During the 1970's research was conducted into the need for replacement wall ties. The actual durability of the protective zinc or galvanised coatings of the original built in wall ties was studied and this now affects the design and use of walls ties for both new build and replacement. The results of the research was found to be quite beyond any previous expectations:
- Vertical twist ties ("fish tails") were found to have a life expectancy of only 30 years - half of that originally intended.
- Wire ties ("butterflies") were found to have a service life of only 15 years.
- Mortar is alkaline which actually protects and enhances the working life of wall ties
- A reaction between mortar and the air causes a process called carbonation which turns the mortar acidic which then attacks the wall ties.
Early attempts at producing a method for replacing existing wall ties highlighted many of the pitfalls that were to be encountered. The expansion type of tie has been found to induce additional stresses into the masonry - similar to the expansion caused by the existing, rusting walls ties - and were costly to make and fit. The use of heavy section re-bars was soon outlawed because of the need for flexibility to allow the necessary differential horizontal and vertical movement between each leaf using a bar of 8mm diameter or above was found to act like mini crowbars and would eventually work themselves loose. The introduction of BSI DD140, BRE Digest 329 and the more recent BRE Digest 401 at last gave guidance for manufacturers and specifiers of remedial wall ties.