A numerical procedure for the thermal analysis of RC slabs at elevated temperatures is the subject of this study, where the moisture increase due to the so-called moisture clog occurring in the cooler parts of reinforced concrete (RC) slabs is introduced as well. Starting from a systematic investigation on the existing concrete constitutive laws available in the literature and in the codes, improved concrete constitutive laws are proposed to describe concrete thermal and mechanical properties at elevated temperatures. The proposed laws – called also models – are validated against well-documented full- and small-scale tests on simply-supported RC slabs. Parametric analyses on the behaviour of RC slabs in fire are carried out as well, to clarify the role that the different constitutive laws may have in the numerical prediction of RC slabs behaviour under fire conditions.

Introduction

In recent years, several full-scale fire tests of reinforced concrete (RC) slabs were conducted to investigate the influence of the tensile membrane action on the behaviour of floor slabs. However, due to the huge costs and complexities associated with the full-scale fire tests, a number of numerical models have been developed by various researchers to predict the fire response of concrete slabs [1-5]. When modelling the behaviours of the slabs in fire, it is very important to accurately and efficiently determine the temperature distribution within the slabs. So far, two main calculation methods have been commonly used to predict the temperature distributions of RC slabs: a combined heat and mass transfer analysis and a heat transfer analysis only. In 1998, Hurst and Ahmed [6] developed and numerically solved a mathematical model to predict heat and mass transfer in concrete structures subjected to fire. The model simulates the changes in pore pressure, temperature and moisture with corresponding changes in concrete properties and has also been able to predict moisture clog. In 2001, Tenchev et al. [7] proposed a comprehensive coupled heat-and-mass transfer model to predict the pore pressures and the moisture migration within concrete at elevated temperatures. In 2003, Gawin et al. [8] developed one mathematical model to predict the pore pressure, moisture transfer and evaporation within concrete at high temperatures. In 2009, Dwaikat and Kodur [9] developed one-dimensional numerical model to predict the fire-induced spalling and pore pressure in concrete structures based on principles of mechanics and thermodynamics. Although these models are capable of predicting moisture concentrations and pore pressures, they are complex and expensive to process. For instance, moisture evaporation or migration is a complex process and depends on many material properties, such as the porosity and permeability of concrete, which are not well known at elevated temperatures. Thus, very often the use of simpler, faster models, which are capable of predicting temperatures with reasonable accuracy, is valid. This is especially true if concrete spalling is unlikely and the moisture can migrate to an unheated surface and escape. As a matter of fact, under physically realistic conditions, if only the temperatures of slabs are required for the structural analysis, a simple approach, in which the effect of moisture is treated explicitly, can be used to predict the temperature distribution of the slabs in fire. For instance, Wickstrom [10] used a modified enthalpy curve applying an average slope to take into account the effect of moisture subjected to evaporation in concrete. Huang et al. [11] proposed a model based on the theoretical heat and mass transfer in concrete. Lamont et al. [12] developed one adaptive heat transfer program HADAPT and considered moisture evaporation by using an enthalpy method.