A good review of strengthening of reinforced concrete structures is given by Ballinger, Maeda and Hoshijima. Repair and strengthening of concrete structures are discussed as is the us of carbon fiber and rods or bars. Mitsubishi Chemical corp, produce leadline, made from Dialead pitch based carbon fibers, which has been used for rods and tendons for pre-stressing concrete.
Reinforced concrete deteriorates with time due to corrosion of the steel reinforcement and environmental effects on the concrete, excessive loading due to earthquakes and wind, coupled with increased loads on, say bridges, due to heavy traffic. These situations necessitate repairs. When steel had been used previously, there were a number of attendant drawbacks such as increased weight of steel—the steel plates had to be welded together—and there was considerable increase in overall thickness due to the protective jacket of concrete.
In Japan, the original bridge column specifications did not provide sufficient strength against earthquakes and a new specification to avoid brittle failure provided for the shear strength to be higher than the flexural strength, maintaining the lateral resistance when the bridge columns were subjected to a large deformation. This lateral resistance was provided using reinforcement applied to the bottom portion of the column. The seismic design strength was 1.64Gpa, based on an assured strength of 2.45Gpa. Sheets of cfrp were bonded in the vertical direction to provide adequate flexural strength, and the shear strength and ductility were achieved by circumferentially wrapping sheets and/or fiber tows.
Kikukawa et al describe the use of carbon fiber textiles for the reinforcement of concrete floor slabs.
Chimneys were reinforced using a similar principle, but employing automated equipment. Initially, attachments such as lightening conductors were removed, then badly damaged areas patched, followed by bonding cfrp sheets in the vertical direction, circumferentially wrapping cfrp sheets or carbon fiber tow and finally repositioning any attachments. At this stage, only one prestressed concrete highway bridge and the floor of an apartment building had been repaired with bonded cfrp materials.
However, work in Switzerland was at a more advanced stage, where strengthening beams by bonding cured cfrp plates to effect repairs was being carried out. In 1991, the multispan prestressed concrete box girder Ibach Bridge in Lucerne County was repaired by bonding 2mm thick*150mm wide cfrp strips to the underside of the damaged beam. The repair was undertaken at nigh using a lightweight mobile man-lift platform.
In 1996, the Co-op City store in Winterthur, Switzerland was expanded to take two new freight elevators by a specialty contractor Sika AG, who had started to specialize in concrete strengthening work in 1994. The installation of the elevators entailed reinforcing the neighboring concrete in the vicinity of the newly cut floors. This was achieved at the top and bottom of the floors by bonding pultruded expoxy composite plates, which weighed about one fifth of the conventional steel reinforcement—12mm thick and 100mm wide for steel against 1.2mm*100mm for the cfrp. Moreover, since the cfrp plates were thinner, they could be threaded under existing pipes and electrical conduits to achieve the requisite crossovers to satisfy the requirements of biaxial reinforcement. The cfrp plates were pultruded by Stesalit AG, Zullwil, Switerland. The concrete was initially grit blasted and gound and a thin layer of adhesive applied to the concrete and the cfrp plate.