Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 138

in impact crushers, the reduction in size takes place without forces being
exerted on the individual particles. On the other hand, in impact crushers,
the chippings break at their weakest points, such as at incipient cracks, so
that the result is a particle with better properties. Reduction by impact also
causes the adhering mortar to be removed to a greater extent.
Through a series of comprehensive tests, Akbarnezhad et al. [13] showed
that the effects of crushing on the mortar content of RCA depend signifi-
cantly on the properties of parent concrete, including the strength and size
of the NAs used in the parent concrete. Because of the overlapping nature
of the mechanisms and interactions of the effects of the various influencing
parameters, drawing general conclusions about the relationship between
the crushing procedure and RCA properties is difficult. A systematic
approach to investigating the effects of one parameter whilst keeping the
others constant is necessary to optimise the crushing regime with a view
to minimising the mortar content of the RCA produced. By evaluating the
properties of the RCAs sourced from two different grades of concrete (30
and 60 MPa) produced with two different coarse aggregate gradings (maxi-
mum size of 12 and 20 mm) and crushed using different crushing pro-
cedures, Akbarnezhad et al . [13] concluded that, for RCAs sourced from
normal-strength concretes (such as C30), the RCA particles produced by
crushing the debris to a relatively similar size range as the maximum size
of NAs used in parent concrete tended to contain slightly lower amounts of
mortar. Therefore, it was recommended that crushing the concrete debris
to a maximum size equal to that of their NAs may result in some improve-
ments in the quality of RCA produced. This may be because the smaller
difference between the size of the NAs in debris and the space between the
jaws of the crusher leads to the production of a smaller amount of RCA
particles comprising two or more NAs adhering together and surrounded
by mortar (type II) and a higher amount of RCAs comprising a single par-
ticle surrounded by a thin layer of mortar (type I), the latter generally hav-
ing a lower mortar content. However, it was reported that when RCAs are
produced from stronger concretes, this trend is not as obvious.
4.3.2 Water absorption and density
Water absorption and density of aggregates are key parameters in the mix
proportioning of concrete constituents and are among the simplest and
most commonly used tests for determining the overall quality of RCA at
concrete-recycling plants and ready-mix concrete plants. Water absorption
and density of RCA are interrelated parameters and have been reported to
show an almost-linear inverse relationship, mainly because the variations
in either of these properties stem from a variation in the mortar content
of RCA. As a result of the presence of the intrinsically less-dense mortar,
RCAs have been reported to have generally lower bulk density and higher