Experimental Study on Strength Reduction Factor in Concrete Specimen Subjected To High Temperature

Abstract

The concrete loses its strength and stiffness when exposed to high temperatures as they accumulate damage. The primary question remaining after a exposure to high temperature is what level of residual strength and the stiffness exists for material. This residual strength is particularly important for high-rise structures where post-fire decision-making hinges on functionality of the columns. This study specifically addresses the experimental program carried out to assess the residual compressive strength of cube specimens. Three type of concretes viz normal , high strength and fly ash concrete specimens were considered. These specimens are tested in two separate condition states. The first representing air-cooled state involves heating the specimen to the required temperature for prescribed duration and air cooled to the room temperature. Phase two involves heating the specimen to the required temperature for the prescribed duration and immediately cooled to the room temperature by immersing it in the water or called water quenched. The strength and stiffness degradation of concretes for various grades has been studied and compared with the similar studies carried by other researchers. 1 INTRODUCTION In general concrete was good in resisting high temperature. In situations like fire accidents the concrete was exposed to high temperatures above 200° to 300° C, the concrete tends to loose its strength and stiffness depending on the temperature, the exposed duration. The different types of structural concrete have been used in the modern construction viz normal, high strength and fly ash concrete. The behaviour of these concrete when exposed to high temperatures has significance in design to safely withstand the effect of high temperature loads in addition to sudden cooling by spraying water on heated specimens such as quenching. The slender columns of high-rise buildings made of high strength concretes are vulnerable for the progressive collapse of the structure itself due to the effect of high temperature exposure. The primary question remaining after a fire accident scenario is what level of residual load carrying capacity and the stiffness exists for the concrete member. The residual strength is particularly important for high-rise structures where post-fire decision-making situations hinges on the overall functionality of the columns. The recent codes such as IS have made progress towards designing the structure for the possible exposure to high temperatures in the concrete structural elements. Unless complete failure occurs in the structural element, concrete subjected to high temperature has some level of residual strength and stiffness. Quantifying this level of residual strength and stiffness for various grades of concrete has seen significant treatment in current research. Chakrabarti et al [1] and many researchers have looked at the issue of residual strength, defined it as the level at which a concrete structure fails to carry its prescribed dead load. Studies during heating along with the loads were carried out by Carlos Castillo and AJ Durrani [2], as well as several other types of transient load strategies. Gowripalan et al [3], Phan et al [4] has carried out experiments on transient and residual strengths of high performance concrete subjected to high temperatures and recommended the modified reduction factors. Phan et al [4] studied the residual strength and deformation characteristics of high performance concrete specimens. Kumar et al [5] conducted an experimental and analytical program to investigate the residual capacity. The experimental results showed greater strength and stiffness degradation after exposed to high temperature. This study focus on the residual strength and stiffness degradation of three types of concretes made of Indian cements and fly ash. The specimens were exposed to high temperature and tested for the residual compressive strength. In this paper, an experimental work carried out on the three types of concretes exposed to high temperatures in addition to quenched specimens was presented. EXPERIMENTAL DETAILS Materials and Mixture proportions: Three types of concrete mixes representing ordinary, normal, and fly ash concretes are considered for the study. Crushed granite (specific gravity of 2.65) having the maximum aggregate size of 20mm and river sand (specific gravity 2.62)were used as coarse aggregate and fine aggregate respectively. In this investigation for all the mixes ordinary Portland cement of grade 53 confirming to IS12269 (1987), was used. The details of the mix proportion are shown in Table.1.