| Summary: | 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. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
|
|---|
