Assessment of post-fire residual strength of reinforced concrete using ultrasonic pulse velocity

High temperature may cause the degradation of concrete strength and subsequent loss of steel-concrete bond. Based on the results of a previous study on the correlation of pulse velocity and residual strength, this research aims to examine further the feasibility of nondestructive testing toward in-situ assessment of concrete structures after fire exposure or under constant exposure to high temperature. Ultrasonic pulse velocities of cylindrical concrete specimens are measured both before and after simulated fire exposure. Residual compressive strengths are obtained from compressive tests. The effects of exposure time, up to two hours, and maximum exposure temperature, up to 800 degree Celsius, are both analyzed. It is confirmed that residual compressive strength of standard 12 by 24 centimeter concrete specimens decreases as exposure time is increased. Regression analyses show strong linear relationship exists between pulse velocity and residual strength over the exposure time ranging from 30 to 120 minutes and the maximum exposure temperature ranging from 500 to 700 degree Celsius. The captured waveforms are also processed using continuous wavelet transform that provides simultaneously frequency and time information. Signal features can thus be identified. The findings can be used as a basis for assessing the post-fire residual concrete strength.