Studies on Performance Enhancement by Recuring of Thermally Deteriorated Concrete and Appraisal of Plaster Compositions as Heat Sheilds

Abstract

Concrete is the most versatile construction material which finds its applications in all the civil engineering structures. The properties of the concrete deteriorate when it is subjected to elevated temperatures. Compressive strength being the most desired property of concrete, it is essential to study the strength retention characteristics of concrete at various elevated temperature levels in order to evaluate the usefulness of concrete. Residual strength of concrete subjected to elevated temperatures depends on the temperature to which it is exposed and the duration for which it is exposed. The first objective is to evaluate quantitative evaluation of effects of exposure duration, temperature and the soaking periods at those designated temperatures. Exposure durations of ½, 1, 1½ 2, 3 and 4hr for elevated temperature levels ranging from 200°C to 800°C at 100°C interval have been considered for the study. Two heating rates (slow and fast) and cooling methods (furnace cooling and water quenching) have been adopted and investigated for their influence on strength characteristics. Concrete residual strength has been found to be affected by the temperature levels especially for the temperatures above 600°C. At each of the temperatures studied concrete retains lower strength for higher exposure duration. Slower heating signifies presence of heat for larger duration resulting into larger deterioration of strength. Cooling of concrete by quenching in water results in higher deterioration of strength owing to thermal shock. Thermally deteriorated concrete when comes in contact with moisture, rehydrates, thereby resulting into partial recovery of strength. The second objective of the study was to study the efficacy of Recuring as a means of strength recovery. To facilitate rehydration thermally deteriorated concrete specimen were subjected to water curing till 56 days and recovery in strength has been noted after 7, 14, 28 and 56 days. Encouraging results were obtained for concrete exposed to temperatures upto 600°C, for higher temperatures however recuring did not result into substantial recovery. The effectiveness of recuring depends on the rehydration capacity of the dehydrated cement paste, which decreases with the increase in exposure temperatures. Partial recovery ofstrength after recuring suggests that if situation permits, recuring can be a potential technique that helps reduce restoration costs. Plastering of concrete elements is an usual practice in order to render a smooth finish and to enhance architectural features. The mortar used for plaster, if made with materials that can resist high temperatures, can protect the structural element. Objective of the research was also to study the effectiveness of mortar as heat shield. Experimental investigations on efficacy of use of vermiculite aggregates in mortar for plastering to enhance fire endurance characteristics have been detailed. The thesis presents strength deterioration of concrete at elevated temperatures with emphasis on exposure levels, duration and rate of heating and cooling. Potential benefits of recuring in strength recovery have been appraised. Efficacy of vermiculite aggregates in mortar for plaster as heat shield has been evaluated.