Models for uniaxial stress-strain relationship and thermal properties of fine recycled aggregate concrete exposed to elevated temperatures

The models for the stress-strain relationship under elevated temperatures and thermal properties of recycled aggregate concrete (RAC) are essential in the fire resistance design of RAC structures, while no such models are available for RAC made with fine recycled aggregate (FRA). To establish the above models, 33 RAC prisms with three FRA replacement levels (0%, 50%, and 100%) under five temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C) were tested for the uniaxial compressive stress-strain relationship; Meanwhile, thermal properties (including the specific heat, thermal conductivity, and thermal expansion) of these concrete mixes under a temperature range of 20–800 °C were also tested. A prediction model for the RAC stress-strain relationship under elevated temperatures was proposed by extending the model for RAC under normal temperature. Based on the test data, models for the thermal properties of RAC with FRA were also established. Results showed that the FRA does not change the shear failure mode of concrete, while decreased the peak stress by up to 33% and elastic modulus by up to 31%; The influence of FRA keeps similar when temperature varies from 20 °C to 800 °C; the specific heat increases by 16% on average while the thermal conductivity decreases by 8%–15% with the FRA replacement ratio increased to 100%; the thermal expansion of concrete under temperature higher than 500 °C decreased by up to 22% when using 100% FRA; the proposed models have good accuracy for the stress-strain relationship and thermal properties of RAC.