THE MECHANICAL PROPERTIES OF MODIFIED (HMA) SMIXTURES FOLLOWING SHORT-TERM AND LONG-TERM AGEING

Abstract

Aggregate and asphalt binder provides the main structure skeleton of the hot mix asphalt (HMA). Due to the nature of high inhomogeneity between aggregate and asphalt binder, significant stress and strain concentration occurs at the interface between the two mix components. The asphalt binder is known to stiffen during the mixing and laying- down operations (short term ageing), as well as during the service life of the pavement (long-term ageing). Asphalt modifiers are usually used to improve the asphalt mixtures properties. One of those modifiers is a polymer, which usually used in asphalt mixtures as a binder modifier (wet process). In wet process, polymer particles react with bitumen at high temperatures during the manufacturing stage. Polypropylene (PP) is the representatives of the category, but due to its non-polar nature, it is almost completely immiscible with asphalt binder. Moreover, the high tendency to crystallize further limits the interactions between the asphalt binder and Polypropylene (PP). This paper presents a novel idea for using a polypropylene (PP) to mitigate the stress and strain concentration between aggregate surface and bitumen layer by introducing an intermediate thin plastic layer between aggregate and asphalt binder as a third layer in HMA. That mediator layer is manufactured by coating of aggregate surface using (PP) before mixing with asphalt, forming Polypropylene treated composite mixtures called plastiphalt. The percentage of (PP) was about 1.7 % by weight of course aggregate, which represent about 0.60 % by weight of the total bitch. Laboratory aging procedures have been set up to simulate the ageing process through the use of accelerated tests. The two stages of ageing, is the short-term ageing (maximum six hours at 140°C at the loose condition to simulate the production to laying period) and long-term ageing (24 hrs, 72 hrs, and 120 hrs) at 85°C, of compacted specimen to simulate mixtures at different durations in service condition. Moisture damage effect through subjected the samples to vacuum water pressure up to 66 cm mercury with in 5 hours has also conducted. It’s found that polypropylene treated composite mixtures significantly improve the mechanical properties. The results demonstrated that the influence of short-term ageing on hardening asphalt is highly significant compared to long-term ageing. The short term and long term ageing evaluations, and resistance of moisture damage for plastiphalt mixtures indicated significant improvement of pavement performance.