| Abstract |
In recent years, modified asphalt mixtures have become increasing in the construction of flexible pavements. Several classes of modified binders have been used in asphalt pavements. The main modifiers are fillers, fiber rubber, and polymers. Because conventional asphalt mixtures can not resist high stress due to the fact that the axle loads and tire pressures are continuously increasing, attention have been increasing towards the use of polymers modified asphalt.
The main purpose of this investigation is to evaluate the effect of adding the polypropylene pellets on the 80/100 and 60/70 Alex. asphalt cements and their asphalt mixtures. Moreover the effect of adding polypropylene on the long-term performance of the modified 80/100 asphalt mixtures is investigated, and is compared with the unmodified 80/100 & 60/70 asphalt mixtures. To achieve the objective of this investigation, the study consists of two phases:
Phase 1 concerns with evaluating of the effect of adding polypropylene on the different properties of asphalt cement such as penetration, softening point, kinematic viscosity, absolute viscosity, specific gravity, durability and penetration index.
Phase 2 concerns with evaluating the effect of adding polypropylene on the different properties of asphalt mixtures such as Marshal stability, Marshall flow, Marshall stiffness, air voids, voids in mineral aggregates, unit weight, and the long term performance. The long-term performance is investigated by using two different tests, the wheel tracking machine test and the static creep test.
For phase 1, the variables considered are the effect of adding different ratios of polypropylene on asphalt cement properties for two types of Alex. asphalt cements, namely 80/100 and 60/70 grades. The added ratios are 1, 2, 3, 4, 6 and 8% by weight for 80/100 asphalt grade & 2 and 4% by weight for 60/70 asphalt grade.
For phase 2, the variables considered for asphalt mixtures are the effect of the above modified asphalt cements on Marshall properties and on rutting results as evaluated by wheel tracking machine at 25 and 60°C and on creep results at three stress levels of 4.2, 6.25 and 9.38 Kg/cm2.
The results of this study indicate that the addition of polypropylene to Alex. asphalt cement and its asphalt mixtures at all ratios improved the all properties. The addition of 3and 4%PP by weight of Alex. 80/100 asphalt cement alters its physical properties to be similar to 60/70 asphalt cement according to the AASHTO M-20 & ERBA specifications. The addition of polypropylene up to 4% by weight of Alex. asphalt cement improves its thermal characteristics by increasing its penetration index, above this percent the improvement decreases thus, the optimum additives percent can be taken as 4% by weight of Alex. asphalt cement. Increasing the percentages of polypropylene increases air voids, stability and Marshall stiffness and decreases the flow of Alex. asphalt concrete mixtures. The unit weight, voids in mineral aggregates and optimum asphalt content of the Alex. asphalt concrete mixtures are not sensitive to the addition of polypropylene. The 80/100 asphalt mixture at 4% polypropylene ratio provides much better long-term performance than 60/70 asphalt mixture, 80/100 asphalt mixture at 3% polypropylene ratio and 80/100 asphalt mixture. Addition of polypropylene to asphalt cement results in shifting the rutting time from 0 to 35 minutes for 80/100 asphalt mixture at 3% polypropylene and from 0 to 45 minutes for 80/100 asphalt mixture at 4% polypropylene. Moreover the 80/100 asphalt mixture at 4% polypropylene ratio gives the lowest rate of increasing of creep when increasing the stress level. For all mixes the increasing of temperature increase the rutting depth. The rate of tracking in the rutting depth improves with increasing the polypropylene ratio. Finally, correlation between wheel tracking test and creep test is developed at 25°C and stress level of 6.25 Kg/cm2 with curing time of 3 days.
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