40-Year-old full-scale concrete bridge girder strengthened with prestressed CFRP plates anchored using gradient method

A 17 m long internally prestressed concrete girder taken from a bridge in southern Switzerland was strengthened flexurally using prestressed CFRP plates and then tested to failure. The prestressing level in the plates was 32% of their nominal tensile strength. The load carrying behaviour of this test girder was compared to a reference girder and a girder strengthened using non-prestressed plates. For the anchorage of the prestressed CFRP plates, a method developed at Empa was used. In this method, the force in the CFRP plate is reduced to zero at the plate ends and permanent anchorage system is not required.The experiments proved the feasibility of anchoring CFRP plates using the gradient method. Repeated loading showed stable tensile strains in the plates and shear stresses between the plates and concrete. Furthermore, prestressing resulted in decreased deflections and strains and a 45% increase in maximum recorded load.