Spalling concrete

Spalling concrete is when the material begins to flake and fall away in sheets or small pieces, at the early stages striated lines, discolouring or coarse texture can be signs of spalling.

This kind of failure is usually related to moisture where ever the concrete has been installed, such as in basements, where moisture, often combined with salt, pushes out from within concrete. It may also be related to the curing of the original pour, where it may not have dried properly, so moisture seeped into the concrete during the drying and curing process.

Heat and mechanical pressure can result in uneven expansion of concrete and subsequent fracture. Particularly concrete that includes reinforcing bars, which absorb heat at a different rate and may expand or corrode. Applied heat may also release the water contained within the concrete. A number of factors will impact the spalling rate of concrete:

• The configuration of materials.
• Composition of the material.
• Temperature and temperature change.
• Local conditions.
• Length of time since curing (water content is higher in concrete that has recently been poured).
• Ability of the surface to absorb liquid.
• Expansion and contraction of the freeze/thaw cycle.
• Presence of chemicals such as de-icers and fertilisers.

The steps to prevent spalling when the concrete is first poured, includes proper air entrainment of about 4% in mixture, allowing for enough airspace within the concrete to accommodate water expansion and ensuring the correct mix. The right mix of sand, cement and aggregate will help prevent spalling, for example where there is insufficient aggregate it may result in a weak top layer which is prone to spalling. The mix should be kept as dry as possible, as high water content can compromise the concrete’s strength, with the correct curing time allowed.

Action should be taken quickly if spalling is detected in concrete installations, as the risk of damage rises the longer remediation is delayed. If left untreated spalling concrete can lead to crumbing concrete which can lead to strength risks.

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