Fire Safety Journal 41 (2006) 155–163 The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete Metin Husem à Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey Received 29 April 2004; received in revised form 10 October 2005; accepted 5 December 2005 Abstract The variation of compressive and flexural strengths of ordinary and high-performance micro-concrete at high temperatures was examined. Compressive and flexural strengths of ordinary and high-performance micro-concrete which were exposed to high temperatures (200, 400, 600, 800 and 1000 1C) and cooled differently (in air and water) were obtained. Compressive and flexural strengths of these concrete samples were compared with each other and then compared with the samples which had not been heated. On the other hand, strength loss curves of these concrete samples were compared with the strength loss curves given in the codes. Experimental results indicate that concrete strength decreases with increasing temperature, and the decrease in the strength of ordinary concrete is more than that in high-performance concrete. The type of cooling affects the residual compressive and flexural strength, the effect being more pronounced as the temperature increases. Strength loss curves obtained from this study agree with strength loss curves given in the Finnish Code. r 2006 Elsevier Ltd. All rights reserved. Keywords: High-performance concrete; Ordinary concrete; High temperature; Compressive strength; Flexural strength; Loss of strength 1. Introduction Concrete has been defined as a composite material obtained using cement, aggregate, water and when necessary chemical and/or mineral additives, placed into moulds of various sizes and shapes and hardened under convenient conditions [1–3]. Today concrete has been used with an increased strength and durability in connection with the developments in technology in pre-stressed concrete, concrete and reinforced concrete structures. Pre-stressed concrete, concrete and reinforced concrete structures are sometimes exposed to fires and many structures become damaged and/or out of use [4]. As it is known, high temperatures caused as a result of fire decreases the concrete strength and durability of such structures. Fire resistance of concrete is affected by factors like the type of aggregate and cement used in its composition, the temperature and duration of the fire, sizes of structure members, and moisture content of concrete [5–7]. Fire resistance of the aggregates is generally high. However, having non-uniform high temperature effect of aggregate or cooling the heated aggregate using water spray may cause internal pressure in the aggregates. This pressure may make the aggregate spall. Some of the deformation in the concrete is due to the expansion of cement in its composition. Hydrated Portland cement contains a significant amount of free calcium hydroxide and will decompose into calcium oxide due to loss of water at 400–450 1C . If this calcium oxide is wetted after being cooled or is kept in a moist environment, it transforms into calcium hydroxide again. The concrete may crumble as a result of such changes in volume [2,3,8,9]. An increase in the size of structural members increases fire resistance. The effects of high temperature on the mechanical properties of concrete have been investigated since the 1940s [8,10–34]. These studies examined the behavior of cement paste, mortar, concrete samples and reinforced concrete members exposed to high temperature [14]. Results of these studies constituted the technical basis for ARTICLE IN PRESS www.elsevier.com/locate/firesaf 0379-7112/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2005.12.002 à Tel.: +90 462 377 2622; fax: +90 462 377 2606. E-mail address: mhusem@ktu.edu.tr.