Asphalt Concrete Mixing Plants - Ohio Department of Transportation

Asphalt Concrete Mixing Plants - Ohio Department of Transportation
STATE OF OHIO
DEPARTMENT OF TRANSPORTATION
SUPPLEMENT 1101
ASPHALT CONCRETE MIXING PLANTS
April 18, 2014
1101.01
1101.02
1101.03
1101.04
1101.05
1101.06
Scope
Requirements for All Asphalt Concrete Mixing Plants
Requirements for Batch Plants
Requirements for Drum Mix Plants
Aggregate and Recycle Weighbridge Calibration
Asphalt Binder Metering System Calibration Simulation
1101.01 Scope. This supplement contains equipment requirements for asphalt concrete mixing
plants performing work for ODOT projects. This supplement also contains procedures required
for performing an asphalt mixing plant calibration.
1101.02 Requirements for All Asphalt Concrete Mixing Plants
A. General Computerized Plant System Requirements.
Produce all asphalt concrete in a plant with a computerized plant system approved by the
Laboratory.
Ensure that all printouts contain the following information:
1.
2.
3.
4.
5.
6.
7.
Date.
Time.
JMF number.
Moisture content of the reclaimed pavement.
Percent asphalt binder in the reclaimed pavement to the nearest 0.1 percent.
Percent virgin asphalt binder to the nearest 0.1 percent.
Percent total asphalt binder calculated to the nearest 0.01 percent.
Ensure that all printouts are preapproved by the Laboratory and are turned over to the DET at the
end of the project or the end of the production year.
Ensure that the computerized plant system has an audible alarm system that notifies the plant
operator when the amount of asphalt binder, aggregate, or reclaimed pavement being mixed into
the asphalt concrete is outside the tolerances established by the Contractor‟s Quality Control
Manager. Make appropriate adjustments when production is outside the tolerances.
Use scales and test weights that conform to the regulations of the Ohio Department of
Agriculture. Seal scales as often as the Laboratory directs to ensure their continued accuracy.
1
Seal test weights at least every 3 years at places designated by the Ohio Department of
Agriculture. Equip the plant with one 50-pound (20 kg) test weight for each 400 pounds (200
kg) of the maximum batch weight with a minimum of ten test weights.
Provide a truck scale or recording batch plant scales for the purpose of obtaining the net weight
of each load of asphalt mixture as required in 401.21. Use truck scales that indicate the total
weight within 20-pound (10 kg) increments and have a rated capacity of at least 10 percent
greater than the largest load weighed. Provide a platform large enough to receive the largest
truck used for a single weighing.
Ensure that the adjustments for total and proportional feed are continuously variable and capable
of being locked at any position.
B. Mineral Filler and Dust Control. Provide adequate dry storage for mineral filler, when
required, and provide equipment for accurate proportioning. When mineral filler is specified,
provide a separate bin and feeder with its drive interlocked with the aggregate feeders. Provide
equipment for accurate proportioning of plant collected baghouse fines.
C. Cold Aggregate and/or Reclaimed Asphalt Pavement Feeder.
Ensure the plant has
mechanical means for uniformly feeding the aggregates into the dryer so that uniform production
and temperature are assured. Ensure aggregates are blended from two or more bins at the cold
feed to meet the requirements of the paving mixture specifications and that a synchronized
proportioning method is provided. If recycling capability is required, ensure the plant is
equipped with mechanical means for feeding the desired weight of reclaimed asphalt pavement
into the mix.
D. Emission Controls. Provide a dust collecting system. Ensure the system is made to waste
the dust collected and can return all or any part uniformly to the mixture.
E. Thermometers. Equip the plant with an adequate thermometric instrument, clearly legible
from the mixer operator‟s station, to monitor the temperature of the asphalt binder. Locate the
sensing element or unit in the feed line near the charging valve at the mixer. Where a pyrometer
is used, connect the indicator to the thermocouple by a weatherproof extension wire.
Also, equip the plant with an approved dial-scale, infrared thermometer, mercury-actuated
thermometer, electric pyrometer, or other approved thermometric instrument placed at the
discharge chute of the dryer to register automatically or indicate the temperature of the heated
material.
F. Storage. Provide storage bins capable of storing hot asphalt concrete mixtures up to 24
hours. Insulate or heat storage bins if mixtures are stored for more than 8 hours. Ensure that the
system is capable of maintaining the required temperature without creating hot spots within the
stored mixture. In addition, if hot asphalt mixtures are stored from 8 to 24 hours, introduce a
silicone admixture into the asphalt binder at the rate of 1 fluid ounce per 5000 gallons (1 mL/640
L). Do not use silicone with warm mix asphalt mixtures.
2
G. Asphalt Binder. Provide a sampling valve between the last piping „tee‟ and inlet into the
plant for sampling asphalt binder.
If more than one kind of asphalt binder is to be used concurrently, store each kind separately.
Equip storage tanks to heat the asphalt binder and effectively control the required temperature at
all times. Ensure heating by steam or oil coils, electricity, or other means such that no flame
contacts the binder.
Ensure the circulating system for the asphalt binder is of adequate capacity to provide proper and
continuous circulation between the storage tank and proportioning units during the entire
operating period. Keep the discharge end of the asphalt binder circulating pipe below the surface
of the asphalt binder in the storage tank to prevent discharging the hot asphalt binder into the
open air. Ensure all pipe lines and fittings are steam or oil jacketed or otherwise properly
insulated to prevent heat loss. When the asphalt binder is emulsified asphalt, provide an asphalt
binder transfer system that will enable the operator to turn off or reduce the heat media from all
lines, pumps, and jacketed asphalt binder buckets as soon as the system is open and circulating
properly.
H. Safety Requirements. Provide adequate and safe stairways to the mixer platform, if
applicable. Locate guarded ladders to other plant units where required. Cover all gears, pulleys,
chains, sprockets, and other dangerous moving parts. Provide a stable sampling platform of
suitable size and capacity to support collection of all required samples in a safe manner and that
provides safe viewing of asphalt mix in trucks.
1101.03
Requirements for Batch Plants
A. Computerized Plant System for Batch Plants. In addition to the general computerized
plant system requirements in section 1101.02, print the information on each weight ticket if the
asphalt concrete is directly loaded into the truck. If the asphalt concrete is loaded into a storage
silo, print the information for every 16 tons (15 metric tons) or less of production. Ensure
printouts contain the following additional information:
1.
2.
3.
4.
5.
Tare weight of the asphalt binder scale.
Tare weight of the aggregate scale.
Pounds (kilograms) of virgin asphalt binder.
Pounds (kilograms) of virgin aggregate.
Pounds (kilograms) of reclaimed asphalt pavement measured by a batch scale or Tons
per hour (metric tons per hour) of reclaimed pavement measured by a belt scale.
B. Plant Screens. Equip plants with plant screens located between the dryer and hot aggregate
bins. Ensure adequate screen capacity and size range to separate the heated aggregate into the
sizes required for proportioning so that they may be recombined consistently within the
specification limits. Determine the nominal maximum size aggregate in the fines bin.
Determine the screen type and size so that no aggregate is larger than the maximum determined.
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C. Hot Aggregate Bins. Provide hot bin storage of sufficient capacity to ensure uniform and
continuous operation. Divide bins into the specified number of compartments arranged to ensure
separate and adequate storage of appropriate fractions of the aggregate.
Provide each compartment with an overflow chute of such size and at such a location to prevent
any backing up of material into other compartments or into contact with the screen.
Equip bins with “tell-tale” devices to indicate the position of the aggregate in the bins at the
lower quarter points. Provide an automatic plant shut-off to operate when any aggregate bin
becomes empty.
Use batch plant hot bins that have an oversized material discharge pipe of not less than 6 inches
(150 mm) in diameter. Ensure pipes discharge material at points outside the plant operation and
do not create a hazard or discomfort.
D. Aggregate Scales. Use batch plant scales that have a capacity at least 10 percent greater
than the largest weight required for any one batch. Scales for any weighbox or hopper may be
either beam, springless dial type, or load cells. Ensure the accuracy of the weighing device is
within 1.0 percent of the actual weight being measured when that weight has been determined
using another measuring device and is within 0.5 percent when that weight has been determined
using test weights.
E. Weighbox or Hopper. Provide a means for weighing aggregate from each bin into a
weighbox or hopper, suspended on scales, and of sufficient size to hold a full batch. Ensure
gates, both on the bins and the hopper, do not leak.
F. Asphalt Binder Bucket and Scales. Use a non-tilting asphalt binder bucket with a loose
sheet metal cover. Ensure that the length of the discharge opening or spray bar is not less than
three-fourths the length of the mixer and that it discharges directly into the mixer. Ensure that
the asphalt binder bucket, its discharge valve or valves, and spray bar are fully jacketed or
heated. Use jackets that drain efficiently and ensure that all connections are constructed to not
interfere with the efficient operation of the asphalt binder scales. Use an asphalt binder bucket
with a capacity at least 10 percent greater than the weight of asphalt binder required in any batch.
Ensure that the plant has an adequately heated, quick-acting, non-drip, charging valve directly
over the asphalt binder bucket. The time required to add the asphalt binder shall not exceed 20
seconds.
Ensure scales for the weighing of asphalt binder meet the requirements for aggregate scales,
except ensure a device to indicate at least the last 20 pounds (9.1 kg) of the approaching total
load is provided. Ensure beam type scales are equipped with a tare beam or adequate
counterbalance for balancing the bucket and compensating periodically for the accumulation of
asphalt binder on the bucket.
I. Mixer Unit. Ensure the mixer is designed to provide means of adjusting the clearance
between the mixer blades and liner plates to ensure proper and efficient mixing. If not enclosed,
ensure the mixer box is equipped with a dust hood to prevent loss of dust by dispersion. Ensure
the mixer is constructed to prevent leakage of the contents.
4
Ensure the mixer is equipped with a positive means for governing mixing time and an accurate
time lock to control the operation of a complete mixing cycle by locking the weighbox gate after
the charging of the mixer until the closing of the mixer gates at the completion of the cycle.
Ensure it locks the asphalt binder bucket throughout the dry mixing period and locks the mixer
gate throughout the dry and wet mixing periods.
The dry mixing period is defined as the interval of time between the opening of the weighbox
gate and the application of asphalt binder. The wet mixing period is the interval of time between
the start of the application of asphalt binder and the opening of the mixer gate.
Ensure the timing control is adjustable and capable of being set at intervals of not more than 5
seconds throughout cycles up to 3 minutes.
J. Maximum Aggregate Size. Reject oversize aggregate by suitable methods or devices before
the aggregate enters the cold feed or by plant screens.
K. Accuracy. Ensure the automatic proportioning system is capable of consistently delivering
materials within the full range of batch sizes within the following tolerances:
Total Batch Weight of
Paving Mix, %
Batch aggregate component
Mineral filler
Asphalt binder
Zero return (aggregate)
Zero return (asphalt binder)
1101.04
± 1.5
± 0.5
± 0.1
± 0.5
± 0.1
Requirements for Drum Mix Plants
A. Computerized Plant System for Drum Plants. In addition to the general computerized
plant system requirements in section 1101.02, print the information every 5 minutes during
production. Ensure printouts contain the following additional information:
1.
2.
3.
4.
Tons per hour (metric tons per hour) of virgin asphalt binder.
Tons per hour (metric tons per hour) of virgin aggregate.
Tons per hour (metric tons per hour) of reclaimed pavement.
Moisture content of the virgin aggregate.
Make provisions for including the moisture content of the total cold feed in the belt weighing
signal and correcting wet aggregate weight to dry aggregate weight. Adequate and convenient
facilities shall be provided for diverting aggregate flow into trucks or other suitable containers to
check the accuracy of the aggregate delivery system.
Ensure an automatic plant shut-off or alarm is provided to operate when any aggregate bin
becomes empty or the flow from any bin gate becomes restricted.
5
B. Asphalt Binder System. Provide satisfactory means to assure positive interlock between
dry weight of aggregate flow and the flow of asphalt binder through an approved meter. Ensure
the interlock is capable of adjusting the flow of asphalt binder to compensate for any variation in
the dry weight of aggregate flow.
Ensure an automatic digital record of the flow of asphalt binder is displayed, recorded, and
totaled in appropriate units of volume or weight and time at least once every 5 minutes of plant
operation, and on demand, at least once every minute of plant operation for a period of at least 5
minutes. Ensure the digital record of asphalt binder flow is coordinated with the digital record of
dry aggregate flow at the point at which the asphalt binder is discharged into the aggregate.
1101.05
Aggregate and Recycle Weighbridge Calibration.
All safety procedures must be followed when performing plant calibrations. Notify the ODOT
district 24 hours in advance of conducting a complete calibration for any ODOT project.
ODOT‟s presence on site during calibration is not required. Check each virgin aggregate and
recycle (material) weighbridge for compliance with the required tolerance in 402.02.
Construct and maintain the material weighbridge according to the manufacturer‟s
recommendations.
2. Operate the conveyor belt empty, prior to calibration, for a length of time to ensure the
belt is traveling at normal operating speed and to perform a zero belt function.
3. Ensure the plant control system is set to zero percent moisture or ensure the control
system has an automatic function that sets moisture to zero or records the total material
weight in actual tons during the weighbridge calibration.
4. Use a material that provides a uniform flow during the calibration procedure.
5. Record the gross, tare, and net weight of the weighing vessel (truck or silo) and record
as the actual weight.
6. Convey material into a suitable weighing device or truck for weighing on truck scales.
All weighing scales must be checked and approved by a qualified scale company that
has performed a scale calibration prior to the calibration of the weighbridge.
7. Set the material feed to ton per hour (tph) to approximate the normal operating range.
8. Set the plant control material total counter to zero.
9. Ensure material runs across the weighbridge into a truck or silo weighing system.
Avoid major changes in material flow.
10. Record the total amount of material displayed on the control system or counter after a
sufficient amount of material (based on operator‟s experience with control system, the
manufacturer‟s recommendation, and the vessel or silo capacity) has ran over the
material weighbridge and the tph on the plant control display has gone to zero.
11. The percent error is calculated as the difference between the actual weight in tons or
pounds (metric tons or kg) and the plant control displayed weight divided by the plant
control displayed weight times 100. If the percent error is below the specification limit,
1.
6
12.
13.
14.
15.
1101.06
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
then perform one additional test to verify. Two consecutive calibration procedures
must be within specification.
Perform repairs or adjustments if a calibration exceeds the specification limit. After the
adjustment conduct another calibration procedure. Two consecutive calibration
procedures must be within specification.
A change in the tph of the material delivery is permitted based on the control system
requirements.
Document and maintain all calculations in the plant control room for review upon
request from ODOT personnel. Record or print out any plant calibration constants that
the plant control system provides. Attach all plant control system printouts to the
calculation form.
Ensure a test weight or calibration bar is configured to hang from the material
weighbridge.
Asphalt Binder Metering System Calibration Simulation
The calibration vessel may be a distributor or calibration tank equipped with load cells.
All weighing scales must be checked and approved by a qualified scale company that
has performed a scale calibration prior to the calibration of the asphalt.
Tare the calibration vessel.
Determine the accuracy of the binder control/delivery by pumping asphalt in the
automatic mode into the calibration vessel.
Ensure a simulated aggregate tph is entered in the plant controls. This can be done
electronically or by hanging a weight on the aggregate weighbridge. Ensure the
aggregate moisture is set to 0%. Set binder spray delay to as close to zero as controls
permit.
Use a job mix formula of 5% asphalt and 95% aggregate. Enter these values into the
plant computer.
Fill all connecting hoses with binder or use an acceptable method to account for the
binder in the hoses (volumetric calculation) prior to calibration.
Ensure the plant is placed in “hot stop” mode after filling hoses so no pumping or
aggregate tph will be recorded.
Set all counters to zero or record initial tonnage.
Activate plant control system to start the flow of asphalt binder.
Ensure plant is in “hot stop” mode after pumping a quantity of binder that the
calibration vessel can accommodate or capture the initial and final plant control
readings if in a continuous mode.
Record the accumulated theoretical dry aggregate weight displayed on the plant control
system.
Record the calibration vessel gross, tare, and net weight. The net value will be the
actual value.
The percent error is calculated as 100 times the difference between the actual binder
weight in tons or pounds (metric tons or kg) and the theoretical binder weight
(calculated from accumulated theoretical dry aggregate weight) in the same units as the
7
above divided by the theoretical binder weight. If not in tolerance investigate, correct,
and re-calibrate. Perform the above calibration procedure to ensure two consecutive
calibrations meet specification requirements.
14. Document and maintain all calculations in the plant control room for review upon
request from ODOT personnel. Record or print out any plant calibration constants that
the plant control system provides. Attach all plant control system printouts to the
calculation form.
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