| Summary: | Reinforced concrete buildings and structures are exposed to fire, and the concrete's qualities can vary in the case of an uncontrolled fire. It is essential to know how concrete's properties may change as a result of exposure to high temperatures and loading for normal concrete with or without fiber reinforcement. Understanding the strength characteristics of concrete structures exposed to high temperatures is important in order to be able to predict how these structures will perform after exposure to this condition. This study investigates the combined impact of loading and high temperature on the properties of concrete samples by testing the compressive, flexural strength, and ultrasonic pulse velocity (UPV) of normal-strength concrete (NSC) with or without steel fiber. The type of steel fiber employed in this investigation is hooked-end steel fiber with a 1% volume fraction of concrete. The concrete samples were subjected to four sustained load cases (0, 20, 40, and 60% of ultimate load) and exposed to elevated temperatures (25, 300, and 500°C) for about 2 hours. In addition, using an unrestrictive test before and after loading and heating, the results indicated the maximum deference of compressive strength decreasing is 4% for the steel fiber (SF1) mixture compared with NSC mixture at 300°C for all load cases. While at 60% sustained load and 500°C, the maximum deference of compressive strength decreasing is 12% for the NSC mixture as compared with the SF1 mixture. The maximum difference in flexural strength is 20% for mixtures with steel fiber (SF1) compared to NSC mixtures at 500°C for all load cases. In addition, the UPV test was used to investigate the microstructure and quality of concrete. This test indicated no difference or approximately equal values for SF1 and NSC mixtures at 25 and 300°C for all load cases, while at 500°C, the UPV test for NSC had a higher value than the UPV value of the SF1 mixture.
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