Iron ties which were used to tie the leaves of the early cavity walls were later replaced by mild steel ties that became standard for many years.
In contact with moisture, mild steel progressively corrodes by the formation of oxide of iron, called rust, which expands fiercely to the extent that brickwork around ties may become rust stained and disintegrate. Standard mild steel ties are coated with zinc to inhibit rust corrosion. The original zinc coating for ties, which was comparatively thin, has been increased in thickness in the current British Standard Specification, for improved resistance to corrosion. As added protection, the range of standard wall ties can be coated with plastic on a galvanised undercoating.
On the majority of building sites wall ties are not commonly protected during delivery, storage, handling and use against the inevitable knocks that may perforate the toughest coating to mild steel and the consequent probability of rust occurring. There are, on the market, a range of standard and non-standard section wall ties made from stainless steel that will not suffer corrosion rusting during the useful life of buildings. It seems worthwhile to make the comparatively small additional expenditure on stainless steel ties as a precaution against staining and spalling of brickwork or blockwork around rusting mild steel ties.
The standard section wall ties, illustrated in Fig. 70, are the vertical twist strip, the butterfly and double triangle wire ties. As a check to moisture that may pass across the tie, the butterfly type is laid with the twisted wire ends hanging down into the cavity to act as a drip. The double triangle tie may have a bend in the middle of its length and the strip tie has a twist as a barrier to moisture passing across the tie. Of the three standard types the butterfly is more likely to collect mortar droppings than the others.
The wall tie illustrated in Fig. 71 is made from corrosion resistant Austenitic stainless steel. The ridge at the centre of the length of the tie is designed for strength and to provide as small as possible a surface for the collection of mortar droppings. The perforations are to improve bond to mortar.
The length of wall ties varies to accommodate different widths of cavity and the thickness of the leaves of cavity walls. For a 50 mm cavity with brick leaves, a 191 mm or 200 mm long tie is made. For a 100 mm cavity with brick leaves, a 220 mm long tie is used.