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Flexural Behavior of Small-sized I-shaped UHPC Beams Hybrid Reinforced with Steel Plate and BFRP
Abstract
A new type of small-sized I-shaped ultra-high-performance concrete (UHPC) beam was proposed and tested in this paper. The bottom flange of the I-beam was hybrid reinforced with an embedded steel plate and basalt fiber reinforced polymer (BFRP) sheets. The web was hybrid shear reinforced with embedded steel threaded rods and vertically surface bonded BFRP sheets. The flexural behavior was tested using the four-point loading method, the test parameters include the cross-sectional area of the steel plate, the containing or not of the threaded rods in the web, and the number of the surface bonded BFRP layer. Based on the test data, the calculation and analysis of the cracking, yielding, and ultimate loads of the UHPC beam were conducted. The results showed that the proposed new hybrid beam has excellent mechanical properties. The proposed hybrid reinforcement method can greatly increase the bearing capacity of the UHPC beam from 10.2 kN to 76.6 kN and can increase the ultimate mid-span displacement from a very small 0.52 mm to an astonishing 45.02 mm. Thanks to the ultra-high durability of BFRP and UHPC itself, it has ultra-high durability that ordinary I-steel beams do not have, which has promising application prospects in extremely corrosive environments.
... Ultra-high-performance concrete (UHPC) has excellent mechanical properties [6] and high toughness, and has low porosity and good durability [7], which is very suitable for marine engineering infrastructure [8]. Rational utilization of the excellent mechanical properties of FRP reinforcement and UHPC would improve bearing and deformation capacities [9] of structures as well as reduce the potential risk of microcracks [10], expanding within structures and prevent the growth of macrocracks, enhancing their durability. ...
... Since there is no yield point for FRP bars, the definition of ductility for RC beams is no longer applicable to FRP bars. Here, referring to ACI 440.1R-15 [40], the ductility (μ) of all the specimens is defined as the ratio of energy dissipation in ultimate state (E u ) to energy dissipation in normal service state (E scr ), as shown in Eq. (9). According to GB 50608-2010 [41], the deflection of the normal service state is defined as the mid-span deflection limit (∆ limit ), which equals 1/200 of the total span (l). ...
Finite-element (FE) analysis of fiber-reinforced polymer (FRP)–reinforced concrete beams cast in U-shaped ultra-high-performance concrete (UHPC) permanent formworks is presented in this paper. Concrete damage plasticity (CDP) and FRP brittle damage models were used to simulate the damage behavior of concrete and FRP bars. The results of FE simulation are in good agreement with the experimental results. Furthermore, parametric studies were conducted to investigate the effect of concrete and UHPC strengths, yield strength of steel bars, elastic modulus of FRP bars, ultimate tensile strength of FRP bars, types of UHPC–normal strength concrete (NSC) interface and thickness of UHPC under different reinforcement conditions. Flexural performances, in terms of cracking, yield, ultimate loads and corresponding deflections, failure mode, energy dissipation and ductility, were investigated. Traction–separation model was used to describe the bonding degradation and the maximum slip of two types of bonding interfaces (smooth surface and medium-rough surface). Both flexural capacity and resistance to deformation of composite beams are significantly improved by the utilization of hybrid FRP/steel reinforcement. The UHPC formwork can also delay the occurrence and development of cracks. By appropriately increasing the strength of UHPC or elastic modulus of FRP bar, the flexural capacity of composite beams is effectively improved. It is expected that the results presented in this paper can guide the design and construction of U-shaped UHPC permanent formwork–concrete composite beams reinforced with FRP bars.
... Due to its flat panel structure, the common approach for repairing and restoring the loadbearing capacity of reinforced concrete bridge decks often involves the use of Fiber-Reinforced Polymer (FRP) [1] or adhering steel plates [2] to supplement the steel reinforcement in the tension zone. Typically, a layer of normal concrete (NC) needs to be poured on top, connecting this layer to the existing bridge deck. ...
Rehabilitation the flexural capacity of reinforced concrete structures has been the subject of many studies. However, there is currently limited research on repairing such structures using Ultra-High-Performance Concrete (UHPC) and the combination of UHPC with Fiber-Reinforced Polymer (FRP) materials. This article presents the results of an experimental study on the flexural behavior of 4 specimens: 1 specimen of reinforced concrete (RC), 1 specimen strengthened with FRP sheets, and 02 specimens with the same height as the RC specimen but with a replacement of 3 cm of the protective concrete layer with a UHPC layer, either bonded or not bonded to the existing concrete, with or without the reinforcement of FRP sheets. The experimental results of the 4-point bending tests indicate that, while maintaining the height of the RC specimen constant, replacing the protective concrete layer with a UHPC layer and incorporating FRP in the tension zone led to an increase of approximately 83.35% in flexural capacity, with the deflection at the failure stage rising by only 12.8%. This research opens up a new and promising direction for improving and extending the service life of RC bridges using UHPC and FRP
... Therefore, there are two cases to be analyzed as explained below. According to the sectional force equilibrium, Eq. (5) can be obtained, where A s is the cross-sectional area of section steel, x is the height of equivalent compressive UHPC zone, that is x = β c x c , and a c and β c are shape factors related to the UHPC compressive stress-strain curve, typically taken as 0.89 and 0.75, respectively [35,36]. The tension lower flange and web of section steel are assumed to have yielded. ...
To investigate the flexural performance of steel-UHPC (ultra-high performance concrete) composite beams with welded steel shear keys (SSK), eight specimens were experimental studied by four-point bending test. The finite element (FE) models were established based on the experimental results, then, the failure mode, load, deflection, strain and relative interface slip were parametric analyzed. The influences of strength, dimensions and configuration of upper concrete slab, steel beams as well as SSK on flexural performance, in terms of load-deflection response, ductility and ultimate energy dissipation, were studied. The experimental results show that steel-UHPC composite beams have superior bearing capacity, deformation capacity, ductility and energy dissipation ability when compared with steel-NSC (normal strength concrete) composite counterparts. Increasing the height of upper concrete slab has a significant effect on improving bending capacity and flexural stiffness, while increasing the width has a significant effect on enhancing deformation, ductility and ultimate energy dissipation. Increasing the yield strength, thickness of web and flange of steel beams has significant effect on improving bending capacity. Reducing the SSK spacing or increasing the yield strength of SSK, height and thickness slightly improve the cracking, yield and ultimate loads, reduce deflections, enhance the flexural stiffness, slightly weakens the ductility and ultimate energy dissipation. Besides, four types of failure modes were defined, based on reasonable assumptions, formulae for bearing capacity were proposed, and the predicted results fit well with experimental results. The results can be taken as reference for the design and application of steel-UHPC composite beams in long-span and heavy-load structures.
To adapt to harsh engineering environments, the use of structural components with fiber-reinforced polymer (FRP) grid-reinforced ultra-high-performance concrete (UHPC) is considered a new development trend. Twelve FRP grid-reinforced UHPC I-beams are fabricated to investigate the effects of the thicknesses of FRP grids (0 mm, 1 mm, 3 mm, and 5 mm) and the steel fiber contents of UHPC (0%, 1%, and 2%) on their flexural performance. Four-point bending tests are conducted to obtain the failure modes, load-deflection characteristics, and strain responses of each testing beam, and a simplified calculation method for cracking loading and flexural bearing capacity is proposed. The results reveal that the increase in FRP grid thickness, the ultimate load bearing capacity of FRP grid-reinforced UHPC structural beams increases and the ductility also can be enhanced. The presence of steel fiber effectively inhibits the cracking and increases the first cracking load of the beam during the test process, although its influence on the ultimate bearing capacity is limited. Besides, until to the cracking stage, there is no noticeable change in the tensile strain of the FRP grids as the applied load increases. It has been used to simplify theoretical calculations, which shows good agreement with the experimental results.
Basalt fiber reinforced polymer (BFRP) is a promising composite material for concrete restoration as an external sheet reinforcement. This work aims to propose a methodology that can effectively monitor and evaluate the damage of external BFRP sheet reinforced concrete during its industrial service life. For better understanding of flexural performance and damage evaluation on BFRP sheet reinforced concrete, four-point bending tests were conducted on specimens produced with different bonding conditions (bonding BFRP sheet at 0%, 80%, 90%, and 95% of failure load) to explore suitable repair strategies for engineering. Multi-view digital image correlation technique is used to analyze the deformation on concrete part and BFRP sheet simultaneously. Such a non-contact measurement makes it possible to observe crack development and evaluate damage degree of concrete beams during loading process and could be applied in industrial measurement. In addition, crack development could be characterized by length and width, which is calculated by strain distribution of both concrete part and BFRP sheet. Results reveal that BFRP sheet could improve about 78.56% of flexural strength and 304.38% of ductility. Besides, it is found that the growth of damage degree is steadier in BFRP reinforced specimens. According to the early warning levels proposed in this work, it is suggested that BFRP reinforcement should be adopted before concrete is loaded to 90% of failure load to allow for timely repair and facilitate subsequent health monitoring.
The ability of the resistivity measurement to study the crack propagation behavior of the ultra‐high‐performance concrete (UHPC) under flexural loading was analyzed, and focused on investigating the effects of crack length and the fracture process zone (FPZ) on the change in electrical resistivity in the post‐cracking stage. The results indicated that the strain‐dependent resistivity characteristic was able to drawn by a linear‐fitting equation before the crack initiated (stage I). After the crack initiated, the change in electrical resistance of the UHPC can be good to reflect the crack development in the post‐cracking stages (II and III), then, the relationship between the crack propagation behavior and the fractional change in electrical resistance (FCR) can be quantitative analysis by using an exponential function. It was also found that the evolution of the crack length and FPZ also had an appreciable impact on the variation tendency of the FCR. The crack length increased dramatically at first and then increased smoothly with the increase of FRC, while the FPZ showed the reverse change trend.
Ultra-high performance concrete (UHPC) combines advanced fibrous and cementitious material technologies to achieve high strength and exceptional durability. The material tends to have microscopic pores that prevent harmful substances such as water, gas, and chlorides from entering. UHPC can also achieve compressive strengths above 200 MPa and tensile strengths above 20 MPa. It also shows significant tensile strain hardening and softening behavior. Owing to all these characteristics, UHPC has excellent performance, making it a potentially attractive solution for improving the sustainability of construction components. Despite UHPC’s outstanding mechanical properties, superior toughness and ductility, and extraordinary durability, various challenges prevent its widespread use. At the same time, several challenges are currently being faced in the applications of UHPC, which include i) design aspects such as material properties; ii) production technology for large-volume and/or long-span elements with low workability, high spalling, and high shrinkage strains; and iii) unknown durability characteristics after the appearance of long-term concrete cracking. With a lack of industry experience, UHPC specialists face additional challenges in spreading hands-on practice to concrete industry professionals so that the latter can be well versed in applying this sophisticated concrete technology. For these reasons, a comprehensive literature study on recent development trends of UHPC should be conducted to determine its current status and prospects. This article scientifically reviews the current status, carbon capturing capabilities, sustainability aspects, challenges and limitations, and potential applications of UHPC. This state-of-the-art review is aimed at helping scientific researchers, designers, and practitioners widen the use of UHPCs in advanced infrastructure applications. This review will help specialists to develop the design guidelines to enable the widespread application of sustainable UHPC. In doing so, design engineers will be provided with an assurance to fully exploit the high strength and other special properties of UHPC and develop models that can efficaciously estimate the ultimate bearing capacity of UHPC sections under various loading conditions.
During the past few decades, strengthening and rehabilitation of reinforced concrete (RC) structures using fiber reinforced polymer (FRP) composites have been considered one of the most promising techniques to achieve the required strengthening and structure life span. Old structures require maintenance and repair because of harsh environments, natural disasters, changes in applied loads, corrosion of reinforcement, and poor maintenance. Many studies have been carried out on flexural and shear strengthening of RC structures using different FRP materials under different schemes. Externally bonded (EB), anchorage, and near-surface mounted (NSM) approaches were the main strengthening techniques addressed by researchers, both experimentally and theoretically. The goal of this work is to give a summary of the literature on the flexural and shear behavior of RC beams reinforced with FRP materials under various schemes. The efficiency of various types of FRP materials and processes has also been considered. Moreover, it illustrates a cost comparison of different strengthening schemes and the limitations of FRP strengthening. This paper also outlines knowledge gaps and possible research directions for FRP-strengthened structures, leading to a greater understanding and the establishment of design guidelines.
In this paper, the shear behavior of glass fiber reinforced polymer (GFRP) bars reinforced Ultra-High-Performance Concrete (UHPC) beams was investigated through experimental tests. Eight GFRP bars reinforced UHPC I-beams were tested until shear failure with various stirrup ratio, reinforcement ratio and shear span to depth ratio. The shear capacity, load-deflection relationship, cracking pattern and failure mode were investigated in detail. The results show that the shear span to depth ratio has the greatest influence on the shear capacity of the beam among the three parameters, followed by the stirrup ratio and reinforcement rate. The stirrup configuration can significantly improve the shear capacity and deformation resistance of the beam and can effectively reduce the stress concentration caused by the uneven distribution of steel fibers, which affects the failure mode of the beam. Increasing the stirrup ratio and reinforcement rate can improve the stiffness of the beam after cracking, and the larger the shear span to depth ratio is, the more significant the improvement effect is. Moreover, stirrup enable the full development of the tensile capacity of GFRP bars. The existing equations of shear strength from five design codes and 7 literatures are compared to the experimental results of 54 UHPC beams. It showed that the formula from the codes AFGC-2013 and JSCE-2006 codes are more accurate. The equations by Kwak, Jin and Thiemicke's provide best predictions.
Beams made of ultrahigh-performance concrete (UHPC), a fiber-reinforced concrete with high compressive strength and tensile strain-hardening characteristics, exhibit flexural behaviors that are different than those traditionally associated with steel-reinforced conventional concrete beams. These behaviors necessitate the development of new predictive design tools that reflect the effect of the material-level properties on the flexural behavior. The research presented in this paper assessed the flexural behavior of UHPC beams through the displacement-controlled testing of a prestressed UHPC bridge girder to failure. The girder was 18.90 m (62 ft) long and contained a total of 26 17.8-mm-(0.7-in.)-diameter steel strands and no mild steel reinforcement. The testing focused on capturing the intermediate and final behaviors, including first cracking, yielding of strands, moment capacity at the development of a single dominant crack, and rupture of strands. Building on observations from this study and prior research by the authors and others, a flexural design framework, founded on the concepts of equilibrium and strain compatibility, is proposed for beams made with UHPC and reinforced with conventional steel reinforcing bars, prestressing strands, or both. The proposed framework includes considerations to avoid the local straining and subsequent hinging of UHPC beams and to address the ductility of flexural members. The framework is verified by comparing the experimental results of flexural tests performed by the authors and others to the analytical predictions, which predominantly relied on input material parameters obtained from independent material tests.
In this study, a novel form of tubular columns that is made of ultra-high-performance concrete (UHPC) internally reinforced with fiber-reinforced polymer (FRP) grid (herein referred to as FRP grid-UHPCtubular column) was developed. The axial compression test results of FRP grid-UHPC tubular columns with and without in-filled concrete are presented and discussed. Effects of the number of the FRP grid-reinforcing cages, the presence of in-filled concrete, and the presence of external FRP confinement were investigated. The test results confirmed that the FRP-UHPC tubular columns have a satisfactory compressive strength, and the strength and ductility of FRP-confined concrete-filled FRP grid-UHPC tube columns are enhanced due to the confinement from the FRP wrap. The proposed FRP grid-reinforced UHPC composite tubes are attractive in structural applications as pipelines or permanent formworks for columns, as well as external jackets (can be prefabricated in the form of two halves of tubes) for strengthening deteriorated reinforced concrete columns.
Corrosion of steel reinforcement is the major cause of deterioration in reinforced concrete structures. Strengthening of concrete structures has been widely studied. However, most research was conducted on sound structures without considering the effects of corrosion. This paper presents an experimental study of the feasibility of using externally bonded FRP laminates combined with U-jackets, applied directly without repairing the deteriorated concrete cover, to strengthen beams with corroded reinforcement. Ten beams were tested in four-point bending. Two beams were not deteriorated and non-strengthened; these served as references. The other eight were pre-loaded to induce flexural cracks and then exposed to accelerated corrosion. Two of the deteriorated beams were not strengthened, three were strengthened with glass-FRP (GFRP) laminates and three with carbon-FRP (CFRP) plates on the beam soffits. On the six strengthened beams, CFRP U-jackets were installed along the span. Local corrosion levels were evaluated with a 3D-scanning technique. Pitting corrosion significantly reduced the load-carrying and deformation capacity of the deteriorated beams. Despite average corrosion levels of 20%, local corrosion levels up to 57% and corrosion-induced cracks up to 1.9 mm wide, the FRP-strengthening method (applied directly to the beams without repairing the deteriorated concrete cover) was effective in upgrading the load-carrying capacity and flexural stiffness. The applied U-jackets effectively suppressed the delamination of the concrete cover and led to the rupture of GFRP laminates and a utilisation ratio of CFRP plates up to 64%. However, improvement in the deformation capacity was not noticeable; this requires further research.
The application of fiber-reinforced-polymer (FRP) bars to reinforce concrete structures could mitigate the corrosion-induced damage of steel reinforcements. No study has been reported in open literature on flexure-critical or shear-critical concrete beams reinforced with Basalt FRP (BFRP) bars under impact loads. In this study, six concrete beams reinforced with BFRP bars were tested under quasi-static and impact loads. The test results showed the flexure-critical beams experienced the failure mode changing from flexure-governed under quasi-static loads to flexure-shear combined under impact loads. The shear-critical beams still failed in diagonal shear under impact loads, but experienced severer concrete spalling and more critical diagonal cracks on both sides of the beams. The impact performance of concrete beams with higher strength concrete may not be necessarily superior to that of beams with normal strength concrete due to the increased brittleness. Moreover, a numerical model of the tested beams under impact loads was developed and calibrated in LS-DYNA. Numerical results showed increasing tension reinforcement ratio could change the failure mode from flexure-governed to flexure-shear combined along with the reduced maximum midspan deflection. The concrete beams reinforced with BFRP bars had comparable impact resistant performance with the conventional steel reinforced concrete beams.
An advanced development in construction industry was achieved by applying ultra-high-performance concrete technology (UHPC). Intensive research efforts had been concentrated in construction to produced amazing levels of qualities with strength greater than 150 MPa and high durability that had never been thought possible before. With this technology, it is possible to construct structures beyond the usual designs but with limited use in construction since it is not commercially viable to replace conventional concrete in most applications. This is attributed to the high cost of materials, the lack of their availability, limited design codes, and complicated manufacturing and curing techniques. This paper reviews the evolution of UHPC and the suggested ideas to replace its expensive composites by cementitious materials. However, concrete made with these alternative materials will not be of the same quality as the standard UHPC. Another promising choice, which seems to be more practical and easier to promote UHPC technology in construction, is looming on the horizon. It is based on the utilization of UHPC in hybrid structures by combining UHPC with other construction materials. The cost of production will hopefully be reduced with such composite structures that have the advantages of the combined materials. Therefore, it is recommended to continue research into this choice which will increase the potential of UHPC to be more accepted in many different construction applications.
Accelerated bridge construction (ABC) is becoming progressively popular in China due to its advantages in sustainable development. However, some challenges still prevent it from being further widespread, especially in some harsh environments. As one of the greatest advances in material science, ultra-high-performance concrete (UHPC) is regarded as a competitive option for addressing the challenges of ABC due to its excellent material properties. A new cost-effective UHPC was developed with some modifications to weaken some adverse factors influencing the applications of UHPC in ABC, that is, high initial cost of material and steam/extreme heating curing requirements. Cost-effective UHPC is deemed to have the potential to build the critical zones of precast bridges, such as stress concentration zone, fatigue stress zone, inelastic deformation zone, harsh environment exposure zone, and late-cast joint zone, considering its material properties. In recent years, a series of tests have been done to investigate the cost-effective UHPC's material properties and its structural elements' mechanical behaviors. This paper systematically reports the cost-effective UHPC alternative for ABC from the laboratory tests on material properties and structural elements to real bridge implementations. Some challenges and future opportunities are presented for reference. The experimental results show that the cost-effective UHPC can have superior material properties and its structural elements can have satisfactory mechanical behaviors. Real engineering examples demonstrated that the cost-effective UHPC enables precast bridges to be lighter in weight, have higher strength, and support longer spans. The biggest challenge may be that more research and engineering examples are required to validate the feasibility of UHPC codes for cost-effective UHPC when it is applied in ABC.
The article describes four-point bending tests of three reinforced concrete beams with identical cross-sections, spans, and high-ductility steel reinforcement systems. Two beams were strengthened in the compressed section with a thin layer of reactive powder concrete (RPC) bonded with evenly spaced stirrups. Their remaining sections, and the third reference beam, were made of ordinary concrete. Measurements of their deflections, strains and axis curvature; ultrasonic tests; and a photogrammetric analysis of the beams are the main results of the study. For one of the beams with the RPC, the load was increased in one stage. For the two remaining beams, the load was applied in four stages, increasing the maximum load from stage to stage in order to allow the analysis of the damage evolution before reaching the bending resistance. The most important effect observed was the stable behaviour of the strengthened beams in the post-critical state, as opposed to the reference beam, which had about two to three times less energy-absorbing capacity in this range. Moreover, thanks to the use of the RPC layer, the process of concrete cover delamination in the compression zone was significantly reduced, the high ductility of the rebars was fully utilized during the formation of plastic hinges, and the bending capacity was increased by approximately 12%.
This paper presents an experimental investigation and a stress-strain model for ultra-high performance concrete (UHPC)-filled fiber reinforced polymer (FRP) tubes under cyclic axial compression. Test results from 12 cyclically loaded and 18 monotonically loaded cylindrical specimens with different tube thicknesses, curing regimes, and steel fibers are presented. The influence of the tested variables on the envelope curve, plastic strain, and stress deterioration was clarified to establish a theoretical stress-strain model. Through a comprehensive assessment of well-known cyclic stress-strain model developed for FRP-confined conventional concrete, a new model for FRP-confined UHPC under cyclic axial compression is proposed based on a more rational consideration of the key characteristics of FRP-confined UHPC. Both the monotonic envelope response and the cyclic response showed good agreement between the analytic predictions based on the proposed model and the test results, confirming the capability of the proposed model to predict the cyclic axial behavior of FRP-confined UHPC.
The application of fiber-reinforced polymer (FRP) bars and ultra-high performance concrete (UHPC) in the field of civil engineering is promising. An innovative FRP bar-reinforced UHPC short-ribbed bridge deck slab, with low self-weight and high structural performance, was proposed in this study. The behavior of one-way basalt FRP (BFRP) bar-reinforced UHPC slabs under concentrated load was experimentally investigated, and compared with that of a steel bar-reinforced UHPC slab. The ultimate capacity of the one-way BFRP bar-reinforced UHPC slab was 0.59 times that of the steel bar-reinforced UHPC slab, while its ductility was better. Increasing the reinforcement ratio and loading area was beneficial to the ductility of one-way BFRP bar-reinforced UHPC slabs. Moreover, the model proposed by EI-Gamal et al. was found to be suitable for evaluating the punching shear capacities of one-way BFRP bar-reinforced UHPC slabs. However, the model failed to consider the unique strain-hardening characteristics of UHPC, which led to conservative prediction.
Steel bent reinforcing bars (rebars) are widely used to provide adequate anchorage. Bent fiber-reinforced polymer (FRP) rebars are rarely used because of the difficulty faced during the bending process of the FRP rebars at the construction site. Additionally, the bending process may cause a significant decrease in the structural performance of the FRP rebars. Therefore, to overcome these drawbacks, a headed glass fiber-reinforced polymer (GFRP) rebar was developed in this study. The pull-out tests of the headed GFRP rebars with diameters of 16 and 19 mm were conducted to evaluate their bond properties in various cementitious materials. Moreover, structural flexural tests were conducted on seven precast concrete decks connected with the headed GFRP rebars and various cementitious fillers to estimate the flexural behavior of the connected decks. The results demonstrate that the concrete decks connected with the headed GFRP rebar and ultra-high-performance concrete (UHPC) exhibited improved flexural performance.
The flexural behavioral properties of ultra high performance concrete (UHPC) low-profile T-beams reinforced with a combination of steel fibers and steel reinforcing bars were investigated in this paper. Five large scale T-beams were tested and analyzed regarding their deflection, ductility, strain, curvature, load capacity and crack development. The experimental variables include the reinforcement ratio, the slenderness (length to diameter ratio) of the fiber reinforcements, and the fiber type. The experiments showed that all specimens exhibit flexural failure with the yielding of steel bars and excessive expansion of flexural crack, and the compression zone in the reinforced UHPC low-profile T-beam is not crushed because of the ultra high compressive strength and area of UHPC. In addition, it was concluded that using hooked-end fibers can effectively increase the specimen’s durability-based cracking load in comparison to straight fibers of same slenderness, whereas the reinforcement ratio and the slenderness of the fibers have little influence on this. Increasing the reinforcement ratio and using hooked-end instead of straight fibers increase the load capacity and bending stiffness of the specimen, as well as reduces the crack width at comparable applied load. A model was established to compute the ultimate capacity of UHPC low-profile T-beams and the prediction agrees well with the experimental results in the present and published investigations.
This paper presents a pilot numerical study to investigate the flexural performance of simply-supported layered ultra-high performance concrete (UHPC)-normal strength concrete (NC) beams consisting of two UHPC layers with an NC core layer sandwiched in between. A three-dimensional finite element model (FEM) is developed for the beam and is validated against the existing experimental results of UHPC-NC bi-layer composite beams. The effects of the bonding between UHPC and NC layers, the fraction of UHPC and NC in the beam, and the mechanical properties of UHPC material on the flexural performance of the beam are discussed in detail. Numerical results show that the flexural behavior of the layered UHPC-NC beam is significantly influenced by the interfacial bonding between UHPC and NC layers that is represented by the cohesive stiffness which should be higher than 3 to avoid possible delamination failure in the beam. The material cost effectiveness of the layered UHPC-NC beam is also evaluated. It is found that with properly designed UHPC layer thickness, the beam with a thick NC core layer can not only sustain the same loading as a pure UHPC beam with the same thickness but is also much less costly, indicating the excellent mechanical performance and great cost effectiveness of the proposed layered UHPC-NC beam.
This study investigates the effectiveness of reducing alkalinity on the durability enhancement of basalt fiber reinforced polymer (BFRP) bars to address their durability in the alkaline environment of seawater sea sand concrete (SWSSC). After accelerated aging at different temperatures and durations, the tensile properties of BFRP bars immersed in simulated SWSSC pore solution and embedded in seawater sea sand mortar (SWSSM) with varying alkalinities were evaluated by tensile testing. In addition, the scanning electron microscopy, X-ray micro-computed tomography, and low-field nuclear magnetic resonance were used to examine the microstructure of embedded BFRP bars and SWSSMs. The experimental results indicate that reducing the pH of the solution and SWSSM to around 12 or below can significantly mitigate BFRP bar tensile strength loss. The maximum improvement of tensile strength retention for bare and embedded BFRP bars are 43.52% and 40.27%, respectively. Compared to BFRP bars wrapped in normal SWSSM, the degradation of resin, basalt fiber, and fiber-resin interface of embedded BFRP bars in the low-alkalinity SWSSMs is considerably mitigated due to the reduced alkalinity and porosity. According to the Arrhenius prediction, the tensile strength retention of BFRP bars embedded in normal SWSSM will decrease to 70% within 1.70–5.10 years, but it will be more than 85% for those embedded in low-alkalinity SWSSMs. This study indicates that it is an effective way to mitigate the deterioration of BFRP bars by using low-alkalinity SWSSC when BFRP bars are used as reinforcement in marine SWSSC structures.
It is a promising way to strengthen and repair the damaged normal concrete (NC) structure with ultra-high performance concrete (UHPC), and the interfacial performance between NC and UHPC is a critical issue. This paper aims to reveal the debonding failure performance between UHPC (repair material) and NC (substrate material) under tensile loading and shear loading. A multi-scale analysis method is established to simulate the mechanical behavior of the NC-UHPC interface, considering the complex interface geometry on the meso-scale. The failure of NC matrix is described by the elastic-plastic damage model in form of coupled gradient-enhanced damage evolution, the interface is represented by the cohesive zone model (CZM), and the UHPC is characterized by the second-order mean-field homogenization (MFH) method. After extracting the roughness characteristic parameters of the tested interfaces, the representative volume element (RVE) corresponding to the roughness is established, the load transfer and the debonding failure behavior are investigated, and the effective traction-separation law (TSL) is obtained. Finally, the tensile and the shear separation mechanisms of the NC-UHPC interface are revealed, and the effects of the roughness and strength of the interface on the macro shear and tensile strength are evaluated. Both the interface roughness and the substrate concrete strength have positive effects on the mechanical properties of the interface.
In this short communication, a series of novel forms of tubular members made of fiber-reinforced polymer (FRP) grid reinforced ultra-high-performance concrete (UHPC) (referred to as "FRP-UHPC tubular members") are developed. FRP-UHPC tubular members have excellent mechanical properties, and their excellent performances are demonstrated through three preliminary studies: i) behavior of FRP-UHPC tubular beams under bending; ii) behavior of FRP-UHPC tubular columns under axial compression; and iii) behavior of FRP-UHPC plates with a novel grouting sleeve connection system under bending. The experimental programs and results are briefed in the paper. The proposed FRP-UHPC tubular members are attractive in various structural applications such as pipelines, bridge box girders, permanent formwork of beams/columns, especially in marine environments (e.g., floating structures).
Over the past few years, there has been a rise in the use of Stay-In-Place (SIP) structural formworks made of fibre reinforced polymer (FRP). This study investigates the performance of decks made of FRP SIP structural formworks with an improved configuration using conventional and ultra-high performance concrete under static loadings. The proposed new composite deck consisted of a glass fibre reinforced polymer plate stiffened with Y-shape ribs for resisting tensile and shear stresses while reinforcements was placed on the compressive side of the deck. The new composite deck was compared with conventional reinforced concrete solid decks and a composite deck consisting of square hollow section (SHS) stiffeners. Results showed that the deck made of Y-shape stiffeners failed in punching shear, and the use of Y-shape stiffeners significantly enhanced the ultimate strength and shear capacity of the proposed deck. It was also concluded that the voids in the concrete deck with SHS stiffeners resulted in lightweight structures but had a negative influence on the ultimate strength of the deck cast on FRP SIP formwork. Increasing the concrete strength from 34 MPa to 140 MPa resulted in a 71% enhancement in the maximum load-carrying capacity but did not considerably influence their stiffness.
A lightweight, easily erected, hybrid glass-carbon FRP bridge girder is developed and assessed. A four-point bend test of a 12.45 m-span specimen is performed to assess load–deflection response, flexural strains and girder-concrete composite action. Web buckling occurred near peak load, but maximum measured bottom flange tensile strain reached more than 90% of its expected ultimate value. Strength and fatigue resistance of the girder-to-deck shear connection is experimentally assessed. Live-load testing of a new, 22.9 m-span bridge using these girders is performed. The test results verify the viability of this FRP girder for short- and medium-span bridges.
Ultra-high performance concrete (UHPC) overlays have become increasingly used for retrofitting concrete bridge decks, beams and slabs. A particular concern for the strengthening system is the localized cracking that develops within the overlay which can lead to failure and reduced strength and ductility. Fibre reinforced polymer (FRP) rebar can be used as a hybrid overlay reinforcement to overcome this issue and result in much thinner and lighter overlays. In the present study, the behaviour of reinforced concrete (RC) beams strengthened with carbon-FRP (CFRP)-reinforced UHPC overlays is examined through a robust finite element (FE) model. The model was validated against two existing experimental campaigns, resulting in excellent predictions for load–deflection and load-slip responses, cracking, yielding, and ultimate loads as well as the failure modes. A parametric study, comprising 68 models, was conducted on 5 key parameters, namely: reinforcement ratios for the beam and overlay, concrete compressive strength for the beam, overlay thickness and the interface between cast-in situ overlay and the beam. In general, the system resulted in significant increases in ultimate load and ductility compared to the control beam and those strengthened with un-reinforced or steel-reinforced overlays and eliminated the overlay cracking failure. Varying the CFRP reinforcement ratio in the overlay for the strengthened beam results in a significant increase in ultimate load in range of 112–463%, compared to the control beam. An analytical procedure was also undertaken, using parametric study results and regression analysis, resulting in the development of an analytical model for estimating the capacity of strengthened beams and can be used for design purposes.
Fiber reinforced polymer (FRP) composites are being extensively considered for construction of ships and marine structures. Due to harsh environmental operational conditions, failure prediction of such structures is an imperative in this industry sector. This paper presents the final results of a 2-year research of real marine environment induced changes of mechanical properties in FRP composites.
Realistic environmental input parameters for structural modeling of marine structures are crucial and can be obtained by conducting tests in real sea environment for prolonged periods, as opposed to usual accelerated laboratory experiments. In this research, samples of epoxy/glass and polyester/glass with various fiber layout configurations have been submerged under the sea for periods of 6, 12 and 24 months. An analysis of mass changes, marine microbiology growth, tensile strength and morphological structures of the coupons was performed and compared with samples exposed to room environment.
All samples exhibited an increase in mass due to seawater absorption and microorganism growth in the organic resins (matrix). The tensile strength loss variation through the periods of submersion showed a correlation with the fiber layout configuration. The results of optical and scanning electron microscopical investigation indicated significant matrix morphological changes primarily due to salt crystal formation and the impact of sea microorganisms embedding in and attaching to the resin.
The outcome of this research will be the basis for a set of realistic input parameters for a failure analysis numerical tool currently in development that can be applied for life-time behavior predictions of marine structures.
A novel joint, inspired by ancient mortise-and-tenon joints in timber structures, was proposed for pultruded FRP beams and concrete-filled FRP columns. FRP structures are widely known to lack appropriate joint solutions. In this regard, the proposed joint effectively provides a feasible solution for FRP frame structures. In this work, the conceptual development of this joint was first addressed. Then, the construction schemes were presented in detail, and a total of four full-scale joints were tested. To assess the structural behavior of the proposed joint, experiments including four specimens were conducted under symmetric and anti-symmetric load, i.e. the loading conditions in sway and non-sway frames. The typical failure modes were identified, including pull-out or tensile fracture of the tensile flanges and end crushing of the compressive flange. Additionally, an analytical study is carried out to develop the design method for this joint. The predicted moment capacity is found to have an excellent accuracy as compared to the experimental results. Moreover, the proposed joint can be classified as the rigid and full-strength joint based on the classifications prescribed by the European design code. It is noted that the proposed joint is completely composed of FRP composites and concrete (i.e., it is a steel-free joint). Thus, the joint should exhibit excellent corrosion resistance, which is of particular interest for marine structures on/near oceans.
This study proposes using glass fibre-reinforced polymer (GFRP) as stay-in-place structural formwork for casting bridge decks with ultra-high performance concrete (UHPC). The GFRP stay-in-place formworks completely replace the bottom layer of rebars, and the top steel reinforcement is also replaced by a GFRP mesh to mitigate the corrosion damage. The formworks were either a flat GFRP plate with square hollow section (SHS) stiffeners or a flat GFRP plate with new Y-shape stiffeners. Concentric static tests on five 1:2.75 scale decks were performed to investigate the effect of stiffener’s configuration and the influence of the concrete strength on the performance of bridge decks. Rotational fixity support was used to simulate a real bridge deck connection of supporting girders. All specimens with stay-in-place formwork exhibited punching shear failure. It was found that the use of Y-shape stiffeners significantly improved the load-carrying capacity of the proposed deck. Replacing normal concrete with UHPC further improved the loading capacity of the deck. The decks demonstrated excellent performance, with the load-carrying capacity 3.8 to 9.5 times higher than the established equivalent service load depending on the concrete strength and configuration of the GFRP stay-in-place formwork. Deflection at service load was less than span/1,600 for all the decks. Compared with normal strength concrete (34 MPa), UHPC improved the maximum load-carrying capacity of the deck from 91.4 kN to 149 kN.
In this study, a novel form of tubular permanent formwork that is made of ultra-high-performance concrete (UHPC) internally reinforced with fiber-reinforced polymer (FRP) micro-bars (herein referred to as FRP-UHPC tubular permanent formwork or simply FRP-UHPC tubular column) is developed. The axial compression test results of FRP-UHPC tubular columns with and without in-filled concrete are presented and discussed. Effects of the FRP micro-bar spiral pitch, the steel fiber addition in the UHPC, the tube thickness and the presence of external FRP confinment are investigated. The test results confirmed that the FRP-UHPC tubular columns have an excellent compressive strength, and the strength and ductility of existing concrete columns jacketed with an FRP-UHPC composite tube are substantially enhanced due to the confinement and axial contribution of the FRP-UHPC tube. The proposed FRP micro-bar-reinforced UHPC composite tubes are attractive in structural applications as pipelines or permanent formworks for columns, as well as external jackets (can be prefabricated in the form of two halves of composites tubes) for strengthening deteriorated reinforced concrete columns.
Current axial stress–strain models of FRP-confined concrete mainly focus on FRP-confined normal concrete, the research work on studying the stress–strain behavior of FRP-confined ultra-high performance concrete (UHPC) is limited so far. To fill this research gap, this study develops a new design-oriented stress–strain model to predict the stress–strain relationship of FRP-confined UHPC cylinders under axial compression. The developed model has two versions, i.e., the Version I model is applicable to the specimens experiencing stress reduction, and the Version II model is a typical bi-linear model with a parabolic first portion and an ascending linear second portion. A large amount of compressive test data collected from five previous experimental studies involving 117 FRP-confined UHPC cylinders was used for the model development. The assessment results reveal that both versions of the developed design-oriented model have the capability of accurately estimating the characteristic stresses and strains at characteristic points. In addition, it is demonstrated that the full stress–strain curves predicted by the developed model agree reasonably well with most of the test results.
Debonding failures of FRP have been frequently observed in laboratory tests of reinforced concrete (RC) beams flexurally-strengthened with near-surface mounted (NSM) fibre-reinforced polymer (FRP). A number of numerical and theoretical studies have been carried out to predict debonding failures in NSM FRP-strengthened beams, and several strength models have also been proposed. The existing studies, however, were all based on the scenario of a simply supported beam tested under one or two-point loading, while the influence of load distribution has not yet been investigated. This paper presents the first ever study into the effect of load distribution on the behaviour of NSM FRP-strengthened RC beams. A series of large-scale RC beams flexurally-strengthened with NSM FRP strips were first tested under different load uniformities; then a finite element (FE) model, which can give close predictions to the behaviour of such strengthened beams, was developed; finally, the proposed FE model was utilized to investigate the influence of bond length of NSM FRP on the load uniformity effect. It was found that the load uniformity has a significant effect on the beam behaviour, and the degree of this effect varies with the bond length of NSM FRP.
This research is aimed at investigating the flexural and cracking behaviors of ultra-high-performance concrete (UHPC) beams. Nine UHPC beams with different reinforcement ratios (0, 1.0%, 2.9%, 4.8%, and 7.1%) and fiber volume fractions (2.0% and 3.0%) were considered under flexure. A section analysis was also performed to predict the flexural and cracking behaviors of UHPC beams and was verified by experimental results. The test results showed that the reinforcement and steel fibers can play a significant role in limiting crack development. Considering the performance of the steel fibers, a high-precision equation for predicting the average crack spacing of UHPC beams was developed. The formulas in the French standard NF P 18–710 overestimated the maximum crack width by a significant margin for the UHPC beams with a high reinforcement ratio (>4%). The flexural stiffness of the reinforced UHPC beams increased as the reinforcement ratio increased, whereas their initial stiffness without reinforcement was larger than that with a low reinforcement ratio of 1.0% because of the weak bond interface between the UHPC and the reinforcement. The flexural capacity of the UHPC beams basically increased linearly with the reinforcement ratios. The result of the sectional analysis indicated that the contribution of steel fibers to the flexural capacity decreased significantly with the increase in the reinforcement ratio. In the ultimate limit states design, the contribution of UHPC tensile capacity could be considered only as a safety reserve when the reinforcement ratio is greater than 4.0%, whereas its contribution needs to be considered for the UHPC beams with a reinforcement ratio of less than 2.9%.
To investigate the shear behavior of post-tensioned concrete beams with longitudinal fiber-reinforced polymer (FRP) reinforcements and without stirrups, seven large-scale beams, with a shear span-to-effective depth ratio of approximately 3.0, were tested. The test variables included the prestressing level, type of flexural reinforcement, and tendon profile. The shear crack width and slip were tracked using digital image correlation technique. It was found that the post-tensioned concrete beams failed in shear-compression or shear-tension, while the nonprestressed concrete beam failed in diagonal tension. The post-tensioned beams designed to have approximately the same longitudinal reinforcement stiffness, exhibited similar shear strength. Draping FRP tendons resulted in an increase of 8.8% in shear cracking strength, while it had slight effect on the maximum shear strength. In this study, it was demonstrated that arching action played a major role in the shear resistance of FRP post-tensioned beams, while the contribution of aggregate interlock was negligible.
In this study, a novel ultra-high-performance concrete (UHPC) composite plate that is reinforced with a fibre-reinforced polymer (FRP) grid (herein FRP-UHPC composite plate) is developed and reported. The durability and mechanical performance of FRP-UHPC composite plates in harsh environments are expected to be superior to conventional materials when used in optimal configurations; because i) both UHPC and FRP composites in isolation have excellent mechanical properties in specific directions, and ii) they are also durable materials. The flexural and tensile behaviour of FRP-UHPC composite plates, with and without the inclusion of steel fibres in the UHPC mix, were investigated via experimentation. The test results, which demonstrate the excellent interaction between FRP and UHPC, confirm the viability of the system: i) the inclusion of an FRP grid enhances the ultimate flexural capacity by over 150% and the ultimate tensile capacity by over 200%; ii) the FRP-UHPC composite plates exhibit tensile elastic-strain hardening behaviour and the average ultimate tensile stress of the composite plates with an FRP reinforcement ratio of about 0.69% is over 25 MPa; and iii) the interaction between the FRP grid and UHPC is sufficient to transfer stresses in the longitudinal FRP strips to the UHPC through the transverse strips of the FRP grid, while the steel fibres in the UHPC passing through the openings of the FRP grid reduce the likelihood of FRP debonding. The proposed strain-hardening FRP-UHPC plates are expected to be promising for structural elements with various purposes.
This paper presents a study of the shear capacity of reinforced concrete (RC) beams strengthened in shear with externally bonded (EB) fiber reinforced polymer (FRP) composites, based on the performance of existing published tests. The main objective of this study is to create a comprehensive overview of the performance of different shear strengthening configurations and the failure modes observed in experimental tests. In addition, the influence of the following parameters on the strengthening efficiency has been analyzed: shear span to depth ratio (a/d), existing steel transverse reinforcement ratio (ρsw), concrete strength (fck), size effect (d), longitudinal reinforcement ratio (ρsL), strengthening configurations, type of FRP: sheets or laminates (CFRP, GFRP), and FRP axial stiffness (AfEf).
Fiber reinforced polymer (FRP) and ultrahigh performance concrete (UHPC) are two durable and high strength construction materials. FRP-UHPC hybrid beam is a durable and efficient bridge superstructure, however, its capacity is limited by the low shear strength of FRP profiles. This paper investigated two ways to improve the shear performance of FRP-UHPC hybrid beams: externally bonded (EB) carbon FRP (CFRP) sheets to the webs of glass FRP (GFRP) profiles, and increasing the number of webs of FRP profiles. Nine FRP-UHPC hybrid beams were tested under bending loads, which indicates: (i) EB CFRP sheets to GFRP webs substantially increased the shear capacity and the rigidity of FRP-concrete hybrid beams. (ii) the built-up II-shaped FRP profiles had almost doubled the shear capacity of FRP-concrete hybrid beams using I-shaped profiles. (iii) With UHPC height increased from 30 mm to 80 mm, the failure mode changed from the shear fracture of UHPC slab to the interfacial disconnection. (iv) With the decrease of shear span length from 700 mm to 120 mm, the shear capacity was largely improved. In summary, EB CFRP sheets and using double web GFRP profiles are two effective ways to improve the shear performance of FRP-UHPC hybrid beams.
Back in the 90′s, the shear design of FRP-reinforced concrete beams was developed based on the shear design of conventional steel reinforced concrete beams. Since then, significant changes were implemented in the shear design provisions of internationally recognized design codes for conventional beams as well as being inconsistent and lack the agreement. In addition, a much large number of FRP reinforced concrete beams without stirrups were tested, which included beams with different cross section shape as well as deep and shallow beams. Therefore, it is our mandate to update, refine, unify the current shear design of FRP reinforced concrete beams without stirrups. The purpose of this study is to examine the design of the shear strength of FRP-reinforced concrete beams without stirrups in an attempt to refine and unify that design. An extensive experimental database was gathered and compiled with a total of 510 FRP-reinforced beams without stirrups from over 67 investigations. In addition, selected design codes and available models were used to calculate the shear strength of the tested beams. These design codes and available models were assessed, and recommendations were outlined. Moreover, a unified model was proposed. The strength predicted using the proposed model, which was compared with that measured during testing and that calculated using selected models from the literature. The proposed model was found to be more refined and unified, compared to the available models from the literature.
Strengthening of deficient reinforced concrete (RC) beams using near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars has in recent years been gaining greater interest and increased field applications. While considerable research has explored the behavior of NSM-FRP strengthened rectangular-section RC beams and the effects of influential parameters, there is a dearth of similar studies on RC T-section beams. Moreover, analytical models for predicting the flexural strength of NSM strengthened RC beams are yet to be confirmed experimentally. Thus, the present study investigates the behavior of RC T-section beams strengthened with NSM FRP bars under monotonic flexural loading and compares the experimental results with predictions of a flexural model derived from first principles. Ten RC T-section beam specimens strengthened with NSM FRP bars and three standard specimens were considered. The failure mode, cracking resistance, yielding, ultimate capacity, flexural stiffness, and ductility of specimens were compared and analyzed. Based on the experimental results, a general increase in flexural stiffness of the strengthened specimens was observed, especially at the post-yield stage of loading. Analytical flexural strength predictions were calculated and corroborated with the experimental results. An adjustment parameter to the flexural stiffness prediction model was proposed to account for reductions in the effective area of the FRP bars used in the sectional strength calculations.
In this paper, ultrahigh-performance concrete (UHPC) combined with fiber-reinforced polymer (FRP) composites is proposed for the shear strengthening of corroded reinforced concrete (RC) beams. The UHPC–FRP composites are utilized to replace the spalled concrete cover in corroded RC beams. UHPC helps controlling the crack width and preventing the corrosive substances in the environment from invading the internal steel stirrups, whereas the carbon FRP (CFRP) meshes are embedded in UHPC to increase its tensile strength. The experimental study shows that the UHPC–FRP shear strengthening significantly increased the shear capacity of the RC beams and suppressed the formation of shear cracks. The introduction of thin CFRP meshes in UHPC greatly decreased the crack width in concrete. No debonding was observed between FRP and UHPC. The shear capacity contribution between the steel stirrups and UHPC–FRP composites was affected by the corrosion level in the existing internal shear stirrups. An analytical model is proposed for the prediction of shear capacity contributions of all materials to the strengthened RC beams, which shows satisfactory precision as compared to the experimental data.
In this study, the efficacy of traditional versus innovative systems for enhancing the flexural strength of RC (reinforced concrete) beams is investigated experimentally as well as numerically. Four-point bending tests were conducted on seven RC beams. Test matrix comprised of two control and five strengthened beams. Strengthening techniques included: bonded steel plate, externally attached CFRP (carbon fiber reinforced polymer) composite, NSM (near-surface mounted) steel rebars, externally attached CFRCM (carbon fiber reinforced cementitious matrix) composite, and innovative hybrid system comprising of ultra-high performance concrete (UHPC) layer combined with NSM CFRP strips. Different strengthening systems were designed to provide approximately the same flexural strength enhancement. The performance of strengthened specimens was compared in terms of load–deflection characteristics. The peak load of the tested specimens was analytically predicted using the equations of ACI 318-19 code and ACI 440.2R-17 guidelines. Nonlinear FE (finite element) modeling was also carried out, and a comparison was conducted between the experimental and FE results showing good agreement. The validated FE models were extended for some useful parametric studies of interest.
In this study, the ultra-high-performance concrete (UHPC) with high tensile strength and cracking resistance was used in lieu of concrete, aiming at improving the mechanical performance and durability of continuous beams under hogging moment. A novel high-performance and high-durability continuous FRP-UHPC hybrid beam was thus investigated, consisting of an I-shaped pultruded GFRP profile with web strengthened by externally bonded carbon FRP (CFRP) sheets, and an overlying UHPC slab. One FRP-concrete and six FRP-UHPC two-span continuous hybrid beams were tested to evaluate effects of slab material, thickness of UHPC slab, reinforcement area and loading pattern. Test results revealed that most continuous FRP-UHPC hybrid beams failed in compression at the bottom flange of FRP profile beam at the middle support. The employment of UHPC slab substantially improved the cracking load by 259.2%, maximum load by 206.4%, stiffness by 38.9% and ductility by 35.0% compared with FRP-concrete hybrid beam. The cracking performance and load-carrying capacity were increased with the increase of the thickness of UHPC slab and reinforcement area. The cross-section strain distribution indicated that the plain section assumption was valid in both positive and hogging moment zones. The small interfacial slippage demonstrated a full composite action between FRP and UHPC.
The composite beam with inverted-T steel girder and UHPC slab connected by studs arranged in the web of the girder was developed to further the economy in material and best exploit the properties of UHPC. However, the composite beam may experience premature failure due to the occurrence of slip at the shear interface. This so-called partial interaction needs to be addressed in the flexural design for field application of the developed composite beam. The present study proposes a nonlinear analysis method for the evaluation of the flexural behavior of the composite beam considering partial interaction. An analysis method using the Fourier series to approximate the internal forces in the beam considering slip is coupled with a sectional analysis method accounting for inelastic behavior of the steel girder and nonlinear behavior of UHPC. The model considering the slip effect induced by partial interaction is seen to realistically and accurately simulate the actual behavior of the composite beam.
This paper presents an investigation on the eccentric compressive behavior of fiber-reinforced polymer (FRP)-confined ultra-high performance concrete (UHPC). Twenty specimens with varied tube thicknesses and loading eccentricities were tested. The results indicated that nearly all specimens showed a strain-hardening behavior, except that several specimens with a very large loading eccentricity showed a strain-softening behavior. With the increase of the loading eccentricities, the load-carrying capacity and deformability exhibited a substantial reduction, while the tube thickness only showed a significant effect in concentric or small eccentricity cases. A finite element model was established, where UHPC was treated as a two-phase material and simulated with UHPC matrix and explicitly modeled steel fibers. An equivalent stress-strain relationship derived from the single fiber pullout test was proposed for the embedded steel fibers to provide a more accurate prediction for the fiber-matrix interfacial behavior. Based on a regression analysis of the experimental and numerical results, a design equation was developed to predict the ultimate load-carrying capacity of FRP-confined UHPC under eccentric compression.
Firstly, this study developed a new type of sandwich composite beam using ultra-high performance concrete (UHPC) and novel enhanced C-channels (ECs). Then, this study developed a nonlinear finite element model (NFEM) for steel-UHPC-steel sandwich beams with ECs (SUSSB-ECs) using general software package ABAQUS. The developed NFEM detailed simulated the novel sandwich composite beams including the complex geometry of ECs, the complex interactions between the ECs and UHPC core, and nonlinearmechanical properties of UHPC and steel. The comparisons of NFEM simulation results with eight four-point bending tests indicated that the NFEM simulated well the ultimate strength behaviour of SUSSB-ECs in terms of load versus strain/deflection curves, deformations and cracks in UHPC, and stiffness/strength indexes. Parametric studies were then performed with the developed NFEM to study the key parameters influencing the ultimate strength behaviour of SUSSB-ECs. The parametric studies showed that increasing the thickness of tension and compression steel faceplates had significant influences, but the spacing of ECs had marginal influences on ultimate strength behaviours of SUSSB-ECs. Reducing the shear span ratio did not change the flexural failure, but reduced the flexural bending resistance of SUSSB-ECs. Increasing the grades of steel faceplates had ignorable influences on the stiffness, but significantly increased the bending resistance of SUSSB-ECs.
A R T I C L E I N F O Keywords: Ultra-high performance concrete (UHPC) Steel-UHPC composite beam Hogging moment Flexural behavior Stud Bolt A B S T R A C T Using ultra-high performance concrete (UHPC) in the hogging moment regions of composite beams might significantly enhance their cracking and flexural performance. In the present paper, the flexural test was performed on steel-UHPC composite beams with stud connectors (SU-S) and bolt connectors (SU-B) at the interface. Crack resistance, ultimate flexural capacity, failure modes, and deformation characteristics of SU-S and SU-B under hogging moment were investigated. The test results showed that steel-UHPC composite beams exhibited excellent cracking and flexural performance under the hogging moment. As compared to the steel-normal strength concrete (NSC) composite beam (SC-S), cracking load and ultimate flexural capacity of steel-UHPC composite beams increased by around 340% and 26%, respectively. Moreover, the length and width of cracks in the UHPC flange plate developed slowly with load. Many short and small cracks were observed, having a close spacing in the UHPC flange plate. However, ductility and rotation capacity of both SU-S and SU-B under the hogging moment were smaller than those of SC-S. Due to the bolts' slip in SU-B, the tensile stress in the UHPC flange plate was reduced, resulting in higher crack resistance and rotation capacity than SU-S, while its flexural stiffness and ultimate flexural capacity were slightly smaller than those of SU-S. Finally, theoretical formulas were proposed for calculation of the slip moment, moment at crack width of 0.05 mm and ultimate moment of the steel-UHPC composite beams under the hogging moment. The test results verify the applicability of these formulas to predict flexural capacity of the steel-UHPC composite beams.
The performance of ultra-high performance concrete (UHPC)-steel composite beams is dependent on the connection between the UHPC and steel. In this study, bending tests were conducted on UHPC-steel composite beams with different interfaces: UHPC directly cast on different plate textures, bonding using an epoxy-based adhesive, and traditional headed studs. The connection between the UHPC and the plates was extremely weak, but the adhesive performed well, failing after the steel beam yielded. Three-dimensional finite element models of the adhesively bonded beams were then established using ABAQUS and agreed well with the test results, successfully predicting the failure of the adhesive bonds.
To study the mechanical property and structure design method of the ultra-high performance concrete (UHPC), a large-scale prestressed T-shaped beam was tested and intermediate and final behaviors, such as cracking loads, flexural stiffness and moment capacity, were obtained. The calculation method for the cracking loads and ultimate bending moment was improved by considering the tensile constitutive relationship and the adjustment of the tensile softening of UHPC. Furthermore, the nonlinear analysis based on the structural parameters, including reinforcement ratio, tension force and depth-span ratio, was conducted. The predicted results based on the calculation program agree well with the tested ones. It is concluded that the prestressed UHPC beam possesses good ductility and deformation capacity. The calculation of the cracking moment for prestressed UHPC beam can refer to the JTG D62-2004 Code. The tensile behavior contributes little to the ultimate bending capacity. The flexural behavior, especially the ultimate bending capacity, can be effectively improved by increasing the area of tendon to take full advantage of the ultra-high compressive strength of UHPC. © 2018, Editorial Office of China Civil Engineering Journal. All right reserved.
In order study the calculation method for flexural capacity of high strain-hardening ultra-high performance concrete (UHPC) T-beams, a four-point loading experiment of five girder specimens was conducted and the load-deflection curve of reinforcement and high strain strengthening UHPC was drawn. The T-beam failure process was divided into the elastic stage, the fracture development stage, and the load-to-failure stage. Different from common concrete, when the force of high strain-hardening UHPC beam reach the ultimate load carrying capacity, the relationship between stress and strain of UHPC of compressive region is still linear. Meanwhile, since the contribution of UHPC tensile strength of tensile region to flexural capacity, after UHPC cracking, should be considered, an equivalent rectangular stress coefficient β of UHPC of tensile region is proposed. The calculation method for flexural capacity of reinforced UHPC T-beams is deduced, based on the flat section assumption. This method is compared with the foreign calculation method and the accuracy of each method is analyzed. The results show that the calculated value of the proposed method fits the test value well, providing reference for theory analysis and structural design of reinforced UHPC T-beams. © 2018, Editorial Department of Journal of Tongji University. All right reserved.
This study presents the results of an experimental program on the compressive behavior of fiber reinforced polymer (FRP) confined ultra-high performance fiber-reinforced concrete (UHPFRC). A total of 38 specimens were prepared and tested under axial compression. In addition to FRP confined UHPFRC, FRP confined ultra-high performance concrete without fiber addition (UHPC), high strength concrete (HSC), and normal strength concrete (NSC) were also tested to investigate their comparative performances. The test results indicate that the FRP confined UHPFRC can exhibit ductile behavior if sufficient FRP confinement is provided. However, due to their ultra-high strength as well as the unique microstructure, FRP confined UHPFRC is likely to exhibit more brittle behavior than FRP confined NSC and HSC. Compared to FRP confined NSC and HSC, the confinement efficiency is less for FRP confined UHPFRC. Sudden stress reduction or stress fluctuations are observed shortly after the initial peak stress (axial stress at the first peak point) for FRP confined UHPFRC. Based on the confinement level, the stress-strain behavior of FRP confined UHPFRC may experience a second ascending branch or a continuous descending branch after the sudden stress reduction or stress fluctuations. The influences of FRP layers, FRP types, and fiber addition on the compressive behavior of FRP confined UHPFRC are observed to be significant. Moreover, existing stress-strain models available for FRP confined UHPFRC are evaluated by using a database collected in this study.