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 20th 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 20th Century

•In such cases, stiff ties

worsened the load bearing

capacity by increasing the

eccentricity of loading.

•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, un-

compacted 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.

More Recent Developments

Research

•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.

●Towards the end of the 1970's, research began on various

matters which affect the design, production and use of wall ties.

•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

•There was also one or two cases of outer leafs being pushed out

by injection of foam insulation into the cavity.

Destruction

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.

Types of Mortar

•Black ash mortar:

pulverised furnace ash added

to the mortar in industrial

areas.

Types of Mortar

•Dredged mortar:

•using sand dredged from

estuaries and pits in coastal

areas containing sea salts.

Types of Mortar

•Plasticised mortar: mortar

with the addition of domestic

detergent, e.g. "Fairy

Liquid".

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.

Corrosion Protection

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.

●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

Table 5

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?