Enhanced Sustainability Concrete Mixtures: Effects of Elevated Temperature Exposure on Changes in Microstructure and Elastic Properties and in Development of Modified Layered-Sectional Analysis for Forensic Investigation

Abstract

Part I of this report discusses the findings of an investigation of the relationships between changes in microstructure and elastic properties of lightweight and conventional, enhanced sustainability concrete (ESC) mixtures resulting from elevated temperature exposure. The shear modulus (G) and dynamic elastic (Young’s) modulus (Ed) was determined from resonant frequency of nominal 25 mm (1 in.) thick by 100 mm (4 in.) diameter disk specimens, tested wet and dry, before and after exposure to 150 C (300 F) and 300 C (570 F). The crack densities () before and after exposure were estimated from wet and dry G values of the disk. A critical finding was that the relationship between initial crack density and changes in crack density were similar regardless of the fly ash content implying that ESC mixtures can be used similarly to conventional mixtures in elements exposed to elevated temperatures. This study confirmed the well-established effects of strength on damaged concrete members due to elevated temperature exposures and found statistically significant differences between changes in crack density of mixtures containing fly ash and those containing slag cement. Part II of this report describes the development of a modified layered-sectional analysis (MLSA) providing the engineer with a tool to assess structural behavior of concrete beams with localized damage, a problem not well suited to classical, closed form solutions. The MLSA framework was then used to examine how concrete materials with enhanced sustainability would perform in service after damage associated with a short intense fire. The elastic (Young’s) modulus of the ESC mixtures was determined in a companion study for undamaged and damaged conditions before and after exposure to 300 C. The elastic properties were incorporated into the MLSA, which predicted satisfactory structural performance of the evaluated ESC beams with localized damage due to fire.