This paper presents an experimental investigation into the improvement of efficiency in carbon fibre reinforced polymer (CFRP)-to-concrete bonded joints by an introduction of a glass fibre reinforced ...polymer (GFRP) interlayer. By introducing a ±45° biaxial GFRP interlayer between the CFRP and concrete, applied force on the CFRP laminate was distributed over a larger area of concrete surface, thereby increasing the load carrying capacity of the bonded joint. A total of 18 single-shear pull test specimens were prepared and tested to investigate the effect of CFRP width (varying from 25 mm to 50 mm) and GFRP/CFRP width ratio (ranging from 2 to 4) on the behaviour of the CFRP-to-concrete bonded joints with a GFRP interlayer. Addition of the GFRP interlayer was found to increase both ultimate load capacity and the deformation capacity significantly. Increase in the GFRP interlayer width, while keeping the CFRP width the same, increased the ultimate load of the bonded joint. Increasing the GFRP width while keeping the GFRP/CFRP width ratio the same also increased the ultimate load of the bonded joint but had little effect on the bond strength. An idealized mechanism was presented to explain the behaviour of the CFRP-to-concrete bonded joints with a GFRP interlayer. It was shown that due to 2D stress transfer of the newly proposed bonded joints, underlying mechanisms may be different to the conventional FRP-to-concrete bonded joints. Therefore, design theories developed for conventional FRP-to-concrete bonded joints may not be directly applicable to the CFRP-to-concrete bonded joints with a GFRP interlayer. Much more investigations are necessary to better understand the behaviour of the newly proposed bonded joints.
•A biaxial GFRP interlayer was introduced between CFRP and concrete in CFRP-to-concrete bonded joints to improve the load carrying capacity.•Both ultimate load and the ultimate displacement of the bonded joints were significantly increased.•Fracture area within concrete was significantly increased with the introduction of the biaxial GFRP interlayer.•Width of the GFRP interlayer increased the ultimate load of the CFRP-to-concrete bonded joints.
•The dynamic bond-slip behaviour between steel bar and concrete was numerically investigated by using the finite element model.•The effects of the concrete specimen shape, c/d ratio and concrete ...strength on the dynamic pull-out test were examined.•An empirical DIF relation of bond strength was proposed for modelling dynamic bond strength.•The modelling accuracy of the impact response of RC beam was improved by considering the DIF of bond strength.
The bond behaviour between steel bar and concrete affects the response of reinforced concrete (RC) structures subjected to static and dynamic loads. Many studies have investigated the influencing factors on static bond behaviour between steel bar and concrete, e.g., cover depth to rebar diameter ratio (c/d) and concrete strength and various empirical formulae have been proposed to quantify the static bond strength, but the study of dynamic bond strength is very limited. As a result, in the analysis of dynamic structural responses, either perfect bond is assumed or static bond strength is adopted, which may lead to inaccurate structural response predictions. In this study, dynamic bond-slip behaviour between steel bar and concrete is numerically investigated by using the finite element (FE) model developed in LS-DYNA and verified against testing results. The effects of the concrete specimen shape, c/d ratio and concrete strength on the dynamic pull-out test are examined. It is found that the splitting of concrete dominates the failure mode of dynamic bond-slip behaviour. The ultimate bonding load is enhanced with the increase of loading rate. It is observed that the increase of bond strength relates to the dynamic increase factor (DIF) of concrete strength. The parametric studies show that among all considered factors, the shape of the pull-out test specimens has a minor effect on the ultimate bonding load, which on the other hand is highly dependent on the concrete strength and c/d ratio owing to the strain rate effect. An empirical relation of DIF of bond strength as a function of c/d ratio, concrete strength and strain rate is proposed based on the numerical results. A case study of drop weight test on an RC beam is then carried out to demonstrate the influence of considering the dynamic bond strength in numerical prediction of structural responses under dynamic loading.
For a wide application of carbon-reinforced concrete, the sufficiency of the bond between concrete matrix and reinforcing elements must be ensured, even at high temperatures such as in the case of ...fire. Mineral-impregnated carbon fibre reinforcement offers a promising alternative to conventional polymer-impregnated carbon yarns or bars, whose performance deteriorates markedly with increasing temperature. One of the challenges in the production of mineral-impregnated carbon fibre reinforcement is the selection of a processing technology and adequate raw materials. The work at hand addresses this challenge while focusing on the influence of the particle-size distribution of mineral fines on the impregnating behaviour of suspensions. Subsequently, the concentration shifts to the mechanical performance of thin reinforcement bars made of mineral-impregnated carbon fibre, as observed in three-point bending and pull-out tests. In comparison to polymer-impregnated reference materials, the new reinforcement demonstrates significantly enhanced mechanical properties at elevated temperatures of 100 °C and 200 °C.
•Reinforcement approaches for digital construction are summarised.•3D-printing of steel reinforcement based on gas-metal arc welding is described.•3D-printed steel bars’ mechanical performance is ...shown to be similar to that of conventional bars.•Printed steel bars’ failure is shown to be ductile, on both the micro- and macro-levels.•Printed steel bars’ bond to printable, fine-grained concrete is shown to be satisfactory.
Digital concrete construction has recently become the subject of very rapidly growing research activities all over the world. Various technologies involving 3D-printing with concrete have been developed, and the number of demonstration projects and practical applications has been increasing exponentially. Most of these approaches are focused on the placement of concrete, while the suggested solutions for incorporation of reinforcement are still rudimentary, and as such they lag behind the concepts for printing concrete. Since the use of (steel) reinforcement is mandatory in most structural applications, there is an urgent need to bring the technology of reinforcing 3D-printed structural elements forward. The article starts with a brief overview of the existing approaches in using reinforcement in digital concrete construction. Then the authors’ own research work is presented, namely a feasibility study on 3D-printing of steel reinforcement using gas-metal arc welding. A description of the newly developed 3D-printing process is followed by a demonstration of its feasibility in producing vertical steel reinforcement bars with and without extra ribs. The mechanical performance of the printed bars was investigated by means of uniaxial tension tests. The samples exhibited comparable mechanical properties to common steel reinforcement of the same diameter. The investigation of fracture surfaces confirmed a ductile mode of failure of the printed steel bars. Finally, the bond between printed steel bars and printable fine-grained concrete was tested by means of pull-out experiments. Here the overall performance could be rated as satisfactory, even though it could be improved by introducing extra ribs in the process of bar manufacturing.
•A new bond-slip model CFRP-to-concrete bonded interface under fatigue loading is proposed.•Models for damage accumulation rate was proposed based on existing experimental data for CFRP-to-concrete ...bonded joints.•The model verified against existing experimental data of CFRP-to-concrete bonded joints.
This paper presents a theoretical study aimed at predicting the behaviour of carbon fibre reinforced polymer-to-concrete bonded joints under fatigue-cyclic loading. A model considering the plastic deformations of the interface, the damage, and the damage accumulation due to fatigue-cyclic loading is proposed. The damage accumulation model is calibrated through the experimental bond-slip relation. Then, a numerical algorithm is formulated to simulate the fatigue bond behaviour using the calibrated damage accumulation model. Numerical simulation was found to provide conservative predictions for fatigue life, which was attributed to the neglect of beneficial effects from the compression stress state near the loaded end in the single-shear pull-off test.
•A stress field is proposed for modelling bond in axisymmetric RC ties.•Bond is determined based on the concrete bearing capacity in tension and conpression.•Interface compatibility is used to ...account for the effect of steel strains on bond.•The results match well with test results obtained using distributed fibre optical sensing.•New insights on the behaviour of embedded reinforcing bars are gained.
The interaction of the reinforcing bars and the surrounding concrete plays an important role in structural concrete as it is decisive for the reinforcing bar anchorage and the load-deformation behaviour, including stiffness and deformation capacity. Therefore, understanding this interaction – commonly known as bond – is highly relevant to a safe and efficient structural concrete design. Bond depends on many parameters, which affect the local reinforcing bar-concrete interface as well as the global load transfer between the reinforcing bars and the surrounding concrete. The latter governs the formation of transverse and longitudinal cracks and the activation of transverse confinement, which are as decisive for bond as the local interface characteristics. Nonetheless, most bond models and code provisions focus on the local interface and merely account for the global configuration by means of empirically calibrated factors accounting, e.g. for good or bad bond conditions in confined or unconfined situations, respectively.
This study investigates the potential of stress fields for modelling bond based on the lower-bound theorem of plasticity theory. Stress fields facilitate the investigation of the local interface and the global load transfer consistently in one model, providing valuable insight into the flow of forces. For simplicity, the fundamental case of a concrete tie reinforced with one deformed steel bar is studied in this paper, superimposing axisymmetric discontinuous stress fields originating at each reinforcing bar rib. Each stress field consists of a triaxial nodal zone adjacent to a rib and a conical compression field that spreads the bond force radially, and is equilibrated along its outer perimeter by axial tensile stresses and confining hoops. The model relies on geometric parameters and basic material properties. The validation against experiments on reinforced concrete ties shows a good correlation between the predicted and observed bond strength and the crack spacing. The predictions can be improved by assigning stiffnesses to the stress field components and requiring interface compatibility, i.e. ensuring contact between the concrete and the reinforcing bar at every rib. Even when assuming a simplified, linearly elastic constitutive behaviour for concrete, the consideration of interface compatibility allows for reproducing the steel strain dependency of the local and average bond stresses and the occurrence and impact of splitting cracks.
•The fatigue bond behaviour between BFRP bars and RAC is tested.•The fatigue bond stress-slip relationship and bond mechanism are investigated.•The fatigue bond interface damage is observed.•The bond ...stiffness, slip, bond strength, and residual bond strength are discussed.
The bond performance between fibre-reinforced polymer (FRP) bars and recycled aggregate concrete (RAC) affects their combined working ability and further influences the bearing capacity and deformation resistance of FRP bar-reinforced RAC structures. In addition to static loads, many structures such as roads, bridges and subways usually bear fatigue loads during their service periods. Therefore, to provide theoretical and experimental references to the fatigue bond design of FRP bar-reinforced RAC structures, this study systematically investigated the fatigue bond behaviour between basalt fibre-reinforced polymer (BFRP) bars and RAC. Pull-out tests were carried out considering the effects of the surface shapes of BFRP bars, bar diameters, concrete types, fatigue stress levels, and fatigue cycles. Based on the damage at the bond interface, the corresponding bond mechanism was analysed and the bond stress–slip characteristics were summarised. Furthermore, the effect of design parameters on the bond stiffness, slip, bond strength, and residual bond strength was revealed. Different surface shapes of BFRP bars induced different bond mechanisms. Moreover, fatigue loads mainly affected the bond behaviour by eliminating the imperfections and accumulating the damage.
•Discussion of experimental bond tests on concrete elements bonded with steel plates.•Review of existing bond strenght models.•Analysis of experimental data on bond tests on concrete elements bonded ...with steel plates avalaible in literature.•Assessment of a specific bond strenght model for concrete elements bonded with steel plates.
The use of steel plates as externally bonded reinforcement for existing reinforced concrete (RC) elements can be still considered a reliable typology of strengthening intervention thanks to their relatively lower prices in comparison with fiber reinforced plastic materials, FRPs, largely used in the last decades, the ductile stress–strain behavior, and the high stiffness properties. Despite steel plates represented the first attempt of external strengthening for RC elements, design indications and practical execution rules concerning the use of such a technique nowadays are still very few. There is, indeed, a lack of experimental and theoretical studies in literature for predicting accurately the behavior of existing RC members externally strengthened with steel plates. In particular, special attention should be paid to the bond behavior at the steel–concrete interface, since it can strongly influence the performance of the strengthened elements. To this aim, an experimental campaign concerning bond tests on steel plates bonded over concrete elements and realized according to the single shear test set-up was carried out by the Authors. The experimental results are analyzed in the paper in terms of failure loads and modes, load–displacement behaviour, axial strain distributions along the steel plates, and distribution of shear stresses and slips. Then, the experimental results are compared with some well-known literature bond strength models for external FRP reinforcements in terms of failure load, transfer length and bond law. The comparisons with existing models in terms of debonding loads are developed also for an extended database of bond tests on steel plates available in literature and were prodromal to calibrate new coefficients specifically assessed for externally bonded steel plates.
The strengthening of masonry structures is nowadays performed by means of high-strength fibers embedded in inorganic matrix (FRCM) where lime or cement-based matrix is used instead of epoxy adhesive ...to reduce debonding issues between substrate and matrix. However, some sliding phenomena and cohesive failures between fibers and the matrix mortar can occur. The paper examines the effect on the FRCM efficiency of the mechanical properties of fiber and matrix and potential geometrical defects, which are possible in real field applications or in qualification tests. The model application to simulate bond tests on typical PBO-FRCM and Glass-FRCM allowed to analyse slips as well as normal and shear stresses both in the bundle and in the matrix constituting the FRCM, for different defects due to application issues. The result of numerical simulations seems to interpret well the results of the qualification tests with a multi-bundle effect that justifies their scatter.
The approach can be applied by varying main mechanical properties of the materials (e.g. elastic modulus, fiber cross section, bond properties) to consider their intrinsic variability in the assessment of the performance of the FRCM system or by changing the type of materials (i.e. mortar and fibre) to optimize the FRCM system.
•Bond of GFRP bars in high strength concrete was investigated using hinged beams.•Bond performance was studied with showing the effect of main parameters on bond.•Design code predictions were ...compared to the experimental results.
Although several research studies have been conducted on investigating the bond stress – slip behaviour of Glass-Fibre Reinforced Polymer (GFRP) bars embedded in high strength concrete (HSC) using a pull-out method, there is no published work on the bond behaviour of GFRP bars embedded in high strength concrete using a hinged beam. This paper presents the experimental work consisted of testing 28 hinged beams prepared according to RILEM specifications. The investigation of bond performance of GFRP bars in HSC was carried out by analysing the effect of the following parameters: bar diameter (9.5, 12.7 and 15.9 mm), embedment length (5 and 10 times bar diameter), surface configuration (helical wrapping with slight sand coating (HW-SC) and sand coating (SC)) and bar location (top and bottom). Four hinged beams reinforced with 16 mm steel bar were also tested for comparison purposes.
The majority of beam specimens failed by pull-out. Visual inspection of the test specimens showed that the bond failure of GFRP (HW-SC) bars usually occurred owing to the bar surface damage, while the bond failure of GFRP (SC) bars was caused due to the detachment of sand coating. The GFRP bars with helical wrapping and sand coated surface configurations showed different bond behaviour and it was found that the bond performance of the sand coated surface was better than that of the helically wrapped surface. Bond strength reduced as the embedment length and bar diameter increased. It was also observed that the bond strength for the bottom bars was higher than that of the top bars. The bond strength was compared against the prediction methods given in ACI-440.1R, CSA-S806 and CSA-S6 codes. All design guidelines underestimated the bond strength of both GFRP re-bars embedded in high strength concrete.
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