Concrete cracks may occur in concrete construction for a variety of reasons. Cracking in concrete construction is almost 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 common, is not the only form of cracking. 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 temperature 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, joints at appropriate intervals will control cracking and ensure that it does not occur in a random fashion to the detriment of the appearance and long-term durability of the structure.
Cracks that form before concrete has fully hardened (e.g. less than eight hours) are known as prehardening cracks.
There are three main types:
• Plastic shrinkage cracks.
• Plastic settlement cracks.
• Cracks caused by formwork movement.
All occur as a result of construction conditions and practices although faulty formwork design may lead to its movement and/or failure. Prehardening cracks are usually preventable by the adoption of good construction procedures.
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.
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.
Hardened concrete shrinks, i.e. it reduces in volume as it loses moisture due to:
• the hydration of the cement; and
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.
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 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”.
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.
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.
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.
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.
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.