Design and performance of porous asphalt mixture for predestrian/cyclist applications in Singapore
Date of Issue2016-05-11
School of Civil and Environmental Engineering
Porous asphalt mixture (PAM) is a particular type of asphalt mixture with high content of air voids (usually ≥ 18%), which is attributed to the open-graded design, namely coarse aggregate dominates the aggregate gradation while fine fraction is strictly limited. The benefits of PAM include driving safety (e.g. aquaplaning alleviation, splash and spray mitigation, glare reduction and skid resistance enhancement) and environmental friendliness (e.g. cooling effect and noise reduction), while the two major drawbacks are ravelling and clogging. With the rapid development in non-motorised traffic facility, it is appropriate to apply PAM in Singapore, a tropical country with frequent thunderstorms during monsoon periods, for low-strength pavement application, such as walking and cycling pathways. The main objective of the research is to design appropriate PAM for specific application, namely low-strength pavement in Singapore. Firstly, the impact of three critical design factors, namely aggregate gradation (G), gyration compaction level (GCL), and asphalt binder type (ABT), on PAM’s properties is investigated. The evaluation shows that all three factors can significantly influence PAM’s air voids content, which is the most important volumetric parameter directly related to PAM’s permeability and mixture strength. Generally desired air voids content is achieved by a proper open-graded design, while modified asphalt binder and sufficient compaction during preparation are needed to achieve adequate strength and durability. As the packing structure created by the coarse aggregates is the major contributor of PAM’s mixture strength, packing mechanism is analysed via both laboratory measurement and Discrete Element Method (DEM) simulation. Coarse aggregates within different size ranges are divided into three components: main-coarse (19.0-6.3 mm aggregates), interceptor (6.3-4.75 mm aggregates), and quasi-fine (4.75-2.36 mm aggregates). Subsequently, 7% fine fraction (namely aggregates passing 2.36 mm sieve) is selected in the open-graded design that meets the required permeability in Singapore, namely 130 ×10-3 cm/s, based on the gradation-permeability relationship. Four potential PAMs are designed with different aggregate gradations: (a) G_cont, the overall coarse fraction is generally continuously distributed, (b) G_coarse, main-coarse aggregates are dominant in the coarse fraction, (c) G_inter, interceptor aggregates are dominant in the coarse fraction, and (d) G_fine, quasi-fine aggregates are dominant in the coarse fraction. Test results show that all the four PAM designs can meet the required mixture strength for low-strength pavement (namely possessing Marshall stability no lower than 4.0 kN), while only G_coarse and G_inter can achieve adequate permeability. Packing condition in the designed PAMs is subsequently evaluated via two kinds of PFC3D simulation models, a model of compacted coarse aggregate blend and a model of compacted PAM specimen. It is found that stable contact as well as chain contact is gradually developed with assistance of finer fraction in the coarse fraction, and large forces carried by the large-size particles (e.g. main-coarse and interceptor components) are partially transferred by the support of finer fraction (e.g. quasi-fine component). A simulation parameter, porosity_CP ratio is proposed, which is defined as the ratio of porosity within the coarse particles in the PFC3D model of compacted coarse aggregate blend and that in the PFC3D model of compacted PAM specimen. It turns out that porosity_CP ratio is an effective and useful parameter to assess PAM’s packing degree. Based on modified experiment methods for ravelling resistance and clogging resistance in three testing scenarios (namely unconditioned, ageing-conditioned, and moisture-conditioned), G_inter has the better performance generally. On the whole, open-graded design with a low content of fine fraction (e.g. 7%) and a high content of interceptor aggregates in the coarse fraction is suggested in the PAM design for the low-strength pavement application, while modified asphalt binder and sufficient compaction application are needed in order to achieve adequate performance.