A new design model for adhesive joints used to bond FRP laminates to steel beams

The strengthening and repair of existing structures using bonded carbon fiber reinforced polymer, CFRP, laminates has attracted a great deal of attention in the past two decades. Investigations clearly indicate the great potential of this method for restoring the capacity of corroded steel beams and improving their fatigue life. One important issue regarding the use of this technique in strengthening steel structures is the design of adhesive joints used to bond FRP laminates to steel substrates. Very limited research work has been conducted in this area and, at the present time, there is a lack of suitable design models for FRP-strengthened steel members. This paper is mainly concerned with a proposal for and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams for strengthening and repair purposes. Quasi-static tests were performed on steel plate and full-scale beam specimens bonded with CFRP laminates to evaluate the new design model proposed in this study. The failure, in all specimens, took place at the steel-adhesive interface. The new design model presented in this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints

Innovative flexural strengthening of RC beams using self-anchored prestressed CFRP plates: Experimental and numerical investigations

This paper presents an innovative method of prestressing carbon fibre reinforced polymer (CFRP) plates used as externally bonded reinforcement for flexural strengthening of reinforced concrete (RC) beams. The proposed method aims to achieve self-anchorage of the prestressed CFRP plate and thus eliminate the need for conventional mechanical anchorage at its ends. Experimental tests of RC beams in four-point bending were conducted to investigate the strengthening efficiency of the self-anchored prestressed CFRP plate. The experimental results showed that using the self-anchored prestressed CFRP significantly improved the flexural performance of the strengthened beam in terms of bending stiffness, crack widths, and load-carrying capacity. The utilisation ratio of the prestressed CFRP plate reached 81% at its debonding. Numerical analyses using nonlinear finite element (FE) method were conducted to model the tested specimens. Based on the reliable simulation of flexural cracks and crack-induced CFRP debonding, parametric studies were conducted using FE analyses, in order to investigate the effect of prestressing levels and the CFRP plate\u27s stiffness on the flexural behaviour. Recommendations were then made for selecting a proper prestressing level and the mechanical properties of CFRP plates

Flexural strengthening of reinforced concrete beams using externally bonded FRP laminates prestressed with a new method

This paper presents a new method and a device for applying prestressed carbon fiber reinforced polymer (CFRP) laminates to flexural structural members without the need for mechanical anchorage of the laminates. An experimental test was conducted aiming to verify the feasibility of the stepwise prestressing method and investigate the flexural behavior of beams strengthened with passive (non-prestressed) CFRP and prestressed CFRP, respectively. Three RC beams with 4.2-meter-span were tested under four-point bending--one beam not strengthened as the control group, one with EB passive (non-prestressed) CFRP, and one with EB prestressed CFRP using the new prestressing technique. The strain values monitored during prestressing process demonstrated that a gradually decreasing prestressing force profile towards the CFRP ends was achieved by using this new prestressing method which eliminated the need for mechanical anchorage at laminate ends. The test results from four-point bending also revealed that using prestressed CFRP led to higher flexural stiffness, postponed yielding load, increased ultimate load bearing capacity, higher utilization of CFRP material tensile strength and reduced crack width

Innovative prestressing method for externally bonded CFRP laminates without mechanical anchorage

Strengthening of reinforced concrete (RC) structures by externally bonded carbon fiber reinforced polymer (CFRP) laminates has been widely accepted as an effective and cost-efficient method. It is well known that advantages offered by the bonded CFRP laminates can be further increased by prestressing the laminates prior to bonding. Mechanical anchors are essential, in this case, to prevent debonding since interfacial stress at areas close to the ends of the strengthening laminate is several times higher than the strength of the concrete substrate. Common anchorage solutions often consist of bolted metallic plates to clamp the prestressed CFRP laminate. Besides labor-intensive installation operation, the anchor plates are vulnerable to galvanic corrosion, which further complicates the inspection and increases the maintenance costs. There are also doubts about the long-term performance of such anchorage systems as it highly depends on the quality of the adhesive layer between the plate and laminate, and the level of pre-tension in clamping bolts. This paper presents the work conducted at Chalmers University of Technology on the development of an innovative prestressing method and a tool which allow for the application of prestressed CFRP laminates without mechanical anchors. The principles of the novel method and the prestressing system are explained. Experimental and numerical work carried out on an RC beam strengthened with this method is presented. Results indicate that CFRP laminates with high prestressing forces (approximately 30% of CFRP tensile strength) can be safely anchored without the need for mechanical anchors. Numerical results based on finite element analyses show that the proposed prestressing method can reduce the interfacial shear stresses in the CFRP-concrete adhesive joint below the bond strength with a reasonable safety margin

Development of a unified design buckling curve for fibre reinforced polymer plates subjected to in-plane uniaxial and uniform compression

Presented are 24 non-dimensional buckling curves to estimate the strengths of Fibre-Reinforced Polymer (FRP) square plates having four simply supported edges and subjected to in-plane uniaxial and uniform in-plane compression. The curves are constructed by the authors from a parametric numerical analysis using ABAQUS® software with changing variables for: material properties; initial geometrical imperfections; laminate lay-ups; and plate thicknesses. These strength curves express relationships for the buckling reduction factor with plate slenderness, and account for post-buckling strength. We observe that regardless of the laminate lay-up (except for purely unidirectional), the choice of FRP material and the magnitude of the initial geometrical imperfection the predicted buckling reduction factors display a meaningful correlation with plate slenderness. Presented is a proposed unified buckling design curve, defined as the lower bound to 18 of the 24 ABAQUS®-generated buckling curves. This new curve is benchmarked by the authors against experimental test results extracted from the literature and it is found that there is a reasonable agreement. The authors recommend that the proposed buckling design curve has the potential to be introduced into structural design standards as a procedure to design the buckling strengths of FRP plates

A new design model for adhesive joints used to bond FRP laminates to steel beams

The strengthening and repair of existing structures using bonded carbon fiber reinforced polymer, CFRP, laminates has attracted a great deal of attention in the past two decades. Investigations clearly indicate the great potential of this method for restoring the capacity of corroded steel beams and improving their fatigue life. One important issue regarding the use of this technique in strengthening steel structures is the design of adhesive joints used to bond FRP laminates to steel substrates. Very limited research work has been conducted in this area and, at the present time, there is a lack of suitable design models for FRP-strengthened steel members. This paper is mainly concerned with a proposal for and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams for strengthening and repair purposes. Quasi-static tests were performed on steel plate and full-scale beam specimens bonded with CFRP laminates to evaluate the new design model proposed in this study. The failure, in all specimens, took place at the steel-adhesive interface. The new design model presented in this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints

A new method for application of pre-stressed FRP laminates for strengthening of concrete structures

Using bonded fiber reinforced polymer (FRP) laminates for strengthening and repair of structural members has been proven to be an effective and economic method. High strength and stiffness, light weight and good fatigue and durability properties of FRP composites together with advantages offered by adhesive bonding have made FRP bonding a suitable alternative for traditional strengthening and repair techniques. It has also been recognized that pre-stressing the FRP laminates prior to bonding would bring additional advantages such as reduced crack widths, postponing the yielding in tensile reinforcement, increasing the load bearing capacity and saving reinforcement material. Using pre-stressed laminates, however, is associated with very high interfacial stresses in the bond line at the laminate ends, which necessitates the use of mechanical anchors. This paper presents a new method and a device for applying pre-stressed FRP laminates to flexural structural members without the need for mechanical anchorage of the laminates

A new design model for adhesive joints used to bond FRP laminates to steel beams - Part A: Background and theory

The strengthening and repair of existing structures using bonded carbon fiber reinforced polymer, CFRP, laminates has attracted a great deal of attention in the past two decades. A large number of concrete structures all over the world have been strengthened and repaired using this technique. In recent years, there has also been a trend towards using this method to upgrade steel structures. Investigations clearly indicate the great potential of this method for restoring the capacity of corroded steel beams and improving their fatigue life. One important issue regarding the use of this technique in strengthening steel structures is the design of adhesive joints used to bond FRP laminates to steel substrates. Very limited research work has been conducted in this area and, at the present time, there is a lack of suitable design models for FRP-strengthened steel members. This paper, which consists of two parts, is mainly concerned with a proposal for and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams for strengthening and repair purposes. The first part of the paper reviews the existing design methods and presents the background to the new proposed model. The second part of the paper deals with experimental work undertaken to verify the new model

A new design model for adhesive joints used to bond FRP laminates to steel beams Part B: Experimental verification

A new design model for adhesive joints used to bond fiber reinforced polymer, FRP, laminates to steel beams was presented in part A of this paper. Quasi-static tests were performed on representative and full-scale beam specimens to evaluate some of the most important failure criteria discussed in part A and the new design model proposed in this study. Three strengthening systems, from two manufacturers (Sika (R) and STO (R)), were used to prepare the specimens. It was observed that, in all cases, the failure took place at the steel-adhesive interface. It was found that, failure criteria based on material strength, considered in this study, fail to provide reasonable predictions of the strength of joints. The new design model presented in part A of this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints