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Last edited 05 Mar 2019
In construction ‘spalling’ refers to the flaking, cracking, peeling, crumbling or chipping of concrete or brickwork, particularly where parts of the surface might be said to have ‘blown’ off. This can occur as a result of water penetration, heating (such as during a fire) or by mechanical processes. Spalling is typically caused by poor installation, poor maintenance, poor repairs, or by environmental factors.
A common cause of spalling in concrete is the presence of moisture. This is a particular problem in basements, where moisture, often combined with salt, pushes out from within concrete. Striated lines, discolouring and coarse texture are physical signs of spalling.
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.
Whether or not concrete will spall varies greatly and is dependent upon a number of factors, such as:
- 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 fertilizers.
Steps can be taken to prevent spalling when the concrete is first poured, as the mix of concrete will influence the likelihood of spalling in the future. Proper air entrainment should be assured, typically a minimum of 4% air in mixture, allowing for enough airspace within the concrete to accommodate water expansion.
The appropriate mix of sand, cement and aggregate should be achieved, as insufficient aggregate may result in a top layer that is weak and 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.
One of the most common causes of spalling in old, solid walled buildings is the use of cement pointing, over soft lime mortar joints. Interstitial moisture is lost primarily through the mortar joints - the lungs of the wall - and when blocked with cement, this moisture then tries to escape through the brick faces, blowing them.
Water is absorbed by the porous surface of bricks, and in cold periods, expansion by freezing can cause bricks to crack. Fluctuations in weather, humidity and temperature are a major factor, as even the slightest movement in the structure can leave holes in mortar through which moisture can penetrate to the brickwork. Very soft lime mortar will alleviate this problem, being highly porous, and it soaks up moisture, releasing it as conditions change. Other elements of the building can lead to similar problems, for instance, windows and gutters that leak, damaged chimneys, doors and windows and around the foundations.
Masonry sealants should never be used. They trap moisture and salts and exacerbate spalling. Alternatively, drainage installation around the building can help prevent water pooling near the foundation level, and repairing leaks and other defects can remove sources of dampness. Water splash from badly fitted gutters is a common cause.
Crystallisation of salts just below the surface of bricks can also cause spalling. This is known as cryptoflorescence. The problem is often associated with magnesium salts. Cryptoflorescence is associated with a large build-up of salts and usually occurs where old, relatively weak, bricks are re-used inappropriately, particularly in areas of excessive dampness. It can also occur if the brickwork has been covered by a surface treatment because the salts may crystallise behind the treated surface and force it off.
In February 2019, researchers from the University of Sheffield used fibres extracted from the textile reinforcement used in tyres to reduce concrete’s tendency to spall explosively under the intense heat of a fire. Ref http://www.constructionmanagermagazine.com/news/old-tyres-improve-concrete-fire-resistance/
 Related articles on Designing Buildings Wiki
- Admixtures in concrete.
- Alkali-silica reaction (ASR).
- Cracking and building movement.
- Damp proofing.
- Defects in brickwork.
- Defects in stonework.
- Diagnosing the causes of dampness (GR 5 revised).
- Mould growth in buildings.
- Penetrating damp.
- Reinforced concrete.
- Rising damp.
- Rising damp in walls - diagnosis and treatment (DG 245).
- Wall tie failure.
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