Bar Anchorage Research Articles

Cast-in and post-installed rebar anchorage systems in RC column foundation joints: System specific assessment and design

For assessment and design of post installed reinforcing bar (rebar) anchorages in reinforced concrete (RC) construction, two different approaches (i) conventional RC approach and (ii) fastening technology approach, coexist. For high strength adhesive systems, it is likely that the design anchorage lengths obtained using the fastening technology approach are significantly smaller in comparison to that obtained using the conventional RC approach. The main reason for this gap between the two approaches is, that while conventional RC approach considers qualified post installed (PI) systems as equivalent to cast in straight systems (CIS), PI systems are considered to be system specific in fastening technology. The system specific consideration of PI rebar anchorages in fastening technology makes it possible to take advantage of the higher bond strengths of the innovative adhesive systems, which is not possible in conventional RC design.This paper considers a comparison of the available approaches for assessment of rebar end anchorages in RC column foundation joints (CFJ). To this end, the benchmark test data generated by the authors [1] is used for the comparative discussions in the background of possible modes of failure. To include system specific definition of bond resistance in conventional RC approach, the potential of the system specific bond model in fastening technology to replace the general bond model in conventional RC practice for rebar anchorages is discussed. The alternate possibility of evolving the existing bond model in RC practice for including system specific consideration is also discussed. It is observed that the system specific definition of bond resistance within the framework of RC design practice is more suitable for rebar end anchorages. A proposal for failure hierarchy based assessment approach employing this system specific definition is made in this paper. The proposed approach is successfully validated to demonstrate its capability to assess both the failure loads and failure modes in a realistic manner, irrespective of the type of anchorage (cast in or post installed). The proposed approach is in concurrence with RC design practice and at the same time does not conflict with fastening technology. The presented is thus, a comprehensive breakthrough towards harmonizing the coexisting design concepts for rebar end anchorages.

Experimental study on the mechanical behavior of HTRB600E steel sleeve grouting based on UHPC

Eight HTRB600E steel sleeve grouting joint specimens filled with ultra-high performance concrete (UHPC) grouting material were fabricated for monotonic tensile testing. The effects of the grouting material mixing ratio and steel bar anchorage length on the mechanical properties of the connector joint specimens were investigated. The failure modes included rebar tensile fracture and rebar slip failure. The flexural performance, compressive performance and residual displacement of UHPC grouting material were significantly improved with the addition of steel fiber. However, the difficulty of grouting work increased with the addition of steel fiber. Increasing the anchorage length of rebar can significantly enhance the specimen bearing capacity but reduce the average bond strength. When UHPC is used as the grouting material, the recommended anchorage length of HTRB600E steel is at least 7d. As demonstrated by the test results, for the simulation of steel sleeve grouting joint monotonic tensile tests, an effective finite element (FE) model was established in ABAQUS.

Enhancement of the bond behaviour between sand coated GFRP bar and normal concrete using innovative composite anchor heads

Anchor heads could efficiently address the bond weakness between Fibre-Reinforced Polymer bars and concrete. This experimental study enhanced the bond behaviour between GFRP bars and concrete using three innovative anchorage systems made from glass fibre cloth and epoxy resin. A direct pullout test was used to study the bond-slip performance between the bar and the concrete. Test variables were GFRP bar diameter (3 diameters), concrete compressive strength (20.4 and 40.2 MPa), and anchor system (three different types). Based on the test results, in low-strength concrete (i.e. 20.4 MPa) samples, the anchor system efficiency was not promising, and the failure occurred between the concrete and anchors. However, for higher strength concrete (i.e. 40.2 MPa) samples, the ultimate developed tensile load increased between 14 and 68% for different bar sizes and anchorage systems compared to the unanchored control specimens.

Pullout Strength of Embed Plates with Welded Anchor Bars in Concrete

Pullout Strength of Embed Plates with Welded Anchor Bars in Concrete

Experimental Investigation of Exterior Reinforced Concrete Beam-Column Connections Subjected to Reversal Loading

Eight full-scale reinforced concrete beam-column subassemblages, 4 for each non-ductile and ductile detail, were tested under quasi-static cyclic loading. All test specimens were mainly designed for gravity service loads. Four specimens named as non-ductile connections were constructed with typical reinforcement details using in Thailand. The others were assembled with details described by the seismic design building code of Thailand, known as the 1301/1302-61 code. Their performance was examined in terms of lateral load resistance, ductility index, stiffness, energy dissipation, failure modes, including joint shear strength. The test results indicated that the specimens with ductile details achieved better seismic performance in every aspect, particularly specimens with adequate joint shear strength designed according to the SST method. The ductile details have no effect to retard the stiffness degradation in the elastic range, but helped to slow the rates of stiffness degradation after the specimens reached their peak loads. The transverse reinforcement in the joints was more efficient to enhance the ductility of joints with lower joint shear strengths (J1 and J2) than the sufficient joints (J3 and J4). Based on the SST method, the connections with low shear strength capacity, J1, J1D, J2, J2D, J3, and J4 suffered with severe joint shear failure (JS), although their beam sections reached the flexural moments. On the other hand, specimens with high joint shear strengths, J3D and J4D, experienced the beam failure (BF) with mild joint shear failure (JS). Test data also demonstrated that a specimen with lesser amount of joint reinforcement exhibited satisfactory seismic behavior, as long as the joint was provided with the adequate shear strength designed by the SST method. Finally, the ACI 318 Building Code overestimated the shear strength of exterior beam-column joints. Moreover, the simplified SST model much better predicted the demand joint shear strength than the ACI 318 Code.
 HIGHLIGHTS
 
 Numerous surveys of RC buildings after earthquakes also confirmed the inadequate detailing of beam-column joints such as presence of splices, lack of hoops, deficiency of beam bar anchorage, and no steel joint stirrups in the joint
 The ductile details have no effect to retard the stiffness degradation in the elastic range, but helped to slow the rates of stiffness degradation after the specimens reached their peak loads. The transverse reinforcement in the joints was more efficient to enhance the ductility of joints with lower joint shear strengths (J1 and J2) than the sufficient joints (J3 and J4)
 The ACI 318 Building Code overestimated the shear strength of exterior beam-column joints. Moreover, the simplified SST model much better predicted the demand joint shear strength than the ACI 318 Code
 Based on the SST method, the beam-column connections with low shear strength capacity, J1, J1D, J2, J2D, J3 and J4 suffered with severe joint shear failure (JS), although their beam sections reached the flexural moments. On the other hand, specimens with high joint shear strengths, J3D and J4D, experienced the beam failure (BF) with mild joint shear failure (JS)
 The deterioration of beam-column joints under seismic could be effectively restrained by the ductile details according to 1301/1302-61 code with adequate shear strength capacity determined by using the equation of SST model
 
 GRAPHICAL ABSTRACT

Cover Picture: Geomechanics and Tunnelling 5/2022

AbstractAs part of the modernisation of the railway line between Šentilj and Maribor, the existing Počehovo tunnel will be replaced by a new tunnel. The new double‐track tunnel through the Pekel Mountains is around 1530 m long and will be equipped with various safety systems. DSI provided AT – Casing System 114 with squeeze connection, self‐drilling Hollow Bar Anchors as well as other ground support products in one line including lattice girders, drill steel, and pumps. Breakthrough on the new Pekel railway tunnel was achieved on 18 August 2022. (photo: DSI Underground)

Seismic performance of concrete-encased column connections for concrete filled thin-walled steel tube piers

An improved type of concrete-encased column connections (CECCs) for circular concrete filled steel tube (CFST) piers is proposed in this paper, which does not require the use of base plates and anchor bolts. The seismic performance of the proposed CECC for circular CFST piers was experimentally investigated. Four CECC specimens were tested under a constant axial load and cyclic lateral loads to investigate the effects of the reinforcement ratio in the steel tube and the thickness of outer reinforced concrete component. The test results show that all test specimens failed flexurally primarily and the lateral load versus displacement hysteresis curve was generally full, indicating a good energy dissipation capacity (with a ductility coefficient >3.5). The strength degradation, stiffness degradation, and energy dissipation coefficient were calculated to study the seismic performance of proposed CECCs. The axial and lateral load versus the strain development of steel tube and anchor bars curves were further investigated to analyze the load transfer mechanism. The calculation methods for the sectional strengths of both CFST section and bottom section are proposed, which are found to be in good agreement with the test results.

Experimental study on spatial mechanical properties of grout anchor lap joints

To study the anchoring performance of non-concentric and multi-medium lap joints of grout anchors, pull-out experiments of 18 groups of specimens were implemented. The main focus of this study is the influence of steel bar diameter and anchorage length on the bond and anchorage performance when the diameter of metal bellows and spacing of spiral stirrups are fixed. Qualitative and quantitative analyses are conducted to experimentally ascertain the mechanical safety of structures. The finite element method is employed to replicate crack development and mechanically simulate the segment model. The simulation results are analyzed and compared with experimental data. When the anchorage length is 35d–40d (where d is the diameter of the embedded steel bar), no component is damaged, indicating that the anchorage length is stable and reliable. However, when the anchorage length is less than 15d, slip failure occurs. In this work, a comprehensive description of the mutual restriction and force transmission variation law of each component of the node under the action of stress based on the strain distribution analysis of the multi-media coupling model is provided. Moreover, optimization suggestions for the structure are proposed. The results of this study provide experimental support for the application of this type of joint in prefabricated building structures.

Performance of exterior beam-column connections with innovative bar anchorage schemes: Numerical investigation

Proper detailing of the joint reinforcement is essential for achieving an optimum framing action of the connected members. The ACI 352R-02, ACI 318–19 and EC-8 codes specify minimum requirements for the components of the standard 90-degrees hook; horizontal part, bend, and vertical tail as a reliable detailing alternative for achieving satisfactory performance of the joint. This numerical study aims to investigate the optimum tail length needed for ensuring adequate performance of the exterior joint and to develop a measure for the joint shear strength under various vertical tail length conditions, utilizing two experimentally tested joints with different reinforcement details as reference. The impact of a wide range of vertical tail lengths on the joint’s ultimate strength has been evaluated. It has been found that the provided tail length governs the joint's ultimate shear strength, where providing a vertical tail length of thirty times the bar diameter or greater achieves the superior performance, which is attributed to the improved confinement to the joint and the better bond condition in the joint region. However, the available column depth determines how the vertical tail length affects the joint strength, where the influence is more significant for joints with narrow depth columns. Thus, this study reveals that the dependence on increased vertical tail length, as a non-conventional alternative, can be safely considered to ensure sufficient joint shear strength, especially in joints with narrow depth columns. Hence, the limits of tail length given by several international codes; e.g., ACI 352R-02, ACI 318–19, and EC-8, are inadequate. Furthermore, an accurate equation for predicting the joint shear strength has been developed based on an extensive parametric study on various sets of exterior joints, taking into account the tail length under various conditions of column depths, joint aspect ratio, column axial stress, and the concrete strength.

Flexural behavior of precast concrete hollow-core slabs with high-strength tendons

Precast concrete hollow core (PHC) slabs have been widely used in brick and concrete structures of rural buildings due to their economic and rapid construction in China. A new PHC slab with high-strength tendons and spliced slab made of the short slabs were proposed in this paper. To investigate the flexural behavior of the slabs, full-scale tests were conducted on three PHC slabs, including one long slab and two spliced slabs with side beams. In detail, the spliced slab was assembled by two short slabs through U-type bars, anchorage bars, and post-pouring concrete. The mechanical properties of specimens were evaluated including the failure modes, strain of concrete and bars, flexural capacity and deflection. The tests results shows that the flexural performance of the spliced slab is close to that of the long slab. Subsequently, finite element models were established and validated by the test data. Based on the validated models, an additional specimen made up of long slab and side beams, was studied to further verify the reliability of the spliced slab. And parametric analysis were carried out on the spliced slab to investigate the influence of prestress of tendons, diameter of tendons, concrete strength, etc. Moreover, the cracking and failure moment of the slabs were calculated by the formulas of GB50010 and ACI318, respectively. The calculated results were compared with the test and simulation results. The results proves that the prediction values calculated by the codes on the flexural capacity of the spliced slab is not conservative. Therefore, considering a reduction factor to correct the results calculated by the codes is necessary.

Investigation on in-plane mechanical behavior of joint connections for discrete connected new-type precast concrete diaphragms

This paper proposes three new types of connectors adopted in discrete connected new-type precast concrete diaphragms (DCNPD). An experimental investigation of innovated connectors under shear load was conducted to evaluate the failure mode, strain and bearing capacity of joint connections in DCNPD. Based on the experimental results, the hairpin connector (HPC) was selected for simulation verification and further analysis under combined loads to provide crucial information on bearing capacity, ductility, reduction rule, deformation and stiffness in order to reveal its working mechanism. The results indicate that three types of connectors can provide effective connection for slab joints. The ideal fracture failure mode of anchor bar or cover plate is realised in all joint connections. The shear capacity of HPC is weakened by simultaneous tension when it is under combined loads. With the increase of tension-to-shear force ratio, the ductility of HPC is improved, and the shear ductility is relatively better. The evaluation of yield displacement and bearing capacity, degradation law in strength, deformation and stiffness of HPC provides a theoretical basis for the development of joint connection and seismic design of DCNPD, which is vital for the application of DCNPD in precast concrete structures.

Experimental study on lap behavior of CFRP indented bars in UHPC

The lap behavior of Carbon Fiber Reinforced Polymer (CFRP) indented bars in Ultra-High Performance Concrete (UHPC) were investigated based on pullout test under direct tension on 27 specimens in total. The test variables include splice length, UHPC strength and stirrup ratio. The lap failure mechanism of CFRP bar embedded in UHPC and the influence of test parameters on lap performance were analyzed. Test results showed that the failure modes, pull-out and rupture of CFRP bar, in those lap-spliced specimens were governed only by the splice length of the CFRP bar embedded in UHPC. The pullout failure was caused by the shearing off of indented ribs on the surface of the bars and the fracture failure was governed by the tensile strength of the bars. The bond strength of lap-spliced indented CFRP bars in UHPC is mostly dependent on the shear strength of indented ribs on CFRP bars, little on the UHPC strength and stirrup ratio. For CFRP bars with a 2480 MPa tensile strength and a 9.8 mm diameter (d), a splice length of 30d in UHPC can fully develop the tensile strength of the bar. A simplified equation for predicting the critical splice length of CFRP bars in UHPC was proposed. Compared with the results on anchorage of CFRP bars in UHPC, the critical splice length of CFRP bar in UHPC was 1.45 times the corresponding anchorage length.

Experimental study on mechanical behaviors of segmental joints with ductile-iron joint panels for deep-buried shield tunnels bearing high inner water pressure

The mechanical properties of high-stiffness segmental joints (HSSJs) in deep-buried drainage shield tunnels bearing high inner water pressure are different from those of segmental joints that do not face inner pressure; therefore, full-scale bending experiments for two kinds of HSSJs were conducted to reveal the mechanical behaviors of HSSJs under the action of inner water pressure and evaluate the joint type that is applicable for deep-buried drainage shield tunnels. The experimental results show that the joint opening grows and the rotational stiffness diminishes with the increasing inner water pressure, mainly due to the significant decrease of axial force at the joint position. With joint opening increases, local failure in the concrete around hand holes of HSSJs will occur. Moving the anchorage sections of the anchor bars in HSSJs behind the hand holes is an effective method to solve the local failure problem, which results in the improvement of anti-crack capacity of concrete in HSSJs, and also, the opening of HSSJs under the action of positive bending loading will decrease. For deep-buried shield tunnels bearing high inner water pressure, this structural adjustment could be considered if the crack development of concrete in HSSJs or the opening of HSSJs bearing a positive bending moment does not meet the tunnel design requirements.

Experimental investigation on compressive behavior of corroded thin-walled CHS T-joints with grout-filled GFRP tube repairing

This paper presents the compression tests of grout-filled GFRP-repaired chord-corroded CHS T-joints. A total of eight specimens, including uncorroded or corroded bare steel joints, grout-filled GFRP- or PVC-repaired joints were tested under axial compressive loading via the brace. The design parameters included the brace-to-chord diameter ratio, weight loss ratio due to corrosion, steel bar anchors on chord and material of external tube. The corrosion and repairing process of specimens are presented, followed by analysis of typical test results such as the failure modes, compressive strength and initial stiffness. The test results indicated that the uniform corrosion with 10% chord weight loss did not change the failure mode of compressive CHS T-joints, but caused near 20% decrease of the compressive strength and the initial stiffness. The strength of external tube and brace diameter were two key factors that guarantee the repairing effect, and the effect of steel bar anchors on chord was minimal. The grout-filled GFRP tube is effective to strengthen chord-corroded CHS T-joints for enhanced compressive strength and stiffness, with the enhance ratio of over 2.0. The repairing mechanism of the grout-filled GFRP tube is to provide a strong constraint effect on the chord, which changes the failure mode of the specimens from chord face plastification to combined failure of chord punching shear and plastification. A modified formula is proposed for estimating the confinement effect of the grout-filled GFRP tube repaired specimens.

Roof deformation characteristics and experimental verification of advanced coupling support system supporting roadway

AbstractThe advanced hydraulic support group combined with bolts (cables) constitutes the advanced coupling support system, which can ensure the stability of coal mine roadway safely and effectively. To explore the influence of the initial bearing capacity of the advanced hydraulic support on roadway roof deformation under anchor bar (cable) action. According to the actual geological conditions and roadway support of the 15106 working faces in Pingshuo Coal Mine, Yangquan, and Shanxi. Based on the change in the stress and displacement of the half‐space body under normal force in elastic theory and combined with the linear superposition principle, the contact mechanics model of the advanced hydraulic support group and roadway roof under anchorage is established. Selecting the measured anchor bar (cable) preload in the coal mine as the input load of the model, the stress and displacement distribution of the roadway roof under different initial bearing capacities are calculated. The calculation results show that the stress and displacement of the roadway roof along the axis exhibit a “symmetrical distribution” and the whole roadway exhibits a “saddle surface” gradient distribution during the active support of advanced hydraulic support under anchorage. The increase in the initial bearing capacity of the advanced hydraulic support has a great influence on the support effect in the middle area of the roadway and has a relatively small influence on the support effect in the surrounding area of the roof. With an increase in an initial support capacity, the stress gradient changes violently but substantially less than it would under the crushing strength of the surrounding rock. Using the similar simulation experiment theory, a similar simulation experiment roadway was established at 1:40, and the simulation experiment of an advanced hydraulic support coupling active support under anchor protection was carried out. The maximum error percentage of the theoretical calculation and experimental results of roof deformation under different initial bearing capacities is 19.51%, and the maximum error percentage of stress is 13.77%. The accuracy of the distribution law of roof stress and displacement under the initial bearing capacity in the process of active support obtained by theoretical calculation is verified. The research results provide a theoretical basis for advanced coupling adaptive support and the control of fully mechanized mining faces.

Tensile behavior of newly developed undercut anchor in cracked and uncracked concrete

The authors have been designing a post-installed anchor that fixes itself into concrete material by expanding the anchor tip in an upward direction, and conducted tensile loading tests to confirm its fundamental dynamic characteristics. The test results on three types of test specimens, with anchor morphology as a parameter, indicate that the final failure modes were all anchor bar fractures, and a stable yield strength was confirmed. Additionally, the yield strength characteristics of the proposed anchor in cases where cracks are present on the concrete surface, wherein the anchors are fixed, were experimentally confirmed.

Identification of the bond between ribbed CFRP bars and novel ARs anchorage

The novel additional aluminum alloy ribs (ARs) anchorage, fabricated through radially extruding the aluminum alloy (AA) tube to attach to the CFRP bar, was recently developed by the authors. Though effectiveness of using the ARs anchorage to improve the bond quality of CFRP bar in concrete has been already validated, the bond at bar-anchorage interface itself has yet been unclear. In order to afford an overall insight into the bond performance between ribbed CFRP bar and ARs anchorage, the present study initiated a pull-out test for a total of 81 bar-anchorage bonded joints. The investigated design parameters of the ARs anchorage consisted of the initial wall-thickness, radial deformation, and the extruded length. Scanning-Electron-Microscope (SEM) observations were also performed to accurately observe the interfacial zone before and after pull-out test. Based on the analysis of the tested results, four typical bond stress-slip laws of the bar-anchorage interface were identified, and recommendations for the anchorage design were further given. Meanwhile, this paper proposed a mathematical model for predicting the nominal bond strength for bar-anchorage interface.

Experimental investigation of a new precast column-foundation connection under cyclic loading

In the prefabricated industry, socket connection is generally preferred to be used in column-foundation connection. In this connection type, the foundations and columns produced in the factory are assembled at the construction site. During the assembly, a wet connection is formed by filling the space between the socket and the column with concrete. Socket connections increase the labor costs by extending the installation time. In addition, adjusting the column balance during installation becomes very important for the superstructure. On the other hand, the use of the wet type of connection complicates installation as to be an active earthquake region of Turkey. An earthquake load that may affect the structure during the installation may cause a total collapse in the structure.Studies are carried out on new connection types for the prefabricated industry all over the world. New connection types are being developed. In this study, a new type of connection in mind that disruptions in the production and assembly processes in Turkey were developed. The proposed connection type was produced by enlarged part of the column cross section from the bottom of the column to a certain height. Anchorage bars on the foundation passed through the holes formed in the enlarged part of the column. The proposed connection type, socket connection, and monolithic connection were tested under cyclic lateral loading. The behavior of the connections was interpreted with the data obtained from the experiments. With the proposed connection, while the lateral load carrying capacity increased slightly, a significant increase was obtained in the energy dissipation capacity. Significant mechanical improvements have been achieved under the effect of lateral load compared to the conventional socket connection.

Exploratory study on the short- and long-term bond between ribbed CFRP bars and additional aluminum alloy ribs anchorage

Innovative additional aluminum alloy ribs (ARs) anchorage has been recently developed to enhance the bond for FRP bars in pretensioned prestressed concrete. Studies have demonstrated the feasibility of using the ARs anchorage to improve the bond of FRP bars to concrete. However, the bond performance between FRP bar and ARs anchorage remains unclear. The present study reported an experimental programme to explore both the short- and long-term bond performance at ribbed CFRP bar-AR interface. The failure modes and bond-slip curves of short-term pull-out specimens were presented. Microstructure at the bar-AR interface was identified to explain the bond failure characteristic. Besides, the short beam test for the CFRP bars was conducted to support the findings obtained from the short-term pull-out test. The long-term bond at bar-AR interface was evaluated by subjecting the pull-out specimens to sustained loading for 500 days. Through the testing results, the bond mechanism of the bar-AR interface was identified, and the reasons behind bond enhancement caused by the prsence of ARs anchorage for CFRP bars in concrete were revealed. This exploratory study could be regarded as the most basic support for the application of ARs anchorage for enhancing the performance of prestressed CFRP reinforced concrete.

Flexural strengthening of RC beams with NSM-GFRP technique incorporating innovative anchoring system

Reinforced concrete (RC) beams may require flexural strengthening due to the corrosion of reinforcing bars or changes in the design load. The near surface mounted (NSM) and externally bonded (EB) techniques, using either glass or carbon fibre-reinforced polymer (GFRP or CFRP) composites, are used for strengthening deficient RC beams. However, RC beams strengthened with both techniques are susceptible to the debonding failure of the FRP composites. This paper proposes an innovative anchoring system for NSM strengthening of the RC beams, which comprises an L-shaped GFRP bar anchor (L-anchor), U-shaped CFRP strips (U-strips) and double plate (DP-anchor). The developed strengthening technique is termed as NLUD technique. Seven RC beams were cast and tested under four-point bending. The experimental results showed that RC beams strengthened with the NLUD technique achieved significantly higher flexural load. The NLUD technique prevented the debonding failure of GFRP bars of the strengthened RC beams.