COLUMN WRAPPING with fibre reinforced plastic (FRP) is not recommended on large rectangular columns in a forthcoming authoritative guide to structural repair with FRP.
Concrete Society Technical Report 55 (TR55) Strengthening of concrete structures using fibre composite materials was first published in December 2000 and is being revised ahead of a planned September 2004 publication date.
The first edition approved the use of FRP wraps to increase the load capacity of circular columns, but warned that there was insufficient research to support its use on square and rectangular columns.
To update the recommendations the University of Bath has carried out a review of worldwide research into column wrapping, a technique that has been widely used in the US and Japan to upgrade the seismic performance of older bridges.
In the UK the Highways Agency has used the technique to increase the impact resistance of vulnerable bridge columns.
But Bath's review has cast even more doubts on the effectiveness of the technique on large rectangular columns.
'Basically there's a problem of scaling up research results from the usual small test specimens, ' Bath lecturer Antony Darby said this week. 'Scaling up works pretty well with circular columns, but the results from square or rectangular columns can be very misleading.'
His colleague Tim Ibell said the problem is centred on the bond between the FRP and the concrete. 'If you overload a rectangular column eccentrically it will try to fail in shear. The bond will fail long before the FRP can develop any meaningful strain, leaving only the actual corners restrained.'
Darby said that the latest research suggested the technique does work fairly well on small columns 250mm x 250mm or less, provided the corners where the FRP was to be applied were ground down to at least 25mm radius to minimise stress concentrations.