Compressive strength and bond behaviour of recycled coarse aggregate concrete

Abstract

Sustainable resource management and development have been at the forefront of important issues concerning the construction industry for the past several years. Specifically, the use of sustainable building materials and the reuse and recycling of previously used building materials is gaining acceptance and becoming common place in many areas. As one of the most commonly used building materials in the world, concrete, composed of aggregate, sand, cement and water, can be recycled and reused in a variety of applications. Using crushed concrete as fill and sub-grade material under roads, sidewalks and foundations has been the most common of these applications. However, research has been ongoing over the past 50 years in many countries including Germany, India, Canada, Japan, the United States, China, and Australia investigating the use of crushed concrete from demolished old concrete structures to fully or partially replace the virgin aggregate used to produce new concrete for use in building and pavement applications. Producing concrete using recycled concrete aggregates (RCA) has several advantages, namely, the burden placed on non-renewable aggregate resources may be significantly decreased, the service life and capacity of landfill and waste management facilities can be extended, and the carbon dioxide emissions and traffic congestion associated with the transport of virgin aggregates from remote sites can be reduced. This research investigates the inter-relationships between aggregate properties, concrete properties and the bond properties between reinforcing steel and RCA concrete. Forty pullout tests were carried out in order to investigate the bond behaviour between recycled coarse aggregate concrete and steel rebars. Four recycled coarse aggregate (RCA) replacement percentages (i.e., 0%, 30%, 60% and 90%) with water-cement ratio 0.42, 0.45, 0.48, 0.51, 0.55 are considered in this paper. Based on the test results, the influences of both recycled coarse aggregate replacement percentages and water- cement ratio on the bond strength between the recycled coarse aggregate concrete and steel rebars were investigated. It was found that under the equivalent mix proportion (i.e., the mix proportions are the same, except for different recycled coarse aggregate replacement 5 percentages), the bond strength between the recycled coarse aggregate concrete and rebar initially decreases with an increase of the recycled coarse aggregate replacement percentage, whereas afterwards the bond strength increases with increase in replacement level of coarse aggregate. Bond strength is maximum for 90% replacement level. With the bond strength, compressive strength is also studied and it also shows the same trend as bond strength. A power series relationship exists between compressive strength and bond strength of recycled aggregate concrete. The existing models (ACI 408, Ogura-Koichi model) proposing the square root law underestimates the bond strength of concrete.