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TECHNICAL PAPER COLLECTOR’S EDITION

aggregate having a coat of weak mortar already reasons BS 8500-2 does not permit use of fine RCA
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attached, and (ii) this attached mortar raising the in concrete. However, it should be noted that as
porosity of the concrete. 9-11,12,13 fine RA contains significantly lower quantities
of ‘cement dust’, its use as fine aggregate is of
To promote further use and confidence in RCA, considerably greater potential.
research has been carried out to investigate RCA
in high strength concrete of cube strength 50 N/ In general, the flexural strength and modulus of
mm to 80 N/mm 2.14 For this work, only coarse elasticity of RCA and RA concrete have been reported
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RCA obtained from rejected structural precast to be proportional to compressive strength.
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elements was used, and it was shown that use of As a result, typical relationships between static
30% RCA had little effect on the ceiling strength of modulus of elasticity and compressive strength,
concrete and that cube strengths greater than 80 and flexural strength and compressive strength
N/mm could be achieved (Figure 2). Furthermore, as derived from design codes are compatible with
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high strength RCA concrete gave equivalent the use of RCA and RA, as illustrated in Figure 3. 18,19
performance to natural aggregate concrete of the Furthermore, when designed for equal strength (by
same strength. This was meant to give engineers reducing the w/c ratio), RCA and RA concretes give
further confidence in the use of RCA in structural equal performance to that of natural aggregate
concrete. However, there is evidence that the concretes as can be seen from Table 1.
“negative” effect of RCA on strength increases as
the w/c ratio of concrete reduces, and that these Notwithstanding the above, there have been some
aggregates are most effective in lower strength recent concerns that the elastic modulus of RCA
concretes. 9,15 concrete is not proportional to strength, and that
lower values than usually associated with strength
Similar results to that with coarse RCA have been of concrete are observed; 20, 21 but even so, the
found when using coarse RA, and researchers have elastic modulus values usually still fall within the
shown that it may be used up to 20% replacement bands given in Eurocode 2, and the elastic modulus
of natural aggregate by mass without having of natural aggregates also varies widely depending
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any significant effect on performance; although on the type of aggregate used. Consequently, this
performance is obviously dependent on the property may be more dependent on the original
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composition of the RA. Higher quantities of RA source of aggregate used in the concrete. A study
tend to cause greater reductions in strength of undertaken by Xiao et al has shown that for a given
concrete than when using RCA. Primarily the reason cube strength, elastic modulus of RCA concrete
for the lower strength is due to the inclusion of may vary by as much as 15 kN/mm . This is
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weak and porous aggregate particles, for example perhaps an area for further research.
low strength bricks and plaster. However, RAs
with large stone contents have shown superior RCA and RA concrete tend to have higher levels of
performance to RCA. drying shrinkage than natural aggregate for the
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same compressive strength. This is mainly because
Whilst research has shown that fine RCA could recycled aggregates provide less restraint to
be used in very low quantities it tends to cause movement (lower elastic modulus), and because of
difficulties with the stability of the mix and the the higher cement contents that are often used in
strength of the resulting concrete. For these these concretes to achieve the required strength.
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112 The Indian Concrete Journal | November 2018