Flexural Behaviour Of Reinforced Concrete Members At Transient High Temperatures PDF Download

Structural members experience significant creep deformations in later stages of fire exposure and are susceptible to failure due to temperature induced creep strains. Fire in a concrete structure can burn for several hours, and temperatures in concrete and reinforcing steel can exceed 500 °C. At such temperatures, high levels of creep strains can develop in concrete and steel, especially in reinforced concrete columns. However, temperature induced creep strains are not fully accounted for in evaluating fire resistance of concrete members even through advanced analysis, and there is a lack of data on high-temperature creep strains for specific types of concrete. To overcome current limitations, comprehensive experiments on evolution of transient creep strain are undertaken under various heating and loading regimes. Transient creep tests are conducted in the temperature range of 20 °C to 750 °C on four types of concrete; normal strength concrete, steel fiber reinforced concrete, high strength concrete, and high strength concrete with polypropylene fibers. The test variables include temperature, load level, rate of heating, strength of concrete and presence of fibers. Data from these tests indicate that transient creep strain constitutes a significant portion of the total strain developed during high-temperature exposure. Data also affirm that temperature range and stress level have significant influence on transient creep strain. However, rate of heating and presence of fibers have only a moderate influence on the extent of transient creep in concrete. Presence of steel fibers in normal strength concrete slightly reduce transient creep strain, while the presence of polypropylene fibers in high strength concrete leads to higher transient creep strain. Generated data from experiments is then utilized to propose temperature and stress dependent creep strain relations for concrete. These transient creep strain relations can be implemented in fire resistance evaluation of concrete members. To account for transient creep in undertaking fire resistance analysis of reinforced concrete (RC) columns, a three-dimensional finite element based numerical model is developed in ABAQUS. Temperature-induced creep strains in concrete and reinforcing steel are explicitly accounted for in this advanced analysis. The model also accounts for temperature induced degradation in concrete and reinforcing steel, and material and geometrical nonlinearities. The validity of the model is established by comparing fire response predictions generated from the model with measured response parameters in fire tests on RC columns. Results from the analysis clearly indicate that transient creep strain significantly influences the extent of deformations when the temperatures in concrete exceed 500 °C for stress level of 40% or more, and this in turn influences fire resistance of RC columns. The validated model is applied to assess the influence of transient creep on fire response of RC columns under different conditions, including different fire scenarios, load level, and number of exposed sides in a column. Results from the numerical studies clearly indicate that severe fire exposure induces higher creep strains in RC columns in much shorter duration than exposure to a standard building fire. Moreover, asymmetric thermal gradients resulting from two or three side fire exposure on a column, can increase transient creep effects and, thus, affect fire resistance. The extent of the developed transient creep in concrete columns under various scenarios of fire exposure is highly dependent on the type of concrete. Overall, results from the analysis infer that neglecting transient creep can lead to a lower prediction of deformations and, thus, overestimation of fire resistance in RC columns, particularly when subjected to severe fire exposure scenarios, with higher thermal gradients.