Concrete Slabs – Shrinkage

Curing is a process which takes place when a concrete slab is laid. As concrete is being poured, fresh concrete is converted into a solid mass when the cementeous materials hydrate. The action taken after the slab is laid to ensure hydration is called curing.

Curing is the protection of fresh concrete from evaporation. When a concrete slab is laid on ground or above ground for a two-storey home, there is a need to control the rate of moisture loss from the concrete during cement hydration. Curing is necessary to ensure that the concrete does not dry out too quickly so that the slab will build up strength, long-term durability and wear resistance. The compressive strength of properly cured concrete can be double that of concrete that has not been cured.

If concrete is allowed to dry too quickly, it may cause shrinkage, cracking or a weak powdery surface on the slab. Depending on the climate, slabs may be kept wet with the use of hessian bags, plastic membranes or similar. There are also chemical compounds which can be sprayed or brushed on the slab surface to ensure adequate curing. However many concrete structures receive no curing other than that they are left idle for several days after the concrete has been placed.

Concrete slabs must be installed in accordance with Australian Standard AS 2870. “Slab” issues that most often arise in disputes between builders and owners relate to the size of cracks that may appear, and the thickness of the concrete slab installed. In Western Australia the most common slab thickness is between 85mm and 100mm. This thickness will be stated in the specifications document which forms part of your contractual arrangements with the builder.

Most Perth building sites are flat and sandy so an 85mm slab would be sufficient for single storey residential construction. If you are building a two-storey home and/or in the hills, engineering requirements usually require a 100mm slab, with thickening under internal walls.

If you have a long slab, perhaps greater than 20 metres in length, any savings from an 80-85mm slab will be offset by the requirement for more steel reinforcement. Steel reinforcement increases the tensile strength of concrete and controls the width of shrinkage cracks.

Cracking of concrete slabs is a common occurrence, but it is not necessarily a major problem.  Some slab cracking is almost certain because concrete shrinks as it loses moisture, when temperatures change or when there is ground movement.

While slab cracks can look unsightly and affect the application of tiling or other floor finishes, many cracks do not affect the structural integrity of the slab.

Cracking can be reduced particularly in hot weather if the builder uses an evaporative retarder after screeding and bull-floating.  Water spraying reinforcement, using water ponding or damp-hessian to provide shading to the concrete, or in certain circumstances providing an expansion joint are some ways to prevent cracking.

Other more general measures that a builder could use to prevent cracking during placing, finishing and curing include using higher slump concrete if high workability is required, dampening the subgrade prior to concrete placement, ensuring formwork and reinforcing is optimal and that concrete is placed quickly and compacted adequately.

Despite these preventative measures, on some occasions cracking can be severe.  If you are still worried about your slab or the width or length of the cracks, contact your builder immediately.  A structural engineering report may be needed in extreme circumstances to determine what remedial work may be necessary.