Concrete Chemistry and Protective Coatings
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Permanent Coatings
There are also more permanent coatings
called cure and seal coatings. These are
typically water-based styrene acrylic or
acrylic emulsions which are applied while
the concrete is wet but then form a more
permanent film over the concrete. These
coatings slow moisture loss during the
early stages of curing, but they also have
some benefits as sealers. They can give
protection from stains and can help minimize dusting from floor abrasion. In addition, they can give protection from some
types of degradation caused by moisture
and certain salts. Cure and seal coatings
are tested by measuring the weight loss of
water from a coated, curing concrete test
block. ASTM C309 specifies less than
0.55 kg/m2 of water loss, and the more
restrictive ASTM C1315 specifies less
than 0.40 kg/m2 over 72 hours (in addition to UV durability and non-yellowing
characteristics). Specially designed styrene acrylics can meet the ASTM C309
specifications at a relatively low cost and
acrylics are often used in the more durable, non-yellowing cure and seal coatings
designed to meet ASTM C1315 Class A.
Failure Modes
Although it is very durable under UV
light and plain water, cured concrete is
subject to several types of chemical attack which can degrade the surface or
cause cracking. In most cases salts carried into the concrete by water can cause
the degradation.
When there is an outside source of
sulfate ions from ground water or exposure to sea water, then ettringite can be
reformed from the monosulfate. This is
an expansive process because the ettringite is a larger volume than the monosulfate. The expansive sulfate attack
reaction puts stresses on the concrete and
can lead to cracking, a loss of the bond
between the cement paste and the aggregate and this results in a significant loss
of strength. Resistance to sulfate attack
is built through the use of low water/ce-ment ratios (lower porosity), the use of
silica fume or fly ash possolans, or by using surface coatings. 8
Concrete is also subject to degrada-
tion from an alkali silicate reaction (ASR)
if there is incorrect selection of aggregate.
Some silicate minerals used as aggregate
will react with sodium or potassium alka-
li in high pH conditions to form a higher-
volume product which can put expansive
stresses on the concrete and cause crack-
ing. While the alkali metals sodium and
potassium that are present in cement are
typically low, external sources of alkali
like deicing salts or sea water can lead to
ASR. Reducing the ingress of external
alkalis through less permeable concrete
(lower w/c) or using coatings or sealers
can reduce the potential for the ASR. 3
Chloride ions can also be a signifi-
cant problem in steel reinforced concrete.
Steel reinforcement rods in wet concrete
quickly form a protective rust layer at
the surface which protect the rods from
further degradation. The chloride ions
destroy the protective Fe2O3 passivating
layer on steel and, if there is water and
air present in the concrete pores, then an
electrochemical current is set up and rust
is formed at the anode. In addition to
causing pitting in the steel, the rust has a
larger volume than the steel and it gives
expansive pressure on the surrounding
concrete and can cause cracking of the
concrete. 5 Decreasing the porosity with
a lower water to cement ratio and the use
of fly ash can mitigate chloride attack,
however these preventative measures are
often used in conjunction with concrete
coatings or coatings for the steel surface.
If deicing salts are used, they can
collect in the voids at the surface of the
concrete. The crystallization of the salts
can put stress on the surface layers of
concrete and cause scaling. This is worse
in concrete exposed to repeated wet/dry
cycles. Coatings which protect the sur-
face from moisture can prevent this type
of degradation. 4
Concrete can also degrade on ex-
posure to acid. Acids can dissolve both
hydrated and unhydrated cement com-
pounds and can dissolve calcium carbon-
ate (calcareous) based aggregate. Acid
rain (pH 4. 5-5) can lead to slight etching
of the surface but does not significantly
affect performance. Reducing the poros-
ity of the concrete with coatings can re-
duce the degradation. 4
Another common concrete failure
is caused by freeze thaw cycles. The
freezing of water in the pores creates
stresses in the concrete which can lead
to spalling. It is common to use air
entraining additives when mixing wet
concrete to prevent damage from freeze
thaw cycles. The entrained air pockets
in the dried concrete give water room
to expand and prevent damage from
freezing. Another way to help prevent
damage is to reduce the porosity which
can help prevent the ingress of water.
The porosity can be reduced by reduc-
ing the water to cement ratio as much
as possible or by using sealers.
Sealers
Polymeric sealers can help prevent the
chemical or physical degradation of concrete. Sealers are typically described as
penetrating or film forming and are differentiated from cure and seal coatings
since they are applied after the concrete
has cured for 28 days.
Silanes or siloxanes are penetrating
sealers. They do help seal the surface
against liquid water and stains but they
do not prevent water vapor loss from the
concrete during cure. The silanes have a
low molecular weight while the siloxanes
are prereacted and have an EO chain giving higher molecular weight. Because of
the lower molecular weight, the silanes
can give more penetration into the pores
than the silanes. Since they are beneath
the surface, they are more abrasion resistant than the aqueous sealers which form
a film on the surface.
Aqueous emulsions used as sealers
can also help prevent ingress of salts and
moisture which can lead to degradation.
These are typically acrylic or styrene
acrylic and have good resistance to liquid
water but are breathable and allow vapor
to pass through the film. In addition to
water resistance, water whitening resistance is also needed to avoid discoloration
if exposed to puddled water. Polymers
need to be designed to be blush resistant
by keeping the particle size small, using
special polymerizable surfactants and
keeping the level of other hydrophilic
materials (such as surfactants) low. These
emulsion sealers are typically designed to
be applied at low dry film thicknesses and
