Masonry facades

The corrosion of metal fittings, ties or other fixings can seriously disrupt brickwork or other porous materials in which they are embedded. The main problem is the formation of hydrated iron oxide (rust), which is accompanied by an increase in volume of around 400%. One of the most common forms of failure is the corrosion of galvanised steel wall ties.

The earliest wall ties were fabricated from wrought iron, but various types have been used over the years including painted mild steel, bituminised mild steel, galvanised mild steel, stainless steel and plastic generally of the vertical twist, double triangle or butterfly type.

In 1945, BS 1243 was introduced. The standard covered cavity wall ties and required a minimum zinc coating weight for wire ties of 305g/m2 and 610g/m2 for vertical twisted types. In 1964, the standards were reduced to 275g/m2 and 460g/m2 respectively. The life expectancy of bitumen and zinc coatings on these ties is frequently well under the 60 years that was originally predicted. In fact, in 1981 BS 1243 tripled the minimum allowed zinc coating thickness on wire ties.

While the reduction of galvanising standards under BS 1243 led to some failures, it is by no means certain that many failures were simply the result of non-compliant galvanising in the first place. Zinc coatings as low as 45g/m2 are not unheard of. However, BRE suggests that average zinc loss is between 1.4-2.8mm per year (Corrosion of metal components in walls, Digest 461, 2001). For pre-1978 ties this results in a predicted coating life of 13-26 years for wire ties and 23-46 years for vertical twist ties. Bitumen coatings have been found to last equally as well as some of the thicker zinc coatings.

On the inner leaf, where the circumstances are less aggressive, the zinc coating can be expected to last much longer.

If wire ties have been used these have the unpleasant tendency of corroding away without substantial physical disruption to the brickwork. Damage becomes manifest by the sudden collapse of an outer leaf, particularly in conditions of high wind. This is why areas of particularly exposed wall should be inspected as a priority.

BRE Information Paper 13/90 sets out useful guidance on the buildings most likely to be a risk from wall tie corrosion. Together with the later BRE Digest 461, it paints a fairly pessimistic view of the life expectancy of galvanised ties generally.

Areas such as the south, north-west and north east coasts of England and the south coast of Wales are particularly vulnerable to corrosion problems in view of the harsh marine climate and the propensity of the walls to stay wet for long periods of time. Investigations have shown that in many cases the embedded faces of the tie will corrode first, with the outer leaf, followed by the inner leaf, showing signs of deterioration, sometimes before corrosion is evident on the portion within the cavity.

Also of particular risk are:

• buildings constructed using black ash mortar;
• buildings located in areas of higher than normal exposure;
• buildings constructed in the period 1900-1940;
• post-war buildings constructed using poorly galvanised ties - especially during the early 1970s;
• buildings constructed before about 1955 with vertical twist ties;
• buildings constructed before about 1970 using galvanised wire ties; and
• timber framed buildings constructed before 1975 using galvanised frame ties.

Buildings constructed from black ash mortar are known to be particularly at risk from the corrosion of wall ties. Black ash mortar, which is commonly found in the Midlands, industrial north and south Wales, is generally composed of a mixture of hydrated lime and finely ground ash or coal dust. Initially highly alkaline, the material soon loses its alkalinity as a result of exposure to atmospheric carbon dioxide and other acidic atmospheric gases. Moist ash increases corrosion rates, particularly if it contains significant amounts of sulphides - these oxidise into sulphates. The presence of chlorides in the ash accelerates corrosion rates yet further.

Walls most likely to be at risk are in exposed situations, with frequent wetting or saturation, defective pointing or other similar defects - perhaps identifiable in dry weather by staining or moss or algae growth. Certain parts of walls may also be at risk from failure during high winds. In particular, large gable walls, large areas of cladding and narrow unreturned strips of brickwork should be examined as they can collapse given suitable exposure. (This happened during the 1987 and 1990 gales in the UK.)

Factors affecting the rate of corrosion

Moisture plays a significant role in the rate of corrosion. It is not so much the quantity as the duration of the moisture that will have an effect. In dry air, a film of zinc oxide forms on the surface of a zinc coated tie, mainly as a result of the reaction of zinc with atmospheric oxygen. The zinc oxide converts to a hydroxide in the presence of moisture and in turn this converts to zinc carbonate through reactions with atmospheric carbon dioxide. The zinc carbonate is very protective and reduces the rate of corrosion considerably. Up to about 70% RH the corrosion of zinc is little affected by humidity but beyond 70% there can be a marked change as a result of the formation of an invisibly thin layer of moisture on the surface. The moisture functions as an electrolyte and forms a corrosion cell.

Rainfall performs very much as humidity in terms of the formation of a surface layer. It also serves to wash away any surface deposition that might, in itself, have harmful consequences. However, rainfall tends to dry out and as long as the zinc is not held in contact with moisture the effects are not severe. It is the degree and duration of wetness that are the governing factors.

Exposure to salt spray in coastal locations can lead to contamination by chlorides, although investigations have shown a marked drop in chlorides a kilometre or so back from the coast. Chloride ions are highly conductive and can form aggressive corrosion cells. However, except in cases where the metal is in regular contact with salt spray, the effects of chlorides on galvanised zinc are probably over-estimated. High exposure probably carries with it the same risks as exposure to an industrial atmosphere, although opinions are mixed on this point.

Sulphur dioxide is one of the most harmful atmospheric pollutants and can alter corrosion rates drastically in test specimens. Sulphur dioxide is present as gaseous emissions in the atmosphere - primarily from the burning of fossil fuels. The gas is dissolved in rainwater and distributed as particulate matter. Contamination can have the effect of reducing the pH of the mortar, preventing the formation of a passive film.

Mere depletion of the zinc does not mean that the component will fail immediately; simply that the base metal is no longer protected from corrosion. The rate of corrosion depends more or less on the factors listed above and the amount of oxygen, moisture and electrolytes present.

Visual indicators

Unless wall ties have failed completely, there is often little visual evidence as to the possible failure of individual ties or a patch of ties. However, indicators such as bowed or cracked brickwork (possibly horizontal cracking at around 450mm centres) and random bulges can indicate a problem. Corroded steel occupies about 4 times its normal volume and so if vertical twist ties are used, corrosion of these usually becomes manifest in the form of cracking, either through brick joints or reflected in rendered surfaces. Cracking may be more pronounced towards the top of a wall.

Expansion of an outer leaf as a result of cavity tie corrosion can sometimes be restrained by the roof construction, in which case bulging of the leaf is likely to occur.

Twisted wire ties are much thinner in section. Corrosion of these is unlikely to lead to the same volumetric expansion as vertical twisted types. In these circumstances, cracking may not be apparent.

Detection

Wall tie detectors can be used to attempt to locate the position of ties. Tie detectors comprise what is in essence a hand-held metal detector with interchangeable heads to scan for ferrous or stainless steel. Care in the use of this equipment is necessary as signals can sometimes be very weak: try several scans at different intensities before drawing any conclusions. Sometimes, ferromagnetic or paramagnetic particles can give misleading results and scanners with large coils can make it difficult to locate the tie accurately. A great deal of effort can be expended attempting to determine reliable locations with little practical result.

Cover meters can sometimes be used to give a guide but they are calibrated to find steel running parallel with the face of an element and may not give satisfactory results.

A more reliable assessment can be achieved by using portable pulsed radar equipment, although such a form of diagnosis involves specialist equipment that is not normally hired. Specialist testing consultancy practices are best able to provide this type of service. A hand-held scanner is passed over the wall. This identifies any major discontinuity of dielectric constant in relation to the antenna surface and thus enables the position of the cavity and the position of the ties to be located with reasonable certainty.

Experiments with infrared photography have had some measure of success, although reviews are mixed and the picture can become confused. Equipment can be hired reasonably easily and used with a minimum of training. The system enables a fairly rapid assessment to be made. This can be validated by means of physical opening up or metal detection.

A visual examination using a fibre optic probe or a physical opening in the brickwork will give a better indication. The problem with fibre optic probes is that they do not necessarily reveal whether the tie has corroded - the area most at risk is that section buried in the wall.

The placing of ties is sometimes erratic, but depends on the thickness of the cavity. For an outer brick leaf of standard thickness, and a cavity of 50-75mm, ties should be placed at 900mm centres horizontally and 450mm centres vertically in a staggered pattern. If the cavity width is increased to 100mm, horizontal centres should be reduced to 750mm. For a cavity width between 100-150mm, ties should be at 450mm horizontal centres.

Replacement strategies

BRE Digest 329 (2000) sets out very useful data in terms of the selection of suitable replacement strategies for wall ties and includes a decision making tree to assist the process.

There are many techniques for the replacement of defective ties, involving epoxy, grouted or expanding type bolt systems. Generically they can be summarised as follows:

• mechanical expansion types;
• resin/grouted anchors;
• resin/grouted and mechanical combination ties;
• friction fixed ties; and
• inflated sock ties.

Care must be taken however to select the correct remedial method, as some of the expanding types cannot be used in lightweight inner block leaves. Pull out tests should invariably be carried out to ascertain the capacity of the inner leaf. Replacement ties should be provided with a suitable level of flexibility in situations where long-term creep of the frame is likely to occur. For example, fitting rigid ties to a timber-framed building could have serious effects.

It is important also to remove the remains of the old ties as further corrosion in these could create further disruption to the brickwork.

Polyurethane foam

One method of repair identified by Digest 329 is the use of polyurethane foam injected into the cavity. A suitable polyurethane foam system may be injected into a masonry cavity wall by injecting liquid polyurethane through properly spaced holes in the outer leaf. The foam expands in the cavity and sets to become substantially closed-celled, cross-linked, rigid polyurethane foam which strongly adheres to the inner and outer leaves, so that they become bonded together.

The foam also serves the dual purpose of increasing thermal insulation values and, because it is substantially closed cell, does not take up water.

The foam stabilises a wall by adhering to the surfaces of the cavity and providing a continuous structural connection between the 2 leaves. It can form a good bond with surfaces that are unprepared and in a raw state. The adhesive bond strength is better than 60kN/m2, which is generally in excess of the wind suction likely to occur on the wall. (British Urethane Foam Contractors Association)

Stabilisation and thermal insulation of cavity walls is covered by British Standard Code of Practice, BS 7456:1991 Code of practice for stabilisation and thermal insulation of cavity walls (with masonry or concrete inner and outer leaves) by filling with polyurethane (PUR) foam systems.

Foam filling is not suitable for timber-framed or metal-stud construction.