Headed Bars Research Articles

Effect of Recycled Concrete Aggregates on the Concrete Breakout Resistance of Headed Bars Embedded in Slender Structural Elements

Recycled concrete aggregates are potentially interesting for the precast concrete industry as they provide a new use for high-quality waste from its products’ life cycle. In precast concrete structures, it is common to use headed bars in several connection types between structural members. This paper presents the results of experimental tests to investigate the impact of replacing coarse natural aggregates with coarse recycled concrete aggregates in the concrete breakout strength of cast-in headed bars embedded in slender structural elements. Results of 12 tests on 16 mm headed bars embedded in 500 × 200 × 900 mm concrete members with an effective embedment depth of 110 mm are presented. The percentage of replacement of natural aggregates by recycled concrete aggregates was 0%, 30%, and 100%, and the flexural reinforcement ratio of the structural elements varied from 0.5% to 3.5%. The behavior and strength of the tested specimens are discussed, and comparisons with theoretical strength estimates are presented. The results showed that the concrete breakout strength of the headed bars was not affected by the use of recycled concrete aggregates and that the flexural reinforcement ratio significantly impacts the load-carrying capacity of the headed bars as they control the crack widths before failure.

Bond-slip model of headed bar and its application to component-based model for precast concrete joints under accidental loads

This study focused on investigating bond-slip behaviour of headed bars embedded in well-confined concrete. Test results based on eight pull-out specimens under displacement-controlled incremental loading showed that recommended development lengths from ACI318–19 are conservative in developing ultimate strength of headed bars associated with smaller slips than straight bars. Moreover, it was found that different (but sufficiently long) embedment lengths had a significant effect on the free-end slip but negligible effect on the loaded-end slip of headed bars. Additionally, the study highlighted the significance of head bearing in anchorage formation, especially during inelastic stage when bond strength had deteriorated. To address inaccuracy in predicting structural behaviour caused by inadequate anchorage strength of headed bars, a macro bond-slip model was proposed based on correlation in strain distribution obtained from the pull-out tests. Subsequently, the proposed model was incorporated into a component-based model (CBM) and validated against test results of PC joints under seismic loading. A numerical investigation using the validated CBM was conducted to study behaviour of exterior PC joints with headed bars and special detailing. These included Type X joint with X-bent bars and plastic hinge relocation (PHR), and Type A joint with an additional bar layer (ABL) in the beam-joint region, subjected to extreme loads such as seismic action or progressive collapse. The results revealed that seismic behaviour of exterior joints was governed by ductile failure of the beam at the PHR and vertical joint interface in Type X and Type A joints, respectively. Progressive collapse was primarily caused by column failure, with flexural and catenary action effectively preventing collapse. Notably, under both loading conditions, presence of ABL in Type A joint and X-bent bars with PHR in Type X joint improves flexural strength of beams at critical sections and joint shear strength, ultimately enhancing structural performance of joints. Design recommendations include replacing hooked bars with headed bars as well as implementing X-bent bars with PHR and ABL in PC joints. The authors advocate the strong-column-weak-beam design approach as an effective measure to prevent progressive collapse.

Progressive collapse behaviour of earthquake-damaged interior precast concrete joints with headed bars and plastic hinge relocation

This study investigated progressive collapse behaviour of earthquake-damaged interior precast concrete (PC) joints with headed bars. The main objective was to quantify the impact of slight and moderate earthquake damage on their residual collapse resistance. The research began with a numerical investigation to understand the interaction between a 2D frame’s global response to seismic loading and its local response during a threat-independent progressive collapse event. Additionally, a separate set of numerical studies examined interior PC joints under cyclic loading to determine maximum drift ratio demands, yielding 2.5% for slightly and 3.5% for moderately damaged joints. Subsequently, an experimental programme under quasi-static loading conditions was conducted on five interior PC joints under cyclic loading, followed by progressive collapse resistance tests. Test results indicated that damaged joints exhibited flexural and catenary action during progressive collapse, but compressive arch action could not be mobilised due to residual cracks from the initial cyclic loading phase. Comparing damaged joints to fully intact ones from a companion study revealed deterioration in stiffness and deformation capacity during a subsequent progressive collapse, with degree of severity proportional to the extent of damage sustained in the cyclic loading phase. Relative to the intact joints without being subjected to cyclic loading, moderately damaged specimens exhibited a degradation of 63% in stiffness and 38% in deformation capacity, compared to slightly damaged specimens, which showed reductions of 38% in stiffness and 28% in deformation capacity. In contrast, strain hardening of steel bars in both slightly and moderately damaged specimens led to slightly greater collapse resistance than the intact joints. Additionally, failure mode shifted from ductile fracture to premature pull-through failure of headed bars in moderately damaged specimens. In this regard, in post-earthquake progressive collapse assessment, it is recommended to use maximum crack width thresholds of 0.2 to 1.0 mm for slight damage and 1.0 to 2.0 mm for moderate damage.

Seismic performance and design of precast shear wall with headed bar overlap connections

A new type of precast steel reinforced concrete shear wall is proposed to serve as a lateral resistant member for residential structures. The vertical steel bars in precast shear wall within the horizontal joints are connected using confined headed bar connection (short in CHBC) while the profiled steels are bolted connection. Experimental investigations on the seismic behaviors of CHBC connected precast shear walls under combined axial and cyclic lateral loads have been conducted, with four specimens designed and tested, including three precast shear wall specimens and one cast-in-situ specimen. Through the hysteretic load-displacement curves and crack development patterns from tests, failure modes, bearing capacity, ductility, stiffness degradation, and energy dissipation properties of these specimens have been compared and investigated. Test results indicate that the CHBC connected precast specimen exhibits seismic performance as good as that of the cast-in-situ specimen, validating the effectiveness of the proposed horizontal joints. The precast specimen with ductility ratio of 5.89 exhibits 7.4% higher load bearing capacity than that of cast-in-situ specimen with ductility ratio of 5.70. All the specimens show the strength degradation ratio greater than 0.9 before peak load, demonstrating the stable load-bearing performance. Configuration details of the horizontal joints are suggested. Additionally, a simplified lateral resistance calculation model considering stirrup-confined concrete stress-strain relations is developed for CHBC connected precast shear walls under combined axial and horizontal loads. The average ratio between the predictions by the proposed model and test data is equal to 0.953. Besides, the simplified skeleton curve model is proposed, and the hysteresis rules are introduced which can demonstrate the hysteresis curves of the CHBC connected precast shear walls.

Development of Post-installed Headed Bars Embedded into Grouted Holes with Enlarged Ends

Development of Post-installed Headed Bars Embedded into Grouted Holes with Enlarged Ends

A parametric finite element study of concrete cone failure in headed bars under tensile loading

A parametric finite element study of concrete cone failure in headed bars under tensile loading

High-Strength Headed Bars in Joints of Special Moment Frames under Cyclic Loading

High-Strength Headed Bars in Joints of Special Moment Frames under Cyclic Loading

Research on the Distributive Relationship between Bond Force and Bearing Pressure for Anchorage Force by Headed Bars

Anchorage force comprises bond force and bearing pressure when headed bars are used. Pull-out tests were conducted on 120 concrete specimens to derive a method for calculating the bond force for reinforcement in straight anchor sections at the yield moment. The parameters include the diameter d, embedded length lae, and yield strength fy of the reinforcement, as well as the strength grade of the concrete fcg. The experimental results indicated that the specimens underwent three failure modes depending primarily on the embedded length lae. The nominal average bond stress τu concept was proposed and the difference between τu and the actual average bond stress τ caused by the headed bars was observed. To reduce the difference between the τu and τ values, the correction coefficient γ was proposed. Analysis indicated that γ increased with an increase in the lae/d (on average, 146% higher than the initial value) and decreased with an increase in the fy/ft (on average, 33% less than the initial value). A formula was developed to calculate γ, and the bond force in the straight anchor sections at the yield moment for the reinforcement was determined. Thus, a distributive relationship was established for the anchorage force, the bond force, and the bearing pressure.

Tensile behaviors and configurations of double-headed bar overlap connections for precast concrete members

The superiorities in construction time, cost, and quality of precast RC structures are accompanied by the challenges in connection, particularly in steel bars connections between RC components. The extensively used steel bar connections for either grouted sleeve splice connection or grouted spiral-confined lap connection requires on-site grouting, which is a concealed construction process with increased the risk of grouting defects, thereby impairing connection reliability. The confined headed bar connection (CHBC) with visible on-site construction has been proposed to address the unseen grouting defects concerns. However, modern structures that use large-diameter or high-strength rebars present higher demands for steel bar connection capacity. For CHBC, it is suggested to use either a long connection length or a large-size head to satisfy the capacity demand. However, the long connection length can increase field wet operation, which is disadvantage in precast concrete construction. The large-size head can help reduce the connection length, but it can cause inconvenience including construction congestion and the increased eccentric bending moment under tension. This paper presents the CHBC with double-head configuration, which can reduce the single head size. The proposed CHBC consists of confinement stirrups and two overlapped headed bars and the stirrup confining the core concrete can also improve the CHBC capacity. Experimental investigation on 21 CHBC specimens has been carried out. The failure modes, bearing capacity, and stirrup strain development have been studied. Effects of the double-head configuration and confinement stirrup ratio have been analyzed through experimental and numerical investigation. Results show that the double-head configuration has a minor impact on the tensile capacity of CHBC while the increase of confined stirrup ratio can improve the tensile capacity of CHBC. Configuration details of size and spacing for stirrup reinforcement in CHBC are proposed. Based on the test results, CHBC tensile capacity prediction method is suggested, which can take the head configuration into account.

Progressive collapse behaviour of advanced precast reinforced concrete joints with headed bars and plastic hinge relocation

This study proposed three types of precast concrete (PC) wet joints, viz., using Headed bars (Type H) to replace hooked bars, placing an Additional bottom bar layer (Type A), or implementing plastic hinge relocation method with X-bent bars (Type X) in the beam-joint region. These detailing features could compensate for negative effects caused by geometric discontinuity at the joint interface and prevent pull-out failure, as observed in conventional PC joints with hooked bars. An experimental investigation based on four interior joints showed that based on the steel content in Type H joint, by increasing respective steel contents of 7% and 8% in Type A and Type X joints, structural resistance to collapse could be enhanced by 34% and 47%, respectively. A fourth specimen of Type A joint incorporated the slab. It was found that although the slab could slightly enhance flexural resistance, it also reduced the joint rotational capacity. Subsequently, component-based joint models (CBM) with a bond-slip model of headed bars were proposed for the three types of PC joints and validated against the test results with satisfactory accuracy. Based on the validated CBMs, parametric studies were conducted for different cases, such as exterior and penultimate joints, under a penultimate column removal scenario. It was found that if the exterior column was too weak, compressive arch action and catenary action could not be effectively triggered, and column failure governed joint behaviour. Hence, to mitigate progressive collapse, the exterior columns and joints must be required to possess a sufficient level of horizontal resistance and stiffness.

Pullout Tests on Concrete Breakout Strength of Headed Reinforcement Bars Embedded in Roof Exterior Beam-Column Joints

Recently, the use of headed bars as mechanical anchorage in reinforced concrete beam-column joints has increased rapidly. This type of anchorage is used for T-shaped beam and exterior column joints at middle floors and for L- and T-shaped beam-column joints at the rooftops of buildings. However, few experimental data are available regarding the anchorage capacity of headed bars embedded in L-shaped beam-column joints. Nine pullout tests are conducted to investigate the anchorage capacity of headed bars embedded in L-shaped beam-column joints. Parameters affecting the anchorage capacity, such as the diameter, embedment length, location of the headed bars, and the number of supplementary bars are considered. The anchorage capacity of the headed bars is evaluated using empirical formulas proposed by Kubota and Murakami and a formula for concrete cone failure. The failure behavior of the specimens is concrete failure with cracks propagating in the diagonal direction at the maximum applied load. The anchorage capacity increases with the embedment length and diameter of the headed bars, number of supplementary bars, and concrete area. For most specimens, the Kubota and Murakami formulas overestimate the anchorage capacity, whereas formula for concrete cone failure yields lower anchorage capacities as compared with the test results.

Structural Performance of Precast Concrete Column Joint with Clamped Headed Bar during Construction

Structural Performance of Precast Concrete Column Joint with Clamped Headed Bar during Construction

Side-Face Blowout Strength of Large-Diameter High-Strength Headed Bars in a Single Layer or Two Layers Terminated within Exterior Beam–Column Joints

Due to a lack of test data on large-diameter headed bars, design provisions limit the bar diameter of headed bars to 36 mm or less. In exterior beam–column joints with two-layer headed bars as beam bars, if the layer-to-layer spacing of the headed bars is narrow, the side-face blowout area of an individual headed bar is overlapped. Existing design provisions do not consider the effects of layer-to-layer spacing. In this study, 34 simulated exterior beam–column joints were tested to examine the effects of bar diameters over the limitation of existing design provisions using large-diameter (41- and 51-mm) headed bars. The side-face blowout strengths of the two-layer headed bars were experimentally investigated through simulated exterior beam–column joints with layer-to-layer spacing of 2db (two times the bar diameter). The test results showed that the side-face blowout strengths had the same characteristics for bar diameters from 22 to 51 mm. For the two-layer headed bars, crack propagation and failure mode were the same as those for single-layer headed bars, but due to the overlap of the blowout area, the strength of the individual headed bar was reduced. A new model was proposed to predict the side-face blowout strengths of large-diameter high-strength headed bars in both a single layer and in two layers by incorporating a spacing factor for an individual headed bar into a model from the literature. A total of 173 test results from this study and previous studies were compared with predictions from the proposed model, and the average and coefficient of variation (COV) of test-to-prediction ratios were 1.08 and 17.4 %, respectively.

FE analysis of pulled-out eccentrically spliced longitudinal headed bars for precast beam-footing connections

ABSTRACT Load transfer in structural elements of RC constructions and the behavior of such constructions are basically dependent on the detailing of both the structural elements and their connections. Assessing analytically the characteristics and behavior of such structural elements and their connections under likely occurring loads is an important issue. For common detailing of footings in steel and precast structures, longitudinal reinforcement of foundation beams is bent horizontally and spliced with reinforcement of cast-in-place footings to insure an adequate juncture for load transfer. In this study, as concerned with reducing construction time and cost, instead of bending longitudinal reinforcement bars of both, beams and footings, headed reinforcement bars are adopted. By doing so, a discontinuity region is created where longitudinal bars of footings become eccentric to those of beams that are embedded in the footings. To allow the flow of tensile forces from longitudinal bars of beams to longitudinal bars of footings, a set of reinforcing ties is provided between them. As such setting of headed reinforcement bars is not common, thorough investigations have been carried out to confirm the relevance of the proposed tie reinforcement. In this paper, results of a finite element numerical investigation of pulled-out eccentrically spliced longitudinal headed bars with different detailing of transverse reinforcement, proposed for precast beam-cast-in-place footing connection, are presented and compared to test results. The elaborated modeling could fairly reproduce the global behavior of the six specimens, where the numerical results acceptably approached the experimental results in terms of initial stiffness, ultimate strength, crack pattern and strain of reinforcement.

Behavior of Confined Headed Bar Connection for Precast Reinforced Concrete Member Assembly

The mechanical performance of precast RC structures relies on the connections, especially the connections of steel bars, between precast RC members. Grouted sleeve splices and grouted spiral-confined overlap connections are widely used in engineering practice in China. Both of these two connection splices require on-site grouting. The process is concealed and invisible, leading to difficult on-site inspection. The unseen defects cause a challenge for detection and repair, which may impair the reliability of precast RC members’ behavior. This paper presents an RC member assembly connection with visible on-site construction quality-monitoring. The proposed confined headed-bar connection (CHBC) consists of two overlapping headed bars and confinement stirrup. With CHBC, the potential construction defects are diminished, and subsequently the construction quality as well as the reliability is upgraded. Experimental investigation on 18 CHBC specimens was carried out; the main parameters considered were overlap length and bar-head size. The failure modes, bearing capacity, stirrup strain development and bond versus slip response are studied. Working mechanism of CHBC is investigated in terms of bond behavior force and concrete compression force at head experimentally and numerically; distributive relationship of these two forces is revealed. The results show that for Φ12 reinforcement, a 90 mm overlap length under test parameters is adequate to reach headed bar ultimate strength in CHBC. Finally, a CHBC-bearing capacity prediction method is suggested based on the superposition method and strut-and-tie model theory.

Reinforced concrete beam‐columns anchored with headed bars subjected to reversed cyclic loading: Experimental and numerical investigation

AbstractPast seismic events have shown poor performance of reinforced concrete (RC) moment resisting frames, due to failure of beam‐column joints, attributed to insufficient anchorage and poor workmanship. Inadequate development length in beam‐column joints may lead to joint strength deterioration due to detailing constraints and site errors. Alternatively, headed bars have potential to replace conventional anchorage system of development length in beam‐column joint. The paper deals with extensive experimental and numerical program conducted on RC beam‐column joints with plain and deformed (grooved and ribbed) headed bars and compared with the specimens with conventional development length. Effect of steel fibers on strength of RC beam‐column joints is also investigated. The test specimens of beam‐column joints were subjected to displacement‐controlled reversed cyclic loading and structural behavior was assessed in terms of failure mode, load carrying capacity, stiffness, drift, ductility, and energy dissipation. The strength parameters of headed bar specimens were well comparable to that of conventional specimens. Grooved‐headed and ribbed‐headed bar specimens exhibited superior load resistance as compared with plain‐headed bar specimens. Numerical analysis was carried out, whose results were in agreement with the experimental results, thus demonstrating the efficacy of headed bars as mechanical anchorage system for RC beam‐column joints.

Behavior of headed bars in steel fibers based concrete

Behavior of headed bars in steel fibers based concrete

Method for calculating local bearing capacity of concrete under bond stress from reinforcement in straight anchor section

A pull-out test is performed on 30 concrete specimens to determine the effect of bond stress on the local bearing capacity. The parameters include the dimensions of the headed bars, strength grade of the concrete, embedded length of the reinforcement, and diameter of the reinforcement. The dimensions of the headed bars are 50 mm × 50 mm × 20 mm and 60 mm × 60 mm × 20 mm, and the strength grades of concrete are C30, C40, and C50. The diameters of the reinforcement are 25 mm and 32 mm and its embedded length is divided into five types in the test. The local bearing capacities of 30 concrete specimens are obtained, and three failure modes are observed in specimens. The different modes of failure primarily depend on the combined action of the shell structure and wedge of the concrete. The pull-out test results indicate that the local bearing capacity of concrete under headed bars increases with an increase in the embedded length of reinforcement and the strength grade of concrete, with an almost linear relationship. In addition, the local bearing capacity decreases with an increase in the relative protective-layer thickness of the concrete specimens. Finally, a calculation method is proposed for the local bearing capacity of concrete under the action of bond stress from the reinforcement in a straight anchor section. The calculations considered the relative embedded length and the relative protective layer thickness of concrete as independent variables and the ratio of the local bearing capacity of concrete to the bearing capacity of concrete under the headed bars as a dependent variable. This calculation method provides a theoretical basis for standard revision and engineering applications.

Bond and anchorage performance of beam flexural bars in beam-column joints using slag-based geopolymer concrete and their effect on seismic performance

An available study on the bond and anchorage performance of beam flexural bars in beam-column joints using slag-based geopolymer concrete (GC) is limited. The applicability of current design codes to beam flexural bars design in GC beam-column joints is unknown. In this study, to investigate the bond and anchorage performance of beam rebars at the GC beam-column joints and their effect on seismic performance, cyclic loading tests were performed on 2 reinforced GC interior beam-column joints and 6 reinforced GC exterior beam-column joints. The test results showed that the increase in column depth-to-beam rebar diameter ratio (hc/db) from 21 to 25 improved the bond performance of the beam flexural bars in the reinforced GC interior beam-column joints, which significantly improved the energy dissipation capacity. The limits of hc/db larger than 20 in GB50010-2010 (except seismic grade I that requires hc/db ≥ 25) and ACI 318-19 were not safe for reinforced GC interior beam-column joints. The limits of hc/db in Eurocode 8 and NZS3101 for low-strength concrete (fc′ ≤ 30 MPa) were applicable. In reinforced GC exterior beam-column joints, the bond and anchorage performance of 90˚ hooked bars was superior to that of the corresponding headed bars. Regardless of the anchorage details, the bond and anchorage performance of the reinforced GC exterior beam-column joints increased with the increase of the development length of beam flexural bars. In reinforced GC exterior beam-column joint design, the bar development length requirement of GB50010-2010 was applicable to both 90˚ hooked bars and headed bars. Eurocode 8, NZS3101, and ACI 318-19 were applicable to the development length of 90˚ hooked bars.

Pullout Behavior of Anchored Steel Bars and Grouted Headed Bars

This paper aims to study the tensile behavior of several types of anchored steel bars used in the cast-in-place connections considering the bonding condition between steel bars and concrete and in the precast concrete connections based on the bond behavior of the headed steel bars grouted in ducts. The pullout tested bars have been experimentally conducted using a static tensile loading in order to evaluate the pullout behavior, the failure mode, and the required development length. Nine pullout tests have been conducted for the cast-in-place straight and 90-degree hooked steel bars, and the grouted headed bars. In addition, a comparison of the calculated development length has been introduced considering different building codes. Results have showed that the peak pullout load has been observed in the cast-in-place 90-degree hooked end bars. Furthermore, the pullout load and the required development length of the cast-in-place straight end and the grouted headed bars have been similar, and they have been higher than 97% of the cast-in-place 90-degree hooked end bar. For the grouted-headed bars and the cast-in-place 90-degree hooked, ACI 318-19 and ECP 203-2020 have showed more rational results compared to experiments while it has been very conservative for cast-in-place straight end bars.