Jun 2017 AN OVERVIEW OF THE CAUSES OF CONCRETE CRACKS
We highlight the few common types of concrete cracks and other blemishes that occur most frequently and investigate its typical causes.
The main classification of concrete cracks
All concrete cracks can broadly be classified as either dormant or active. The difference can be summarised as follows:
- Dormant cracks remain unchanged; and
- Active cracks change over time, in direction, width or depth.
Cracks can be diagonal, longitudinal, vertical or random and the severity of a crack is also characterized by its direction, depth, width. Both dormant and active cracks create opportunities for moisture and other deleterious materials or liquids to penetrate and can lead to future damage if left unrepaired.
Types of concrete cracks and its causes
Some of the concrete cracks that are seen most often include:
- Plastic shrinkage cracks: If it occurs, it will normally be visible shortly after placing the concrete. The shrinkage is caused by the forces and subsequent stresses that are created by the rapid removal of water from the concrete. Consequently the severity of cracking is largely influenced by the rate of drying of the concrete surface. The drying and removal of water from the surface depends on such factors as : –
• The temperature of the air and the concrete.
• The relative humidity of the atmosphere.
• The velocity of the wind.
• The absorbency of the surface in contact with the fresh concrete.
- Crazing, also known as checking or map cracks: This is a network of fine, superficial cracks that feature across the surface of the concrete. Craze cracking is usually caused by excessive floating and trowelling of the wet concrete surface or by applying driers comprising neat cement or a dry mixture of sand and cement onto the wet surface and then trowelling. This surface bear bad wear resistance and has a tendency to delaminate easy.
- D-cracks: D-cracks run deeper than surface cracks and either originates from a concrete joint or runs parallel to it. It is caused by moisture penetration at the joints.
- Hairline cracks: Hairline cracks are very thin, but run deep and could sometimes be a bit wider at the top than at the bottom. These cracks typically occur as the concrete settles during curing. It is also known as plastic settlement cracks.
- Pop-outs: These are conical, slightly sunken depressions that appear on the concrete surface. It commonly appears due to pieces of aggregate close to the surface of the concrete that are highly absorptive, such as clinker ash with sometimes high free lime present. As a result, it expands and protrudes through the surface.
- Scaling: Scaling causes minor pock-like marks on the concrete surface and exposes the aggregate underneath it.
Scaling can be caused by different issues. Firstly, it appears when concrete is not adequately finished and causes moisture to infiltrate. The moisture expands when there are temperature changes and pushes off pieces of concrete from the surface.
Then there is delamination. Ineffective finishing methods and too early floating before all or most of the bleed water has evaporated can lead to excessive moisture or air in the concrete, which will in turn rise to the top and form pockets just below the surface. These pockets may create blisters, which can break open and cause scaling.
- Spalling: Spalling is similar to scaling, but the surface depressions are larger and deeper and often linear to the length of a rebar (the reinforced steel rod used in concrete).
There are multiple issues that can lead to spalling. This includes pressure from under the surface of the concrete, inadequately constructed joints or decomposition of the rebar in the concrete.
- Offset cracks: Offset cracks occur where the concrete on the one side of the crack is of a different height than on the other side of it. It generally arises when the surface under the concrete has not been levelled or inadequately/ inconsistently compacted.
- Diagonal corner cracks: These cracks run diagonally from one joint to its vertical joint at the other corner of the concrete slab. It appears because of curling or warping. Curled up or warped corners are not very stable as it generally has empty spaces below it. As a result, it often cracks after curing, the moment weight is applied.
Prevention is better than cure
When analysing the causes of cracking, it becomes clear that prevention is better than cure. Be sure to avoid errors in design and detailing and apply sound construction practices right from the word go. Feel free to get in touch with Sephaku Cement for expert advice on construction of concrete fixtures.