Civil Engineering Reference
In-Depth Information
their subsequent desorption would favour the reformation of ettringite in the
hardened material, causing expansion and cracking in the concrete during
its service life (Ramlochan et al., 2003, 2004).
General trends
Several parameters are necessary for DEF to be initiated and develop in
concrete, and SCMs can affect most of them. Concrete must be subjected
to a high temperature (65-90 °C), a temperature for which the stability of
ettringite cannot be guaranteed, for a certain duration in the first few days
of its hydration (Taylor et al., 2001; Carles-Gibergues and Hornain, 2008).
It is recognized that fly ash and GGBS reduce the peak temperature of
concrete when they are used in large quantities (Neville, 1995). These SCMs
could thus help to limit the increase of heating by a cement dilution effect,
especially in mass concretes.
The presence of water and humidity are essential to DEF since water
is involved in the diffusion process and in the formation of the reaction
products. DEF is essentially present in structures that are exposed to high
humidity, in contact with water, or subjected to water ingress. The beneficial
effect of pozzolan in the production of a denser and less permeable paste
pore structure (Section 8.2.2) could help reduce the transport properties of
the concrete.
The alkali content in the concrete has an effect on the solubility of ettringite,
since high amounts of alkalis in the pore solution favour its dissolution and
inhibit the precipitation of primary ettringite, making sulphate ions available
for further delayed ettringite formation. Alkalis do not need to be at high
concentrations in the concretes affected. For instance, in a study of eight
bridges affected by DEF (LCPC, 2007), the alkali content in the concretes
ranged from 2 to 4.6 kg/m
3
. A decrease in the amount of alkalis increases
the critical value of temperature causing DEF (LCPC, 2007).
As already stated for ASR, SCMs help to produce low Ca/Si C-S-H that
fix more alkalis than normal C-S-H. All other things being equal, the decrease
of alkali content in the pore solution reduces the initial sulphate uptake by
C-S-H, thus limiting their future availability after desorption (Ramlochan
et al., 2003).
The sulphate and aluminate contents have to be high enough to allow the
formation of delayed ettringite and increase the risk of expansion (Odler
and Chen 1995, 1996; Fu and Beaudoin, 1996). The critical SO
3
content is
within the normal dosage in Portland cement (up to 3-4%) (Taylor, 1998).
The amount of Al
2
O
3
in Portland cement depends on the C
3
A content, and
damage due to DEF has been identified in bridges for quantities of C
3
A as
low as 7% (LCPC, 2007). However, DEF seems to remain insignificant for
C
3
A contents below 5% (Carles-Gibergues and Hornain, 2008).
