End Of Specimen Research Articles

A novel experimental method for studying rock collision

This paper introduces a new experimental method for studying rock collision by making full use of the beauty of stress wave theory. In this method, a newly developed energy transmission component was placed between the gas gun and the transmitted bar of a split Hopkinson pressure bar (SHPB). The forementioned component consists of an incident bar which moves frictionlessly within a specified distance, a circular steel plate welded to the incident bar, and a support base which is bolted to the SHPB bed. A rock specimen is attached to the farther end of the incident bar. When the striker bar, propelled by the gas gun impacts the incident bar, a compressive stress wave is transmitted from the incident bar to the rock specimen. When the compressive wave arrives at the free end of the rock specimen, it is reflected into a tensile wave. Then when the pure stress becomes tensile and it is over the tensile strength of the glue at the interface between the rock specimen and the incident bar, the rock specimen is ejected, and then the ejected specimen will collide with the transmitted bar. During specimen flight, the velocity of the rock specimen can be measured by a laser instrument, while the remained energy transferred to the transmitted bar is measured by strain gauges attached to it. The process of rock specimen flight before collision and fragment flight after collision can be photographed using a high-speed camera. This experimental method can be used to not only study a collision between a moving rock and another object, but also imitate a drop weight test. By using this new method, seven rock collision tests were successfully conducted.

Experimental study on concrete-filled L-shaped steel tubular columns and beam-columns

Concrete-filled L-shaped steel tube (CFLST) is an efficient structural form for corner columns in high-rise buildings, which avoids column protrusion. This paper presents an experimental and theoretical study on the structural behavior of CFLST columns and beam-columns. A total of nine full-scale specimens (i.e., three stub columns, two slender columns, and four slender beam-columns) was tested under compressive loads and the specimen ends were free to rotate about a geometric axis. Plastic local buckling was observed for the stub columns and the confinement effect was not obvious. Flexural buckling occurred for the slender columns and beam-columns. With the increase in the applied load, the neutral axis shifted to the compressive side of the beam-column. Performance of current design codes, which apply for rectangular CFSTs, on predicting the ultimate capacities of CFLSTs was assessed. The prediction for section capacity was conservative and it was likely caused by the reduction in the design strength of concrete. The buckling curve in design codes for rectangular CFSTs was applicable for CFLST columns whereas the interaction relationship specified in design codes underestimates the ultimate capacities of CFLST beam-columns. Based on fiber element analysis, out-of-plane bending moment was found in the tested CFLST slender members and the values were quantified. A new interaction relationship was proposed for CFLST beam-columns, which exhibited good agreement with experimental and numerical results. Finally, design methods were recommended for CFLST columns and beam-columns.

Interfacial bond-slip of horizontally embedded CFRP strips and concrete considering the effect of the groove depth-to-width ratio

Concrete structures with shallow grooving depths require minimal adjustments to accommodate horizontal near-surface-mounted (NSM) carbon fibre-reinforced polymer (CFRP) strips. This method proves advantageous in minimizing damage to reinforced concrete structures, establishing itself as an effective reinforcement technique. To delve into the interfacial performance of the horizontal CFRP strip reinforcement, single shear pull-out testing is utilized to analyze and compare the interfacial bonding performances of concrete specimens reinforced with external bonding (EB), NSM, and horizontal NSM CFRP strips. The study also explores the influence of groove depth-to-width ratio on the interfacial bonding performance of horizontal NSM CFRP strip reinforcement. An interfacial bond-slip principal structure model of horizontal NSM CFRP strips and concrete is established, factoring in the groove depth-to-width ratio. Subsequently, the proposed interfacial bond-slip model is validated through finite element numerical simulation. The results show that the horizontal NSM CFRP strips reinforcement method exhibits commendable bonding capacity and interfacial bond stiffness. Within a specific range of groove widths, increasing the groove depth-to-width ratio diminishes the distance between the peak bond shear stress location and the loading end of the horizontal NSM CFRP strip-reinforced specimens. This improves the interfacial bonding performance between the horizontal NSM CFRP strips and concrete. The theoretical curve derived from the interfacial bond-slip principal structure model aligns well with test data across all reinforced specimens. FEM numerical simulations based on this interface bond-slip principal structure model demonstrate minor relative errors compared to experimental values. Furthermore, the theoretical distribution curve of CFRP strip strain closely follows the measured distribution curve. This model concerning the bond-slip principal structure of the interface between horizontal NSM CFRP strips and concrete, accounting for the groove depth-to-width ratio, exhibits applicability and stands as a valuable reference for practical engineering applications.

Uncertainty quantification and global sensitivity analysis of bearing capacity of push-out specimen considering randomness in bond-slip behaviour

As an artificial composite material, the uncertainty in mechanical property of concrete significantly impacts the bond-slip behaviour of the shaped steel concrete structure. To investigate the effect of the uncertainty of concrete mechanical properties on the bearing capacity of push-out specimens, based on the data of a small number of push-out test with identical design parameters, the improved Bootstrap method is utilized to execute sample augmentation for the bond stress-slip curves, and the stochastic bond slip constitutive model is established after considering five parameters (K1, …, K5) in the deterministic bond-slip model as random variables. Simultaneously, considering the randomness in elastic modulus of concrete (Ec), uncertainty quantification (UQ) of the bearing capacities of the push-out specimens is conducted using the arbitrary polynomial chaos (aPC) method. Subsequently, the global sensitivity analysis of the ultimate and residual bearing capacities is accomplished through the aPC surrogate model. The results indicate that the probability distribution types of the parameters in the stochastic bond-slip constitutive model can be arbitrary when considering the randomness of concrete material, and the aPC method with at least a third-order expansion is recommended to execute UQ for the push-out test due to the strong nonlinearty between the bearing capacities and the random parameters. The ultimate bearing capacity is primarily affected by the parameter K2, followed by K4, K3, and K5. Whereas the residual bearing capacity is predominantly influenced by the parameter K4, followed by K5, the remaining parameters exhibit negligible influence. In contrast, K1 and Ec have the greatest influence on equivalent plastic strain of the concrete near the loading end of the push-out specimen.

Introducing a novel sheet test specimen for biaxial stretching under uniaxial loading

This paper introduces a novel sheet test specimen that plastically deforms under biaxial stretching when subjected to uniaxial loading in conventional testing machines. The specimen was designed using finite element simulation and consists of a two-dimensional reticular structure with interconnected arms in the form of triangles that converge at the center and are connected to full-width sheets at the specimen's ends. Accumulation of damage at the center is ensured through spherical cups that are milled at opposite sheet surfaces. Experimental strain loading paths obtained by digital image correlation confirm the capability of the novel sheet test specimen to provide biaxial stretching strain paths. This is achieved under friction-independent conditions and without requiring complicated multiaxial setups and machines, provided appropriate values of the inner and outer angles of the interconnected arms are used.

Study on Bearing Strength and Failure Modes of Single Bolted Joint Carbon/Epoxy Composite Materials.

The growth of the Urban Air Mobility (UAM) industry emphasizes the need for considerable study into assembly procedures and dependability to guarantee its effective integration into air transport networks. In this context, this study seeks to evaluate the mechanical characteristics of bolted joint Carbon Fiber Reinforced Plastic (CFRP), with a particular emphasis on bearing strength. By altering the w/D (specimen width to hole diameter) and e/D (distance between hole center and specimen end to hole diameter) ratios, the study investigates how edge and end distances affect material performance. The study discovered a shift from tension to bearing failure at w/D ratios of 4.0, with maximum bearing strength decreases of 90.50% and 69.96% compared to full bearing failure. Similarly, for e/D ratios of 1.5, 2.0, and 3.0, transitioning from shear to bearing failure at 2.0 resulted in maximum bearing strength losses of 94.90% and 75.96%, respectively. Maintaining a w/D ratio of at least 6.0 and an e/D ratio of at least 3.0 is critical for maintaining maximum performance and stability in CFRP structure design.

Improvement of the Cracking Moment-Based Asphalt Mixture Splitting Test Method and Splitting Strength Research

The asphalt mixture splitting test is one of the most important methods for measuring asphalt’s tensile properties. To characterize the limitations of the traditional splitting test and the influence of the specimen size and loading conditions on the accuracy of the test, the factors affecting the strength of the splitting test were analyzed to reveal the splitting failure state and establish a unified representation model between the splitting and direct tensile tests. Initially, the moment of specimen cracking was taken as a key indicator, combined with image processing technology, to establish a set of criteria to judge the splitting test. Subsequently, standardized splitting tests were conducted and compared to tests of different specimen sizes and loading methods. Based on the octahedral strength theory, the stress points before and after the improved test were compared to the existing failure criteria. Direct tensile and splitting tests were conducted at different rates, and a unified strength–rate function model was established, realizing the unified representation of direct tensile and splitting tests. The research results indicate that the standardized splitting test is prone to the phenomenon wherein the specimen end face cracks before the center, affecting the accuracy of the test and potentially leading to redundant material strength evaluations. Using a loading method with a “35 mm specimen thickness” and a “0.3 mm rubber gasket + 12.7 mm arc-shaped batten” can essentially achieve the test hypothesis of “cracking at the center first”, resulting in less discrete outcomes and closer alignment to the three-dimensional stress failure state. The tensile and splitting strengths are both power function relationships with the rate as the independent variable, establishing a unified function model of the tensile and failure strengths. The research provides a more reliable testing method and calculation model for asphalt pavement structure design, and it also provides an effective basis for the improvement of splitting tests on materials such as concrete and rock.

Dynamic Splitting Performance and Energy Dissipation of Fiber-Reinforced Concrete under Impact Loading.

In this paper, the influence of different fiber materials on the dynamic splitting mechanical properties of concrete was investigated. Brazil disc dynamic splitting tests were conducted on plain concrete, palm fiber-reinforced concrete, and steel fiber-reinforced concrete specimens using a split Hopkinson pressure bar (SHPB) test device with a 100 mm diameter and a V2512 high-speed digital camera. The Digital Image Correlation (DIC) technique was used to analyze the fracture process and crack propagation behavior of different fiber-reinforced concrete specimens and obtain their dynamic tensile properties and energy dissipation. The experimental results indicate that the addition of fibers can enhance the impact toughness of concrete, reduce the occurrence of failure at the loading end of specimens due to stress concentration, delay the time to failure of specimens, and effectively suppress the expansion of cracks. Steel fibers exhibit a better crack-inhibiting effect on concrete compared to palm fibers. The incident energy for the three types of concrete specimens is roughly the same under the same impact pressure. Compared with plain concrete, the energy absorption rate of palm fiber concrete is decreased, while that of steel fiber concrete is increased. Palm fiber-reinforced concrete and steel fiber-reinforced concrete have lower peak strains than plain concrete under the same loading duration. The addition of steel fibers significantly impedes the internal cracking process of concrete specimens, resulting in a relatively slow growth of damage variables.

The Effect of Buckling Restraint on Axial Strain Life Fatigue Data

Abstract Focused on the common problem of easy specimen buckling in the axial strain–controlled fatigue testing of thin automobile steel sheets, a strain fatigue test method for thicknesses less than 2.5 mm is proposed. Because of the unrestrained grip end of specimens between the anti-buckling device and the testing machine fixture, specimens were prone to buckling in this area. In addition, if the designed specimen sizes were unreasonable, buckling may have occurred in the width and thickness directions of the gauge length of the specimen. Both situations can lead to invalid tests. This paper adopted a testing machine fixture and an anti-buckling plate as a mortise-and-tenon structure for the anti-buckling method; the upper and lower fixtures of the testing machine were cut in straight grooves, and the left and right anti-buckling plates were designed to be connected with a mortise and tenon. This is equivalent to narrowing the width of the unrestrained part of the specimen’s grip end to avoid buckling failure due to plane strain. The factors affecting the stiffness of fatigue specimens are also discussed herein. In order to avoid common instability in the thickness and width directions of the gauge length and unconstrained part of the specimen’s grip end at a strain ratio of Re = −1 during testing, six recommended specimen sizes are given on the premise of ensuring maximum specimen stiffness. When using the anti-buckling device, the recommended specimen sizes and test points of this study (i.e., the lower limits for yield strength and thickness of strain control testing) are 110 MPa and 0.5 mm, respectively, with which continuous and smooth stress–strain curves that result in accurate and reliable experimental data can be obtained.

Evaluation of Fatigue and Impacted bond Strengths of Denture Base Repaired by Using Different Type of Surface Treatment

Objective: Since the 1930s, a variety of resins have been introduced into dental treatments for the construction of dental prostheses and their efficacy has been based on physical, chemical and biological properties. Denture breakage is usually related to faulty design, faulty fabrication, and/or poor materials choice. Purpose: of this study is to investigate (in vitro) the fatigue bond strength and impact bond strength of the denture repaired by light-cured acrylic resins by using different chemical solvents such as acetone, monomer and thiner. Materials and methods: The acrylic resin used are heat-cure, specially designed molds are use to prepare 6 groups of specimens following the manufacturer's instruction (5 specimens for each cured polymerization). Specimens were cut in guiding by standardized positional jig. The ends of specimen saturated by different solvents acetone, thiner and monomer before repairing. To evaluate the impact strength, plastic strips were fabricated as per the dimensions (50×5×4) mm. Alternating bending fatigue machine was used to test the ten samples with the dimension of (70102.5) mm. Result: impact bond strength with monomer solvent higher than thiner solvent and acetone solvent. Fatigue bond strength with acetone solvent higher than thiner solvent and monomer solvent. Acetone, thiner and monomer were applied as solvents, the results of all surface treatments revealed significant difference at (P- value <0.05) in mean value (no significant for fatigue bonde strength but significant with impact bond strength.

The influences of international standards on structural performance and fracture behaviour of 3D printed long fiber composite structures

The material properties of the additive manufactured composite structures are usually done following international standards, that show some difference dimensionally. This study focuses on the impact-induced on the tensile properties of the 3D printed continuous carbon fiber reinforced composite part due to the standards applied for sample preparation. The specimens were fabricated by Markforged® Mark two 3D printing machine using carbon fiber as reinforcement and Onyx® as matrix material based on ASTM D638 and ASTM D3039-D3039M standards. The experimental results revealed that the specimens fabricated based on ASTM D638 showed a premature failure at the location where the straight gauge section of the specimen ends, and the curved transition regions begin due to stress concentration. The tests based on ASTM D3039-3039M standard showed better tensile strength and less stress concentration compared to ASTM D638. Fracture test with SEM reveals fiber breakage, debonding, and fiber pullout, which created cavities and voids between layers as the reasons for the tensile failure.

Effect of Loading Direction on Deformation and Strength of Heterogeneous Paleo Clay Samples

Landslides result from weak surfaces with varying rock-soil properties, posing a significant concern for engineering and accurate deformation analysis. This study investigated the macroscopic physical and mechanical properties of paleo clay specimens during triaxial compression testing, aiming to elucidate the deformation mechanisms exhibited by these specimens under varying loading directions at both the loading and unloading ends, and numerical simulation methods were carried out to simulate actual engineering scenarios. The analysis encompasses deformation patterns, stress–strain relationships, Mohr stress circles, and numerical simulation failure cloud diagrams for soil samples under different loading directions. The results showed that the loading end of heterogeneous specimens exhibited noticeable deformations. Alteration of the loading direction induced variations in the failure mode. The position and size of the deformations for the only iron-manganese clay, loading end iron-manganese clay, and loading end reticulated clay samples changed with the clay layer at the loading end of the sample. Moreover, the stress–strain curves under different loading directions were different, with strain hardening and strain softening appearing in the two loading directions, respectively. The results of this study contribute to an in-depth understanding of the impact of the loading direction on the deformation and strength of paleo clay, thereby providing a foundation for landslide prevention and control measures.

Residual Stresses in Cold‐Formed Square Hollow Sections of High Strength Steel

AbstractIn this study, the residual stresses were experimentally studied for cold‐formed square hollow section (CFSHS) with dimensions of 150×150×10 mm. The residual stresses were measured in both longitudinal and transverse directions for S355J2H/S420MH and S700MLH tubular sections. The results showed that measuring the longitudinal residual stresses on the outer and inner surfaces at a distance of 60‐70 mm from the end of the tubular specimen is an accurate and convenient method. For both steel grades studied, the highest residual stresses on inner and outer surfaces were on the flat regions close to corners. On the face opposite to the weld seam, the residual stresses scattered clearly among different repetition tests and differed from values reported in the literature. The influence of the steel grade on the residual stresses was more observable on the faces with the weld and opposite to the weld. As on the face with the weld, the combined effect of cold forming and temperature affect residual stresses, it is recommended to quantify residual stresses separately for the face with the weld and other faces. In the flat regions, the longitudinal residual stresses were so dominating that the transverse residual stresses can be neglected. The scatter of the longitudinal residual stresses on the face opposite to the weld and the irregular values of the transverse residual stresses in the corners indicate that additional tests are needed to quantify the randomness characteristic of local residual stresses.

Determination of Final Strand Slips of Prestressed Precast Hollow-Core Slabs Subjected to Flexural Load Using Machine Learning Algorithms

Precast prestressed concrete hollow-core slabs (HCUs) are structural elements with less self-weight, providing improved structural effectiveness in withstanding the straining action and allowing for a long span. This study investigated the additional strand slips and developed machine learning (ML) models for evaluating the final strand slips (Śf) of the precast HCUs. Two groups of HCUs, with nominal widths of 1.2 m and 0.55 m, were subjected to flexural loading conditions. One sample from each group was selected to form composite specimens by casting a concrete topping slab, and the restrain mechanism was attached at the ends of the additional HCU specimens. The experimental datasets used to train the ML models, including the support vector machine (SVM), multi-linear regression (MLR), and improved eliminate particle swamp optimization hybridized artificial neural network (IEPANN) models for the prediction of Śf. The efficacy of the IEPANN model compared to the nonlinear predictive models was evaluated, and the performances of the developed ML models were checked using the evaluation matrices. The results indicated that the prestressing strands with relatively higher initial strand slips may result in larger additional slips during flexural loading. The restraining mechanism and cast-in-place topping slab influenced the additional strand slip rate. The hybridized IEPANN model outperformed other classical models in estimating the additional slips with the R2 values greater than 0.9 in the two modelling stages, indicating the efficacy of the IEPANN compared to the nonlinear predictive modes.

Use of bending test to determine the tensile strength and elastic modulus of GFRP bars

Fiber-reinforced polymer (FRP) composite bars have been increasingly employed to reinforce concrete members when the use of traditional steel bars would represent an issue for the durability of the structure. In fact, FRP bars have high strength-to-weight ratio, do not suffer of corrosion, and do not conduct electricity. Mechanical characterization of composite bars is complex due to the remarkable difference between bar properties in the direction parallel and orthogonal to the fiber. Therefore, direct tensile testing of FRP bars requires proper specimen preparation, which usually involves expensive bonding of metal pipes at the specimen ends. In this paper, a 3-point bending test is proposed as a possible alternative to the direct tensile test to obtain a quick and reliable estimate of the bar tensile strength and elastic modulus. An analytical model that accounts for bar shear deformation and large displacements is used to describe the specimen bending behavior. Forty bending tests of glass FRP (GFRP) bars with thermosetting or thermoplastic resin and four different diameters are presented and used to validate the test set-up and analytical model proposed. The results are compared with those of corresponding direct tensile tests showing the validity of the proposed bending test.

Detection of Anomalies in Additively Manufactured Metal Parts Using CNN and LSTM Networks

The process of metal additive manufacturing (AM) involves creating strong, complex components by using fine metal powders. Extensive use of AM methods is expected in near future for the production of small and medium-sized batches of end-use products and tools. The ability to detect loads and defects would enable AM components to be used in critical applications and improve their value. In this study, the Surface Response to Excitation (SuRE) method was used to investigate wave propagation characteristics and load detection on AM metallic specimens. With completely solid infills and the same geometry, three stainless steel test bars are produced: one conventionally and two additively. To investigate the effect of infills, four bars with the same geometries are 3D printed with triangular and gyroid infills with either 0.5 mm or 1 mm skin thickness. Two piezoelectric disks are attached to each end of the test specimens to excite the parts with guided waves from one end and monitor the dynamic response to excitation at the other end. The response to excitation was recorded when bars were in a relaxed condition and when compressive loads were applied at five levels in the middle of them. For converting time-domain signals into 2D time-frequency images, the Short-Time Fourier Transform (STFT) and Continuous Wavelet Transform (CWT) were implemented. To distinguish the data based on fabrication characteristics and level of loading, two deep learning models (Long Short-term Memory algorithm (LSTM) and Convolutional Neural Networks (2D CNN)) were utilized. Time-frequency images were used to train 2D CNN, while raw signal data was used to train LSTM. It was found that both LSTM and 2D CNN could estimate solid parts' loading level with an accuracy of more than 90%. In parts with infills, CNN outperformed LSTM for the classification of over five classes (internal geometry and loading level simultaneously).

Optimization of parameters in rotary friction welding process of dissimilar austenitic and ferritic stainless steel using finite element analysis

Solid state joining techniques are increasingly used in joining different types of materials, in this work TP347HFG austenitic stainless steel will be welded with HiPerFer ferritic stainless-steel alloy due to their similarity in their properties to some extent after making heat treatment to ferritic alloy in addition to difference in their cost which save for economic reasons. In this work, a three-dimensional finite element model was developed. The thermal analysis and profiles between process parameters were predicted by using an ANSYS software tool. Design of experiments by Anova was used to examinate the simulation results and to evaluate contribution ratio of each parameter on responses, while grey relational analysis was used to specify the optimum trial. It was observed that trial 11 give best results (A4B2C1D1) which got higher grey relational grade. It was concluded that the friction pressure and friction time have more impact on interface temperature, whereas forging pressure and friction time affect the equivalent Von-Mises stress directly. The speed and forging pressure have a more influence on total deformation. It was also concluded that the temperature was fade out by diverging toward the end of specimen and the amount of decrease in temperature is less in ferritic side. The amount of flash extruded from both steels are approximately similar due to similarity in properties as heat treatment of ferritic alloy had been achieved. The investigation results show that trial 8 achieved highest Von-Mises stress (300 MPa) while trial 7 induced lower stress (255 MPa), maximum deformation was found equivalent to 0.012 mm with trial 16 whereas, it was found (0.008 mm) with trial 11 which were both less than the allowable deformation in end application. Tensile strength of weld joint was found equivalent to 80% of softer base metal. The hardness of optimum trial was equivalent to 156 HB.

Investigation of the crack evolution characteristics of coal and rock bodies around boreholes during progressive damage based on stress threshold values

The stress threshold is a key stress indicator of crack evolution of coal and rock bodies around boreholes during progressive damage. To investigate the crack evolution characteristics of coal and rock bodies around boreholes, digital speckle correlation method tests were conducted on intact specimens, specimens with boreholes, and specimens with grouted boreholes, and the characteristic parameters, such as the stress threshold, surface deformation field, crack opening and shear, and strain energy density, were quantitatively analyzed. The results show that (1) the distribution characteristics of surface cracks on the specimen are affected by the borehole structure and grouting; (2) the crack displacement evolution of coal and rock specimens remain basically stable at the initial stage of loading. In the middle and later stages, the cracks are continuously opening and shearing, which directly leads to the damage of the specimen; (3) the energy accumulation and release law of coal and rock bodies is related to the evolution of localization zone. In different areas outside the localization zone, the strain energy density shows different characteristics; (4) the tensile crack is the main reason for the destruction of coal and rock bodies around boreholes. During the progressive damage, the tensile cracks at the upper and lower ends of specimen experience the process of opening, closing and reopening. The presence of cohesion g(x) reduces the crack tensile stress intensity factor, so that the crack is in a balanced state before penetration, which is conducive to the connection and penetration of other cracks. The results provide theoretical guidance for the optimization design of gas extraction boreholes construction and grouting.

Transmission of SARS-CoV-2 among recruits in a US Army training environment: a brief report.

In 2020, preventive measures were implemented to mitigate the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) among 600-700 recruits arriving weekly at a basic combat training (BCT) facility in the southern United States. Trainees were sorted into companies and platoons (cocoons) at arrival, tested, quarantined for 14days with daily temperature and respiratory-symptom monitoring and retested before release into larger groups for training where symptomatic testing was conducted. Nonpharmaceutical measures, such as masking, and social distancing, were maintained throughout quarantine and BCT. We assessed for SARS-CoV-2 transmission in the quarantine milieu. Nasopharyngeal (NP) swabs were collected at arrival and at the end of quarantine and blood specimens at both timepoints and at the end of BCT. Epidemiological characteristics were analyzed for transmission clusters identified from whole-genome sequencing of NP samples. Among 1403 trainees enrolled from 25 August to 7 October 2020, epidemiological analysis identified three transmission clusters (n=20 SARS-CoV-2 genomes) during quarantine, which spanned five different cocoons. However, SARS-CoV-2 incidence decreased from 2.7% during quarantine to 1.5% at the end of BCT; prevalence at arrival was 3.3%. These findings suggest layered SARS-CoV-2 mitigation measures implemented during quarantine minimized the risk of further transmission in BCT.

Seismic performance of precast double-column pier with UHPC-filled socket connections

Precast piers have been extensively investigated and applied in low seismic intensity areas, owing to the faster construction speed, limited traffic interruption, and environmental friendliness. However, few studies and engineering practices of the precast double-column piers (PDPs) located in high-intensity seismic areas have been reported. Therefore, in this study, a novel socket connection filled with ultra-high-performance concrete (UHPC) was proposed for the PDPs. The socket connections with concrete filled steel tube (CFST) are used to enhance the strength of the column-cap beam joints, at the same time the socket connections with the tooth-shaped shear keys are adopted at the column-footing joints to reduce the required anchorage length. The PDP with a scale ratio of 1:3 was constructed and then tested under the quasi-static load to investigate its seismic performance. Finally, a simplified analytical model of the socket connections was proposed based on the concept of the elastic foundation beam. The test results demonstrate that the damage of the specimen PDP was mainly concentrated at the bottom of the columns, and the damage at the damage ends of the PDP specimen was severer than that of the cast-in-place (CIP) specimen. The load-bearing capacity of the PDP is slightly higher than that of the corresponding CIP specimen, while the energy dissipation and displacement ductility of PDP are slightly smaller than those of the CIP specimen. The proposed model could accurately and efficiently capture the hysteretic behavior of the proposed PDP with socket connections.