Civil Engineering Reference
In-Depth Information
others such as aggregate and cement types, water content and void content. The
density of fresh concrete also controls various hardened-state properties of con-
crete. For example, given the same quantity of cement and aggregate, lower fresh
concrete density indicates lower strength because the density decreases as the
water and voids content increases.
The density of fresh concrete containing CDW aggregate is slightly lower than
that of the mix containing natural aggregate since the density of CDW aggregate is
lower than that of natural aggregate. The presence of lower density residual
cement mortar particles attached to the aggregate is the main factor for lowering
density of concrete due to the addition of CDW aggregate (Hansen and Narud
1983
; Gonzalez-Fonteboa et al.
2011
). Table
5.4
shows some typical values of the
density of various concrete mixes containing natural as well as CDW aggregate
along with the density of the aggregate.
Katz (
2003
) observed no significant difference in densities of concrete mixes
containing RCA prepared from old concrete of three different ages (1, 7 and
28 days), which suggested that the amount of adhered mortar content in the var-
ious concrete aggregates was similar regardless of their crushing age (Table
5.4
).
Soutsos et al. (
2011
) observed marginally lower wet density in concrete containing
coarse RCA than in concrete containing coarse limestone aggregate due to volu-
metric rather than weight-based substitution of coarse aggregate. Lopez-Gayarre
et al. (
2009
) reported that the variations of properties of RCA have little effect on
the resulting density of concrete. They observed a reduction of about 5 % in the
density of concrete mix when all natural coarse aggregate was replaced by RCA.
They used the analysis of variance (ANOVA) method to study the effects of
various parameters of aggregate on the density behaviour, which is presented in
Table
5.5
.
5.2.3 Air-Content
The presence of a certain amount of air bubbles trapped during concrete mixing
has several beneficial effects for fresh and hardened concrete properties. In fresh
concrete, an air-content of around 3 % can reduce the water demand of concrete
and make the mix stickier, which helps to reduce segregation and bleeding.
However, if the air-content is higher than the specified amount, the increased
stickiness makes concrete finishing more difficult. Thus the air-content of a con-
crete mix is an important property and a few studies are available on the behaviour
of air-content of fresh concrete due to the incorporation of CDW aggregate.
Table
5.6
shows the air-content of different concrete mixes containing natural as
well as CDW aggregates of various types.
In some of the studies, it was reported that the addition of CDW aggregate to
concrete increases the air-content (Katz
2003
; Lopez-Gayarre et al.
2009
). Lopez-
Gayarre et al. (
2009
) observed an increase in the air-content of concrete with the
RCA aggregate content, which was visible above 50 % replacement of aggregate
