Assessing Resistance to Top-down Cracking in Flexible Pavements: An Innovative Experimental Approach

Abstract: The layered elastic theory is commonly used to analyse and design flexible pavements, considering parameters like the resilient modulus, Poisson’s ratio, and layer thickness to determine critical strains. This is a conventional way to analyse dense graded asphalt mixtures under uniform loading conditions. However, this method may not accurately simulate real-field conditions. This is because, in a real-field scenario, the surface strains are non-uniform due to the presence of tyre treads and the additional traction forces imparted by heavy loads during acceleration and braking, especially at elevated surface temperatures. Neglecting these critical factors in pavement design often leads to premature failures, such as top-down cracking, which initiates at the surface.
This study compares the performance of two asphalt mixtures, a dense-graded asphalt concrete mix (DG) and a gap-graded stone matrix asphalt (SMA) mix, both with a nominal maximum aggregate size (NMAS) of 13.2 mm, prepared using the SBS-modified binder PG 76E-10. The mixtures were tested for indirect tensile strength (ITS), cracking tolerance index (CTindex), and resilient modulus (MR). To address the limitations of the layered elastic analysis and to experimentally demonstrate the variability in tensile strains on DG and SMA, an innovative experimental test setup was developed to replicate real-field tire-pavement interactions. The test setup consisted of a box with dimensions 1m x 1m x 1m, in which flexible pavement and granular layers were placed to simulate field conditions. Advanced instrumentation tools were also installed to measure the critical strains at the surface and the bottom of the asphalt layer.
Though the DG mixture exhibited a higher resilient modulus, the SMA mixture demonstrated a superior ability to reduce the surface tensile strains by up to 50%. Furthermore, tensile strains at the bottom of the asphalt layer were 30-40% lower for pavement with SMA as the surface layer in the experimental box setup. These findings from the experimental box setup suggest that SMA mixtures offer improved resistance to top-down cracking under varying temperatures and loading conditions compared to conventional dense-graded mixtures.