Recycled aggregate concrete for structural applications

The use of recycled aggregate in concrete opens possibilities in the ways in which recycled materials can be used for structural applications; indeed, it may be an important breakthrough towards sustainable development. The utilisation of recycled aggregate is an effective solution to the problem of possessing excess waste materials while simultaneously maintaining satisfactory concrete quality. The utilisation of waste construction materials should be related to the application of quality guarantee systems to achieve suitable product properties. A complete understanding of the characteristics of new materials is, therefore, extremely important so that it’s potential in applications can be thoroughly studied. Recycled aggregate has been employed for a long time, and is in limited use for roadwork, earth work and non-structural applications. The use of recycled aggregate in concrete construction or structural applications is scarcely reported. However, in some countries recycled aggregate is used in new concrete for construction projects through the careful control of its production process, such as the Waldaspirale and Vilbeler projects in Germany. Thorough study of the characteristics of recycled aggregate can establish improvement on its properties and promote its use in concrete construction applications. Information survey was carried out by interviewing construction and demolition industry representatives in south-eastern Australia. According to these discussions, the main difficulties in using recycled aggregate are lack of specifications, and lack of experience in using recycled aggregate. Recycled aggregate samples were collected from various sources in south-eastern Australia for property testing. A simplified testing approach was established by constructing a correlation among different aggregate test results. Thirteen physical, mechanical and chemical properties of aggregate were investigated, these being (i) uncompacted bulk density, (ii) compacted bulk density, (iii) particle density, (iv) water absorption, (v) aggregate crushing value, (vi) dry strength, (vii) wet strength, (viii) contaminant, (ix) weak particle, (x) flakiness index, (xi) particle shape, (xii) chloride content and (xiii) sulfate content. Eleven recycled aggregate iii samples collected from construction and demolition sites and centralised recycling plants in Brisbane, the Gold Coast and Sydney were used in this study. A linear regression analysis was used to construct the correlation among the refined four group of tests, these being (i) Group A (density, water absorption, strength and contaminant),(ii) Group B (weak particle), (iii) Group C (particle shape) and (iv) Group D (chemical contents). The number of tests can be reduced from their correlation of about 76.92%. Recycled aggregate can be classified to three classes: A, B and C. The optimal mixing approach for recycled aggregate concrete with eight prediction equations was developed. The developed equations can be used to predict characteristic compressive, tensile and flexural strength by providing parameters of the mix proportions, or from one of the characteristic strengths. It was also proved that the developed equations are accurate from the verification dataset. A guideline for the use of recycled aggregate for recycled aggregate concrete was developed. The industry can classify the recycled aggregate collected from its construction and demolition sites as Class A, B or C by testing five types of recycled aggregate properties. The industry can also use the results of five recycled aggregate properties in estimating the other recycled aggregate properties from the developed equations. After knowing the quality of the recycled aggregate collected, the industry can use the optimal mixing approach developed with the developed equations to design its concrete mix by water-to-cement ratios and recycled aggregate replacement ratios. The characteristic compressive, tensile and flexural strength can then be predicted. This guideline on the use of recycled aggregate for recycled aggregate concrete is able to provide confidence to the industry through knowledge of the reasonable prediction of concrete strength from its recycled aggregate, and thereby encourage its use