Deansbrook Viaduct, M1 Motorway, Mill Hill, London

Innovative spray concrete repair products were used for the permanent remedial repair of the Deansbrook Viaduct after it was badly damaged in April 2011 when fire broke out in a scrap yard beneath. 

Extensive investigation, assessment and temporary structural reinforcement was undertaken to restore the six-lane M1 motorway at Mill Hill in London as quickly as possible. On completion of further examination and despite the severity of the blaze, the damage, while serious, was not as extensive as first anticipated. Concrete repairs, parapet and bearing replacement, and new bridge joints were essential to ensure the continued safety of the structure.

The viaduct was constructed with reinforced precast concrete beams 1m wide, 20m long. When subjected to the severe heat, the soffits of the concrete beams spalled and delaminated. In fire situations, the silica aggregate undergoes a crystalline transformation and expands in volume. The resultant concrete surface became very weak up to the depth where the heat reached around 600°C. Visually, the concrete turned pink where it had been weakened. A thick layer of soot covered the soffits and in the process of high pressure water jetting, these weak layers fell away.

The gaps that formed between the beams were filled with webercem spray DS, a polymer-modified, dry-spray, structural cement-based concrete ideally suited for the repair of fire damaged concrete structures. It is ready for use and designed for dry process spray application to give high early strength, reduced rebound and maximum application thickness. 

About 4,000 25kg bags of webercem spray DS were used during the 16-week programme and a further 1,200 25kg bags of webercem spray RS, the rapid setting equivalent, was used where a more rapid strength development was required. It is particularly valuable when application is hampered by restricted access and difficult time constraints.  Work was carried out at night when traffic flow was at its lowest and two lanes closed in each direction.

The installation of temporary bearing props required the use of webertec EP mortar, a high-strength epoxy resin mortar for repairs, bedding and fixing. This high performance mortar is stronger than concrete in less than 24 hours and three times stronger in flexure than normal concrete. It is impermeable to water, oil, petrol and chemical spillage and easy to mix and apply.

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