Category:404 Bituminous Mixing Plants

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404.1 Construction Inspection for Sec 404

404.1.1 Description (Sec 404.1)

This article contains information on the calibration of the bituminous mixing plants. Detailed information on the operation of the plant and the equipment used to produce bituminous mixes is given in Batch Plants and Drum-mix Plants in Plant Inspection.

404.1.2 Requirements for All Plants (Sec 404.2)

Aggregate (Sec 404.2.1)

See Batch Plants and Drum-mix Plants in Plant Inspection.

Asphalt Binder (Sec 404.2.2)

See Batch Plants and Drum-mix Plants in Plant Inspection.

Binder Sampling (Sec 404.2.2.3) See Asphalt Binder Content in Plant Inspection.

Cold Aggregate Feeder (Sec 404.2.3)

See Batch Plants and Drum-mix Plants in Plant Inspection.

Drier (Sec 404.2.4)

See Batch Plants and Drum-mix Plants in Plant Inspection.

Dust Collector (Sec 404.2.5)

See Batch Plants and Drum-mix Plants in Plant Inspection.

Filler Storage (Sec 404.2.6)

See the applicable portions of Mineral Filler, Hydrated Lime, and Baghouse Fines.

Thermometric Equipment (Sec 404.2.8)

See Temperature Control.

Plant Calibration (Sec 404.2.9)

By specification, the contractor is required to perform the plant calibration. However, the procedures used in calibration will be outlined in this article for informational purposes. The contractor has the option of hiring a certified commercial scale company. The contractor is required to provide the RE with the calibration records proving that the plant has been calibrated and meets the requirements of the Standard Specifications. This includes all measurements, test results, etc.

The plant must be calibrated at the beginning of the season before any mix is produced for MoDOT projects, at any time that the plant is moved, and at any time that the accuracy of the proportioning equipment is in question. QA and QC personnel are not required to be present during the calibration process. However, MoDOT reserves the right to witness the calibration.

If the plant does not meet the requirements of the Standard Specifications, production cannot begin.

Surge Bins (Sec 404.2.11)

The following procedure may be used to calibrate a surge bin or storage silo:

(1) Check the scale for cleanliness and freedom of movement.
(2) A sufficient amount of standard test weights (50 lbs.) must be available to efficiently calibrate the scales.
(3) A device must be available to hang weights from the scale.
(4) Zero the scales, then add standard test weights. Computerized scales are then programmed to read the known weight of standard test weights in use. Remove the weights and check zero.
(5) Add test weights in increments sufficient to accurately calibrate the surge bin.
(6) Remove the weights and add material to the surge bin up to the checked point.
(7) Repeat steps 5 and 6 until the capacity of the surge bin is reached.
(8) The sensitivity of the scale should be checked at various intervals by adding the proper weight.

404.1.3 Requirements for Batch-Type Plants (Sec 404.3)

Aggregate Scales (Sec 404.3.2)

The following procedure may be used to calibrate the aggregate scales at a batch plant:

(1) Charge the hot bins to capacity.
(2) Allow the plant to settle for approximately 24 hours.
(3) Check all moving parts (levers, bearings, knife edges, and beams) for cleanliness and freedom of movement. The scale should read zero when no load is applied.
(4) Suspend a platform below the hopper so that the applied check load will be in the same position as the load, which will be applied when material is actually being weighed.
(5) Balance the scale with no load.
(6) Add standard test weights (50 lbs.) to the suspended platform, centered as nearly as possible under the hopper, one at a time. An increment of 100 lbs. may be used until 75% of the anticipated normal working load has been applied.
(7) Record the scale reading for each increment.
(8) When the last increment has been weighed and recorded, remove the weights.
(9) Charge the hopper with material equal to the weight shown on the scale.
(10) Standard test weights are then added to the platform as before.
(11) This process is repeated until the required capacity is reached.
(12) Check the sensitivity of the scale at each increment within 200 lbs. of the anticipated normal working load. The sensitivity weight shall be no greater than 0.1% of the nominal scale capacity. This weight is placed after the 100 lb. weight has been added and the reading is recorded. This additional weight should cause an immediate, observable reaction on the scale reading. Otherwise, the necessary repairs must be made.
(13) The scale should be calibrated 400 lbs. beyond the anticipated normal working load.

Asphalt Bucket (Sec 404.3.3)

See Batch Plants.

Asphalt Scales (Sec 404.3.4)

The asphalt scales at a batch plant are calibrated using the same procedure as the aggregate scales. However, the total check load only needs to exceed the working load by 50 lbs. and sensitivity is checked using a smaller weight. While checking the tare weight of the empty asphalt bucket, watch carefully to see that the bucket is drained completely. Adjust the tare weight to compensate for any binder and dust clinging to it. If the weighing error exceeds the permitted tolerance, plant operations should not start until the scales are properly adjusted.

Mixer Unit (Sec 404.3.5)

See Batch Plants.

404.1.4 Requirements for Drum Mix Plants (Sec 404.4)

Feed Indicators (Sec 404.4.2) Each cold feed bin should be calibrated to determine the output in tons per hour throughout the anticipated operating range to ensure the correct amount of each aggregate is being supplied to the mix. It will be the contractor’s responsibility to adjust the individual bin settings so that the combined aggregate flow will conform to the gradation requirements of the Standard Specifications and the JMF.

Cold feed bins may be calibrated by using the previously calibrated belt scale. Aggregate passing over the belt scale at various bin gate or belt speed settings will provide a ton per hour (TPH) reading for plotting a calibration curve. Set the master feed control on the control panel at a point that the producer anticipates will be run during maximum production. This probably will be in the 50 – 100% range.

Set an individual bin control feed at three intermediate points such as 30%, 50%, and 70% and check the top point first. Observe the displayed flow rate until it stabilizes and record it in terms of dry tons per hour as a curve point for that setting. The top selected point should provide a flow rate equal to or greater than the maximum rate needed from that particular bin. Plot dry tons per hour vs. bin control settings and approximate the curve for each bin with a straight line. Record the bin gate and master feed control settings on each graph for future reference.

A combined cold feed sample shall be taken with the plant operating at anticipated normal production. By selecting the desired TPH rate of dry aggregate flow, one may determine from the JMF the percentage and thus the TPH to come from each bin. By use of the previously plotted graphs one may then determine the approximate individual bin control settings to use for obtaining the trial gradation. Gradation results will determine the need for fine adjustment of the individual bin controls within two percentage points of the percentage shown for each fraction on the JMF.

Asphalt Meter (Sec 404.4.3) Enter the binder percentage into the control panel at the percentage designated in the JMF. Some plant control systems may be adjusted to enter this figure either as a percentage of total mix or as a percentage of total aggregate. Set the specific gravity compensator using the information on the binder delivery ticket, making temperature corrections if the plant is not equipped to do so automatically.

Record the initial totalizer readings, including the CPT display reading, if provided. Divert approximately 800 to 1000 gallons of the binder flow into a tared asphalt distributor or other suitable equipment providing an accurate gross and net weight. The amount indicated on the totalizers should be within 2% of the net weight obtained from the distributor. If not, the contractor shall make appropriate adjustments and the test shall be repeated until acceptable tests are obtained.

Mineral Filler (Sec 404.4.4) The mineral filler feeder system shall be variable, continuous, and interlocked with the aggregate weigh belt so that the total dry aggregate, including mineral filler, is indicated to the asphalt proportioning system.

Scales for continuous batch weighing shall be accurate to within 0.4% of the indicated weight. The change in load required to change the position of rest of the indicating element shall not be greater than 0.1% of the nominal scale capacity.

The rate of flow of the mineral filler into the delivery system shall have an accuracy of 10% by weight of the material actually being measured in any given period of time.

Belt Scales (Sec 404.4.5) Check the belt scale conveyor for alignment visually to detect any obvious binds, strains or tensions that might prevent the load cell from functioning properly. The conveyor shall be blocked in a solid, even position. If the scale conveyor is portable, all weight should be off the tires. The conveyor belt should be run empty for at least 30 minutes before calibration begins. The scales and the conveyor at the scales shall be protected from wind and weather effects.

To begin calibration of the belt scale, take initial, or zero, readings on the aggregate totalizer located on the computer control panel. On plants equipped with both mechanical and cathode ray tube (CRT) totalizer displays, both readings should be recorded. Divert the aggregate flow from the belt scale conveyor so that all the material passing over and being weighed by the belt scale is run into a tared truck for weighing on a calibrated truck platform scale. A five-minute run is desirable. The truck should be large enough to hold the anticipated flow of material for the entire run. The longer the run, the more accurate the test will be. It is not necessary to time the test. Tests should be run at the lower, middle and upper limits of the anticipated normal production rate.

The net weight of aggregate recorded on the belt scale totalizer must be within 2% of the net weight in the truck. If not, the contractor shall make proper adjustments and the test shall be repeated until acceptable test results are obtained. Depending on the trouble incurred, more than one acceptable test may be desirable to ensure that the scales are properly calibrated.

Upon completion of the calibration, apply the belt scale weights (if plant is equipped) to the empty belt while running, and record the indicated flow rate for future reference. The weights may then be applied to the empty belt scale whenever a quick check of its initial calibration is desired. The belt weight check shall not be used as a substitute for the actual verification.

System Interlocks (Sec 404.4.7) The blending system reads the aggregate flow rate from the belt scale, converts this to a dry flow rate using the percent moisture set in the moisture compensator, computes the required binder flow rate as indicated by the asphalt percent and specific gravity settings, and adjusts the metering pump to increase or decrease the flow of binder. In this way the aggregate, mineral filler, hydrated lime, RAP, and binder flow rates are continuously interlocked.

Set the asphalt percentage, specific gravity or weight per gallon, and aggregate moisture content at the anticipated normal values. Place a load on the belt scale and run the plant with binder, at normal operating temperatures, circulating through the meter. The test should run at least 5 minutes. It is not necessary to time the test.

Using the accumulated tonnage from the aggregate totalizer, determine the required binder. The amount accumulated on the binder totalizer should be within 2% of the required amount of binder calculated. If not, the contractor shall make appropriate adjustments to the blending system. The procedure shall be repeated until acceptable tests are obtained. If the belt scale or binder flow meter is adjusted to eliminate accumulated errors, it may be necessary to require additional verification of the individual units at the adjusted settings.

Moisture and specific gravity compensators may be checked by placing a load on the belt scale, then changing the settings and observing the change in the control panel displays. An increase in the specific gravity setting should produce a decrease signal to the binder pump. The binder flow rate display should not change. An increase in the moisture compensator setting should produce a decrease in the dry aggregate flow rate and a corresponding decrease in binder flow. Opposite adjustments would produce correspondingly opposite effects.

Calibration (Sec 404.4.9) If truck platform scales are used to weigh the mix in a drum plant, the scales can be calibrated as follows:

(1) Check scale floor, lever arms, knife-edges, etc. for cleanliness and freedom of movement.
(2) Balance scales with no load.
(3) Weigh an empty truck on the scales. The tare weight has been determined on a certified commercial scale.
(4) Leave the empty truck on the scale. Add two standard test weights (50 lbs.) on one corner of the scale and reweigh, checking the scales for accuracy in weighing the additional 100 lbs.
(5) Add two more standard test weights to another corner of the scale and repeat the weighing procedure in step 4.
(6) In successive increments, add two standard test weights to each of the remaining corners and check the weight. This will give a check for 400 lbs. in addition to the empty truck weight.
(7) Remove the truck and weights from the scale.
(8) Place a truck with a minimum load, which has been weighed on a certified commercial scale, on the scale and repeat steps 4 through 6.
(9) After step 8 has been completed, remove the standard test weights and add 400 lbs. of material to the truck and repeat steps 4 through 6. Repeat this procedure until the maximum load to be weighed has been checked.

The contractor may utilize in the calibration all of the standard test weights that are available and, therefore, increase the load increments in step 9 accordingly.

Sensitivity of the scale should be checked by adding proper weights to the platform at various intervals as noted above to see if the indicator will “break”.

Contractors may elect, at their expense, to have the truck scales calibrated by the Division of Weights and Measures of the State Department of Agriculture or by a reputable scale manufacturer or service company. The contractor must provide a complete record of the calibration to the RE, which will be maintained with the project records.

Commercial scales must be calibrated annually and approved by the Division of Weights and Measures. Approved scales are marked with a seal giving a number, date tested, and name of testing agency. If calibrated commercial scales are used during the calibration process or during production, calibration will not be required. The location of the scale, the number, and the date tested from the seal of approval must be recorded and maintained with the project records.

Periodic checks of scales should be made to assure that accurate weights are being obtained at all times. This may be by the above calibration procedure or by weighing empty and loaded trucks on certified scales. All scales are to be checked for balance and sensitivity at least once daily when in use. A project record should be made of all scale checks.

404.1.5 Stone Matrix Asphalt Mixtures (Sec 404.5)

See Fibers in Asphaltic Concrete Pavement.

404.1.6 Liquid Anti-Strip Additive Systems (Sec 404.6)

See Liquid Anti-Strip Additives.

404.1.7 Hauling Equipment

See Haul Trucks.