Use of recyled aggregate in shotcrete and concrete
Creators: name: Chan, Cesar
Publisher: name: University of British Columbia
Year: 2009

description: As the problem with waste concrete escalates in most industrialized nations, growing efforts have been directed towards recycling this material. Recycling of wastes, besides providing an alternate route for their management, also contributes to the conservation of natural resources. With waste concrete, this is primarily done by crushing the material to desired particles sizes and reusing it as aggregates suitable enough for specific applications. However, to cope with the increasing levels of demolished concrete being generated, more potential applications for its recycling must be sought. The purpose of this research was to investigate the use of recycled aggregates in shotcrete. Both, wet and dry processes were investigated and compared to companion cast concrete. The mechanisms by which recycled aggregates affect the performance of concrete were also analyzed. For a fair comparison between recycled and virgin aggregates, they were both made to have particle size distributions as close as possible to each other by matching the virgin aggregate gradation to the recycled aggregate gradation using a regression style analysis. The resulting mixes were investigated for their fresh properties as well as hardened properties. In the wet process, recycled aggregates were found to produce a quick stiffening and a rapid loss of workability of the fresh mixture. This is due to the higher water absorption and cohesiveness of the material due to the higher content of the fine material liberated during the dry-mixing of recycled aggregates. Such properties, however, brought a significant positive contribution to shotcrete in that the rebound was found to be much lower compared to virgin aggregate mixes in both wet-mix and dry-mix shotcrete. Reduction was seen in both material as well as fiber rebound in fiber reinforced shotcrete mixes. While some attention must be paid when shotcreting, because of a different shooting and pumping behaviour of recycled aggregates, casting of concrete with recycled aggregates can be carried out in the normal ways. Compressive stress-strain tests performed on drilled cores revealed that the fracture process of cylinders with recycled aggregate mixes is very stable and gradual, unlike the unstable and rapid crack propagation in cylinders with virgin aggregates. Although the compressive strengths in recycled aggregate mixes were 40-56% lower than virgin aggregate mixes, the former had greater deformability. This greater deformability, accompanied by stable failure, allows for a much higher energy absorption capacity. Splitting tensile strengths, flexural strengths, flexural toughness of fiber-reinforced mixes and moduli of elasticity of recycled aggregate systems were also found to be lower than virgin aggregate mixes. Microscopic observations revealed that large quantities of dust and loose debris exist in recycled aggregate mixes which are likely a contributing cause to the above observations. It appears that recycled aggregates affect shotcrete mixes in the same way as they affect cast concrete mixes such that there is no apparent gain or loss in using them in shotcrete. Fiber reinforcement of recycled aggregate shotcrete mixes is not as effective as it is with virgin aggregate mixes although suitable enough for many applications. Finally, more research on durability related issues of recycled aggregate systems must be carried out to obtain a more complete understanding of these materials.