Geoscience Reference
In-Depth Information
DAMAGED CONCRETE
The most significant reaction is that involving the
reaction of calcium hydroxide to calcium carbonate and
therefore carbonation of concrete is generally
summarized as:
Ca(OH)
2
O
VERVIEW
Although most concrete is durable and will achieve good
design life service, it can sometimes suffer from distress.
Dense, good-quality concrete is highly resistant to
weathering owing to its low permeability. However, most
external concrete surfaces will eventually show evidence of
deterioration caused by weathering mechanisms. The type
of mechanisms operating will depend on the environment
of use and may variously include driving and/or acid rain,
cyclic wetting and drying, thermal cycling, leaching,
freeze-thaw mechanisms, and salt crystallization.
Examination of the microscopic appearance and textures
of concrete gives a good indication of its condition and
enables screening for evidence of deterioration. Concrete
can sometimes suffer from deterioration caused by
deleterious reactions or attack by aggressive agents. The
petrographical study of crack and microcrack patterns,
mineralogical changes, and secondary deposits provides
crucial diagnostic information for determining causes of
concrete distress. When investigating concrete
deterioration it is important to establish whether the
mechanisms of deterioration are intrinsic or extrinsic, as
this will have important implications for repair and
remediation of the affected concrete structure. Intrinsic
mechanisms (e.g. AAR) are essentially internal reactions
involving deleterious substances cast into the concrete at
the time of construction and, consequently, can affect the
whole section of the concrete member. In contrast,
extrinsic mechanisms (e.g. the thaumasite form of sulfate
attack [TSA]) involve agents from outside the concrete and
produce a zone of deterioration confined to near the outer
surface.
+
2
CaCO
3
+
2
O
→
Calcium hydroxide
Calcium carbonate
Carbonation usually advances inwards from the exposed
concrete surfaces to form a carbonated layer (with a
carbonation front). It may also advance along cracks or
be associated with porous aggregate particles. The rate
of carbonation depends on the environmental conditions
and permeability of the concrete and can be predicted by
modelling if these factors are known. The environmental
conditions likely to promote high rates of carbonation
are relative humidity in the 50-75% range, high carbon
dioxide concentrations, and high temperature.
Carbonation has important implications for the durability
of concrete, notably reinforcement corrosion. Carbon-
ation causes the alkaline protection of steel
reinforcement bars to be lost by lowering the pH from
13-14 in uncarbonated concrete to 8.6 in fully
carbonated concrete. In damp conditions this can cause
deleterious corrosion of steel reinforcement.
Depth of carbonation is commonly assessed by spraying
freshly broken concrete surfaces with phenolphthalein
indicator solution (phenolphthalein in a mixture of ethanol
and water), which stains concrete of >pH 9 purple. This
indirect staining method has the advantage of being quick
and inexpensive. However, it also has the disadvantage of
detecting only fully carbonated cement paste, and as
partially carbonated areas are missed, tends to
underestimate the maximum depth of carbonation (St John
et al
., 1998). Direct optical microscopical examination (in
thin section) is the definitive method for determination of
carbonation depth as both fully and partially carbonated
cement paste are readily observed. Depth of carbonation in
relation to the outer surface and the position of
reinforcement are routinely assessed during petrographic
examination of hardened concrete.
In thin section, carbonated cement paste is readily
detected by the presence of clumps of calcium carbonate
crystals. Calcium carbonate (CaCO
3
) has three
polymorphic forms, these being calcite, aragonite, and
vaterite. Calcite, the most stable, is the ultimate form
found in carbonated concrete. In terms of optical
properties, calcite is extremely birefringent, exhibiting
pale high-order interference colours in cross-polarized
light. Figure
193
shows the appearance of carbonated
cement matrix in comparison with an uncarbonated area.
For a particular part of the cement matrix the degree of
C
ARBONATION AND REINFORCEMENT CORROSION
Carbonation of concrete is a reaction between carbon
dioxide (CO
2
) from the atmosphere and the various
components of the hardened cement matrix to form
carbonate minerals, as follows:
Portland cement
Carbonation reaction
matrix component
products
Calcium hydroxide
Calcium carbonate and water
Calcium silicate hydrate
Calcium carbonate, silica gel,
and water
Calcium aluminate
Calcium carbonate, alumina
hydrate
gel, and water
Hydrated ferrite phases
Calcium carbonate, ferric
oxide, alumina gel, and water
Ettringite and calcium
Gypsum, alumina gel, and
monosulfate
water
