Research trends

Fatigue behavior of notched steel beams reinforced with bonded CFRP plates: Determination of prestressing level for crack arrest

E. Ghafoori, A. Schumacher, M. Motavalli, Engineering Structures, Volume 45 (December 2012), pp. 270-283


A recent survey indicated that almost 22% of European railway bridges are metallic constructions, of which 40% are 50-100 years old and 28% are older than 100 years. Over the last few decades, higher speeds and heavier trains have resulted in intense vibration and dynamic deflections far greater than before, which may create fatigue-induced cracks in bridge elements. In the recent years, several studies have shown the effectiveness of using the externally bonded carbon fibre reinforced polymer (CFRP) plates to reinforce metallic members. The benefits of using CFRP compared to traditional methods (i.e. welding of additional steel plates) for the strengthening of structures include the high ratio of strength-to-weight of the material, corrosion resistance and high fatigue performance. Some researchers have investigated the effectiveness of prestressed CFRP plates in increasing the service life of the reinforced members.


This paper presents the findings of a study on the behaviour of notched steel beams reinforced by non-prestressed and prestressed bonded CFRP plates under cyclic loading. A fracture mechanics based model is proposed in order to estimate the required prestressing level needed to arrest the crack propagation in the notched beams. Experimental results show that the fatigue life of a beam reinforced by the prestressed CFRP plate increases more than five times of that of an identical beam reinforced by non-prestressed CFRP plate. Prestressing also significantly reduces the residual deflection during the FCG (fatigue crack growth) process. It is also shown that as the stiffness of the FRP-to-steel bond joint reduces the FCG rate increases, while as the crack grows (in steel web) the FCG rate decreases.

Chemical recycling of glass fibre reinforced composites using subcritical water

Géraldine Oliveux, Jean-Luc Bailleul, Eric Le Gal La Salle, Composites Part A: Applied Science and Manufacturing, Volume 43, Issue 11 (November 2012), pp.1809-1818


The total volume of end-of-life and production waste generated by the glass thermoset composites market in Europe is expected to reach 304 000 tonnes by 2015 (source: European Recycling Services Company ECRC). The current inability to recycle thermoset composites may lead to customers preferring more recyclable materials and so the recycling of thermoset composites has therefore been studied for many years. Three methods have been researched: material recycling; thermolysis; and chemical recycling. Material recycling involves grinding of glass fibre reinforced composites to a recyclate that can be used as fillers or partial reinforcement in other products but the incorporation rate is limited. Thermolysis involves the volatilisation of the resin releasing the fibres and, eventually, fillers from the composite. But in the case of glass fibre reinforced composites, the fibres are strongly degraded. Chemical recycling operates at lower temperatures and uses a solvent to depolymerise the resin and release the fibres and, eventually, fillers. Chemical recycling in supercritical conditions of water (T >374°C and P >221 bar) and alcohols has been more applied to carbon composite materials because of the value of carbon fibres, but without any interest in the products issued from the resin. 


The objective of this paper is to present a more complete evaluation of recycling by subcritical hydrolysis of glass fibre reinforced polyester composites, in terms of quality of recovered fibres, nature of recovered products and process efficiency.

The glass fibres were separated from the resin, but contaminated by a residual organic substance. A washing with an appropriate organic solvent was necessary and proved to be an important step of the process in batch conditions. A degradation of the mechanical properties of the glass fibres was observed, which increased as the reaction temperature increased. The organic products recovered from the resin comprised monomers of the resin, which could present an interest in terms of valorisation. But, as for the fibres, a too high temperature led to a degradation of those monomers. The batch conditions used in the study showed several disadvantages.

Bending behavior of concrete-filled tubular FRP arches for bridge structures

Habib J. Daghera, Daniel J. Bannon, William G. Davids, Roberto A. Lopez-Anido, Edwin Nagy, Keenan Goslin, Construction and Building Materials, Volume 37 (December 2012), pp. 432-439.


Concrete-filled fibre reinforced polymer (FRP) tubes (CFFTs) have been used for various infrastructure applications, including waterfront piles and bridge girders. They have been developed both for rehabilitation of existing members using FRP composite wraps and new construction using the FRP tube as concrete formwork.

In contrast with prior research that has focused on straight CFFT members under bending and/or compression loading, this study investigates the use of CFFTs for buried arch bridge structures. The tubes considered here are multilayer, hybrid braided composites made from three distinct layers: an inner layer of braided E-glass fibre and two outer layers of braided carbon fibre. The fibres are rigidified with a vinyl ester thermoset resin using a vacuum infusion process. These thin-walled hybrid composite tubes were developed at the University of Maine’s Advanced Structures and Composites Centre, and function as confinement, tension and shear reinforcing, eliminating the need for conventional steel rebar. While an all-glass or all-carbon tube could be produced, the combination of glass and carbon used here provides good performance while remaining cost-effective, easily fabricated and lightweight.


The objectives of this study were to experimentally assess the bending response of the hybrid, braided CFFT beams and arches, compare test results with analytical models, and illustrate the field application of this technology.

To achieve these objectives, the bending load-deformation and moment-curvature response of CFFTs were investigated through quasi-static laboratory testing of three beams and four arches. The specimens were instrumented with displacement transducers and strain gauges at several locations. Two additional arches were subjected to 2,000,000 fatigue cycles and subsequently loaded to failure to assess the effect of fatigue on capacity. A non-linear beam finite-element model that accounts for concrete confinement and cracking is presented. Model-predicted load-deformation and moment-curvature behavior as well as specimen capacity are shown to compare well with measured data from the arch testing, with arch capacity predicted within 4.2% of the average measured failure load.

Flexural strengthening of concrete beams using CFRP, GFRP and hybrid FRP sheets

N. Attari, S. Amziane, M. Chemrouk, Construction and Building Materials, Volume 37, December 2012, pp. 746-757


The use of composite materials such as fibre reinforced polymers in strengthening and repairing structural elements, particularly those made of reinforced concrete, is spreading. However, for successful and cost-effective applications, engineers must improve their knowledge of the actual behaviour of strengthened structures. Glass fibre reinforced polymers, because they are more ductile and cheaper than carbon fibres, can be considered as an alternative solution to repair and strengthen concrete elements.


The objective of this study was to examine the efficiency of external strengthening systems for reinforced concrete beams using FRP fabric (glass-carbon). Different strengthening configurations are considered: separate unidirectional glass and carbon fibres with some U-anchorages; and bidirectional glass-carbon fibre hybrid fabric. A total of seven flexural strengthened reinforced concrete beams were instrumented and tested under repeated loading sequences to complete a failure analysis. The results for strength, stiffness, ductility and failure modes are discussed for the various strengthening solutions considered. An analytical model to predict the flexural failure of strengthened concrete elements is also developed. The results demonstrate that the model predicts strengthened concrete beam behaviour under applied loads accurately. The present tests also reveal the cost effectiveness of twin layer glass-carbon FRP fabric as a strengthening configuration for reinforced concrete structures.

Buckling strength of slender circular tubular steel braces strengthened by CFRP

X.Y. Gao, T. Balendra, C.G. Koh, Engineering Structures, Volume 46 (January 2013), pp. 547-556


Traditional methods of retrofitting steel members typically involve steel plates that are bolted or welded to the member. But steel plates require heavy lifting equipment and add more dead load to the structure. The added steel plates are also susceptible to corrosion, which will lead to increasing costs in future maintenance. Due to their light weight, high tensile strength and good durability, carbon fibre reinforced polymers (CFRP) have become an alternate approach in upgrading and repairing steel structures.


Although the application of carbon fibre reinforced polymer (CFRP) on strengthening of concrete structures have been reported widely, there are relatively few applications to steel structures, especially to tubular steel structures. In order to investigate the performance of circular tubular steel braces retrofitted with CFRP, long hot finished circular hollow steel braces (88.9 mm x 4 mm) wrapped with CFRP sheets were tested to study the effects of CFRP on overall buckling behaviour. A numerical model was developed to predict the axial load capacity, lateral and axial displacements accounting for the initial out-of-straightness, material and geometric nonlinearities. It was found that CFRP layers could increase both the strength and stiffness of the brace considerably.

The results presented will be useful for application in tubular structures, in particular in the offshore industry. Studies conducted on application of CFRP under water and have been encouraging with respect to bonding between CFRP and steel.

Positive and negative moment behaviours of hybrid members comprising cellular GFRP bridge decking epoxy-bonded to reinforced concrete beams

Wendel Sebastian, Giorgi Gegeshidze, Sam Luke, Composites Part B: Engineering, Volume 45, Issue 1 (February 2013), pp 486-496.


Owing to their low weight (only 20% that of reinforced concrete deck slabs), their modularity and their corrosion resistance, voided cellular units made of glass fibre reinforced polymer (GFRP) show strong potential for use as rapidly assembled, easily lifted into position, durable decks of traffic bridges. Consequently, both laboratory and field studies are being reported into the structural performance of FRP deck-on-beam bridges. 

The experimental study reported in this paper builds on the previous investigations by addressing three novel issues: 

  • reinforced concrete (RC) beams were used with the FRP deck; 
  • a bonded connection was used between deck and beam; 
  • both the positive and negative moment responses to applied load were investigated for the bonded FRP deck-RC beam hybrid members. 


This paper reports tests to failure of hybrid members comprising cellular GFRP decking connected to RC beams via an epoxy adhesive. RC beams of low crack widths under service loads are useful with GFRP decking owing to the economy of and modest expertise needed to install such beams.

The results show the adhesive to be a significant deck-to-beam shear connector under positive and negative moments. Whether the deck was a tensile or compressive top chord, the primary failure mode, which occurred at high loads, was delamination within the GFRP at the deck’s flange-to-web joints, and was neither failure of the GFRP-adhesive interface nor of the adhesive between deck units. For the positive moment specimen there was also fracture of the concrete near the deck as observed for FRP-plated RC beams, despite the absence of flexural cracks from this near-deck concrete.

Further work should consider the effects of surfacing and of long-term environmental and traffic (fatigue) loads on the integrity of the GFRP decks acting in tandem with RC beams. ♦

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