Damage to brickwork
Damage to brickwork
The majority of clay bricks contain sulphates of sodium, magnesium or calcium.
These are soluble in water with calcium sulphate being the least soluble.
Usually these sulphates are seen as the harmless efflorescence .
The reaction causes a significant increase in volume. Wet ordinary bricks, in contact with mortar based on ordinary Portland cement, can lead to the formation of calcium sulpho-aluminate and to expansion.
It is the mortar which is attacked, as opposed to the bricks. The volume increases due to the formation of calcium sulpho-aluminate. This in turn causes vertical expansion of brickwork, which is often as high as 0.2%.
Most brick walls with mortars based on ordinary Portland cement are liable to sulphate attack. In practice however, sustained exposure to water is necessary. The most vulnerable walls are retaining walls and parapet walls as they are always damp and are regularly exposed to high levels of water.
The usual sign of a problem on facing brickwork are cracks in the horizontal mortar joints, which generally occurs in several of them.
The vertical expansion can result in spalling of the surface of the bricks and bowing of the walls. The brickwork may also step out over the damp-proof course upon which it can readily slide due to a lack of bonding. Similar to irreversible moisture expansion.
There is a difference between irreversible moisture expansion which takes place in the early months of the life of the building, and sulphate expansion which takes several years before it will show itself.
On rendered brickwork, sulphate attack is often noted by cracking of the render. The cracks are usually horizontal and likely correspond to the mortar joints underneath. The render may adhere quite well to the brickwork at first, but ultimately areas are likely to become detached as the expansion of the underlying brickwork causes severance of the bond between the two materials.
There are three primary ways of preventing sulphate attack in mortars:
1. by ensuring that walls do not get, and stay, unduly wet;
2. by using bricks low in soluble sulphates;
3. by the use of cements low in tricalcium aluminate.
Renderings generally keep the walls beneath them moderately dry, and well-designed and applied renderings will help prevent sulphate attack. Those liable to shrinkage cracking, however, can allow rain to penetrate the cracks but not escape afterwards.
Under such conditions, the brickwork can remain wet for long periods, this assists the reaction to take place. The rendering itself may be attacked: expansion can then exceed 0.2%. A 1:1:5 to 6 cement:lime:sand mix, or its equivalent, is usually suitable for most conditions: denser, stronger mixes are best avoided.
Where un-rendered brickwork has expanded through sulphate attack, remedial measures will much depend upon the extent of damage. A first essential is to prevent the brickwork from continuing to become wet (and to eliminate the source of dampness, if this is a defect such as a leaking gutter or a defective downpipe).
When bricks are damp and the pores filled with water, freezing temperatures exert a force, the magnitude of which much depends upon the amount of water present and the pore structure of the material. Under-fired bricks are more susceptible to frost damage than well-fired bricks.