Uniaxial tensile properties of multi-scale fiber reinforced rubberized concrete after exposure to elevated temperatures

With the rapid development of transportation and population growth, there is a surge hike in demand for automobile tires. Waste rubber disposal has become a big environmental concern worldwide due to the non-biodegradability of waste rubber. Based on the previous literature, waste rubber has great potential to make green concrete. However, the mechanical behavior of concrete, especially the tensile strength after being heated, will be reduced by adding waste crumb rubber. To improve the residual tensile behavior of rubberized concrete, steel fibers, PVA fibers (or PE fibers), and CaCO3 whiskers were added to make green concrete with better fire resistance. This study investigated the influences of the crumb rubber and multi-scale fibers on the failure mode, first cracking strength and strain, ultimate strength and strain, and the stress-strain relationships of MSFRRC subjected to elevated temperatures (up to 800 °C). In addition, the damage evolution of rubberized concrete under uniaxial tension was studied by the acoustic emission (AE) technique. Results show that specimens without fibers exhibited a single straight crack and a brittle failure. Adding crumb rubber could not improve the brittleness of concrete and eliminate explosive spalling. However, the brittleness of rubberized concrete could be improved and a multi-cracking behavior could be achieved by incorporating multi-scale fibers. Meanwhile, the ultimate strength decreased significantly when the crumb rubber volume replacement ratio increased from 0% to 30% since the increase in the porosity of the concrete specimen. The incorporation of multi-scale fibers could mitigate the strength loss and contribute to the first cracking strength, ultimate strength, and strain-hardening capability. The tensile strength of MSFRRC was enhanced by 54.4%–64.3% and the ultimate strain increased by 26.9–50.4 times. Nevertheless, the strain-hardening capability was eliminated owing to the weakened interfacial bond between the steel fiber and the matrix and the decomposition of PVA fibers when the temperature reached 400 °C. In addition, the large and medium capillary pores were reduced due to the additives of multi-scale fibers and the content of mesopores increased. Furthermore, the empirical formulas to predict the residual uniaxial tensile stress-strain relationship of MSFRRC considering parameters of crumb rubber, multi-scale fibers, and elevated temperatures were proposed based on the test results.