Today, however, cavity wall tie corrosion is deemed to be a significant structural defect.
Initially, such tie failure was not recognised and, quite often, the symptoms were treated without
full investigation of the real cause. Lateral cracking of the brickwork joint for example, was often
attributed to frost, storm damage or ground movement.
A wide variety of ties and mortars are susceptible , combined with age, climate and environmental exposure. As the mortar ages, alkalinity is reduced by the absorption of atmospheric carbon dioxide.The presence of chlorides or sulphates in the mortar mix hastens the passive layer breakdown and removes any protective coating to the metal. The time period for this to occur varies, but after as little as 10 years, serious corrosion can occur. This corrosion results in extreme expansion of the steel tie to four or five times its original thickness.
Once corrosion is established, deterioration is comparatively rapid. Rust can attain a crushing strength equal to concrete and it is well within the power of corroded ties at normal spacing, to lift 5 metres of brickwork. Such upward movement can result in an upturning of the roof, particularly at the gable end, to alleviate the pressure commonly known as the Pagoda effect, or in bulging of the outer wall as the weight of the roof resists upthrust. Other visible evidence of the problem, can be cracking around windows or doors and at internal wall-to-ceiling junctions as floor joists are pushed up by expansion.
It is the condition of the outer wall that determines the remedial action to be taken. In some cases, the wall must be rebuilt, tying it back to the inner leaf. However, if the outer wall appears to be in good condition it can be stabilised by retying, It was back in the 1970s that the first instances of wall tie failure were identified. In South Wales, the use of black ash mortar had made the problems particularly severe. ITW was called in, and with the co-operation of the local authority in Newport, developed the first ITW remedial wall tie.
In developing a system, a number of important criteria had to be met. Firstly, ties had to be completely corrosion proof, which was solved by manufacturing from 300 series 18/8 austenitic stainless steel. Also, ties should be capable of being fixed externally to avoid re-decoration and disruption to tenants. An effective moisture barrier should be incorporated, which was solved by positioning a neoprene drip ring on the rod.
Taking into account these criteria, SPIT have developed a wide range of remedial products to cater for every application and all substrate materials encountered in the construction of walls.