Geoscience Reference
In-Depth Information
221
minerals (e.g. feldspars, phyllosilicates) have been found
to release alkalis into concrete (Goguel & Milestone,
2007). Figure
221
shows ASR in concrete with granite
coarse aggregate. During ASR, hydroxide ions may
attack feldspars and micas in granite to release silica,
alumina, and alkalis that contribute to continued ASR in
the concrete (Yan
et al
., 2004). An external source of
alkali found in highway structures is de-icing salt
(Katayama
et al
., 2003). De-icing salt is halite (sodium
chloride) and in solution this may react with portlandite
in the cement paste to release alkali as follows:
2NaCl
+
Ca(OH)
2
2NaOH + CaCl
2
→
221
ASR associated with granite coarse aggregate. A
granite particle (right) exhibits internal cracking
(yellow) and the crack continues into the surrounding
concrete where it is seen to be filled with alkali-silica
gel (light grey); PPT, ×35.
Sodium
Portlandite
Alkali
chloride
It is thought that rather than causing ASR, alkali from
de-icing salts accelerates the reaction once it has already
been initiated by alkali from inside the concrete (West,
1996). Figures
222
and
223
show concrete suffering
from ASR that has been contributed to by de-icing salts.
This particular example is described in a publication by
Dunster (2002a, Case Study D) and involves crosshead
beams of an elevated link road, which exhibited ASR
only on outer surfaces and in drains that had been
heavily contaminated with de-icing salt. The reactive
aggregate constituent was flint (chert), of 'normal'
reactivity. A combination of petrography and chemical
analysis was used to determine that ASR had only
occurred in areas of the concrete where the alkali content
was >5 kg/m
3
Na
2
Oeq.
222
222
ASR in concrete contributed to by de-icing salts.
Flint (mottled brown/grey) aggregate particles exhibit
internal cracking (blue) and cracks in the cement
matrix are filled with alkali-silica gel (white); PPT,
×35.
223
223
Close view of alkali-silica gel (white) formed at
the edge of a flint aggregate particle (light brown) in
concrete suffering from ASR (related to de-icing salt
exposure). The gel exhibits desiccation cracks (blue);
PPT, ×150.
