Wall Tie Failure
Wall ties are an essential part of the stability of a cavity wall structure - they tie its weather protecting masonry facade to the main body of a building at regular spacings. An effective wall tie system transfers static and live loads across the cavity, which enables load-sharing by both inner and outer walls. Typically, cavity wall ties are bedded in a mortar bed joint as a building is constructed - or in the mortar bed joint as the outer leaf is constructed.
What is Wall Tie Failure?
Wall tie failure can be a consequence of a construction defect. For example:
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.
Signs of Wall Tie Failure
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.
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.
What is the result of Wall Tie Failure?
In the most extreme cases of wall tie failure, the outer leaf may have to be part- or completely demolished. This should only be taken as a last resort where cracks in the masonry cannot be repaired or where the outer leaf has moved beyond remedial work.
Results of Wall Tie Failure
Target's Retro Flex wall tie replacement system is available in three different diameters of 6mm, 8mm and 10mm. It offers the advantages of a non-expanding mechanical fixing on the far leaf and a polyester resin or cementitious grout fixing on the near leaf.
Proof testing of the far leaf using a Target Load Test Unit can be performed randomly as installation proceeds. Because the fixing method employed does not induce additional stresses into the substrate Retro Flex can be used in many and varied materials, from poured concrete columns to aircrete blocks, with satisfactory results and there is no concern to achieve the vital edge distance spacing necessary with any expansion fixing.
The design of the Retro Flex remedial tie ensures that any potential for installer error can be minimised. The multiple drip design of each fin allows the Retro Flex to be installed at an angle of up to 25° towards the inner leaf without the possibility of any water transfer across the cavity. It is recommended that each Retro Flex is installed horizontally.
The Retro Flex installation procedure is outlined below: