2 COMMON TYPES OF CRACKS IN HARDENED CONCRETE

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

Crazing
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.

Drying shrinkage cracks in hardened concrete
Drying shrinkage cracks in hardened concrete

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.

2 thoughts on “2 COMMON TYPES OF CRACKS IN HARDENED CONCRETE”

  1. to the concerned person.
    I refurbish tennis and netball courts.
    What I find on concrete netball and tennis courts in many cases is a network of Craze cracks or alligator skin cracks.
    I have tried many overlays and fillers, but the cracks seem to return and show through the repaired areas.
    I have contacted many concrete contractors and was told that they are able to repair these craze cracks but can not issue a warranty or guarantee that these cracks will not reflect through the repaired surface again.
    They say that as soon as there is sufficient fluctuations in temperatures and moisture these cracks will return.
    I have specific courts that I need to repair in an area that also may be subject to heavy downpours of rain and coastal moisture.
    Do you have any product or method of repair that may prevent these craze / alligator skin cracks from returning?
    If so will you issue a warranty / guarantee in writing in this regard.

    Regards
    Chris De Wet

    Reply
  2. The use of poor quality raw materials has resulted in problems like cracks occurring frequently in hardened concrete. I started seeing these cracks develop all over the house both indoors as well as outdoors. Having unsuccessfully tried a number of solutions to deal with this perplexing situation, I was about to give up and call a contractor when a friend of mine suggested that I try Roff Crackfill. He said that he had a similar problem last year and had used it to fill in the cracks. Since it had held strong through the monsoon and prevented any leakages, I decided to give it a shot. Well, since I have applied the solution, the polymer-modified crack filler has been able to level the undulations of the surface of the wall.

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