Civil Engineering Reference
In-Depth Information
5.2.1 Workability
The workability of fresh concrete is a very important property, which controls
various other fresh and hardened-state properties of concrete such as density, air-
content and strength. The workability of concrete depends on various properties of
its constituents. The workability performance of concrete containing CDW
aggregate was studied extensively since various properties of CDW aggregate,
which controls the workability of concrete, do not match those of natural aggregate
(NA). The workability of concrete is determined by various methods; the most
versatile one of which is slump.
The slump of concrete containing any type of CDW aggregate should be lower
than that of conventional concrete due to the higher water absorption capacity of
CDW aggregate than that of natural aggregate. The surface texture and angularity
of CDW aggregate have also considerable influence on the workability perfor-
mance of concrete (Buyle-Buddin and Zaharieva
2002
).
Topçu (
1997
) observed 75 mm of slump of concrete containing coarse RCA
generated from crushed concrete, while that of the equivalent conventional con-
crete was 100 mm. A lower workability of concrete containing CDW aggregate
than of concrete containing natural aggregate was reported in several earlier
publications and therefore additional amount of water was added to control the
workability (Topçu
1997
; Rasheeduzzafar and Khan
1984
; Mukai and Kikuchi
1978
; Buck
1977
; Frondistou-Yannas
1977
; Malhotra
1978
; Hansen and Narud
1983
).
Due to the higher water absorption capacity and consequent porosity of CDW
aggregate by comparison with natural aggregate, in several studies, a water pre-
saturation of the CDW aggregate was adopted before adding it to the mix.
However, depending on the pre-saturation technique, the slump varies greatly.
Table
5.1
shows some typical literature data on the slump performance of concrete
containing pre-saturated CDW aggregate along with the water absorption capacity.
Poon et al. (
2009
) studied the slump performance of concrete mixes containing
coarse granite aggregate as well as various amounts of coarse recycled concrete
aggregate (RCA) at three different moisture states: air-dried (AD), oven dried at
105
8
C for 24 h and then cooled down to room temperature prior to mixing (OD),
and saturated surface dried (SSD). The water and aggregate contents were adjusted
to keep the design proportions similar in all concrete mixes.
Due to the higher water absorption capacity of RCA at OD and AD states,
higher amounts of water need to be added to the mix containing this aggregate.
The initial slump (0 min) was measured immediately after mixing and then slump
values were measured in 15 min intervals. The changes in slump of the mixes
containing various types of aggregates are presented in Fig.
5.1
. Their results can
be summarised as follows: (1) the initial slump of the mix containing OD
aggregate was higher than that of the other two types of aggregate; (2) the initial
slump of mix containing OD and AD aggregates increased with their contents in
the mix; (3) the higher slump of the mix containing OD aggregate or with higher
