Book Description

In recent decades, the use of High Performance Concrete (HPC) has grown vastly, being used in multiple applications with high requirements. However, the use of recycled aggregates (RA) has been mostly limited to conventional concrete. Many studies have defined limiting properties of RA, replacement ratios of natural aggregates and particular techniques to achieve suitable conventional concrete containing RA. Nonetheless, very few studies have been focused on the use of RA in the production of HPC. This study examines the behaviour of High Performance Recycled Aggregate Concrete (HPRAC) in physical, mechanical, durability and structural properties according to the RA content and its quality. RA were sourced from Construction and Demolition Waste of several categories: Recycled Concrete Aggregate (RCA) obtained from 40, 60 and 100 MPa concretes, Ceramic Waste Aggregates (CWA) and Recycled Mixed Aggregates (RMA). In the first experimental phase, the limiting replacement ratios of RA were established in order to achieve comparable HPRAC to the reference HPC with a design strength of 100 MPa. The physical, mechanical and durability properties were studied for concretes containing 20, 50 and 100% of coarse RCA and RMA, and 15 and 30% of fine CWA. According to the mechanical properties, 100% of coarse RCA can be used, as long as RA is sourced from a 60 MPa minimum-strength concrete waste. Nevertheless, durability behaviour was more influenced by the use of RA and replacement ratios of RCA could only be maintained on those obtained from parent concretes with the same quality as the new HPC. Moreover, significant reductions of the RA quality (RCA sourced from 40MPa - strength concretes or RMA) only permitted 20% replacement ratios. On the other hand, the concretes containing fine CWA (up to 30%) reached higher performances than those from conventional HPC. On the second experimental phase, fly ash was used in replacement of 30% of Portland cement in order to enhance the RCA performance. Keeping in mind prestressed concrete as potential application which requires high early-age strength, the concrete mixtures were also subjected to an initial steam curing cycle. The natural aggregates could be completely replaced by RCA sourced from the same quality HPC, producing improved mechanical properties and pore structures. It was determined that when using lower quality aggregates, the use of steam curing was mandatory to fulfil the standard requirements for prestressed concrete. The steam curing had negative effects on the long-term mechanical properties, however the steam-cured HPRAC had greater improvements on the pore structure and the mechanical properties than conventional HPC. The third experimental phase assesses the role of RCA in internal curing whose effect is significant in HPC. The effects of RCA were investigated in the plastic, autogenous and drying shrinkage of HPC, being the second of special interest in concretes with low water-cement ratio. The results revealed that the plastic and drying shrinkage became higher as the quality of the RCA decreased and the replacement ratio increased. However, a reduction in the autogenous shrinkage was proved to be possible by the use of a high content of lower quality RCA, since they acted as internal curing agents. The suitable behaviour of the HPRAC mixtures containing 50 and 100% of RCA sourced from 100 MPa-strength concretes enabled the production of prestressed concrete sleepers. The structural properties of HPRAC were tested on the conventional HPC and on both HPRAC sleepers. The prestressed concrete sleepers were subjected to static and dynamic load tests at rail-seat and centre sections. The structural requirements for prestressed concrete sleepers were extensively verified by sleepers made with HPRAC. Regardless of the replacement ratio, the HPRAC sleepers' results barely differed from those of conventional HPC sleepers.