PSI - Issue 14

D. Anupama Krishna et al. / Procedia Structural Integrity 14 (2019) 384–394 A. Krishna et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Fire temperature All the experiments were carried out using an electric heating furnace of 42kWh with a maximum temperature of 1200 o C in the Structural Engineering Laboratory at College of Engineering Trivandrum, India. The fire curve followed the Chinese National Standards (CNS, 2005) which is same as the curve from International Organization for Standardization (ISO, 1999). The temperature change is expressed as in Equation (1). After two hours the temperature reaches 1049 o C, and after 4hrs the temperature reaches 1153 o C as shown in Fig.1.

345log10 (8 1) 20    t T

(1)

Fig.1. Fire curve from CNS (2005) and ISO (1999).

4. Mechanical properties of concrete influencing fire resistance

The parameters that control the concrete behaviour like compressive and tensile strengths, modulus of elasticity, creep strain, thermal conductivity and thermal strain are non-linear functions of temperature [Diederichs et al. (1987)]. The mechanical properties that influence the fire performance of RC members are compressive and tensile strength, modulus of elasticity, and strain response of constituent materials at elevated temperatures. Many constitutive models for compressive and tensile strength for concrete at normal temperature are available in literature. The constitutive law for concrete material under fire condition are complicated and the current knowledge of thermal properties is based on limited material test data. There are either limited test data for some high temperature properties, or there are considerable variations and discrepancies in the high-temperature test data for other properties of concrete [Phan et al. (1998, 2003)]. These variations and discrepancies are mainly due to the differences in test methods, condition of procedures, and the environmental parameters accompanying the tests [Flynn (1999)]. Thus, at present, there are no reliable constitutive relationships in codes and standards for many of the high temperature properties of concrete [Bastami and Aslani (2010)].

4.1. Compressive strength

Compressive strength of concrete at an elevated temperature is of primary interest in fire resistance design. At ambient temperature it depends upon water-cement ratio, aggregate-paste interface transition zone, curing conditions, aggregate type and size, admixture types, and the type of stress [Mehta and Monteiro (2006)]. At high temperature, compressive strength is highly influenced by room temperature strength, rate of heating, and binders in batch mix (such as silica fume, fly ash, and slag) [Kodur (2014)]. The most important models of the compressive strength of concrete at high temperature are summarized in Table 3. The variation of normalized compressive strength with temperature for various models and the present study is plotted in Figure 2.