Civil Engineering Reference
In-Depth Information
In this specification, the mix composition, proposed for concrete prepared by
replacing 100 % coarse NA by RA is: Portland cement: 100 kg; fine aggregate:
180 kg; RA with 20 mm size: 180 kg; RA with 10 mm size: 90 kg.
7.2.3.2 Workability
In this specification, only pre-soaked RA is allowed to be used in concrete prep-
aration. This measure seeks to avoid the change of actual water to cement ratio due
to the higher water absorption capacity of RA than that of NA, which will affect
the cement hydration reaction. It is also recommended that the slump of RAC must
be greater or equal to 75 mm at the time of casting.
7.2.3.3 Compressive Strength
This specification also suggested some recommendations for testing the com-
pressive strength of RAC. Accordingly, to determine the strength of concrete with
100 % replacement of coarse NA by RA, at least four concrete cubes should be
prepared in the same day to determine the 7- and 28-day compressive strengths;
out of these, two should be used to determine the 7-day compressive strength and
the other two the 28-day compressive strength. These cubes should have a mini-
mum compressive strength of 14 and 20 MPa at 7 and 28 days respectively. This
imposition is unique and deserves to be studied in detail. However, a major
drawback is the limitation on the use of large amounts of RAs in the preparation of
concrete with CS above 20 MPa.
7.2.3.4 Requirements for RA
The coarse RA should be produced by crushing old concrete and comply with the
requirements presented in Table
7.8
. This specification prohibits the use of fine RA
Table 7.8
Requirements of coarse RA (WBTC No. 12/2002)
Properties
Limits
2,000 kg/m
3
Dry density of aggregate (minimum)
Water absorption capacity (maximum) (%)
10
Wood or similar material less dense than water (maximum) (%)
0.5
Other contaminants (maximum) (%)
1
Particles finer than 63 lm (maximum) (%)
4
Particles finer than 4 mm (maximum) (%)
5
Sulphate content (maximum) (%)
1
Chloride content (maximum) (%)
0.05
