Wall tie failure
Main article: Defects in Brickwork
Failure of wall ties has become a significant problem in recent years. The main cause of failures is rusting of metal ties, although there can be other causes, such as failure to properly bed the tie in the mortar joint, poor quality mortar reducing the bond between tie and mortar, or not installing the requisite number of ties.
- The rust will have a significantly greater volume than the original metal. This expansion of the tie may cause cracking and distortion of the structure, particularly where strip ties have been used. The rust-induced expansion in strip ties can lead to secondary damage, such as a redistribution of loads, buckling and bulging of wall, and damage to the roof as the external leaf increases in height.
- The less bulky wall ties will not generally produce enough expansion to induce cracking unless the joint is abnormally thin or the mortar is very dense. Unfortunately, wire ties produced in the UK before 1981 had less rust protection than strip ties and therefore are likely to have a shorter life expectancy; a particular problem because failure can occur without the outwardly visible warning signs produced by cracking.
- Cracking will also reduce the weather resistance of the wall, which in turn accelerates the rusting process.
A wall tie from a house built in the 1920's - When placed, both sides of the wall tie would have been symmetrical in size and shape to each other. The right hand side of the tie was in the outer leaf, and has been heavily corroded since then.
 Corrosion and Rusting
Corrosion refers to a partial or complete wearing away or a dissolving or softening of a substance by a chemical or electrochemical interaction between the metal and its environment. Rusting is a type of corrosion and it is caused by the interaction of water and iron or steel in the presence of oxygen. Rust is a hydrated iron oxide, which although solid in form, is weak and brittle.
Galvanising is a means of protecting iron or steel by coating it with zinc. The zinc protects the steel electrochemically. Of the two metals zinc is the more chemically inactive and it will slowly corrode in the preference to the steel. The thicker the original coating of zinc, the longer it will last. The zinc protection reduces in thickness but it does not undergo a chemical change. Eventually the zinc coating will disappear.
The process of rusting can accelerate due to:
- Aggressive chemicals: black ash is a product of coal mining which was sometimes added to mortar to give it a black colouring. The high sulphur content of black ash produces a weak sulphuric acid when wet for long periods. This sulphuric acid can attack a galvanised coating.
- Chloride salts, which may come from marine sands or may have been added to mortar as accelerators, can, even in small amounts, speed up corrosion.
- Carbonation: where ties are well bedded in mortar, protection against rust is provided by calcium hydroxide, which is formed as the cement hydrates. Unfortunately, as the mortar slowly carbonates this protective alkaline layer is destroyed. Carbonation will occur more quickly in permeable mortars.
- The building, or particular sections/elevations, experiencing high levels of moisture ingress because of their exposure.
The portion of the tie in the outer leaf will usually be the most severely affected because of exposure to rain penetration. However, condensation may also produce enough moisture to allow rust failure to occur.
 British Standards and Premature Failure
In the UK during 1945, a British Standard (BS) was introduced which set out, amongst other things, a minimum thickness for the galvanising layer. The standard identified the two basic shapes of tie – vertical twist and butterfly. The standard specified that the galvanising should be twice as thick on the vertical twist as it was on the butterfly ties. Unfortunately this minimum thickness was reduced in 1968, because it was thought that the standard was excessive.
In 1981 the standard was increased and specified the same thickness for both types. The reason for this was the increasing evidence regarding performance in use and the realisation of the scale of the problem. The problem of rusting was thought at one time to be localised as it was believed that it occurred mainly in areas where a catalyst, for example black ash mortar, increased the likelihood of breakdown. It was later appreciated that the problem was more widespread and the causes more complex.
The Building Research Establishment has suggested that all steel or iron wall ties inserted prior to 1981 are at risk of (premature) failure (premature relates to the notional 60 years life of a building.) Because cavity walls tended to be first introduced in areas with high rain penetration, including salts, there can be a potent combination of age and aggressive environments.
 Stainless Steel Ties
Stainless steel walls ties are available. Stainless steel is a ferrous alloy which contains at least 10% chromium. Since chromium is more reactive than iron, a ‘self healing’ impermeable layer of chromium oxide forms naturally on the surface of the steel. Chromium oxide is durable in a wide range of exposures and prevents the formation of rust. Where there is exposure to chlorides, for example in marine conditions, care must be taken in selecting the appropriate grade.
 Detection and Investigation
Wall tie failure can often be identified by the horizontal cracking that results from the expansion caused by the rusting process. Unlike sulfate attack the cracks will coincide with the position of the ties (and will obviously only occur in cavity walls). However, because butterfly ties will usually not cause cracking it may be necessary to carry out investigations where the problem is suspected or anticipated.
Ideally, strip ties should be identified before the cracking occurs. This might involve a visual inspection of the suspect ties, either by using an optical probe or by removing bricks. It should be kept in mind however, that a tie can be in good condition in the cavity but poor in the outer leaf and therefore exposure of the end embedded in the outer leaf is a sensible step.
Ties can be assessed visually for rusting. The presence of red rust, the product of the oxidation of iron, indicates severe rusting, while the presence of white deposits or a blackening of the galvanising indicates that the zinc is corroding. Measurements of zinc thickness will enable the projection of the remaining life (The Building Research Establishment give some useful guidance on this.) Metal detectors can determine the exact pattern of tie placements and can help in establishing if a defect is caused by wall tie failure.
Disguising wall tie replacement is difficult, particularly where houses are rendered. Painting will help, but it is very difficult to often match the texture of the original render. In plain brickwork, extensive repointing is usually necessary.
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