Shear Behavior Of Joints Research Articles

Effect of joint width on shear behaviour of wet joints using reactive powder concrete with confining stress

The filling concrete in a wet joint without steel reinforcements of precast segmental bridges is one of the weakest part of the bridges under shear. The joint width tends to be narrowed in construction, but structural shear behaviour might be different when the joint geometry varies. Therefore, it is crucial to study the shear behaviour of wet joints with various joint widths. In this study, the shear behaviour of wet joints with confining stress induced by the effect of joint width was investigated via experimental research and numerical simulation. A calculation model was proposed to evaluate the shear capacity of wet joints considering the effect of joint width. The experimental results revealed different failure modes of wide and narrow joints with a change in the force transmitting path. The wide joint exhibited brittle failure with one diagonal crack, whereas two symmetrical cracks were observed in the narrow joint before its failure. The load–displacement curve of the narrow joint exhibited two fluctuations, whereas that of the wide joint exhibited only one. These fluctuations represented the number of the cracks occurring in the joints. The numerical results demonstrated that the rotation effect, which was induced by different joint widths, caused a longer crack in the wide joints. Additional concrete was involved in the shear load transfer because the vertical area of shear component along the crack expanded. The average ratio of the calculations from the proposed formula to the results of this study and previous researches was 1.14, which showed to be accurate to predict the shear capacity of the wet joints.

Response of Adhesively Bonded Single Lap Joint Doped with Graphene in Adhesive to Shear Loads

The adhesively bonded joint creates a strong bond between the adherend and the adhesive and are replacing the riveted joints in numerous fields such as aerospace, automobile and civil sectors. In this study, the consequence of compositions of graphene nanoparticle doped in the epoxy adhesive on the shear behaviour of single lap joints of adhesively bonded glass and carbon fibre as adherends were investigated. Glass and carbon fibre laminates were produced using Rotex EP-306 resin and Rotex EH-758 hardener. Adhesively bonded single lap joints were then made by using a two-part epoxy structural adhesive. Graphene was added in weight percentages of 0.1%, 0.5%, 1% and 2% into the resin and dispersed by using the method of ultrasonication. A shear test was carried to determine the bonding strength. Results indicated that joints with 0.5% wt. of graphene showed better bond strength compared to other configurations for both glass and carbon adherend. It was noted that with further increase in percentage of graphene, the shear strength decreased. Furthermore, the joints made with carbon as adherend showed better strength properties in comparison with joints made of glass adherend.

Towards boundary conditions in concrete-rock joint shearing with STAs profile by PFC2D

Boundary conditions significantly affect the shear behavior of concrete-rock joints. Most of the joint models have been developed for the constant normal load (CNL) conditions but cannot be directly extended to the more general constant normal stiffness (CNS) conditions, as a ‘normal stiffness (K)’ serves as a spring in such a condition to constantly adjust the normal confinements in terms of the dilation. In this study, the authors attempted to set up numerical modeling of concrete-rock joint shearing by layered over-compaction method based on PFC2D. An obvious extension is that explicit spring-based kinetic equations were employed to govern the travel of the upper shear chamber, by which the system can be dynamically balanced at each timestep to permit CNS conditions. Also, an alternative joint profile consisting of multiple similar triangular asperities (STAs) was defined in the presented modeling, taking account of both varied asperity heights and sizes. The comparisons with laboratory observations, recently published by the authors, demonstrated that the model makes reasonable predictions of shear behavior, especially well capturing the essential mechanisms of STAs profile, including dilation, shearing, and local lift-off. Compared with CNL conditions, the CNS conditions can better mobilize the peak shear strength, maximum normal stress, micro-cracks, and force chain. Results indicate that an adequate choice of CNL or CNS conditions is of significance to evaluate the side resistance, loading capacity, or engineering stability in geotechnical engineering practices.

Experimental and numerical investigation on the shear behavior and damage mechanism of segmental joint under compression-shear load

The segment dislocation at circumferential joints severely impacts on the structural safety of shield tunnels during construction and operation. Especially for the underwater tunnel with a super-large cross-section, the circumferential joint usually consists of oblique bolts and distributed mortises and tenons to control the dislocation. Given this, a series of full-scale loading tests of the circumferential joint is carried out based on Jinan Yellow River Tunnel, and a three-dimensional refined numerical model with concrete damage plastic (CDP) constitutive is also established to reveal the condition of stress and damage. The deformation process of joints, the stress of bolts, and the damage characteristics of mortises and tenons are investigated through the full-scale tests and numerical simulation. Moreover, the mechanical behavior and damage evolution of circumferential joint has been revealed under compression-shear loads. Besides, the influence of mortises and tenons, bolts on the shear resistance of joints is unraveled. The results show that the deformation process of the circumferential joint can be divided into three stages under the ordinal shearing loading scenario (OSLS) and reverse shearing loading scenario (RSLS). The bolt plays an important bearing role under OSLS and the bolt is in an unstressed state under RSLS, which makes the shear stiffness and shear capacity of the joint under OSLS larger than that under RSLS. The tenon and bolt hole damage presents local concrete crushing, while the damage of mortise occurred with concrete spalling. Compared with bolts, the mortises and tenons can significantly improve the shear stiffness and the ultimate bearing capacity of circumferential joints.

Numerical study on the shear behavior of concrete-rock joints with similar triangular asperities

The shear behavior of concrete-rock joints with similar triangular asperities (STAs) was investigated by numerical direct shear tests under constant normal stiffness (CNS) conditions based on particle flow code (PFC), and numerical models were validated by laboratory observations recently published by the authors. The failure patterns and shear load transfer of asperities on the joints were revealed in terms of shear micro-cracks and shear force chains on the micro-scale. For comparison, joints with conventional profile, regular triangular asperities (RTAs), were also examined. The comparisons showed that a prominent ductile behavior at post-peak most likely occurred for the STAs profile owing to asynchronous asperity failure rather than brittle shear caused by a synchronous failure of the RTAs profile. Results also indicated that there would exist a critical inclination to limit the further mobilization of joint dilation and it may be useful to guide the preliminary design of the rock-socketed pile foundation.

Punching shear behavior and strength prediction of UHTCC-enhanced RC slab-column joints

RC flat slabs are widely employed in medium-height buildings due to their constructional and architectural advantages. However, due to the tensile brittleness of normal concrete, an RC slab-column structure may suffer progressive collapse induced by punching shear failure of slab-column joints. Considering the excellent tensile strain-hardening behavior of ultra-high toughness cementitious composite (UHTCC), the punching shear behavior of slab-column joints enhanced by UHTCC could be substantially improved. Three slab-column joint specimens with different UHTCC application ranges were tested under vertical monotonic loading. The cracking development process, peak bearing capacity, ductility, and post-punching capacity were recorded and analyzed. Yield line theory and critical shear crack theory were adopted to estimate the flexural capacity and punching shear capacity of the specimens, respectively, and the post-punching capacity was also calculated. Finally, the failure patterns of the specimens were determined by comparing the estimated flexural capacity with the estimated punching shear capacity and taking into account the ductile coefficient. The main conclusions are as follows: (1) applying UHTCC can considerably enhance the peak bearing capacity and ductility of a joint, and a more significant improvement effect is achieved by increasing the UHTCC application range; (2) adopting UHTCC affects the failure pattern of a slab-column joint from premature punching shear failure to flexural-triggered punching shear failure achieving better ductile performance, and flexural behavior becomes more evident with the increase of the UHTCC application range; (3) the post-punching capacity of a joint is improved when the UHTCC application range covers the entire punching shear region. Therefore, the progressive collapse resistance of the RC slab-column structures could be substantially improved by introducing the UHTCC-enhanced joints.

Experimental investigation on microstructure and surface morphology deterioration of limestone exposed on acidic environment

Understanding evolution of microstructure and surface morphology is essential for studying the strength of building material in acidic enviorment. In this study, the microstructure of limestone samples immersed in acidic solutions of varying pH values was evaluated by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), while the joint surface morphology evolution process was determined by three-dimensional (3D) scanning experiments. Further, direct shear tests on the immersed jointed samples were conducted for shear properties analysis. It is found that the pore size distribution gradually changes from nanopore to micropore with the decrease in pH value, while the single pore area and perimeter increased and the fractal dimension decreased. Therefore, the increased pore distribution, enlarging in area, and change in the shape of the pores altered the microstructure and caused the increase in porosity. Also, with the decrease in pH value, the decrease rate of 3D relative fluctuation height, angle, and 3D area ratio increase. The 3D average fluctuation angle decreases the most, whereas the 3D area ratio decreases the least. In an acidic environment, the 3D surface morphology is significantly influenced by the 3D average fluctuation angle, which eventually changes joint roughness. Under the condition of acidic underground water, the microstructure of the joint wall and surface morphology of the joint degraded, thereby affecting the joint shear behavior. Therefore, the degradation of joint roughness coefficient and joint wall compressive strength should be considered for evaluating the joint mechanical properties in an acidic environment.

Shear Behavior and Asperity Damage of 3D Rough Joints under CNS Boundary Conditions Based on CZM Simulation

Although boundary conditions can significantly impact the shear behaviors and asperity damage evolution of jointed rocks, numerical studies on the damage of 3D rough rock joints under the constant normal stiffness (CNS) boundary condition have rarely been reported. In this work, the three-dimensional model of the irregular joint surface is established by using point cloud reconstruction technology. Based on the cohesive zone model (CZM), we simulate the shear behavior of three-dimensional rough rock joints under the CNS boundary condition, which is realized by using embedded spring elements implemented with a Python subroutine. We conducted laboratory direct shear tests under CNS boundary conditions. The agreement with the laboratory experimental results verifies the fidelity of the numerical method. Our results show that boundary conditions can significantly affect the shear behavior of rock joints, especially in the post-peak stage. Under the same initial normal stress, the peak shear stress and the number of microcracks in the asperities increase significantly with the increase of normal stiffness. The proportion of shear cracks positively correlates with the normal stiffness, indicating that the normal stiffness affects the joint failure mode. The damaged area and the volume of asperities increase with the increase of normal stiffness. Moreover, the distribution of shear-induced asperity loss becomes more nonuniform, and the loss of joint roughness increases rapidly and nonlinearly.

Effect of roughness on the shear behavior of rock joints subjected to impact loading

The shear behavior is regarded as the dominant property of rock joints and is dramatically affected by the joint surface roughness. To date, the effect of surface roughness on the shear behavior of rock joints under static or cyclic loading conditions has been extensively studied, but such effect under impact loading conditions keeps unclear. To address this issue, a series of impact shear tests was performed using a novel-designed dynamic experimental system combined with the digital image correlation (DIC) technique. The dynamic shear strength, deformability and failure mode of the jointed specimens with various joint roughness coefficients (JRC) are comprehensively analyzed. Results show that the shear strength and shear displacement characteristics of the rock joint under the impact loading keep consistent with those under static loading conditions. However, the temporal variations of shear stress, slip displacement and normal displacement under the impact loading conditions show obviously different behaviors. An elastic rebound of the slip displacement occurs during the impact shearing and its value increases with increasing joint roughness. Two identifiable stages (i.e. compression and dilation) are observed in the normal displacement curves for the rougher rock joints, whereas the joints with small roughness only manifest normal compression displacement. Besides, as the roughness increases, the maximum compression tends to decrease, while the maximum dilation gradually increases. Moreover, the microstructural analysis based on scanning electron microscope (SEM) suggests that the roughness significantly affects the characteristics of the shear fractured zone enclosing the joint surface.

Experimental Study on Shear Behavior of Rock Composite Material under Normal Unloading Conditions.

As a composite material, the stability of rock mass is usually controlled by a joint. During the process of excavation, the normal stress of the joint decreases continuously, and then the shear strength of the joint decreases, which may eventually lead to the instability and failure of rock mass. Previous studies have mainly focused on the shear behavior of joints under constant normal stress, but have rarely considered the unloading of normal stress. In this paper, a direct shear test of joints with different roughness was carried out, in which the shear stress remained unchanged while the normal stress decreased. The strength characteristics of joints were explored, and the deformation and acoustic emission-counting characteristics of joints were analyzed by digital image correlation (DIC) techniques and acoustic emission (AE). A new method for predicting the instability of joints under normal unloading was proposed based on the evolution law of normal deformation energy (Un), tangential deformation energy (Us) and total deformation energy (U0). The results show the following: (1) The unloading amount of normal stress was enlarged for greater initial normal stress and roughness, while it decreased with an increase in initial shear stress. (2) AE events reached their maximum when the normal stress was equal to the failure normal stress, and the b-value fluctuated more frequently in stable development periods under normal unloading conditions. (3) U0 would change with the loading and unloading of stress, and this may be used to predict the unloading instability of rock mass using the abrupt change of U0.

Laboratory-Scale Investigation on Shear Behavior of Non-Persistent Joints and Joint Infill Using Lattice-Spring-Based Synthetic Rock Mass Model

Discontinuities mainly control the mechanical behavior of rock mass and cause a significant reduction in the rock mass strength. Joint persistency and joint infill conditions are considered the most significant joint parameters that control the mechanical response of rock mass. In this study, numerical and statistical analyses were performed on pre-cracked specimens with two flaws to investigate the effect of joint persistence parameters on shear strength. In addition, an extensive study was conducted to explore the effect of infilled mineral strength, infill thickness, and infill wall roughness on shear strength. The Lattice-Spring-Based Synthetic Rock Mass (LS-SRM) approach was utilized to perform the numerical models. The results showed that the tensile crack propagation is limited at higher normal stresses as tensile damage is largely suppressed. The increases in rock bridge angle slightly increased the shear strength and caused a change in the failure mechanisms of the rock bridge from tensile to shearing. The results of the models with infilled minerals revealed that infilled minerals mainly controlled the shear strength of specimens when the infill thickness was 4.0 mm or greater. The infill wall roughness had no apparent effect on the shear strength. In contrast, it governed the failure mechanisms; cracks initiated at the asperity of the rough filling wall and propagated through the hosted rock mass.

Effects of Sn-Ag-x leveling layers on the microstructure and shear behavior of Sn-58Bi solder joint under thermal cycling

Effects of Sn-Ag-x leveling layers on the microstructure and shear behavior of Sn-58Bi solder joint under thermal cycling

Retraction Note to: Effect of grain size on the interface structure and shear behavior of lead-free solder joints

Retraction Note to: Effect of grain size on the interface structure and shear behavior of lead-free solder joints

Geometrical aspects in the consideration of actual opposed surface along shearing direction in rock joints

Shear behaviour of rock joints is strongly influenced by the roughness of its surface as the different failure mechanisms will be conditioned by the shape and distribution of the asperities and undulations in it. Currently, this surface can be digitalized using scanning methods directly in the field or in representative laboratory samples. After that, the surface is digitally reproduced using triangulation methods. This allows to analyse it using stablished parameters that do not depend on the arbitrariness of the technician. Despite the fact that 2D roughness parameters are still of great importance, these techniques are nowadays provoking a tendency to implement 3D measuring methods that allow to treat all the surface simultaneously and thus gathering valuable information of the rock joint as a whole. The vast majority of the 3D most powerful methods consider the area of the surface that is opposed against the shearing direction in order to determine the roughness, anisotropy and peak shear strength of the rock joint. However, not always the whole surface of each triangle that faces against shearing direction should be considered. In this work, a new methodology to consider the actual area of each triangular element that is involved in the shearing process is mathematically derived. Therefore, improving the accuracy of current methods, as well as the truthfulness of the mechanical characterization of rock joints.

Method for visualizing the shear process of rock joints using 3D laser scanning and 3D printing techniques

This study presents a visualized approach for tracking joint surface morphology. Three-dimensional laser scanning (3DLS) and 3D printing (3DP) techniques are adopted to record progressive failure during rock joint shearing. The 3DP resin is used to create transparent specimens to reproduce the surface morphology of a natural joint precisely. The freezing method is employed to enhance the mechanical properties of the 3DP specimens to reproduce the properties of hard rock more accurately. A video camera containing a charge-coupled device (CCD) camera is utilized to record the evolution of damaged area of joint surface during the direct shear test. The optimal shooting distance and shooting angle are recommended to be 800 mm and 40°, respectively. The images captured by the CCD camera are corrected to quantitatively describe the damaged area on the joint surface. Verification indicates that this method can accurately describe the total sheared areas at different shear stages. These findings may contribute to elucidating the shear behavior of rock joints.

Effect of roughness and pore pressure on shear behavior of rock joint under true triaxial loading conditions

This paper presents the direct shear test to investigate the effect of the joint roughness and pore pressure on the shear behavior of rock joints. Recently, we have developed an experimental method for the direct shear test using true triaxial apparatus that can control three mutually perpendicular loads and pore pressure independently. The morphological properties of the joints were quantified using a non-contact 3D laser scanner to investigate the change in surface roughness due to shear and the effect of pore pressure. In addition, the time variation of shear load, normal displacement, and flow rate on the joint during shear sliding were measured, and the relationship with the change of the discontinuous surface after the experiment was discussed.

Evaluation of the effect of low normal stresses on the joint shear behavior using numerical simulation

Evaluation of the effect of low normal stresses on the joint shear behavior using numerical simulation

Shear strength of steel fiber reinforced lightweight self-consolidating concrete joints under monotonic and cyclic loading

The shear behavior of precast concrete segmental bridges depends on the type of joints between the segments. However, there are few studies evaluating the shear strength of steel fiber reinforced lightweight self-consolidating concrete for application to precast concrete bridges. The purpose of this study is to compare the shear behavior of joints made of general concrete with developed concrete that includes lightweight aggregate and steel fiber under monotonic and cyclic loading. In this study, the push-off shear test was performed in accordance with experimental parameters of different concrete mixtures, key type, confining stress, and application of epoxy between the segments. The tests were carried out to assess the shear strength, shear behavior, crack propagation and failure mode of joints. The test results represented shear capacity of joints decreased 15.2% by replacing normal coarse aggregates with lightweight aggregates but could be enhanced through the incorporation of hooked-end steel fibers. The shear capacities of joints fabricated using developed mixture tended to be underestimated compared to analytically calculated strengths. Furthermore, the normalized stiffness of the joint was decreased with an increased number of load cycles under high-amplitude low-cycle load. The shear resistance through aggregate interlock may gradually decrease with increasing number of cycles resulting in failure at 85% of the failure load of the monotonically tested specimen. An appropriate level of confining stress and application of epoxy shall be considered to prevent sudden failure of joints at high amplitude cyclical loading conditions.

Effects of different design factors on the shear performance of assembled steel–concrete composite beam with local laminated joint

This paper aims to experimentally investigate the effects of constructional details on the shear resistance of assembled steel–concrete composite beam with the local laminated joint. A series of push-out experiments were carried out to clarify the key structural factors determining the shear behavior and failure mechanism of post-cast wet joints under the combined action of the longitudinal loading and bending loading. Results indicated that the anchorage length of bottom transverse reinforcement placed in the post-pouring region had a significant impact on the shear resistance of assembled concrete slab, especially the ultimate bearing capacity and ductility of push-out specimens decreased when the anchorage length decreased to a critical value. In addition, using the U-type bottom transverse reinforcement could optimize the size of reserved slots, but this method presented a poor performance in limiting the cracking and failure behaviors of assembled slabs. Compared with the surface roughness treatment of the prefabricated concrete slab, the steel bar truss applied in the local laminated part could effectively strengthen the interface bond strength between the precast concrete and post-cast concrete, avoiding the premature cracking phenomenon. Furthermore, the reasonable layout of bottom transverse reinforcements could make full use of the plastic properties of steel bars and increase the shear resistance of assembled concrete slab, but the plastic deformation of steel bars might lead to the crushing of the post-cast concrete near the interface between the precast concrete and post-cast concrete. Finally, discussing the change of stress distribution on the section of post-cast wet joints caused by the negative bending moment based on the failure analyses, and the formulas of the shear capacity of local laminated joints considering the effects of the negative moment and constructional details were established.

Development of neuro-fuzzy models for predicting shear behavior of rock joints

The purpose of this article is to present predictive models of dilation and shear stress of rock discontinuities by applying the neuro-fuzzy technique, which uses a) the high capacity of artificial neural networks (ANN) to understand and to model complex multivariate phenomena, and b) the concepts of fuzzy sets theory to consider the variability of the input parameters in the proposed models’ responses. To develop the proposed models, experimental results were obtained from large-scale direct shear tests performed on different types of rock discontinuities and boundary conditions. The input variables of the proposed neuro-fuzzy models are the normal boundary stiffness, the ratio of fill thickness to asperity height, the initial normal stress, the joint roughness coefficient, the uniaxial compressive strength of the intact rock, the basic friction angle of the intact rock, the friction angle of the infill, and the shear displacement. The proposed models for dilation and shear stress provided results that fitted satisfactorily the experimental data, and the analyses of their performances indicated that they can represent the influence of the input variables on the shear behavior parameters of the rock discontinuities. The results from the neuro-fuzzy systems developed are also closer to the experimental data than those estimated by using traditional analytical methodologies existing in Rock Mechanics. This occurs because once considering the uncertainty of the input data, a more representative shear behavior prediction can be made by the neuro-fuzzy models.