Civil Engineering Reference
In-Depth Information
SCMs can sometimes have a negative effect on entrained air voids,
by increasing the demand for air-entraining agent (AEA) to attain a
correct air-void spacing length. At equivalent AEA content, the use of
certain SCMs can lead to larger spacing factors. This effect principally
concerns fly ash (Gebler and Klieger, 1983; Zhang, 1996) but is also
observed with GGBS (Fernandez and Malhotra, 1990; Saric-Coric and
Aitcin, 2003) and silica fume (Khayat and Aitcin, 1992). This could
be due either to a greater surface area or to high carbon content of the
pozzolans. The carbon absorbs a portion of the air-entraining agent, thus
limiting its availability for producing the required air bubble network.
When the carbon content is low, pozzolans have practically no effect
on air void structure (Pigeon et al., 1989).
∑
Water-binder ratio -
Relatively high w/b (0.7-0.8) leads to mediocre
freeze-thaw durability, even though the concrete contains air-entrained
(Gagné and Linger, 2008). Water-binder ratios of less than 0.5 or 0.3
for air-entrained or non-air-entrained concretes, respectively, are usually
needed to reach a correct level of durability. At these levels, there is less
freezable water and concrete has a higher resistance to internal stresses
due to the formation of ice. Moreover, a decrease of w/b tends to decrease
the size of pores, leading to less freeze-water at a given temperature,
since the ice needs a lower temperature to be formed in smaller pores
(Gagné and Linger, 2008). In many cases, SCMs also tend to decrease
the size of pores (see Section 8.2.1), which can be beneficial for frost
resistance.
∑
Maturity of the concrete
-
As for air-void, many frost problems (especially
for surface scaling) are eliminated when an adequate maturity of the
concrete is achieved, since it helps decrease porosity and permeability.
Concretes with SCMs are particularly vulnerable to short curing or to
early freeze and thaw cycles. Numerous examples of SCMs having low
reaction kinetics (e.g. GGBS and fly ash) are reported in the literature
concerning short curing times leading afterward to insufficient frost
resistance: see, for instance, Saric-Coric and Aitcin (2003).
Specific effects of SCMs
Internal cracking and surface scaling should be taken separately when analysing
the effect of SCMs on the frost resistance of concrete conserved in a humid
environment.
Internal cracking
is generally only slightly affected by SCMs
(Fig. 8.9 left) for concretes having equivalent strength and adequate air
void structure. Several authors have noted little or no significant influence
of fly ash (Gebler and Klieger, 1986b; Langley et al., 1989; Naik et al.,
1995a), GGBS (Malhotra, 1983; Sakai et al., 1992), silica fume (Bilodeau
and Carette, 1989; Batrakov et al., 1992; Sabir and Kouyiali, 1991) and
