No abstract

The results of a test programme to study the use of recycled concrete aggregate (RCA) in high-strength, 50 N/mm 2 or greater, concrete are described. The effects of coarse RCA content on the ceiling strength, bulk engineering and durability properties of such concretes have been established. The results showed that up to 30% coarse RCA had no effect on concrete strength, but thereafter there was a gradual reduction as the RCA content increased. A method of accommodating the effects of high 1KCA content, involving simple adjustment to water/cement ratio of the mix is given. It is shown that high-strength RCA concrete will have equivalent engineering and durability performance to concrete made with natural aggregates, for corresponding 28-day design strengths. The practical implications of the study for concrete construction are discussed. Sont d&its ici les r&ultats d' une s&ie d' essais destin& a dtudier l'utilisation de granulats provenant du recyclage d'~18ments en b~ton (RCA) clans des b&ons de haute r&istance (50 MPa et plus). Les effets de la teneur en gros granulats recycl& sur la r&is-tame des plafonds et des

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& The paper describes a study undertaken to examine the suitability of recycled concrete aggregate (RCA) for use in BS 5328 designated mixes. The results for the aggregate characteristics showed that plain as well as reinforced demolished concrete debris can be crushed using existing plant to provide RCA of physical properties that satisfy the current BS 882 requirements. The density of both ®ne and coarse RCA was found to be lower than that of natural aggregate (NA) and water absorption was three to six times higher. The results showed that original strength of the recycled concrete had a negligible eect on the fresh properties of the designated concrete mixes produced. Those mixes with high proportions of RCA were found to be harsh or less cohesive and exhibited higher bleeding, but this could be overcome using a ®ller material, such as coarse PFA. The test results also indicated that up to 30% coarse or 20% ®ne RCA had no eect on the strength of concrete, but thereafter there was a gradual reduction in strength with increase in RCA content. A method for taking account of strength reductions with RCA concrete was proposed, which involved simple adjustment to the water/cement ratio. Studies of concrete durability, comprising (i) near surface absorption, (ii) carbonation rates, (iii) sulphate resistance (in nonaggressive conditions), (iv) freeze/thaw in water and (v) abrasion resistance showed similar performance for RCA and NA concrete mixes of equivalent strength. The practical implications of the study for the construction industry are also considered. IntroductionThe principles of sustainable construction development require prudent use of natural resources and maximum recycling of construction wastes. 1±5 In keeping with this approach, many governments have actively sought to encourage the use of recycled concrete aggregate (RCA) as an alternative to primary aggregates, 4±6 through a combination of research and development work and land®ll taxes. However, a recent survey has shown that of the 70 million tonnes of demolition and construction wastes produced annually in the UK, 7 only 10% is converted into aggregate, mainly for use as a highway ®ll material. 8 The use of RCA in concrete construction represents a further potential outlet for the material. Several investigations have been made to study the eects of RCA on the fresh and engineering properties of concrete. 9±12 However, there are very limited data available on the durability or long-term performance of RCA concrete. This, coupled with the general concern that concrete performance may be compromised with the use of RCA, has tended to restrict its wider use. 2. Given this background, a study was undertaken by the Concrete Technology Unit at the University of Dundee to examine the suitability of RCA for use in BS 5328 designated mixes, 13 namely, the GEN, PAV and RC series. These mixes have modest performance requirements, but within their design range encompass dierent concrete mixes which account for over half of the total concrete used in the UK....

a b s t r a c tThis paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes.