ABSTRACT
A parameter termed ‘Normalised Modulus’ (NM) is used as the basis for developing the ASTM: D7460 (2010) standard for determining the fatigue of compacted asphalt concrete subjected to repeated flexural bending. The parameter (NM) is calculated and the cycle corresponding to the peak of NM curve is referred as the fatigue life of the material. However, it is observed that the material response need not always follow the assumptions related to NM.
In the present investigation, four point beam bending tests were conducted on asphalt mixtures using unmodified and modified binder. The control binder and modified binder used for the study were Viscosity Grade 30 (VG30) and Polymer Modified Binder 40 (Elastomer) as per Indian standards. Experiments were carried out on beam samples of size 380 × 63 × 50 mm fabricated with 4 ± 0.5% air voids at 10 Hz frequency for five strain levels (200, 400, 500, 600 and 800 micro-strain) at 20 and 0°C. While for some testing conditions, a clear peak value was seen when the data corresponding to normalised modulus was analysed, for different strains and materials, such peak was not seen. In fact, four different types of NM curve trends were observed and each such trend was quantified using the evolution of the stress-strain-time curve.
