Prevention of Plastic Shrinkage Cracks

The best protection is to understand when the risks of plastic cracking are greatest so that appropriate actions can be taken.

The main variables that control the evaporation rate are:

• wind speed;

• relative humidity;

• concrete temperature;

• air temperature.

The greatest risk of plastic cracking occurs on hot, dry, windy days. Figure 1 provides a method of estimating the evaporation rate given the above information. When the estimated evaporation rate exceeds 1 litre/m2 per hour, precautions need to be taken to prevent plastic cracking. New Zealand experience suggests this limit is adequate. It is recommended that a conservative approach be adopted when deciding to take precautions, as in countries such as the United Kingdom it is advised that plastic cracking protection should be instigated at half this evaporation rate.

Typically one of the more significant variables is the wind speed. This is why in Britain it is often called ‘wind cracking’ as a reminder that it is primarily caused by air movements causing drying of the surface. In New Zealand’s warmer, drier environment, temperature and humidity are equally important. Some protection can be obtained by preventing air movement over the slab with the use of a windbreak. The use of polythene will prevent both evaporation and air movement. It should be used with caution though when trying to obtain a consistent colour to the slab. Generally the use of polythene produces shade differences due to differing moisture conditions associated with wrinkling of the polythene. Polythene can be laid on the surface with sufficient rolled back in sections to allow finishing to be completed.

Other precautions that can be adopted include:

• The use of proprietary anti-evaporant alcohols. These are simply sprayed onto the surface, to provide a thin layer of alcohol that reduces water evaporation rates at the concrete surface. These products are inexpensive and can be applied with a weed sprayer. It is important to note that these products are not curing agents, and will need to be re-applied if the surface is disturbed.

• Water misting – this can be diff icult to achieve in windy conditions.

• Polypropylene fibres – these are typically added at the batching plant and therefore their use requires planning.

Plastic Settlement Cracks

Most concrete after it is placed bleeds, i.e. 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. See Figure 2

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. See Figure 3

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;

Prevention of Plastic Settlement Cracking

Plastic settlement cracks may be prevented or closed, by revibrating the concrete after settlement is virtually complete, and it has begun to set e.g. 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, i.e. after the concrete has begun to harden, may damage the bond with reinforcement or reduce its ultimate strength.

On formed surfaces, crazing tends to occur on smooth faces cast against low-permeability form-face materials.

It is generally accepted that crazing is a cosmetic problem. There is much anecdotal evidence of industrial floor slabs that exhibit crazed surface cracking which have been in service for many years without deterioration. Autogeneous healing of fine cracks can occur, and although ‘healed’ the cracks will still be visible.

Prevention of Crazing

To avoid crazing on trowelled surfaces:

• avoid very wet mixes;

• do not use ‘driers’;

• do not overwork the concrete;

• do not attempt finishing whilst bleed water is present;

• do not steel trowel until the water sheen has gone;

• commence continuous curing promptly; and

• do not subject the surface to wetting and drying cycles.

On formed surfaces, very wet and over-rich mixes should be avoided and curing should be continuous. The concrete should not be subjected to wetting and drying cycles.

Drying Shrinkage Cracks

Hardened concrete shrinks, i.e. it reduces in volume as it loses moisture due to:

• the hydration of the cement; and

• evaporation.

The shrinkage caused by moisture loss is not a problem if the concrete is completely free to move. However, if it is restrained in any way, then a tensile stress will develop. If that stress exceeds the ability of the concrete to carry it, the concrete will crack.

A number of factors influence the shrinkage of concrete, in particular the total water content. Others include:

• the content, size and physical properties of the aggregate;

• the relative humidity;

• admixtures, especially those containing calcium chloride; and

• the curing conditions.

The cement content of concrete influences shrinkage drying almost only to the extent that it influences the amount of water used in a mix.

In order to reduce the total shrinkage of concrete:

• the water content should be minimised (consistent with the requirement for placing and finishing);

• the amount of fine material should be minimised;

• the highest aggregate content should be used;

• the largest possible maximum aggregate size should be used; and

• good curing practices should be adopted.

Simply reducing the shrinkage of a concrete will not necessarily reduce cracking since this is also influenced by the restraint, detailing, geometry, construction practice, etc.

Preventing Cracking Due to Drying Shrinkage

The prevention of uncontrolled cracking, due to drying shrinkage, starts with the designer. Appropriate design and detailing is essential. Specifically, attention must be given to the following:

• The provision and location of adequate reinforcement to distribute the tensile stress caused by drying shrinkage. This is particularly important in floors, slabs-on-ground, and similar applications where reinforcement may not be required for load carrying or structural reasons.

• The provision, location and detailing of joints to isolate restraints and permit movement between discrete parts of the construction.

Construction practice is also important for it must:

• ensure that the concrete is properly placed, compacted and cured in order to minimise the magnitude of drying shrinkage;

• ensure the designer’s details are correctly put in place; and

• ensure removal of restraint by the formwork.

Thermal Movement Cracks

Thermal cracking is attributable to the heat generated during the cement hydration process. The subject is complex, and therefore with the space limitations of this information bulletin we will look only at:

• Developing some initial understanding of the issues.

• Determining when designers and builders should think carefully about this subject.

• The types of cracks that can form.

• Design and construction strategies to remove, or reduce the incidence of this type of cracking.

For those wanting a more detailed discussion on this subject, use CIRIA Report 91 “Early-age thermal crack control in concrete”.

Heat of Hydration

The mixing of cement with water starts a chemical reaction that gives of heat. The amount of heat generated is influenced by several factors, including:

• The amount of cement used.

• Whether supplementary cementitious materials are used.

• The type of cement, for example, High, Early or General Purpose cement.

• The properties of aggregates.

• The placing temperature of the concrete.

• The ambient temperature.

• The type of formwork, and when it is stripped.

Table 1 provides an indication of the temperature rises above mean ambient for various cement contents, section thickness and formwork types.

The table specifically relates to an assumed concrete placing temperature of 20oC and a mean ambient temperature of 15oC. Higher placement and ambient temperatures will increase the rate of hydration, and higher temperature rises above ambient will occur. The table also assumes that the formwork remains in place until after the peak temperature has been reached. For a 500 mm thick section the peak temperature will typically be reached between 20-48 hours.

If it is considered desirable to reduce the temperature build-up in the concrete, there are several mix design related options that could be explored. It is worth discussing the options with your local ready mix company to assist in evaluating both the technical and economic implications of the options.

Options which may be considered include:

• Using supplementary cementitious materials such as granulated ground blast furnace slag, silica fume, or fly ash.

• Using larger aggregates.

• Using water reducing admixtures.

• Lowering the placement temperature.

Heat of Hydration cracks

Cracking associated with hydration heat, can be roughly split into two categories:

• Cracks that are due to the development of a large thermal gradient through the member (internal restraint).

• Cracks that develop due to external restraint from free contraction as the member cools.

Internal Restraint

The usual rule of thumb used to prevent the first type of cracking, is to ensure that the temperature difference through the member is less than 20 degrees Celcius. Temperature differences larger than this can occur in large members such as raft foundations, or potentially when the formwork is removed early. It is suggested that this issue should be carefully considered when the member thickness is greater than 500 mm.

External Restraint

As concrete cools it contracts. If this contraction is prevented by external restraints it can crack.

The key to the prevention of this cracking lies in ensuring that the coefficient of expansion x temperature drop x restraint factor is less than the tensile strain capacity. Therefore reducing the thermal movement or the restraint, or increasing the tensile strain capacity reduces or prevents early age thermal cracking. If it is not possible to prevent early age thermal cracking, the crack widths can be controlled by reinforcement.

Table 2 provides a summary of factors that help prevent or control early age thermal cracking.