Wall ties are an essential element for the stability of a cavity wall. Wall ties anchor the weather protecting facade of a cavity wall to the main structure of a building. The wall tie system enables the static and live loads that are carried by the inner leaf to be transferred by the wall ties and shared by both walls. Typically, a cavity wall tie is bedded in a mortar bed joint as a building is being constructed.Wall tie failure can be a consequence of a construction defect; for example where the original 'built-in' cavity wall ties have been incorrectly fitted, fitted with wall ties that are too short, omitted in part or omitted in full. Alternatively wall tie failure may be a result of a buildings aging process, and caused through wall tie corrosion.
Over time mortar bed joints, which host the wall ties, undergo a chemical change through carbonation. The mortar becomes aggressive to the mild steel wall ties and their protective coatings, reducing the life expectancy of the brick ties to as little as 26 years. The design life of the building is typically much longer than this period and it therefore follows that at some point the wall may need replacement of its wall ties if the stability and the load sharing capacity of the wall structure are to be maintained.
As wall tie corrosion sets in the steel wall ties generate a build up of iron oxide (rust) layers which occupy a greater volume than that of non-corroded steel. In most cases, particularly when wire wall ties have been used, the increase in volume is accommodated within the mortar bed as the cavity wall tie erodes. In such cases there may be little sign that the outer facade is free-standing and the walls un-supported, save that the wall may appear bulged or out of plumb.
In other cases, particularly where sheet steel or vertical twist wall ties have been used in less forgiving mortars, the iron oxide build up may have the effect of lifting the masonry above wall tie positions. If expansion is widespread and present along a row of corroding wall ties, a tell-tale pattern of horizontal cracks may be produced.

Where any type of cavity tie failure has been established the walls should be immediately stabilised with new stainless steel 



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