Civil Engineering Reference
In-Depth Information
to 1,800 kg/m
3
, with a greater dependency than the RCA on the composition and
level of performance target level of the future concrete.
Water absorption of the aggregates is a function of their density. For the
standards that consider both requisites, this relationship is valid up to an acceptable
degree, and therefore the maximum water absorption values fall between 7 and
10 % for RCA and from 10 to 20 % for MRA and RMA.
Neither the British nor the Swiss standard contemplates any limitation on either
the density or the water absorption of the aggregates. In these cases it is assumed
that the demands made for the aggregates' composition are enough to ensure the
quality of the aggregates, bearing in mind the aim proposed.
The maximum content allowed for chlorides and sulphates is usually stated in
the normative documents and, if no values are stipulated for any of these
parameters, a reference is normally made to the need for the person/entity
responsible to specify a value through a case-by-case analysis. The maximum
authorised content of sulphates varies from 0.8 and 1.0 % of aggregates (in mass)
while for chlorides the range of values is much wider, changing according to the
demand level of the use, even within the same standard. For structural concrete,
values between 0.03 and 0.05 % are common, usually associated with RCA
because they are the most suitable RA for this type of application. The Brazilian
standard allows much higher amounts of chlorides because it does not envisage the
use of RA in structural concrete.
7.3.3 Field of Application
Taken individually the aggregates' requirements do not reveal the specification's
approach. It can only be stated that a regulation is conservative or permissive after
the aggregates' requirements and the proposed use of concrete are examined
simultaneously.
Therefore, on this point the standards for both the application of concrete made
with RA and the conditions in which concrete is produced are compared, i.e.
replacement ratios of natural by RA, level of demand of the application, and
maximum strength level authorised. Table
7.44
shows the field of application of
RAC allowed in the different countries, and the conditions by which this appli-
cation is governed.
Due to its propensity to contain contaminants that may affect the mechanical
properties of concrete, the use of a fine fraction as concrete aggregate is barred in
most of the standards. The Brazilian and Japanese (BCSJ) norms are exceptions,
explained partly by the low demand for this use (non-structural concrete). The
Danish and Swiss norms are even more complacent and allow the use of this
fraction in structural concrete.
The coarse fraction provides the best conditions for use in concrete. None-
theless, it is regarded conservatively by some of the standards. The Spanish norm
allows a replacement ratio of natural by RA of only 20 %, even though the
