Concrete in Fire Conditions
Concrete structures have a reputation of behaving well in fires (Buchanan, A.H. "Structural Design for Fire Safety"). This is partly due to the low thermal conductivity of concrete which means that the side away from heating takes a long time to heat up and may never be expected to reach high temperatures, this is shown in the graph below.
The increase in temperature has several effects on the behaviour of the concrete, the magnitude of change can vary depending on the type of concrete used also, since previous work focused on normal weight concrete, only the behaviour of normal weight concrete will be discussed here as it will be the concrete type used in the analysis. The general behaviour of normal weight concrete can be described as follows:
Strength and stiffness degrade
The tensile strength of concrete can conservatively be assumed to equal zero as is the case at ambient temperature. For compressive strength, there is little effect on the strength until approximately 350°C, the strength then rapidly decreases to approximately 20% of its ambient value by 600°C and at approximately 1000°C the concrete has lost all of it's strength (Schneider, U. "Concrete at high temperatures - a general review" 1988). The Youngs Modulus follows a similar pattern, with ductility remaining constant until approximately 150°C, the modulus of elasticity then decreases to approximately 10% of its ambient value at 600°C (Buchanan, A.H. "Structural Design for Fire Safety")
The deformation of the concrete becomes more complex
When concrete is heated, there are four strain components responsible for the total strain. These are:
- Thermal Strain - εth(T): The thermal elongation is relatively simple and easy to calculate, there are linear relationships between the strain and temperature for different concrete types, which are available in Eurocode 2.
- Stress-related Strain - εσ(σ,T): This includes both the elastic and plastic components of the stress. As the temperature increases, the strain at peak stress increases as the member becomes more ductile. As mentioned earlier, there is also a drop in the compressive strength. This is shown in the figure below:
- Creep Strain - εcr(σ,T,t) and Transient Strain - εtr(σ,T): These two terms are closely related, when a member is subjected to heating and a stress being applied, the concrete will exhibit signs on creep. Creep is time dependent so the maximum creep strain may never be reached for a real fire as the temperature will decrease before maximum strain has occurred. The magnitude and even the direction of the creep is dependent on the stress, if a small stress is being applied, the strain will increase over time if the temperature is great enough. However, if there is a sufficiently large stress, the concrete can contract causing an overall shrinkage or a negative strain. This phenomena is known as Load Induced Thermal Strains (LITS). The figure below shows the total deformation of concrete for different stresses applied when heated. Load induced thermal strains are non-recoverable, this is due to the transient strain term which occurs during the first heating cycle. Here the concrete undergoes changes in the moisture content and chemical composition of the cement paste which is non reversible hence the strain is permanent. The modelling of these phenomena is made easier by Law, (Law, A. & Gillie, M. "Load Induced Thermal Strain: Implications for Structural Behaviour" 2008) who states that it is only necessary to distinguish between elastic and plastic components of strain for short term heating conditions.
The total strain is a sum of the four terms above giving:
ε = εth(T) + εσ(σ,T) + εcr(σ,T,t) + εtr(σ,T)
In flat slabs, it is assumed that the underside of the slab is subjected to heating whilst the atmosphere at the top side is ambient. This will lead to thermal bowing if unrestrained. However slabs will typically have some restraint built in usually from a continuation of the floor plate. This will prevent the full strain from developing, the work conducted by Thornsteinsson (2011) showed that when the slab was restrained, there was also a large amount of compressive damage in areas where thermal expansion would be high, there was also a large amount of tensile damage around the centre of the column. This damage was caused by thermal stresses, which could in turn lead to effects such as Load Induced Thermal Strain to become more prominent, however there was no inclusion of this into the numerical analysis. The increase in thermal stresses is also noted by Annerel et al. who states that the punching load will increase due to the restraint of the thermal curvature (E. Annerel, et al. "Thermo-mechanical analysis of an underground car park structure exposed to fire", Fire Safety Journal 2012).
Spalling
Typically an explosive process where pieces of concrete are blown off from the slab, this can expose the reinforcement causing a reduction in the bending capacity of the slab. There are several mechanisms associated with spalling including pore pressure increase and different rate of expansion between the aggregate and cement paste. It is still an area of research to accurately quantify the mechanisms responsible for the behaviour, however it can be observed that spalling is more likely to occur in enclosed concrete where expansion is restricted and also for concrete with high strength, density or with a large moisture content.