Strengthening of reinforced concrete chimneys, columns, beams and retrofits:
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 1996, the Co-op City store in Winterthur, Switzerland was expanded to take 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 epoxy composite plates, which weighed about one fifth of the conventional steel reinforcement—12 mm thick and 100mm wide for steel against 1.2 mm * 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 puotruded by Stesalit AG, Zullwil, Switzerland. The concrete was initially grit blasted and ground and a thin layer of adhesive applied to the concrete and the cfrp plate.
To prevent cracking in pre-stressed 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.
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 *7.5m long cfrp plates, 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 Redmile Canal Bridge. This used plates 14mm thick tapering to a thickness no more than 2-3mm to reduce end peeling stresses, obviating the need of bolting. These plates are manufactured from prepreg and are considerably more expensive that a puotruded plate, but pultruded section of at least a thickness of 30mm would have been required and would also have been more expensive to install.
New structures with cfrp:
Herning bridge, an 80m footbridge in Denmark, will be built as a cable stayed structure with a tower holding the cfrp reinforced concrete deck by means of 40 mm diameter puotruded cfrp cables tested to a capacity of 100 tons. The bridge will be monitored electronically with all cable stays equipped with strain gages, whilst the cfrp reinforcement in the deck will have vibration wire gages. A change in pitch will reveal a change in stress.