Fire Theory Concrete
Fire Theory Concrete - Concrete properties change continuously during and after fire - Decrease in compressive strength relatively smaller with calcareous or lightweight aggregate compared to siliceous aggregate - Modulus of elasticity influenced in the same way as compressive strength and is influenced by the same factors (% reduction in Ec is larger than that for F'c) - Tensile strength decreases at a faster rate than the compressive strength - Thermal properties change with respect to increasing temperature. Thermal conductivity is reduced while the specific heat increases.
Steel - Yield strength decreases with increase in temperature. - Modulus of elasticity decreases at a faster rate than yield strength - Thermal properties change by increase in temperature = Decrease in thermal conductivity; slight increase in specific heat. NOTE not relevant to concrete structures as the amount of steel is so low it can hardly influence the temperature distribution.
Both - The geometrical sizes of both concrete and steel are modified by temperature changes. Thermal expansion is not a linear increasing function of temperature. It can reach approximately 1% at very high temperatures in the range on 800°C. This is quite important in the behaviour of concrete structures because it induces: - very large displacements that may generate second order effects, or - indirect effects of actions if the expansion is restrained - Bond at steel-concrete interface decreases with an increase in temperature - Similar to the reduction in concrete tensile strength
- rare in concrete structures, however more critical in pre-stressed structural elements
Spalling - Related to high temperatures - shedding of exterior layers (exposes inner parts and reinforcing bars to elevated temperatures) - can result in explosive spalling at elevated temperatures (sudden destruction of local cover) - Influencing factors: rapid temp increase, high moisture content, high compressive stress levels, young age of concrete, low porosity, small (thin) member sizes, geometrical effects (corner spalling) Notes on Fire Design - Need to achieve satisfactory performance during exposure to fire. (can be minimized by compartmentalization - Fire risk and exposure are difficult to evaluate - Entire structure behaviour must be considered (parts of structure not exposed to fire may get loaded critically) - Direct and indirect effects need to be considered. (EX: heat introduced by convection and radiation) - Actions resulting from restrained thermal expansion (compression in beams) - Thermal gradient in slabs result in large increase in curvature and deflection (results in tensile membrane effect) can be beneficial - Restraint to thermal expansion modifies the bending moment diagrams in continuous elements, generally increasing negative bending moment. Calculation Method Available (way beyond scope of the course) See pg 1-17 (concrete handbook) for start of fire requirements
Steel - Yield strength decreases with increase in temperature. - Modulus of elasticity decreases at a faster rate than yield strength - Thermal properties change by increase in temperature = Decrease in thermal conductivity; slight increase in specific heat. NOTE not relevant to concrete structures as the amount of steel is so low it can hardly influence the temperature distribution.
Both - The geometrical sizes of both concrete and steel are modified by temperature changes. Thermal expansion is not a linear increasing function of temperature. It can reach approximately 1% at very high temperatures in the range on 800°C. This is quite important in the behaviour of concrete structures because it induces: - very large displacements that may generate second order effects, or - indirect effects of actions if the expansion is restrained - Bond at steel-concrete interface decreases with an increase in temperature - Similar to the reduction in concrete tensile strength
- rare in concrete structures, however more critical in pre-stressed structural elements
Spalling - Related to high temperatures - shedding of exterior layers (exposes inner parts and reinforcing bars to elevated temperatures) - can result in explosive spalling at elevated temperatures (sudden destruction of local cover) - Influencing factors: rapid temp increase, high moisture content, high compressive stress levels, young age of concrete, low porosity, small (thin) member sizes, geometrical effects (corner spalling) Notes on Fire Design - Need to achieve satisfactory performance during exposure to fire. (can be minimized by compartmentalization - Fire risk and exposure are difficult to evaluate - Entire structure behaviour must be considered (parts of structure not exposed to fire may get loaded critically) - Direct and indirect effects need to be considered. (EX: heat introduced by convection and radiation) - Actions resulting from restrained thermal expansion (compression in beams) - Thermal gradient in slabs result in large increase in curvature and deflection (results in tensile membrane effect) can be beneficial - Restraint to thermal expansion modifies the bending moment diagrams in continuous elements, generally increasing negative bending moment. Calculation Method Available (way beyond scope of the course) See pg 1-17 (concrete handbook) for start of fire requirements
