Actual Stress State Research Articles

Flexural bond evaluation of deformed steel rebars in recycled aggregate concrete

An accurate quantification of the bond between embedded rebar and recycled aggregate concrete (RAC) is a prerequisite for confident use of RAC. However, there is a noticeable lack of experimental data that can faithfully represent the actual bond stress state in real RAC structures. To address this limitation, a total of 37 beam specimens were tested in strict conformity with the RILEM testing protocol. The strength grade of old concrete was among several control variables studied for their impact on the bond strength. The findings reveal that the incorporation of coarse or/and fine recycled aggregate (RA) does not alter the expected mode of bond failure for a beam; however, it significantly affects the beam’s bond strength in flexure. Using RA sourced from weaker parent concrete leads to much inferior interfacial bonding, but there exists an almost linear relationship between the bond strength of RAC relative to that of natural aggregate concrete (NAC) and their relative water absorption ratio. This relationship seems to hold true regardless of the strength of RA. Based on the probabilistic analyses conducted herein, the anchorage length specified in existing codes developed for NAC is found inadequate and unsafe for RAC. A modifier is therefore proposed to remedy this unconservatism.

Design and development of an in-flight simulator for flooding and drawdown

Geotechnical structures such as levees, dykes, canals and tailing dams, when subjected to extreme climatic conditions such as flooding and drawdown, are prone to failure on the upstream and downstream sides due to transient seepage conditions. Therefore, it is imperative to physically replicate such conditions in the actual stress state. Hence, the objective of this paper is to present the performance of an in-flight simulator developed to simulate flooding and drawdown (ISFD) events at enhanced gravities. The working principle, design details, various components of the developed simulator and calibration at normal (1g) and high gravities are discussed. In this study, the calibration and performance of the ISFD set-up were demonstrated on a model levee section in a 4.5 m radius large beam geotechnical centrifuge facility at the Indian Institute of Technology Bombay, India. In total, three centrifuge model tests were conducted on a scaled-down levee section (with and without internal drainage layers) constructed with silty sand-type material to validate the ISFD capabilities. In addition, seepage and slope stability analysis for the centrifuge models was carried out using Plaxis-2D geotechnical software, which compared favourably with physically observed tests results.

Evaluation of flow stress in nylon 66 via digital image correlation method and response surface method

Nylon 66 is an engineering plastic with the advantageous characteristics of light weight, heat resistance, high strength, and high rigidity; it has been used in automobile parts as a substitute for aluminum alloys. There have also been many studies on the mechanical properties of nylon 66. However, the flow stress in nylon 66 after necking has not been thoroughly investigated yet due to the difficulty in identifying local stress. In this study, a flow stress optimization method based on the digital image correlation (DIC) method and response surface method, is proposed. The averaged true stress–true plastic strain curve is first obtained by a quasi-static tensile test via DIC. Subsequently, the flow stress is optimized via the response surface method to obtain the actual flow stress state after necking. The accurate flow stress in the specimen can be obtained based on the relationship between the global load force and distance between the two shoulders of the specimen grip parts that is unaffected by the cross-sectional area change during necking. The results indicate that the average true stress recorded from the experiment overestimates the flow stress after necking. The optimized flow stress exhibits the maximum decrease of 9%. The accuracy of the optimized flow stress is evaluated by comparison with the true strain distribution on the specimen that is accurately measured via DIC.

Mechanical Action Mechanism and Crack Response Analysis of Slide Plate of No.12 Turnout

In the course of maintenance and repair, the railway workshops of the Lan-Qing railway found that the slide plate was prone to injury and damage. In order to study the actual stress state of the slide plate during the running of the train, a finite element model of the slide plate and related components was established based on ANSYS, Hypermesh was used for meshing and mesh quality control, and the Mooney-Rivlin constitutive intrinsic model was selected to simulate the rubber plate. The stress state of the slide plate when passing through the turnout in the straight and lateral directions is analyzed respectively. The results show that there is stress concentration at the root of the tongue depressor of the slide plate, and large deformation exists in the middle of the base plate, the front end of the platform plate and the position of the guide iron when passing through the turnout directly and laterally. The stress state of the slide plate is simulated when the rail brace fails, and the maximum stress is found to be 279.24 MPa, which exceeds the material yield limit. Therefore, we should pay attention to the inspection of the condition of the rail brace during the maintenance and repair. Presenting cracks at the weld seam location, it is found that the crack has a significant effect on the dewelding of the slide plate, and the heel position is the weakness of the slide plate. When the crack length reaches 1.2 mm, there is a tendency to crack at the weld seam position of the slide plate, and when the crack length continues to increase, the crack will further expand and jeopardizing structural safety. The research results can provide a theoretical basis for the improvement, disease prevention and maintenance of the slide plate.

Full-scale loading experiments on performance of UHPC joints for prefabricated mountain tunnel

This study used ultra-high performance concrete (UHPC) to design joints reinforced by various bolt types: straight, inclined, and curved bolts. Full-scale samples were prepared in the laboratory. Loading tests on these joints and the unbolted one were carried out based the actual stress state of the tunnel, and various axial force magnitudes and bending moments were combined to obtain the deformation characteristics. The fitting law of the bending stiffness of joints was presented. It is found that axial force increases the bending stiffness of the joint with better elastic performance. The bending stiffness of the straight bolt joint is the highest under positive bending moment during the first loading process, while that of the curved bolt joint is the greatest under negative bending moment during the second loading process. The test results indicate that joints made using UHPC (especially the curved bolt joint) can be used to build a more resilient prefabricated tunneling system.

The Effects of True Triaxial Loading and Unloading Rates on the Damage Mechanical Properties of Sandstone

Coal is the main energy source in China. In the process of coal resource mining, the surrounding rock of roadways is often in the complex stress environment of “three heights and one disturbance”. At the same time, rocks in the stratum are often in a three-way unequal pressure state under the action of geological structure, and conventional rock mechanics tests cannot study the mechanical properties of rocks under actual stress conditions; thus, this is based on the self-developed true triaxial multifunctional fluid–structure coupling test system to study the damage mechanical Properties of Sandstone. The results are shown as follows: With an increase in loading rate, the peak damage Dcr of sandstone decreases, but the initial damage Da increases in the elastic stage, and the brittleness of sandstone weakens. With the increase in the unloading rate, Dcr increases, but Da decreases in the elastic stage, and the sandstone brittleness increases first, then decreases. In addition, the peak maximum principal strain ε1maxfirst decreases rapidly and then slowly; the peak minimum principal strain ε3max increases first, then decreases slowly, and increases slowly; the peak intermediate principal strain ε2max decreases slowly; and the peak volume strain εvmax increases rapidly first and then slowly with increases in the loading rate. With an increase in the unloading rate, ε1max increases rapidly first, then decreases slowly, then increases rapidly and finally increases slowly; ε3max first decreases slowly, then increases slowly, and finally decreases slowly; and ε2max increases slowly then decreases slowly. εvmax decreases rapidly first and then increases slowly with increasing loading rate.

Experimental study on the effective stress law and permeability of damaged sandstone under true triaxial stress

The effective stress law and the permeability play a notable role in dealing with the gas-solid coupling problem of porous media . The key parameter to quantify the influence of pore pressure to effective stress is the effective stress coefficient α ij . However, current relevant researches are mainly focused on the pore elastic flow characteristics in the elastic stage or hydrostatic pressure under conventional triaxial stress state, which are difficult to reflect the actual formation stress condition. To this end, the present study was conducted to investigate the influence of induced damage on the effective stress coefficient and the permeability of sandstone under true triaxial stress. The results showed that the effective stress coefficient and the permeability were closely related to the damage evolution and fracture propagation . α ij exhibited obvious anisotropy, α 1 was larger than α 2 and α 3 , and α 3 > α 2 appeared in the horizontal direction. α ij increased with increasing the fracture density and opening, even greater than unity. Increasing the initial horizontal stress could reduce α ij under the damage state. The elastic modulus E ij also exerted anisotropy during loading, E 1 basically exhibited an increasing trend followed by a decrease as the axial strain increased, E 2 and E 3 were continue to degrade with increasing the axial strain. Loading the deviatoric stress could cause damage, which induced the initiation and propagation of microcracks , and in turn led to the nonlinear stress-strain relationship and volume expansion of the rock, so the permeability increased, and it had increased sharply before reaching the peak strength. Under the influence of σ 2 , α ij first increased and then decreased with increasing the intermediate principal stress coefficient b under the damage stage. The porosity effect also affected the relationship between α ij and the permeability related to the induced damage, and α ij revealed an increasing trend as the permeability increased. • Effective stress coefficient a ij increases with fracture growth, even beyond unity. • a ij and E ij exhibit anisotropy with loading. a 1 is the largest, and a 3 > a 2 . • Increasing the horizontal stress can reduce a ij related to the induced damage. • a ij first decreases and then increases with increasing b under the damage state. • a ij increases with the increase in permeability attributed to the induced damage.

EXPERIMENTAL STUDY ON THE VERTICAL SELF-WEIGHT STRESS DISTRIBUTION LAW OF SLOPE WITH GRANULAR MATERIALS UNDER DIFFERENT CONDITIONS

Topography is one of the important factors affecting the distribution of the self-weight stress field. However, granular materials are different from general continuum materials (such as fluids and solids). Only adopting the continuum theory research still has certain limitations, while the use of experimental methods can better reflect the actual stress state of the granular materials. Therefore, in order to further obtain the vertical self-weight stress distribution of single slope with granular materials, the indoor experimental study of quartz sand based on the point source method is carried out in this paper. The research results indicate that: The measured value of the vertical stress on the bottom surface of the quartz sand slope is generally smaller than the γh (Gravity × Buried depth) value of the corresponding point, and the closer the measuring point is to the slope top, the greater the difference between the test value and γh. Besides, the influence of slope heights and slope ratios on the vertical self-weight stress about slope with granular material is also analyzed. Stress depressions appear in some test conditions, that is, the measured stress peak on the bottom of the slope does not appear at the measuring point closest to the slope top. Whether there is a stress depression and the scope of the depression is mainly related to the slope height, while the slope ratio has little effect on it.

Experimental investigation and design optimization on flexural behavior of new UHPC deck panel with longitudinal ribs reinforced by steel plates

Due to high strength and superior durability, ultra-high performance concrete (UHPC) is usually used in bridge deck panels to reduce self-weight and improve the anti-cracking capacity. In this study, two schemes of the UHPC deck panel with longitudinal ribs reinforced by steel plates were investigated basing on the Binzhou Yellow River Bridge, including the design considerations and advantages. The experiments were conducted to investigate the flexural behavior of this UHPC deck panel. The experimental results showed that the steel plate can improve the anti-cracking performance and stiffness of the deck more effectively compared to conventional measures of using steel bars. Cracks developed at the position where headed studs were placed due to the weakening of cross-section by high studs. Specimens failed with the appearance of the slightly compressive crack. In addition, the experimental test was simulated by finite element (FE) model in software ABAQUS. The cracking position, load-displacement relationship and strain response of testing specimens were verified by the FE model. For the design purpose, a numerical method was established to analyze flexural behavior including the stiffness, ultimate capacity and strain. The prediction results agreed well with the test results. Further, a parameter analysis was performed and it was clear that the narrow and high rib scheme provided superior flexural properties. Compared with UHPC tensile strength, the steel plate thickness had a more significant effect on the stiffness and ultimate capacity for the specimens. Finally, the segmental FE models were developed for the Binzhou Yellow River Bridge to obtain the actual stress state of the new UHPC deck. The calculation results indicated that the maximum longitudinal nominal stress in the UHPC deck was 7.6 MPa. In addition, when the diaphragm spacing was between 3.0 m and 5.0 m, the longitudinal stress would increase by about 12% with the diaphragm spacing increasing by 0.5 m. The comparisons between the actual stress and experimental results ensured the feasibility of the design.

Methodology of Leakage Prediction in Gasketed Flange Joints at Pipeline Deformations.

The paper presents the proposal of a leakage prediction method in flange joints, after pipeline deformation, based on FEM (Finite Element Methods). The stages of developing the design are discussed, and a complex, multi-stage method of applying the loads is presented in detail. Moreover, the gasket material data obtained in experiments were used. The paper also presents the results of calculations on a non-uniform stress distribution in the radial direction of the gasket. In addition, it has been shown that the deflection of the pipeline with a minor displacement causes an increase in the diversification of the circumferential pressure of the gasket, and also has a significant influence on the determination of the actual state of stress to which the gasket is subject. Moreover, it was found that the distribution of contact pressure on the deflection of the pipeline has a significant influence on the level of leakage. The results of tests are compared to the results of the numerical calculations of the stress in bolts. By comparing the bolt tension changes obtained by numerical and experiment analyses, it has been shown that the assumptions made in developing the numerical model are correct.

Unification of the Mechanical Model and Parameter Analysis of the Elastic Foundation Beam of Pipe-Roof

Based on the existing model of pipe-roof considering the arching effect, combined with the mechanical model of pipe-roof when the tunnel is excavated to the end, a unified mechanical model of the elastic foundation beam for pipe-roof is established. Deflection and internal force calculation formulas of the model were derived. Combined with actual engineering cases, the model was compared and analyzed, and the parameters affecting the pipe-roof were analyzed by taking the unified model as an example. The results show that the established unified elastic foundation beam model can better represent the actual stress state of a pipe-roof and the model has strong applicability. The stress state of the pipe-roof at the end of excavation can be calculated by changing the boundary conditions of the model. When the diameter of the steel pipe is 108–114 mm, the supporting effect of the pipe-roof is similar. When Ec (elastic modulus of converging) > 40.0Eg (elastic modulus of ground rock), the excavation footage and the diameter of the steel pipe have little effect on the deflection and bending moment of the pipe-roof. Therefore, increasing the elastic modulus of the reinforced area is the most effective method to reduce the deflection and internal force of the pipe-roof. The longer the residual length of the pipe-roof in surrounding rock, the safer the tunnel will be during excavation. The economically reasonable value of the residual length of the pipe-roof in surrounding rock is 2–3 m.

Analysis of mechanical properties of steel-concrete joint of large-span hybrid beam bridge

The mechanical properties of the steel-concrete joint determine the mechanical properties of the whole bridge. The mechanical properties of the steel-concrete joint of Jianhua railway rigid-frame bridge are analyzed by using nonlinear finite element method. The results show that the overall stiffness transition of the joint is smooth, the maximum stress of concrete section and steel box girder section are lower than the design value of the strength, which indicates that the structural stiffness and strength of the steel-concrete joint section meets the requirements. The error range of the finite element analysis results and the measured data is within 6 %, which shows that the finite element method can simulate the actual stress state of the bridge and it could save a lot of time and energy compared to the actual test.

In situ dynamic X-ray imaging of fluid-rock interactions inside tight sandstone during hydraulic fracturing: fluid flow process and fracture network growth

Hydraulic fracturing plays a fundamental role in unconventional hydrocarbon production and the development of enhanced geothermal systems. Thus, studying the fluid flow process and fracture network growth during hydraulic fracturing is necessary to provide guidance for creating and sustaining fracture networks in target reservoirs. The actual formation stress state and hydraulic fracturing process can be simulated in the laboratory for small-scale rock samples in hydraulic fracturing experiments. High-energy industrial X-ray computed tomography (CT) has been used to directly track the fracture propagation in millimeter-scale samples under triaxial stress loading. In this study, a large-scale fracture network with economic benefits for unconventional energy exploitation was the target of the experiment; therefore, centimeter-scale samples were used. A hydraulic fracturing simulation experiment was conducted on tight sandstone by applying triaxial compression and injection pressures. With the assistance of the PEEK core holder and third-generation medical X-ray CT scanner, in situ dynamic X-ray imaging of the fluid-rock interactions inside the sample was conducted for the first time. The fluid-rock interactions and fracture network growth can be dynamically observed, revealing the fluid-rock interactions during the hydraulic fracturing process and how macroscopic failure develops from the microfracture.

Deformation recoverability of longitudinal joints in segmental tunnel linings: An experimental study

The shield-driven tunnel is a fabricated lining structure. The existence of longitudinal joints between segments in a ring makes the entire tunnel lining vulnerable to deformations, because the rigidity of longitudinal joints is lower than that of the main concrete segments. Large transverse deformation, accompanied by joint opening, is a common disease of shield tunnels. The aim of this work is to explore the deformational behaviors and mechanical properties of tunnel longitudinal joints under overloading, unloading, and soil grouting conditions, with emphasis on the recoverability of joint deformation. For this purpose, a series of laboratory tests have been carried out on the full-scale longitudinal joints at tunnel crown adopted in Shanghai metro tunnels, including three parallel tests with different overload levels and one cyclic overloading and unloading test. The full-scale testing considers the actual stress state of the longitudinal joint under different working conditions in real environments. The specimen of the longitudinal joint at tunnel crown adopts the actual three-segment-two-oblique-seam structure, which is different from the simplified two-segment-one-straight-seam form used in previous studies. Multiple deformation indicators, including joint opening, concrete strain, and bolt strain, are monitored and analyzed. Based on the test results, the effects of restoration actions, i.e., unloading and soil grouting, on joint deformation recovery under different overload levels are discussed. The change of recovery efficiency with the degree of joint deformation is also obtained. In addition, the deformation recoverability and rotational stiffness are compared between the longitudinal joint at tunnel crown which is subjected to positive bending moment and the longitudinal joint at tunnel springline which is subjected to negative bending moment. The test results show that the larger the joint deformation caused by overload, the lower the recovery efficiency. From a mechanical perspective, soil grouting is an effective measure to recover the large deformation of shield tunnels.

Influence of the Roofing System on the Seismic Performance of Single-Layer Spherical Reticulated Shell Structures

The seismic performance of a single-layer spherical reticulated shell is the key problem to be solved in the design and analysis of this structure. In previous studies, the influences of roofing systems on the seismic performance of shells were usually ignored, resulting in large discrepancies between the results of analyses and the actual stress states of shells. In this paper, the finite element analysis method is applied to a shell with a roofing system, and the applicability of the method is proven by static loading experiments. The influences of roofing systems on the seismic performance of shells are obtained from seismic response curves, the proportions and distributions of plastic members and the failure behaviours of the shells during strong earthquakes. The mechanism of the influence of the roofing system on the seismic response of a shell is revealed by analysing the damage of purlin joints and the energy consumption of the components of the shell. The relationships that describe the influence of different parameters of reticulated shells and roofing systems on the seismic response of the shells are studied, and the results show that the roofing system can greatly change the seismic response and failure of a shell under strong earthquake conditions.

Eccentric compression behaviour of rectangular concrete-filled steel tube columns with self-compacting lower expansion concrete

The use of a self-compacting lower expansion concrete in a concrete-filled steel tube (CFST) structure not only promotes the quality of concrete pouring but also improves the bond behaviour between the steel and the concrete. In combination with the actual stress state of the columns in the engineering structure, it is necessary to study the eccentric compression behaviour of the column. In this study, experimental studies involving both uniaxial and biaxial bending tests of rectangular self-compacting lower expansion CFST columns were carried out. The variation laws of the load–displacement curves, the lateral deflection curves and the stress–strain curves during the loading phase were analysed. Furthermore, the failure modes and the mechanical properties of the specimens under eccentric compression loads were investigated. Subsequently, the numerical models of CFST columns with self-compacting lower expansion concrete were considered and established. In order to verify the rationality of the finite element modelling, the numerical calculation results were compared with test results. Then, a parametric analysis of the compression and the bending bearing capacities of each column was carried out by changing the eccentricity of the load, and the N–M curves or N-Mx-My surfaces describing the ultimate bearing capacity of the column were obtained. Finally, by the parametric finite element analysis of the rectangular CFST columns regarding to the bearing capacity under the same eccentricity, a conclusion was obtained: when the expansion agent content γ of a specimen increased from 0% to 10%, the bearing capacity of the columns increases significantly, but when continue increasing the expansive agent content, the expansion agent content has little effect on the compression–bending bearing capacity.

Experimental study on the cyclic bond behavior of corroded rebar based on modified beam test

The corrosion of rebar results in the degradation of bond between rebar and concrete, thus is unfavorable to the safety of reinforced concrete structures. So far, many studies were undertaken to investigate the bond behavior of corroded rebars under monotonic loading, however, only a few considered the effect of cyclic loading. Moreover, in these researches, the bond behavior of corroded rebars under cyclic loading were generally studied with pull-out specimens, which cannot ensure an actual stress state of the rebar and concrete during loading. Therefore, in this paper, an experimental study is made to further investigate the cyclic bond behavior of corroded rebars based on a modified beam test, in which the stress state of the rebar and concrete is consistent with reality. Based on the test results, the effects of corrosion level and loading history on the bond stress at a given slip are evaluated. A cyclic bond-slip model is proposed, in which the degradation rule of bond stiffness is calibrated by the test results. The model is implemented in a user-defined bond element and validated with the test results. The predictions of the model show great agreement with the test results, meaning that the proposed model is feasible to simulate the bond stress-slip relationship of corroded rebars under cyclic loading.

Multi point monitoring method of prestressed tendon based on distributed sensors

This paper presents a multi-point strain monitoring method based on distributed sensors, which can monitor the multi-point stress of prestressed tendons in concrete. Firstly, the special sensor protection device is installed in the designated position of the component. Then, the sensor is placed in the device and attached to the monitored prestressing tendon. After concrete pouring, the device can form a closed space around the sensor to ensure that the sensor can freely expand and move in a small range when the prestressed tendon is tensioned. The proposed method is applied to the monitoring of the construction period and initial operation period of a large-span spatial structure, and the research results show that the method proposed in this paper can well realize the multi-point monitoring of the prestressed tendon strain in the concrete, and effectively solve the engineering problems; through the analysis of the long-term monitoring data, we can accurately grasp the influence of each construction condition on the structure stress, and clarify the actual stress state and long-term change trend of the structure. The obtained monitoring data play a positive role in guiding the construction of prestressed structure, and the construction safety can be objectively evaluated by analyzing the monitoring data.

Estimation of bond strengths and design parameters for precast concrete-UHPC shear key interface in box beam bridge

Adjacent precast prestressed concrete box beam bridge is often the choice for short-to-medium span bridges. However, it has been reported that box beam bridges experienced reoccurring longitudinal cracks at beam-shear key interfaces. The interface experiences a complex stress state represented by tensile/compressive stress perpendicular to the interface and longitudinal and vertical shear stress parallel to it. In this study, small-scaled specimens were cast together with the box beams and UHPC shear keys of a real bridge and were tested using four different test methods that try to replicate the actual stress conditions. The interface experienced satisfactory bond strength under different stress scenarios even at early age. The cohesion and friction coefficients were determined based on analysis of the experimental data and can be used to estimate the interfacial bond capacity at early age of UHPC. The interfacial bond capacity of a typical bridge was also determined and compared with the stress due to design truck load. The results indicate that the early age shrinkage and thermal load on the interface is more critical compared to the effects of the truck load. When subjected to the design truck load specified by current code, only 4.9% of the bond capacity was used, while the interfacial bond strength during the early age was exceeded. This indicates that the load is not the critical factor that causes bond failure at beam-shear key interface in box beam bridges. Furthermore, the cohesion value is sufficient to resist the applied load and the shear reinforcement can be optimized in the future application of UHPC in box beam bridges. Meanwhile, the early age tensile bond strength was exceeded and therefore the use of shear reinforcement to carry the early age tensile stress is necessary to prevent cracking.

Hysteretic Behaviour of Shear Stud Connectors in Composite Steel Moment‐Resisting Frames

AbstractIn steel moment resisting frames (MRFs) with composite floor slabs, seismic loads are transmitted from the slab into the beam through shear studs. Shear strength degradation of the studs due to cyclic loading results into the loss of the load transfer mechanism and composite action. To date, several low‐cyclic, high‐amplitude push‐out tests have been conducted to characterize the shear stud hysteretic behaviour. These isolated tests showed considerable degradation in the shear stud resistance; nevertheless, they do not simulate the actual hysteretic behaviour of shear studs within a composite steel beam. To elaborate, conventional cyclic push‐out tests do not replicate the actual stress state in the concrete slab, nor do they account for the force redistribution between the shear studs. This paper investigates the hysteretic behaviour of shear studs in steel MRFs with composite floor slabs. The load‐slip hysteretic behaviour of the shear studs is examined through continuum finite element (CFE) models of 2‐bay sub‐systems and their corresponding cruciform subassemblies. Moreover, shear force and interlayer slip envelopes are obtained along the composite steel beams. Results of the CFE models are used to propose new design recommendations for shear studs in composite‐steel MRFs.