Evaluation of thermal bond integrity and residual strength properties in concrete mixes with hybrid fibres and recycled aggregates

Purpose The present investigation uses recycled aggregates (RA), fly ash (FA) and manufactured sand (MS) to examine the bond performance of concrete. Various mixes (M1, M2, M3, M4 and M5) were developed by incorporating hybrid fibres with FA and MS. After being exposed to high temperatures (2000C, 4000C and 6000C) for 2 h durations, the compressive strength, flexural strength (fL) and bond strength were assessed and compared to control concrete (M1). Numerous researchers have investigated the bond strength and residual strength of natural and alternative aggregates. There is an observed gap in the literature on the bond properties and residual concrete strength, including MS and RCA as well as hybrid fibre-based mixes containing MS after being exposed to sustained elevated temperatures. Hence, this research can be a value addition, since it provides a solution for the usage of RCA, FA and MS in the manufacturing of concrete, which reduces the consumption of natural resources and finds a solution for waste disposal. Design/methodology/approach The tests were conducted for different mixes, which include control concrete, concrete with fibres and concrete containing recycled concrete aggregates (RCA). M30 grade concrete with a 0.45 w/c ratio has been considered for the present study, and the natural coarse aggregates (NCA) were replaced with 50 and 100% of RCA. In addition, the behaviour of these mixes was studied at elevated temperatures of 2000C, 4000C and 6,000C for 2 h of sustained temperature in a specially designed electric oven with well-controlled instrumentation. The residual compressive strength, flexural strength, thermal conductivity and pull-out test with parameters, namely mean bond strength, mean slip and mean normalised bond strengths, were evaluated at these temperatures. Findings It was observed that the mixes with blended fibres are found to perform better in terms of actual and residual strengths. Though the recycled aggregate concrete mixes performance is marginally lower than control concrete, the results are well acceptable from the point of practical applications. Originality/value Unlike prior research, which focused primarily on post-fire mechanical strength, this investigation focuses on the bond behaviour of specimens made of RCA, fly-ash and hybrid fibres. It has been found that specimens containing hybrid fibres perform better than other combinations in terms of bond strength.