RESIDUAL COMPRESSIVE BEHAVIOUR OF CONFINED 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 temperatures. 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 earlier studies were performed on the effect of elevated temperatures on unconfined concrete. 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 concrete mix, confinement and heating & cooling regimes. First component of the study attempts to investigate the post-fire residual stressstrain behaviour of unconfined plain and fibrous high strength and normal strength concretes under axial compression. The experimental variables of the study were concrete strength levels, volume fractions of flat crimped steel fibres and polypropylene fibres, inclusion of hybrid fibres and temperature of exposure. A total of 147 cylindrical specimens (150 x 450 mm) were castand tested under this study. The specimens were first exposed to temperatures ranging from room temperature to 800°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 n unconfined concrete. The test results indicate that up to a temperature of exposure of 400°C, both the plain concrete and fibre reinforced concrete did not show any appreciable loss in their initial compressive strength. However, the compressive strength reduced remarkably as the temperature increased beyond 400°C in all the tested specimens. The addition of steel fibres resulted into relatively lower losses in the compressive strength of concrete at high temperatures. The residual behaviour of normal strength concrete was different from that of high strength concrete at high temperatures. Explosive spalling was observed in high strength concrete specimens at 600°C and 800°C temperatures. However, no spalling in high strength concrete specimens was observed when steel or polypropylene fibres were incorporated into the mix. The observed strains in the specimens increased significantly with the increase in temperature of exposure beyond 300°C. Based on the test data obtained, a simple empirical model is proposed to describe the complete residual stress-strain relationships of plain and fibre reinforced concrete after exposure at elevated temperatures. The residual mechanical properties of high yield strength rebars of relatively smaller diameters, typical of transverse confining steel, were investigated after exposure to high temperatures. Accurate prediction of the material properties, particularly of steel bars exposed to high temperature, is necessary for determining the load carrying capacity of structures during and after fire. Therefore, in this context several tensile tests on steel rebars were conducted after exposing them to different temperatures ranging from ambient to 800°C. The test variables included temperature of exposure, yield strength of rebars and diameter of rebars. It was observed that the properties like ultimate strength and yield strength reduced with the increase in temperature while ultimate strain and percentage elongation increased. Influence of temperature on residual properties of reinforcing bars was evident mainly after 500°C. However, residual stiffness of rebars remained unaffected within the range of temperatures considered in this study. Based on the results, a mathematical expression for determining the residual yield strength of reinforcing bars has been proposed. In the next part of the investigations, the effects 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 cylindrical specimens exposed to elevated temperatures ranging from room in temperature to 800°C. The test variables included temperature of exposure, amount of confining reinforcement and yield strength of transverse confining steel. A total of 63 confined cylindrical specimens of size 150 mm x 450 mm were tested in this test program. The effects 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 400°C. However, the peak confined stress falls and the corresponding strain increases in the temperature range of 500 to 800°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. The effect of various heating and cooling regimes on the residual compressive stress-strain behaviour of confined high strength concrete was also investigated. To this end, a total of 57 hoop confined high strength concrete cylindrical specimens of size 150 x 450 mm were tested. The specimens were exposed to seven different temperatures ranging from room temperature to 800°C. Two different heating rates (5°C/minute and 15°C/minute) and two cooling rates (natural air cooling and water quenching) were employed in the study. Measurements were taken for thermal gradient, spalling and residual axial load-displacement properties of confined high strength concrete. Test results indicate that the residual strength, strain corresponding to the peak stress andthe post-peak strains ofconfined high strength concrete are affected only in the temperature range of500 to 800°C. Experimental results show that a faster rate of heating does not have any detrimental effects on the residual behaviour of confined concrete. Lesser thermal induced effects were noticed in specimens exposed to faster rate of heating than in specimens subjected to slower rate of heating irrespective of the temperature of exposure. However, the results suggest that cooling with water quenching has adverse effect on the residual stress-strain properties of confined high strength concrete. Compared with natural cooling, thermal shock induced by water quenching caused more severe damage to confined high strength concrete, in terms of greater losses in confined concrete strength and increase in strains. IV The concept ofusing a combination of suitable randomly distributed discrete fibres with nominal amount of lateral steel has been discussed in the literature to ease the requirement ofhigh amount ofconfinement in the plastic hinge regions ofhigh strength concrete columns. However, the effect of high temperatures such as during fire, on the effectiveness of combined confinement of lateral reinforcement and fibres, remains to be investigated. The objective of this study is to gain the knowledge of the strength and deformability of confined fibre reinforced high strength concrete after exposure to a thermal cycle at high temperature. A total of 105 confined cylindrical specimens of 150 mm in diameter and 450 mm in height were cast and tested under this program. After exposing to the desired elevated temperatures ranging from room temperature to 800°C, the concrete specimens were allowed to cool down naturally in the furnace before testing them under axial compression the next day. The variables considered in this experimental study included different exposure temperatures and volume fraction of steel and polypropylene fibres. The effects of temperature on confined fibre reinforced high strength concrete were studied and quantified with respect to strength and ductility gains. Based on the results, a simple analytical model has been proposed to estimate the residual compressive behaviour ofconfined plain and fibrous high strength concrete.