Load-slip Response Research Articles

Experimental and analytical investigations on debonding response of embedded through-section ribbed GFRP bar-to-concrete joints at elevated temperatures

Embedded Through-Section (ETS) technique has gained a great deal of increasing interest in the shear strengthening of existing reinforced concrete members, which is based on using fibre-reinforced polymer (FRP) bars installed through predrilled holes into concrete via adhesive. One of the key issues associated with the bonding of ETS FRP bars to concrete is the debonding of FRP bar-to-concrete interfaces which leads to premature and brittle failure of strengthened members, particularly under elevated temperatures. However, little research has been conducted on the influence of elevated temperatures on the bond behavior between ETS FRP bars and concrete. This paper presents experimental and analytical evaluations of the bond response of ETS-ribbed glass FRP (GFRP) bar-to-concrete joints subjected to elevated temperatures. A total of 90 pullout specimens were tested to quantify the influences of the temperature, bar diameter, and concrete strength on the failure mode, load response and bond strength. An analytical model to reproduce the full-range debonding process of ETS-GFRP bar-to-concrete interfaces was presented, which allowed simultaneously considering both long and short embedded lengths, interfacial friction, and snap-back behavior. The load-slip response, stress field, and peak load were solved analytically, resulting in a closed-form solution for the critical length for snap-back and a non-closed-form solution for the effective embedment length. After an inverse analysis procedure was proposed for determining the bond parameters, the accuracy of the analytical solution was verified through comparisons with the self-conducted test results and existing experimental data. A new temperature-dependent bond strength model was also developed. The findings revealed a remarkable decline in the bond strength of up to 92.3 % as the temperature increased from 20 °C to 160 °C, with about 60–70 % of this degradation occurring close to the glass transition temperature (Tg) of the adhesive. At temperatures well above Tg, the bond strength decreased with the bar diameter and displayed insignificant improvement with higher concrete strength. The results also demonstrated that the proposed analytical approach and bond strength model can predict the bond response of ETS ribbed GFRP bars embedded in concrete with reasonable accuracy.

Cyclic inelastic performance evaluation of locking bolt demountable shear connectors in steel-concrete composite structures

This study extends the research on the Locking Bolt Demountable Shear Connector (LBDSC), previously introduced and extensively analysed under monotonic loading through both experimental and numerical methods. The focus here shifts towards the inelastic cyclic performance of the LBDSC, a critical aspect for applications in seismic regions. The LBDSC, designed for use in composite floors or decks, such as in-situ solid slabs, profiled deck slabs, or precast slabs in buildings and bridges, features a grout-filled steel tube bolted to the top flange of steel sections. The design incorporates a unique 'locking bolt' technique to counteract the influence of construction tolerances on undesirable initial slip behaviour, significantly facilitating the deconstruction and reuse of steel sections without the need for recycling. Following the Eurocode 4 guidelines, a series of standard push out tests were conducted to explore the LBDSC's inelastic cyclic characteristics. The experimental results confirm the LBDSC's capability for high strength, high initial stiffness, and adequate slip capacity under cyclic loading, with ductile shear failure modes observed at the bolt shank and local concrete crushing above the steel flange. The cyclic loading tests reveal a larger zone of damaged concrete compared to monotonic loading conditions, yet the monotonic and cyclic force-slip curves closely align. Based on these findings, and supported by validated finite element analyses, this paper proposes theoretical monotonic and cyclic models for LBDSC under push out loading, aimed at accurately predicting the load-slip response. These models are intended to facilitate the efficient analyses, design and numerical modelling of composite structures incorporating LBDSCs, and to support the broader adoption of LBDSCs in earthquake-prone areas, promoting sustainable steel-concrete composite construction practices.

Shear Performance of Demountable High-Strength Bolted Connectors: An Experimental and Numerical Study Based on Reverse Push-Out Tests

Steel–concrete composite beams, essential for large-span structures, benefit from connectors that reduce cracking at the supports. The crack resistance and alignment with sustainable building trends of high-strength bolted connectors have been extensively researched. Nevertheless, only a few studies exist on their load–slip behavior in hogging sections. In this study, the shear performance of high-strength bolted connectors subjected to tension due to hogging moments was studied based on experiments and numerical modeling according to numerous reverse push-out tests. The results revealed that tensile and splitting cracks were produced in the concrete. Their distribution was affected primarily by the concrete strength and bolt diameter; this distribution became denser at decreasing concrete strengths and increasing bolt diameters. Subsequently, an analysis of the out-of-plane displacement and load–slip response was performed to investigate the phenomenon of anchor rod sliding. A cost-effective and time-efficient finite-element (FE) model was developed to investigate the internal microstates of the specimens. It revealed a correlation between bolt cracking, specimen hardening, steel yield, and failure. A correction factor is also proposed for the shear capacity of bolts within concrete subjected to tension. The findings offer insights into the load–slip response of high-strength bolted connectors subjected to hogging moments, aiding in safer, more durable supports for steel–concrete composite beams.

Test and analysis of the interfacial bond behaviour of circular concrete-filled wire-arc additively manufactured steel tubes

Interfacial bond behaviour of the circular concrete-filled wire-arc directed energy deposition (DED) steel tubes was investigated experimentally, in which wire-arc DED, commonly referred to as wire-arc additive manufacturing (WAAM), represents a metal 3D printing method. Firstly, a 3D laser scanning was employed to generate the 3D models of the WAAM steel components to obtain the geometric features. A parameter relating to the surface undulation was proposed to evaluate the roughness of the WAAM steel tubes. The results of tensile testing undertaken to obtain the mechanical properties of the WAAM material were summarised. Twelve push-out specimens were tested to obtain the load-slip response and bond strength, while the influence of surface undulation, diameter-to-thickness ratio and interface length was also assessed, respectively. The test results demonstrated that: 1) after reaching the peak load, a slowly descending region appeared until a relatively stable residual load was achieved; 2) the average bond stresses of the push-out specimens were greater than those of the push-out specimens fabricated by conventional steel tubes and even checkered steel tube. Comparisons of the bond strength of the push-out specimens against existing structural design standards indicated the design guidelines of various codes were quite conservative. There needs to consider the influence of the surface undulation of the WAAM steel tube. Finally, based on the nonlinear regression of the test data generated in the present study, an empirical equation was proposed for the prediction of the average bond strength for concrete-filled WAAM steel tubes.

Bolted shear connectors in steel–concrete composite structures: Shear behavior

The use of bolted shear connectors is of great importance to the sustainable development of steel–concrete composite structures. In this paper, an experimental program consisting of four push-out specimens is performed to investigate the effects of bolt length and fabrication method of concrete slabs on the shear behavior of single-nut embedded bolted shear connectors in terms of failure mode and load-slip response. The concrete slabs are fabricated either as cast-in-situ monolithic slabs or precast slabs with reserved pockets which will be filled with grout in the final construction stage. The obtained results demonstrate that the fabrication method based on grouting, which is commonly utilized for strengthening and retrofitting works, does not affect the behavior of the bolted shear connection. Based on the experimental observations, a finite element (FE) model of the bolted shear connection is developed, after the obtained numerical results are verified against the test results obtained in this paper and those presented in other existing literature, a parametric study is carried out to further investigate the effects of the concrete strength, bolt strength, bolt diameter, bolt pretension load, and the length-to-diameter ratio of bolt on the performance of the bolted shear connection. Moreover, based on the obtained results, a design formula is proposed to obtain the ultimate shear resistance of the bolted shear connection, and the efficacy of the proposed design formula is proved through the comparison with the test results shown in the existing literature.

Generalised analytical solutions for linear and non-linear bond–slip models for externally bonded FRP to a concrete substrate

This article presents a set of novel analytical derivations from generalised bond–slip models using first principles, which can be used to predict experimental outcomes, including maximum load resistance, load-slip response, and strain throughout the bonded length. The generalised bond–slip models are constructed in a way that can be translated into various shapes, including linear, nonlinear and exponential forms, all of which have been used in the literature to explain the bond–slip behaviour of epoxy-bonded FRP on concrete. When the prerequisites for the derivations are satisfied, as demonstrated in this paper, a comprehensive validation of a bond–slip model is possible. Furthermore, the transformability of the generalised bond–slip models and the analytical nature of the derivations also allowed for the comparison of several bond–slip models simultaneously.

Nonlinear Semianalytical Prediction of Debonding at the FRP–Generic Material Interface with Mechanical End Anchorages

The full-range debonding process of a fiber-reinforced polymer (FRP)–generic material substrate interface with mechanical end anchorages requires a better understanding. In this study, we developed a nonlinear semianalytical model based on a cohesive crack model to predict the full-range debonding process of such an interface in a single-shear mode, considering the relatively deformed region in the interface as a cohesive zone. Specifically, we produced a double exponential bond–slip constitutive model with residual bonding shear stress and boundary conditions of slip constraints at both anchoring ends. Implicit solutions for the relative slips, bonding shear stresses, and axial forces in the FRP along the bond length, as well as the load–slip responses at the loading end and effective bond length, were obtained using the Chebyshev polynomial approximation strategy. The accuracy of the proposed model was verified using experimental results available in the literature. Then, the evolution of full-range interfacial bonding behavior and load–slip responses for different bond lengths was discussed. Finally, a parametric study highlighting the effect of geometry and material properties on the load–slip response at the loading end was presented.

Flexural behaviour of deconstructable composite beams

In order to promote the development of full life-cycle deconstructable steel–concrete composite beams, and to better understanding their flexural behaviour, an experimental study on the flexural performance of a novel deconstructable steel–concrete composite beam is carried out in this paper. With various connector (bolt) diameters (M20, M24) and material strengths (C40, C50 concrete) being incorporated, the failure mode, load–deflection response, load-slip response, strain distribution, ductility, characterised loading capacities and deconstructability are analysed thoroughly base on two-point loading tests. By comparing the test results with that specified in national standards for the flexural capacity and stiffness of the composite beams, new calculation formulae for this new deconstructable composite beams are put forward. By employing the shear-slip constitutive model proposed previously for shear connectors of such deconstructable composite beams, a finite element model is developed and further validate against experimental results. Subsequent parametric analyses are carried out, and the mechanism of shear connection is analysed accordingly. In addition, effects of different parameters (including sectional dimension, strength of concrete and degree of shear connection) on their flexural capacity, stiffness and interface slip are clarified.

Numerical studies on the behavior of cold‐formed double‐channel shear connector

AbstractThe longitudinal shear forces are transferred through mechanical shear connectors at the steel beams and concrete slab intersection in composite beams. The shear connectors can be directly welded to the hot rolled steel beam, which is difficult in thin, cold‐formed steel beams. Thus, a cold‐formed double‐channel shear connector is proposed, and its suitability is investigated in this study. The double channel shear connector is a channel shaped shear connector formed from a flat plat and it is mounted on the flange of steel beam in pairs. The finite element model of the experimental Pushout test assembly was developed with the equivalent numerical constraints and interaction commands in a finite element software (ABAQUS). The developed models were fed with initial imperfection, load slip response, and material nonlinearity to achieve the proper experiment simulation. Initially, the developed models were validated against nine experimental test data reported by authors in the previous study. The parametric study was conducted to assess the behavior of cold‐formed channel shear connectors. The key parameters adopted in this study are the length of the shear connector, the grade of concrete and the grade and thickness of the steel connector. Three common failure patterns were witnessed among 107 numerical models. In this study, the factors influencing the control of failure patterns are presented, along with the factors influencing the shear connector's ultimate capacity.

Effects of the liquid rubber modified adhesive on the bond-slip response of the CFRP-steel interface

This paper presents an experimental and analytical study, purposely exploring the effects of the liquid rubber incorporation into epoxy resin on the bond performances between the carbon fiber reinforced polymer (CFRP) plate and steel. The experiment consisted of 36 double-shear bonded joints, and the investigated variables were the liquid rubber contents (i.e., 0, 5%, 10%, and 15% by weight) and adhesive thickness (i.e., 0.5 mm, 1.0 mm, and 2.0 mm). The failure mode, ultimate load, strain distribution, as well as the load-slip response were compared and discussed between specimens using the non-modified and liquid rubber modified epoxy adhesives, respectively. On the basis of the experimental results, a bi-linear bond-slip model considering the influence of liquid rubber content was proposed, and the critical parameters including the maximum interfacial shear stress, initial slip, fracture energy, and the maximum slip were identified, respectively. The present study lays a foundation for the subsequent research on the long-term bond enhancement for the CFRP-steel interface under freeze-thaw environment.

Repetitive behavior of an advanced demountable bolted shear connection in push-off tests

Currently, the repetitive behavior of the advanced bolted shear connector embedded in non-shrink self-compacting high strength pouring material (HPG) is unclear, restricting its application in steel–concrete composite beams (SCCBs). This study examined the repetitive performance of 21 push-off test specimens using different bolt diameters, bolt loads, loading paths, and loading programs. The failure models, load-slip response, shear bearing resistance, initial slip load, shear stiffness, peak slip, and ductility of this advanced bolted shear connection were evaluated. Bolt shearing off is the typical failure mode for all specimens, along with some concrete crushing underneath the bolts. Under repetitive loading, bolt shear capacity and load-slip response are similar to those under monotonous loading. Peak slip grows with the increase of bolt diameter under repetitive loading compared to monotonic loading. The shear stiffness under repetitive loading is lower than that under monotonic loading and improves with the bolt diameter and the bolt pretension increase. The ductility ratio under repetitive loading is smaller than that under monotonic loading and enlarges with the increase of the bolt diameter. Some design recommendations are proposed based on the existing formulae for predicting this advanced bolted shear connection.

Study on mechanical behavior of steel-UHPC-NC composite beams under negative bending moment

To investigate the negative moment region flexural performance of a steel-ultrahigh performance concrete (UHPC)-normal concrete (NC) composite beam, steel-UHPC-NC composite beams with a uniform and cluster arrangement of studs and three push-out specimens were designed and fabricated. Based on the test results, the finite element model of steel-UHPC-NC composite beams was established and verified. The results of the study show that in the push-out test, the failure mode of the specimens is the shear failure of the stud, and the specimens with the NC-UHPC composite slab and UHPC slab are crushed and spalled at the root of the studs, but the damage extent and damage range of the specimens with UHPC slabs were significantly smaller than the NC-UHPC composite slab specimens. The load-slip response of the push-out specimen is obviously affected by the concrete slab material. The ultimate bearing capacity of the single stud of the steel-UHPC specimen, the steel-UHPC-NC stud single layer arrangement and the double-layer arrangement specimens were 249 kN, 158 kN and 192 kN, respectively. In the negative bending moment zone loading test, the development trend of the mid-span load–displacement curves of steel-UHPC-NC composite beams with uniform and cluster studs is consistent, with the cracks in the former concentrated at the mid-span position and the cracks in the latter concentrated near the nearest group of shear studs in the mid-span. The load–displacement curves of composite beams that studs are evenly distributed and studs are cluster type are consistent, with ultimate displacements in the span of 65.8 mm and 68.5 mm, respectively, and ultimate bearing capacities of 723.8 kN and 712.3 kN, respectively. The calculation results obtained by the simulation method are in good agreement with the test results, which could effectively predict the load–displacement response of the steel-UHPC-NC composite beam.

Direct shear performance of UHPC Multi-Keyed epoxy joint

The modular Ultra-High Performance Concrete (UHPC) girder elements show outstanding potential to produce lightweight superstructure with high quality, low lifecycle cost and excellent durability for precast segmental bridge; and the required connection between segments is critically important to ensure desirable overall structural performance. This paper presents the results of direct shear tests on 18 UHPC joint specimens considering the influence of the number of shear keys, confining stress, depth of shear key, steel fiber volume content of UHPC and joint type. The shear behavior was characterized in terms of load-slip response, shear resistance, cracking pattern, and failure mode. Two types of failure modes, namely synchronous and stepwise shear failure, were observed. These specimens with stepwise shear failure showed smaller shear resistance and higher ductility compared with those with synchronous shear failure. And the ultimate shear stress decreased with the increase of key number, while the positive effect from confining stress was identified for the shear resistance in general. In addition, an updated model based on the existing ones was proposed and validated against the test data (standard deviation of 0.11).

Experimental investigation of the overall pull-out behavior of group anchored straight-type steel strands

The Precast Prestressed Assembled Structural (PPAS) system frame is a novel prefabricated concrete structure, which has been widely applied in building and storage structures due to the advantages of the large structural bearing capacity, high construction efficiency, low cost, and the overall structural performance is not lower than the cast-in-place structure system. Good beam-to-column connection performance is a prerequisite for the PPAS system to bear the external load. However, the non-prestressed steel strand group anchor technology used in the beam-to-column connection of the PPAS system is still lacking corresponding research. For this reason, the overall pull-out tests on 2 single anchor specimens and 10 group anchor specimens of straight-type steel strands with a nominal diameter of 15.2-mm were performed in this research. The results indicated that the pull-out failure of steel strands is the main failure mode for single anchor and group anchor specimens. Meanwhile, the anchorage type of test specimens had no significant effect on the pull-out load-slip response, except for the slight difference in the linear elastic ascending stage. Furthermore, as the arrangement form (layout quantity) of the steel strands increased, the ultimate anchorage capacity of the specimens presented a nonlinear growth trend, whereas the anchoring efficiency of group anchor specimens exhibited a three-stage variation law of sharp attenuation, slow attenuation, and fast attenuation. the ultimate anchorage capacity of group anchor specimens increased linearly with the embedment length of the steel strand.

Evaluation of shear lag effect in HSS-UHPC composite beams with perfobond strip connectors: Experimental and numerical studies

In order to evaluate the shear lag effect of high strength steel (HSS)-Ultra high performance concrete (UHPC) composite beams with perfobond strip connectors (PBLs), this study involved the preparing and testing of six HSS-UHPC composite beams with different shear connections, deck width-to-thickness ratios, and deck width-to-span ratios. Failure mode, load-deflection curve, load-slip response, and stresses distribution of the HSS-UHPC composite beams were presented and discussed. Experimental results indicated that uniform elastic stresses in the UHPC deck cross-section were observed, and the stress distribution became uneven as the composite beam entered the plastic stage. Under a load of 0.3Mu, the bending stress at the cross-section top center of UHPC deck was 5.3% larger than that at the edges, and this stress discrepancy went up to 9.4% as load approached 0.9Mu. To gain insight into the plastic performance of the tested beams, the finite element (FE) model was further established for the composite beams. Numerical results indicated that the sectional stress hysteresis coefficient (λ) at L/4 near loading point was briefly similar to that of the mid-span in elastic stage, and the stress unevenness increased as the shear connection degree dropped. As the connection degree decreassed from 1.02 to 0.89 and 0.76, the λ of the investigated beams were magnified by 2.7% and 6.7%, respectively. Results also showed that the shear lag effects degraded as deck width-to-thickness ratio increased. The λ for the beams using width-to-thickness ratios of 4.50 and 4.09 were 13.3% and 18.6%, both smaller than that using the ratio of 5.63. Moreover, as the width-to-span ratio increased from 0.20 and 0.24, the stress hysteresis effect of HSS-UHPC composite beams barely varied.

Composite connections between CFS beams and plywood panels for flooring systems: Testing and analysis

A composite floor consisting of cold-formed steel (CFS) beams connected to structural plywood panels is a sustainable and economical alternative to concrete only floors or the traditional floor systems supported by solid lumber joists. The structural performance of the composite CFS-plywood floor depends on the strength and stiffness properties of the shear connection. This study presents an experimental investigation into the load-slip behaviour of connections between CFS and plywood panels constructed with self-drilling screws, coach screws, and nut and bolt. The influence of structural adhesives at the shear interface alongside fasteners was also investigated. The stiffness, ductility and load-carrying capacity of the eight different types of connections were evaluated through a series of push-out tests. Nut and bolt connections were demonstrated to be the best choice amongst the shear connectors, followed by self-drilling screws; however, the size of the nuts and bolts should be determined based on the crushing strength of the timber itself. Connection with coach screws was less stiff and relatively less ductile than other tested shear connections. Composite connections with fasteners along with adhesives demonstrated higher stiffness and load-capacity than the fastener alone. Furthermore, a simple analytical expression for the load-slip response of the connections is proposed based on the Foschi formula, which showed a good correlation with test results. Based on the observations in this study, self-drilling screws and M8 nuts and bolts are recommended as shear connectors for lightweight cold-formed steel and timber flooring systems.

Bond–Slip Models for Corroded RC Members Exposed to Fire

This paper presents the latest research findings investigating RC members’ residual load–slip response to combined damage caused by corrosion and fire. The idea is to simulate an accidental fire in old or aging structures situated in an aggressive environment such as along the coastlines. The research involved evaluating the response of an RC bond exposed to corrosion and subsequently to elevated temperatures. Pullout bond tests were carried out on cylindrical bond specimens as per standard recommendations following the exposure to accelerated corrosion and elevated temperatures at steady state. Specimens were corroded in the range (2%–20%) as mass loss followed by exposure to elevated temperatures to achieve the steady state (200°C–800°C). The residual load–slip response is examined with the aid of displacement-controlled pullout tests. A set of LVDTs equipment was used to precisely obtain the ultimate bond strength and the steel bar’s slip relative to the concrete. The study presents distinct load–slip response curves depicting the overall load–slip mechanism of an RC bond. The results indicate significant changes in the load–slip response due to the two nonlinear phenomena’s superimposition. The bond strength degradation and the load–slip response have been modeled to predict RC members’ bond behavior exposed to corrosion–temperature interaction.

Mechanical Behaviour of Aluminium-Timber Composite Connections with Screws and Toothed Plates.

This paper presents an investigation of the load-slip behaviour of aluminium-timber composite connections. Toothed plates with bolts are often used for connecting timber structural members with steel structural members. In this paper, toothed plates (C2-50/M10G, C2-50/M12G or C11-50/M12) have been used as reinforcement in aluminium-timber screwed connections for the first time. The push-out test specimens consisted of laminated veneer lumber slabs, aluminium alloy beams, and hexagon head wood screws (10 mm × 80 mm and 12 mm × 80 mm). Of the specimens, 12 additionally had toothed plates as reinforcement, while 8 had no reinforcement. The load carrying-capacity, the mode of failure and the load-slip response of the strengthened and non-strengthened screwed connections were investigated. The use of toothed plate connectors was found to be effective in increasing the strength of aluminium-timber composite connections and ineffective in improving their stiffness. The examined stiffness and strength of the connections can be used in the design and numerical modelling of aluminium-timber composite beams with reinforced screwed connections.

Bond performance between textile reinforced concrete (TRC) and brick masonry under conventional environment

The interfacial bonding properties between textile-reinforced concrete (TRC) and masonry substrate is one of the critical indicators to evaluate the strengthening effectiveness. The effects of textile type, matrix type and interfacial bond length on the bond properties between TRC and masonry were analyzed according to single shear test in this paper; especially, engineered cementitious composite (ECC) was considered as the matrix to improve material properties. Failure mode, load-slip response, failure mechanism, bond properties and fracture energy between TRC and masonry were analyzed. The results showed that failure mode is influenced on the textile types and textile slippage from the matrix is the most common failure mode which may be accompanied by tensile rupture of textile. The bond properties between TRC and masonry using coated-carbon textile was the best. The ultimate load and bond strength are the largest when the bond length exceeds the effective bond length. The effective bond length is related to the textile and the matrix type, the effective bond length under different conditions is obtained. The fracture energy is obtained and its influencing factors are analyzed in this paper.

Experimental investigation of stirrup confinement effects on bond-slip responses for corner and middle bars

Stirrup confinement on bond between concrete and reinforcement is vital for RC (reinforced concrete) structures as it strengthens the friction and mechanical interlock between the two kinds of materials. In this paper, an experimental study has been carried out to look into the stirrup effects on bond-slip responses of RC with corner embedded reinforcing bars and bars embedded in the middle area of cover (corner and middle bars). This aims to quantify the difference of the bond strength caused by the embedment position of reinforcement while confined with hooked and straight stirrups. Different material properties have also been considered in the experiment including concrete cover, bar diameter and stirrup diameter. The failure modes for different specimen groups are presented and discussed. Besides, the influence of stirrup confinement effects on global load-slip response is also investigated by conducting another experiment with long embedment length of reinforcing bars which also includes the corner bars and middle bars. Furthermore, the debonding resistance compared with yielding force of the reinforcing bars are compared and analyzed for the global load-slip responses with stirrup confinements.