Porthcawl Breakwater, UK

Admixture Conplast UW with Conbextra GP grout minimised washout to keep the Porthcawl Breakwater project within budget. To beat the tides, fast setting and low rebound Renderoc DS and DSR dry sprays were used for concrete repair.


Porthcawl Breakwater protects the small former fishing harbour that now houses a gated marina. The breakwater, which extends into the sea by several hundred metres, consists of a masonry wall protected by a revetment, formed partly of the natural bedrock and rock armour placed over the years. The exposed structure faces the brunt of storms and tides, and is placed within the Bristol Channel, known to have the second highest tidal range in the world. There is also an insitu concrete section toward the end of the breakwater where a small lighthouse is situated. The masonry walls and revetment were found to have large voids during a sonic survey. There was also severe corrosion to the steel reinforcement in the concrete section, causing spalling. Arup and Fosroc have a long history of working together on numerous projects and the Engineers approached Fosroc on advice for filling the voids in the structure and for repair solutions.


The original specification for filling the voids was based on an underwater grout. However due to the high cost of such a grout, trials were undertaken using Conbextra GP as a pumped void-filling grout, with the occasional addition of an anti-washout agent when required. Fosroc and Betonbauen established a cost-effective workable solution with Fosroc’s Conplast UW admixture. When added to concrete or grout, Conplast UW produces a gel in the water phase which surrounds the cement particles and other fine components of the concrete mix. This protects them from washout during placement and once the concrete is finally in position. Unlike other anti-washout materials, Conplast UW contains plasticising agents which counteract the effects of the increased cohesion necessary to minimise washout and allow normal workability levels to be maintained without the addition of extra water. Conplast UW provided a degree of retardation to the concrete mix, allowing more time from batching to placement and better control of the placing process to fill the voids and avoid washout by incoming tides. For the concrete repair element of the project, Renderoc DS was used alongside Renderoc DSR where tides would again affect the set of a standard material.


The volumes of void filling made the use of a proprietary underwater grout prohibitive from a cost perspective. Fosroc’s Conbextra GP with Conplast UW proved to be the ideal solution, with approximately 600 tonnes of the grout used to fill the voids. The partnership between Betonbauen and Fosroc ensured the successful application and a high-quality finish, avoiding product wastage and keeping project costs minimised.

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