Asphalt - With Types

The subject of this term project was an asphalt paving process utilizing a paving machine and 20 tons capacity tri-axle trucks. The location of the process was at the corner of Main and Madison in Greenwood ( South of Indianapolis ). The project is being run by the Reith-Riley Construction Company. - Indianapolis. The overall process involved :

• Hot-mix batch plant cycle
• Tri-axle truck cycle
• Roller cycle
• Spreader cycle
• Crew cycle

Because of the complexity of the overall construction process, we chose to observe, report on, analyze and model the paving process on the base layer of the 15' lane road. At that time, the other lane of the road was not paved yet. The road has slightly increasing grade and curve along the process. The preliminary process of gathering the data used in this project, the efficiency of the operation, a model and MicroCYCLONE simulation of the process, and illustrations will be discussed and presented.

Asphalt has been used by man for its adhesive and waterproofing properties. Asphalt was used in 3800 B.C. in the Euphrates and 2500 B.C. in Egypt. The Sumerians used asphalt in 6000 B.C. for its shipbuilding industry. Today, asphalt is applied to roofing, sealants, caulking, brake linings, paints, enamels, and most widely used in the paving industry (Asphalt - Science and Technology, 1968).

Process Description

Batch Plant Production

First, aggregate travels through the cold feed bins, where initial proportioning of the aggregate takes place. The quantity of material leaving each bin is regulated by the size of the gate opening, or the speed of a belt, or a combination of the two. The aggregate is sent to a drier. Here the moisture is removed and is heated to provide the proper mixing temperature in the pugmill. The aggregate continues to the hot elevator by screens to the hot bins. The screens provide the final separation of the aggregate.

The different sizes of aggregate are released into the weight hopper one bin at a time. The aggregate is dropped into the pugmill for mixing with the asphalt. The mixture is then dropped into a waiting truck or moved to a storage silo. Samples are taken from each hot bin for testing. A sieve analysis is conducted as well as gradation test. From the gradation information, the weight of the aggregate must be equal to the design gradation. A trial run should be performed and the weights adjusted until the desired mix is produced. (U.S. Department of Transportation, December 1984)

Placing Asphalt Pavement

Placing the Coat

Before the paving operation starts, an asphalt distributor is used to spray asphalt on the unpaved surface. This film of asphalt serves as the prime and tact coats. The coats are then allowed to cure before the actual paving resume. The purpose of having these coats is to prevent any slippage between the surface and overlay during or after the compaction. (The Asphalt Institute)

Placing the Asphalt Mix

To start the paving operation, the paver is positioned properly onto the road. The screed of the paver is lowered onto block of the same depth of the loose asphalt mat that is going to be laid on the road. (The screed is responsible for the setting the depth of the asphalt mix.) After that, the block can be removed and paving can start. As soon as the haul truck arrives at the job site, the paving inspector must check that the asphalt delivered must be in a satisfactory condition. The paving inspector usually check for these criteria listed below:

1. blue smoke - blue smoke indicate that the mix is too hot.
2. stiff appearance
3. mix slumped in truck.
4. lean, dull appearance - this indicates that the mix has insufficient asphalt.
5. rising steam - too much moisture.
6. segregation.
7. contamination.

If there is any of the signs above is observed, the mix will be sent back to the batch plant to be reprocessed. After all conditions are satisfied, the haul truck can load the mix into the receiving hopper of the paver.
When loading the mix into the receiving hopper, the haul truck is placed carefully in front of the paver. The rear wheels of the truck should be in contact with the truck roller of the paver to avoid any misalignment with the paver. The paver will push the truck forwards as it paves the road. If skewness happens, the whole process will be delayed because they have to reposition the truck in front of the paver.
Most paver used are self-propelled paver. Each of them consists of two main units:

• tractor unit. -it includes the receiving hopper, slot conveyor, flow control gates, spreading crew, power plant, transmission, operator control for use on either side, and operator's seat. This unit will move the whole system forward.
• screed unit. -it is attached to the tractor unit by long screed arms on both sides of the machine. It consists of screed plate, vibrators or tamper bars, thickness control, crown control, and screed heater.

As soon as the the first load of asphalt mix has been spread, the uniformity of the asphalt texture should be checked. Operators will adjust the the appropriate adjustment points to correct any nonuniformity. Any segregation of materials also should not be allowed. Operation should be stopped immediately if any segregation is detected. The operators should also be aware of is the crown control. Pavement with crown has to be redone all over again. In addition to that, operators should continuously loosen the mix that clings to the sides of the hopper and push it back into the active mix. If the asphalt mix grow cold, it cannot be properly compacted and thus, looses its strength.
The last process of paving is compaction. This process is highly influenced by major mix proportion; (1) asphalt content: aggregate size, shape texture and distribution gradation; (2) filler content, and; (3) mix temperature. Appropriate rollers and rolling methods should be used in accordance with these proportion. There are several roller combinations used for maximum results:

1. steel-tired static and pneumatic-tired rollers,
2. vibratory and steel-tired static rollers, or
3. vibratory rollers used in vibrating and static modes.

These combinations are highly recommended by the asphalt institute.
Rollers should be moved in a slow but uniform speed to achieve the best result. (See table) These rollers should also be in good conditions. Any irregularities in the rollers' performances will result in poor compaction of the asphalt; thus, the pavement will not last long. The rollers should not reverse suddenly while compacting because this action can displace the mix. If displacement happens, the whole mat should be loosened with lutes or rakes and restored to the original grade before rolling can restart. A pattern that is economical and provides the maximum compaction result should be established. (The Asphalt Institute)

Testing Method

Why Evaluate Density of Hot Asphalt Concrete

As we know, lacking of density during construction of asphalt concrete causes many problems. It is necessary to obtain high density to insure that the asphalt concrete will provide the necessary stability and durability for performance. For instance, low density generally causes long-term deterioration when the asphalt begins cracking. Therefore various methods have been used to measure the density in the asphalt concrete.

Procedures Used to Obtain Density

Proper aggregate gradation and asphalt content are important parameters to ensure that the density of asphalt concrete meets the requirement. Generally, poor gradation results in a reduction of voids in the mixture; thus, reduces the asphalt content which serves as the lubricant for aggregates in the mix. The stiff mix is more difficult to compact. Both the aggregate gradation and the asphalt content are interrelated and equally important.
Paving asphalt is really difficult in cold climate. The hot mix cools down faster and harder to compact. To overcome this, contractors usually increase the temperature of the mix. Unfortunately excessively increasing the temperature of asphalt mixture may cause problems during compaction and increase oxidation of the asphalt cement. This may result in a hard and brittle pavement. The mix temperature should be selected so that the mixture would be able to support the roller immediately behind the paver. Since there is less time to roll the mixture before it cools, more rollers or larger rollers are required for the compaction process.
After the mixture is transported to the site, the next step is to ensure proper density while laying down the asphalt with a spreader. A continuos availability of the asphalt mix for the paver is crucial. The spreader cannot afford to start and stop while waiting for the materials. It important to have material that has the same texture and appearance.

Evaluation of In-Place Density

An evaluation of the in-place material is necessary to ensure that a satisfactory density is obtained. Most of the time, a nuclear gage is used to estimate the density. However, the results obtained using this equipment are not accurate. It has to be calibrated by taking a number of measurements from different location as soon as the project starts. After calibrations, a series of readings are taken and then, the readings are compared to the density obtained from the laboratory. The laboratory results are the density of core samples taken from the same location. (Placement and Compaction of Asphalt Mixtures, 1982)

Laboratory and Field Pavement Stability Tests

The American Association of State Highway Officials (AASHO) and the American Society for Testing and Materials (ASTM) are two agencies which set forth methods and test procedures the paving industry must follow. Five test methods readily used in the field and laboratory are:

1. The Hubbard Field Stability Test (ASTM-D-1138-52 or AASHO Test 169) - tests the resistance to plastic flow of fine aggregate mixtures.
2. The Unconfined Compression Test (ASTM-D-1074-52-T) - measures the cohesion of paving binder and performance of the internal friction of the aggregate.
3. The Marshall Test (ASTM-Method-D-1559) - measures the flow value or flow index by distorting the specimen until fracture.
4. The Hveem Test (ASTM-Method-1560) - uses stabilometer and cohesiometer apparatus. The stabilometer determines the maximum amount of asphalt which will obtain the greatest stability by measuring the internal fiction of the mineral aggregate. The cohesiometer determines the cohesion properties and the strength of asphalt films by bending and breaking a specimen.
5. The Triaxial Compression Test - is useful in determining the cohesion of the mix and asphalt contents and the angle of internal friction of the entire mixture by applying lateral pressures. Theses are the most widely used although a variety of other methods exist. It must be stated that different local, state, and federal organizations will require certain tests to be performed and may accept a different range of values.