STUDY ON BEHAVIOUR OF STEEL FIBER CONCRETE UNDER ELEVATED TEMPERATURE

Abstract

Concrete, as the predominant building material worldwide, is widely utilized but exhibits weakness in tension due to its inherently brittle nature. To address this concern, the incorporation of steel fibers into concrete has emerged as a viable solution to enhance its ductility. Given the extensive usage of concrete and its exposure to elevated temperatures resulting from fire incidents and other factors, investigating the impact of temperature on concrete with steel fiber reinforcement assumes significant importance. Although numerous studies have investigated this subject, limited emphasis has been placed on exploring the influence of the aspect ratio of steel fibers in concrete. Hence, the present study aims to investigate the efficacy of incorporating steel fibers with different aspect ratios into the concrete through experimental methods. The objective is to evaluate the performance enhancement of concrete components subjected to both low and high temperatures. The experimental work was conducted in multiple stages, in the initial stage, several tests were carried out at ambient temperature, including workability, compressive strength, and split tensile strength tests. The intent was to investigate the response of concrete and determine the optimum fiber dosage for aspect ratios of 65 and 55 mixed in the range of 0% to 1.50% at intervals of 0.25%. The subsequent stage focused on evaluating the mechanical properties of the specimens with the determined optimum fiber dosages. This evaluation was conducted at elevated temperatures of 125°C, 250°C, and 375°C. The mechanical tests performed included compressive strength, split tensile strength, and flexural strength tests. The results indicated that the inclusion of steel fibers with various concentrations and aspect ratios significantly impacted the mechanical properties of concrete.

It was observed that the workability of the concrete decreased as the fiber aspect ratio increased. The maximum decrease, compared to the control mix, was 39% and 36% for the 65 and 55 aspect ratio mixes, respectively. However, the inclusion of steel fibers improved the compressive strength by approximately 13% to 38% and the split tensile strength by 7% to 65% at ambient temperature. Based on the results obtained, the optimum fiber volume percentages for achieving maximum increases in compressive and split tensile strength were determined to be 0.75% for aspect ratio 65 and 0.50% for aspect ratio 55. When the specimens were exposed to elevated temperatures for 3 hours, ranging from 125°C to 375°C, a reduction in weight was observed. This weight loss could be attributed to the evaporation of water content from the concrete at higher temperatures. Mild cracks were also observed on the surface of the specimens exposed to 375°C due to the loss of water and the generation of stresses at high temperatures. The compressive strength of the fiber-reinforced concrete increased with rising temperatures, although the rate of increase decreased. However, the split tensile strength and flexural strength exhibited an increase up to 250°C, followed by a decrease at higher temperatures. Furthermore, a mathematical model was developed to predict the residual strength of steel fiber-reinforced concrete for each mix proportion under elevated temperatures. The model took into consideration factors such as temperature, fiber volume, and fiber content. The established relationship between these factors and the residual strength was compared to the experimental results. The analysis revealed that the developed mathematical model provided a rational and accurate prediction of the residual strength of steel fiber-reinforced concrete under elevated temperature conditions, as it aligned well with the experimental findings.