THE COMPANY				top
	As the originator of the replacement wall tie, REDIFIX has focused on the remedial market and is committed to a strategy of product development to satisfy the needs of this sector. The company has a well-founded reputation for high quality products and outstanding service. REDIFIX products are manufactured within BS EN 1S09000.
	THE PROBLEM				top
	Cavity wall ties have been used in almost all types of construction throughout the UK since the early 1900s - commercial, industrial and high-rise buildings, as well as domestic housing.Their purpose was to join the inner and outer leaves of the wall at pre-arranged centres, and be capa ble of transferring loads between the two leaves. Often they were protected by a bitumen coating or galvanising before installation. 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 environ mental exposure.As the mortar ages, alkalinity is reduced by the absorption of atmospheric car bon 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 I 0 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, particu larly 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.
	THE SOLUTION				top
	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. REDHEAD/REDIFIX was called in, and with the co-operation of the local authority in Newport, developed the first REDHEAD/REDIFIX 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, REDHEAD/REDIFIX have developed a wide range of remedial products to cater for every application and all substrate materials encountered in the construction of walls.
	THE SITE SURVEY				top
	A site survey must be made to ascertain the cause of any wall defects and to establish the condition of existing walls. Wall failures can be attributed to insuffient numbers of ties or badly installed ties, as well aswall tie corrosion.The survey should analyse whether old ties should be removed or isolated. The type of construction of both leavesof the wall carefully identified to establish the best replacement wall tie for the remedial work required.
	REPLACEMENT WALL TIE SELECTION				top
	Variations in substrate call for corresponding variations in remedial fixing techniques.Traditional brick inner and outer leaves are usually re-tied by mechanical expansion in each leaf. The Mactie replacement tie comprises two parallel expander sleeves assembled on a central rod, each being set to a pre-determined torque with a pre-set torque device giving fast, controlled installation at low in-place cost. Where the inner leaf is lightweight block, or suspect material incapable of sustaining expansion stresses, secure fixing is achieved by injecting BRC3 or styrene free resin. It is preferred that mechanical fixing is retained in the outer leaf to allow vertical and horizontal differential movement of the walls due to thermal movement. The REDIFIX Product Selector recommends the most suitable product for the site substrates concerned.
	TIE SPACING				top
	The location and density of replacement wall ties is the responsibility of the structural engineer. Site tests should be carried out to obtain the actualpullout values of the selected tie in the particular site substrate. As ageneral guide, ties are fixed at 900mm horizontal centresand 450mm vertical staggered to give a domino five pattern. Around openings the density is normally increased to 300mm centres and max 225mm from opening.
	Density 2.5 Ties per metre 5 ties per metre Horizontal 900mm 450mm Vertical 450mm 450mm