WALL TIE FAILURE

The outer leaf of a cavity wall is generally a weather resistant facade that is attached to the main structure of a building with a series of cavity wall ties so that the static and live loads carried by the main inner leaf can be shared.  Typically cavity wall ties are metal and are bedded into mortar bed joints as a building is constructed.
Wall tie failure can be a consequence of a construction defect; for example where the original ‘built-in’ ties have been incorrectly fitted, fitted with ties that are too short, omitted in part or omitted in full.  Such defects often manifest themselves in the bowing, leaning or bulging of walls having no corner returns.  Vulnerable areas of masonry include  gable apexes and panels between window openings.
Wall tie failure can also be a result of a buildings natural aging and deterioration process.   Over time the building’s mortar beds undergo a chemical change through carbonation and the mortar becomes aggressive to the mild steel wall ties and their protective coatings.  The life expectancy of poorly coated ties can be as little as 26 – 46 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 corrosion sets in the ties generate a build up of iron oxide (rust) layers, which occupy a greater volume than that of non-corroded steel.  In some instances, particularly when wire wall ties have been used, this increase in volume is accommodated within the mortar bed as the cavity tie erodes, leaving 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 as it is restrained only by the strength of corners and return walls.
In other cases, particularly where sheet steel ties of greater mass have been used in less forgiving mortars, the iron oxide build up may have a theoretical fourfold expansion in volume.  Such irresistible ‘expansion’ may have the effect of lifting the masonry above each line of wall ties, producing a pattern of horizontal cracks along the bed joint courses that host the ties.