This exercise requires attention at various
stages of design and concrete construction.
It can be painful to see cracks in otherwise
good structures. Cracks are quality control problems, regardless of whether
they are superficial surface cracks or deep cracks that endanger water
tightness and structural integrity. Controlling cracking requires attention at
various stages of design and concrete construction. Cracking can be
significantly reduced by considering the basic causes and by adopting
preventive steps to control them.
There are several types of cracks:
This is a system of shallow cracks, mostly
resulting in cosmetic problems. They develop at early age. They manifest
themselves when the concrete is drying after the surface had been wet. They can
be treated with any good surface treatment method and do not cause any structural
problems. They are rarely visible when treated properly.
Water evaporates from the surface of any
freshly made concrete. If the rate of evaporation is faster than the rate at
which it is replaced by bleed water, the surface of concrete will shrink. They
are caused by the shrinkage of drying surface on concrete that is restrained by
wet concrete in the interior where shrinkage is absent. This action produces
tensile stresses in the surface layer resulting in shallow cracks which are of
varying depth and fairly wide at the surface.
The following factors contribute to excessive
plastic shrinkage cracks:
• High water to cement ratio
• Over –vibration which brings too much of
paste to the surface.
• Too much of floating or trowelling for
finishing
• Sprinkling of dry cement on a finished
surface to absorb bleed water
• Poor curing that permits rapid surface
drying, especially in summer
Control measures to avoid this problem
include use of lowest possible water-cement ratio consistent with workabilityrequirement, avoiding over-vibration and excessive floating before bleed water
dries up and curing properly without allowing wet and dry spells on concrete
surface.
In spite of considerable knowledge about the
advantages of low water-cement ratio, almost all concrete is mixed with more
water than what is needed for cement hydration. The excess water evaporates
causing the concrete to shrink.
The restraint to shrinkage provided by
reinforcement, sub-grade or other parts like re-entrant corners cause tensile
stresses to develop in the hardened concrete. Restraint to drying shrinkage has
been the most common cause of cracking. They are commonly visible in
rectangular or square openings at the corners.
Proper mix proportioning with low water
content and high aggregate content is the first defence against drying
shrinkage cracks.
Control measures include provision of
diagonal reinforcements near corners of the openings, installing control joints
to allow propagation of cracks along the predetermined path, easing the stress
concentration by providing a 45 degree splay at the re-entrant corners.
Welded wire fabric or small diametre wire
meshes can be used to control shrinkage cracks. There should be enough concrete
cover to protect these reinforcements from corrosion. Currently use of
synthetic fibers is also advocated to control micro cracking.
This type of cracking is usually a problem in
mass concreting work. As the concrete sets the temperature rises due to heat of
hydration. As the interior of concrete heats up and expands due to this, the
surface cools due to drying and shrinks. This sets up thermal gradient between
surface and interior. A temperature differential of 20 degrees centigrade is
enough to cause cracking. However, within 24-hours of placement, concrete
temperatures can reach anywhere from 10 to 25 degrees hotter than ambient
temperatures
The width and depth of cracks depend on this
gradient.
Adopting heat reducing admixtures and
physically cooling the aggregates and adding ice instead of water are some of
the measures, which can help in overcoming this problem.
Settling or wash out of soil below the
sub-base can lead to cracking and eventual structural failure. This can also
occur due to ineffective formwork or premature removal of formwork.
Using proper sub-grade preparation sub-basematerial with adequate moisture content will ensure prevention of cracking due
to this cause.
When steel embedded in concrete corrodes, the
rusted steel occupies a volume much larger than original volume of the rebar.
This expansion causes tensile stresses, cracking and spalling.
Using good quality concrete with proper cover
and properly placed consolidated, finished and cured concrete will ensure
protection against corrosion. Any admixture containing calcium chloride should
be avoided.
This reaction occurs with certain types of
aggregates. The active mineral component of the aggregate reacts with alkali
hydroxides in concrete. Two forms of reactions occur. Alkali silica reaction
and alkali carbonate reaction. Due to this network of cracks appear and disrupt
the structural integrity.
Due to the natural accumulation of water in
the base and sub-base of pavements, the aggregate may become saturated. Then
with freezing and thawing cycles, cracking of the concrete starts in the
saturated aggregate at the bottom of the slab and progresses upward until it
reaches the wearing surface.
Such cracking is usually visible at pavementjoints. This is more a problem in colder climates and is generally solved by
the use of proper aggregates.
Unexpected cracking of concrete is a frequent
cause of complaints. Cracking can be the result of one or a combination of
factors described above. Cracking can be significantly reduced when the causes
are taken into account and control measures are utilized.
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