Prestressed carbon fibre's debut as a beam strengthening technique endowed an ancient cast iron bridge with a fivefold increase in load carrying capacity.
The five times increase in load capacity achieved at central Oxford's Hythe Bridge for just £120, 000 points to a bright future for prestressed carbon fibre reinforced polymer structures. Hythe Bridge's two 7.8m long cast iron beam and jack arch spans carrying a key main road over a Thames backwater have been almost invisibly transformed from 7.5t weaklings to 40t capacity stalwarts.
Remarkably, this massive upgrade in load carrying performance has been achieved with minimal reconstruction and interference to road traffic apart from the preliminary work of replacing the road slab and deck waterproofing.
Oxfordshire County Council engineers were anxious not to compromise the appearance of the elegant cast iron structure. Hythe Bridge's iron girders had adequate shear capacity, but limited tensile strength reduced their bending resistance.
Strengthening with steel plates attached to the four beams in each of the two spans would have required a 135mm deep addition to the bridge soffit. An unsightly 70mm thick bandage would have been the result had bonded unstressed polymer composites been used as reinforcement.
Instead the 4mm thick stressed CFRP pultruded plates proposed by consultant Mouchel resulted in a repair that is barely visible apart from the anchorages. Work carried out from a scaffold deck over the river involved grit blasting and grinding the iron soffits, then bonding anchorages near to the ends of the old beams.
Once the connection bonds were cured the beams were coated with epoxy resin. Four CFRP plates were stressed to 18t each, encapsulated and clamped tightly up to the prepared soffit so that they would bond over their full length.
The team involved in the operation believes that this is the world's first such application of stressed carbon fibre. The technique appears to have very significant advantages for future projects such as the strengthening of underground railway covered way structures where clearances are very often critically tight.
'Other industries have hitherto been quicker to exploit the properties of composites but this application may be as significant in construction as the introduction of prestressed concrete. While other uses for prestressed composites will surely follow, their enormous potential in structural upgrading is clearly demonstrated on this modest but important bridge.'
Oxfordshire County Council
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