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abstractAlthough concrete is a non-combustible material, it is found that when exposed to high temperatures,uch as fire, the physical, chemical and mechanical properties of concrete can drastically change. Thus,t becomes important to assess the relative properties of concrete under high temperatures in order toevaluate and predict the post-fire response of reinforced concrete (RC) buildings and structures. Thispaper assesses the effects of elevated temperatures and cement dosages on the mechanical propertiesof concrete. Two concrete mix designs were considered in this research in an attempt to study theeffects of cement dosage (250 and 350 kg/m3) on the post-fire response of concrete. Once cast, the testamples were first exposed to elevated temperatures ranging from 100 to 800 1C, and then allowed tocool down slowly to ambient room temperature of 20 1C before being tested to failure. 43191
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Several testswere then carried out to determine the mechanical properties of the cooled concrete specimens. Theest results indicated that at temperature above 400 1C, concrete undergoes significant strength losswhen compared to the strength of non-heated concrete. In addition this strength reduction was foundo be unaffected by the cement dosages. The experimental results were also compared with currentEuropean standard (BS EN 1992-1-2:2004 standard) strength equations and American Concretenstitute standard (ACI 216.1).2012 Elsevier Ltd. All rights reserved. 1. IntroductionExposure to high temperature during fire is one of the mostdamaging environmental effects for reinforced concrete struc-tures. Although concrete is a non-combustible material, itschemical, physical and mechanical properties change onceexposed to high temperature [1]. In fact, the effects of hightemperature on the mechanical response of concrete have beeninvestigated since the middle of the twentieth century [2–5] andis still undergoing up to today. The research found in the technicalliterature can be classified into two categories: materials testingand element testing. The results of materials testing provideinformation on the effects of temperature on the mechanicalproperties of concrete (such as concrete compressive and tensilestrength, modulus of elasticity, etc.), while the results fromelement testing is used to assess the fire resistance ofreinforced-concrete structural elements (such as beam, columns,slabs). The results of these studies provided a large pool of datafor development of rules, provisions and guidelines for fire design of concrete structures. For example, methods incorporated in ACI216.1 [6] are based on fire research performed from 1958 through2005 and are by far the most commonly used in typical designsituations in the North American continent.When subjected to heat, concrete responds not just in instan-taneous physical changes, such as expansion or cracking, but byundergoing various chemical changes [7]. Because the hydratedcement paste amounts to only 24 to 43% in volume [8] of theconstituent-materials comprising the concrete mix, the propertiesof concrete with temperature are found to vary widely with thetypes of aggregates used. Thus predicting the post-fire response ofconcrete is a complex phenomenon since the constituent-materials react to high temperature in a variety of ways. Theeffects of additives, such as silica fume and fly ash, on themechanical response of concrete at elevated temperatures wasalso assessed [9–11] and was found to enhance the concreteproperties when compared to the normal concrete not containingthese additives.It is found [7] that the loss of strengths of concrete with hightemperature can be attributed to the thermal incompatibilitybetween the cement paste and the aggregates, the building ofinternal-pressure due to the evaporation of water during theheating process, as well as the chemical changes in the cement paste and aggregates. Experimental studies have also shown thatwhen subjected to elevated temperature, there is a tendency forthe hot surface layers to separate and spall from the coolerinterior.