Matrix revolution for AA HQ

Concrete, Apr 2007 by Whittaker, Tim

At almost 84m high, the AA's headquarters in Basingstoke was reputed to be the tallest building between London and New York when originally constructed. Matrix Solutions faced technical challenges in repairing spalled areas of concrete on this landmark structure. The team had to carefully match the unusually ribbed, aggregate finish and find a permanent way of protecting the building against any future moisture damage.

When the 18-storey AA headquarters building was constructed in Basing View, Basingstoke in the early 1970s, the external façade of its lift shaft and stair tower was clad in reinforced precast concrete sections with a ribbed profile and exposed aggregate finish. The building still dominates the town's skyline but after 30 years some of the embedded reinforcement in the facing panels on its service tower was showing initial signs of corrosion with some areas of spalling.

Repair work

Before starting any remedial work, a detailed concrete condition survey was commissioned to better understand the nature, cause and extent of the damage. The study examined cover depths, carbonation and chloride levels but highlighted that the most significant contributory factor for corrosion was the reinforcement cover at the base of the ribbed sections. Each decorative panel is approximately 50mm thick with a 50mm shaped rib on top but the average cover was approximately 20mm. The underlying reinforcement was starting to suffer from expansive corrosion as a direct result of surface weathering, moisture ingress and carbonation. Although the process was still in its early stages, the developing pressure was likely to produce further microcracking and surface degradation. Urgent action was needed to prevent the deterioration accelerating as the porosity of the panels increased, particularly with the constant threat of driving rain and wind erosion that occurs around such a tall structure.

Matrix Solutions was appointed to carry out a complete hammer survey and to repair and reinstate all the damaged areas of concrete. The work started in December 2006 and is expected to be completed in late February 2007. At any one time, and weather permitting, up to four concrete repair specialists have been working from a counter-balanced cradle access platform to survey the tower's cladding and to identify, break out and treat any defective areas. As a final step, Chem-Crete Pavix CCC100 concrete impregnator is spray-applied to all the external elevations. This one application solution will provide a permanent, 'active' barrier to protect the building against any further moisture damage and carbonation but without changing its distinctive external appearance.

As a first step before carrying out the repairs, the external faces of the service tower were pressure washed with water and all the damaged areas of concrete were identified either visually or with a sounding hammer. The loose, carbonated concrete was fully removed to expose the underlying reinforcement that was cleaned and treated with Fosroc NitroPrime Zincrich to prevent future corrosion. The areas were then filled and reprofiled using Fosroc Renderoc HB25, a high-performance, polymer-modified lightweight mortar that is perfect for vertical and overhead repairs but also offers low permeability to water, carbon dioxide and chlorides. The mortar was blended with specially sourced 10mm aggregate to match the existing external finish.

Reinstatement mix

The reinstatement mix was applied in a two-stage process. The first step was to fill the areas flush and then with the mortar still wet to embed steel pins every 50mm in a vertical line so that approximately 30mm of each pin was left protruding. The pins were then linked together with standard steel wire to form a cable anchor that could support the V-shaped fins being formed and built on top. Additional mortar/aggregate mix was then added and compacted around the anchor points and hand-profiled to form a 50mm-depth rib. When fully dry, the surface was gently wire brushed to re-expose the aggregate and ensure the repaired areas blended perfectly with the existing ribs and surface.

Three options were then considered for protecting not just the repaired areas but the whole concrete tower against frost or moisture damage - applying an anti-carbonation coating, two coats of silane/siloxane or treating it with Pavix solution.

Ted Jenkins from ASI Solutions pic, UK distributor for Pavix, explains that the solution was the best choice because while an anti-carbonation coating would certainly prevent water ingress, chloride migration and carbonation, ultimately it's a coating. It would eventually need replacing and therefore presented a future maintenance issue plus the other significant factor was colouring. Pigmented coatings can start to look unsightly as they age and a clear coat would result in a shiny finish, which with a building of this size and prominence would significantly alter its overall appearance.

Silane was also actively assessed by the engineering team but rejected for health and safety reasons and the practical issues in using it during the winter months on an 18-storey structure. The solution is toxic so treated areas need to be completely screened to prevent mists drifting onto passing vehicles, nearby homes or polluting waterways. It is also a two application process. To be fully effective as a frost protector, each treatment has to be applied in dry and calm conditions making it virtually impossible to use in damp, wintry weather.