Bond between composite materials and concrete in reinforced concrete members strengthened with CFRP composites

Abstract

The research project concerns the problem of adherence of composite carbon fibre materials to concrete. The test model suggested by RILEM (the tests of steel reinforcement bonding to concrete, 1982) has been adopted to the bond test of composite materials to concrete.
The test included 50 concrete reinforced beams unstrengthened and strengthened with carbon fibre composite materials. The beams were divided into two basic Series distinguished by a kind of composite reinforcement (E and N).
In E Series, the beams were strengthened with strips and sheets bonded on the tension surface of the elements, in series N, the strips were bonded into grooves made in the concrete cover of the beams. In each of the basic series (E, N) the low beams of type I, tall beams of type II, long beams of type L, and short beams of type S were distinguished. Two different tapes of the steel reinforcement were adopted: type A – beams reinforced with 2#8mm, and type B – beams reinforced with 2#16mm. In N Series beams, reinforced with NSM strips, three bond lengths were assumed: 80mm, 120mm, 160mm. In both Series (E, N), two classes of concrete of compressive strength 20MPa and 40MPa were adopted. In order to test the influence of the longitudinal steel reinforcement on the bond conditions of the composite material to concrete, the steel bars in four beams were cut through.
The test results indicated that the increase in concrete strength delayed the debonding of the strips and increased the ultimate strain of the composite at its debonding. Due to the mode of failure of the beams under bending the concrete strength had an insignificant influence on the load bearing capacity of the tested elements, both in the case of beams strengthened with the NSM strips, and in the case of the beams externally strengthened with sheets and strips.
The measurements of the composite strain along the bond length proved the influence of the position of the tested section on the composite strain. If the section was situated within the pure bending zone, the debonding occurred later (with higher ultimate strains of the strips) than in the case of the position of the tested section within the zone of the simultaneous action of the moment and the shear force.
Beams reinforced with rigid strips externally bonded on the surface of concrete demonstrated higher bond stress, compared to the beams strengthened with flexible sheets. The influence of the steel reinforcement on bond behaviour between the strips and concrete was confirmed, and cutting through the reinforcement delayed the debonding of the strips, and thus made a significant increase in their ultimate strain.