COMPARISON OF CHLORIDE PERMEABILITY OF METAKAOLIN AND FLY ASH CONCRETES AND MORTARS UNDER ELEVATED TEMPERATURES Abid Nadeem, Hong Kong College of Technology, Hong Kong Johnny Y. N. Mok, Hong Kong College of Technology, Hong Kong Salman Azhar, Auburn University, AL, USA Brian H. Y. Leung, Hong Kong College of Technology, Hong Kong Gary K. W. Tse, Hong Kong College of Technology, Hong Kong Abstract This paper investigates the chloride permeability performance of high strength concrete and mortar specimens containing varying proportions of metakaolin (MK) and fly ash (FA) as partial replacement of ordinary Portland cement (PC). A total of seven concrete and three mortar mixes were tested after exposing each mix to 200, 400, 600 and 800°C. In concretes, the dosage levels of MK were 5, 10 and 20% and for FA were 20, 40 and 60%. In mortars, the dosage level of MK and PFA was 20%. For unheated specimens, the chloride permeability values were extremely low. At 200°C, the chloride permeability was higher than that at 27°C but it was still in the acceptable limits. After the exposure of 400°C, most of the values for chloride permeability indicated moderately permeable condition in concrete. However, after the exposure of 600 and 800°C, all the concrete specimens were highly permeable. All MK concrete specimens had lower chloride permeability than PC concrete for temperatures up to 200°C , whereas FA concrete specimens had lower chloride permeability than PC concrete at all temperatures. At normal temperatures, the chloride permeability of mortar was greater than concrete. The ratio of mortar to concrete chloride permeability decreased with the increase in temperature and at exposures of 600°C or above, the chloride permeability of concrete was slightly greater than mortar. This is an indication of damage mechanism in the interfacial transition zone and hardened cement paste in concrete microstructure due to exposure of elevated temperatures and has been discussed in the paper. Keywords: High Strength Concrete, High Temperatures, Metakaolin, Fly Ash, Interfacial Transition Zone. 1. Introduction Chloride permeability is one of the measures of the diffusion properties of concrete. Its test which first developed in 1981 by Whiting [1] is widely applied in research and practice to describe the performance of concrete in relation to its permeability which is one of the various durability characteristics of concrete. The widespread use of chloride permeability test is due to its relative simplicity and shorter duration to conduct the test as compared with other durability tests on concrete [2]. Besides, the chloride permeability tests have been standardized by testing organizations such as AASHTO [3] and ASTM [4]. The method has been critically reviewed. Its criticism focuses on two problems: “(1) the theoretical tenets and intrinsic effects, including the temperature rise during the test; and (2) interpretation of results for concrete systems containing various chemical and mineral admixtures” [2]. The results of chloride permeability are not an absolute measure of the permeability “REVISED TEXT”