Civil Engineering Reference
In-Depth Information
Fig. 5.1 Changes in slump of concrete containing various types of aggregate (Poon et al.
2009
) a
100 % crushed granite, b 80 % crushed granite ? 20 % recycled aggregate, c 50 % crushed
granite ? 50 % recycled aggregate, d 100 % recycled aggregate
content of OD and AD aggregates was due to the increase in free water in the mix;
(4) the initial slump of the mix containing RCA in SSD state was almost constant
for all replacement levels of natural aggregate; (5) the slump loss was faster and
slower for mixes containing OD and SSD aggregates, respectively.
Poon et al. (
2009
,
2004a
) also reported that the replacement of natural coarse
aggregate by RCA in concrete prolonged the slump loss time. The slump value and
the rate of slump loss with time of mixes containing RCA respectively increased
and decreased with the addition of fly ash (Poon et al.
2004a
). On the other hand,
Thangchirapat et al. (
2008
) observed faster water loss in mixes containing coarse
RCA aggregate than in the mix containing natural aggregate, when water was
added to reach the required slump range of 50-100 mm. Sagoe-Crentsil et al.
(
2001
) did not observe any difficulty in achieving the desired workability and
subsequent compaction of concrete mixes containing RCA even though the
amount of water absorbed by RCA was higher than for basalt aggregate.
Several mixing methodologies were developed to control the workability-
related problems of concrete containing CDW aggregate. They can be summarised
as: (1) increasing the amount of added water according to the water demand of
concrete mixes containing dry CDW aggregate; (2) pre-soaking CDW aggregate in
water for 10-20 min or for 24 h before use; (3) increasing the moisture content of
CDW aggregate up to 70-80 % of total water absorption capacity for 24 h before
followed by covering with plastic to control water loss due to evaporation; (4)
increasing the super-plasticizer amount; (5) increasing cement content in the
concrete composition.
