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Fibre-reinforced polymers (FRPs) are a promising corrosion-resistant alternative to steel reinforcement. FRPs are, however, generally costly and have a high energy demand during production. The question arises whether the high performance of FRPs and possible savings in concrete mass can counterbalance initial costs and environmental impact. In this paper, a parametric design study that considers a broad range of concrete infrastructure, namely a rail platform barrier, a retaining wall and a bridge, is conducted to assess the mass-related global warming potential and material costs. Design equations are parametrised to derive optimum reinforced concrete cross-sectional designs that fulfil the stated requirements for the serviceability limit state and ultimate limit state. Conventional steel reinforcement, glass and carbon FRP reinforcement options are evaluated. It is observed that the cross-sectional design has a significant influence on the environmental impact and cost, with local extrema for both categories determinable when the respective values become a minimum. When comparing the cradle-to-gate impact of the different materials, the fibre-reinforced polymer-reinforced structures are found to provide roughly equivalent or, in some cases, slightly more sustainable solutions than steel-reinforced structures in terms of the global warming potential, but the material costs are higher. In general, the size of the structure determines the cost competitiveness and sustainability of the FRP-reinforced concrete options with the rail platform barrier application showing the greatest potential.

Automatization and additive manufacturing provide a platform to move away from traditional monolithic concrete forms towards more material efficient functionally-graded or structural composite elements. This has a great potential to tailor the element’s mechanical or thermal performance which allows to efficiently use material and furthermore safe costs and reduce the environmental footprint of an element. However, the deposition of different wet-on-wet and weton- hardened concrete layers introduces the risk of a weak interlayer zone (ILZ). Furthermore, functionally layered concrete made of multi-mixes introduces a new dimension whereby the mix compositions and the deposition delay times potentially interact to influence the interlayer behaviour. To provide a better insight on the interlayer zone behaviour of functionally graded concrete, a detailed investigation of the interlayer fracture properties of multi-mix concrete elements cast in layers and the effect of the pour delay between layers is presented. Wedge-splitting-tests are used to infer the fracture behaviour of layered specimens fabricated using three different concrete mixes in single and multi-mix combinations. The influence of a pour delay between layers of up to 4.0 h is also interrogated. The results show that the multi-mix fracture behaviour depends on not only the mix compositions but also on the pour delay time. Cases where the fracture properties were at least as good as, or even better, than those of the comparator weaker single mix suggest that judicious mix designs and controlled fabrication processes for functionally layered concrete can mitigate the formation of weak ILZs.

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.

Functionally graded concrete, where multiple mixes are layered in structural elements, is a promising technology for minimising cement use. However, a challenge when fabricating wet-on-wet graded concrete is the control of the fresh state deformations of multiple mixes cast into the same mould. Horizontally cast elements with two different concrete mix layers were investigated to ascertain the influence of mix density, workability and layer sequence on the intended material placement. Horizontal layers were achievable unless the workability of the top layer was much stiffer than that of the bottom, or the top layer was denser and both mixes were fluid. The findings underpin opportunities for the exploitation of lower carbon materials and cement efficiency.

The bond between Carbon Fiber Reinforced Polymer (CFRP) tendons and concrete depends on the resin mechanical properties that can degrade under environmental conditions. An analytical model to predict the combined effects of temperature and humidity fluctuations on the bond behaviour of CFRP tendons embedded in concrete is presented. The interaction between a CFRP tendon and concrete is studied by considering different competing mechanisms such as tendon swelling and plasticisation effects due to the concrete internal humidity, differential thermal expansion, concrete autogeneous shrinkage, Poisson’s ratio effects and radial bond stresses generated during the pulling out of a tendon from concrete. The analytical bond failure predictions correlate reasonably well with experimentally observed radial-cracking failure modes of CFRP tendons in high strength concrete. Matrix plasticisation appears to influence the long-term bond strength of FRP rods in normal or high strength concrete. Swelling and shrinkage effects become significant in uncoated tendons due to their impact on the frictional bond component. Deviations between measured and predicted bond strength values are attributed to a possible overestimate of the moisture-induced swelling expansion of CFRP tendons. The research findings suggest the introduction of environmental bond factors to consider durability effects in the design of FRP prestressed concrete structures.

Reinforced concrete (RC) beams strengthened with transverse pre-stressed unbonded carbon-fiber-reinforced polymer (CFRP) straps have shown substantial increases in shear capacity when subjected to short-term static loading. However, understanding the time-dependent behaviour is a further prerequisite in order to better evaluate this strengthening technique and its engineering applications. This paper presents time-dependent experimental results from an un-strengthened RC beam and four RC beams strengthened with CFRP straps. The parameters under consideration included the concrete strength, applied load level and pre-stress level in the CFRP straps. The beams were subjected to sustained loads that were fixed percentages of the short-term failure load. To investigate the development of the external concrete strains, the internal steel shear link strains, the strap strains and the shear crack patterns, the beams were monitored over a period of around 320 days. The long-term beam deformations were also recorded and decomposed into bending and shear components. When compared with an equivalent unstrengthened beam, the presence of the CFRP straps reduced the proportion of the shear deflection. However, in the strengthened beams the shear proportions of the total mid-span deflection were still 24% to 28%, which were not negligible. An analysis using a Mohr’s circle of strain confirmed that the long-term tensile behaviour in the shear span appears to dominate the development of the shear deformation., Cambridge Commonwealth and Overseas Trusts University of Cambridge Department of Engineering Homerton College Empa

The structural performance of Carbon Fibre Reinforced Polymer (CFRP) pretensioned structures is controlled by the bond between the CFRP tendons and concrete. The bond strength of CFRP sand-coated tendons can be affected by humid environments due to the porous epoxy matrix structure or by defects in the external sand coating layer of CFRP tendons e.g. due to storage conditions. Pull-out tests were carried out to assess the bond strength performance of sand-coated tendons embedded in high strength concrete and immersed in water at either 23°C or 40°C. Sand-coated CFRP tendons with two different core diameters of either 4.2 mm or 5.4 mm were studied. To assess the effect of the sand coating coverage on the bond, half sand-coated and uncoated tendons were also tested. An image processing technique was developed to help correlate bond strength variations with variations in the sand coating. An average difference of 24% between the bond strengths of the half sand-coated and full sand-coated tendons was recorded. A large scatter in the pull-out results for the sand-coated tendons of diameter 5.4 mm was observed and this was attributed to the manufacturing process. There was no clear trend of bond strength degradation in the sand-coated tendons even after roughly 1.5 years of full immersion in water irrespective of the exposure temperature. However, an increase in the bond strength of the uncoated tendons and in the bond stiffness of all CFRP tendons was observed. This was felt to be the result of concrete autogenous shrinkage in high strength concrete and the potential swelling effects of the tendons in a humid concrete environment. Analytical models are used to describe the bond stress-slip behaviour and their suitability for sand coated CFRP tendons is studied., SACAC Ltd, Onassis Foundation, CEMEX, BASF

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.

Due to the relatively low tensile strength of concrete, cracks are inherent in reinforced concrete structures. A common cause of cracking is the corrosion of internal steel reinforcement, a deterioration process that can affect the bond behaviour and anchorage capacity of reinforcing bars. Corrosion leads to a reduction of the reinforcing bar diameter, the formation of a weak layer of corrosion products around the bar and expansive forces on the surrounding concrete (that can lead to cracking). In the past, the impact of corrosion on bond has been investigated by means of accelerated corrosion tests. However, the more fundamental impact of cracking as distinct from corrosion products on the bond reduction is still not fully understood.This study applies a novel test method to investigate the bond behaviour of reinforcing bars in cylindrical cracked reinforced concrete specimens. The influence of the number of cracks, crack orientation, confinement and concrete cover are investigated.The results indicate a significant loss of bond strength for single cracked specimens. This reduction becomes as high as 65% for double cracked specimens in the absence of confinement. It is shown that the crack orientation with respect to the rib pattern is of minor influence, but the concrete cover and confinement play a significant role in the obtained bond characteristics.

There is an opportunity to revisit and generalise classical theories for concrete cracking in light of increased interest in the use of non-conventional reinforcing materials and material efficiency. Fracture-based models to describe concrete cracking have potential but a limitation has been that many variables and different phenomena have to be incorporated to produce realistic material models. In this paper, an integrated fracture-based model (IFBM) is developed to predict the behaviour of lightly reinforced concrete beams. The proposed model is a closed-form solution that integrates different local phenomena to more precisely describe the onset of cracking, crack propagation and crack rotation. The IFBM incorporates post-cracking tensile stresses in the concrete, the bond-slip behaviour between the reinforcement and concrete, and compression softening in the concrete compressive zone. The model can predict parameters such as the crack length development and crack mouth opening displacement in Mode I lightly reinforced concrete flexural specimens subjected to three-point bending. The predictions show a fairly good agreement with experimental results for small-scale reinforced concrete beams with low reinforcement ratios (0.15-0.5%). The ability of the IFBM to identify specific failure modes and to capture the crack propagation and crack rotation stages of behaviour in lightly reinforced concrete beams are particular advantages. Such an approach provides a powerful tool to study the problem of minimum reinforcement requirements.

Half-joint beams, also referred to as dapped-end beams, have been the subject of several studies, primarily focussing on the design optimisation of new reinforced concrete beams and bridge decks. Existing half-joint structures, however, often show signs of deterioration and can exhibit improper reinforcement detailing. In order to gain a better insight into the impact of local corrosion, anchorage cracking, limited amounts of provided shear reinforcement, and improper reinforcement detailing, a test program was designed. Full-scale tests on nine half-joint beams were performed. The results of the study show that even though the impact of an individual shortcoming on the load carrying capacity of reinforced concrete half-joint beams might not be substantial, inspectors and assessors should pay attention to the possibility of combined effects. When multiple deterioration processes are noted and/or questions are raised with respect to the reinforcement detailing, the impact on the load carrying capacity of the beam might be larger than the linear combination of the individual effects.

Relatively few fracture-oriented experimental studies have been conducted on concrete that is reinforced. An experimental investigation was therefore undertaken to explore the cracking process in lightly reinforced concrete (RC) beams and to observe the details of the localised fracture process zone development. More specifically, the aims were to investigate the relationships between beam height (120 mm, 220 mm and 320 mm), steel reinforcement ratio (0.1–0.5%), ductility and the onset of crack branching. RC beams were tested in three-point bending and experimental surface strains and crack openings were inferred using digital image correlation (DIC). It was found that the presence of the reinforcement prevented premature fracture and led to crack branching where a single crack bifurcated in the region of the compression zone. In the larger beams the branching developed at a lower relative height and a greater reinforcement ratio led to a shallower branching angle. These observations were associated with ductility measures for lightly reinforced concrete beams., The authors would like to thank the Yousef Jameel Foundation and Cambridge Overseas Trust (COT) for their financial support of this research.

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.

Concrete prestressed with carbon fiber–reinforced polymer (CFRP) tendons beneficially utilizes the strain capacity and durability characteristics of the CFRP material. However, changes to CFRP tendon material formulations or production processes present a challenge when building up a body of knowledge to inform the long-term behavior of a prestressed concrete system. Initial qualification tests can help identify the potential for fiber–reinforced polymer (FRP) tendons to meet long-term mechanical and durability requirements. Protocols that can be undertaken on commercially produced cylindrical FRP pultrusions are particularly advantageous because the constituent materials and manufacturing processes are representative of the actual product. Selected mechanical and durability characterization approaches, and their suitability for adaptation for wet specimens with cylindrical geometries, were critically assessed. A series of qualification tests were then conducted on CFRP tendons with three different resin systems. Tendon samples were exposed to wet environments to evaluate the long-term solution uptake and associated mechanical durability implications. While characterization measures such as the glass transition temperature, optical imaging, and moisture uptake provided comparative results, the correlation with mechanical properties obtained from uniaxial tension, double notch shear, and torsion tests was unclear. Using a subset of the mechanical test protocols, a retrospective analysis of CFRP tendons extracted from a prestressed concrete lighting pole under sustained load for 16.5 years was also reported.

External prestressed carbon fibre reinforced polymer straps can be used to strengthen shear-deficient reinforced concrete structures. For an efficient shear retrofitting system, the optimum combinations of parameters such as the number of straps, strap locations, strap stiffness and initial strap prestress need to be identified. The modified compression field theory and the shear friction theory have previously been applied to carbon fibre reinforced polymer strap strengthened beams. As implemented, both of these methods are iterative. Particle swarm optimisation and genetic algorithm stochastic optimisation methods were used to reduce the computational cost associated with the shear strength evaluation and also to search the design space for carbon fibre reinforced polymer strap strengthened beams. An initial comparison across several test functions showed that the preferred optimisation algorithm depended on the characteristics of the design space. When applied to a reinforced concrete case study, the genetic algorithm was better for searching the shear friction theory shear strength design space that was characterised by several peaks. However, for the smoother modified compression field theory shear strength evaluation space, and for the design space for the carbon fibre reinforced polymer strengthened beams calculated using either the modified compression field theory or the shear friction theory, the particle swarm optimisation converged more quickly and accurately. The optimised solutions reflect the assumptions within the underlying evaluation methods.

This study applies a plastic approach to the analysis of shear at a joint or interface in reinforced concrete. Push-off tests have been used historically to investigate combinations of shear and compression across an interface in reinforced concrete. Recent work by the authors has shown that such tests can also be modified to model combinations of shear and tension; as is often found at critical interfaces such as joints in reinforced concrete structures. New experimental results are presented for modified push-off tests subject to a range of combinations of shear and tension. These results, along with a number of historical results reported in the literature are analysed using the upper bound theory of plasticity for interface shear. It is shown that the behaviour predicted by the upper bound theory is consistent with the new experimental results for an initially uncracked concrete interface subject to combinations of shear and tension. Effectiveness factors for the plastic analysis indicated by the experimental results are proposed.

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.

Carbon fibre reinforced polymer (CFRP) prestressed concrete poles offer a durable, light-weight alternative to conventional steel-reinforced or prestressed concrete poles. In particular, the corrosion resistance of the CFRP tendons can result in lower maintenance costs and a reduction in the required concrete cover. For lighting poles used in pedestrian or low-trafficked areas, wind loading represents a dominant load case. The wind acts as a transient force and can blow from any direction. It is therefore of interest to investigate how CFRP prestressed lighting poles perform under repeated cyclic loads and/or load reversals. Experiments were carried out on pole sections tested horizontally. These included a static control test and a conventional fatigue test where a cyclic load was applied in a downwards direction for 2 million cycles. Three further fatigue tests where the load direction changed, either after a defined number of cycles or within a load cycle, were also conducted. It was found that all the poles performed adequately for 1–2 million cycles of loading and that the majority of any deflection increases occurred within the first 50,000 cycles. It is believed that repeated cyclic loading may have increased the tendon debonding at the crack locations. Localised debonding potentially relieves the stress in the outermost tendon layer which delays the onset of failure and allows the inner tendon layers to take up further stress thus leading to a higher load carrying capacity. Loading orientation reversal from a downwards to an upwards direction within a loading cycle did not result in a greater stiffness degradation when compared to the other fatigue tests. The initial findings suggest that in-service cyclic loading and load reversals will not be detrimental to the performance of CFRP prestressed concrete poles.

The corrosion resistance of glass-fibre reinforced polymers (GFRPs) is a key advantage in pipeline applications. Adhesive bonded joints are often used in tensile resistant GFRP pipe systems and the behaviour of small-diameter GFRP tubular pipe-sections joined by using an adhesive bonded coupler is of interest. In particular, the performance of the joint when subjected to a combination of high tensile axial loads and relatively low internal pressure is investigated. Experiments are conducted where a self-equilibrating pressurisation system is used to control the internal pressure and the coupled sections are tested in tension until failure. The cases of no pressure and a 32 bar working pressure are considered. A preliminary one-dimensional linear-elastic model is developed to predict the behaviour in the early stages of loading and to quantify the influence of internal pressure on the joint performance. Based on the experimental and analytical findings, it is concluded that the most critical case for the joint is likely to be the presence of large tensile loads in absence of internal pressure.

Two groups of cylindrical carbon fiber-reinforced polymer (CFRP) tendons were exposed in distilled water at 23°C and 60°C to study the diffusion mechanisms and the effect of moisture uptake on the tendon shear modulus. The two tendon groups had different manufacturing processes, so DMA tests and optical microscopy were used to help characterize the materials. Mass uptake readings of tendon samples were recorded and the uptake generally agreed with Fickian predictions. To study the time-dependent changes in the matrix stiffness due to exposure, torsion tests within the elastic range of loading were conducted. The tendon shear modulus was then derived from the torque versus twist plots. For both groups of tendons the measured shear modulus decreased due to exposure in water. A long-term shear modulus prediction model was developed to relate the tendon torsional shear stiffness and the moisture concentration and the results appeared to agree well with the experimental findings.

Shear deficient reinforced concrete (RC) structures can be effectively strengthened using external prestressed carbon fibre reinforced polymer (CFRP) straps. Due to the presence of the external elastic straps, a strengthened beam can continue to carry significant load beyond the stages of crack plane slipping and internal shear stirrup yielding, and the concrete is subjected to high tensile strain levels. As a consequence, the concrete material models play a significant role in the context of modeling such behavior. The modified compression field theory (MCFT), which is a widely accepted shear theory for unstrengthened RC structures, incorporates the details of the stress-strain behavior of concrete. The MCFT also considers compatibility as a governing factor, which facilitates the inclusion of the strap system into the MCFT formulation. In the current study, modifications were investigated to model CFRP strap retrofitted RC beams associated with either uniform or non-uniform strap spacings. An experimental investigation on strengthened and unstrengthened rectangular RC beams was carried out to validate the MCFT predictions for various strap layouts. The validation process revealed that, in general, the MCFT was able to model the shear response of the retrofitted RC beams but the representation of the softening of the concrete compressive strain, and stress, was found to be influential in the determination of the ultimate load capacity.

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.

As a means of characterising the diffusion parameters of fibre reinforced polymer (FRP) composites within a relatively short time frame, the potential use of short term tests on epoxy films to predict the long-term behaviour is investigated. Reference is made to the literature to assess the effectiveness of Fickian and anomalous diffusion models to describe solution uptake in epoxies. The influence of differing exposure conditions on the diffusion in epoxies, in particular the effect of solution type and temperature, are explored. Experimental results, where the solution uptake in desiccated (D) or undesiccated (U) thin films of a commercially available epoxy matrix subjected to water (W), salt water (SW), or alkali concrete pore solution (CPS) at either 20 or 60 °C, are also presented. It was found that the type of solution did not significantly influence the diffusion behaviour at 20°C and that the mass uptake profile was anomalous. Exposure to 60°C accelerated the initial diffusion behaviour and appeared to raise the level of saturation. In spite of the accelerated approach, conclusive values of uptake at saturation remained elusive even at an exposure period of 5 years. This finding questions the viability of using short-term thin film results to predict the long-term mechanical performance of FRP materials.

In fibre reinforced polymer (FRP) prestressed concrete applications, an FRP tendon must sustain high axial tensile stresses and, if cracks occur, additional dowel forces. The tendon may also be exposed to solutions and so the combined axial-shear stress performance after long-term environmental exposure is important. Experiments were conducted to investigate the combined axial-shear stress failure envelope for unidirectional carbon FRP tendons which had been exposed to either water, salt water or concrete pore solution at 60 °C for approximately 18 months. The underlying load resisting mechanisms were found to depend on the loading configuration, restraint effects and the initial stress state. When saturated, CFRP tendons are likely to exhibit a reduced shear stiffness. However, the ultimate limit state appeared to be fibre-dominated and was therefore less susceptible to reductions due to solution uptake effects.

Advances in material technology allow for the exploration of new structural forms and systems. In recent years, fiber-reinforced polymers (FRPs) have emerged as candidate materials for civil engineering applications, and the use of FRPs in construction has been an area of growing interest. Unidirectional high-strength FRPs are well-suited for use as tensioning elements, but anchorage details present a challenge. An alternative is to self-anchor the FRP tensioning element by winding thin layers of material around supports and then laminating all the layers together (a laminated strap) or by securing only the outermost layer to form a closed outer loop while the inner layers remain nonlaminated (a nonlaminated strap). Nonlaminated FRP straps have been found to have higher efficiencies than equivalent laminated straps, which is advantageous in high-tension applications. The suitability of nonlaminated FRP straps for use as unbonded tension elements provides scope for use in new construction and for the strengthening of existing structures. A review of nonlaminated carbon FRP strap system properties and applications in the context of reinforced concrete, timber, and masonry structures is presented.

A retrofitting technique that uses prestressed unbonded carbon-fiber-reinforced polymer (CFRP) straps to provide additional shear capacity has previously been shown to be successful under short-term static loading conditions. The current study explores the longer-term behavior of this retrofitting technique through two experiments (a sustained load and a cyclic load experiment) and the development of a model based on the modified compression field theory. The experiments indicated that the strain in the CFRP straps changes with time due to changes in the load sharing with the concrete (caused by creep) and the steel stirrups (caused by yield of these elements). The predictive model was initially validated against static experimental results before being applied to the longer-term experiments. The model predicts the trends in behavior well although it is conservative in its estimates of strap strain. The model was then used to determine the influence of stirrup yielding, the load level before and after retrofitting, and the duration of loading on the CFRP strap strains. The initial results suggest that the largest increases in long-term strap strain will occur when the straps are installed early in the structure’s service life although further experimental validation is required.

A prestressed carbon fiber-reinforced polymer (CFRP) strap retrofitting system has been found to significantly enhance the shear capacity of existing reinforced concrete beams. In previous studies, the CFRP straps were supported on metal pads placed on the top and bottom of a beam necessitating top surface access. The goal of the current work was to develop a system where the straps were installed from underneath a slab without compromising the strengthening efficiency. A series of T-beam experiments was conducted where the CFRP straps were inserted through holes that were drilled from below the flange, thereby avoiding the need for access to the top surface. The depth of penetration of the CFRP straps into the compression flange, the concrete strength, the CFRP strap spacing, the presence of holes in the compression flange, and the size of the loading pads were all found to affect the shear performance. Using the most successful installation technique, the resulting CFRP strengthened beam failed at a load that was approximately 50% higher than that of an unretrofitted control beam.

A retrofitting technique has been developed that uses carbon fiber-reinforced polymer (CFRP) straps to increase the shear capacity of reinforced concrete beams. The vertical straps are not bonded to the beam but are instead anchored against the beam, which makes this technique potentially more effective than bonded FRP retrofitting techniques. However, it also means that models for bonded FRPs are not appropriate for use with the straps. Instead, a model based on a shear friction approach has been developed where the strain in the straps is calculated based on a term that accounts for the effects of prestress and additional strain in the strap due to shear crack opening. The model can either consider the shear reinforcement to be smeared along the length of the beam or discrete elements. The “smeared” model was checked against an experimental database consisting of rectangular, T-, and deep beams, both in terms of predicted capacity and predicted strain in the straps. Overall the smeared model predicted the capacity of the specimens and, with some adjustments, the strains quite accurately. There were, however, cases when it was more appropriate to use the “discrete” model such as when the transverse reinforcement ratio was low or when the transverse reinforcement spacing was high. Further experimental data are required to fully validate the models and to determine appropriate limits on the use of the smeared model and the discrete model. However, the initial results are promising.

Carbon-fibre-reinforced polymer (CFRP) tendons can be used as a corrosion-resistant alternative to steel for reinforcing or prestressing concrete in aggressive marine environments. The design lives of many civil marine structures often span decades and so the long-term durability of the internal reinforcement is important. One significant, yet overlooked, aspect of CFRP tendon durability is its susceptibility to swell on absorption of aqueous solutions including water and salt water. This paper reviews uptake-induced swelling in CFRP materials and draws comparisons with studies of swelling caused by thermal effects. The diffusion of aqueous solutions and their interaction with the polymer matrix are identified as the primary mechanisms responsible for the swelling. Experimental details are presented that correlate the uptake of water and salt water to swelling observed in CFRP tendons. The results are contrasted with both uptake- and thermal-driven behaviour cited in the literature. Lamé's equations are used to estimate the magnitude of the resulting stresses induced in the surrounding concrete. The practical significance of the findings is discussed, including the potential for cracking in the surrounding concrete cover and the implications for the tendon–concrete bond.

Smeared-crack finite element (FE) analyses of reinforced-concrete beams strengthened with prestressed carbonfibre-reinforced polymer (CFRP) shear straps have shown that, at high load levels, the stresses in the straps predicted by the FE analyses were less than the observed experimental values. As the straps are elastic and brittle it is important to investigate this anomaly since otherwise the FE predictions of CFRP shear-strengthened structures could prove to be unconservative. It was postulated that the concrete crack models used in the analyses were inadequate and thus the current work focuses on the FE modelling of cracks subjected to tension and shearing actions. Details of nonlinear FE analyses of the crack behaviour are presented and it is concluded that it is necessary to incorporate advanced crack models in order accurately to predict the stress in the CFRP reinforcing elements.

One promising means of increasing the capacity of existing shear-deficient beams is to strengthen the structure using external prestressed carbon fiber reinforced polymer (CFRP) straps. In this system, layers of CFRP tape are wrapped around a beam to form a strap that acts like a discrete unbonded vertical prestressing tendon. Experiments were undertaken to investigate the influence of the strap spacing, the strap stiffness, the initial strap prestress level and/or any preexisting damage on the strengthened behavior, and mode of failure. An unstrengthened control beam was tested and failed in shear. In contrast, all of the strengthened beams showed a significant increase in their ultimate load capacity with several of the strengthened beams failing in flexure. A number of different failure modes were noted and initial guidelines on the design parameters that influence the propensity for a particular failure mode were developed.

The use of fiber-reinforced polymers (FRPs) for the strengthening and repair of existing concrete structures is a field with tremendous potential. The materials are very durable and, hence, ideally suited for use as external reinforcement. Although extensive work has been carried out investigating the use of FRPs for flexural strengthening, a fairly recent development is the use of these materials for the shear strength enhancement of concrete. The current system investigates the use of posttensioned, nonlaminated, carbon fiber-reinforced polymer (CFRP) straps as external shear reinforcement for concrete. Experiments were carried out on an unstrengthened control beam and beams strengthened with external CFRP straps. It was found that the ultimate load capacity of the strengthened beams was significantly higher than that of the control specimen. Existing design codes and analysis methods were found to underestimate the ultimate resistance of the control specimen and the strengthened beams. Nevertheless, the modified compression field theory provided insight into possible failure mechanisms and the influence of the strap prestress level on the structural behavior. It is concluded that the use of these novel stressed elements could represent a viable and durable means of strengthening existing concrete infrastructure.

Summary The paper focuses on recent developments in the use of fibre reinforced polymers (FRPs) in reinforced and prestressed concrete applications. The influence of the FRP material properties on the design of concrete structures is considered and applications are discussed in conjunction with the structural function of the FRP reinforcement. The review includes examples of the practical implementation of the technology and addresses important durability issues. A number of future considerations/research needs are identified and the requirement for the provision of standard, sustainable and cost-effective FRP solutions is highlighted. It is concluded that although there have been significant advances in our knowledge of the behaviour of FRP-concrete structures, the timeframe in which FRPs will no longer be considered as new materials remains ambiguous.

Beams prestressed with partially bonded fiber-reinforced plastic (FRP) tendons have high strength and rotation capacity but cannot be modeled by conventional techniques. Herein, it is assumed that all deformation takes place at cracks between rigid bodies. By setting up appropriate compatibility and equilibrium equations, the behavior at a single crack can be modeled, which then allows predictions to be made as to which of four possible events will occur next. These lead either to beam failure, or to changes in the geometry that can be analyzed using the same techniques. Comparisons are made with test results, and reasonable agreement is shown.

There is a growing interest in the use of new materials in construction. Materials such as fibre-reinforced plastics (FRPs) are lightweight, strong, and, for the most part, non-corrodible. Hence, the use of FRPs as prestressing tendons for concrete represents a viable, durable alternative to conventional practice using prestressing steel. This paper addresses the nature of the bond between an FRP tendon and concrete in the transfer zone of a pretensions concrete beam. In particular, two different types of aramid fibre-reinforced plastics (AFRPs) are considers. In the experimental investigation, the tendon pull-in behaviour during detensioning of a number of small-scale (100 X 200 X 2800 mm) pretensions concrete beams was monitored. These results were then used to make inferences about the bond stress distribution through the transfer zone. Both a constant and a non-linear relationship between the bond shear stress and the tendon slip were considers and the transfer behaviour of the two types of AFRP and steel were compared. It was found that by assuming a non-uniform bond stress distribution some interesting results about the distinctly different natures of the bond behaviour of the three materials were obtained.

Fibre reinforced plastics describes a group of materials composed of inorganic or organic fibres embedded in a resin matrix. frps are strong, non-magnetic, light-weight and for the most part, non-corrodable. There is great scope for the use of frps as concrete reinforcement and the high strength of the materials is conducive to prestressed applications. However, finding a suitable method of anchoring the tendons without inducing stress concentrations in the fibres has been identified as a problem. The current paper investigates the potential for the use of expansive cement couplers as a means of pretensioning frp tendons. An experimental study was carried out on couplers to join steel reinforcing bars and then extended to include the coupling of frp materials to steel prestress wire.

A reinforced concrete half-joint beam has a complex geometry that includes both a locally disturbed nib region and a full depth section. While this configuration simplifies the design and construction procedures, half joint structures rely on the internal steel reinforcement to transfer force from the nib into the bulk of the beam. When assessing existing reinforced concrete half-joints, engineers can be confronted with internal reinforcement layouts that do not correspond to the as-designed drawings and/or do not comply with current design practice. Bars may be missing or the location, percentage and spacing of the steel reinforcing bars may be non-compliant.To provide a better understanding of the contribution of the internal steel reinforcing bars found in a typical half joint detail, an experimental test program on full-scale half-joint beams was undertaken. Four different scenarios were tested to identify the impact of specific reinforcing bars. A reference specimen was designed in accordance with existing practice. The reference beam, and beams with either missing diagonal reinforcement, missing horizontal reinforcement or a reduced amount of shear reinforcement were tested.All the beams exhibited nib failures with the exception of the beam where the shear reinforcement was reduced. In this case, the failure mode changed from a nib failure to a full-depth failure. The results indicated that if certain bars are missing the overall load bearing capacity of a half-joint could be approximately 40% lower than that of a properly designed detail, but that a redistribution of forces was noted.