Cracks in Hardened Concrete

Cracking in concrete after setting occurs for a variety of reasons. The two most common forms of cracking in hardened concrete are crazing and drying shrinkage cracking.

1. Crazing

This is best described as a network of very fine cobweb-like or alligator-skin cracks, which appear on the surface of concrete after it has been exposed to the atmosphere for some time. They occur in the surface cement mortar and are generally more common in surfaces with a highly steel-trowelled or burnished finish as the additional time required for finishing leaves the surface exposed for a mlonger period prior to commencement of curing

Crazing does not normally extend below the top 2 to 3 mm, nor does it lead to durability or other serious problems; it generally affects only the appearance.

Causes of Crazing

The main causes of this form of cracking are:

• Rapid drying of the highly compacted surface (from trowelling) while the concrete is still weak (between trowelling and commencement of curing);
• The surface of the concrete expanding and shrinking during alternate cycles of wetting and drying;
• Steel trowelling the concrete before the water sheen has gone and hence trowelling bleed water into the surface of the concrete causing increased shrinkage and weakening of the surface mortar;
• Using cement-rich mixes on the surface as ‘driers’ to remove excess water (these may create a surface layer with substantially different properties to the concrete below, eg higher shrinkage);
• Overworking the surface and bringing excess mortar to the surface (again creating a surface layer that may have increased shrinkage characteristics);
• Adding excess water to the mix;
• Inadequate or inconsistent curing.

Preventive Methods

To avoid crazing on trowelled surfaces:

• Avoid very wet mixes and do not add excess water on site.
• Do not use ‘driers’ such as cement, pigments or colour hardeners to soak up bleed water.
• Do not work the bleed water into the surface but wait until the water sheen has gone. Excess Bleed water can be dragged off the surface with a hose.
• Do not overwork the concrete by unnecessary trowelling of the surface.
• Avoid wetting and drying cycles and excessive ‘wet wiping’ of the surface where water is sprinkled onto the surface to aid with trowelling.
• Protect surfaces from rapid drying during finishing.
• Commence curing promptly.

2. Drying Shrinkage Cracks

These are caused by moisture in the concrete drying out over time leading to shrinkage of the concrete. This is not a major problem if the concrete is free to move, but, if restrained, tensile stresses can develop in the concrete and, if these exceed the tensile strength of the concrete, cause it to crack.

The water content of the mix is the major factor influencing drying shrinkage. Other factors that may also affect the risk of cracking in hardened concrete include restraints, curing conditions, aggregate size and content, detailing geometry and construction practices.

Preventive Methods

Methods to reduce the risk of cracking due to drying shrinkage include:

• Do not add excess water to the concrete on site.
• Compact the concrete adequately to achieve the maximum density.
• Provide adequate curing and commence promptly.
• Place joints in correct locations, provide correct geometry (size/shape of slab panels) and ensure proper construction detailing to limit restraint of the concrete.
• Use good construction practices when placing concrete.
• Provide adequate reinforcement and ensure correct placement.