. Bleeding is usually represented by a longitudinal streak in the wheelpaths of the pavement. Fat spots are isolated areas where asphalt binder has come to the surface of the mat during the laydown and compaction operations. These spots can occur erratically, or may be numerous in a regular pattern. Fat spots are primarily caused by excessive moisture in the mix. This is a common problem with mixes that have a high percentage of fine aggregate and/or aggregates with high porosity. If all of the moisture in the aggregates is not removed at the plant, the moisture vapor will force the binder to the surface as the moisture escapes from the mix and evaporates. Fat spots occur more frequently when aggregate stockpiles are wet or when the moisture content varies within the stockpiles. In addition, fat spots can be associated with segregated areas in the mat. Variations in the mat temperature behind the paver indicate that the moisture content of the mix may also be variable. Where moisture has evaporated, the temperature is lower. This can contribute to pavement bleeding under traffic and fat spots in the mix during construction. Therefore, the aggregate should be thoroughly dried, especially when dealing with highly absorptive aggregates. Bleeding problems associated with excess binder can be solved by reducing the binder content of the mix. If rutting also occurs, the only solution is to redesign the mix. Occasional fat spots in the mix are not detrimental to the long-term pavement performance. However, a large number of fat spots in the wheelpaths or bleeding can affect the longterm pavement performance because of the variations of binder and air void content throughout the mat. Shoving, rutting, and reduced skid resistance may occur in a pavement that contains many fat spots or bleeding. Surface Waves. Surface waves, both short and long, can be caused by an inconsistent head of material in front of the screed. Variations in the amount of mix in the auger chamber causes the screed to “hunt” for an angle of attack and move up and down, resulting in variations in the thickness of the mat and a wavy surface. Variations in the stiffness of the mix as a result of temperature or composition changes can also cause surface waves. As the stiffness varies, the mix pushes on the screed with varying force, causing the screed to rise and fall. Surface waves are also a result of the poor mechanical condition of the screed, improper mix delivery, or improper automatic grade control procedures. Long waves may also be caused by continual adjustment of the thickness control cranks, the condition of the underlying surface, and if the direction of the rollers is suddenly changed. If the distance between the waves corresponds to the distance between truckloads, the waves may be a result of emptying the hopper and conveyors between loads. The only way to eliminate surface waves is to prevent their formation. One way this can be accomplished is by keeping the head of material as consistent as possible by the proper setting of the flow gates and continuous operation of the conveyors and augers. In addition, the stiffness General Construction Manual © Missouri Department of Transportation 2006 400-37 of the mix should be maintained as consistent as possible. Mix stiffness is controlled at the plant by keeping the mix temperature, aggregate gradation, and the binder and moisture contents as constant as possible. Finally, proper mix delivery, paving, automatic grade control, and rolling procedures should be used. Long-term pavement performance is affected by surface waves in two ways. First, the waves reduce the smoothness of the pavement. If the waves are severe enough to increase the dynamic or impact loading of the pavement under heavy truck traffic, the structural integrity of the pavement will decrease. Secondly, the factors that cause surface waves can affect the pavement density. Washboarding is a roughness built into the pavement during compaction. A vibratory roller operated at a high speed or at an improper frequency and/or amplitude setting usually causes washboarding. Proper operation of the vibratory roller is the only way to eliminate washboarding. Because washboarding affects the density obtained during compaction, the long-term durability of the pavement can be significantly reduced. Tearing. Tearing is defined by an open texture in the mat behind the screed. Tearing may appear as longitudinal streaks. These streaks are usually caused by improper crown in the screed. A streak in the center of the mat may be caused by a lack of lead crown. On the other hand, an excess of lead crown may result in streaks that occur along both edges of the mat. The proper relationship between the lead and tail crowns will result in a uniform mat texture across its full width. A longitudinal streak may also occur in center of the mat as a result of a lack of mix being pushed under the gearbox because of missing, worn, or improperly set reverse augers or paddles. Gearbox streaks are more prevalent in mixes with a large nominal maximum size aggregate or a low binder content. A gearbox streak can be eliminated by increasing the amount of mix forced under the gearbox by adjusting or installing reverse augers or paddles on each side of the gearbox. Tearing that occurs at the edge of the mat can be caused by improper flow gate settings or incorrect installation of the screed extensions. Partial-width tearing of the mat can result if the screed has not been uniformly preheated before paving begins. Full-width tearing of the mat can be caused by warped or worn screed plates, high paver speed, a mat thickness that is too thin for the size of aggregate in the mix, and low mix temperatures. The long-term pavement performance is affected because of the decreased density and increased air voids that occur as a result of tearing. In addition, the torn areas will be more susceptible to raveling and to the effects of moisture (stripping). Shoving and Rutting. Shoving is the displacement of mix in a longitudinal direction and may occur during compaction or under traffic. In most cases, shoving during construction is accompanied by a large bow wave in front of the breakdown roller, particularly at the reversal points near the paver. However, shoving can occur under any roller that is operated improperly. Generally, shoving and rutting occur in an unstable mix because of excess binder and/or moisture content, an improper aggregate gradation, or the binder and aggregate properties. Excess crack and joint sealant or tack coat from the underlying surface may be pulled into the mix, increasing its tenderness and fluidity. Shoving may also occur if the underlying surface was not properly cleaned and tacked. Rutting is distortion of the mat in the vertical and transverse directions. Consolidation (purely vertical) rutting occurs when heavy traffic passes over an unstable mat because adequate Division of Construction and Materials 400-38 © Missouri Department of Transportation 2006 compaction was not achieved during construction. The traffic is essentially finishing the compaction process. The most common form of rutting is transverse distortion. In this case, the mat shoves as a result of the lateral flow of the mix under traffic. The solution to a mix that shoves under the compaction equipment is to increase its internal stability. This can be accomplished by reducing the binder and moisture content of the mix, lowering the production temperature, or adjusting the aggregate gradation. The compaction process may need to be changed in order to obtain the required density during construction and reduce the amount of shoving and/or rutting that may occur under traffic. Also, the underlying surface should be cleaned and a tack coat properly applied. Generally, shoving and rutting are caused by an unstable mix. These mixes will continue to distort under traffic, both longitudinally and transversely. Shoving during construction is a strong indication that the pavement will rut and not perform adequately under traffic. Checking. Checking is defined as short transverse cracks that occur in the surface of the mat during the compaction of a tender mix. Severe checking results in longer cracks that are spaced closer together. However, checking does not extend through the full depth of the mat. When checking occurs, the cracks are formed in the bow wave as a result of the mix shoving in front of the steel wheel rollers. Checking rarely occurs during the first or second pass of the rollers. If checking is going to occur, it will normally happen after the temperature of the mix has cooled to a certain point and additional passes are made with the steel wheel rollers. Checking usually does not occur with a pneumatic tire roller. Checking may be caused by excessive deflection of the mat under the compaction equipment because of a weak or yielding underlying surface. A mix that checks does not have enough internal stability at elevated temperatures to support the weight of the rollers causing excessive movement of the mat. The checks are then formed when the surface of the mat is pulled apart as the underlying surface deflects during compaction. The checks should only appear where the underlying surface yields. The most common cause of checking is a poor mix design. Excessive binder and/or moisture content, particularly in addition to low VMA, results in a tender mix that is easily displaced under the rollers. If the mix is tender because of excessive binder, checking should occur on a regular basis. If the mix is tender because of excessive moisture, checking should only occur when the aggregate has not been sufficiently dried. Mixes that contain a high percentage of fine aggregate or lack adequate VMA will be tender and check continuously. If the gradation varies, however, checking may occur only when the gradation is improper. Finally, the aggregate properties can significantly affect the amount of checking. Checking is greatly increased if the mix temperature is too high for the binder grade. Secondary causes of checking are an overheated mix, improper rolling procedures, and a poor bond between the mat and the underlying surface. Occasionally, checking may be caused by temperature differences within the depth of the lift (heat checking). If checking is the result of a poor underlying surface, it should be repaired and properly prepared. Soft spots should be removed and replaced. Areas of excessive deflection should be stabilized or removed and replaced. If the mix contains an excess of binder or moisture content, the binder content should be decreased or the aggregate properly dried. In order to reduce the moisture content of the mix, the production rate of the plant may need to be slowed down or the plant operating conditions may need to be changed. The VMA may also need to be increased. If General Construction Manual © Missouri Department of Transportation 2006 400-39 checking is caused by excessive fine aggregates in the mix, the gradation should be adjusted. The best approach to rolling a mix that has tendency to check is to compact the mat as much as possible while the mix is hot and before checking starts, stay off the mat in the middle temperature zone when checking is most likely to occur, and then finish the mat once it has cooled enough to support the weight of the finish roller. Also, if the mix is at a high temperature when it reaches the paver, it should be allowed to cool before compaction. Improper rolling procedures should be corrected and the underlying surface should be properly prepared. Although checking only extends a short distance into the mat, they are highly detrimental to the long-term performance of the pavement because of the lack of density associated with the checks. Inadequate density leads to increased air voids. Therefore, the pavement life will be significantly reduced. Poor Compaction. The mat should be compacted so that the density and air void content are within specifications. If the air voids are too high, the mat will be permeable and will not have the desired durability. If the air voids are too low, the mix may become unstable under traffic resulting in shoving and/or rutting. When the mix is too stiff or too tender, compaction is difficult. Some possible causes of poor compaction are inadequate support from the underlying surface, improper type and weight of rollers, improper tire pressure in rubber tire rollers, improper rolling procedure, improper mix design, segregation, high moisture content, and a low mix temperature or variations in the mix temperature. Solutions to compaction problems include taking the necessary steps to ensure adequate support, producing a satisfactory mix, and using proper paving and rolling procedures. If the binder content is too high, the mix may compact too easily. If the binder content is too low, the mix may be stiff and difficult to compact. A satisfactory mix design will produce a mix with an optimum binder content that can be compacted to the required density with reasonable effort. Reducing the speed of the rollers and/or increasing the weight and number of rollers can increase density. Inadequate compaction is detrimental to the long-term pavement performance because the mix will be permeable to air and water. Water can flow through the mix and reduce the strength of the underlying base. The high air voids also result in excessive oxidation of the mix, which leads to raveling, cracking, and general deterioration of the mix over time. When the air voids are excessively low after compaction, because of an unsatisfactory mix, the mix is likely to rut and shove under traffic. Joint Problems. Joint problems are caused by improper construction of the joint, improper start-up procedures, or inadequate compaction. Poor transverse joints are associated either with a bump at the joint and/or a dip in the pavement near the joint. Poor longitudinal joints are usually characterized by a difference in elevation between the two lanes and/or raveling of the joint. See GCM Sections 400.7.7 “Compaction.” on page 31 and 400.7.8 “Transverse Joints.” on page 33 for the proper joint construction procedures. A poor transverse joint will not affect the long-term pavement performance if density is achieved. However, the joints will be felt by traffic. An improperly constructed longitudinal joint, on the other hand, can seriously decrease the serviceability of the pavement. A poorly Division of Construction and Materials 400-40 © Missouri Department of Transportation 2006 placed and compacted longitudinal joint will ravel and, eventually, wear away under traffic. Also, the joint may be porous, allowing the environmental elements to damage the underlying subgrade or base material, which may lead to