RESIDUAL STRENGTH OF CONCRETE SUBJECTED TO ELEVATED TEMPERATURE

Abstract

In the present day practice, the reinforced concrete structures are designed to behave in a • ductile manner to resist natural and man-made hazards like earthquake, fire and blast loading. Thus inelastic, deformability of reinforced concrete elements is essential for overall stability 'of structures in order to sustain these hazards. Deformability of reinforced concrete structural components is generally achieved through proper confinement of the core concrete. The behaviour of concrete confined by well-detailed lateral confinement reinforcement is well-documented now, at ambient conditions, although most of the concrete structures are subjected to a range of heat no more severe than that caused by the weather, there are important cases in which structures are exposed to much higher temperature. Examples include building fires and some industrial applications where reinforced concrete structural elements are close to furnaces and reactors. Such fires or elevated temperatures result, in most cases, in considerable damage to structures. Therefore, it becomes important to evaluate the effectiveness of confinement devices in confining core concrete after exposure to elevated temperatures, especially in seismic resistant structures. A critical review of the literature indicates that most of the studies were performed on the effect of elevated temperatures on unconfined concretes. Thus, the main aim of the present research is to examine the complete residual compressive uni-axial behaviour of confined concrete at elevated temperatures by considering the effects of various key parameters of confinement and temperature of exposure. First component of the study attempts to investigate the post-fire residual stress-strain behaviour of unconfined plain normal strength concrete under axial compression. The experimental variable of the study was temperature of exposure. A total of 9 cylindrical specimens (150 x 150 x 450 mm) were cast and tested under this study. The specimens were first exposed to temperatures ranging from room temperature to 750°C and then tested under uni-axial compression after cooling to obtain complete residual stress-strain response. The effects of the chosen variables of the study were studied on the stress-strain curves of the unconfined concrete. The test results indicate that when the concrete is exposed to low temperature ranges, the plain concrete did not show any loss of their initial room temperature compressive strength. However, the loss in residual compressive strength increases as the temperature increases. Based on the test data obtained, a simple empirical model is proposed to describe the complete residual relationships of plain concrete after exposure at elevated temperature. 11 In the next part of the investigations, the effect of elevated temperatures on the concentric compressive behaviour of confined normal strength concrete are presented. An experimental program was designed and carried out involving testing of hoop confined concrete specimens exposed to elevated temperatures ranging from room temperature to 750°C. The test variables included temperature of exposure and amount of confining reinforcement. A total of 24 confined cylindrical specimens of size 150mm x 150mm x 450 mm were tested in this test program. The effect of various key variables of confinement were studied and quantified with respect to strength and ductility gains. The results indicate that the residual strength, strain corresponding to the peak stress and the post-peak strains of confined normal strength concrete are not affected significantly up to an exposure temperature of 500°C. However, the peak confined stress falls and the corresponding strain increases in the temperature range of 500 to 750°C. An increase in the temperature of exposure makes the stress-strain curve of confined concrete flatter. It is shown that an increase in the degree of confinement reinforcement results into an increased residual strength and deformability of confined concrete. An empirical stress-strain model has been proposed to predict the residual compressive stress-strain behaviour of confined concrete. 111