CPD: Masonry stabilisation and reinforcement
Of the nine million dwellings in the UK constructed with cavity walls, BRE research shows that two million suffer wall tie corrosion and are in need of remedial wall tie replacement. Richard Walker from repair and preservation specialist Peter Cox explores how to identify and determine the extent of the problem and outlines the repair and replacement process.
Wall ties are designed to help prevent the movement of a wall and improve its resistance to vertical loads and to horizontal pressures from weather-related forces, such as wind, cold and damp. In cavity walls, the wall ties must also prevent water transfer across the cavity and be flexible enough to allow differential movement between the two walls without undue stress to the mortar.
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Given the important role they play in ensuring the structural integrity of a building, wall tie failure can cause significant problems, including:
- Cracks and bulging in masonry and mortar
- Cracks to render coats
- Bowing walls
- The lifting of roof tiles (or the ‘pagoda’ effect)
- The peeling away of the outer leaf of bricks
- Potential building collapse
Identifying wall tie problems
Here in the UK, there are a number of reasons why wall tie failure is such a prevalent problem. These include:
- Insufficient ties installed during construction
- Insufficient protection – prior to the 1945 some ties were not galvanised
- Substandard galvanising – the British Standard for weight of galvanising was reduced between 1964 and 1981
- Chemical action – for example, where black ash mortar has been used
- Location – for example, marine environments with driving rain and salt-laden atmospheres
Signs of bulging, bowing and cracking in mortar joints can all be signs of wall tie failure. In particular, deformed masonry with regular horizontal cracking is likely to be suffering from wall tie failure generally and the entire structure may need to be treated. In addition, internal cracks and the lifting of roof edges can also indicate wall tie failure. Where such visual signs are evident, a specialist wall tie survey must be undertaken.
Peter Cox was established in 1951 and today boasts over 60 years’ experience in providing a range of property services, including damp proofing, waterproofing, timber preservation and wall stabilisation.
Peter Cox has built an enviable reputation as the market leader in the repair and preservation of all types of property, from private housing to historic and listed buildings.
It is an approved installer of the world-renowned Cintec systems and its branded AnchorbondVtek masonry anchors are used in heritage buildings across the UK and Ireland.
The first step in a repair programme is a survey to determine the extent of the problem. The positions of existing wall ties are plotted using a metal detector and then an endoscope is used to examine their condition in the cavity. Ties should be inspected in each wall and at different heights. Bricks should usually be removed for the physical examination of sample ties or if cavity wall insulation is present.
During the inspection process, it is important that care is taken not to create debris that could cause cavity bridging, especially where there are insulated cavities, and also to minimise the disturbance to existing finishes.
The tables in the BRE Digest 401 should be referred to when estimating and classifying the two visible sections of the wall ties being inspected in situ; namely, the cavity spanning section and the section buried in the outer leaf. Table 2 specifies the corrosion level and a description of the condition, while Table 4 specifies the predicted life based on the current condition and recommended action.
Depending on the observed current state of the wall ties, recommended actions range from inspections within 10, 5 or 2 years or the need to fit remedial ties.
The surveyor will also need to consider whether the problems can be resolved by using remedial wall ties or lateral restraint ties or whether additional work is required. It is particularly important to consider whether additional work is required when dealing with non-standard constructions.
Remedial wall ties
Where there is total failure, red rust and erosion, or red rusting to parental metal it is vital to fit remedial wall ties immediately. It is also recommended to effect immediate repairs where there is terminal corrosion, zinc white rust or heavy to moderate corrosion to zinc or bitumen.
A range of alternative wall tie designs is available – resin, mechanical or cementitious. The choice of tie will depend on the type and condition of the masonry. In most cases, the existing ties are isolated to prevent further corrosion and cracking of the mortar joints.
Formed in austenitic stainless steel, replacement ties are available in different lengths to suit different cavity widths. In cavity wall repairs, designs should incorporate a centre drip to prevent transmission of water across the cavity.
Installation of remedial wall ties
Whatever wall tie is used for remedial work, the number one rule to observe is to fit the remedial ties first, then deal with the old ties.
Work should never be attempted the other way around.
For replacement wall tie solutions, at Peter Cox, we favour our DriveTie wall tie. This is because they provide an effective and cost-effective method of tying most masonry types including brick, block and timber frame.
Once the new remedial wall ties have been installed, the old ties can be dealt with. Typically, all ‘fishtail’ ties must be dealt with by isolating the ties within the mortar joint and enveloping them with a waterproof foam to prevent further corrosion and expansion because, if not, they rust and can continue to cause further cracking of the mortar joints. However, it is possible to leave narrow gauge ‘butterfly’ ties in situ because these are unlikely to cause additional damage as they corrode.
Anchorbond Vtek masonry anchors
The anchor is comprised of a hollow stainless steel bar surrounded by a woven polyester fabric mesh sleeve into which a specially formulated cementitious grout is injected under pressure. The flexible sleeve expands and moulds itself into the spaces within the wall, providing a strong mechanical and chemical bond when cured. Various attachments can be welded to the anchorhead.
Benefits of the AnchorbondVtek masonry anchors include:
- Purpose-designed for each application
- Versatile in use
- Works well in weak substrates
- Effective in poor quality materials and for bridging cavities
- Invisible when installed
- Fire resistant
- Cementitious-based and, therefore, sympathetic to the original structure
- Approved by heritage authorities
They can be used as replacement and supplementary wall ties, lateral restraints, for stabilising masonry (including solid, cavity, hollow pot and rubble filled), crack stitching, stitching anchors (for example, in arch consolidation), as stud anchors, in parapet wall strengthening, and retaining wall anchoring.
Where there has been extensive damage to masonry as a result of wall tie failure, additional masonry stabilisation work may be required. This is typically one or more of the following:
- Lateral restraint
- Steel reinforcement
- Crack stitching
Subject to the correct design and specification, installation of grout-inflated anchors, such as the AnchorbondVtek masonry anchors, will also serve to address these issues. In addition, grout inflated anchors can serve as secure fixing anchors, where required.
Finally, some additional rebuilding of brickwork and / or repairs to external finishes may be required.
Grout-inflated anchors offer a versatile and cost-effective solution for masonry stabilisation and strengthening in a variety of applications. Some typical applications are illustrated below.
Lateral Restraint - Wall to Floor Joists (not relevant on bridges)
- 20 or 30mm diameter hole – to suit anchor length
- Anchor expands to provide rigid support
- Neoprene drip in cavity
- Cavity and solid walls can be tied to the joists running parallel to the wall
- Bespoke length to suit individual requirements
- ‘Armlock’ fixed with stud anchor to joists at 90° to wall
Stitching Anchor for Rubble Filled Wall
- Sock expansion into the soft friable core
- Anchor body design to suit load - std 15 x 15, 20 x 20 or 30 x 30mm square hollow section
- Length to suit
External Wall Anchored to Internal Wall
- External wall and Internal wall tying options
- Anchor positioned at floor levels and hidden in the void below the floorboards
- Length to suit individual situations – bespoke designs
- Bottle screw to tension the anchor
- Solid bar connector
- Choice of 30mm and 60mm diameter anchors
- Delaminated and poor quality masonry can be stitched together
- 30mm anchor in 60mm diameter drilled holes
- Anchors positioned to suit individual constructions
- Stitching anchor at 90° to the normal
- Length to suit individual situations
- Bespoke design – anchors made to measure
- Typically 30 x 30mm hollow square section steel anchor filled with grout to carry main load
The installation of grout-inflated anchors
Installation of grout-inflated anchors is typically a five-step process.
Drilling: installation holes are created in the masonry using a wet diamond drilling process with extension drill bits added as required to achieve the required hole depth. The waste is removed in the form of cores.
Inserting anchor: for stabilisation work anchor lengths are typically 1m to 11m in length. Care needs to be taken not to puncture the polyester sock.
Injecting grout: the cementitious grout is site mixed and then sieved before pouring into a pressure pot which operates at between 3 and 4.5 bar. The grout is forced into the sock around the anchor. This expands to fill the cavity drilled out, starting to harden after approximately one minute.
Anchor sections: the grout is injected through the rod in the case of hollow section anchors but if solid single or multi bar sections are used, a separate injecting tube is inserted in the fabric sock. As illustrated here, threaded rods can be used to facilitate fixing attachments to the anchor head – for instance, for tie bar extensions and support brackets.
Surface repair: drilling holes are made good so that the repair will be almost invisible – this is particularly important in the case of historic properties.
Where are grout-inflated anchors used?
At one of the finest medieval castles in England, our team installed a series of specially designed bespoke anchors into the Nevill tower which had been falling apart. During the process, we worked closely with English Heritage to ensure we met their exacting standards.
Manchester’s light railway system was first opened in 1992 and continues to undergo massive expansion. Our technicians installed high load fixings into existing buildings adjacent to routes. These fixings take the straining wires where the overhead cables are fixed.
Dysart Toll Booth
This medieval toll booth in Fife was comprehensively repaired in 2009. We repaired major cracks with six metre stainless steel rods drilled through the building in hidden locations.
In order to carry the new gantries which will hold the electric cables for electrification of the line – which create huge loads of up to 200kn – the viaduct walls had to be strengthened. Our technicians installed 6m long anchors, going in at a 45˚ angle, for the gantries over the two-mile stretch. In some places, it was necessary to drill through the viaduct horizontally to enable installation of the cantilever gantries.
The access bridge to this 200-year-old, Grade II listed building – now a golf course and country park – was cracking, so our technicians installed anchors to ensure the bridge was secure from further deterioration.
The signs of wall tie degradation or insufficiency, such as bowing walls or horizontal cracking, should be investigated immediately and a structural assessment carried out.
As a result of changing building standards, wall tie-related problems are extremely common in the UK, especially in older properties or homes built in the 1970s.
Because of the risk of possible masonry collapse, early intervention is vital and can halt serious damage occurring. Where more serious masonry problems exist or a structural solution is required, grout-inflated anchors are a well-proven and versatile solution. Their installation has very littlecosmetic impact on a building, making them an ideal solution for listed properties.
Richard Walker is National Technical and Development Manager at Peter Cox
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