The corrosion of wall ties is due to several factors. Principal factors are:
Porosity of Brickwork
Bricks vary considerably in their moisture absorption rate. Engineering bricks are almost impervious to moisture and water. Softer bricks however, behave like sponges.
Quality of Mortar
Weak lime mortars are more porous than dense sand and cement mixes. The chemical action of mortars can vary between two extremes, either pacifying or encouraging corrosion. Ordinary Portland Cement mortars are alkaline in nature and as such discourage the corrosion of the wall tie. Other mortars will contain a proportion of sulphates which, when combined with moisture, creates a more aggressive condition than normal.
The location of any given elevation will determine the life expectancy of the wall tie, e.g. coastal, industrial or locations of persistent driving rain, are all vulnerable. The presence of salt within the atmosphere and coastal locations result in accelerated corrosion of wall ties. Industrial conditions will increase the acidity of the moisture reaching the ties, this can lead to a more aggressive and corrosive condition. Driving rain, by its very nature, increases the period at which the wall and its ties may be persistently wet. South and west facing elevations are more vulnerable than those on the north and east of the building. Poor design and construction problems including the lack of an effective damp proof course, blocked cavities, inadequate flashings and lack of cavity trays, are recognised as contributing factors to the ingress of moisture.
As wall ties corrode, bed joints widen and the outer leaf grows upwards causing tension in the inner leaf, therefore bending movement in the wall as a whole. Outward movement, or bulging, of the wall usually results with the inner leaf of the wall often being carried outwards as well as the wall ties which are normally intact to some degree at this stage.
Other common symptoms of wall tie failure include vertical and horizontal cracking, sagging of lintels, upward cambering of window sills, lifting of roof edges, separation of the window reveals and the separation of floors.
Old Corroded Ties
New Replacement Ties