The Canterbury North Control Building, part of the UK power network infrastructure, was suffering from “concrete cancer”. Cracked and crumbling concrete window cills, mullions and lintels needed specialist concrete repairs to remedy the underlying issues and prevent further deterioration.
The Problem
AWT was contracted by AMEC Foster Wheeler to provide structural repairs on part of the UK power network infrastructure. Corroded reinforcing bars within the concrete window mullions and lintels was causing the concrete to crack and crumble from within. This is commonly known as “concrete cancer”. Weathering had also taken its toll on the concrete framework around the windows and doors.
Failing lintels and mullions can cause difficulties opening and closing doors and windows as the frames become distorted. There is also a danger of injury from falling masonry. More than superficial repairs by traditional building techniques was required.
The Solution
Action Wall Ties’ skilled operatives used specialist cutting tools to cut away larger areas of damaged concrete. Specialist tools were used to scabble the remaining areas to remove layers of loose scale, corrosion and unsound concrete.
The existing steel reinforcing bars were restored by this process to a clean, bright condition and coated with a suitable steel primer to prevent further corrosion. New stainless steel reinforcing bars were installed as necessary.
The bare concrete surface was treated with a Flexcrete 842 Bonding Bridge to prepare the surface. The damged areas of concrete were then restored using Flexcrete Monomix concrete repair mortar system. The finishing touch was achieved by applying Monolevel FC, a cosmetic grade mortar, to seal the repairs and leave a smooth surface finish ready for painting.
The concrete repairs achieved by AWT’s skilled staff restored the integrity of the masonry by getting to the root of the problem. A long term solution was provided that maintained the appearance of the property.
Greater London, Essex, Sussex and Kent are all covered by the professional services of Action Wall Ties.
Action Wall Ties (AWT) are specialists in structural defect remediation, masonry reinforcement and repair, providing professional solutions to a wide variety of structural problems. Contact us on 01227 721 255, or email us.
Inspection of this 9 storey concrete frame tower block revealed serious defects in the previous remedial works by another company. AWT installed remedial head restraints and wall ties to restore structural integrity.
The Structural Problem
This circa 1970s nine storey social housing apartment block in Dagenham that was over clad some two years previously with a polystyrene insulation system, that was deemed unsafe and removed to be replaced with PermaRock mineral Fibre EWI system. AWT was contracted to undertake an intrusive inspection of the wall ties, head restraints and visible concrete elements.
This concrete framed building’s cavity infill walls rely on wall ties to provide a degree of interaction between the two leaves, increasing vertical load and resistance to lateral movement (generally wind load) than two leaves acting separately and head restraints to tie the inner and outer walls to the concrete frame. Sound, correctly installed wall ties and head restraints are vital to the structural stability.
The British Standards Institution and the Building Research Establishment Specification Digest provide clear guidelines for the fitting of wall ties and head restraints in concrete framed buildings. Not only must the appropriate type and size of wall tie and head restraint be fitted but the number of fixings per square metre and the pattern of fixings within the masonry must also be considered.
Our Structural Repair Inspection
The AWT inspection comprised a limited inspection of the existing wall ties by drilling 12mm holes in the outer leaf of the brickwork to view within the cavity using an Endoscope. A metal detector was used to establish the remedial wall tie density and distribution and condition.
Our inspection revealed the original wire butterfly wall ties were in poor condition and found to be over their useful lifespan. Many of the original wall ties were also incorrectly installed or not embedded at all.
The remedial wall ties installed to replace the original butterfly ties were installed poorly and with insufficient population. Insufficient or incorrectly fitted head restraints dramatically reduce the load bearing capacity of the brickwork. If wall ties are broken, missing or incorrectly fitted this can allow the masonry to bow and crack.
The remedial wall ties were installed in an erratic pattern rather than the recommended pattern for maximum load capacity. The majority of the remedial wall ties had been installed in the mortar bed joints rather than into the bricks. The wrong length of ties had been fitted and insufficiently resin bonded. Many of these wall ties could be removed by hand!
The head restraint fittings were also incorrectly fitted with many being too long and protruding out of the brickwork rather than being securely fixed within the masonry.
The Structural Repair Solution
AWT provided high quality remedial work that is designed to last. Our skilled operatives take pride in their work and are committed to ensuring that every project they complete is safe.
High quality remedial ties were carefully installed in the correct pattern and density. Head restraints were also fitted to restore structural integrity.
AWT provide cost effective, safe remedial repairs to concrete framed buildings throughout London and the South East.
Concrete boot lintels to window openings were in common use in the 1970s and have the unique appearance of a visible one course high concrete toe that appears not to have any bearing onto the adjacent brickwork (i.e. the visible concrete toe is in between the brickwork reveals).
Many low rise blocks from the 60s and 70s develop defects that require concrete boot lintel repairs due to cracks, gaps and sagging brickwork above the windows. Failure to address these issues promptly can lead to dangerous, collapsing masonry as well as extensive damage to internal walls.
AWT are able to provide long term, cost-effective boot lintel repairs with minimal disruption to residents.
Concrete Boot Lintel Problems
Rotating Boot Lintel Causing Gaps & Cracks
Stepped Cracks Due To Failing Boot Lintel
Gaps Above Concrete Boot Lintel
Boot Lintel Defect Causing Compression Cracks
As shown in the gallery, in the 60s and 70s, many concrete boot lintels were incorrectly fitted onto unsuitable bearing allowing the lintel to rotate and the supported brickwork to drop and crack.
This fundamental flaw in the installation of concrete boot lintels means that the weight of the heavier brick outer skin is too great for the relatively soft and lightweight inner blocks to support. The lintel rotates, causing gaps to open up, compressing the blocks of the inner wall and causing the outer brickwork to sag.
Common symptoms of boot lintel problems:
Cracks in the visible part of the concrete lintel
Stepped cracks in the brickwork above windows/doors
Dropping of the brickwork above windows/doors
Bulging or bowing brickwork above windows/doors
Cracks in the internal walls and ceilings above windows/doors
Warped window/door frames making them difficult to open and close
Chris Nichols explains, “This type of lintel has no bearing onto the external brickwork the concrete lintel rarely fails. (As with any concrete the toe of a boot lintel can be affected by corroding reinforcing bars and require concrete repairs.) The boot lintel is reliant on the bearing onto the internal skin. The internal skin bearing the lintel should be of a material capable of withstanding and spreading the load (e.g. pad stone concrete blocks or engineering bricks). “
“In many cases the lintel was bedded directly onto lightweight blocks that are unable to take the imposed load and compress the outer edge of the bearing, rotating the lintel and allowing the external brickwork above to crack and drop as described in BRE Good Building Guide 1.”
Cracking Concrete Boot Lintels
Gaps & Cracks Above Concrete Boot Lintel
Gaps & Cracks Around Concrete Window Lintels
Rotated Boot Lintel Causing Gaps
Boot Lintel Problems Due To Incorrect Installation
Concrete Boot Lintel Repairs
Action Wall Ties provide a cost-effective, long term solution that retains the existing appearance of the brickwork with the minimum disruption to the occupants and the environment.
Action Wall Ties have developed a remedial system and methods, including specifically designed equipment, to remedy the problem of cracked concrete boot lintels. Action Wall Ties slightly anti-rotate the lintels back in place, drilling for and then resin bonding 16mm stainless steel shear pins to both end bearings. Once the resin bonded fixings are set the props are released, pre-loading the shear pins, thus reducing any further cracking.
To larger openings and with advanced movement it may be necessary to introduce two rows of twin cord reinforcement, forming a composite deep masonry beam. To the underside of the concrete toe, we drill for and inject grouted 8mm helical stainless steel pins passing into the line of the deep masonry beam to arrest any further rotation of the concrete lintel.
Action Wall Ties concrete boot lintel repair and replacement solutions are available to clients in London and the South East.
Action Wall Ties (AWT) are specialists in masonry reinforcement and repair, lintel repairs, replacing a damaged lintels, and providing professional solutions to a wide variety of structural problems. We work with home owners, London boroughs, social housing providers, architects, project managers, local authorities and insurance companies.
1970s style low-rise block with defects to balustrade walls on balconies
1970s style low-rise blocks with defects to balustrade walls on private balconies – a combination of single corded bed joint reinforcement, AWT diamond drilled for grouted stainless steel pins, removing the existing reinforcing bars, cut handrail and injected grouted steel pins to repair.
The subject building is a 1970s style low-rise block of domestic apartments in South East London, of conventional construction with open access decks and private balconies.
Rusting of steel reinforcing bars and the handrail baluster fixings had caused potentially dangerous cracking of the balcony walls.
Action Wall Ties’ solution for fixing the cracked brickwork involved reinforcing the brickwork.
The rusted reinforcing bars and other corroded steelwork balusters were removed to prevent further cracking. New stainless steel fixings designed by Action Wall Ties were then fitted and the balcony walls were rendered and made good.
Detailed Case Study:
Balcony Problems & issues
Visible cracking of brickwork and previous repairs (in a similar pattern) was noted to the inside of the balustrade ½ brick thick walls to the private balconies.
The balconies’ balustrade walls were all of a similar construction – ½ brick thick stretcher bond brickwork, with faced brickwork externally and render finish to the inside.
They were finished with a concrete coping stone and a steel tubular handrail supported by tubular balusters embedded through the coping stones down into the first two courses of brickwork.
Generally the external bricks and pointing were found to be in good condition with little spalled pointing and spalled bricks noted.
There was visible cracking and previous repairs in a similar pattern to the internal render finish, and cracking to the coping stone and first two courses in line with the embedded steel baluster.
Action Wall Ties’ inspection revealed a heavily corroding 10mm mild steel reinforcing bar, set into the concrete slab adjacent to the brickwork and fixed to the brickwork with mild steel wire ties bent round the upright bar and built into the brickwork bed joint.
The cracking and previous repairs noted corresponded with the reinforcing bars built into the render finish.
In line with the embedded tubular handrail baluster some cracked bed joints and dislodged coping stones were noted.
In some cases the cracking continued down through the first two courses of brickwork, in line with the depth of the embedded baluster. The coping stones have no mechanical fixing required by modern day standards.
Discussion
From Action Wall Ties’ findings it is apparent that the cracking to the render finish and cracking to the coping stones and brickwork noted above were due to two generic defects – that of the advanced expanding corrosion to each of the embedded reinforcing bars and the handrail tubular baluster.
The brickwork has admitted moisture, resulting in expanding corrosion (i.e. the formation of ferrous oxide that expands several times the thickness of the metal it has resulted from) on the embedded mild steel, thus cracking brickwork and further fuelling the process by reducing the section of reinforcing bar and the effectiveness of the reinforcing bars.
The ½ brick thick balustrade wall’s lateral stability is dependent on this reinforcement.
The brickwork has admitted moisture running down from the baluster into the masonry resulting in expanding corrosion to the mild steel baluster, thus cracking the brickwork and dislodging the coping stones further fuelling the process.
From our findings an engineering appraisal and remedial strategy was adopted.
The judgment was to determine the extent of the remedial works that should be instigated to accommodate a maintenance program and safety issues assessment.
Structural Repair Solution
Action Wall Ties’ remedial works were designed to provide a cost effective, long term repair solution ameliorating all the defects noted, retaining the existing appearance with the minimum of disruption to occupants and impact on the environment.
Our proposed remedial scheme provided a viable repair with the introduction of a combination of single corded bed joint reinforcement – to remove the handrail and baluster from the brickwork, drill for and grout in AWT designed stainless steel fixings, cut and weld original handrail in place.
AWT Diamond drilled the full height of the balustrade wall into the concrete slab and injected grouted stainless steel pins, removing the existing reinforcing bars from the brickwork and made good the render, drilled for resin bond stainless steel ties and made good coping stones.
Action Wall Ties (AWT) are specialists in masonry reinforcement and repair, replacing corroded embedded steel and re rendering the surface, providing professional solutions to a wide variety of structural problems in the south east including Kent and London. Contact us on 01227 721 255, or email us.
Eight Storey Concrete Frame Block with Bowing Brickwork and Brick Slips Movement
Five circa 1950/60s eight storey blocks of concrete frame construction with cavity brickwork infill walls in London SW19. Bowing brickwork and brick slips movement, due to concrete frame shortening. Repaired by some rebuilding brickwork and brick slips fixed insitu – mechanically fixed with Action Wall Ties designed method .
The subject development comprised of five circa 1950/60s high-rise blocks of flats in South West London. The blocks are of concrete frame construction with cavity brickwork infill walls below windows and full height to flank elevations.
The Structural Problem
The infill cavity brickwork walls were constructed with an external skin of LBC type bricks, 75mm to 95mm cavity, and a hollow clay pot internal skin.
The infill walls were constructed from each concrete floor beam to the underside of the floor beam above with approximately 65mm support to the external brickwork.
This left 40mm protruding to accommodate six/seven courses of 35mm brick slip bedded onto the floor beam, forming the appearance of continuous brickwork. The bricks have weathered well, with little sign of spalling, though some damaged bricks were noted.
The manufactured brick slips were 35mm thick with a bedding groove to the rear.
The brick slips were bedded down onto the last brick course of the infill wall and onto the face of the concrete beam, with sand and cement mortar up to the first course of the brick wall above (no horizontal soft joints were found).
A cast-in wire butterfly tie was uncovered that had not been bent out and bedded into the mortar as intended. Previous repairs, bulging leaning movement, and hollow and missing brick slips were all noted.
Hollow clay blocks formed the internal skin of the cavity walls constructed in-between the concrete floor/beams and columns.
The wall ties were found to be galvanised vertical twist, generally adequately bedded, clear of building debris, installed to a good pattern and of adequate density.
No head restraints were located with the last row of ties six/seven courses down from the head of the wall. No significant corrosion to the existing ties was noted.
Discussion
We found that the buildings had generally performed well, including the buildings’ brickwork infill walls, with the exception of the brick slips and the head of the infill walls.
Since the buildings had been completed, the concrete used in their construction had shrunk, the concrete frame had shortened and the bricks’ first irreversible expansion had taken place.
Where brickwork and concrete frame abut these opposing stresses were putting the brickwork in compression, locking in the brickwork to the continuous brick slips to the flank walls.
This effect was further augmented by thermal expansion, especially to the south-facing walls.
Where the walls were not adequately tied the compression force had been released laterally, causing bulging and leaning of the brick slips and the head of the wall down to the first row of effective ties.
Most of the brick slips had become detached from the concrete beam, probably due not only to the compression force described above but to differing movements and poor fixing details. Modern-day standards require a mechanical fixing to each brick slip.
Long Lasting Structural Repair Solution
AWT completed a remedial scheme providing a viable repair by rebuilding sections of the brickwork and brick slips, introducing a pattern of mechanical fixings to the brick slips and angled head restraints along with a number of horizontal soft joints.
The gallery below shows the blocks 13 years after work was completed. The quality repairs by AWT are still going strong. In the long term, quality structural repairs provide better value for money than repeated, ineffectual repairs using traditional building methods.
All of our work to low and high rise blocks follows the guidelines provided by the British Standards Institution and the Building Research Establishment Specification Digest.
Action Wall Ties (AWT) are specialists in masonry reinforcement and repair, providing professional solutions to a wide variety of low, mid and high rise block structural problems.
AWT work with councils, housing associations, project managers and architects across London, Kent, Surrey & Sussex.Contact us on 01227 721 255, or email us.
This overview of the history of cavity walls looks at how the use of cavity walls started. It also looks at some of the problems that property owners need to be aware of and how AWT can help to solve these issues. Read on to find out about:
Development of Cavity Wall Construction
Wall Tie Corrosion
Wall Tie Life Expectancy
Wall Tie Damage by Corrosion
Modern Wall Ties
Masonry Damage from Corroded Wall Ties
Cracked Bricks from Failing Wall Ties
Development of Cavity Wall Construction
Cavity wall construction has almost entirely replaced solid wall construction in the United Kingdom. It evolved in the latter years of the nineteenth century and became common in dwellings in northern and western Britain in the early 1900s.
Its widespread adoption as virtually standard in the construction industry happened throughout the building booms of the 1920s, ‘30s and ‘40s.
The gallery above shows the typical outer leaf (the external brickwork) of a cavity wall and the type of damage often associated with failing cavity wall ties.
When identifying whether a wall is of solid or cavity construction, something to be aware of is that the presence of headers in the brickwork (bricks laid with the smallest side facing out) is not always indicative of solid brickwork.
From the mid-1940s to the mid-1950s, “snapped headers” were often used in cavity wall construction to emulate the English bond brickwork pattern that was common in solid brickwork.
Action Wall Ties can help identify whether your property has cavity walls or solid brickwork and diagnose the underlying cause of structural defects.
In the early years, the skins of these cavity walls were held together by metal ties made from cast or wrought iron, mild steel or copper.
Wall Tie Corrosion
The corrosion of wall ties was first officially recognised in the 1960s in South Wales. At first, it was thought to be due to a combination of poor tie protection and exposure to the elements. Time has shown that many of the early ties and mortars were just more susceptible to corrosion.
Mild steel ties initially were either left unprotected or given a bitumen coating, but in the early 1930s zinc coatings or galvanising became accepted.
The most common type of mild steel ties (under BS 1234) are strip ties (commonly known as vertical twist or fishtail) and wire ties (commonly known as butterfly or double triangle). The photo shows a range of traditional wall ties, all of which are prone to corrode and fail over time. In contrast, modern wall ties are carefully designed for strength and durability.
Wall Tie Life Expectancy
It is very difficult to predict the life expectancy of ties used in the construction of houses before 1945.
However, it can be fairly accurately predicted for those built between 1945 and 1964 :
Strip ties will probably last somewhere between 31 and 61 years
Wire ties will probably last somewhere between 15 and 31 years.
The expected life of ties used in construction between 1964 and 1981 is estimated at:
23 to 46 years for strip ties
13 to 26 years for wire ties.
ALL mild steel wall ties will eventually corrode – the only question is when!
As shown in the photo, Action Wall Ties can survey cavity wall ties by drilling small inspection holes and using an endoscopic camera to assess the wall ties. Contact us to arrange a wall tie survey.
Wall Tie Damage by Corrosion
The key here is catching the problem as early as possible. If the symptoms are recognised before deterioration has progressed too far, the walls may be re-stabilised rather than needing to be re-built.
Old ties must be located using an electronic detector and replaced with a suitable corrosion-resistant remedial fixing.
Finally, existing ties will need to be isolated to prevent further damage to the outer leaf of brickwork. Corrosion of a tie within the inner leaf of a cavity wall is unlikely to become significant as they are embedded in a dry environment.
AWT’s specialist equipment allows them to isolate failing wall ties and install remedial wall ties with minimal disruption by working from the outside of the building.
Ferrous oxide (rust) will result when embedded mild steel wall ties corrode. This will expand to several times the thickness of the metal it has resulted from, often breaking through the outer leaf of the brickwork (sometimes splitting the bed joints) and causing either lifting or bowing of the walls and damage to internal finishes.
Rust Damaged Wall Tie
Rusty Wall Ties
Failed Wall Tie
Crumbling Wall Ties
Rusted Wall Tie
Corroded Wall Ties
From a structural perspective, this leaves the wall vulnerable to vertical and wind loads, especially in the case of large gable and unreturned walls. The instability of the wall will eventually result in the ties corroding away completely, necessitating the rebuilding of the wall. The gallery shows how corrosion causes the cavity wall ties to lose strength and eventually crumble away.
Modern Wall Ties
As shown below, Action Wall Ties uses a range of modern cavity wall ties that are far superior to traditional wall ties. Using their many years of experience, AWT selects the most appropriate remedial wall tie style for each project.
There are 3 main types of replacement wall ties:
Mechanical
Helical
Resin or grout bonded
All AWT’s replacement wall ties are made from high-grade stainless steel. They are designed to restore structural integrity and provide a long lasting solution to cavity wall tie issues.
Equipment designed by AWT can also realign brickwork that is bulging and bowing due to failing wall ties before installing new wall ties and lateral restraints.
Kent based Action Wall Ties (AWT) are specialists in wall tie surveys and replacement, providing professional solutions to a wide variety of cavity wall problems. We serve domestic and commercial customers, local authorities, housing associations and insurance companies across London, Kent, Essex, Sussex and Surrey.
Bulging Brickwork and Brick Slips on 60s /70s Apartment Blocks
Concrete frame blocks of social housing in Wandsworth, London with clearly visible bowing to the brickwork and infill brick slip. Defective support to infill wall with cracked and detached brick slips.
The Structural Problem
The structural survey report on this South West London social housing block
The survey of these circa 1960s/1970s blocks of flats revealed, that due to a construction defect, the external brickwork had insufficient support. AWT also found heavily corroded wall ties plus a lack of weep vents and no horizontal expansion joints.
The infill wall external brickwork was originally only supported onto the concrete floor beam by less than 50mm. Where the wall had bulged outward this was reducing the support to just 30mm with the loads onto the brick-slips causing them to bulge out.
Our Structural Repair Solution
The infill wall brickwork was leaning and in danger of becoming detached. AWT promptly made the elevation floor safe (erecting a scaffold to provide access) by installing AWT’s specially developed props.
The remedial work included:
Installation of stainless steel support angles and cavity trays
Clearing debris from the wall cavities
Rebuilding the brickwork by removal, re-bedding and fixing of brick slips
Installation of stainless steel wall ties
Provision of horizontal expansion joints.
Repair and Replacement of Brick Slips
The original bricks and brick slips were reused when possible. Replacement bricks, mortar and sealants were colour matched to maintain the visual integrity of the apartment blocks.
Action Wall Ties are the apartment block structural repair specialists. Our brick slip and wall tie replacement solutions are available to clients in the South East i.e. in London & in Kent, Sussex, Surrey & Essex.
Action Wall Ties (AWT) are specialists in structural repairs to concrete framed buildings and tower blocks. From movement issues to wall tie replacement and brick slip repairs. We provide specialist professional solutions to a wide variety of structural problems. Contact us on 01227 721 255, or email us.
