Foundation, Concrete and Earthquake Engineering

Wall Ties -Making the cavity walls stable

Cavity wall is a load sharing system which use ties in specific patterns and provides extra resistance to weather conditions because of the cavity. The other main advantages of cavity walls include the thermal insulation, sound insulation, prevents moisture entering from outside and the structural stability. The material of construction or the structure of the cavity wall can vary, and the strength of these walls also depend upon the tying mechanism and tying product (wall tie) being used. Wall ties are the important member which significantly determines the stability of a cavity wall.

Builders use wall ties in cavity walls to hold the two walls together and stop them bulging or bowing. Horizontal cracking in brickwork, external rendering or building blocks can often be caused by the failure of cavity wall ties. In old properties it is common to find steel wall ties - these rust and corrode and have to be replaced. When the cavity wall ties corrode they expand to several times their original thickness, which causes the wall to crack at the mortar joints and often result in major damage and sometimes collapse of the wall.

The type and density of wall ties depends upon certain factors like the area of uncontrolled brickwork, the extent of exposure to wind, size of the cavity height of the building and the typical nature of the structure. If the wall ties used are not selected carefully, this can lead to damp penetration, the failure of the whole structure or even collapse.

If you are facing a wall tie failure, you should contact the specialist company. Their qualified surveyor will carry out a survey on the existing cavity wall ties using a metal detector, borescope and damp meter and by drilling into the wall at appropriate intervals he will view inside the cavity to check for cavity wall tie corrosion. He will also determine the extent of any external cracking and advice you of any appropriate repairs that are required or if the services of a structural engineer are needed. They will then issue a detailed report, our recommendations and estimate for all necessary cavity wall tie replacement and remedial work.

The several key elements of the wall survey includes location and approximate age of the property, width of cavity, detecting cracks and any signs of movement, cavity insulation, condition of ties, the pattern in which the ties are arranged, etc. The survey is carried by using specialist instruments like metal detectors and endoscopes, etc.

The detailed survey will help in identifying the exact problem and will provide a good starting point to combat wall ties failure problems by carefully selecting the appropriate repair strategy. The repair techniques that can be adopted may include the use of

• Structural wall ties and restraints

• Embedded wall stitching

• Strapping

• Structural Pinning

• Resin bonding or resin beam repairs – When the base material is not adequate or unable to take the stress of the mechanical member, then the resin helps in bonding the tie to the base material.

All the above mentioned repair strategies should always be carried out or supervised by a competent person as many of these require the old wall ties to be removed and replaced by new wall ties. Tapco HomeDry use a double expansion remedial cavity wall tie that is manufactured from corrosion resistant Austenitic 304 grade stainless steel tie bars with Neoprene sleeves. These cavity wall ties have a unique ‘tall-nut’ which when turned with a setting tool, rotates the whole tie bar, forcing the inner leaf sleeve to open and grip the masonry. At a factory set torque level, the end of the ‘tall nut’ shears away and the remaining nut is forced down the bar to expand the sleeve and grip the outer leaf.


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