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Early Age Crack Control
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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:

  1. Plastic shrinkage cracks
  2. Plastic settlement cracks
  3. 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 25mm to as much as 2m in length. They are fairly straight and vary from a hairline to perhaps 3mm in width. They generally occur in hot weather.

This nomogram 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 1kg/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.

Prevent Plastic Shrinkage Cracking
The most effective way to reduce the risk of plastic shrinkage cracking is to prevent rapid loss of moisture from the surface of the concrete. Practices to achieve this are:

  • Dampen sub-grade and forms ensuring any excess water is removed prior to placing concrete.
  • In hot weather, lower the temperature of the fresh concrete by using cool aggregates and chilled mixing water.
  • Add polypropylene fibres to the concrete mix.
  • Erect wind breaks to reduce wind velocity over the concrete surface.
  • Use aliphatic alcohols sprayed over the surface prior to and after finishing before curing can commence to reduce rate of evaporation from the surface.
  • Commence curing promptly after finishing is complete and ensure the surface is subject to continuous curing.

Re-vibration of Concrete
If plastic cracking does become evident before the concrete has taken its initial set, the cracks may be closed by re-vibration 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 re-vibration of concrete only if the vibrator will sink into the concrete under its own weight. Surface re-vibration 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
  • Depth of reinforcement relative to total thickness
  • Total time of settlement
  • Depth of reinforcement/size of bar ratio
  • Constituents of the mix
  • Slump

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
  • 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.