Performance of Recycled Aggregate Concrete Containing Pozzolanic Materials and Fibers Exposed to Elevated Temperatures

Abstract

Sustainable development and environmental awareness have become important attributes of societal growth. An important role in this direction is attained through the sustainable development of the built environment, preservation of natural resources, reduction of pollution, sustenance of construction materials, and energy savings. Due to their storage and processing, aggregates that comprise between 70 and 80 percent of the volume of concrete have a significant impact on environmental system. Natural coarse aggregate (NCA) resources are dwindling due to the increased use of concrete in the construction industry, posing one of the most significant challenges. Recycling construction and demolition waste (CDW) and using it as aggregates in the production of new concrete is one solution to this problem. This recycling of aggregates and production of recycled aggregate concrete (RAC) can reduce CDW and natural resource consumption. The development of concrete technology has enabled the construction industry to produce concrete with recycled aggregates. With the anticipated structural applications of recycled aggregate concrete (RAC), it is necessary to characterize its behavior under various service conditions, such as fire. As fire is a common severe threat to members/structures during their service life, it becomes necessary to describe the mechanical and material performance of RAC at high temperatures. To investigate the possible benefits of utilizing recyclable aggregates as a concrete component. It is essential to research Recycled Aggregate Concrete (RAC) properties and behavior at the material level. This study utilizes pozzolanic materials and fibers as a part of RAC at room and elevated temperatures. Research work was carried out in two phases. In the first segment, the optimum percentage of recycled aggregates was found. From different standardized tests, its mechanical behavior was evaluated under an ambient and elevated temperature of 300◦C and 600◦C. Among different recycled aggregate percentages, RAC25 was found to be the optimum recycled aggregate content. In a second phase of the research pozzolanic materials ( Silica fume and Flyash) and fibers ( Steel fibers and Polypropylene fibers) were added to the RAC25 to enhance its properties. The specimens were evaluated at room temperature and elevated temperatures at a rate of 5C/min until the desired temperature was reached. According to data collected at room temperature, the addition of mineral admixtures to recycled aggregate concrete increases its own strength properties, i.e., silica fume increases its strength to such an extent that it exceeds the properties of natural aggregate concrete. Based on the results, it was determined that all of the materials increased the tensile strength of the RCA concrete. Nonetheless, the strength of steel fibers at room temperature varies significantly. With a temperature upsurge the remaining compressive strength of RAC with pozzolanic and fibers drop by around 20% at 300◦C and 50% at 600◦C. At elevated temperatures, pozzolanic materials enhance their tensile properties. Moreover, only Flyash addition to RAC shows the elastic modulus recovery at high temperatures. Ductility index and Toughness index were maximum by RAC with fibers at all temperature ranges. However, silica fume RAC ductility and toughness increase with temperature rise. Visible cracks at different temperature variations along with color differences help predict different chemical variations. Flexural tests of reinforced beams show that optimum silica fume addition to RAC enhances its load-carrying capacity and eventually increases its rapture modulus. It was concluded that for most of the mechanical properties, optimum silica fume addition in recycled aggregate concrete mix performed better as compared to other materials.