Strength prediction model and methods for improving recycled aggregate concrete Khaleel H. Younis a,b,⇑ , Kypros Pilakoutas a a Dept. of Civil and Structural Engineering, The University of Sheffield, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD, UK b Dept. of Civil Techniques, Erbil Polytechnic University, Kirkuk Road, Erbil, Iraq highlights  Particle density, strength of aggregate, water absorption and RA content are used in a strength predictive model.  Particle density is related to both LA value and WA and can be used as a quantity to classify RA.  Treatment of RA with reactive microfillers (e.g. silica fume) can increase the strength of RAC by 17%.  Short RTSF helped improve the strength of RAC by a further 13%.  Appropriately sorted and treated RA can perform at the same level as NA. article info Article history: Received 30 July 2013 Received in revised form 29 August 2013 Accepted 3 September 2013 Available online 25 September 2013 Keywords: Recycled aggregate concrete Microfillers Surface treatment ITZ Multi-linear regression Lime stone powder Silica fume Recycled tyre steel fibres abstract This paper examines the effect of various parameters on the performance of recycled aggregate concrete (RAC) and proposes a strength prediction model. Relations that link the properties of recycled aggregate (RA) to the strength of RAC are developed using multi-linear and non-linear regressing analysis. To enhance the compressive strength of RAC, the effect of surface treatment of RA using small quantities of reactive and non-reactive microfillers is examined. For the same purpose, two mixing methods and the addition of recycled tyres steel fibres (RTSFs) are also investigated. The results show that RTSF as well as reactive and non-reactive microfillers can enhance the strength of RAC by 30%. Furthermore, density separation can be used to produce high quality RA from construction and demolition waste (CDW). Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Large quantities of construction and demolition waste (CDW) materials arise annually worldwide. In the UK alone, this waste amounts to roughly 110 million tonnes per year which corre- sponds to 60% of total waste [1]. Only 40% of this amount is reused or recycled. At the same time, large quantities of natural aggregates are extracted for construction every year. The utilisation of recy- cled aggregates (RAs) in concrete production can potentially con- serve the non-renewable natural resource of virgin aggregates, eliminate unnecessary consumption of limited landfill areas and reduce energy consumption. However, the variability in the characteristics of RA and recycled aggregate concrete RAC are the main engineering concern which hinders the use of RA. Low den- sity and high water absorption and porosity, mainly caused by the heterogeneous nature of RA, can lead to low quality concrete (low compressive, tensile and flexural strength as well as high creep and shrinkage). For example, the use of recycled concrete aggregate (RCA) can lead to reduction of up to 40% in compressive strength [1–3]. As a result, current standards and specifications [4–7] impose limitations on the use of RA in new concrete and particularly in structural concrete. With limitations, such as only 20-30% of NA can be replaced by RA in new structural concrete being common. Therefore, more research in this field is necessary to understand the effect of the various key parameters and explore approaches to improve the properties of RA and RAC. Different approaches have been adopted by researchers to im- prove the characteristics of RA and RAC. Some of these approaches deal with how to improve the RA itself while others focus on concrete production technology. Examples include, detaching the 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.09.003 ⇑ Corresponding author at: Dept. of Civil and Structural Engineering, The University of Sheffield, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD, UK. Tel.: +44 (0) 114 222 5795; fax: +44 (0) 114 222 5700. E-mail addresses: cip09khy@sheffield.ac.uk, khaleelyounis@gmail.com (K.H. Younis). Construction and Building Materials 49 (2013) 688–701 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat