The only road link between west Wales and other parts of the country is the Carmarthen Bridge, a reinforced concrete structure opened to traffic in 1938. It carries two major highways over a river, a railway and a road. Today, like many other such structures, the bridge and its approach viaducts must carry a far heavier load than it was designed for- in this case increased traffic resulting from the growth of tourism in west Wales, as well as traffic generated by the oil industry. Two different methods of strengthening the reinforced concrete beam/slab and slab decks were used. Spans in public view were to be strengthened with stressed external strand, while other spans were to be strengthened with weathering steel support frames, beams, or towers. The use of grout bags above these steel frames and beams, to take up the variable space and provide uniform support, proved the key to the success of this second operation. Several methods of achieving continuous contact were considered, but the real problem was the difficulty of access after the steel beams were installed. Furthermore, the soffits of the decks tended to undulate, resulting in varying gaps. Packing with a sand-cement mortar or a sand-epoxy resin mixture seemed impractical. What was needed was a system that would be easy to install where access was difficult, could take up any shape to fill the gap and could provide enough rigid contact to transmit loads from the concrete deck to the frames and beams. The use of bags pumped full of cement-sand grout to fill varying gaps between the steel frames and beams and the deck soffits was then investigated. Heavy-duty polyethylene was considered first, even though it seemed possible that this material would be too flexible and likely to stretch excessively under pressure. However, after testing, the bags proved satisfactory.