106.4 Plant Inspections

This article establishes procedures for calibration of scales, material proportioning plants, bituminous distributors and field testing equipment.

With the advent of quality control / quality assurance (QC/QA) many of the plant calibrations and verifications performed by MoDOT personnel have become the responsibility of the contractor or producer. In those cases, the inspector’s job now is to review the calibration or verification for accuracy, timeliness, and correct procedures.

When possible, it is beneficial for the inspector to be present during any calibration or verification procedures to observe and learn about the process and equipment.

Producers using their own weights for verifications must submit a certificate of test from the Department of Agriculture annually. If a producer performed calibration or verification does not appear to meet the intent of the specification, the District Construction and Materials Engineer should be notified.

Calibrations may be performed in either English or metric units depending on the equipment being calibrated. Please see EPG 106.11 Metric Implementation for additional information relating to the conversions and procedures for metric measurement.

All calibrations should be performed when dictated by specifications or more often when deemed necessary by field performance or equipment manuals. When in doubt, a new calibration should always be performed to assure accurate measurement.

Verifications are performed to confirm the accuracy of a calibration. If verification fails, the calibration is in doubt and the device should be recalibrated. A failing verification may be repeated. However, any additional verification should not take precedence unless it is clear that the earlier verification was incorrectly performed.

106.4.1 Apparatus

(a) Rule with suitable graduations to accurately measure the material to be inspected.

(b) Stop watch with at least 0.5 second graduations.

(c) Two sets of standard weights, one milligram to 100 grams, and one gram to 2000 grams.

(d) Standard 50-pound (20 kg) weights as required in the specifications.

106.4.2 Procedure

Calibration of truck scales, material proportioning plants, and bituminous distributors shall be performed when requested by Construction personnel. Ready-mix concrete plants shall be calibrated annually when they are producing steadily for MoDOT projects. Central paving plants shall be calibrated annually and whenever the plant is moved to a new location. If calibration forms are not required or provided, all original calibration data and calculations shall be kept on file.

106.4.2.1 Truck Scales

Calibration of truck scales shall be performed in accordance with EPG 400 Flexible Pavement.

106.4.2.2 Reports

A copy of the calibration and verification results including manufacturer's name, serial number, and any other pertinent data shall be saved electronically per district protocol. The data should be accompanied by a letter of transmittal indicating that the scales have been found to comply with Specification requirements.

106.4.2.3 Bituminous Mixing Plants

General plant checkout of equipment shall be performed in accordance with EPG 400 Flexible Pavement.

Calibration of weighing equipment for batch plants shall be performed in accordance with EPG 400 Flexible Pavement.

Calibration of proportioning equipment for continuous pugmill mixing plants shall be performed in accordance with EPG 400 Flexible Pavement.

Calibration of proportioning equipment for drum mix plants shall be in accordance with EPG 400 Flexible Pavement.

Reports. A copy of the calibration results including manufacturer's name, serial number, and any other pertinent data shall be furnished to the Resident Engineer involved. The data shall be accompanied by a letter of transmittal indicating that the plant has been found to comply with specification requirements.

106.4.2.4 Portland Cement Concrete Proportioning Plants

Scales, water measuring devices, admixture dispensers, and volumetric proportioning units shall be calibrated in accordance with EPG 501.1 Construction Inspection.

Truck mixers used for mixing of Portland Cement Concrete shall be inspected in accordance with EPG 501.1 Construction Inspection.

Central mixers used for mixing Portland Cement Concrete shall be inspected in accordance with EPG 501.1 Construction Inspection.

Mixing time for a central mix plant shall be established on the basis of uniformity tests as required by Sec 501. Frequency of uniformity tests is to be as shown in EPG 501.1 Construction Inspection. It is necessary to establish uniformity at each batch size furnished, however, a practical way of doing this is to perform the tests on the smallest and largest batch size the contractor proposes to furnish. Batch sizes between these two sizes may then be mixed at the mixing time established by the uniformity tests. The largest batch tested is never to exceed the manufacturer's rated mixing capacity. The smallest size batch tested is to be of a size that will allow uniform addition of all components, especially air-entraining agent and water, using the same sequence and discharge times as larger batches. It will be the inspector's responsibility to determine whether all of the mix components can be uniformly and continuously batched at that batch size, with that plant. Tests shall be performed according to the following procedures to determine compliance with the maximum permissible difference in results shown in EPG 501.1 Construction Inspection. For plants having more than one mixer drum, the prescribed tests may be performed on only one drum if the drums are made by the same manufacturer, are of the same size, and are in comparable condition as to wear of blades, etc.

Test Procedure. The procedure for uniformity testing is found in TM-82, Concrete Uniformity.

Reports. A copy of the calibration results, including uniformity test results for mixing time reduction, serial numbers, plant manufacturer's name and any other pertinent data shall be furnished to the Resident Engineers involved. The data should be accompanied by a letter of transmittal indicating that the plant has been found to comply with Specification requirements and the approved mixing time. A copy of the report and results of the uniformity tests including calculations should be saved per project in the Contract Information Archive.

106.4.2.5 Portland Cement Treated Base proportioning plants

The procedure to be used for the calibration of portland cement treated base proportioning plants is shown in EPG 501.1 Construction Inspection.

Reports. A copy of the calibration results including plant manufacturer's name, serial numbers, and any other pertinent data shall be furnished to the Resident Engineer. The data should be accompanied by a letter of transmittal indicating that the plant has been found to comply with Specification requirements.

106.4.2.6 Bituminous distributors

The calibration of all distributors will be approved by the engineer prior to use. The contractor is required to furnish all equipment, materials and assistance necessary. A calibration is required before a distributor is used for MoDOT work. Further calibrations are not required unless the district has reason to believe that the distributor has been altered or damaged in such a manner as to make the original calibration invalid. The district participates in the calibration of distributor tanks and assigns a permanent plate to be securely attached to the distributor indicating that it has been calibrated and the results are on file in the district office. Calibration tags are available from Construction and Materials.

106.4.2.6.1 Method for Calibration of a Distributor Tank

(a) The distributor is emptied completely.

(b) An accurate flow meter certified by a reputable agency or a container calibrated to measure exactly a multiple of five gallons (20 L) is obtained.

(c) The distributor is reasonably leveled both longitudinally and transversely.

(d) The tank is filled with liquid in increments of from 5 to 25 gallons (20 to 100 L) depending on the size of the distributor and the available equipment.

(e) After each increment of liquid is added, the liquid in the tank must be allowed to settle to a quiet surface. Then a gauge stick held vertically against the side of the manhole, using the top of the rim or other readily identifiable portion of the distributor as a reference, is carefully lowered, centered and plumbed to just touch the surface of the liquid. The stick is then marked at reference line, removed, carefully measured to the nearest 1/16 in. (2 mm) , and recorded with the corresponding gallon amount. Another acceptable method to determine the depth is to lower the stick until it touches the bottom of the distributor tank. The stick is then removed and carefully measured to the nearest 1/16 in. (2 mm) and recorded with the corresponding gallon amount. Other methods may be used at the districts' discretion providing acceptable results are obtained.

106.4.2.6.2 Reports

A complete copy of the calibration results on Form M-45 shall be furnished to the contractor. The contractor should make a permanent gauge stick, preferably from aluminum 1/8 in. x 1 3/4 in. (3.175 mm x 44.45 mm) x length required, from the measurements with markings and the corresponding volumes obtained from the calibration data. A tag number shall be assigned by the district. The gauge stick shall be checked before giving the tag to the contractor. The tag number shall be stamped on the gauge stick and recorded on Form M-45 (page 7). A completed copy of Form M-45 should be maintained with the distributor.

New calibrations should be performed using English measures and the gauge stick marked the same way. It is acceptable to also convert the calibration to metric and scribe the back of the same gauge stick with those results. The same stick is to be clearly identified as "English" or "metric" on the respective sides.

106.4.2.7 Field testing equipment

The calibration of field testing equipment used by Construction and Materials is a responsibility of the district and shall be performed at least once a year. Certified test weights, one milligram to 100 grams and one gram to 2000 grams shall be maintained in the district and used for calibration purposes only. These weights shall be submitted to the Laboratory for recertification before the date of expiration on the "Certificate of Test" or at least once every two years. In instances when the district does not have the ability to calibrate a piece of field testing equipment, the Laboratory should be contacted for assistance.

106.4.2.7.1 Aggregate quality control scales

The balance or scale for field use shall be calibrated from 0 to maximum capacity to an accuracy of 0.5 percent computed to the nearest gram of the net load applied, or one gram, whichever is the greater, and shall be sensitive to a weight not greater than the minimum graduation value.

106.4.2.7.2 Analytical balance

The weights for use on an analytical balance shall be calibrated to within the tolerances shown in Table 1, AASHTO M 231.

106.4.2.7.3 Air Meter

Calibration of meters designed to determine the percent air-entrainment in Portland Cement Concrete by the pressure method shall be in accordance with EPG 501.1 Construction Inspection.

106.4.2.7.4 Report

A copy of the calibration data including manufacturer's name, serial numbers or identification numbers and any other pertinent data shall be filed in the district office for a minimum of five years. A sticker, label, or tag shall be affixed to each item of test equipment that has been calibrated, showing the date of last calibration.

106.4.2.8 Commercial Scales

Scales used by MoDOT at the supplier's or manufacturer's plants shall be calibrated or verified at the discretion of the district and reports kept on file in the district office.