Construction and Demolition Waste Aggregates - Recycled Aggregate in Concrete - page 87

in Fig. 3.3 (De Juan and Gutiérrez 2009 ), which shows that the thermal method
gives the lowest value and the acid treatment method the highest one.
Apart from increasing the mechanical processing steps, several other methods
are proposed to remove the mortar content and therefore improve the quality of
CDW aggregate (Akbarnezhad et al. 2011 ). These are thermal treatment,
mechanical treatment, thermal-mechanical treatment, acid soaking, chemical-
mechanical treatment and microwave-assisted treatment.
3.5 Properties of CDW Aggregate
3.5.1 Density/Specific Gravity
Density is one of the fundamental parameters of aggregate and is important to
design concrete mixes and control several properties of the resulting concrete.
Table 3.4 shows the density of recycled aggregates reported in several works. The
density of CDW aggregate is lower than that of natural aggregates. This is due to
the existence of porous and less dense cement paste in the CDW aggregates. Due
to their origin and size, CDW aggregates may have different densities depending
on the amount of adhered mortar paste. The CDW aggregates with less mortar
paste have higher density than the CDW with higher content of cement paste
(Fig. 3.4 ).
Poon et al. ( 2004a , b ) reported a lower density for CDW aggregate obtained
from high performance concrete (HPC) than that of CDW aggregate made from
normal strength concrete (NC). Both concrete types were prepared by using the
same type of granite aggregate but the HPC contained fly ash and silica fume as
mineral additions. Santos et al. ( 2002a ) found slight variations in the density
parameters along with other properties of two types of the CDW aggregates
prepared from two concrete types with different 28-day compressive strengths of
45 and 56 MPa (Table 3.4 ). The mortar contents in the 56 MPa and 45 MPa
concretes were, respectively, 36.3 and 49.4 %. Gomez-Soberon ( 2002 ) found that
the density of CDW aggregate increased with increasing particle size. De Brito and
Robles ( 2010 ) and De Brito and Alves ( 2010 ) reported that the density of the
mixture of CDW and normal aggregates showed high correlation coefficients in the
graphical analysis for the various hardened concrete properties.
The bulk density of CDW aggregate is also lower than that of normal aggre-
gates (Table 3.5 ). The bulk density of CDW aggregates is generally in the range of
1,150-1,400 kg/m 3 with a few exceptions. According to Ferreira et al. ( 2011 ), the
lesser bulk density of CDW aggregate compared to natural aggregate is due to the
greater volume of voids between particles in CDW aggregate. Ferreira et al. ( 2011 )
found 48.8 and 50.4 % of bulk void contents in the natural and CDW aggregates,
respectively.