Civil Engineering Reference
In-Depth Information
14.3.2 Characteristics of recycled aggregates from CDW
To be satisfactory in use and in certain applications, the recycled material must
meet certain requirements of maximum grain size, presence of contaminants
and other requirements of stability and durability. Usually recycled aggregates
have a more elongated, irregular and a rougher surface texture, as well as
being more porous than natural aggregates, and therefore often presenting
a fissured surface (Ravindrarajah and Tam, 1987b; Carneiro et al., 2001;
Zaharieva et al., 2003; Tu et al., 2006).
These characteristics are reflected directly in the water absorption of
recycled aggregates, often with values much higher than those of natural
aggregates. This increased absorption is assigned to the mortar adhered
to the natural aggregate concrete duly recycled (ravindrarajah and Tam,
1985; Hansen, 1986; Katz, 2003; rakshvir and Barai, 2006). However, the
presence of ceramic materials in the composition of recycled aggregate can
lead to a high absorption recycled aggregate. Agrela et al. (2011) obtained
average values of absorption for recycled concrete aggregate (more then
90% of concrete) between 3.7% and 7.1% and for mix recycled aggregate
(ceramic content <30% and concrete content <90% and >70%) between
5.3% and 7.2%. However, for ceramic recycled aggregate (ceramic content
>30% and concrete content <70%) the absorption values are very superior
to those obtained for the other two groups, ranging from 9.9% to 13.5%.
According to Tam et al. (2005), aggregates with high absorption rates
usually lead to concrete with lower performances, thus affecting properties such
as strength, durability, creep and shrinkage. As for the red ceramic recycled
aggregates, it appears that the porosity thereof is directly proportional to the
porosity of the ceramic artifacts created and, consequently, the mechanical
strength of the same (Schulz and Hendricks, 1992).
The specifications of Rilem (1994) set a ceiling for the water absorption of
recycled concrete and ceramic coarse aggregate, 20% and 10%, respectively.
As for Japanese Standards, the maximum absorption of recycled concrete
coarse and fine aggregate should be 7% and 13%, respectively (Hansen,
1992). According to NBr 15116, the maximum absorption for coarse and
fine RCA are 7% and 12%, respectively. For aggregates of mixed waste, these
rates rise to 12% and 17% for the coarse and fine aggregates, respectively.
Another striking feature of these recycled aggregates is the speed with
which water is absorbed. According to Schulz and Hendricks (1992), within
30 minutes of submersion in water, recycled ceramic aggregates absorb 98%
of all the water absorbed in 24 hours of immersion. It is not different for
the recycled aggregate concrete: Bairagi et al. (1993) state that for this type
of aggregate, absorption also occurs very quickly in the first 30 minutes
of submersion, and in their experiment it absorbed 76% of all the water
absorbed in 24 hours, while for 4 hours of submersion, the stated figure
rises to 94%. Figure 14.4 shows the behavior of water absorption by time of
