Rutting Evaluation of Hot Mix Asphalt Mixtures Using Static Creep and Repeated Load Tests

Abstract

Permanent deformation of hot mix asphalt (HMA) mixtures is a complex phenomenon in which aggregate, asphalt, and aggregate asphalt interface properties govern overall performance. With passage of time, these properties as well as their relative influence changes as mixture starts to fail due to excessive permanent deformation (rutting). This research investigated the rutting susceptibility of various HMA mixtures subjected to static and repeated loading. Eight (08) HMA mixtures (four wearing and four base course) were selected including Superpave, asphalt institute manual series (MS), dense bituminous macadam (DBM) and national highway authority (NHA)‟s class A & B gradations. Optimum bitumen content was determined using Marshall mix method and Superpave gyratory compactor (SGC) was used to fabricate specimens for performance testing. The Flow test (flow time and flow number test) at selected single effective temperature 54.4°C and stress level of 300 KPa was conducted using asphalt mixture performance tester (AMPT). For flow time test, specimen were subjected to static load while haversine repeated load is applied with a waveform of 0.1 second loading time followed by 0.9 seconds rest time for flow number test. Maximum allowed accumulated axial strain is 50,000 microstrains or test shall continue till 10,000 load cycles. The results were used to determine commencement of tertiary flow for selected HMA mixtures. The test results indicated that all mixtures experienced tertiary flow in flow number test whereas no mixtures reached the tertiary flow state in flow time test. Data smoothening technique (five point moving average method) is employed for removing the resonance in raw data obtained from AMPT software. Performance of mixtures were compared using flow number values, cycle number at which the 50,000 microstrains occurred in specimen and intercept obtained from regression analysis of total permanent strain. Observed accumulated axial strains at the time of termination were used for comparison of mixtures as tertiary phase is not exhibited in flow time tests. The analysis indicated that NHA-A among wearing course mixtures and DBM in base course mixtures found comparatively better resistance to rutting among tested mixtures. The findings of this study would facilitate the pavement engineers/ practitioners in selection of rut resistant wearing and base course mixtures. The results of this study (static creep and repeated load tests) can be correlated with other rutting evaluation tests (Asphalt pavement analyzer & Hamburg wheel tracker) to better characterize the rutting propensity of HMA mixtures in selection of appropriate material for hot climatic conditions of Pakistan.