Brickwork spalling

The word ‘spall’ refers to the breaking of a material into pieces, particularly to cracking below the surface that may cause part of the surface to come off.

In construction ‘spalling’ refers to the flaking, cracking, peeling, crumbling or chipping of stone or bricks, particularly where areas of the surface are said to have blown. This can occur as a result of water penetration, freezing, heating (such as during a fire) or by mechanical processes. 

Spalling can be inconsequential, that is, a purely aesthetic issue, or it can lead to serious structural damage requiring costly repair work.

Older bricks deteriorate through spalling as a result of moisture penetration, eventually resulting in the brick crumbling. 

One of the commonest 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, (often known as ribbon, or strap pointing) this moisture then tries to escape through the brick faces, blowing them.

Water is absorbed by the porous surface of old bricks, and in cold periods, expansion by freezing can cause them to crack. Changes in weather, humidity and temperature are a contributing factor.  Minor movement in the structure can leave cracks in mortar through which moisture can penetrate.  Very soft hot 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, poorly fitted windows, gutters and downpipes that leak and damaged chimneys.

A major contributing factor to damage in cities at low level is nitrate emissions from diesel vehicles - which create nitric acid - this attacks brickwork and causes severe spalling of stonework, especially anything with a carbonate binder to the stone.

Masonry sealants should never be used. They trap moisture and salts and exacerbate spalling.

Masonry paints such as Sandtex, Weathershield and many others form plastic coats over the brickwork or stone, and trap moisture, causing spalling and disintegration and damp walls. Masonry paints should always be removed and replaced with limewash if a coloured finish is desired.

Improving drainage around the building can help prevent water pooling near the foundation level causing damp walls.  Repairing leaks and other defects can remove sources of penetrating dampness. Water splash from badly fitted gutters is a common cause.

If spalling has already occurred, damaged bricks can be replaced, but the cause of the moisture should also be identified and dealt with appropriately. We recommend the use of the arbortech plunge masonry saw.  Angle grinders should NEVER be used, as they damage the top and bottom edges of the brick, removing any glaze, and leaving them exposed to spalling. 

Crystallisation of salts just below the surface of bricks or stone can also cause spalling. This is known as cryptoflorescence. The problem is often associated with magnesium salts (epsom 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 moisture stress. It can also occur if brickwork has been covered by a surface treatment (damp proofing, tanking) because the salts may crystallise behind the treated surface and force it off.

 

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