6.1.1 Asphalt Binder Storage and Handling
There are many kinds of asphalt binder used in highway construction. It is the responsibility of both the QC & QA technicians to insure that the proper grade of asphalt binder as shown on the job mix formula is being used. It is also the responsibility of the technicians to see that the asphalt binder is handled and stored properly. Asphalt binder used for NCDOT work must be certified or tested prior to use (See Delivery and Acceptance of Asphalt Materials in Section 2).
The QC technician should check to see that the asphalt binder is properly stored and protected. Before production is started, the QC & QA technicians should determine that the asphalt binder to be used is the grade and from the source as that stated on the job mix formula. The technicians should also determine if the asphalt binder is certified or tested. If there is no evidence of the asphalt binder being certified or tested, the QC technician should immediately notify the Division QA Supervisor.
The asphalt binder storage capacity at the plant must be sufficient to allow uniform plant operation. Where more than one grade of asphalt binder is required for a project, at least one tank will be needed for each grade or the tank must be completely emptied before a different grade is added. Different grades of binders shall not be mixed.
Asphalt binder storage tanks must be capable of being measured so that the amount of materials remaining in the tank can be determined at any time. They also must be heated to keep the asphalt binder fluid enough to move through the delivery and return lines. Heating is done either electrically or by circulating hot oil through coils in the tank. Regardless of the heating method used, an open flame must never come in direct contact with the tank or contents. The specifications state that the asphalt binder shall not be heated to a temperature in excess of the supplier's recommendation while stored or when being used in production of the mix at the plant. Where circulating hot oil is used, the oil level in the reservoir of the heating unit should be checked frequently. A drop in the level could indicate leakage of the hot oil into the tank, leakage which results in contamination of the asphalt binder. All transfer lines, pumps and weigh buckets also must have heating coils or jackets so that the asphalt binder will remain fluid enough to pump. One or more thermometers must be placed in the asphalt binder feed line to ensure control of the asphalt binder temperature, as it is being introduced into the mixer or drum.
The asphalt binder tanks must be equipped with a circulation system capable of uniformly dispersing and mixing additives throughout the total quantity of asphalt binder in the tank. Required additives, such as silicone, must be added sufficiently in advance of production so as to insure that they are thoroughly distributed throughout the asphalt binder. See Section 5 for more specific requirements for in-line blending of anti-strip additives.
Adequate pumps must be furnished so that asphalt binder can be unloaded from tankers and still continue to operate the plant. A sampling valve or a spigot must be installed in the circulating system or tank to allow sampling of the asphalt binder. When sampling from the circulating system, exercise extreme care, as pressure in the lines may cause the hot asphalt binder to splatter.
6.1.2 Asphalt Mix Temperature Requirements
The temperature of the binder and aggregates must be adequate to allow for proper coating of the aggregate with the binder and sufficient mixing action to produce a uniform asphalt mixture. The completed asphalt mixture must be within a desired range to allow for proper placement and compaction without having damaged the binder in the mix.
The mixing temperature at the asphalt plant will be established on the job mix formula. The mixing temperature will be different depending on which grade of asphalt binder is being used. The normal mixing temperatures for Superpave mixes are as follows unless requested by the Contractor and approved by the Engineer:
These are normal temperatures but certain circumstances may call for a higher or lower mixing temperature. Plant and Roadway Technicians should always refer to the most current Job Mix Formula for the correct mixing temperature. The specifications require that the temperature of all mixes when checked in the truck at the asphalt plant shall be within ±15ºF (8ºC) of the JMF temperature.
Good stockpiling and storage procedures are crucial to the production of top-quality asphalt hot-mix. When stockpiled properly, aggregates retain their proper gradation. When stockpiled poorly, aggregate particles segregate (separate by size), and gradation varies at different levels within the stockpile. All handling degrades (breaks down) individual aggregate particles to some extent, and, where different-sized aggregate particles are involved, may cause particle segregation. Therefore, handling should be kept to a minimum to prevent degradation and segregation that could make the aggregate unsuitable for use. The technician should be aware of the effect of various stockpiling, storage and handling practices on aggregate gradation and should encourage good practices at all times.
Aggregates must be handled and stored in a manner that will minimize segregation and avoid contamination. The aggregates should be stockpiled in the vicinity of the plant on ground that has been cleared of vegetation, that is firm, well-drained and prepared in such a manner as to protect the aggregates from contamination. Stockpiles must be separated to prevent intermingling. This can be accomplished by positive separation of the stockpiles (space), by using adequate bulkheads, by the use of silos, or other means. If bulkheads are used, they should extend to the full depth of the stockpiles and must be strong enough to withstand the pressures that will be exerted under operating conditions.
Stockpiles should be constructed in layers, rather than in cone shaped piles. Individual truckloads should be spotted close together over the entire stockpile surface. When stockpiling with a crane, each bucketful should be deposited adjacent to another, over the entire area, so that the layers will be of uniform thickness. When constructing, maintaining, or removing aggregates from coarse aggregate stockpiles, one should be careful to minimize degradation caused by the equipment. Operating on top of the stockpiles with either rubber-tired or tracked equipment should be minimized.
6.3 AGGREGATE COLD FEED SYSTEM
The flow of aggregate through an asphalt plant begins at the cold feed bins. The cold feed system (See Fig. 6-1 in Printed Version of the 2008 QMS Manual) for both batch and drum mix plants consists of a series of cold bins (usually 3 to 5) with gates and feeders mounted under the bins and the entire unit positioned over a collector conveyor belt. As stated in the Specifications, there must be a separate cold feed bin for each aggregate component used in a particular mix type. The usual setup will be four cold bins. Separate feeder bins must be used when RAP / RAS is being incorporated into the mixture.
Each cold bin should be large enough to hold an adequate supply of aggregate. When the bins are mounted together, there should be a baffle between adjacent bins that will prevent one bin from overflowing into the adjacent bin, causing variations in the gradation and properties of the mix.
Each bin will be equipped with an adjustable gate opening and a feeder belt to draw aggregate out of each bin at a controlled rate. The aggregate from each feeder is then deposited onto a collector conveyor and then fed into the dryer. The gate openings and feeder belt speeds for the various aggregate feeders must be synchronized and calibrated to deliver the proper amount of each material required by the JMF for each mix. Section 6.4 discusses how to calibrate the cold feed blend ratio being fed into the plant based on specific gate openings and belt speeds to determine conformance with the aggregate blend on the JMF.
There are several types of cold feeders used on plants, including:
For a uniform output from the asphalt plant, input must be accurately measured and controlled. Cold feeders must be synchronized so that a change in the feed rate of one is proportional to the feed rate of all others. Each bin must be equipped with a no-flow sensor that will alert the plant operator and/or shut the plant down if aggregate flow stops or becomes restricted.
The importance of feeding the required amounts of each size aggregate into the dryer at the correct rate of flow cannot be over-emphasized. This is doubly important when the plant is a drum mixer, since there is no regrading of the aggregates by a screening unit as at a batch plant.
6.4 CALIBRATION OF COLD FEED BLEND RATIOS
The production and construction of a high quality asphalt pavement meeting the mix design requirements and with the required density begins with the cold feeders at the asphalt plant. The job mix formula is based on a mix design, which was developed based on a specific blend or ratio of coarse aggregate, fine aggregate, and RAP/RAS (when used) with individual specific gravities and gradations. During mix design, these materials are combined in such proportions that will result in a combination gradation and asphalt binder percentage that meets the design criteria for that particular type of mix. This blend ratio must be maintained during production if the design criteria are to remain valid. Excessive deviation from this blend may change the optimum asphalt binder percentage, result in a low stability mix, deficient or excessive air void content, a different mix specific gravity or other changes. It is for these reasons that the blend ratio shown on the job mix formula must be closely maintained at the asphalt plant.
The cold feeder aggregate blend ratio must be calibrated (at a minimum) during mix verification of each job mix formula and as necessary thereafter. While this calibration is the responsibility of the Contractor, the QA technician must be aware of the methods and procedures used and be able to check or verify the calibration. Approval of mix verification by the Division QA Supervisor will not be granted without satisfactory evidence of the cold feed blend calibration. Periodic checks are recommended thereafter.
There are instances where the gradation of one or more of the individual aggregates and/or RAP/RAS may have significantly changed from that used in the initial mix design. This may make it impossible to meet the combined mix gradation if the blend ratio is still maintained. While it is usuallybest to maintain the blend ratio and adjust the gradation target values, there may be situations where it is better to change the aggregate blend ratio. This is especially true when two or more “same” source aggregate are being used. In these situations, QC personnel may make per aggregate cold feed blend changes of +/- 10% or less from the JMF target without prior QA approval. However, when natural sand is utilized in “C” or “D” level asphalt mixes, the maximum natural sand percentage in the mix design and/or production aggregate blend as detailed in Table 1012-1A (See Page 4-6 of this manual) cannot be exceeded. A cold feed blend change in excess of +/- 10% can only be authorized by the Pavement Construction Section. In most cases, a blend change in excess of +/- 10% will cause a new mix design to be considered.
Calibration of the Cold Feeders can be done by various methods. Any method is satisfactory provided the blend ratio can be determined with reasonable accuracy. Reasonable accuracy is normally considered to be within +/- 5% of the JMF target for each aggregate and RAP/RAS component. Whichever method is used, aggregate and RAP/RAS samples must be actually weighed in order to determine the cold feeder percentages. Some of the more commonly used methods of calibration are listed next with a brief description of each.
Fig. 6-5 and 6-6 (in Printed Version of the 2008 QMS Manual) are examples of a length-of-belt method of calibration and a JMF comparision. While this is the example used in this manual, it in no way implies that this is a better or more accurate method than the others listed above.
6.5.1 The Dryer
From the cold feeder the aggregates are delivered to the dryer. The dryer accomplishes two things (1) it removes moisture from the aggregates and (2) it raises the aggregate temperature to the desired level. The component parts of the dryer are: (1) a revolving cylinder usually from 3 to 10 feet (1 to 3 meters) in diameter and from 20 to 40 feet (6 to 12 meters) long; (2) a burner which is either gas or oiled fired; and (3) a fan which may be considered part of the dust collector system, but its primary function is to provide the draft air for combustion in the dryer. The dryer is equipped with longitudinal troughs or channels, called lifting flights, which lift the aggregate and drop it in veils through the burner flame and hot gases.
The slope of the drum, its speed of rotation, diameter, length, and the arrangement and number of flights control the length of time required for the aggregate to pass through the dryer. The aggregate then passes to the hot elevator through a discharge chute near the burner end of the dryer.
The dryer will have an automatic burner control device with an approved thermometric instrument located in the aggregate discharge chute to actuate the automatic burner control. The purpose of the automatic burner control is to insure a uniform mix temperature and to prevent overheating of the aggregate which can cause damage to the asphalt during mixing. Fluctuating mix temperatures are often the cause of poor laydown and compaction results. Uniform density at the required degree of compaction cannot be achieved when the mix temperature varies from one batch or load to the next.
6.5.2 Screening Unit
The screening unit (See Fig. 6-8 in Printed Version of the 2008 QMS Manual)) includes a set of several different sized vibrating screens. The first in the series of screens is a scalping screen which rejects and carries off oversized aggregates. This is followed by one or two intermediate-sized screens, decreasing in size from top to bottom. At the bottom of the stack is a fine screen, occasionally referred to as a "sand" screen. (See Figure 6-9 in Printed Version of the 2008 QMS Manual).
The screens serve to separate the aggregates into specific sizes. To perform this function properly, the total screen area must be large enough to handle the total amount of feed delivered. Here again, the screens must be clean and in good condition. The capacity of the screens must be in balance with the capacity of the dryer and the capacity of the pugmill.
When too much material is fed to the screens or the screen openings are plugged, many particles which should pass through, ride over the screens and drop into a bin designated for a larger size of particle. Similarly, when screens are worn or torn, resulting in enlarged openings and holes, oversized material will go into bins intended for smaller-sized aggregate. Any misdirection of a finer aggregate into a bin intended to contain the next larger size fraction is called "carry-over". Excessive carry-over can add to the amount of fine aggregate in the total mix, thus increasing the surface area to be covered with asphalt binder. If the amount of carry-over is unknown or if it fluctuates, particularly in the No. 2 bin, it can seriously affect the mix design in both gradation and asphalt binder content. Excessive carry-over can be detected by a sieve analysis of the contents of the individual hot bins and must be corrected immediately by cleaning the screens or reducing the quantity of material coming from the cold feed, or both. Some carry-over is permitted in normal screening operations, provided it remains relatively uniform.
The No. 2 bin (intermediate fine aggregate) is the critical bin for carry-over. This is the bin that will receive the finest aggregate in carry-over and which will affect the asphalt binder demand of the mix the most. Typically, the carry-over in the No. 2 bin should not exceed 10 percent. Running a sample of the No. 2 bin material over a No.8 (2.36mm) sieve will indicate the amount of carry-over. To prevent excessive carry-over, daily visual inspection of the screens for cleanliness and overall condition is recommended, preferably before starting each day's operation. When conditions warrant, the screens shall be cleaned or replaced.
6.5.3 Hot Bins
Hot bins are used to temporarily store the heated and screened aggregates in the various sizes required. Each bin is an individual compartment or a segment of a large compartment divided by partitions (See Fig. 6-9 in Printed Version of the 2008 QMS Manual). Properly sized hot-bin installation should be large enough to hold sufficient material of each size when the mixer is operating at full capacity. The bin partitions must be tight, free from holes, and high enough to prevent intermingling of the aggregates. Each hot bin shall be equipped with an overflow pipe to prevent aggregate from backing up into other bins. The overflow pipes shall be checked frequently to make sure they are functioning and thus prevent contamination by intermingling sizes from adjacent bins. The bottom of each bin is fitted with a discharge gate which shall close tightly so that no material is allowed to leak into the weigh hopper.
All hot bins are to be equipped with a device to permit sampling of the aggregate from each bin. Samples of aggregates from these bins may be secured from "gates" or "windows" in the sides of the bins, or by diverting the flow of aggregates from the bins into the sampling container. It is essential that the sampling devices or methods be such that representative samples of the materials in the hot bins can be obtained. Gradations of the samples from the different bins will then be analyzed to determine the hot bin pulls, as discussed later.
6.5.4 Aggregate Weigh Hopper
Aggregates are released from the hot bins directly into the weigh hopper, generally beginning with the largest size aggregate and progressing down to the finest size, where the mineral filler usually is used, sandwiched between the larger aggregates. The amount from each bin is determined by the batch size and the proportions or percentages required to be blended. Determination of hot bin percentages and hot bin pull weights will be discussed later in this section.
The weigh hopper is suspended from a scale beam and the amounts of aggregate are weighed cumulatively.
Before withdrawal starts, there should always be sufficient materials in the hot bins for a complete batch. If a bin is near depletion or is running over, chances are that an adjustment in the cold feed rates or hot bin pulls are required.
6.5.5 Asphalt Binder Bucket or Meter
Asphalt binder may be weighed in a special bucket, or it may be measured by a meter for each batch. When weighed into a batch, asphalt binder is pumped into a bucket of known weight and weighed on a scale.
When metering devices are used, a volumetric measurement is made. The volume of asphalt binder changes with temperature. Some asphalt binder meters have built-in temperature-compensating devices that correct the flow of asphalt binder when changes in temperature occur. The volume of asphalt binder pumped between two meter readings may be weighed as a means of calibrating the meter.
6.5.6 The Mixer Unit (Pugmill)
After proportioning, the aggregate and bituminous material are introduced into the pugmill for mixing. The bituminous concrete batch plant is equipped with a pugmill mixer which consists of twin shafts equipped with paddles for mixing the ingredients into a homogeneous mass. Main parts are the paddle tips, paddle shanks, spray bar, liners, shafts, discharge gate and heated jacket. Efficient mixing is dependent upon the number, shape, and condition of the paddle tips, speed of the mixing shafts, length of mixing time, temperature of the material, quantity of materials in the mixer, and specifically the clearance between the paddle tips and liner plates. NCDOT Specifications require that all batch plants shall have a mixer with a rated capacity of not less than 3,000 lbs (1360 kg ). (See Fig. 6-10 in Printed Version of the 2008 QMS Manual)
Close attention should be given, through visual inspection, to the uniformity and coating of the mix. Excessive clearance between paddle tips and mixer liners, broken, worn or missing paddle tips and the batch size are factors which can contribute to a lack of uniformity and coating of the mixture. Batch sizes should be determined by the rated capacity of the mixer. The rated capacity of the pugmill mixer will be indicated on the plant certification. Underfilling or overfilling should not be allowed.
The mixers of batch mix plants and continuous mix plants are essentially of the same design, except for the variations in arrangement of the paddle tips. In the batch-mix mixer, the materials are dumped into the center of the mixer and the paddles are arranged to give an end-to-center mixing or a run-around (Figure eight) mixing pattern. The material is held in the mixer the required mixing time and then discharged through the discharge gate into the transporting vehicles.
Specifications require that the mixer be equipped with an automatic timing device to automatically regulate the dry-mixing and wet-mixing periods.
6.5.7 Automatic Control of Proportioning and Mixing
NCDOT Specifications require that all plants have fully automated controls for proportioning and mixing. A fully automatic system is defined as one such that once mix proportions and timers are set and the plant operation is started, it will automatically complete the proportioning and mixing cycle without further effort of the plant operator or until a shortage of material or some extraordinary event causes the plant itself to halt the operation.
The mixer must be equipped with an interlocking timing device, capable of being set at intervals of five seconds or less, to control the operations during the mixing cycle. The mixing time will be established by the plant operator or the QC technician. The mixing time should be long enough to get a uniform distribution of aggregate sizes and a uniform coating of asphalt binder on all aggregate particles; however, excessive mixing should not occur due to the hardening effect of the asphalt binder film on the aggregate by exposure to air and heat.
The Specifications also provide that upon a malfunction of the required automatic equipment on a batch plant, the plant may continue to operate manually for the following two (2) consecutive working days. It is the intention of this specification that the automatic equipment be operated at all times, except in situations where legitimate breakdowns occur. The Contractor must make every effort to repair any breakdowns of automatic controls immediately.
6.5.8 Calibration of Batch Plant Asphalt Scales, Weigh Bridges, and Meters
Separate sets of scales are used to weigh asphalt binder, aggregates, RAP and RAS (if applicable), being used in the mix. The Contractor is required to check the calibration of these aggregate scales, asphalt binder scales, and weigh bridges (recycled mixes), on a periodic basis. The frequency for performing these scales checks is monthly for the asphalt binder scales, aggregate scales, and weigh bridges. All plant scales shall be accurate to 0.5 percent of the anticipated scale settings that may be required. The Contractor shall have on hand not less than ten 50 lb. (22.7 kg) weights (Standard Specification Subarticle 610-5C(1) for testing the plant scales. The procedures and documentation of this calibration are described below.
(A) AGGREGATE SCALES & WEIGH BRIDGES (Quarterly Frequency):
The aggregate scales are checked as follows: Scales are first checked with the weigh hopper empty to be sure they show a correct zero. Then ten 50 lb.(22.7kg) standard weights are either placed on or attached to the aggregate weigh hopper and the scales reading is read and recorded. This will be the increment check. Care should be taken in evenly distributing these weights in order to prevent scale misalignment. The weights are then removed and the same amount of aggregate is drawn from the hot bins and deposited into the weigh hopper to replace the standard weights. The ten weights are again placed on or attached to the weigh hopper and another scales reading is taken and recorded. This would be the 1000 lb (454 kg) increment check. This procedure is repeated at 500 lb (227 kg) intervals until the total weight checked is slightly more than the pounds of aggregate that will be used in each batch of mix. At anticipated scale settings, the scales must be within 0.5 percent accuracy. If not, it is the Contractor's responsibility to make the necessary adjustments or have scales repaired by a qualified scales technician. A recheck of the scales would then be made to ensure their accuracy.During normal operations, aggregate scales should be monitored to be sure they zero correctly during weighing operations and that they show no signs of binding or dragging which would cause erroneous readings. Some common causes of scale malfunction are: build-up of asphalt binder, dust, corrosion or dulling of the scales' knife edges, or aggregates lodging in the scale supports.
Belt scales (weigh bridges) are utilized at some batch plants to monitor the RAP material percentage used in recycled mixes. When used, these belt scales will be checked monthly using the same procedure for checking belt scales on a drum mix plant (See Section 6.6.3 for this procedure).
(B) ASPHALT BINDER SCALES (Quarterly Frequency):
The asphalt binder scales are calibrated in nearly the same manner as the aggregate scales, but only one weighing operation is required. The 50 lb (22.7 kg) standard weights are placed on or attached to the asphalt binder bucket one at a time, and readings are recorded as each weight is added. This is continued until the combined weight is slightly in excess of the pounds (kilograms) of asphalt binder required per batch of paving mixture. Asphalt binder scales, if in true adjustment, should indicate the same value as the total of the standard weights used, and must be within the required 0.5 percent accuracy at the anticipated scale setting. If the weighing scales error exceeds the tolerance permitted by specifications, plant operations should not be started until the scales are adjusted or repaired by a qualified scale technician. This corrective measure is the Contractor's responsibility. During normal operations, the tare weight of the empty bucket should be watched carefully to see that the bucket is drained completely and to compensate for any asphalt binder and dust clinging to it. The asphalt binder bucket should be tared at the beginning of each day and checked after the first few loads are discharged. Quite often, asphalt binder accumulates on the side and bottom and reduces the weight of asphalt binder actually used in the mix.
(C) ASPHALT BINDER METERS (Quarterly Frequency):
Asphalt binder meters are volume displacement mechanisms, and when used they should be checked for accuracy. This check should be performed bi-monthly at a minimum. The metering system should also be within 0.5 percent accuracy. Since asphalt binder content is usually expressed as a percent by weight, a correlation between meter readings and weight should be established. A simple method to determine the correlation is to read the meter, pump a quantity of asphalt binder into a tared container, and then read the meter again. The weight of asphalt binder divided by the difference in meter readings determines the weight of asphalt binder pumped per division. For a more detailed explanation of checking an asphalt binder metering system, see Section 6.6.4 of this manual. The viscosity and unit weight of the asphalt binder change with a change in temperature. When the temperature is increased the viscosity decreases. The unit weight decreases about 1 percent for each increment of increase in temperature of 60° F (15.5°C). Pumping efficiency may be affected by a change in temperature, and it may be desirable to calibrate the pump over a range of asphalt binder temperatures. Volumes and viscosity’s can be determined later for calibration and plotting purposes if necessary. Some asphalt binder meters have built-in temperature-compensating devices that correct the flow of asphalt binder when temperature changes occur. When a meter without a temperature-compensating device is used, it is necessary to adjust the delivery setting for each change in asphalt binder temperature. The Technician should refer to the Plant Manual for additional details.
(D) ANTI-STRIP METER SYSTEM (Monthly Frequency) :NOTE: This calibration flowmeter should not be confused with the Totalizer Flowmeter that is also required on the additive feedline if additive is introduced to the binder at the plant. See Section 5.7 of this manual for details on the requirements for the totalizer flowmeter.
If the anti-strip additive is introduced into the binder at the plant site, a calibration flowmeter is mounted in the anti-strip additive feedline near the additive storage tank. This meter is to insure that the additive pump is operating properly and the correct amount of additive is being uniformly introduced into the binder at all times. The metering device will be interlocked with the asphalt binder control equipment in such a manner as to automatically vary the additive feed rate to maintain the required proportions and which will automatically indicate in the plant control room when the additive feed rate is obstructed or stops. This meter should be checked for accuracy at least monthly at a batch plant (bi-monthly at a drum plant) and should be within +/- 10% of the amount of additive specified on the JMF.Since the additive is introduced at minor dosages, it is adequate to pump off only a small quantity to check the meter. The procedure is as follows: Check and record the current meter reading. Pump off 4-5 gallons of additive (based on the meter reading) through the by-pass valve into a tared container. Determine the additive weight by weighing contents of tared container on scales accurate to at least the nearest pound. Calculate the actual gallons pumped off by dividing this weight by the pounds per gallon weight of the additive. The pounds per gallon weight is determined by multiplying 8.33 times the specific gravity of the additive. The specific gravity of the additive can normally be found on the additive delivery ticket or by contacting the supplier. The gallons shown to have been pumped off by the meter reading should be within +/- 10% of the actual gallons. If not, the meter must be adjusted and this procedure repeated until the required tolerance is met.
(E) DOCUMENTATION:6.5.9 Setting of Batch WeightsA plant scales calibration Form QC-2 (Fig. 6-11 in Printed Version of the 2008 QMS Manual) and/or certification by the Department of Agriculture or a certified scale company will suffice for either the initial check or for a periodic check, provided the date of the certification is within the time frame noted above (30 days for asphalt binder, aggregate scales and weigh bridges). A single scales check may and should serve for several different projects if the check was conducted within the above noted time. Form QC-2 (Fig. 6-11 in Printed Version of the 2008 QMS Manual) should be submitted to the Division QA Supervisor at the completion of each scales check.
As noted, the QC Technician should post a copy of Form QC-2 in the QC Laboratory when the scales are actually calibrated. A copy shall also be furnished to the appropriate Division QA Supervisor.
Normally it is the Contractor's responsibility to calibrate the hot bins; however, the QA Technician should observe and be aware of the procedures used to arrive at an aggregate combination that meets the job-mix formula.
To produce the desired asphalt mix; it is necessary to pull a certain amount of aggregate from each hot bin. The amount that is pulled from each bin is dependent upon what the Job Mix Formula calls for and what each hot bin contains (the gradation); which means the content of each bin must be analyzed.
The first step is to start running the plant, the cold feed, the dryer and the screens. After the plant has settled down so that the material in the bins is representative of the proportions established at the cold gates, a sample of aggregate is taken from each bin.
(A) HOT BIN SAMPLING
This method describes a procedure for obtaining representative aggregate samples from the asphalt plant hot bins. An approved sampling device is used that will retain a representative sample of aggregates when passed through a veil of material flowing out of the bin chute. The device must be of a dimension so that it will extend slightly more than the distance from the outside edge of the predominantly fine material to the outside edge of the predominantly coarse material as the device enters the stream of flowing aggregates. A shovel is not acceptable.In the flow of material over the plant screens, finer particles fall to the near side of the bins and coarser particles fall to the far side, particularly in the No. 1 bins (See Fig. 6-12). When material is drawn from the bin by opening a gate at the bottom, the stream consists predominantly of fine material at one edge and coarse material at the other.
This condition is critical in the No. 1 (fine) bin, since the asphalt binder demand is influenced heavily by the material from this bin. Therefore, the relative position of the sampling device in the stream determines whether the sample will be composed of the fine portion, the coarse portion, or will be an accurate representation of the total material in the bin (See Fig. 6-12). In the Standard Specifications, Subarticle 610-5C(3) Hot Bins, states that "Each compartment shall be equipped with adequate and convenient devices to provide for sampling." This means that the plant must be constructed to either (a) enable a sampling device to be inserted and used effectively, (b) to have an approved gate, window or slide under each bin for diverting the flow of aggregates from the bins into the sampling device, or (c) to contain an automatic bin sampling device.The sampling procedure is as follows: Verify that the containers and sampling device are clean. Pass the sampling device through the veil of material as it flows from the bin. The sampling device must be inserted under the stream of material in such a way that the device simultaneously collects the predominantly fine material at the other edge (See Fig. 6-13). The sampling device will be withdrawn before it overflows and the contents deposited in a clean container. This procedure will be repeated for each bin to be analyzed.
Figure 6-12
Correct Use of Sampling Device
(B) SIEVE ANALYSIS OF EACH BIN
A sieve analysis will be run on each sample taken from the hot bins. This is done as soon as the samples cool down to the point they can be handled. The equipment and test procedures are those specified for running sieve analysis of coarse and fine aggregates according to AASHTO T-27 Modified. The information is recorded on the standard form B-75-2 (See pg. 6-22 in 2008 printed version of manual)
(C) DETERMINING HOT BIN PERCENTAGES
Once the gradation of material in each hot bin is determined, the percentage to be pulled from each bin to meet the job mix formula gradation can be calculated. This is best done by use of a trial and error method.In determining proper calibration of the hot-bin feeds, the job-mix formula is the starting point. It is necessary to determine what percentage of each size of the aggregate in the hot bin should be incorporated into the mix in order to meet the job mix formula gradation requirements. It should be stressed that there can be numerous sets of hot bin pull percentages that are correct. Any combination of bin materials that will meet the job mix formula gradations, with the Specification tolerances applied, will be acceptable and satisfactory. The purpose of this trial and error B-75-2 procedure is to arrive at a starting point for plant production to ensure that the mix will be reasonably close to our job mix formula.
First the gradation of material in each of the hot bins is determined. Aggregate proportions are then estimated as follows. (Reference should be made to Fig. 6-14 to work through this estimation process).
The raw material passing the 2.36 mm sieve and the 0.075 mm sieve is used as a starting point. Of the three hot bins used for this S12.5 C mix, bin No. 1 carries most of the minus 2.36 mm and minus 0.075 mm material. This will generally be true in all batch plants since the screen over the No. 1 bin is of such size to normally control the mix gradation from the 2.36 mm sieve down through the 0.075 mm sieve. This being the case, the 2.36 mm sieve through 0.075 mm sieve gradations in the asphalt mix can be controlled by the percentage of material pulled from the No. 1 hot bin.
Beginning with the 2.36 mm, the job mix formula requires that 52 percent pass this sieve. Note that there is 93 percent passing the 2.36 mm sieve in hot bin No. 1. Since this 93 percent represents the majority of minus 2.36mm material in all three hot bins, the 52 percent required in the mix can be controlled from this 93 percent. The 52 is divided by the 93 for a percentage result of 56 percent that is to be pulled from the No. 1 bin. In other words, what percent of the No. 1 bin minus 2.36 mm material is needed to give the desired percent passing the 2.36 mm on the job mix formula. The 0.075 mm sieve is now checked in the same manner, as was the 2.36 mm above. The job mix formula requires that 4.8 percent pass the 0.075 mm sieve. The No. 1 bin has 8.9 percent minus 0.075 mm material. The 4.8 is divided by the 8.9 for a result of 54 percent.
Either the 56 percent (arrived at by use of 2.36 mm) or the 54 percent (arrived at by use of 0.075 mm) will meet the job mix formula requirements. The example in Fig. 6-14 uses 55 percent out of hot bin No. 1.
As the hot bin containing the finest graded material had certain characteristics that enabled us to control certain portions of our mix gradation, so does the hot bin containing the coarsest graded material. This characteristic is different, or opposite, from that in the fine bin in that this is large stone, which is generally retained on the majority of sieves instead of passing most sieves. This retainage characteristic is used to estimate the percentage to be pulled from the bin containing the most coarse material.
The example in Fig. 6-14 uses three hot bins to make this S12.5 C mix, with No. 3 bin containing the coarsest graded material. (The hot bin farthest to the right from the plant's hot elevator will always contain the coarsest material.) Keep in mind that the No. 1 hot bin is already set at 55 percent which controls the 2.36 mm through 0.075 mm sieves; therefore, we need not be concerned about these sieves any more. The concern now is for the sieves above, or larger than, the 2.36 mm.
In order to select a controlling sieve which determines the amount to pull out of the No. 3 bin, the gradation for all sieves above the 2.36 mm in all three bins need to be compared. The first sieve above the 2.36 mm that differs drastically in gradation, from that same sieve's gradation in the Nos. 1 and 2 bins, will be the controlling sieve. The 4.75 mm is the first sieve above the 2.36 mm. The 4.75 mm gradation is 98 percent in the No. 1 bin, 89 percent in the No. 2 bin, and 13.6 percent in the No. 3 bin. The 4.75 mm sieve gradation is very much different in bin No. 3 from that in Bin Nos. 1 and 2; therefore, this will be our controlling sieve. These gradations show that there is basically no plus 4.75 mm material in bin Nos. 1 and 2, with bin No. 3 containing 86.4 percent plus 4.75 mm; therefore, bin No. 3 is the only bin from which the plus 4.75 mm material in the mix can be controlled. The job mix formula requires that 70 percent pass the 4.75 mm sieve, which means that 30 percent would be retained on that sieve. Since the No. 3 bin is basically the only hot bin with plus 4.75 mm material, 30 percent would be pulled from this bin in order to have 30 percent plus 4.75 mm in the mix.
The remaining bin (No. 2 in our example) will be set at the difference between 100 percent and the total percent of bin Nos. 1 and 2. This would be 100 minus 85 (55 plus 30) or 15 percent to be pulled out of the No. 2 bin. At times only two hot bins will be used. In those cases, the No. 1 hot bin will be set as outlined previously and the balance percentage will be pulled from the No. 2 bin. Also, there will be times when four hot bins will be used for 19.0 mm, 25.0 mm and 37.5 mm mixes. In these cases, the Nos. 1 and 4 hot bins will be set as previously outlined and the balance percentage should be split 1/3 out of the No. 2 bin and 2/3 of the balance out of the No. 3 bin. This 1/3:2/3 split comes from experience and has proven to normally meet the job mix formula needs.
After deciding on a set of hot bin percentages, it is necessary to verify that these percentages will meet the job mix formula requirements. This is done by blending the hot bin materials together, on paper, at the percentages decided upon. Following this procedure through in our example, it will be seen that the 55-15-30 percent combination of hot bin materials will produce a mix gradation that is very close to the job mix formula.
Figure 6-14
(D) CALCULATING BATCH WEIGHTS
After determining the proportions required for each hot bin, we can calculate the weight of asphalt binder and the amount of aggregate to be pulled from each bin to produce a single batch of asphalt mix.First, we select a batch size which will be mainly dependent upon the pugmill capacity of the asphalt plant. Assume that we have a 4,000 lbs (1,818 kg) capacity pugmill.
Summarize the information we have available:
From this information, the weight of asphalt binder in each batch can be calculated by multiplying the batch weight by the percentage of asphalt binder in each batch:
4,000 lbs.(1,818 kg) x .058 (5.8%)= 232 lbs.(105 kg)The total weight of aggregates in each batch is determined by subtracting the weight of the asphalt binder from the total batch weight:
4,000 lbs.(1,818 kg) – 232lbs.(105 kg) = 3,768 lbs.(1,711 kg)Knowing the total weight of all aggregates needed for a batch of asphalt hot-mix, along with the estimated hot bin percentages, allows calculation of the weights of aggregate to be pulled from the bins. This calculation is shown below.
(E) SETTING THE BATCH WEIGHTS(1) AGGREGATE SCALES: From the hot bins the aggregates are withdrawn for deposit into a weigh hopper. The weigh hopper is suspended from scale beams and weighs accumulatively the amounts of aggregate entering it. (See Fig. 6-15)
Figure 6-15
How Cumulative Scale Settings Are Used to Control Material Amounts
Pulled from Hot Bins
The weights to be pulled should be arranged in the order best suited for the plant. The Contractor or Producer will select the sequence of loading the weigh hopper. In most plants, the coarse aggregate is drawn first, so that the fine aggregate will not leak through the weigh hopper gate. Once the bin order has been determined, the bin weights are set accumulatively on the aggregate scale dial. The scale settings are established as follows:
These settings should be marked on the scale and used until adjustments
are required. (See Fig. 6-16)
Figure 6-16
Asphalt Plant Scale Settings
6.6 "DRUM MIX" PLANT OPERATIONS(2) ASPHALT BINDER SCALES: From the weigh hopper, the aggregates are deposited into the plant's pugmill (mixing chamber), where they are blended with the proper proportion of asphalt binder. In a typical plant system, asphalt binder is weighed separately in a weigh bucket before being introduced into the pugmill.This weight is set on a separate asphalt binder scale from the aggregate scales. From the previous computations, our asphalt binder scales setting would be 232 lbs (105 kg). (See Fig. 6-16)
(3) AUTOMATIC PLANTS: The batch weights for the mix are normally set on the control panel of an automatic plant by various formula setting devices. The three methods used on most automatic plants are as follows:
- Preset Dial Type - Each dial is set to the individual weight to be pulled from the corresponding hot bin. A separate dial is also used for the asphalt binder.
- Card Set Type - A card set type uses a prepunched or preslotted card which is inserted into a card reader on the control panel. The cards are prepunched or preslotted for the component weight of any mix desired in the proportion specified.
- Batch Plug - A batch plug consists of several potentiometers, each set for the individual component weight and enclosed in a case about half the size of a cigarette pack. A number of these can be stored at the plant for various mixes.
- Central Processor Unit - Computer System capable of storing different mix types, printing batch weights and controlling most plant operations.
Drum mixing is a relatively simple process of producing asphalt hot-mix. The mixing drum from which this type of plant gets its name is very similar in appearance to a batch plant dryer drum. The difference between drum-mix plants and batch plants is that, in most drum-mix plants the aggregate is not only dried and heated within the drum, but also mixed with the asphalt binder. However, there are some more recent model drum mix plants that introduce the asphalt binder outside the drum. The addition of a coater box, which is a pugmill type device, located at the discharge end of the drum allows the asphalt binder to be added into the coater box instead of into the drum. Still other "double barrel" type drum mix plants will add the asphalt binder between an inner and outer drum. The basic concept of all these types is the same though -- a continuous mixing process as compared to the mixing of batches at batch plants. There are no gradation screens, hot bins, weigh hoppers or pugmills in a drum-mix plant. Aggregate gradation is controlled at the cold feed.
6.6.1 Cold Feed System
Since the drum mix plant does not incorporate a gradation screening unit, the aggregate must be accurately proportioned prior to entry into the mixing drum. Mix gradation and uniformity are entirely dependent on the cold-feed system. The plant must be equipped with provisions to conveniently obtain representative samples of the full flow of material from each cold feed and the total cold feed for calibration purposes. Calibration of the cold feeds to determine compliance with the job mix formula is essentially the same as for batch plants. The technician should refer to Section 6.4 for these procedures and frequency of calibration.
Each feeder shall be equipped with an automatic device which activates a warning alarm and/or flasher light when any bin becomes empty or when aggregate flow becomes restricted. The automatic device shall be interlocked into the plant control system so as to automatically stop production if normal aggregate flow is not resumed within 60 seconds. Each feeding system shall be so constructed that samples can be readily obtained.
6.6.2 Vibratory Scalping Screen
Subarticle 610-5E(2) of the Standard Specifications require a vibratory screening system capable of removing all oversize materials for the particular mix being produced shall be provided prior to entry of the aggregate into the dryer-drum mixer. Normally, a screen size just slightly larger than the maximum aggregate size for the type mix being produced is satisfactory. It is also desirable that the scalping screen unit be located just after the material leaves the cold feed bins and prior to the material passing over the weigh bridge of the aggregate weighing system. This serves to uniformly distribute the material on the belt and results in less fluctuation of the aggregate feed rate data being conveyed to the plant control blending system.
6.6.3 Weight Measurement of Aggregate & RAP / RAS (Quarterly Calibration)
Before calibrating the weight measurement of either the aggregate or
RAP/RAS (when used), the Contractor must give the appropriate division
QA Supervisor at least two days notice to allow the QA supervisor an opportunity
to observe this calibration. Drum-mix plants require a continuous
weighing system on the cold feed conveyor belts (including RAP/RAS conveyor
belts). In-line belt weighing devices, also called weigh bridges
(Figure 6-18) are continuous belt-weighing devices used for this purpose.
Combined aggregates or RAP/RAS passing over the conveyor belt are continuously
weighed and a readout (in the control room) indicates the weight of the
flow over the scale at any given instant. No material should ever
be diverted from the conveyor belt after it passes the belt weigh bridge
during actual production.
Fig. 6-18 shows one of the conveyor idlers (designated the weigh
idler) of the belt weigh bridge which is mounted on a pivoted scale carriage.
As the loaded belt passes over this idler, the weight is read in tons and/or
tons per hour and a reading displayed at the control console in the control
trailer. This reading is normally corrected to account for moisture
in the aggregate (since dry-aggregate data is used to establish the required
percentage of asphalt binder) and is a key reading in monitoring plant
operations.
Figure 6-18
Weigh Bridge
There are several various methods for checking these weigh bridges. Any method that will verify the actual weight passing across the weigh bridge as compared to the plant's digital readout is correct. Some of these methods are briefly described next.
Before running either type method, it is suggested that at least 4 tons (4 metric tons) of one size aggregate be passed over the cold feed conveyor belt and into a truck by means of a diversion chute. This quantity can then be dumped back onto the stockpile. This precaution insures proper seating of the conveyor belt on the rollers and insures better accuracy. If this precaution is not taken, the first test may give misleading results.
If the plant controls are equipped with a readout that gives actual weight (tons, not tons per hour) crossing the weigh bridge, this check is as follows. First, set the plant's moisture compensator to zero and then zero out the belt scales. Next, run approximately 15 tons (15 metric tons) of clean coarse aggregate for aggregate weigh bridges or a minimum of 10 tons for RAP and RAS weigh bridges across the appropriate weigh bridge at normal production rate, and check the readout weight by use of a certified set of scales. A coarse aggregate is preferable due to less chance of loss of fines during the weighing process, less moisture in the material, and ease of handling as compared to finer materials. Means should be provided for diverting the aggregate into trucks, or other containers. This is normally done by use of a diversion chute located at the end of the cold feed conveyor belt. The quantity of test material will be run across the weigh bridge, through this diversion chute into a container, and then weighed on an approved set of scales. The net weight will then be determined by subtracting the container weight from this gross weight. This net weight is then compared with the weight reading displayed in the plant control console. The readout should compare within ± 0.5% of this net weight, except that when Auto-Recordation is used for asphalt binder control, it should compare within ± 0.3%.
The plant may be equipped with a weigh bridge digital readout that only gives a rate in tons per hour. In this case, the above procedure can generally be used but the actual weight will be converted to tons per hour. When making these checks, it is very important that the plant be operated at normal production rate and this rate be uniformly maintained during the test. As above, approximately 15 tons (15 metric tons) of material will be run across the weight bridge, thru a diversion chute into a container, weighed, and a net weight determined. The difference being that this is a timed test to be performed over some measured period of time. Time the material from its first crossing the weigh bridge until the last material clears the weigh bridge. The conversion to tons per hour is made by use of the following formula:
This figure is compared to the weigh bridge tons per hour shown on the readout. The readout must be within the ± 0.5% accuracy of the computed rate for either aggregate or RAP/RAS weigh bridge scales.
Any necessary adjustments are made in the electronics of the weigh bridge scale to compensate for the difference between the actual weight or tons per hour and the digital readout. This adjustment is the Contractor's responsibility.
In drum-mix plants the aggregate is weighed before drying. Since the undried material may contain an appreciable amount of moisture that can influence the aggregate's weight, an accurate measurement of aggregate moisture content is important. From this measurement, adjustments can be made to the automatic asphalt binder metering system to ensure that the amount of asphalt binder delivered to the drum is proper for the amount of aggregate minus its moisture content.
The Technician should monitor the moisture content of the cold feed aggregate before beginning each day's operation and again about the middle of the day, and the Contractor should adjust the moisture control equipment accordingly. If the moisture content is believed to vary during the day, it should be checked more frequently. The moisture content may be determined manually or electronically. A minimum of one moisture test per normal day's operation shall be performed by the plant technician. See Section 7 of this manual for moisture test procedures. Provisions must be made for electronically correcting wet aggregate weight readings to dry aggregate weight readings in the plant control system.
6.6.4 Asphalt Binder Meter System (Quarterly Calibration)
Before this calibration is performed, the Contractor must give the appropriate division QA Supervisor at least two days notice to allow the QA supervisor an opportunity to observe this calibration. Most drum-mixers are typically equipped with a device (Fig. 6-19) to add asphalt binder to the aggregate inside the drum mixer. Some more recent model drum mix plants may be equipped to add the asphalt binder to the aggregate outside the drum-mixer into a coater box, which is a pugmill type device.
Still other "double barrel" type drum plants will add the asphalt binder between an inner and outer drum. Either of these will utilize an asphalt binder metering and delivery system which is a continuous mechanical proportioning system interlocked with the aggregate weigh system to ensure the exact asphalt binder content of the mix. The weight of aggregate going into the mixer, as measured by the weigh belt, is the basis of determining the quantity of asphalt binder delivered into the drum or coater box, whichever is applicable.
Most metering systems measure the volume (liters/gallons) of asphalt binder being delivered by use of a volumetric flow meter. This volume must be converted to a weight rate of flow. Since the volume and weight are temperature dependent, the plant control system must include a means to allow for temperature and specific gravity variations in asphalt binders. The system must also have a temperature indicating device in the asphalt binder feed line. Procedures for making these adjustments are usually included in the manufacturers plant operations manual.
Some few drum mix plants are equipped with mass flow meters that measure the mass of asphalt binder being delivered instead of volume. These normally read out in a weight, instead of a volume, and are not temperature and/or specific gravity dependent.
The proportioning of asphalt binder is accomplished by establishing the necessary rate of asphalt binder delivery to match the aggregate delivery rate in tons of dry aggregate per hour. The asphalt binder delivery rate is automatically increased or decreased proportionately according to the corrected dry weight measurement of aggregate passing over the belt scale. The rate of asphalt binder delivery is normally indicated in tons per hour on a rate meter on the control panel.
Means must be provided for checking the accuracy of the asphalt binder metering system. This check should be performed at a minimum of bi-monthly and should check within +/- 0.5 percent accuracy.
Most drum-mixer plants are equipped with a by-pass valve system which will allow the asphalt binder to be pumped through the flow meter and into a container, instead of into the drum-mixer. By means of this by-pass system, a minimum of 500 gallons (1,893 liters) (or the appropriate quantity as recommended by the manufacturer) will be pumped off into a container. Due to the quantity to be pumped off, this container will normally be either an asphalt distributor tanker or a supply tanker. This container will need to have been previously weighed empty, in order to obtain the net weight of the pumped-off asphalt binder. These weights should be made by use of a certified set of scales of sufficient capacity. The net weight of the asphalt binder should be compared to the number of gallons shown pumped through the flow meter, if a volumetric flow meter is being used, or compared to the weight shown if a mass flow meter is being used. If a mass flow meter is being checked, a direct weight to weight comparison is made and no conversion is necessary. If a volumetric flow meter is being checked, a conversion of either pounds to gallons (kg to liters), or gallons to pounds (liters to kg), will have to be made in order to make the necessary comparison. This conversion can be made by use of one of the following formulas.
The specific gravity at 60°F (15.6°C) should be given on the asphalt binder delivery ticket. The above conversion formulas are useable only if the drum-mixer plant is equipped with systems that automatically compensate for varying asphalt binder temperatures and specific gravities. It is very important to be sure these compensating systems are operating correctly before checking the metering system. Most manufacturer's manuals contain instructions for checking to see if these systems are functioning properly.
The plant manual should always be consulted prior to the meter check for the manufacturer's recommended procedures. This is important because some plant meter readouts show weights, some show gallons (liters) @ 60°F (15.6°C), and others show liters(gallons) @ the actual binder temperature. The plant manual will always give the correct method of calibration and conversion.
The flow meter readout should check within ± 0.5% accuracy. Any necessary adjustments to be made to the metering system are the Contractor's responsibility. See Figure 6-20 (in the Printed Version of the 2008 QMS Manual) for documentation of this meter check.
The plant control read-out will show binder tons per hour being added to the mix. This read-out figure is calculated by the plant control computer system utilizing the aggregate weigh bridge weights and the JMF % binder. This read-out should be mathematically checked occasionally to be sure that the binder quantity is being calculated correctly. The following formula should be used for this calculation:
This calculated TPH binder number should be within approximately 8% -10% (not tons per hour) of the readout.
If the plant is equipped with an additive metering system it should be calibrated at the same time the binder meter is calibrated. This system should be accurate within ± 10% of amount specified. A similar process to the binder meter calibration will be followed except that only a minimum of 19 liters (5 gallons) must be pumped off and weighed on calibrated scales of sufficient capacity.
6.6.5 Anti-Strip Additive Meter System (Monthly)
If the anti-strip additive is introduced into the binder at a drum
mix plant, the additive meter system shall be calibrated bi-monthly in
accordance with the procedures in Section 6.5.8 (D) of this Manual.
6.6.6 Documentation
A drum plants weigh bridges / asphalt binder meters calibration Form QC-2 (Pg. 6-34 in the Printed Version of the 2008 QMS Manual) and/or certification by the Department of Agriculture or a certified scale company will suffice for either the initial check or for a periodic check, provided the date of the certification is within the time frame noted above (30 days for asphalt binder, aggregate scales and weigh bridges). A single scales check may and should serve for several different projects if the check was conducted within the above noted time. Form QC-2 (Pg. 6-34 in the Printed Version of the 2008 QMS Manual) should be submitted to the Division QA Supervisor at the completion of each scales check.
As noted, the QC Technician should post a copy of Form QC-2 in the QC Laboratory when the scales are actually calibrated. A copy shall also be furnished to the appropriate Division QA Supervisor.
6.6.7 Drum-Mixer Dryer
The heart of the drum-mix plant is the drum dryer itself (See Fig. 6-19). The dryer is similar in design and construction to a conventional rotary dryer, except that most drum dryers utilize the parallel flow principle as opposed to the counter flow principle used in conventional batch plants. The burner is mounted at the high end of the drum where the cold proportioned aggregates are introduced. By using this approach, the hottest gases and flame are at the charging end of the drum. When the asphalt binder is introduced further down the drum, it is protected from the excessive harmful effects of the burner flame by the evaporating moisture on the aggregate. The exception to this is the newer model double barrel drum mix plants which have the burner located on the lower end of the drum with the aggregate flow being toward the flame. This is because the inner drum of this type plant serves the purpose of a drying chamber only and not that of a mixing chamber.
Some more recent model drum mix plants have been modified from the more conventional method, to methods that will further protect the asphalt binder from excessive heat. The addition of a coater box at the discharge end of the drum and the double-barrel drum mix plants both serve this purpose. The asphalt binder may be added into the coater box or between the inner and outer drums; both being removed from direct flame exposure. The mix temperature is monitored on all type drum plants by a thermometric device in the dryer discharge chute which automatically activates the burner controls, and therefore, controls the mix temperature.
NCDOT Specifications require that dryer drum mixers have a rated capacity of at least 90 tons (82 metric tons) per hour when producing a finished mixture at 300°F (149°C) with removal of 5 percent moisture from the combined aggregate.
6.6.8 Surge-Storage Bins (Silos)
In a drum-mix operation, which produces a continuous flow of fresh asphalt hot mix, it is necessary to have a surge silo for temporary storage of the material and for controlled loading of trucks. A weigh system may be connected to the holding bin of the silo to monitor the amount of material loaded into each truck. Weight measurements are normally recorded by the weigh system control panel, located in the control van or trailer.
Insulated silos or bins can store hot mix up to 12 hours with no significant loss of heat or quality. Capacities range as high as several hundred tons. Non-insulated storage structures are usually quite small and can store hot mix only for short periods of time.
Storage silos work well if certain precautions are followed, but they can cause segregation of the mix if not used properly. It is good practice to use baffle plates, a batching hopper, a rotating chute, or similar devices at the discharge end of the conveyor used to load the silo. These devices help to prevent the mix from coning and segregating as it drops into the silo. It is also recommended to keep the silo at least one-third full to avoid segregation as the silo empties and to help to keep the mix hot.
Hot mix is dumped into the top of the bin and falls vertically into the structure. The bin must be designed so that segregation of the mix is held to a minimum. The subsequent loading of trucks should be made with a minimum of three dumps. Frequent visual checks of the mix must be made by the QC Technician to make certain segregation has not occurred during the charging of the bin and/or during loading into trucks. Coarse mixes must be watched closely since they are more subject to segregation.
Sample of the mix for testing purposes will be taken directly from the truck body in accordance with procedures outlined in Section 7 of this manual.
6.7 SEGREGATION OF PLANT MIXES
Segregation refers to a condition in hot-mix asphalt in which there is non-uniform distribution of the various aggregate sizes across the mat to the point where the mix no longer conforms with the specified job mix formula. Segregation is the tendency of larger particles to separate from a mass of particles of different sizes under certain conditions.
Segregation may be brought about by the methods of mixing, storing, transporting, and handling the mix wherein there is a condition created that favors non-random distribution of the aggregate sizes. Segregation problems are most often associated with drum mix plants. There is, however, nothing to indicate that drum mix plants themselves are more prone to segregation problems than batch plants. Segregation in hot-mix asphalt pavements is more closely associated with surge-storage systems, which are most often used with drum mix plants. The previous Section 6.6.8 explains the uses of these surge-storage systems, inherit segregation problems, and some possible solutions.
Coarse graded mixes, such as the 37.5 mm and 25.0 mm Base mixes, are naturally more prone to segregate due to their stone content, low asphalt binder content, and due to possible gap-grading. Finer graded mixes, such as the 9.5 mm and 12.5 mm surface mixes, do not tend to have severe segregation problems for the opposite reasons of those noted with coarse graded mixes.
Segregation can originate at virtually any point in the process of hot-mix asphalt production. It can get its start in the mix design, in the aggregate stockpile, in the cold-feed bin, in the batch plant hot bin, in the drum mixer, in the drag-slat conveyor, or in the surge-storage bin. In some cases, segregation doesn't start until the truck is being loaded out. The earlier segregation begins during the hot mix process, the worse the problem tends to be due to more movement of the mix in completing the process. Whenever segregation does occur, all of these areas should be closely monitored for their extent of contribution to the problem.
The solution to segregation problems usually lies within several of these problem areas. Modifications in the mix design may be needed; improper handling of the aggregates may need to be addressed; modifications to the plant, drag-slat conveyor, and/or the surge-storage bin may be necessary; and the handling and movement of the mix through the surge-storage bin and into the truck. It's most important to remember that for whatever the reason and at whatever the location segregation begins, after it does, any unrestricted movement, especially down slope movement, will compound the problem drastically. Because of this, down slope movement of the mix should be kept to a minimum throughout the hot-mix asphalt process.
Reference should be made to the segregation diagnostic charts in the Appendix (printed version of 2008 QMS Manual) for possible solutions to various types of segregation problems.
6.8 SCALES AND PUBLIC WEIGHMASTER (ARTICLE 106-7)
Specifications for weighing asphalt materials which are to be paid for on a ton (metric ton) basis can be found in Article 106-7 of the Standard Specifications. This Article requires that any scales which are to be used to determine the weight for payment purposes shall be certified by the North Carolina Department of Agriculture. This may include platform scales and/or the plant aggregate and asphalt binder scales, depending upon which scales are being used by the Contractor's public weighmaster to issue the weight certificate. See Section 5.9 of this manual for specific details of the NCDA & CS certification requirements. Requirements for automatic weighing, recording and printing of tickets are listed under Article 106-7. It should be noted that a Project Special Provision requires that the JMF No. be recorded on each asphalt weigh ticket. Also included in this same Article are provisions for checking the scales by re-weighing a truck load of material on another set of approved platform scales.
It is the Resident Engineer’s and QA Supervisor’s responsibility to assure that the Contractor is meeting the requirements of Article 106-7 before any weight certifications are issued. The requirements of this Article or approval of the weighing equipment is not covered by the plant certification.
Weigh tickets provide essential records for the control of project operations, quality, and quantity of mix delivered. Although different systems are used by various agencies, certain items related to tickets remain generally the same from project to project. Weigh tickets-numbered consecutively-are generally issued at the plant. They must state the project number, the origin of the load, time loaded, the temperature and weight of the load, the truck number, the type of mix, the JMF Number, Plant Certification Number and location (station number) where the mix was placed. It will also list the weight and roadway temperature of the mixture. See Std. Specs. Article 106-7 and the Project Special Provisions for detailed requirements for asphalt weigh tickets.
Several things of importance to the Technician are contained on these tickets. First, the numbering of the tickets consecutively will show whether or not a truck arrived at the paver in an order different from which it was loaded. This could be due to breakdown of the truck, traffic problems, or whatever, but it will give the Technician an idea of the length of time that the material has been loaded. If the period of time was longer than what would be considered normal, then the mix must be checked more thoroughly than usual for proper temperature and for lumps formed due to cooling. If serious temperature problems are detected, the load should be rejected. It is important for the Technician to collect the load tickets from each truck as the truck is dumping. In this manner, he can be assured that no loads intended for his project were diverted. (See also NCDOT Construction Manual - SCALES & PUBLIC WEIGHMASTER)
6.9 HAULING OF ASPHALT MIXTURES
The QC and QA technicians must inspect truck bodies in which the mix is to be hauled to be sure that they comply with the Standard Specifications. The truck body should be inspected to make sure that the bed has been lightly coated with an approved release agent to prevent the mixture from adhering to the bed. After the bed is coated, any excess solution must be adequately drained before any mix is allowed to be loaded.
Excess solution can be extremely detrimental to mixture in which it comes in contact. Fuel Oil shall not be used. For an approved list of truck release agents, contact the Asphalt Design Engineer at the M&T Unit.
The mixing temperature at the asphalt plant will be established on the job mix formula. See Section 6.1.2. for the normal Superpave mixing temperatures. Plant and Roadway Technicians should always refer to the most current Job Mix Formula for the correct mixing temperature. The temperature of all mixes when checked in the truck at the asphalt plant shall be within ± 15°F (± 8°C) of the JMF temperature. The temperature of the mix immediately prior to discharge from the hauling vehicle at the roadway shall be within +15??F (+8°C) to -25°F (-14°C) of the JMF temperature.
The mix should be observed frequently and the temperature checked at regular intervals and recorded (See Fig. 6-21). The Contractor shall provide a platform near the truck loading area from which the mix may be observed and from which the samples of the mix may be secured, as well as the temperature of the mix determined. The QC Technician shall see that all trucks are properly covered and the covers securely fastened before leaving the plant to protect the mix from chilling during the haul, caused either by cool weather or rain showers. Covers must be of sufficient length and width to cover the entire load.
Truck or Platform Scales must meet requirements of Standard Specifications Article 106-7, "Scales and Public Weighmaster". The platform scales shall be certified by the Department of Agriculture before they are used to determine the weight of mixture for payment purposes. (See section 5.9 of this manual for details of DOA certification requirements) The Engineer may require occasional loads to be re-weighed on another set of approved platform scales. Different scales at the same site may be used provided they are DOA approved. When reweighing is being done to check scales accuracy, the weights should compare within 0.4%, plus or minus. (See example pg. 6-39 in printed version of 2008 QMS Manual)
6.10 POLLUTION CONTROL EQUIPMENT
Specifications require that all plants be equipped and operated with the necessary pollution control equipment in order to meet all applicable State, Federal and Local pollution and environmental regulations. The Contractor must make certain that the plant has been properly registered and permitted prior to the plant being certified by the NCDOT. The Plant Certification will indicate the air quality permit number and the date of expiration.
Adequate safety devices must be provided by the Contractor at all points where accessibility to plant operations is required. Accessibility to the top of truck bodies will be provided by a platform or other suitable device to enable the technician to obtain samples and mixture temperatures. All gears, pulleys, chains, sprockets, and other dangerous moving parts shall be thoroughly guarded and protected. A clear and unobstructed passage must be maintained at all times in and around the truck loading area. All work areas shall be kept free from asphalt binder drippings.
6.12 PRODUCTION CONSISTENCY AND AUTOMATIC EQUIPMENT
The Specifications state that any asphalt plant that cannot consistently produce a uniform mix meeting the requirements of the job mix formula and other applicable specifications will be considered in non-compliance and may have its certification revoked. These requirements include proper gradation, proper asphalt binder content, uniform mix temperature, and operation of all required automatic equipment.
The Specifications for batch plants state that upon a malfunction of the required automatic equipment, the plant may continue to operate manually for the following two (2) consecutive working days. If the automatic equipment is not repaired within two (2) working days production of all mix must stop until all repairs are made.
The Specifications also state that when a malfunction of required automatic equipment occurs at a drum-mix plant, manual operation of the plant will not be allowed except that if in the opinion of the Engineer an emergency traffic condition exists, the plant may be allowed to operate manually until the unsafe traffic condition is corrected. This mix is subject to the same specification requirements as the mix that is produced automatically. No other production from this plant will be allowed until the malfunction is repaired.
It is the intent of these specifications that all automatic equipment properly operate at all times, except in situations where legitimate breakdowns occur. The Contractor must make every effort to repair any breakdowns of automatic controls immediately.