Geoscience Reference
In-Depth Information
Wet concrete can be damaged by freeze-thaw cycles in
temperate and cold climates. Frost attack causes irregular
cracks to form subparallel to the exposed concrete surfaces
or corners. The cracks normally run through the cement
matrix and around coarse aggregate particles.
Petrographically, cracking is the main feature observed but
the concrete may also exhibit some degree of leaching.
There is another type of cracking induced by freeze-thaw
cycles, termed 'D-cracking' that is linked to the coarse
aggregate. Coarse aggregates from certain sedimentary
rock sources (both natural gravel and crushed rock) are
susceptible owing to the characteristics of their pore
structure. D-cracking manifests itself after 10-15 years of
exposure, as fine cracks near the edges (and joints) of
slabs, which run through both the coarse aggregate and
the cement matrix, parallel to the edges/joints.
Frost has undesirable effects on fresh concrete. If
concreting has been undertaken in cold weather, slower
than normal hydration may lead to the formation of a
relatively porous cement matrix, with a lighter than
normal colour for the given W/C. Placing concreting in
freezing conditions can cause the concrete not to set
properly, resulting in a weak and friable product.
Concrete that is exposed to salts is prone to salt
weathering. Salts may originate naturally from sea water,
sea spray, coastal air, ground/groundwater or be applied
by man as de-icer salts. Salt crystallization causes flaking
and scaling of concrete surfaces up to several millimetres
at a time, often associated with white salt deposits.
During sample collection and preparation care must be
taken not to lose soluble salts from contact with water.
calcium aluminate phase of the cement matrix to form
ettringite (calcium sulfoaluminate) as follows.
Calcium
aluminate + CaSO
4
.2H
2
0
3CaO.Al
2
O
3
.3CaSO
4
.32H
2
O
→
hydrates
The other sulfate salts (ammonium, magnesium, sodium,
potassium) react with calcium hydroxide (portlandite) in
the concrete to form calcium sulfate (gypsum) as shown
below. This calcium sulfate may then interact with the
calcium aluminate phase to form ettringite, as in the
reaction detailed above (St John
et al
., 1998).
(NH
4
)
2
SO
4
+ Ca(OH)
2
+ 2H
2
O
→
CaSO
4
.2H
2
O + 2NH
3
MgSO
4
+ Ca(OH)
2
+ 2H
2
O
CaSO
4
.2H
2
O + Mg(OH)
2
→
Na
2
SO
4
+ Ca(OH)
2
+ 2H
2
O
→
CaSO
4
.2H
2
O + 2NaOH
K
2
SO
4
+ Ca(OH)
2
+ 2H
2
O
CaSO
4
.2H
2
O + 2KOH
→
Magnesium sulfate also attacks the calcium silicate phases
of the cement matrix to form gypsum, brucite (magnesium
hydroxide), and hydrated silica. Below a pH of 10.6, it may
also attack ettringite to form more gypsum, brucite, and
hydrated alumina (St John
et al
., 1998).
Sulfate attack from an external source of sulfates (such
as groundwater) will exhibit a zone of deterioration that
works inwards from the surfaces exposed to the sulfates.
Deterioration manifests itself, firstly, by causing cracking
associated with the expansive formation of ettringite
and/or gypsum and, secondly, by the dissolution and
weakening of the cement matrix. Petrographically, the
cracks and microcracks will be observed in thin section
along with secondary deposits of sulfate minerals (usually
ettringite or gypsum) filling cracks and air voids. Figure
203
shows concrete from a pile cap of a flyover that
exhibits evidence of sulfate attack consisting of a network
of microcracks and air voids filled with ettringite. Ettringite
is identified from its small, needle-like crystals, first-order
grey interference colours, and its colourless appearance in
plane-polarized light. Fluorescent microscopy can be used
to highlight the cracks produced by deterioration and
Figure
204
shows this for an example of a pile from a
building suffering from external sulfate attack.
Guidance regarding the design of buried concrete
elements that may be exposed to sulfates of other
aggressive ground conditions is given in BRE Special
Digest 1 (Building Research Establishment, 2005).
S
ULFATE ATTACK FROM GROUNDWATER
Sulfate attack is a term used to describe a series of
deleterious chemical reactions between sulfate ions and the
components of hardened concrete, principally the cement
matrix, caused by exposure of concrete to sulfates and
moisture (Skalny
et al
., 2002). The sulfates of greatest
concern for the durability of concrete are salts found in
natural soils and groundwaters such as sulfates of sodium,
potassium, magnesium, and calcium. In addition,
groundwater that has been contaminated with fertilizer or
industrial effluent may also contain ammonium sulfate.
The mechanisms of sulfate attack are complicated,
involving a number of overlapping chemical reactions,
which are not yet fully understood. However, it is known
that the extent of attack depends on the amount of sulfate
in solution and that the aggressiveness of the sulfate salts
is broadly related to their solubility. The solubility of sulfate
salts running from most to least is: ammonium,
magnesium, sodium, potassium, and calcium. Calcium
sulfate (found in gypsum-bearing soils) attacks only the
T
HE THAUMASITE FORM OF SULFATE ATTACK
(
TSA
)
TSA is a special form of external sulfate attack that can
lead to particularly severe deterioration in buried
concrete. In conventional sulfate attack, incoming
sulfate ions react with the calcium aluminate phases
and calcium hydroxide in the cement matrix to form
