Structural engineer in Brisbane photo of slab cracks

Why Does Concrete Crack

Cracks in Concrete Ground Slabs

Cracks occur in concrete and they should be expected. However our clients sometimes ask us to look at concrete ground slabs to determine if there are too many cracks or if the cracks are structural.

These are some of the types of concrete cracks we see:

Plastic Shrinkage Cracks

These cracks normally ‘appear’ within a couple of days of pouring the concrete.

They actually occur within the first few hours of the concrete being poured before the concrete has any strength.

They can look like random lines across a slab and sometimes they run close to and parallel to a sawn joint or tool joint.

Concrete Shrinkage Crack
Typical Shrinkage Crack in Concrete

The cracks are caused by the concrete shrinking as it cures or goes hard. Once cracks have formed it is normally too late to install sawn joints or tool joints so we always recommend sawing the slab as soon as possible after pouring.

These cracks can be controlled by adjusting the shape of the slab to be poured, applying water sprays to the concrete surface, avoiding hot weather pours, applying curing compounds and evaporation retardants and covering the poured surface with plastic membranes to minimise evaporation.

Plastic Settlement Cracks

Cracks in Concrete Pathway
Settlement Cracks in Concrete Pathway

These cracks tend to occur in more uniform patterns over the top of

Plastic settlement slab cracks
Settlement Cracks in a concrete slab

mesh reinforcement (cracks spaced at multiples of 20cm) or along footing beams. They occur when the wet concrete settles under its own weight and is ‘held up’ by the reinforcing mesh. They are caused by inadequate compaction of the wet concrete. These cracks can be prevented by ensuring the concrete is adequately compacted and vibrated when it is poured.

Structural Cracks

These cracks occur when the concrete has been overloaded or the slab has insufficient ground support or the ground is heaving. Their location and width depends on what is changing or has changed. These cracks should be inspected by an engineer to determine their cause.

Crack Repair

Once shrinkage cracks and plastic settlement cracks have formed in concrete slabs on ground they are very difficult to repair. So it is always better to take the effort to prevent them when pouring.

However, if cracks have already appeared, the can be disguised with one of these techniques:

  • Filling with a fine grained cementitious grout.
  • Demolition and removal and replacement of the slab section.
  • Tiling.
  • Spray paving.

Call Cornell Engineers for advice on cracks in your slab and the best rectification method.

One thought on “Why Does Concrete Crack”

  1. Hi were have hairline cracks in our tiles we are currently removing the tiles and have noticed hairline cracks in the concrete slab
    Please contact me

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