Test standardisation for FRP-to-concrete bond characterisation

Brittle debonding of flexural FRP strengthening systems can occur at relatively low load levels, thus significantly reducing the FRP effectiveness and affecting the overall structural safety. Although various design models against debonding are available in the literature, they lead either unsafe or too conservative predictions. This could mainly be attributed to the lack of standardised methodology for experimental bond characterisation. Accordingly, design models continue to be developed or calibrated based on limited sets of experimental data. This paper presents an investigation into the suitability of the double shear test setup to become a standardised test and suggests potential improvements. The experimental program in this study comprised a series of 20 double shear specimens and 6 types of FRP systems. The performance of the adopted double shear test set up is analysed in terms of reliability of bond stress-slip data obtained as well as the ease of specimen preparation and testing. In addition, the influence of the roughness of the concrete substrate on bond capacity is also assessed

Effect of section geometry on development of shrinkage-induced deformations in box girder bridges

Non-uniform shrinkage strains can lead to significant additional deflections in large box girder bridges, leading to serviceability problems. This article examines experimentally and analytically the effect of different cross-section geometries on the shrinkage camber of bridge box girders. Small-scale beams were tested to determine the development of shrinkage strains across the beams of depth. Parameters investigated include cross section thickness, drying conditions, and type of concrete mix. Based on the experimental results, inverse analysis is utilised to obtain a surface factor and a hydro-shrinkage coefficient. In this study, such vales are used to determine, for the first time, shrinkage-induced bending deformations of long-span bridges using a hydro-mechanical approach. The results are then used to examine numerically the effect of different section geometries on the development of shrinkage camber. It is shown that the analytical predictions match the experimental results with an accuracy of 85%. A further parametric study is carried out to investigate the effects of specimen geometry and ambient relative humidity. The hydro-mechanical approach is further validated using shrinkage field data from the 230 m two-span box girder Yiju River Bridge (China). The approach proposed in this study is expected to contribute towards improving the predictions of the long term behaviour of box girder bridges and towards better bridge management

A practical method for determining shear crack induced deformation in FRP RC beams

This article proposes a practical semi-empirical method for determining shear crack-induced deformations in Glass Fibre Reinforced Polymer (GFRP) Reinforced Concrete (RC) beams. Current design guidelines neglect shear and shear crack-induced deformations in the calculation of deflections of GFRP RC beams. However, shear-induced deformations can be up to 30% of the total beam deflection due to the lower stiffness of GFRP bars compared to steel. To calculate the component of deflection due to shear action and crack opening, the proposed model uses a ‘single fictitious inclined crack’ with a width equal to the sum of the individual effective shear crack widths. Twelve shear tests were conducted on six RC beams reinforced internally with GFRP bars considering different reinforcement types and test parameters. The additional deformation due to shear cracks calculated by the proposed model is then used to predict the overall deformations of such beams up to failure. It is shown that, in comparison to current design guidelines, the proposed model predicts more accurately the total deflection of FRP RC beams at both service and ultimate loads. This article contributes towards the development of more accurate models to assess the overall shear deflection behaviour of FRP RC beams so as to balance the performance, serviceability and economic viability of structures

Strength degradation in curved fiber-reinforced polymer (FRP) bars used as concrete reinforcements

Steel reinforcements in concrete tend to corrode and this process can lead to structural damage. Fiber-reinforced polymer (FRP) reinforcements represent a viable alternative for structures exposed to aggressive environments and have many possible applications where superior corrosion resistance properties are required. The use of FRP rebars as internal reinforcements for concrete, however, is limited to specific structural elements and does not yet extend to the whole structure. The reason for this relates to the limited availability of curved or shaped reinforcing FRP elements on the market, as well as their reduced structural performance. This article presents a state-of-the art review on the strength degradation of curved FRP composites, and also assesses the performance of existing predictive models for the bend capacity of FRP reinforcements. Previous research has shown that the mechanical performance of bent portions of FRP bars significantly reduces under a multiaxial combination of stresses. Indeed, the tensile strength of bent FRP bars can be as low as 25% of the maximum tensile strength developed in a straight counterpart. In a significant number of cases, the current design recommendations for concrete structures reinforced with FRP were found to overestimate the bend capacity of FRP bars. A more accurate and practical predictive model based on the Tsai−Hill failure criteria is also discussed. This review article also identifies potential challenges and future directions of research for exploring the use of curved/shaped FRP composites in civil engineering applications

Experimental investigation on torsional strengthening of box RC structures using NSM FRP

The near surface mounted (NSM) technique is a strengthening method that provides additional reinforcement by means of strips or bars embedded into grooves made in the concrete cover of reinforced concrete (RC) elements. The effectiveness of using NSM fibre reinforced polymer (FRP) bars or strips to enhance the shear and flexural capacity of RC elements has been demonstrated over the past decade. However, the idea of using NSM FRP reinforcement to address issues related to deficient torsional performance is yet to be explored. Torsional strengthening of RC elements (e.g. bridge girders, transfer beams) may be necessary due to degradation of materials, changes in the design codes, deficiencies in the initial design, changes in building usage etc. This paper investigates the torsional strengthening of thin walled tubular RC beams using NSM CFRP laminates. The experimental program involved testing of six box sectioned RC beams, including two reference beams (with and without shear reinforcement) and four beams strengthened with different arrangements of NSM CFRP reinforcement, providing varying longitudinal and transverse reinforcement ratios. All the strengthening proposals resulted in significant increase in torsional moment capacity, ductility, stiffness in the elasto-plastic range and were very efficient in arresting crack propagation, proving the effectiveness of NSM strengthening technique for torsional strengthening. The proposed experimental program is described in detail and the main results are presented and discussed.CASA -Center for Arabic Study Abroad, University of Texas, Austin(undefined

Experimental study of torsional strengthening on thin walled tubular reinforced concrete structures using NSM-CFRP laminates

Although the use of near surface mounted (NSM) reinforcement for shear and flexural strengthening of reinforced concrete (RC) structures has been examined extensively in the past twenty years, its performance as torsional strengthening solution has never been assessed. This paper presents an experimental program on the use of NSM carbon fibre reinforced polymer (CFRP) laminates to enhance the torsional behaviour of RC thin walled tubular elements. Six specimens were tested as part of this work, including two reference specimens and the remaining four strengthened with different configurations of longitudinal and transverse CFRP laminates. The research shows that the addition of NSM CFRP laminates is very effective in increasing the torsional moment carrying capacity, stiffness and torsional deformability, and arresting the crack propagation, with imperceptible alteration of the geometry of the strengthened element.European Network for Durable Reinforcement and Rehabilitation Solutions, for the grant received to perform the research. Also, to the industries CASAIS and CiviTest in helping to execute the experimental work. The support provided by FCT through the PTDC/ECM-EST/1882/201

Three-dimensional BF Theories and the Alexander-Conway Invariant of Knots

We study 3-dimensional BF theories and define observables related to knots and links. The quantum expectation values of these observables give the coefficients of the Alexander-Conway polynomial.Comment: 32 pages (figures available upon request); LaTe

Constitutive model for rubberized concrete passively confined with FRP laminates

This article develops an analysis-oriented stress-strain model for rubberized concrete (RuC) passively confined with fiber reinforced polymer (FRP) composites. The model was calibrated using highly instrumented experiments on 38 cylinders with high rubber contents (60% replacement of the total aggregate volume) tested under uniaxial compression. Parameters investigated include cylinder size (100×200mm or 150×300mm; diameter×height), as well as amount (two, three, four or six layers) and type of external confinement (Carbon or Aramid FRP sheets). FRP-confined rubberized concrete (FRP CRuC) develops high confinement effectiveness (fcc/fco up to 11) and extremely high deformability (axial strains up to 6%). It is shown that existing stress-strain models for FRP-confined conventional concrete do not predict the behavior of such highly deformable FRP CRuC. Based on the results, this study develops a new analysis-oriented model that predicts accurately the behavior of such concrete. This article contributes towards developing advanced constitutive models for analysis/design of sustainable high-value FRP CRuC components that can develop high deformability

A new damage factor for seismic assessment of deficient bare and FRP-retrofitted RC structures

The seismic assessment of reinforced concrete (RC) structures before and after retrofitting is a challenging task, mainly because existing numerical tools cannot accurately model the evolution of concrete damage. This article proposes an innovative numerical method suitable to model and assess the ultimate carrying capacity of RC structures. The modelling approach proposes a steel constitutive material model with a damage factor that accounts for accumulated damage within the surrounding concrete domain, which effectively captures bar slippage. The proposed method is validated with experimental results from full-scale cyclic tests on deficient bare and CFRP-retrofitted RC joints tested previously by the authors. The results indicate that the proposed simulation method captures the extreme nonlinearities observed in the tested RC joints, with acceptable accuracy and computational robustness. The results of this study are expected to contribute towards the development of more reliable numerical tools and design guidelines for efficient seismic assessment of RC structures before and after earthquakes

A new damage factor for seismic assessment of deficient bare and FRP-retrofitted RC structures

The seismic assessment of reinforced concrete (RC) structures before and after retrofitting is a challenging task, mainly because existing numerical tools cannot accurately model the evolution of concrete damage. This article proposes an innovative numerical method suitable to model and assess the ultimate carrying capacity of RC structures. The modelling approach proposes a steel constitutive material model with a damage factor that accounts for accumulated damage within the surrounding concrete domain, which effectively captures bar slippage. The proposed method is validated with experimental results from full-scale cyclic tests on deficient bare and CFRP-retrofitted RC joints tested previously by the authors. The results indicate that the proposed simulation method captures the extreme nonlinearities observed in the tested RC joints, with acceptable accuracy and computational robustness. The results of this study are expected to contribute towards the development of more reliable numerical tools and design guidelines for efficient seismic assessment of RC structures before and after earthquakes