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Unanchored U-wrapped externally bonded carbon-fiber-reinforced polymer (CFRP) fabrics are widely used to increase the design shear strength of existing reinforced concrete slab-on-beam structures, but current design models do not accurately predict the degree of enhancement. Experimental investigations further indicate that some T-beams retrofit with externally bonded CFRP fabric fail at lower shear loads than nominally identical nonretrofit reference counterparts, suggesting a negative experimental CFRP contribution. This work finds a new application for the upper-bound theorem of plasticity in analyzing the problem of U-wrapped externally bonded CFRP-retrofit beam behavior. The study provides insight into the poor historical prediction of the CFRP contribution, and demonstrates the limitations of a widely used experimental approach to determining this contribution. The analysis suggests a new way of thinking about the behavior of slab-on-beam structures retrofit with unanchored U-wrapped externally bonded CFRP. The upper-bound plastic analysis provides better predictions of retrofit shear capacity than some widely used design models, indicating that this approach can lead to better design of retrofit interventions in future. This work proposes a new design limit on enhancement that can reduce the likelihood of unsafe design in practice.

The strategic road network (SRN) in England carries 33% of traffic in England and Highways England's bridge management systems plays a crucial role in the maintenance of infrastructure assets along the SRN. Reinforced concrete half-joint structures are susceptible to deterioration and hard to inspect, hence they require special attention. Inspection data relating to half-joint structures on the SRN was gathered. Within this portfolio, 252 structures with half-joint-related defects were interrogated to classify the most common defects and identify any potential shortcomings in current inspection practice. Clear correlations were shown to exist between defect classes, emphasising the need for quality control and proper workmanship. A revised inspection methodology for half-joint structures that provides more comprehensive information about crack details, and a greater alignment between defect information and indicators of structural measures, is proposed. Concurrently, knowledge of the zonal crack location, crack orientation and crack severity helps inform decision making about the structural condition. There is scope for use of the methodology in conjunction with automated processing procedures to identify half-joint structures with particular defect characteristics and profiles. In this way, asset managers will be better able to allocate resources to structures with an increased risk of failure.

The authors wish to gratefully acknowledge the financial support of the United Kingdom Engineering and Physical Sciences Research Council (EPSRC) through Grant EP/I018972/1.

An experimental investigation was undertaken into the effectiveness of unanchored and anchored externally bonded (EB) U-wrapped carbon fibre reinforced polymer (CFRP) shear strengthening for reinforced concrete T-beams at a range of realistic sizes. The T-beam sizes, geometry and reinforcement were chosen to reflect existing slab-on-beam structures with low levels of transverse steel shear reinforcement. Geometrically similar reinforced concrete T-beams were tested across three sizes ranging from 360 to 720 mm in depth and with different amounts of EB CFRP shear reinforcement. The beams were subjected to three-point bending with a span to depth ratio of 3.5. All the beams failed in diagonal shear. The experimental results indicate significant variability in the capacity of unstrengthened control beams, and a number of these control beams showed greater shear capacity than their EB CFRP strengthened counterparts. Greater thicknesses of CFRP reinforcement did not lead to increased shear capacity compared with lesser thicknesses of unanchored or anchored EB CFRP, but anchored EB CFRP did lead to moderate increases in shear capacity compared to both control and unanchored EB CFRP strengthened beams.

This study reports the findings of an experimental investigation into the behavior of an inclined shear plane in reinforced concrete, such as a diagonal crack in the web of a beam, strengthened with externally bonded carbon fiber–reinforced polymer (CFRP) fabric. A modified push-off test of novel geometry was developed for this study. This test generates a diagonal failure plane subject to combined shear and tension. Both unwrapped and wrapped tests were conducted, allowing the load sharing and load-displacement behavior of the reinforced concrete, and the reinforced concrete with externally bonded CFRP fabric, to be investigated. Fully wrapped and U-wrapped CFRP fabric configurations were tested. Results indicate that for the arrangement tested, concrete, steel, and CFRP contributions to resistance are not independent, and that effective anchorage lengths given in the United Kingdom and United States guidance for U-wrapped CFRP may not be adequate in some cases.

Phased nonlinear finite-element (FE) analyses were carried out to predict the behavior of precracked reinforced concrete (RC) T-beams repaired in shear with externally bonded (EB) carbon fiber–reinforced polymer (CFRP) sheets and subjected to two loading patterns (LPs). Appropriate constitutive relationships were employed to model the behavior of concrete, internal steel reinforcement, EB CFRP reinforcement, and CFRP-to-concrete interface and consequently predict the structural behavior and capture the failure modes of the strengthened beams. Three constitutive models for the behavior of concrete in shear were evaluated, namely, a total strain rotating crack model and two fixed-angle crack models with either constant or variable shear retention factors. The majority of published FE studies have considered rectangular sections that were strengthened before testing. The key feature of the FE models presented in this paper is the use of the phased-analysis technique to model realistically the process of strengthening RC T-beams under load and predict the structural response of the beams to different loading patterns. Furthermore, the paper provides insight into and evaluates the accuracy of the three concrete shear models named above. A detailed comparison between the numerical and experimental results included the shear forces at failure, shear force-deflection curves, crack patterns, failure modes, and strains in the internal steel and external CFRP shear reinforcement. The FE models predicted the experimental shear force capacities and crack patterns with sufficient accuracy but underestimated the postrepair stiffness for the beams subjected to Loading Pattern 1 and overestimated the strain in the CFRP sheets.

The results of an experimental and numerical investigation involving unstrengthened reinforced concrete (RC) T-beams and precracked RC T-beams strengthened in shear with prestressed carbon fiber-reinforced polymer (CFRP) straps are presented and discussed. The results provide insights into the influence of load history and beam depth on the structural behavior of both unstrengthened and strengthened beams. The strengthened beams exhibited capacity enhancements of 21.6 to 46% compared to the equivalent unstrengthened beams, demonstrating the potential effectiveness of the prestressed CFRP strap system. Nonlinear finite element (FE) predictions, which incorporated the load history, reproduced the observed experimental behavior but either underestimated or overestimated the post-cracking stiffness of the beams and strap strain at higher load levels. These limitations were attributed to the concrete shear models used in the FE analyses.

A plastic theoretical model is presented for the punching shear failure of laterally restrained concrete slabs, in which a parabolic Mohr failure criterion for concrete is adopted. The proposed method allows for the effect of compressive membrane action and a membrane-modified flexural theory of elasto-plasticity is used to calculate the compressive membrane forces. The predictions by the proposed analysis show good agreement with a wide range of experimental test results.

Many engineering applications often involve the minimization of objective functions. The optimization becomes very difficult when the objective functions are either unknown or do not have an explicit form. This is certainly the case in the strengthening of existing precracked reinforced concrete structures using external carbon fibre reinforced polymer (CFRP) reinforcement. For a given concrete structure, the identification of the optimum strengthening system is very important and difficult, and depends on many parameters including the extent and distribution of existing cracks, loading capacity, materials and environment. The choice of these parameters essentially forms a coupled problem of finite element analysis and parameter optimization with the aim of increasing the serviceability of the structure concerned. In this paper, virtual ant algorithms combined with nonlinear FE analysis are used in the optimization of the strengthening parameters. Simulations show that the location and orientation of the CFRP reinforcement has a significant influence on the behaviour of the strengthened structure. The orientation of the reinforcement with a fixed location becomes optimal if the reinforcing material is placed perpendicular to the existing crack direction. The implication for strengthening will also be presented.

Modelling crack propagation in fracture mechanics is a very challenging task. Different methods are usually robust under different conditions and there is no universally efficient numerical method for dynamic fracture simulations. Most available methods are computationally extensive and usually require frequent remeshing. This comparison study focuses on three major methods: the discrete element method, the adaptive fixed crack method and the element-free Galerkin method. By implementing these methods to study a 2D concrete beam with reinforcement of carbon-fibre reinforced polymer straps, we have shown that for simulations of a limited number of cracks, fixed crack method gives the best results. For multiple crossover cracks, the discrete element method is more suitable, while for moderate number of elements, the element-free Galerkin method are superior. However, for large number of elements, fixed crack method is most efficient. Comparisons will be given in details. In addition, new algorithms are still highly needed for the efficient simulations of dynamic crack propagations.

This study investigates the structural behavior of precracked reinforced concrete (RC) T-beams strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) sheets. It reports on seven tests on unstrengthened and strengthened RC T-beams, identifying the influence of load history, beam depth, and percentage of longitudinal steel reinforcement on the structural behavior. The experimental results indicate that the contributions of the external CFRP sheets to the shear force capacity can be significant and depend on most of the investigated variables. This study also investigates the accuracy of the prediction of the fiber-reinforced polymer (FRP) contribution in ACI 440.2R-08, UK Concrete Society TR55, and fib Bulletin 14 design guidelines for shear strengthening. A comparison of predicted values with experimental results indicates that the guidelines can overestimate the shear contribution of the externally bonded FRP system.

Summary During the 1960’s and 1970’s a number of reinforced concrete (RC) bridges were constructed with ‘half-joints’ introduced into bridge decks. A half-joint is a particular type of RC structure that was introduced into bridge decks as a means of simplifying the design and construction operations. However, a major disadvantage is that there are problems associated with leakage of water through the joint leading to concrete deterioration and corrosion of the reinforcement. When assessing the integrity of existing half-joint structures, determining the influence of material deterioration and/or repair works is a challenge as current code provisions or guidelines do not pro-vide guidance on how to take any associated strength losses into consideration. This paper focusses on the impact of changes in the material properties (such as compressive strength, yield strength of the reinforcement, reinforcement bar diameter, etc.) on the load bearing capacity of reinforced half-joint structures. The vulnerability of a typical half-joint design to these changes is analysed using finite element models. The results allow designers, assessors and decision makers to better quantify the impact of observed deterioration phenomena on the predicted strength of the studied half-joint.