Civil Engineering Reference
In-Depth Information
can also improve the permeability related durability performance of RCAC.
Concrete porosity depends on various factors including aggregate's porosity. The
use of CDW aggregate in concrete can change the porosity of concrete due to the
higher porosity of CDW aggregate than that of the NA.
Gomez-Soberon (
2002
) observed an increase in total porosity of concrete with
the replacement level of coarse NA by RCA. Total porosity was determined by
mercury intrusion porosimetry. After 90 days of curing, total porosity of concrete
containing RCA as the only coarse aggregate was around 3.8 % higher than that of
conventional concrete. They also observed a significant decrease in total porosity
as curing time increased, which was more prominent for RCAC. Kou and Poon
(
2006
) observed that incorporation of coarse RCA in concrete gradually increased
the total porosity and average pore's diameter and shifted the pore size distribution
to larger pores. The replacement of 25 and 35 % OPC by FA respectively
decreased and increased the open porosities of conventional and RCA concrete.
On the other hand, decreasing water to binder ratio decreased the open porosities
and average pore diameters of both types of concrete. Kou et al. (
2011a
) observed
improvement of total porosity of RCAC as curing time increased.
Properties such as water absorption capacity by immersion and capillarity,
chloride and other gas permeation of concrete with CDW aggregate are discussed
in this section.
5.4.2.1 Water Absorption
The water absorption capacity of concrete is an important property, which pro-
vides data on the water accessible porosity of concrete; concrete with high water
absorption capacity is less durable in aggressive environmental conditions. Since
the water absorption capacity of CDW aggregate is higher than that of natural
aggregate, concrete containing CDW aggregate has higher water absorption
capacity than conventional concrete. The water absorption is evaluated by an
immersion test, which measures the open porosity of concrete specimens, and by
capillarity test, which measures the capillary water absorption due to a difference
in pressure occurred between the liquid on the concrete's surface and inside the
capillary pores of concrete. Several references are available on the evaluation of
water
absorption
capacity
of
concrete
containing
various
types
of
CDW
aggregates.
Kwan et al. (
2012
) observed increasing water absorption capacity of concrete as
the replacement level of coarse NA by RCA increased (Fig.
5.64
). They state that
the replacement of 30 % by weight of coarse NA by RCA led to a water absorption
capacity below 3 %, i.e. a concrete considered to have low water absorption
capacity. For an 80 % replacement, the water absorption capacity of RCAC was
2.2 times higher than that of conventional concrete. Rao et al. (
2011
) also observed
a gradual increase of water absorption capacity of concrete with the incorporation
of coarse RCA to replace coarse NA due to the higher water absorption capacity of
RCA, which was about 3.5 times higher than that of NA.
