The petrographic examination of concrete and concrete repairs - Concrete Repair - page 64

affects concretes with or without sulphate-resisting Portland cement
(SRPC). Sulphate attack is also common in concrete exposed to bacterially
derived sulphur dioxide in sewage treatment structures. Figures 2.6 and 2.7
show the development of TSA and ettringite-related sulphate attack in a
concrete repair. In many cases of sulphate attack the transition from severely
weakened concrete to concrete showing little or no evidence for weakening
is sharp and it is important that all the affected concrete is removed prior
to instigating repairs. Petrographic examination can be used to measure the
maximum depth of affected concrete.
Delayed ettringite formation (DEF)
This is a relatively rare cause of on-going damaging expansion encountered
in concrete cured at elevated temperature - usually > 65-75°C resulting
from expansive ettringite formation in the cement paste in the presence of
moisture. Controlling rates of moisture ingress may slow the rate of future
concrete expansion and core expansion testing (BCA, 1988) can be used
as a guide to the maximum future potential for expansion. Petrographic
examination is one of the most reliable means of detecting this form of
deterioration. With concrete placed in situ DEF may only be present in the
concrete at depths where temperatures reached during curing would have
been at their greatest.
Fire damage
Fire damage commonly results in spalling and the development of surface-
parallel cracking. Fire damage may also modify the porosity of the cement
hydrates and generate intense microcracking that may be invisible to
the naked eye. In most cases it is the concrete that has been heated to
temperatures > 300°C that shows the most severe weakening. However,
cracking may develop at much lower temperatures and at greater depths
where reinforcement is affected by temperature increase and undergoes
thermal expansion. A series of changes in the microstructure of the cement
paste and aggregate particles can develop in concrete exposed to fire and
it is possible by petrographic analysis to establish the depth of various
temperature isotherms. The primary use of petrographic examination in
examining fire-damaged concrete is in detecting the maximum depth of
weakened concrete and the depth of concrete that needs to be removed
before repairs can begin ( Figures 2.8 and 2.9 ).
Freeze-thaw damage
Freeze-thaw damage occurs when water-saturated concrete is subjected to
cycles of freezing and thawing and results from the expansion that occurs
when water freezes within cracks or voids. In common with fire damage and
sulphate attack, freeze-thaw damage tends to produce surface-parallel cracks.