Civil Engineering Reference
In-Depth Information
groups were used. The PA was replaced with RA for the following fractions:
2.36-9.5 mm and 9.5-25 mm (both coarse), and 0-2.36 mm. Fine RCA was used
in small amounts and only to improve workability. Families were defined in terms
of the type of cement used (traditional and white Portland) and the age of RA at
crushing. The traditional cement class was lower than the white cement class, and
that had some influence on the results. The hardened concrete was tested for
compressive strength, splitting and flexural tensile strength, modulus of elasticity,
water absorption, carbonation penetration and shrinkage.
6.3.4 Properties
6.3.4.1 Compressive Strength
Compressive strength is the most common tested property of hardened concrete,
and for this reason it was possible to obtain results from four campaigns: Carrijo
(
2005
), Leite (
2001
), Kou et al. (
2004
) and Gomez-Soberón (
2002
). The general
trend identified for this property indicates a reduction of strength with the increase
of the PA/RA replacement ratio.
Figure
6.1
(top) shows the variation of the ratio between the 28- and 90-day
compressive strengths of concrete (f
c
) and the ratio between the densities (D)of
the mixture of aggregates. The correlation coefficient is considered good
(according to the criteria defined in Eq.
6.1
) and a linear relation between the
parameters can be identified. The reduction of the density and mechanical strength
of RA compared with NA, due to the higher percentage of attached mortar in RA,
contributes to the reduction of the ratio between the compressive strengths of
concrete.
The same analysis was performed with the variation of the ratio between the
compressive strengths of concrete and the ratio between the water absorptions (w
a
)
of the mixture of aggregates and is presented in Fig.
6.1
(centre). The correlation
coefficient is not acceptable, indicating however a trend for a linear behaviour
between the ratios.
Figure
6.1
(bottom) shows the variation of the ratio between the 28- and 90-day
compressive strengths of concrete and the ratio between the 7-day compressive
strengths of concrete (f
c7
). The lack of data about the 7-day compressive strength
of the concrete in the campaign of Carrijo (
2005
) excluded the author from this
particular analysis. The negative values in the abscissa axis mean that in the
campaign of Gomez-Soberón (
2002
), some of the results for the 7-day compres-
sive strength of the concrete with RA were higher than the conventional concrete.
This particular behaviour is not to be expected and contradicts most of the
research; the values were nevertheless included in the analysis, contributing to the
reduction of the correlation coefficient, which was, however, acceptable.
