Non-destructive evaluation of residual compressive strength of post-heated reinforced concrete columns

An experimental study was carried out to evaluate the residual compressive strength of post-heated reinforced concrete columns using techniques applicable in the field. A total of thirty-five reinforced concrete columns (along with three standard cubes for each column) were tested. Ultrasonic pulse velocity tests were carried out on un-damaged and heat damaged reinforced concrete columns and on the related un-damaged and heat damaged concrete cubes. The reinforced concrete columns (fourteen square and thirteen circular) along with three cubes for each column were heated to a uniform temperature of 500 °C at a rate of 150 °C/h. In addition, fifteen undamaged and fifteen heat damaged extra cubes cast from the same batches of concrete (used for casting the columns) were also tested in order to develop a relationship between pulse velocity tests and the residual compressive strength when the cubes were heated over a temperature range. The fifteen heat-damaged cubes were heated with the same heating rate as that used for the columns to uniform temperatures of 200 °C, 300 °C, 450 °C, 500 °C or 550 °C. It was shown that the residual compressive strength of concrete on the 7th day of cooling at ambient temperature after exposing to 200 °C, 300 °C, 450 °C, 500 °C and 550 °C was found to be 80%, 76%, 60%, 47% and 30% of the original un-heated strength respectively. A general relationship was developed between the ultrasonic pulse velocity and the residual compressive strength of cubes over a temperature range of 20 °C–550 °C in order to relate the compressive strength of cubes directly to the in-situ compressive strength of concrete in the reinforced concrete post-heated columns. Additionally, the theoretical and experimental tested values of the axial compressive strength of un-damaged and heat damaged columns were compared in order to evaluate the residual compressive strength of columns. It was found that the theoretical and tested values agreed well for un-damaged (20 °C) and heat damaged (500 °C) square and circular columns.