Recycled Concrete Aggregate Modified with Polyvinyl Alcohol and Fly Ash for Concrete Pavement Applications

Abstract

Recycled concrete aggregate (RCA) is traditionally limited to nonstructural applications. Although the crushing process results in weak mortar particles and surface cracks throughout the RCA, RCA concrete can be employed as rigid pavement materials after its mechanical strength has been improved. This research evaluated the novel use of polyvinyl alcohol (PVA) and fly ash (FA) for the improvement of mechanical strengths of RCA concrete. The influence of PVA to binder (p/b) and FA/cement (FA/c) ratios on the mechanical strength of RCA-PVA-FA concrete was assessed by compressive, split tensile, and flexural strength tests. The mechanisms controlling the improvement of mechanical strengths were discussed based on the results of microstructural analysis using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) methods. The results of this research indicate that the addition of PVA resulted in a reduction of the compressive strength of RCA concrete in both the short and the long term. However, with an optimum p/b of 1%, both the flexural and tensile strengths increased significantly. At the optimal p/b, the PVA films infiltrated into cementitious matrixes and enhanced the reinforcing ability within the matrixes. However, excess p/b retarded the hydration process and caused reductions in the cementitious tensile bonding. The long-term mechanical strength (>28 days) was remarkably improved by FA addition at optimum FA/c of 20:80, and the presence of cementitious products on the FA grains and in the voids was clearly observed. The highest long-term flexural and tensile strengths were found at p/b=1% and FA/c=20∶80, whereby the 28-day flexural strength was higher than the requirement specified by the US Army and the US Air Force. The 28-day compressive strengths were also higher than the requirements specified by Department of Highways, Thailand. The outcome of this research will result in the promotion of using wastes, namely RCA and FA, in rigid pavement application, which is practical and preferred from engineering, economic, and environmental perspectives.