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TECHNICAL PAPER COLLECTOR’S EDITION

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within the recommended limit of 3.5 kg/m in BS concrete designed to achieve appropriate design
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8500-2 for normal reactivity aggregates. strengths. An example is shown in Figure 5,
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where a RCA concrete pavement was constructed
No research has reported any deleterious effect of using material crushed on-site to produce new
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recycled aggregates on the long-term resistance to concrete of strength 35 N/mm and designed to
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sulfate attack of concrete. withstand a XF4 environment. The pavement
consisted of a 150 mm deep RCA concrete laid on
top of a 300 mm deep RA sub-base. In addition,
Abrasion resistance of recycled aggregate concrete
has been assessed using an accelerated abrasion pigmentation and imprinting were required since
method. 6,30 For concretes designed for equal the client requested that the finished surface
strength, the results show that there is a tendency resemble an existing road from the first phase of
for abrasion resistance to reduce as recycled development where red coloured block paving
aggregate content increases; up to 1 mm had been used. To provide additional protection,
difference being recorded between the natural polypropylene fibres were added to the concrete
aggregate and 100% recycled aggregate concretes. mix to assist in preventing cracking due to drying
However, up to 30% by mass of recycled aggregate shrinkage. Specimens taken from selective mixes
as coarse aggregate there is only a small difference for investigation in the laboratories have shown
in abrasion resistance (approximately 0.2 mm), excellent performance.
and this is within test repeatability. RELEASE OF REGULATED DANGEROUS
SUBSTANCES
MIX DESIGN AND DEMONSTRATIONS
A comprehensive study has compared the release
Since similar performance for natural and recycled of regulated dangerous substances (RDS) from
aggregates can be met at equal strength, a number unbound recycled and natural aggregates, with
of mix design methods have been proposed to particular emphasis on how revisions to Essential
enable concrete producers to achieve the necessary Requirement 3 (ER3) of the factory production
design strength when using recycled aggregate. control system for materials produced to the
One very efficient method, introduced by Dhir et al, Construction Products Directive will affect the use
is to reduce the w/c ratio in relation to the amount of recycled aggregates in construction in the EU.
of recycled aggregate used as a replacement of The results showed that the leaching of RDS from
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natural aggregate; also recognising that there RCA and RA were of a similar level to that of the
is no need to reduce w/c ratios when the recycled three reference natural aggregates (natural gravel,
aggregate content is below a certain limit (30% granite and limestone) used in this study, Table 2. 34
by mass for RCA). An example is given in Figure 4
which shows the w/c ratio for a natural aggregate
Furthermore, research on leaching from recycled
and the adjustment necessary to the w/c ratio for
aggregate concretes has shown that the cement
various RCA contents to achieve a strength similar
paste matrix around the aggregates has an
to the corresponding natural aggregate concrete.
immobilisation/stabilisation effect on RDS release
and that this significantly reduces the leaching
Based on the mix design described above, a
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levels. Indeed, data gathered in the Netherlands
number of demonstrations have been carried
over six years has shown that recycled aggregate
out to establish case studies suitable for long-
concrete always satisfies the Dutch Building
term monitoring of in-situ performance of RCA

The Indian Concrete Journal | November 2018 115