The use of recycled aggregate concrete of equal compressive strength in reinfornced concrete beams

• Main Author: Hedayatnasab, Arastoo
• Other Authors: Kew, Hsein ; Limbachiya, Mukesh
• Published: Kingston University 2013
• Subjects: 624.1; Civil engineering
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.579141

One area for sustainable engineering is the efficient use of recycled aggregates obtained from construction and demolition and also as by-products derived from industrial waste that optimises economic and environmental benefits. For the past five decades, studies on the effect of coarse recycled aggregates (RCA) on properties of concrete have been going on, and in fact, none of them reported that good quality coarse RCA, from the mechanical point of view, is unsustainable for structural use. However, according to the limits stated in BS EN 8500 part 2 (2006), at least 87% of the coarse RCA shall be obtained by crushing old hardened concrete debris. As it was impossible to obtain such good quality aggregates from existing plants, coarse recycled aggregate (RA*) containing 60% of coarse RCA, 10% asphalt, 10% brick, 15% unbound aggregate, and 5% fines and other materials was used in this study. The main aim of this study was to determine and compare the structural properties of natural aggregate concrete (NAC) and recycled aggregate concrete (RAC) of equal compressive strength by using different percentage of course RA* and to investigate the suitability of coarse RA* for use in steel-reinforced concrete elements. The study was carried out in three phases. Phase 1 involved the characterisation of the aggregates through a testing regime which include physical and mechanical assessments to study their possible application in concrete production. It was found that the aggregates were suitable to produce normal concrete. For the Phase 2, the effect of the coarse RA* content (up to 100%) on the main properties of concrete was investigated. The results showed that there was a gradual decrease in slump (up to 24%), compressive strength (up to 21%), flexural strength (up to 10%) and modulus of elasticity (up to 30%) as the percentage of coarse RA* increased up to 100%. Thereafter, NA concrete and RA concrete with the same slump and compressive strength made with 100% Portland cement (PC) only and also with a combination of 70% PC and 30% pulverised fuel ash (PFA), were produced. Flexural strength, modulus of elasticity, drying shrinkage, and creep were determined and compared. The compressive strength of RA concretes was increased by lowering the w/c ratio through reducing the free water content. For the Phase 3, pull-out test was carried out using 16mm and 20mm reformed bars embedded in concrete cylinders made with equal strength NA and RA concretes to investigate the bonding strength. It was discovered that the coarse RA* content had no significant effect on the values of pull-out force. Then, for studying flexural properties (cracking load, ultimate load, deflection, strain in rebar, strain in concrete, and pattern of cracks) of steel-reinforced recycled concrete (RRC) beams, two types of beam sections (under-reinforced and over-reinforced) were made with equal strength NA and RA concretes and tested. It was discovered that the difference in ultimate load values was negligible. Although higher deflections were observed for RA beams, the deflections were still within acceptable limits. Overall, it was concluded that, the flexural behaviour of steel-RRC beams made with equal strength concrete is not considerably affected by the presence of coarse RA*, as well as PFA, and the differences were minor with no practical significance. However, due to effect of coarse RA* on the shrinkage and creep strains, their use in structural elements prone to such deformations may require some special considerations.