CFRP Strips Research Articles

Fatigue life of reinforced concrete beams under bending strengthened with composite materials

The present paper focuses on the problem of the durability of bent RC beams strengthened with composite materials, subjected to cyclic loading. The theoretical model of determining fatigue life of the cross-section of the bent reinforced concrete beam strengthened with external, non-stressed CFRP strips is presented. The author's own proposal is to introduce the coefficient of the strip position effect on each individual bar and a way of taking into account the energy absorbed by the strengthening. The energy causes a decrease in the velocity of the crack propagation in the rebar, and consequently, an increase in its durability. The theoretical model was verified in the experiments carried out on two series of reinforced concrete beams with different steel and strengthening ratios. For each group of the beams the value of fatigue load was matched to get similar stress in the tension bars.

Prediction of concrete cover separation failure for RC beams strengthened with CFRP strips

External bonding of fibre reinforced plastic (FRP) strips to reinforced concrete (RC) beams has been widely accepted as an effective method for strengthening. The ultimate flexural strength of strengthened RC beams can be improved significantly, but it is often impaired by premature failure modes, such as concrete cover separation. The objective of this paper is to establish a simple and accurate design methodology to predict the load carrying capacity of a strengthened RC beam when concrete cover separation takes place. An analytical expression is developed taking into account the stress concentrations in concrete near the tension rebar closest to the cut off point of the FRP strip. The derivation of the expression involves two major steps: (i) determination of the tensile stress in the FRP strips assuming a full composite action; and (ii) obtaining the local stresses and comparison with the concrete strength. The predictions based on the present analytical model are compared to 58 experimental data from the literature and good agreement has been obtained. The expressions derived in this paper therefore have potential for application in the design of FRP-strengthened beams.

Crack growth induced delamination on steel members reinforced by prestressed composite patch

ABSTRACT Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modifies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive‐plate interface was observed using an optical technique. Moreover, the size of the debonded region significantly influences the efficiency of the crack repair. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the influence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive‐plate interface debond.

Effect of rubber modifier on interlaminar fracture toughness of CFRP-concrete interface

Fibre reinforced plastic (FRP) strips bonded to the tension face of reinforced concrete (RC) beams are an effective means of rehabilitation and strengthening of concrete structures. Epoxy and vinylester resins have been widely used as structural adhesive for this purpose. Amongst various materials and design variables, the properties of adhesive play a significant role in controlling the efficiency of FRP strengthening, which in turn determines the durability of strengthened concrete structure. This paper presents an experimental investigation regarding the influence of reactive rubber modifier added in the adhesive on interlaminar fracture resistance between the CFRP strip and concrete. The interlaminar fracture toughness measured from the asymmetric double cantilever beam (ADCB) test showed significant improvements with 10 and 20% rubber modifier contents. This ameliorating effect is partly attributed to the ability of modified resin to penetrate deeper into the concrete than unmodified resin, as suggested by the micro-hardness measurements. Experiments in bending of RC beams showed substantially enhanced load carrying capacity and ductility for the CFRP plates bonded with modified resins.

Delamination effects on cracked steel members reinforced by prestressed composite patch

Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce crack closure effect. Moreover, it modifies the crack geometry by bridging the crack lips and reduces the stress range at crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A debond crack in the adhesive–plate interface was observed by optical technique. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique. A parametric analysis is performed in order to investigate the influence of some design parameters such as the composite patch Young’s modulus, the adhesive thickness and the pretension level on the adhesive–plate interface debond.

Design of Anchorage of CFRP Strips

Anchorage of externally bonded reinforcement is the main problem for strengthening concrete structures with CFRP strips (Štěpánek & Šustalová 2000, 2001). This paper describes a form of the tests and some results implicating from different properties of anchorage elements. The test results are compared with values obtained from mathematical analysis of anchorage zones behavior and normal stress distribution along the anchorage length.

Shear Strengthening of RC Beams by Externally Bonded Side CFRP Strips

This paper presents results of an experimental investigation on the response of reinforced concrete beams strengthened in shear using externally applied epoxy-bonded unidirectional carbon-fiber plastic side strips. Three series of 1,300-mm-long reinforced concrete (RC) beams are considered: (1) a series of beams having full strength in shear; (2) a series of beams underreinforced in shear; and (3) a series of beams strengthened in shear. Side strips placed either perpendicularly or diagonally to a beam’s longitudinal axis provided the external shear reinforcement. The beams were instrumented and tested under four-point load conditions. Results show the feasibility of using epoxy-bonded strips to restore or increase the load-carrying capacity in shear of RC beams while substantially reducing shear cracking. RC beams strengthened by diagonal side strips outperformed those strengthened with vertical side strips. However, under extreme loading conditions concrete may be torn off at anchorage zone where high concentration of tensile normal stresses is present.

Cylindrical bending of angle-ply strips—A large deflection analysis

The Dynamic Relaxation (DR) method is employed to solve the finite-difference forms of the antisymmetric, angle-ply strip, large deflection equations. Dimensionless results are presented for selected values of uniform pressure applied to simply supported and clamped GFRP and CFRP strips. Particular emphasis is placed on quantifying the influence of shear-bending coupling and lay-up angle on the elastic response of these strips.