Low-Cycle Fatigue of FRP Strips Glued to a Quasi-Brittle Material

This paper aims at further advancing the knowledge about the cyclic behavior of FRP strips glued to quasi-brittle materials, such as concrete. The results presented herein derive from a numerical model based on concepts of based on fracture mechanics and already presented and validated by the authors in previous works. Particularly, it assumes that fracture processes leading to debonding develop in pure mode II, as is widely accepted in the literature. Starting from this assumption (and having clear both its advantages acnd shortcomings), the results of a parametric analysis are presented with the aim of investigating the role of both the mechanical properties of the interface bond–slip law and a relevant geometric quantity such as the bond length. The obtained results show the influence of the interface bond–slip law and FRP bond length on the resulting cyclic response of the FRP-to-concrete joint, the latter characterized in terms of S-N curves generally adopted in the theory of fatigue. Far from deriving a fully defined correlation among those parameters, the results indicate general trends that can be helpful to drive further investigation, both experimental and numerical in nature.

Strain Transfer of Fiber Bragg Grating Sensor Externally Bonded to FRP Strip for Structural Monitoring after Reinforcement

To date, a method of attaching a FRP (fiber-reinforced polymer) to concrete members with epoxy has been widely applied to increase the strength of the member. However, there are cases in which the adhesion of the epoxy deteriorates over time and the reinforcing effect of the FRP is gradually lost. Therefore, monitoring whether or not the reinforcing effect is properly maintained is needed in order to prevent a decrease in the structural performance of the member improved by FRP reinforcement. In this regard, this study examines FRP with OF (optical fiber) sensors to monitor the reinforcing effect of FRP in concrete structural members. In particular, this paper seeks to determine an appropriate adhesion length when FBG (fiber Bragg grating) based OF sensors are externally bonded to FRP strips with epoxy resin. To this end, a tensile test was carried out to evaluate the sensing performance according to the adhesion length. In addition, an analytical approach was performed and the result were compared with test result. The results of the experimental and analytical studies showed that the strain generated in the FRP is sufficiently transferred to the OF if the total adhesion length of it is 40 mm or more in consideration of the error in the epoxy thickness.

Shear Strength of Externally U-Bonded Carbon Fiber-Reinforced Polymer High-Strength Reinforced Concrete

In this paper, we investigate the contribution of Fiber-Reinforced Polymer (FRP) to the load-carrying capacity of shear-strengthened Reinforced Concrete (RC) beams. Specifically, the investigation is focused on the FRP’s contribution in the presence and absence of shear stirrups. To this end, two sets of full-scale RC beam specimens were tested to failure in a simply supported setup. Set 1 consisted of specimens without shear stirrups whereas Set 2 included steel stirrups spaced at 170 mm. One and two layers of FRP discrete strips were bonded to the beams in a U-jacketing configuration. To investigate the contribution of FRP and its interaction with the stirrups, two different locations were considered when bonding the FRP strips: between the stirrups (referred to as Off-beams) and at the same level of the stirrups (referred to as On). Results of the experimental program showed that strengthening the beams with two layers of FRP does not necessarily translate to improved capacity. Furthermore, the location of FRP strips with respect to the location of shear stirrups has an influence on the beam’s overall behavior, especially its displacement ductility. This is an important parameter to consider to avoid premature failure of RC members. Test results were then used to assess the performance and accuracy of the predictions of ACI PRC-440.2-17 and fib-TG9.3. Both design codes were found to be conservative with an average prediction-to-test ratio of 0.7.

Closed-Form Solution Procedure for Simulating Debonding in FRP Strips Glued to a Generic Substrate Material

The present paper proposes a useful closed-form solution for a wide class of mechanical problems, among which one of the most relevant and debated is the deboning process of Fiber-Reinforced Polymer (FRP) strips glued to generic materials and possibly intended as a mode-II fracture process. Specifically, after outlining well-known equations, a novel piecewise analytical formulation based on a cascading solution process is proposed with the aim of keeping the mathematical expressions of the relevant mechanical quantities as simple as possible. Although other analytical solutions and numerical procedures are already available in the literature, the present one is capable of handling the softening or snap-back response deriving from the full-range simulation of the depending process with no need for complex numerical techniques. This is obtained by considering the slip at the free end of the strip as the main displacement control parameter. After some comparisons between the proposed closed-form solution and experimental results available in the literature, some mechanical considerations are highlighted by elaborating on the results of a parametric study considering the variation of the main geometric and mechanical quantities. The numerical code implemented as part of the present study is available to readers in Open Access.

Design, construction and structural response of a lightweight FRP footbridge

<p>This paper presents the design, construction and structural response of a laboratory FRP footbridge. Pultruded elements, Glass-FRP profiles and Carbon-FRP strips, comprise the 10 m long simply supported structure, which linear mass is only 80 kg/m approximately. A value of <i>L</i>/200 (<i>L </i>being the length of the span) was the limit adopted for the long-term deflection at mid-span, and verifications at Ultimate Limit State were carried out according to the recommendations of the FRP manufacturer and the European Guideline to design FRP Structures. To construct the bridge, the following phases were completed: (i) bonding CFRP strips to the GFRP stringers, (ii) connecting the FRP elements, (iii) casting concrete at the support regions, and (iv) installing the deck. Once the construction finished, an experimental static and dynamic campaign was performed to assess the bridge structural behavior. As expected from the design stage, excessive vertical vibrations under different pedestrian actions (walking and bouncing) were measured.</p>

Delamination Buckling of FRP Strips in Strengthened Structural RC Beams and Masonry Walls

This paper deals with delamination buckling of fibre-reinforced polymer (FRP) strips glued to reinforced concrete (RC) beams or to heterogeneous material as masonry. In the field of rehabilitation of existing civil structures, the strengthening using composite materials is becoming a frequent technique although many points have not yet been clarified. The delamination of FRP strips' layer can be often the cause of loss of the strength capacity in strengthened elements. In general, the delamination is due to loss of adhesion of FRP on the adherent material under tensile loading. This type of delamination foresees a slip of FRP strip and development of fracture energy until the detachment. Delamination buckling of FRP strips is instead due to compression loading. Although the FRP is usually adopted to improve the tensile capacity, in civil structural elements subjected to cycle loading, as RC frames in seismic areas or masonry cross walls, the loading is cyclic and the strengthening of FRP strips may be subjected to compressive stresses with separation of the layer from the adherent element. This type of delamination may significantly influence the strength, stiffness and stability. In this paper experiments on the strengthening of RC beams and masonry walls with GlassFRP strips are shown further, analytical and numerical analysis have been developed to study this mechanism of delamination which too often has been missed in the design of strengthening with FRP strips.