Externally bonded FRP - Columns
Fibre composites (FRPs) are widely used to strengthen columns. The most common technique is to impregnate sheets or strips of fabric (either carbon, aramid or glass) and wrap them around the column, building up the layers until the required thickness is achieved. This provides hoop restraint to the concrete in the column, increasing the load-carrying capacity. Fabrics may also be applied longitudinally, increasing the bending capacity of the column. A major advantage of the technique is that the dimensions of the column are not significantly increased.
An alternative approach is to form a fibre composite shell, which is then bonded completely around the column. The gap between the column and the shell can be filled with expanding grout, which has the effect of applying a hoop prestress to the concrete.
The columns of several major highway bridges have been strengthened, mainly to improve their impact resistance. The first in the UK was the Bible Christian Bridge in Cornwall. Columns have been strengthened in a number of buildings and multi-storey car-parks.
Design guidance is given in Concrete Society Technical Report 55, along with guidance on site operations, surface preparation, quality control etc. Guidance on some aspects of bridge strengthening is given in Highways Agency (now Highways England) documents.