REGULARS: TECHNICAL TALK
Soft or pure water attack: Waters with
very low ion contents will cause the
dissolution of the hydrates within
concrete. Soft or pure waters are often
found in waters that have not come into
contact with calcareous rock.
Chemical conversion of
hydration products
Corrosion of steel reinforcement is a major cause of concrete deterioration, particularly in
harsh coastal environments.
cover to it. However, what is sometimes
overlooked is the environment into which
the concrete is placed – a factor that could
substantially negatively affect the planned
durability targets.
Listed below are a few of the better-known
mechanisms that could lead to deterioration
of concrete. Durability issues, generally, can
be categorised under two main headings:
mechanical (and physical) deterioration and
chemical deterioration. Chemical
deterioration, furthermore, can be
subdivided into the chemical dissolution of
the hydration products and the chemical
conversion of the hydration products.
Mechanical (and physical)
deterioration mechanisms
Erosion and abrasion: Erosion is caused by
fluids, most often water, containing abrasive
particles wearing down the surface of the
concrete. Abrasion is caused by wheeled
traffic on floors and pavements as well as
abrasive material, as in ore passes and silos,
wearing down the surface of the concrete.
Strong concrete with little surface laitance,
well cured and finished off along with
abrasive resistant large aggregate will slow
down the rate at which the concrete surface
will wear. Specialist surface hardeners will
also help reduce wear.
Cavitation: The collapse of very high
pressure vapour bubbles, created by fluid
jumping off the surface of the concrete due
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to sudden changes in direction of the fluid,
can lead to massive damage to the concrete
within a short period of time. Aerating the
fluid – such as often done on dam spillways
– to prevent pressure build-up is an effective
way of preventing cavitation.
Salt crystallisation: This is often called
‘salt jacking’ and tends to occur along the
coast where the concrete is subject to high
concentrations of salt. Salt in solution is
transported into the concrete and, when the
concrete dries out, it re-crystallises with the
resulting expanding salt crystal exfoliating
the surface of the concrete.
Freeze-thaw: Ingress of water into the
concrete that subsequently freezes can
cause the formation of cracks due to the
expansion of the ice. Air entrainment can
effectively prevent this. The size of the air
bubbles and, more importantly, the spacing
of the bubbles are key factors in how
effective the air entrainment will be.
Chemical deterioration
mechanisms
Dissolution of hydration products
Acid attack: The calcium in the concrete
hydrates have a high solubility in acids. Often
a sacrificial calcareous aggregate is used to
get an even wear and slower disintegration
of the concrete surface. With strong acids, a
sacrificial layer or an acid resistant coating is
more effective in protecting the concrete.
Alkali Silica Reaction (ASR): Reactive
silicas contained in the aggregate can
react with active alkalis within the
concrete pore structure. This forms an
expansive gel that results in the concrete
cracking. Avoiding aggregates containing
reactive silica, minimising the active
alkalis within the concrete and
preventing the ingress of water into the
concrete are effective measures in
preventing ASR.
Sulphate attack: A combination of
sulphates, calcium hydroxide and
tricalcium aluminates in the concrete can
lead to the formation of two expansive
compounds: gypsum and ettringite. This
can cause the concrete to soften and
crack. Using sulphate resisting cement or
extending the cement with fly ash or slag
can help prevent sulphate attack on the
concrete. Also ensuring the sulphate
content in the aggregate and mixing
water is kept low and preventing the
ingress of water into the hardened
concrete will prevent sulphate attack.
Interestingly, all of the above
described deterioration mechanisms can
be mitigated or even prevented by
designing the concrete mix with a lower
water to cement ratio and implementing
good site practice by ensuring excellent
compaction and optimising curing. By
doing this, the concrete will be less
permeable which is key to a more
durable concrete.
Although the mechanisms mentioned
above are by no means exhaustive with
regards to concrete’s durability, they
serve to illustrate the wide spectrum of
environments that concrete may be
subjected to. There are many well
researched methods of ensuring that
concrete will perform for its service life
within any specific environment it is
place, however it is important that
engineers and concrete practitioners
should take heed of these environments
or potential durability problems.
CLADDING // CONCRETE // INSULATION // STEEL // THATCH // TIMBER // TRANSLUCENT // WATERPROOFING // COMPONENTS
NOVEMBER 2018
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