Ghazi Jalal Kashesh

Crumb rubber (CR) modification offers a sustainable approach to improving asphalt performance while promoting the beneficial reuse of end-of-life tyres. This study evaluates the structural performance of CR-modified hot mix asphalt (HMA) under service conditions through an integrated mechanistic and machine-learning framework. Laboratory-measured resilient moduli for conventional and 15% CR-modified HMA at 20, 40, and 60 °C were incorporated into a three-layer Odemark–Shell analysis considering 11 asphalt-layer thicknesses and three representative seasonal temperature periods established from an 11-year (2015–2025) climatic record for Karbala, Iraq. Pavement performance was assessed for a 20-year design period under 6 × 10⁶ equivalent single axle loads (ESALs), with fatigue and rutting damage accumulated using Miner’s rule. Eight machine-learning algorithms were additionally evaluated as fast surrogate models for predicting the critical pavement responses. Incorporating CR increased the resilient modulus by 8.5–9.2%, resulting in reductions of 9.7–10.4% in horizontal tensile strain and 1.2–2.8% in vertical compressive strain over the investigated temperature range. The continuous interpolation of the fatigue-damage criterion indicated required asphalt thicknesses of approximately 27.3 cm for conventional HMA and 25.8 cm for CR-modified HMA, corresponding to a reduction of approximately 1.5 cm (5–6%). At the common 27.5 cm design thickness, CR modification reduced the governing fatigue damage index from 0.954 to 0.723, representing a 24.2% reduction in accumulated fatigue damage and increasing the estimated service life from approximately 21.0 to 27.7 years. Gradient Boosting achieved surrogate-model fidelity, with R² values of 0.993 and 0.9998 for the critical strain responses, while asphalt-layer thickness emerged as the dominant performance variable, consistent with the known analytical structure of the governing equations. The use of CR could additionally divert approximately 10.9 tonnes of scrap tyre rubber from the landfill per lane-kilometre. The findings demonstrate that the CR modification can reduce the continuously estimated required asphalt thickness and extend the theoretical pavement service life under the adopted assumptions, although the thickness reduction is not resolved when standard 2.5 cm construction increments are considered. Additionally, CR modification provides measurable sustainability benefits through the reuse of scrap-tyre rubber.

Clayey soils are generally characterized with low strength and high plasticity which may affect the stability of the subgrade in road infrastructure and hence encourage research into sustainable stabilization techniques. The objective of this study was to investigate the possibility of using recycled crumb rubber (CR) mixed with biaxial geogrid reinforcement to enhance the engineering performance and interface shear behaviour of problematic clayey soil. The experiments were performed on biaxial geogrid BX1100, waste crumb rubber, clay subgrade soil, type B subbase granular material and other materials. The subgrade soil of clay has been collected from the airport area of Al-Muthanna region, Baghdad. An extensive programme of laboratory tests was conducted on soil mixtures with 5%, 10% and 15% of crumb rubber (CR) and untreated soil to determine the effect of stabilization with crumb rubber. The protocol consisted of Atterberg limits, modified Proctor compaction, California Bearing Ratio (CBR) and large-scale direct shear testing. The results showed that the engineering properties of the clay soil were improved by using CR. Maximum improvement was observed at 15% CR content where CBR increased by 56.6% and plasticity index decreased by 44% over the untreated soil. In addition, the large-scale direct shear tests showed that the interface shear strength increased with increasing CR content under geogrid reinforcement. The calculated interaction coefficients were greater than unity for all the tested mixtures indicating effective bonding and interlocking between the reinforced soil layers. The results indicate that the synergistic effect of CR and geogrid reinforcement could improve the interface behaviour of the weak clay subgrade soils with sustainable reuse of waste tyre rubber.