Cracks may occur in concrete construction for a variety of reasons.
Indeed, unless appropriate measures are taken to control it, cracking
in concrete construction is inevitable because concrete, like most other
building materials, moves with changes in its moisture content.
Specifically, it shrinks as it loses moisture. Being a brittle material
it is liable to crack as it shrinks, unless appropriate measures are taken
to prevent this, e.g. by the provision of control joints.
Shrinkage cracking, although, perhaps, the most common form of cracking
in concrete construction, is not the only form. Cracks may occur also
due to settlement of the concrete, movement of the formwork before the
concrete member is able to sustain its own weight, or due to changes in
the temperatures of the concrete and the resulting thermal movement.
Appropriate measures will at least minimise, if not prevent entirely,
these forms of cracking. In all cases, ensure that it does not occur
in a random fashion to the detriment of the appearance and long-term
durability of the surface.
Prehardening Cracks
Cracks which form before concrete has fully hardened, that
is in less than eight hours, say, are known as prehardening
cracks. There are three main types, viz:
- Plastic shrinkage cracks
- Plastic settlement cracks
- Cracks caused by formwork movement
All occur as a result of construction conditions and practices
although, obviously, faulty formwork design may lead to its
movement and/or failure. Prehardening cracks are usually preventable
by the adoption of good construction procedures.
Plastic Shrinkage Cracks
Plastic shrinkage cracks are formed in the surface of the
concrete whilst it is still plastic, that is before it has
set and begun to harden, although they may not become visible
until some time later. They are due to the too rapid loss
of moisture from the surface of the concrete, e.g. during hot,
dry and windy conditions. Thus, they are a form of drying
shrinkage crack.
Usually, they form without any regular pattern and may range
from as little as 25 mm to as much as 2 m in length. They
are fairly straight and vary from a hairline to perhaps 3
mm in width. They generally occur in hot weather.
Effect of concrete and air temperatures, relative humidity,and
wind velocity on the rate of evaporation of surface moisture
from concrete
Figure 1.1 may be used to estimate the likelihood
of plastic shrinkage cracking occurring and, hence, the need
for suitable precautions to be taken. As may be seen the factors
which affect the rate of evaporation of moisture from the
surface include:
- air temperature;
- relative humidity;
- concrete temperature;
- wind velocity.
Where these factors combine to produce a rate of evaporation
greater than 1 kg/m2/h, then plastic shrinkage cracking is
likely and precautions should be taken. As may be noted, high
air temperatures are not necessary for this to occur; concrete
temperature and wind velocity have a greater effect.
Figure 1.1
Effect of concrete and air temperatures, relative humidity,and
wind velocity on the rate of evaporation of surface moisture
from concrete
Precautions to Prevent Plastic
Shrinkage Cracking
The most effective way to prevent plastic shrinkage cracking
is to prevent rapid loss of moisture from the surface of the
concrete. The erection of sunshades and windbreaks will help
achieve this as will other good hot-weather concreting practices.
A method widely used is the application of an aliphatic alcohol
as an evaporation retardant. This may be sprayed on the surface
of the concrete immediately after initial screeding has been
completed.
Revibration of Concrete
If plastic cracking does become evident before the concrete
has taken its initial set, the cracks may be closed by revibration
of the concrete over the full depth of the cracks. This should
be done, preferably, by an experienced operator, but a good
rule of thumb is to permit revibration of concrete only if
the vibrator will sink into the concrete under its own weight.
Surface revibration may be only partially effective as it
may not close the cracks to their full depth. They will then
almost certainly recur as the concrete dries out.
Further information and guidance on procedures
to prevent their occurrence is available on this site on the
Hot & Cold Weather Concreting page.
Plastic Settlement Cracks
Most concrete, after it is placed, bleeds, ie water rises
to the surface as the solid particles settle. The bleed water
evaporates and there is a loss of total volume - the concrete
has 'settled'.
If there is no restraint, the net result is simply a very
slight lowering of the surface level. However, if there is
something near the surface, such as a reinforcing bar, which
restrains part of the concrete from settling while the concrete
on either side continues to drop, there is potential for a
crack to form over the restraining element
Differential amounts of settlement may also occur where there
is a change in the depth of a section, such as at a beam/slab
junction
Settlement cracks tend to follow a regular pattern coinciding
with a restraint, usually the reinforcement, or a change in
section. Generally, the cracks are not deep but, because they
tend to follow and penetrate down to the reinforcement, they
may reduce the durability of a structure.
Factors which may contribute to plastic settlement include:
- rate of bleeding;
- the depth of reinforcement relative to total thickness;
- the total time of settlement;
- the depth of reinforcement/size of bar ratio;
- the constituents of the mix; and
- the slump.
Figure 1.2 Settlement cracking
Figure 1.3 Differential settlement cracking
Precautions to Prevent Plastic Settlement Cracking
Plastic settlement cracks may be prevented, or rather closed,
by revibrating the concrete after settlement is virtually
complete and it has begun to set, eg after half an hour to
one hour. Revibration closes the cracks, and enhances the
surface finish and other properties of the concrete. Careful
timing is essential to ensure that the concrete reliquefies
under the action of the vibrator and that the cracks close
fully. Applying vibration before the concrete has begun to
stiffen may allow the cracks to reopen. Applying it too late,
ie after the concrete has begun to harden, may damage the
bond with reinforcement or reduce its ultimate strength.
Other procedures which may help reduce plastic settlement
cracking include:
- using lower slump mixes;
- using more cohesive mixes;
- using an air entrainer to improve cohesiveness and reduce
bleeding; and
- increasing cover to top bars.
Where there is a significant change in section, the method
of placing may be adjusted to compensate for the different
amounts of settlement. If the deep section is poured first
to the underside of the shallow section, this concrete can
be allowed to settle before the rest of the concrete is placed.
However, the top layer must be well vibrated into the bottom
layer.
Avoiding the use of retarders is sometimes suggested as a
way of reducing plastic settlement cracking but, for hot-weather
concreting, the advantages of retarders generally outweigh
the disadvantages.
Formwork Movement
If there is movement of the formwork, whether deliberate or unintentional, after the concrete has started to stiffen but before it has gained enough strength to support its own weight, cracks may form. Such cracks have no set pattern.
To avoid cracking from this cause, formwork must be:
- Sufficiently strong and rigid to support the weight of the concrete without excessive deflections; and
- Left in place until the concrete has gained sufficient strength to support itself.
Some guides for the stripping time of formwork assume that Type GP cement is being used. Concretes incorporating supplementary cementitious materials, such as fly ash, may take longer to gain strength and allowance should be made for this.
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