Performance of silica fume slurry treated recycled aggregate concrete reinforced with carbon fibers

Abstract

The utilization of recycled concrete aggregate (RCA) is a sustainable way to produce concrete. However, using recycled aggregate in concrete results in inferior mechanical performance and durability, mainly due to the weak interfacial transition zone (ITZ) and attached mortar. The research aims to overcome the inferior properties of recycled aggregate concrete (RAC) by treating recycled aggregate with silica fume (SF) slurry and carbon fibers (CF) as reinforcement. A total of thirteen mixes were prepared and tested for workability, water absorption, mechanical strength, elastic modulus and performance against acid attack. To optimize the performance of recycled concrete, carbon fiber and silica fume were employed at 0.5% and 10%, respectively, with 25%, 50%, and 100% substitution levels of recycled aggregates. The aggregate was pre-soaked in SF slurry for 24 h before mixing in concrete to ensure its absorption or adherence to aggregate. Compressive, split tensile, and flexure strengths of recycled aggregate concrete were evaluated at 7, 28, and 56 days. Elastic modulus was examined under axial compression at 28 days of curing age. Experimental results showed that SF improves the microstructure of cement paste and the bonding between binder matrix and carbon fibers. Compressive, split tensile and flexural strength results of 45.2 MPa, 5.4 MPa and 7.8 MPa, respectively were observed by combined utilization of carbon fiber and silica fume at 28-day curing age in 100% recycled concrete i.e., for recycled concrete mix CFSF-RC100, which is 20%, 34%, and 46% more than their corresponding control mix i.e., RC100. Elastic modulus for RC mixes were found to vary between 22 GPa for RC100 mix and 35.6 GPa for CFSF-RC25 mix respectively. Silica fume also contributed to the water absorption reduction of up to 14% in recycled concrete. Carbon fiber-reinforced recycled concrete was more acid attack resistant having only 6.3% mass loss after acid attack and hence 40% reduced mass loss was observed compared to that of recycled aggregate concrete. Hence, strong, durable, and environment friendly concrete was produced.