Civil Engineering Reference
In-Depth Information
a logarithmic scale and why the difference between the permeability of two
distinct concretes must be substantial (a factor of about 10) to be statistically
significant.
General trends
The water content and the curing time greatly influence the permeability
of cement-based materials. Generally speaking, these two parameters are
more important regarding permeability variations than the effect of cement
replacement by a mineral admixture. In the latter case, the measured
permeability usually varies by a factor of 1 to 10 (see Fig. 8.3), while it has
been shown that the variations of w/b (e.g. 0.30 vs. 0.70) combined with
different curing times (e.g. > 7 days vs. < 1 day) can lead to permeability
variations of several orders of magnitude (Dhir et al., 1989).
The ITZ is sometimes regarded as a key feature governing the permeability
of mortars and concretes (e.g. (Winslow et al., 1994; Garboczi and Bentz,
1996). The beneficial effect of pozzolans on the ITZ (Bentur and Cohen, 1987;
Xu et al., 1993; Gao et al., 2005), i.e. the densification of the microstructure
and the improvement in the mechanical properties of the bond by a decrease
in the quantity of portlandite crystals and a modification of their orientation
arrangement at the ITZ, could partly explain the reduction of permeability
when pozzolans are used.
Although results in the literature about the effect of SCMs on permeability
are quite dispersed, some general trends, schematized on Figs 8.3 and 8.4,
can be highlighted. On the one hand, some rare results, involving mainly fly
ash or GGBS, imply an increase in the permeability with the use of mineral
admixture (1 on Fig. 8.3) (Kasai et al., 1983; Shi et al., 2009; Elahi et al.,
10
2
n
1
Minearl admixture
vs control
Lower strength
Equivalent or
higher strength
1
10
n
2
2
n
0
Mineral admixture content (%)
8.3
Schematic representation of the mineral admixture effect on
concrete permeability (
n
= -18 (gas permeability) or -20 (water
permeability).
