Civil Engineering Reference
In-Depth Information
Therefore, when much of the natural aggregates are replaced by recycled
aggregates, the concrete has a higher drying shrinkage, since recycled
aggregates have a lower modulus of deformation than natural aggregates,
being more deformable. They also have high water absorption, requiring
higher water content that leads to increase the drying shrinkage (Poon et
al., 2002). Brito and Alves (2010) blame the higher shrinkage of hardened
concrete due to the higher porosity and lower stiffness of the recycled
aggregates.
recycled concrete aggregate generally consists of 60-70% of its volume
by natural aggregate and 30-40% by mortar, being the latter much more
porous than the previous one (Hansen and Narud, 1983). This high content
of mortar causes the appearance of some undesirable effects on the recycled
concrete, such as an increase of drying shrinkage, thereby enhancing the
appearance of cracks. According to Tavakoli and Soroushian (1996), the
greater the amount of adhered mortar in recycled concrete aggregate,
the greater the possibility of having high drying shrinkage in recycled
concrete.
Therefore, it is common to try to release the mortar from the recycled
concrete aggregate. However, the procedure that is often used to accomplish
this is to put recycled aggregates in a concrete mixer and let them mix alone.
The friction between the concrete aggregates and between the recycled
concrete aggregate's mortar and the mixer walls makes the mortar come off
(Tavakoli and Soroushian, 1996). According Hansen (1992), this process
also improves the aggregate's shape, contributing to a better mix.
Drying shrinkage also increases as the content of natural aggregate are
replaced by recycled aggregate (limbachiya et al., 2000; Gómez-Soberón,
2002, 2003; Khatib, 2005). It is also dependent on the quality of the concrete
from which the recycled aggregates came. The greater the resistance of the
original concrete, the greater the shrinkage of concrete made with their
recycled aggregates. This is probably occurs due to the high content of
mortar in higher-strength concretes (ravindrarajah and Tam, 1985).
It is generally accepted that recycled concrete made with the recycled
concrete coarse aggregate performs better on the drying shrinkage than concrete
made with recycled ceramic coarse aggregate (Khalaf and DeVeeny, 2004).
The biggest drying shrinkage presented by recycled ceramic concrete can be
explained due to the lower resistance offered by these recycled aggregates
to deformations of the concrete's cement paste, once these aggregates have
a lower modulus of deformation than recycled concrete aggregates (Schulz
and Hendricks, 1992).
One may associate the drying shrinkage of recycled aggregate concrete
with the type of recycled aggregate, since the more porous the recycled
aggregate is, the less restricted is the shrinkage, thus allowing the concrete
to retract further. Hence, concrete produced with recycled aggregates
