Application of Carbon fibers on reinforced concrete chimneys, columns (2)

In the USA, initial work in repairing bridge columns used glass aramid fiber reinforced plastic, and buildings were repaired with bonded steel plates, but it is now believed that these will be replaced by cfrp.

To prevent cracking in prestressed concrete sheet piles, it is necessary to place the reinforcement near the tension surface of the concrete to provide effective crack control. If steel is used, it requires a substantial thickness of concrete cover to provide protection against corrosion, whereas Makizumi et al found that carbon fiber net required only one seventh of the cover to provide effective crack control, where the transverse strands of netting play an important role in resisting the applied tensile force. The netting had a mesh size of 20mm and each strand was 3*18k tows of pitch type carbon fiber, impregnated with 40% epoxy resin and fully cured.

Californian earthquakes, such as Whittier 1987, Loma Prieta 1989 and Northridge 1994, demonstrated the vulnerability of older reinforced concrete bridge columns to failure under seismic demands.

XXsys Technologies Inc., a Sun Diego company, specializes in the retrofit of concrete structures using cfrp and has automated the process with a wrapper that applies carbon fiber by wet winding to a column to augment the vertical reinforcement and utilizes portable oven curing technology. The process is used for seismic and corrosion retrofits.

The use of bonded cfrp plates is now an accepted cost-effective process in the UK as an alternative to replacement and other traditional methods of strengthening. The cfrp plates can be bonded to concrete, masonry, timber, cast/wrought iron and steel. The substrate must be prepared by grit blasting and then vacuumed to remove any dust. The adhesive is applied to both the substrate and the cfrp plate, manually offering the plate up to the substrate, pressed and rolled into position.

The first early steel bridge to be reinforced with cfrp plates was Slattocks Canal Bridge, Rochdale. Mouchel used two 4mm thick*100mm wide *7.5mm long cfrp paltes, which were factory bonded prior to bonding on site to the steel beams with Exchem Resiflex adhesive, using temporary support clamps to hold up the plates during cure. Throughout the entire operation, the bridge was carrying traffic.

A new development, especially for cast iron bridges, where stiffness is more critical than strength, is to use high modulus cfrp plates and these have been used for a number of projects in the UK such as the Remile Canal Bridge. This used plates 14mm thick tapering to a thickness no more than 2-3mm to reduce end peeling stresses, obviating the need for bolting. These plates are manufactured from prepreg and are considerably more expensive than a pultruded plate, but a pultruded section of at least a thickness of 30 mm would have been required and would also have been more expensive to install.

The Grayson bridge over the Little Sandy River in the east Kentucky has been repaired by applying thin sheets of CF/epoxy to the steel girders with an epoxy adhesive and applying a final protective coat of epoxy, thereby avoiding the expensive replacement of a new bridge.

The 8-span concrete Ebey Island bridge near Everett, WA had spalled badly and in 1999, after removing loose concrete and grit blasting exposed steel rebars, carbon fiber/epoxy plies were applied to the webs both longitudinally and transversely to effect a speedy repair.

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