Civil Engineering Reference
In-Depth Information
ash, silica fume and metakaolin on the permeability of concrete compared to
a control mixture without mineral admixture (2 on Fig. 8.3), with different
efficiencies depending on many parameters (quality and quantity of pozzolan,
time of curing, concrete properties…). Most of the studies concern mixtures
having slightly or much higher strength than control concretes (zone D on
Fig. 8.4). However, the increase in compressive strength alone does not
explain the reduction of permeability of pozzolan concrete, since significant
decreases can be obtained even for equivalent concrete strengths (region near
the interface between zones C and D on Fig. 8.4) (Thomas and Matthews,
1992a). These reductions are often attributed to the formation of smaller
and less permeable capillaries (Manmohan and Mehta, 1981), i.e. a pore
structure refinement due to the pozzolanic reaction, and to a reduction of the
pore continuity (Feldman, 1984; Hooton, 1986). It can be noted that very
few increases in permeability are found when the compressive strengths of
pozzolan concrete are higher than those of control concretes (zone B on Fig.
8.4).
Particular effects of SCMs
GGBS contents
up to 67% are able to maintain permeabilities (±10%) similar
to that of control concrete, even for decreases in strength of up to 30% (zone
C on Fig. 8.4) (Dhir et al., 1996). Significant reductions in permeability
can also be found. For instance, Cheng et al. (2005) obtained 41 and 48%
decreases for GGBS contents of 40 and 60% respectively.
Low-lime and high-lime
fly ashes
used up to the range of 30-50% can
lead to reductions in permeability, especially when the concretes are correctly
cured (Nagataki and Ujike, 1986; Dhir and Byars, 1993a; Naik et al., 1994;
Khan and Lynsdale, 2002).
For
metakaolin
(MK), Bonakdar et al. (2005) showed a continuous decrease
of permeability (-70%) with up to 15% MK. This trend was confirmed by
Shekarchi et al. (2010), who found similar results. Badogiannis and Tsivilis
(2009) and San Nicolas (2011) found an optimum at 10 and 20% respectively.
These differences in the quantity of metakaolin to be used may be related
to the characteristics of the product (e.g. purity and fineness).
Silica fume
is a very active pozzolan which rarely provokes an increase
in permeability. The use of 5 to 10% SF is usually sufficient to divide the
permeability by a factor of 1.5 (Bonakdar et al., 2005; Shekarchi et al., 2010)
or 3 (Nagataki and Ujike, 1986; Batrakov et al., 1992; Zadeh et al., 1998;
Khan and Lynsdale, 2002). Larger replacement rates (17 to 30%), although
they are not easy to achieve in practice, can lead to even larger reductions
(Nagataki and Ujike, 1986; Batrakov et al., 1992). It has also been reported
that silica fume concretes are less dependent on the curing time in water
(Nagataki and Ujike, 1986).
