Investigating behaviour of concrete at elevated temperatures

Abstract

Concrete is one of the most widely used construction materials in the world and it provides superior fire resistance in comparison to other construction materials such as timber and steel. However, the outer walls of high-rise concrete buildings are often covered by foam insulation material and rooms are full of other flammable materials; these have resulted in damage and destruction of concrete structures due to fires in the past. Concrete behaviour at elevated temperatures changes drastically from that at ambient temperature and comprises of several complex thermomechanical responses. The primary aim of this study is to understand how concrete behaves under load and temperature. This study includes experimental work and computer simulation. The study evaluates mechanical behaviour of concrete due to heating and loading. Two series of tests are conducted: those in which heated samples are subjected to loading, load holding, unloading and recovery; and those in which loaded samples are subjected to heating, maintenance of constant temperature, unloading and recovery. Different strain components, free thermal strain, instantaneous stress-related strain, time-dependent creep strain and load induced thermal strain are evaluated and analysed. The experimental work uses digital image correlation to evaluate strains. By analysing the photos taken during the experiment, the value of strain is evaluated. The method of post-processing photos is found to be a simple and inexpensive way to evaluate strains at elevated temperatures. This study evaluates the transient temperature distribution under different heating rates and heating time. As the material with a low thermal conductivity, the thermal gradients increase within concrete at larger heating rates. The differential expansion with the thermal gradients can result in damage to concrete. The heat transfer analysis is conducted to find the most efficient and reasonable heating rates for experiments in order to prevent damage due to differential thermal expansion. Also, the heat transfer analysis shows the chamber temperature needs to be maintained constant for 2 hrs to achieve uniform temperature in the concrete for the size of samples considered. This study develops a method to simulate time-dependent creep using viscoelasticity with Prony series; this provides a simple way to model time-dependent creep. The parameters are evaluated through curve fitting in MATLAB. Then, the parameters are used in finite element analysis for defining viscoelastic material in ABAQUS. The simulation results fit the experimental data well. This study examines the components of load induced thermal strain (LITS). As one of the largest strain components at elevated temperatures, LITS is usually treated as plastic strain and irrecoverable during the first heating. The experiments show that LITS is only partly irrecoverable.