Renata Kotynia

Prestressed CFRP Strips for Concrete Bridge Girder Retrofitting: Application and Static Loading Test

AbstractThis paper presents an investigation on the practicability and structural efficiency of prestressed carbon-fiber–reinforced polymer (CFRP) strips with a gradient anchorage in the framework of a bridge-strengthening application in Poland. The nonmechanical anchorage system avoids the installation of metallic bolts and plates, with the exception of a temporary support frame. Two 18.4-m-long large-scale prestressed concrete girders were produced following the drawings of the existing bridge construction. One girder served as a reference, and the second one was strengthened with two prestressed CFRP strips. In this case, the initial negative cambering was leveled out by a layer of dry shotcrete. CFRP strips with a prestrain of 0.58% were applied for flexural upgrading. Both girders with a total length of 18.4 m were finally statically loaded up to failure to assess the strengthening efficiency in flexure of the retrofitting technique used. Tensile failure of the CFRP strips was reached, indicating an ...

Prestressed FRP Systems

This chapter provides an overview on the state-of-the-art in prestressing systems for the structural retrofitting of reinforced concrete (RC) structures using Externally Bonded (EB) Fibre Reinforced Polymers (FRP). Focus is put on flexural strengthening, which currently is the most common application field for composite materials in structural engineering. The manuscript provides information regarding commercially available prestressing systems and their anchorage procedures. In addition to conventional mechanical anchorages, the innovative ‘gradient anchorage’ that lacks any remaining plates or bolts is also presented. Additionally, the authors mention various current prototypes at the laboratory-scale level. Performed experimental investigations, results, and conclusions represent the core content of this chapter. Several studies from various universities and research institutes worldwide are presented and explained. In these research projects, the previously mentioned systems are applied to specific reinforced or prestressed reinforced concrete members for strengthening purposes. Static and/or dynamic loading indicate the efficiency of the retrofitting concept compared to the reference structure. Generally, prestressed FRP will be demonstrated to follow the principle of conventional prestressed concrete by resulting in higher cracking, yielding, and bearing loads. Especially under service loads, the structural behaviour is improved. A special section is dedicated to prestressed near-surface-mounted (NSM) systems. In addition to the experiments section, calculation techniques for designing prestressed FRP for flexural strengthening are also handled. In shear strengthening and column confinement, prestressed FRP has been limited to notably few research applications to date. Nonetheless, an overview is given and future possible employment is discussed. Eventually, examples from real structural retrofitting projects should provide the practitioners with some background to better disseminate the retrofitting technique in question. The concluding section summarizes the actual situation and identifies needs for future research.

Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips

The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material.

Shear Capacity Assessment of Posttensioned Concrete Girders Strengthened with CFRP Materials

This paper shows a case of the existing prestressed concrete bridge girders, which due to an increase in the service load needed flexural and shear upgrading with prestressed CFRP laminates. A flexural strengthening of the bridge girders with two pre-tensioned CFRP laminates was published and it is not discussed in this paper. An analysis of the design shear resistance of the original bridge girders performed based on the Polish code and Eurocode indicated the lower shear strength than the design shear force for increased service load. Based on this analysis the girders were strengthened in shear with CFRP sheets wrapping. The mechanical model by Mari et al. (2016) considering the concrete shear contribution and the flange effect on the shear strength indicated promising results. According to the model the design shear resistance of the original girder was in fact much higher than the maximum expected shear force.

Shear Behavior of Steel or GFRP Reinforced Concrete Beams Without Stirrups

The paper presents results of experimental test carried out on 12 T-shaped concrete beams reinforced with glass fiber reinforced polymer (GFRP) or steel bars without stirrups. The aim of this research was to analyze the influence of a type of longitudinal reinforcement (GFRP or steel) on shear capacity and deformability of concrete beams without stirrups and to investigate a dowel effect of the reinforcement on the shear strength. The beams varied mainly with a type of flexural reinforcement, its reinforcement ratio (ρ l ) corresponding to about: 1%, 1,4% and 1.80%, a number of bars, their diameter and a number of reinforcement layers (1 or 2 layers). All beams failed in shear, but diagonal shear cracking was affected by a type longitudinal reinforcement. The GFRP reinforced beams failed due to gradual development of diagonal cracks, while in beams with the steel reinforcement diagonal cracking developed rapidly leading a brittle failure. Steel reinforced beams indicated higher shear capacity than the GFRP reinforced beams. Beneficial influence of two reinforcement layers was confirmed especially for the high longitudinal reinforcement ratio equal of 1.80%, while for the low reinforcement ratio about 1.0%, no difference in the shear capacity due to number of layers was observed. Due to almost four times lower elasticity modulus of the GFRP bars than steel, the GFRP reinforced beams indicated higher ductility than the steel reinforced beams.

Assessment of the Existing Formulations to Evaluate Shear-Punching Strength in RC Slabs with FRP Bars Without Transverse Reinforcement

Nowadays, the real application of fibre reinforced polymer (FRP) bars in slabs or in bridge decks submitted to environments susceptible of corrosion is very limited. In spite of its cost, one of the reasons might be the limited knowledge about the punching-shear behaviour of this type of elements in their connection slab-column or under point loads. The punching-shear strength of slabs reinforced with FRP bars without transverse reinforcement is a complex phenomenon. The main difference with the conventional reinforced concrete is that the passive reinforcement has a linear elastic behaviour up to failure. There is a limited number of experimental and analytical studies about this subject. According to the existing experimental programs, it seems that the basic perimeter at failure is lower in an FRP reinforced concrete (RC) slab than in a conventional RC slab, strains are higher due to the lower modulus of elasticity of the FRP bars, and the cracks are wider. In this paper, a comparative analysis of the reliability of the existing formulations to evaluate the punching strength of FRP RC slabs without transverse reinforcement is performed. A database of 92 tests compiled from 21 experimental programs has been assembled. Most of the existing formulations to predict punching of FRP RC slabs without transverse reinforcement are an adaption of the existing formulations for conventional RC slabs, which take into account the lower stiffness of the FRP bars. Based on the unified model for shear and punching developed by Mari et al. (2015), a preliminary proposal has been made and its reliability has also been evaluated by means of the assembled database.