Cement and fly ash-treated recycled aggregate blends for backfilling trenches in trafficable areas

Abstract

The shortage of natural aggregates available for filling excavated pipeline trenches in trafficable areas has prompted the exploration of alternative resources. This study investigates the feasibility of using cement and fly ash-treated recycled aggregates as trench backfill materials subjected to traffic loadings. Blends of recycled glass (RG), plastic (RP), and tire (RT) were treated with different proportions of cement and fly ash, resulting in a total of 8 treated blends. Geotechnical tests including compaction and California Bearing Ratio (CBR) were conducted to evaluate the mixtures according to backfill specifications. Specialized pavement testing, such as repeated load triaxial testing (RLT) and quick shear, simulated real-life stress levels at trafficable areas. Scanning electron microscope (SEM) images were taken to investigate the microstructural characteristics of the cement and fly ash-treated samples. The results showed that the CBR and resilient modulus of the treated blends improved with higher cement, fly ash, and RG contents, while they decreased with increased RT content. Cement-treated blends demonstrated significant improvements in peak shear strength with increased cement and RG contents and decreased RT content. Fly ash-treated blends showed minor improvement in peak shear strength when the fly ash, RG, and RT contents varied. Only cement-treated blends exhibited properties comparable to Class 4 (CL4) crushed rock, which was the control material. Under the same stress levels, cement-treated blends demonstrated up to 1.17 and 2.62 times greater stiffness than CL4 and clay subgrades, respectively. The SEM analyses confirmed that the inclusion of cement in the recycled blends resulted in the formation of greater bonds between particles compared to fly ash, which led to higher strength. These findings highlight the potential of sustainable materials in backfilling pipeline trenches under traffic loadings, reducing the reliance on natural aggregates for this application.