Influence on mechanical properties and microstructure analysis of hybrid fiber recycled aggregate concrete after exposure to elevated temperature

The use of recycled aggregate concrete (RAC) in construction is one of the most effective ways to solve the environmental problems of concrete waste. The purpose of the present study is to improve the high-temperature resistance of RAC by incorporating hybrid cellulose and basalt fibers. The mechanical properties of the specimens with different volume fractions of cellulose and basalt fibers after high temperature were studied. Additionally, in order to study the degradation behavior of the specimens, the microstructure of interface between fibers and the cement matrix of RAC was examined by scanning electron microscope. Results show that hybrid fiber can reduce the explosive spalling and mass loss of RAC after being subjected to high temperatures. Compared with the brittle failure of RAC, the failure mode of hybrid fiber recycled aggregate concrete (HFRAC) was transformed into ductile failure under compression and splitting test. After high temperature, the compressive strength and splitting tensile strength of HFRAC were higher than those of RAC, and the cellulose-basalt fibers displayed an excellent synergistic effect on the mechanical properties of RAC. Among all the RAC specimens with different volume fractions of cellulose and basalt fibers, specimen hybrid with 0.15% cellulose fiber and 0.15% basalt fiber showed the best mechanical properties. Microstructure analysis research shows that small channels can be left in the matrix after the melting of cellulose fiber after elevated temperature, which improves the permeability of concrete and reduces the spalling damage of RAC.