460.3 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.

460.3.1 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.

460.3.1.1 Batch Plants

Batch plant

Components of a batch plant

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.

460.3.1.2 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.

Drum mix plant

Components of a drum mix plant

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.

460.3.2 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) Gross, tare and/or net weights (masses).

(b) Current date and time.

(c) MoDOT job mix type.

(d) Unique ticket number (may be preprinted on the ticket).

(e) Job number, route and county.

(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 Asphalt Binder Sampling.

(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.

460.3.3 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.

460.3.4 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.

400.3.5 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.

460.3.6 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.

460.3.7 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.

460.3.8 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.

460.3.9 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. Approved mineral filler suppliers are listed on "Qualified Mineral Filler for SMA". 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 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.

460.3.10 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.

460.3.11 Reclaimed Asphalt Pavement (RAP)

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 5% from the virgin aggregate, it cannot be used. At a minimum of once per project or stockpile, for stockpiles built to serve multiple projects, 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 or state roadway of an adjoining state. 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.

460.3.12 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 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.

460.3.13 Asphalt Binder Sampling

Take a daily sample, consisting of two one-pint friction top cans of material, from the binder line supplying the mixer to submit to the Central Laboratory. One can is to be retained and one can, labeled identically in accordance with this section, will be given to the producer for their immediate testing or retention. 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 for one week until one of the following occurs:

• The weekly 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. The weekly gathering of samples does not have to begin on Sunday. For example, if the district ships to the Central Laboratory on Thursday, then the samples should be collected from Thursday through Wednesday so they may be shipped at the earliest possible time after collection.

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 to determine 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.

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.

460.3.14 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.

460.3.14.1 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.

460.3.14.2 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.

460.3.14.3 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.

460.3.14.4 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 AASHTO T 329. 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 once per 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 flights inside the drier. These methods are more effective than simply increasing the temperature of the mix.

460.3.14.5 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 guidance ections 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.

460.3.15 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 EPG 1001.2 Sampling Procedures. 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.

460.3.15.1 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 EPG 401.2.7 Failing Test and EPG 401.2.4 Gradation Control 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. Variability may 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.

460.3.15.1 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.

460.3.16 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 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.

460.3.17 Visual Inspection of Mix

460.3.17.1 Visual Inspection

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.

460.3.17.2 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 Segregation in 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.