PSI - Issue 14

D. Anupama Krishna et al. / Procedia Structural Integrity 14 (2019) 384–394 A. Krishna et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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5. Results and discussion Compressive strength : The variation of compressive strength of concrete with elevated temperature is illustrated in Figure 2.The results of the present study shows a good match with the provisions in Eurocode [EN 1992-1-2 2004], ASCE 1992 and the available literatures [Chang et al. (2006), Kodur et.al (2004), Lie et al. (1986)]. The figure shows a large but uniform variation of compressive strength throughout a temperature range of 20 o C to 500 ∘ C among the different models. However, a wider variation is observed in the temperature range above 500 ∘ C.This is mainly because of the variations from different tests using different heating or loading rates, specimen size and curing , testing conditions (moisture content and age of specimen), and the use of admixtures. In ASCE 1992 model the compressive strength of concrete is almost equal to zero at a temperature of 874 0 C. The model predicted by Chang et al.(2006) and Lie et al. (1986)also traces the same path. The strength in the predicted model from the present study also approaches to zero at a temperature of 800 o C. This means that the concrete losses all its compressive strength at a temperature of near to 800 o C. Hence the developed model closely agrees with the literature. Thus the model can be recommended to be included in the Indian Standards for Reinforced Concrete Structures IS 456-2000. Tensile strength : The residual tensile strength of the specimens in the present experimental study have been compared with the published literature [Chang et.al.(2006), Bazant and Chern (1987), Terro (1998)] in Figure 3. The developed residual tensile strength model and the experimental results of Chang et al. (2006), Bazant and Chern (1987) and Terro (1998) for concrete from 20°C to 800°C temperatures is in good agreement with each other. In the experimental model developed in this study the rate of decrease of tensile strength is greater when the temperature is above 400 o C. At a temperature close to 800 o C concrete loses all its tensile strength. Stress Strain response: Generally, the slope of stress-strain curve decreases with increasing temperature because of a decrease in compressive strength and increase in ductility of concrete. The strength of concrete has a significant influence on stress-strain response both at room and elevated temperatures. With rise in temperature the strain corresponding to peak stress increases, especially above 500 ∘ C.This increase is significant and the strain at peak stress reaches four times the strain at room temperature as shown in Figure 4.The initial inelastic behavior is followed by a plastic hardening curve up to ultimate stress, after which a decaying zone represents the post-crushing behavior for concrete. This relationship has the advantage of allowing the definition of a stress level for large plastic deformations, usually reached during fire conditions. Modulus of Elasticity : It can be seen from the Figure5 that the trend of loss of elastic modulus of concrete with temperature is similar for the various models. The degradation of the modulus may be attributed to excessive thermal stresses and physical and chemical changes in concrete microstructure. The variation of modulus of elasticity of the present experimental study has been compared with published literature[ Chang et al. (2006), BS 8110 (1985), Xiao and Konig (2004), Li et al. (2005)]. It can be seen that the developed model is in good agreement with the above literatures for concrete from 20°C to 800°C temperatures. 6. Conclusions In this paper, the constitutive models and relationships for the mechanical properties of concrete subjected to elevated temperatures are developed. They are intended to provide more efficient modeling and also for developing specific fire performance criteria for the actual behavior of concrete structures exposed to high temperatures. The major conclusions derived from the present work are:  The developed model from the experimental studies for compressive strength and tensile strength of concrete at elevated temperatures has a good conformity with the published literature.  The available models for compressive strength at elevated temperatures did not consider the aggregate types.