Fretting is caused by the action of salt migration through brickwork. Water which has salt dissolved in it migrates through brickwork to its surface. As the brick dries, the salt is left behind and forms salt crystals which grow in the voids within the brick. As more salt is left behind by evaporation of water, salt crystals grow larger and larger. The strength of growing salt crystals can be stronger than the elements that hold the brick together. If this occurs, the brick face begins to crumble and fall away. This is also true for mortar joints. For salt attack to occur the following three conditions are required:
- There must be salts present
- There must be water entering the wall
- The water must evaporate from the wall
The absence of any of these conditions will prevent salt attack. The fretting of bricks can be exacerbated at specific locations around a house, which undergo increased wetting and drying cycles. The brickwork will continue to deteriorate unless moisture movement through the masonry is prevented. When treating fretting, "prevention is the best cure". The source of the salt may be airborne salt from sea spray or salts that are naturally present in the soil, or introduced by fertilizers and salt-water swimming pools. The use of bore water may also provide the source of the salt.
The first step in treating fretting is to identify the source of the salt and where the salt might be coming from. The best method of preventing fretting is to prevent the salt from being absorbed into the wall. This may be a physical barrier such as plastic sheeting, digging the soil away from the wall or rearranging the sprinklers so that they do not spray directly onto the wall.
Once fretting has begun it will not stop until the salt source is identified and removed. It could then take another 12-24 months before the fretting process comes to a complete halt. Once the fretting process has stopped the brickwork can then be repaired. Any repairs made to the brickwork while the fretting action is still occurring will generally fail. Fretting brickwork may be treated using breathable sealers that penetrate the brickwork and consolidate loose particles. However, depending on the degree of fretting it may be necessary to render or replace the affected brickwork.