Page 113 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

recycled aggregate concretes have higher cement
content in order to achieve a given strength.
24
Thus, the alkaline reserve acts to protect the
concrete surface against carbonation mechanisms.
In addition, recycled aggregates obtained from
crushed concrete and crushed masonry are partly
constituted with old mortar that increases the
alkaline reserve further. It was noted, however,
that the best performance overall in terms of
carbonation was for concretes with a RA content
of between 20 and 50% by mass. Thus there is
probably an optimum in terms of balancing the
Research suggests that chloride ingress into increased alkaline reserve and the increased
recycled concrete occurs more rapidly than that porosity when using RA in concrete. 24
of natural aggregate concrete at equal w/c ratio
(Table 1); mainly as a result of the increased Tests on air-entrained recycled aggregate concrete
permeability of the concrete. However, when used at up to 100% by mass of coarse aggregate
tested at equal strength results tend to show a have been shown to give good freeze-thaw
reduction in chloride ingress, probably as a result scaling resistance (56 day scaling < 0.5 kg/m )
2
of the lower water or higher cement content and and to perform similarly to air-entrained natural
therefore reduced permeability of the cement aggregate concretes. 30 Therefore, provided
paste. There is, however, evidence that the chloride concretes are air-entrained, resistance to freeze-
ingress of recycled aggregate is not directly thaw can be assumed. For non-air entrained
related to the permeability of the concrete since concrete, recycled aggregate concretes may be
the recycled aggregates are able to supplement somewhat more prone to freezing and thawing
the chloride binding capacity of the cement and degradation than natural aggregate concretes. 23
28
to some extent cancel out permeability effects.
When determining the resistance of concrete
to internal chloride attack, research has shown A number of research projects have attempted to
that chloride contributions from RCA for use identify whether recycled aggregate concrete is
in calculations should be measured by an acid- more susceptible to chemical attack than natural
29
soluble test that provides a worst-case value. This aggregates. A comprehensive study of damaging
has been incorporated into BS 8500-2. 8 alkali-silica reactions (ASR) using expansion tests in
accordance with BS 812-123 at 60°C for both coarse
31
and fine recycled aggregates by Dhir et al. has
In contrast, many tests for carbonation have established that the use of recycled aggregates is
shown that the carbonation depth (for concretes likely to be low risk with respect to damaging alkali-
of equal compressive strength) after a given length silica reaction (ASR). For the materials tested, it was
of exposure decreases as the recycled aggregate noted that there was no correlation between the
content of concrete increases, i.e. recycled alkali release content of the RA and ASR expansion.
aggregate concrete has better resistance to Furthermore, tests to resolve application issues
carbonation than natural aggregate concrete. 16,24 It with regard to the use of RCA in concrete have
has been purported that this is due to the fact that
demonstrated that the alkali content of RCA falls

114 The Indian Concrete Journal | November 2018