Fire effect of the behaviour of reinforced concrete with variable strengths

Abstract

Fire accidents are inescapable. Damages on the structure varies depending principally upon duration and temperature of burn, property of the construction materials, quantity and type of substances subjected to burn, ventilation system of the structure, location and direct exposure of the structural member to the fire, etc. Repair or removal of the burnt structure challenges either the life or the economy. This research has been conducted to analyze the effects of clear cover depth against fire, the ability of the structure to reserve its strength and to generate a technique of evaluating burnt structure thereby to devise a mechanism to assist improved construction technology. A total of 120 pieces reinforced concrete beams with twelve different compositions have been experimented under ten different conditions. The compressive strength, clear cover and the strength of reinforcing steel have been the prime variables for difference in compositions. The testing conditions varied in burning temperatures and durations apart from the ambient one basing on different realistic fire accidents. A full scale natural gas furnace has been constructed in this research having integral gas line. Both element and material testing have been conducted in this research. After completion of the fire test all the beams were checked for different physical properties and the data have been recorded and maintained. It is important to note that no siliceous materials have been used for preparing high strength concrete. The results obtained from the experiments have been evaluated under four analytical studies. These analyses finally explained valuable opinions concerning the comparative spalling effect, reserve strength and their typical behavior trends between high strength and normal strength concrete. The high strength concrete displayed better performances as regards to spalling effect and the reserve strength than the normal strength concrete. Though thicker clear cover provides better protection to the reinforcement, on the contrary, it has adverse effect against fire and causes severe damage in terms of spalling and loss of the residual strength. Strength of reinforcing steel has exhibited a twofold property countering each other. High strength steel exhibited higher performance in case of reserve strength but inferior in case of spalling. These parametric studies also suggested a good number of graphical guide lines for designing better fire endurable flexural members within commonly used concrete compressive strengths. The research has finally proposed two separate methods, one for designing superior fire durable structure and the other for evaluating pre/post fire state of structures. The fire design method consults a group of charts those have been formulated from the experimental results. This design method will assist Engineers to select suitable combination of materials and clear cover depth to produce improved fire durable concrete structures. The later method will assist in designing effective fire defence system or to optimize the use of burnt structures. The paper ends with very important conclusions drawn from the findings of the research.