Civil Engineering Reference
In-Depth Information
Fig. 5.3 Effect of replacement ratio (%) and type of aggregate on the air-content of concrete
containing CDW aggregate
Table 5.7
Properties of two types of RCA aggregate (Lopez-Gayarre et al. 2009)
Aggregate
name
Size
(mm)
Dry
density
(kg/m
3
)
24 h water
absorption
(%)
Los Angeles
coefficient (%)
Attached
mortar content
(%)
Declassified
content (%)
OV
4-20
2,200
5.0
37.2
34.2
2.6
MA
4-20
2,360
3.8
33.1
23.0
1.5
Sagoe-Crentsil et al. (
2001
) observed no difference in air-content between concrete
containing normal aggregate and concrete containing RCA aggregate.
5.2.4 Bleeding
Bleeding of concrete is the upward movement of water during settling of concrete
mix. It is a particular form of segregation, in which some of the water within
concrete comes out of its surface. Sometimes, with this water along comes a
certain quantity of cement. A higher bleeding to the surface increases the water to
cement ratio and therefore decreases the strength of the concrete surface. The
increase in capillary porosity of hardened concrete due to bleeding can also affect
the durability performance. However, in some cases, the bleeding water may not
come up to the surface and is trapped by flat or flaky pieces of aggregates and also
by reinforcement and accumulates below such obstacles. This is known as internal
bleeding, which can affect several properties of hardened concrete such as cement
paste-aggregate bonding and enhance micro cracks. High bleeding of concrete
occurs due to various factors such as high water to cement ratio, low cement
content, coarse cement particles and poorly graded aggregates. Few references are
available on the bleeding behaviour of concrete containing CDW aggregate.
The old cement paste on the surface of the recycled aggregate can absorb some
mixing water. Thus, the total amount of bleeding of concrete decreases as the
