Beneficial use of recycled materials in concrete mixtures

The need to incorporate recycled materials in building products is becoming more important than ever before. The use of recycled materials in concrete mixtures creates landfill avoidance and decreases the depletion of virgin raw materials. The basis for this research was to investigate the effects of using recycled materials, in varying amounts, on the fresh and hardened concrete properties. The recycled materials used in this study consisted of ground granulated blast furnace slag (GGBFS), recycled concrete aggregate (RCA) and crushed waste glass. The GGBFS, or slag cement, was used as a replacement for the cement. The RCA and waste glass were used to replace the coarse and fine aggregates, respectively. The concrete mixtures designed ranged from a 25% replacement to one 100% replacement with recycled materials. In addition, a standard concrete mixture using cement and virgin aggregates was designed for comparison purposes. Fresh and hardened concrete properties were examined including slump, air content, unit weight, compressive strength, rate of strength gain, freeze–thaw durability, permeability, and alkali–silica reactivity (ASR) potential. The 100% recycled materials concrete had very low permeability and a compressive strength of 4200 psi (29.0 MPa) with 6.5% air content. Concrete mixtures composed of 50% and 75% recycled materials achieved strengths of nearly 7000 psi (48 MPa) and 6350 psi (43.8 MPa) respectively. Beneficial and negative effects of using recycled materials in concrete mixtures were investigated, including the potential alkali–silica reactivity (ASR) of using waste glass as aggregate. The slag cement, when used at replacement levels of 50%, was found to eliminate these concerns. The use of recycled materials was beneficial with regards to strength and durability up to 50% when compared with a normal concrete made from virgin materials.