Asphaltic Concrete

ASPHALTIC CONCRETE (ACW20) Asphalt Concrete mixes made with asphalt cement are prepared at an asphalt mixing plant. Here, aggregates are blended, heated, dried, and mixed with asphalt cement to produce hot mix asphalt. Prior of laying the asphaltic concrete, the design mix was submitted for obtain approval and a trial lay shall carry out and tested comply with JKR specification of road work (JKR/SPJ 1988 C14.1) Table 4.8 Gradation limits for asphaltic concrete and Table 4.9 Design bitumen content. Beside a lab compaction test are required to determine the lab density. The asphaltic concrete was mixed at Dakelin Sdn Bhd’s plant which located at Stephen Yong Road at 18th November 2011. The asphaltic concrete was prepared at drum mix plant instead of batch mix plant. Asphalt Concrete plants are basically of two types, batch or drum mix. The two types of asphalt plants derive their names from their particular type mixing operation. In the batch-type mixing plant, hot aggregate and asphalt are withdrawn in desired amounts to make up one batch for mixing. After thoroughly mixing, the material is discharged from the pugmill in one batch. In the drum-type mixing plant, the aggregate is dried, heated, and mixed with the asphaltic cement in the drum in a continuous operation.

Drum Mix Plant Drum mixing is a relatively simple process of producing asphalt mixtures. The mixing drum looks like the familiar dryer. The difference is that the aggregate is not only dried and heated within the drum, but also mixed with the asphalt cement. The drum mix process depends upon cold feed control for gradation of aggregates. The drum mix plant requires the use of a surge silo for mix load out. Drum mixers can produce a true hot mix or a low temperature mix.

Dakelin’s asphaltic mixing plant

Drum mixer

Cold freed bins

Cold freed conveyor

Control House

Surge silo

Drum mixer

Weigh bridge

Cold freed conveyor

COLD FEED BINS Production starts with the cold feed bins. It is necessary to proportion the mixing drum. Total and proportional control with variable speed belt feeders permits adjustments of individual feeder output to desired proportions. Thus total tonnage of aggregate going to the drum mixer can be increased or decreased by the speed of the feeder belts without changing the proportions.

DRUM MIXER The aggregates go into the drum mixer at the burner end so aggregates and hot gases move through the drum in the same direction. By utilizing this principle, the hottest gases and flame exist at the charging end of the drum mixer. Thus, the asphalt is best protected from the harmful effects of oxidation and direct contact with the burner flame by the evaporating moisture on the aggregate. While the drum mix process features a smooth continuous flow of material, inside the drum mixer certain events occur in phases within fairly well delineated zones of the drum mixer. One of the principle differences between the conventional method of mix production and drum mixing is how the aggregate is coated. Heating and mixing of the aggregates and asphalt cement is done in four phases. Phase I - The aggregate has entered the drum mixer. In the early heating phase, surface and, free moisture begin to leave the aggregate as temperature rises. Phase II - Most of the heat rise occurs in phase II as aggregate temperatures reach approximately 170 to 180ºF (77 to 82ºC). The majority of the moisture is driven off in this phase and the rate of increase in mixing temperature levels off. Phase III - As mix temperature reaches between 180 and 200°F (82 and 93°C) asphalt is introduced to the mix. Moisture driven off now causes the asphalt to foam. This foaming action causes the surface area of the asphalt to be greatly enlarged, thus entrapping dust as well as larger particles and coating the aggregate rapidly. Thus, it can be seen that aggregate coating in a drum mixer is not a function of asphalt being forcibly mixed but rather of the aggregate particles being engulfed by the foaming, rapidly spreading asphalt. Phase IV - Most of the moisture has been removed. The aggregate has been coated, and mix temperatures will continue to rise until desired temperature is reached. After the mix temperature has been achieved, the mix is discharged into a hot incline elevator that carries the mix to either a surge silo or storage tank where it is held at a constant temperature until use.

TESTING Bitumen extraction test The method described is a procedure used to determine the asphalt content of asphalt aggregate mixtures.

Asphalt centrifuge extractor

Test Procedure Turn ventilation equipment on and adjust for maximum suction from the extractor and enclosure. Spread the asphalt mix sample evenly in the bowl. Cover the sample with the specified solvent (Solvents - suggested materials are chlorothene or trichloroethylene). Rotate the bowl back and forth gently by hand to distribute solvent and asphalt mix evenly in the bowl.

Scale capable of weighing to 2500 g at a 0.1 g accuracy.

Paper or felt filter rings

Place a filter rings on the bowl rim and secure the bowl cover plate on top of the filter papers. Ensure that for all subsequent extractions the bowl cover plate is tightened. Allow the material to soak for 45 minutes before the first centrifuge is begun. Begin centrifuging slowly, increasing bowl speed slowly to a maximum of 3600 rpm. When the asphalt-solvent effluent stops flowing from the extractor, turn off the motor, slow and stop the bowl using the braking mechanism. Add approximately 250 ml of solvent to the bowl and repeat the above described procedure. The procedure should be repeated until the extracted effluent has a light yellow straw colour. This is usually accomplished in 3 washings.

Bowl lid.

Filter Ring

After wash

Paper or felt filter rings to be placed on the rim of the bowl and beneath the bowl lid After the final wash, remove the filler screw and cover plate. Remove the filters carefully, clean the extracted aggregate from the filter papers and bowl and place in a clean pan. The recommended drying procedure is to place the extracted aggregate in an oven at a controlled temperature of 120o C and dry to a constant weight. Establish the required drying time by weighing after repeated heating and cooling until the weight is constant. To prevent burning of the mineral particles, do not heat the aggregate above 120o C.

MARSHALL TEST The Marshall Test is used to measure physical properties of asphalt specimens that relate to plastic deformation properties of asphalt mixes. Stability and Flow The accuracy measurement of stability and flow of specimens tested in a load frame is important if consistence and representative results are to be achieved. The load frames and ancillary items listed have been designed to enable technicians to test specimens quickly and easily with confident recording of results.

Marshall Automatic Compactor

Ring dynamometer

Flow meter Water bath at 60⁰C Breaking Head

Test sample

Procedure 1. Specimens are heated to 60 ±1oC either in a water bath for 30-40 minutes or in an oven for minimum of 2 hours. 2. The specimens are removed from the water bath or oven and place in lower segment of the breaking head. The upper segment of the breaking head of the specimen is placed in position and the complete assembly is placed in position on the testing machine. 3. The flow meter is placed over one of the post and is adjusted to read zero. 4. Load is applied at a rate of 50 mm per minute until the maximum load reading is obtained. 5. The maximum load reading in Newton is observed. At the same instant the flow as recorded on the flow meter in units of mm was also noted.