The Effect of High Temperature on Mechanical and Material Response of Recycled Aggregates High Strength Concrete

Abstract

Recently, 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 built environment, preservation of natural resources, reduction of pollution, sustenance of construction materials and energy savings. Each of the constituents of concrete has some contributing impact on the environment. Aggregates which are about 70% to 80% of concrete volume add significant disturbances to the ecological system due to its production and transportation. With the increased amount of concrete usage in the construction industry, main challenges being faced are depletion of natural course aggregate (NCA) resources and high consumption of Portland cement with its associated carbon footprint. A substantial amount of demolition waste is being produced from concrete structures that result from the replacement of old and damaged structures. Construction and demolition (C&D) waste management, is amongst biggest environmental challenges in many developed countries which renders recycling of concrete as a necessity Recycling concrete for producing aggregates is one of the leading approaches to sustainability in the construction sector. An increase in the new construction replacing old construction results in the generation of demolished concrete wastes that consume large volumes of the landfill and heavy on new construction resources. The advances in concrete technology have enabled construction industry with the possibility of producing high-strength concrete (HSC) with recycled aggregates. With the envisaged structural applications of recycled aggregates high strength concrete (RA-HSC), it is imperative to characterize its behavior under different service conditions. Fire is a common severe threats to which members/structure are threatened with during their service life, the requirement to characterize the mechanical and material performance of RA-HSC at elevated temperatures becomes important. This research presents an experimental study on unstressed and residual mechanical and material performance of RA-HSC, after exposure to elevated temperatures. Two different concrete compositions were prepared: one with natural coarse aggregates (NCA) as a reference concrete and other with recycled coarse aggregates (RCA). Replacement level of 100% was made for natural aggregates with that of recycled aggregates. Specimens were exposed to a temperature of 23, 100, 200, 400, 600, and 800°C. A heating rate of 5°C/min was maintained, under both unstressed and residual test conditions to achieve the desired target temperatures. Specimens were transported to the testing machine in hot state for unstressed testing and after cooling down to surrounding temperature for residual testing conditions. Mechanical and material properties comprising of compressive strength, splitting tensile strength, stress-strain response, elastic modulus, and mass loss were investigated using both unstressed and residual test conditions. Additionally, visual observations and microstructural analysis were performed to characterize the fire behavior of RA-HSC at temperatures up to 800°C. Data obtained from the elevated temperature tests of RA-HSC suggested that the replacement of NCA with that of RCA decreases the mechanical performance under normal loading condition. But at elevated temperature, the performance of RA-HSC is better compared to that of natural aggregates high strength concrete (NA-HSC). At elevated temperatures, decrease in stress-strain response for both RA-HSC and NA-HSC was observed, however, an increase in the axial strain was observed more in the case of RA-HSC. Compressive strength drop was higher in the case of NA-HSC compared to that of RA-HSC. The split tensile strength of RA-HSC was better compared to that of NA-HSC due to a rougher surface of RCA. Microstructural variation in both types of concrete were studied at elevated temperatures ranging up to 800°C using scanning electron microscopy (SEM). Visual investigation after exposure to elevated temperature exposure discovered that RA-HSC shows low cracking with less shading change as compared to NA-HSC. Results show that RA-HSC performed better at elevated temperature in terms of mechanical properties.