Mechanically Fastened FRP Research Articles

Rapid Prestressed Concrete Retrofit with Prestressed Mechanically-Fastened Fiber-Reinforced Polymer: Field Performance and Observation for a Deteriorated Prestressed Concrete Bridge

This paper presents repairs to rural bridges in North Carolina that deteriorated as a result variously of aging, overweight traffic, and exposure to salts and sulfates. The prestressed concrete C-channel superstructures exhibited prestressing strand loss and displayed significant concrete spalling, with one structure having to be closed to traffic after a routine inspection. Analysis conducted using the American Association of State Highway and Transportations Officials (AASHTO) bridge load rating criteria concluded that repair techniques which strengthen deteriorated flexural elements without also restoring lost prestressing forces are insufficient to maintain load ratings in C-channel structures with heavily damaged prestressing tendons. A prestressed mechanically-fastened fiber-reinforced polymer (MF-FRP) retrofit solution was developed and successfully installed on three structures by the authors and North Carolina Department of Transportation maintenance crews. The most extensive of these three repairs is presented here in detail. The field applications and associated analysis show the temporary MF-FRP repair system is capable of restoring lost prestressing forces, allowing original inventory and operating ratings to remain in place until a permanent superstructure replacement can be scheduled. The most heavily repaired bridge remains in service after 23 months, its performance demonstrated by long-term monitoring data. As currently implemented, the MF-FRP repair is a viable temporary solution for maintaining traffic on a degraded structure while a replacement structure is designed, programmed, and implemented. Efforts to expand the MF-FRP repair into a longer-term solution are underway.

Prestressed MF-FRP: Experimental Study of Rapid Retrofit Solution for Deteriorated Prestressed C-Channel Beams

AbstractThis paper presents design and installation details and full-scale test results for a prestressed mechanically fastened fiber-reinforced polymer (MF-FRP) retrofit solution that restores the...

Modeling and verification of response of RC columns strengthened in flexure with mechanically fastened FRP

One of the most important challenges exists in using of carbon fiber reinforced polymer(FRP) for strengthening of concrete members is the brittle behavior of RC members due to premature de-bonding of the layers. Mechanically fastened FRP (MF-FRP) systems are emerging as a promising method to prevent de-bonding of FRP strips and the prior brittle failure of strengthened RC members. The approach entails the use of fiber-reinforced polymer connected to the concrete substrate by means of steel anchors. In this research, a new anchor designed to delay de-bonding of FRP in reinforced concrete (RC) strengthened columns. Two reinforced concrete columns strengthened using the externally bonded conventional method and the new proposed method in comparison with unstrengthened RC column in the Laboratory. Specimens subjected to constant axial loading and cyclic lateral loading. Besides, numerical simulations performed using finite element models for nonlinear analyses of RC specimens: un-strengthened column; the column strengthened conventionally EB method and the column using new MF method.

Mechanically fastened hybrid composite strips for flexural strengthening of concrete beams

While externally bonded fiber-reinforced polymer reinforcing has become an accepted technology for the flexural strengthening of reinforced concrete members, less information is available on flexural strengthening using mechanically fastened FRP (MF-FRP). This study presents the development of a vacuum-infused, hybrid glass–carbon MF-FRP system for the flexural strengthening of reinforced concrete beams. Two different FRP fabric architectures were considered to assess the impact of fiber orientation on behavior. The resistance of the MF-FRP composite strips to freeze–thaw cycling and saltwater submersion was assessed with single-fastener tension–bearing testing. Test results indicate that the MF-FRP system should display adequate residual capacity in typical cold-climate bridge applications. The flexural strengthening capabilities of MF-FRP composite strips were evaluated using four-point bending tests of MF-FRP-strengthened, steel-reinforced concrete beams and non-strengthened beams. The flexural test results for two glass–carbon MF-FRP reinforcing systems show an average increase in ultimate capacity of nearly 50% with good ductility. Flexural fatigue testing was conducted on four MF-FRP reinforced specimens. The cyclic flexural test results indicate that the strength gains that can be realized with the MF-FRP systems used here may be limited by fatigue performance, and that fatigue is an area that warrants further study.

RC Beams Strengthened with Mechanically Fastened Composites: Experimental Results and Numerical Modeling

The use of mechanically-fastened fiber-reinforced polymer (MF-FRP) systems has recently emerged as a competitive solution for the flexural strengthening of reinforced concrete (RC) beams and slabs. An overview of the experimental research has proven the effectiveness and the potentiality of the MF-FRP technique which is particularly suitable for emergency repairs or when the speed of installation and immediacy of use are imperative. A finite-element (FE) model has been recently developed by the authors with the aim to simulate the behavior of RC beams strengthened in bending by MF-FRP laminates; such a model has also been validated by using a wide experimental database collected from the literature. By following the previous study, the FE model and the assembled database are considered herein with the aim of better exploring the influence of some specific aspects on the structural response of MF-FRP strengthened members, such as the bearing stress-slip relationship assumed for the FRP-concrete interface, the stress-strain law considered for reinforcing steel rebars and the cracking process in RC members resulting in the well-known tension stiffening effect. The considerations drawn from this study will be useful to researchers for the calibration of criteria and design rules for strengthening RC beams through MF-FRP laminates.

지속하중과 환경영향을 받은 MF-FRP 보강보의 구조건전성 평가

Mechanically fastening FRP (MF-FRP) strips using nails and anchors, has been shown to provide a more ductile behavior to the strengthened structural element than using bonded FRP. To further advance the state of the knowledge on this strengthening method, the current study examined environmental effects for six months on MF-FRP beams. Reinforced concrete beams strengthened with mechanically fastened FRP strips and subjected to sustained loads for six months were exposed to outdoor weather and constant high temperatures (). For comparison, the behavior of RC beam with and without sustained loads was evaluated. Results from flexural tests did not show any significant degradation or change of failure mode as a result of sustained load and of environmental effects such as high temperatures and outdoor weather over a period of six months. Failure of the beams was governed by FRP delamination followed by concrete crushing as not much load applied to the nail and anchors because of slip effects.

Analysis and design of RC structures strengthened with mechanically fastened FRP laminates: A review

The use of Mechanically Fastened Fiber Reinforced Polymer (MF-FRP) laminates is emerging as a viable alternative to adhesively bonded FRP laminates for the rehabilitation of reinforced concrete (RC) members such as beams and slabs. A recently published state-of-the-art review of the experimental research has demonstrated the viability and effectiveness of MF-FRP systems. This paper provides a state-of-the-art review of the analytical and numerical studies performed over the last decade with the aim of: (a) predicting the strength, the load-deformation response and the failure mode of rehabilitated RC members, and (b) accounting for the interfacial behavior between the concrete and the MF-FRP laminate. Ultimate strength models and constitutive models are critically reviewed based on their key assumptions and formulations and compares the analytical predictions with previously reported experimental results.

Hybrid shear strengthening system for reinforced concrete beams: An experimental study

This research mainly investigates the efficacy of the mechanically fastened fiber reinforced polymer (MF-FRP) and the hybrid externally bonded/mechanically fastened FRP (EB/MF-FRP) strengthening systems for beam shear strengthening as compared to the well-established EB-FRP counterpart. The results of twenty-one (21) beam specimens categorized into four types, namely, reference specimens, EB-FRP, MF-FRP-FRP and hybrid EB/MF-FRP strengthened specimens are reported. The work presented is part of a research campaign on the use of the newly developed MF-FRP and EB/MF-FRP strengthening systems. A comprehensive study is presented on the effect of using the MF-FRP and the hybrid EB/MF FRP strengthening systems on the modes of failure of the strengthened beams. Utilizing the MF- and hybrid EB/MF-FRP strengthening systems aims mainly to avoid the debonding type of failure that is often experienced when using externally side bonded FRP strips. The effect of the interaction between the external strengthening strips and the internal transverse steel reinforcement on the shear capacity of the beams is qualitatively assessed for the EB-, MF-, and hybrid EB/MF-FRP strengthening systems. This has been achieved by testing specimens with and without transverse steel reinforcement within the critical shear span. Moreover, different parameters have been assessed through utilizing wide versus narrow and vertical versus inclined FRP strips in addition to utilizing two different patterns of fasteners for the MF- and the hybrid EB/MF-FRP strengthening systems.

Modeling of flexural behavior of RC beams strengthened with mechanically fastened FRP strips

An alternative to fiber reinforced polymer (FRP) materials adhesively bonded to the concrete substrate is the implementation of mechanically fastened FRP (MF-FRP) systems using steel anchors to secure the laminate to the substrate. The benefit of MF-FRP, compared to adhesive bonding for FRP flexural strengthening, is the speed of installation with unskilled labor, minimal or absent surface preparation under any meteorological condition and immediate use of the strengthened structures. Some of the potential shortcomings are: possible concrete damage during anchoring and limited opportunity of installation in the presence of congested internal reinforcement in the members to be strengthened. Laboratory testing and a number of field applications have shown the effectiveness of the MF-FRP method. In this paper, an analytical model is discussed for reinforced concrete (RC) members strengthened with MF-FRP strips. The model accounts for equilibrium, compatibility and constitutive relationships of the constituent materials; in particular, it accounts explicitly for the slip between the substrate surface and the FRP strip due to the behavior of the fasteners. The proposed flexural model, coupled with the computation algorithm, is able to predict the fundamentals of the behavior of RC flexural members strengthened with MF-FRP strips, in terms of both ultimate and serviceability limit states. Comparisons between the analytical predictions and the experimental results have been successfully performed.

Hybrid Externally Bonded/Mechanically Fastened Fiber-Reinforced Polymer for RC Beam Strengthening

Externally bonded (EB) fiber-reinforced polymer (FRP) systems and mechanically fastened (MF) FRP strips can both be used to strengthen reinforced concrete (RC) structures, but each has some drawbacks. This paper proposes a new technique for the flexural strengthening of RC beams that combines the EB and MF FRP systems. The technique uses nylon anchors inserted inside the concrete prior to installing the fasteners. These anchors provide a better grasp of the fasteners with the concrete. The hybrid EB/MF-FRP-strengthened specimens showed higher load capacity and post-cracking stiffness than those of the corresponding MF-FRP counterparts. Extending the FRP strips for the entire beam span is necessary for achieving the noticeable enhancement of the load capacity and stiffness with this hybrid EB/MF-FRP system. The failure of the strengthened beams in the system is associated with diagonal cracks at the strip end locations due to fastener rotation and bearing damage.

Analysis of RC beams strengthened with mechanically fastened FRP (MF-FRP) strips

The analytical model for reinforced concrete beams strengthened using mechanically-fastened fiber reinforced polymer (MF-FRP) pultruded strips is discussed in this paper. In this method, FRP strips, reinforced with a combination of carbon and E-glass unidirectional fibers and continuous strand mats, are fastened to the concrete with steel powder-actuated (PA) fasteners and expansion anchors (EA). The model predicts the ultimate strength and failure modes of MF-FRP strengthened beams and was developed based on results of tests on over 75 RC beams of many different sizes using the MF-FRP method, that have been conducted over the last five years. These tests have explored numerous different failure modes and factors affecting the behavior of MF-FRP strengthened beams. The analytical model can be used to proportion the strengthening system for an RC beam so as to cause the beam to fail in a unique ductile failure mode. This procedure was used to proportion strengthening systems for large-scale beams (7.3 m long by 51 cm × 51 cm) that were used to verify the analytical procedure. The strengthened RC beams were designed to fail in a ductile manner. In the test results presented in this paper, the strengthened beams showed increases in yield and ultimate moments of up to 25% and 58%, respectively over an unstrengthened beam. All strengthened beams failed, as intended, in a ductile manner with the ultimate failure mode due to concrete compression failure at large deflections with the FRP strip still firmly attached. Comparisons between the analytical predictions and the experimental results show good agreement.