Remedial Wall Ties - Presented by Robert Hall 1. A brief History how and why wall ties have developed. 2. Research into how existing wall ties perform. 3. Findings of the Research. 4. Stainless Steel. 5. Why and how wall ties fail. 6. Buildings most susceptible to wall tie failure. 7. BRE Digest 401.
A Brief History
The Early 20 th Century During the 1920's double leaf walls with open cavities were introduced, which improved waterproofing and heat retention on the inner walls. The load bearing was now spread between the two leafs at the top. The view was that the ties should improve stiffness, against bending, which resulted in widespread use of vertical twist ties.
The Early 20 th Century In such cases, stiff ties worsened the load bearing capacity by increasing the eccentricity of loading.
More Recent Developments Since the late 1940's almost all houses have had cavity walls, leading to the introduction of block work for the inner leafs. These blocks were usually cement based, using weak, uncompacted mixes with clinker of furnace ash aggregates, e.g. 'breeze blocks'. Differing materials increases the likelihood of differential movement between them, coefficient of thermal and moisture expansion or shrinkage and of elasticity are different.
Research Towards the end of the 1970's, research began on various matters which affect the design, production and use of wall ties. Research was conducted into the effects on different parts of the buildings, the wind gusting and funnelling resulting from the shape of buildings especially tall ones, and of the adjacent ground. Research also considered the effects on different parts of the buildings, the wind gusting and funnelling resulting from the shape of buildings especially tall ones, and of the adjacent ground.
Research also began into the durability of zinc coated wall ties under service conditions as a result of a few cases of cavity walls collapsing in high winds. The classic example is the high rise wall collapse at Plymouth Polytechnic in 1983. Destruction
Destruction There was also one or two cases of outer leafs being pushed out by injection of foam insulation into the cavity.
Findings of Research Mortar is an alkaline material which does not attack galvanising, and actually enhances the working life of bare metal embedded in it. There is, however, an ongoing chemical reaction between the air and the mortar called carbonation, which causes the mortar to move towards acidic. In this state it will attack the steel and any galvanising, leading to corrosion and failure of the ties.
Old Ties Vertical Twist ties were likely to have practical service lives of only half the originally intended 60 years. Wire ties were found to have a reliable service life of only 15 years. If mortar is allowed to decay, water will pass through the outer leaf at an even higher rate than normal and run down the inside surface of the outer leaf. This will lead to the corrosion of the ties in the cavity, starting adjacent to the inner face of the outer wall, and if it penetrates the mortar bed will lead to accelerated decay of the embedded tie end.
Types of Mortar Three kinds of mortar used were found to be highly corrosive.
Black ash mortar: pulverised furnace ash added to the mortar in industrial areas. Types of Mortar
Types of Mortar Dredged mortar: using sand dredged from estuaries and pits in coastal areas containing sea salts.
Plasticised mortar: mortar with the addition of domestic detergent, e.g. "Fairy Liquid". Types of Mortar
Corrosion Protection Years after the original construction, a reaction can take place between the mortar and the air. This process, called carbonation can change the nature of the mortar from being positively alkaline, to become towards acidic. Determined by: The porosity of the mortar and the surrounding brickwork. The available moisture in the air.
Corrosion Protection The effect this has varies greatly according to the quantity, quality and types of finish on the mild steel. Also, if the tie end is close to the outer face then this will be attacked (carbonation) at an early stage and rusting will develop rapidly. This process is commonly seen on concrete structures where spalling has taken place and the reinforcing bars are exposed.
Corrosion Protection Rusting is a process of oxidation of the metal, where the the components of steel and oxygen combine to form oxides which occupy considerably more volume than the original. Hence the possibility of ties splitting the mortar beds as they swell.
Effects of Corrosion The power of rusting wall ties by expansion should not be underestimated.
Effects of Corrosion There are many readily available examples where wall tie expansion has caused issues. It is most pronounced at the gable ends due to the additional height and the reduction in the imposed load due to the apex. A vertical twist wall tie may expand up to 7 times its original thickness when rusting occurs
Effects of Corrosion In this case it is possible to see right through the roof cavity where the tile overlap has been broken as a result of the outer leaf lifting due to wall tie growth.
Effects of Corrosion If rusted wall ties have expanded and weakened or split the horizontal joints then the ability to transfer the wind loads vertically may be lost and practically all the wind loads will have to be transferred to the inner leaf by remedial ties which should be positioned in the bands between every horizontal crack.
Mild Steel Protection Traditionally mild steel is protected against corrosion by covering the surface completely with a protective finish. Common finishes to be found are paint, bitumen, zinc plating and galvanising. The purpose of the finish is to prevent oxygen from attacking the steel and the finish must therefore be complete. If there are pin holes or scratches, corrosion can start at those small points, work under the coating and lifting it away from the surface.
Mild Steel protection Hot dip galvanising was favoured for many types of heavy wall tie and the "Tri-galv" specification was frequently quoted for butterfly ties. This worked well provided that the tie was not knocked with a trowel or bent to line up with the bed joints. If this happened the galvanising may well be broken allowing corrosion to start.
Stainless Steel Corrosion is not halted just by calling for "stainless steel". Correctly specified and applied stainless steel products will last a considerable length of time but a basic understanding will be helpful.
What is Stainless Steel? Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. The chromium is the critical element in giving steel its unique "stainless" properties. The corrosion resistance is achieved by the chromium enabling a tough, invisible, corrosion-resisting chromium oxide film to be formed over the surface of the metal. The film adheres strongly to the surface but if it is damaged mechanically or attacked chemically it will heal itself, provided that even a very tiny amount of oxygen is present.
The Effects of Wind Wind Loading can be devastating The effect of a 45m/s (100mph) wind on the gable end of a house can be quite devastating. The wind lasted only a few seconds.
Wind Loadings The standard Wind Zone Chart is based on the highest mean wind speeds to be expected once every 50 years. Because these are 'mean' wind speeds the effects of gusts lasting a few minutes may be much greater.
Wind Loadings
Wind Loadings
Wind Loadings
Wind Loadings
Wind Loadings When calculating for wind loading, whether for walls or roofs, the greatest factor is always the negative pressure suction. The negative pressure is usually around 50% greater than the positive pressure - compression - from the direct wind force.
Wind Loadings Wind loads initially act on the outer faces of walls and are then transferred, by design, to walls running at right angles to them and into the roof and ground structures. If there is cracking between the cross walls and the outer walls the ability to transfer loads may be lost.
The Survey The very first point to establish before working on a 'cavity wall problem' is that it is a cavity wall. Whilst cavity walls have existed since the start of the 19th century, they only became common since about 1900. Solid masonry walls are also common up to about 1950. It may also be possible that the inner leaf is a timber, concrete or steel frame.
The Survey The construction may change from cavity to solid somewhere in its height. Particular areas to look are where there is a change in finish between storeys and at parapet walls. A common technique is to use fair-faced masonry cavity walling for the ground floor storey and tile hung solid block work above. Extensions to older buildings, either as additional storeys or additions to plan area, are often a different form from the original.
Risks of Wall Tie Failure Construction using 'black ash', 'dredged' or 'plasticised' mortar. Buildings subject to higher than normal exposure, e.g. near the coast, on exposed hills or in areas subject to heavy industrial pollution. Properties built between 1900 and 1940. Post war buildings with vertical twist ties with substandard coatings owing to shortages just after the war, or those built during building booms, particularly the early 1970's.
Risks of Wall Tie Failure Buildings older than 40 years with galvanised vertical twist ties. Buildings older than 25 years built with galvanised wire ties. Timber-framed buildings older than 15 years with galvanised ties.
How to Spot The Risks The survey may rely on existing experience in relation to factors such as age, exposure, mortar type (e.g. black ash) and tie type, or it may involve the recognition of the characteristics of wall tie failure such as horizontal cracks and wall growth.
How to Spot The Risks The cracks produced by the expansion of rusting vertical twist ties can normally be recognised by horizontal joints separating at regular intervals to coincide with the vertical tie spacing. They are more discernible just below eaves level or below openings where the imposed brick loads are at their lowest.
How to Spot The Risks Wire ties probably have insufficient bulk to cause lifting when corroding, except in very hard mortar and/or thin joints. Cracking in the inner leaf is rare in heated dwellings and would normally be treated as part of re-decoration.
Common Pitfalls Do NOT check existing ties with a boroscope only. The tie may seem perfectly sound within the cavity but may be totally rusted within the outer leaf mortar joint. A metal detector must be used to define the existing tie pattern and density. Existing ties may be sound but at too low a density, in a very weak mortar bed or too short to provide adequate embedment.
Site Tests To aid the specification process remedial wall ties should be inserted into the affected elevations and a tensile test effected with the aid of a suitable load test unit. BRE Digest 401 specifies a testing regime. At the same time the cavity width can be checked and the composition and condition of the inner leaf determined. If cavity insulation is present this may necessitate the removal of outer leaf bricks to aid the survey and will reduce the options in which remedial ties to use.
Table 5 Figures are for walls with a tie density of 2.5 ties/m² and which are horizontally spanning between masonry returns or have sufficient edge ties to give simple support of the vertical edges. Bands of basic wind speed are used which correlate with those used to classify ties in DD 130: Part 2. For cladding masonry which is vertical spanning, e.g. between openings, multiply the table values by 1.72 For different tie densities multiply the table values by 2.5 /tie density; for example for vertical spanning classing masonry at 3 ties/m² multiply by 1.72 x 2.5/3 = 1.43. Larger developments or complex terrain may need more detailed designs by a qualified engineer
I hope this brief presentation has given you all an insight into the mysteries of wall ties, why they fail and how to spot the early signs of failure. The BRE produces many documents that are very helpful if you are interested in further background reading. We are always available to offer any help, advice and guidance. Thank you for your time. Questions?