Behaviour of cement concrete at high temperature

The paper presents the impact of high temperature on cement concrete. The presented data have been selected both from the author's most recent research and the published literature in order to provide a brief outline of the subject. The effect of a high temperature on concrete covers changes taking place in cement paste, aggregates, as well as the interaction of these two constituents, that result in changes of mechanical and physical characteristics of concrete. This paper presents the effects of a high temperature on selected physical properties of concrete, including colour change, thermal strain, thermal strains under load, and transient thermal strains. In addition, changes to mechanical properties are discussed: stress-strain relationship, compressive strength, and modulus of elasticity. Moreover, the phenomenon of explosive spalling and the main factors that affect its extent are analysed in light of the most recent research. Interest in the behaviour of concrete at a high temperature mainly results from the many cases of fires taking place in buildings, high-rise buildings, tunnels, and drilling platform structures. During a fire, the temperature may reach up to 1100 ◦ C in buildings and even up to 1350 ◦ C in tunnels, lead- ing to severe damage in a concrete structure (1). However, in some special cases, even much lower temperature, may cause explosive destruction of concrete, thus endangering the bear- ing capacity of the concrete element. Nevertheless, concrete is considered a construction material that satisfactorily preserves its properties at high temperature. Owing to concrete's fairly low coefficient of thermal conductivity, the movement of heat through concrete is slow, and thus reinforced steel, which is sensitive to high temperature, is protected for a relatively long period of time. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the ma- terial is progressive, but because this process is slow, signifi- cant temperature gradients are produced between the concrete member's surface and core inducing additional damage to the element. Fundamental issues related to the impact of high temperature on concrete involve identification of the complex changes that take place in concrete while heated. This con- cerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids). The analysis is complicated due to the fact that cement concrete is a composite consisting of two substantially different constituents: cement paste and aggregates. The effects of the various changes taking place in heated concrete are the alterations of its physical, thermal, and mechanical properties. Research has demonstrated (1, 2), that changes in the strength of concrete as a function of tem- perature are related to, inter alia, concrete composition the type of aggregate used, the water/cement ratio, the presence of pozzolana additives, etc. Important factors are also the rate of heating and the time of concrete exposure to high tempera- ture. The increase in temperature results in water evaporation, C-S-H gel dehydration, calcium hydroxide and calcium alu- minates decomposition, etc. Along with the increase in tem- perature, changes in the aggregate take place. Due to those changes, concrete strength and modulus of elasticity gradu- ally decreases, and when the temperature exceeds ca. 300 ◦ C, the decline in strength becomes more rapid. When the 500 ◦ C threshold is passed, the compressive strength of concrete usu- ally drops by 50% to 60%, and the concrete is considered fully damaged. The Eurocode method of calculating the load- bearing capacity of reinforced concrete members subjected to a fire is based on this assumption. In the 500 ◦ C isotherm method, sections of concrete surface where the temperature had exceeded 500 ◦ C are omitted from the calculations (3).