THE HIGHWAYS Agency this week hit back at fierce criticism of the surface quality on the M5's Avonmouth Bridge.
NCE reported last week that drivers had been complaining about the rough road surface on the bridge near Bristol after a £125M refurbishment.
'Drivers come off a machinelaid, low noise modern surface at high speed onto a hand-laid traditional surface on the bridge that's much noisier, ' said Agency operations directorate group leader Andrew Hewitt. 'Add to that the inherent irregularity of this type of bridge deck, and the initial bad impression is made even worse.'
Steel box girder bridges of the Avonmouth type were only designed to carry a maximum thickness of 40mm of mastic asphalt surfacing, Hewitt added.
'Because the bridge had to be kept open during the entire strengthening project the new surface was hand laid in small panels and then chippings rolled in. This will inevitably produce a more variable surface.'
Deck distortions produced during construction were common on steel box girder bridges, FaberMaunsell bridges and special structures director Jolyon Gill confirmed. 'Wherever there's an internal diaphragm or a deck plate weld there's going to be a surface irregularity, ' he said. 'And with only 40mm of surfacing to play with, you can only smooth out 3mm or 4mm of roughness.'
Traditionally, mastic asphalt had been laid over a bituminous waterproof membrane applied directly to the steel deck.
This time around, however, Stirling Lloyd's long established Eliminator membrane was used, following major problems with bituminous waterproofing membranes in Holland.