Sustained Load Research Articles

Effect of creep on the strength of high strength concrete-filled steel tubes

High strength concrete filled steel tubes (HSCFSTs) are being increasingly used as columns in tall buildings, bridges, and other critical infrastructure and complex structural systems. Very little research has been done to study the performance of these columns under long-term sustained loads where creep effects take place, and no study exists on the long-term performance of these columns under eccentric sustained loads at the large scale. Six large scale (2700 mm height) HSCFSTs were tested in this paper under sustained eccentric compression for a period of five months. After that, the columns were tested to failure and their performance was compared to another six companion HSCFSTs that were kept unloaded for the same time period. A decrease of about 13% in the load carrying capacity was observed after creep. It was also observed that creep significantly changed the force distribution between steel and concrete. A study of the force redistribution showed that the force carried by the steel increased by >50% of its initial value.

Short- and Long-Term Prestress Losses in Basalt FRP Prestressed Concrete Beams under Sustained Loading

The favorable mechanical properties of basalt fiber–reinforced polymer (BFRP) bars, such as their excellent strength-to-weight ratio, resistance to corrosion, lower environmental impact, and electromagnetic neutrality, make them attractive for internal reinforcement of concrete elements with specific service requirements. Prestressing has emerged as a possible method for limiting the deflections and cracking of BFRP-reinforced flexural RC elements. However, long-term behavior of such structural members has not yet been investigated extensively. Therefore, this paper aims to give information on long-term behavior and prestress losses of pretensioned BFRP-reinforced concrete beams based on the collected experimental data. The testing program includes long-term analysis of six BFRP concrete beams pretensioned to different prestress levels—namely, 20%, 30%, and 40% of the ultimate tensile capacity of the bars. The monitored testing stages included initial tensioning of the bars, casting and curing of concrete, transfer of prestressing force to the concrete, long-term unloaded phase after transfer, sustained-loading application, long-term sustained loading phase, unloading, and final destructive testing, conducted in a controlled indoor environment. During all phases of the testing, strain levels in the BFRP bars and deflections were continuously monitored. The results of continuous strain monitoring show that the average loss of strain over the period of initial 90 days of unloaded monitoring was 7% of the initial strain. During the following 6 months of monitoring under sustained loading, an additional 0.3% reduction was recorded on average. The loss was dependent on the initial strain.

Evaluating Strain Limits of Embedded FBG Sensors in Pultruded Hybrid Composite Rods

This paper demonstrates that relatively inexpensive acrylate coated FBGs can survive short exposure times to high temperatures in a pultrusion process and monitor relatively low magnitude bending and axial tension strains in a composite material. However, under higher sustained axial loads, the observed interfacial failure of the optical fiber and the host material resulted in a significant reduction of the indicated strains during constant elongation. These findings are analyzed by experimental measurements supported by finite element simulations. The optimal placement of embedded optical fibers in the rods is discussed based on the limitations of the rods and the mechanical strength of the commercially available optical fibers.

Short-term creep behaviors of seawater sea-sand coral aggregate concrete: An experimental study with Rheological model and neural network

Appropriate utilization of wasted coral aggregates, seawater and sea-sand can effectively resolve the shortage of raw materials for offshore projects and shorten the construction period. However, the low strength and porous nature of coral coarse aggregate will affect the creep properties of seawater sea-sand coral aggregate concrete (SSCAC) under short-term sustained load and this study aimed to investigate the short-term creep behaviors of SSCAC. A total of 72 SSCAC specimens were prepared and subjected to sustained load to analyze the influence of curing age, load duration and applied load level on the short-term creep behaviors of SSCAC; meanwhile, the Rheological model based on the Boltzmann superposition principle and the artificial neural networks (ANN) model were introduced to predict the short-term creep of SSCAC. The experimental results indicated that the creep and hysteretic recovery strain of SSCAC increased with the load duration and applied load level due to its viscoelastic property and the internal micro-damage accumulation, while decreased with the curing ages owing to the cement hydration. Three classical models, ACI209R, B3, GL2000 as well as the present Rheological model, ANN model were compared with the test results. The present models in this study correlated more well with the test results than classical ones. In addition, the creep deformation of SSCAC is 56.1 % and 48.4 % higher than that of ordinary concrete at 3 days curing age and 28 days curing age, respectively. Moreover, the fundamental time-dependent strains of SSCAC could reach its peak within minutes and gradually kept stable during load duration, which implies that the partial hysteretic recoverable deformation gradually transform into unrecoverable deformation with the accumulation of micro-damage in SSCAC. The overall results clearly characterize the short-term creep behaviors of SSCAC and demonstrate that the Rheological model and ANN model can both provide reliable prediction results for the fundamental time-dependent strain of SSCAC.

Assessment of Spinal Stability after Discectomy Followed by Annulus Fibrosus Repair and Augmentation of the Nucleus Pulposus: A Finite Element Study

Lumbar disc herniation (LDH) is a common condition which can lead to back pain. Although surgical treatments for LDH are well established, complications such as spinal instability and narrowing of adjacent facet joints are still frequently reported. The purpose of this study was to use finite element models to evaluate the stability of the L3–L4 segment after conservative or aggressive percutaneous transforaminal endoscopic discectomy (PTED) with and without an artificial material filler to correct LDH. Compared to the intact model, aggressive PTED reduced the stability of the segment (increased ROM) and narrowed the space between facet joints in the medial/lateral (ML) direction during flexion (maximum 6.7 degrees change in ROM and 90.5% spacing between facet joints), extension (maximum 2.1 degrees and 38.6%), and axial rotation (maximum 4.2 degrees and 90.1%). Aggressive PTED had a similar effect in the anterior/posterior (AP) direction during lateral bending (maximum 2.0 degrees and 44.2%). Augmenting the nucleus pulposus with a polyurethane filler after aggressive PTED improved spinal stability in both the ML and AP directions in all simulated motions, with results similar to the intact model. However, using a hydrogel filler did little to stabilize the spine, likely because the material is too soft to support the heavy, sustained loading. In conclusion, this study found that if an aggressive discectomy is required, augmenting the nucleus pulposus with a PU filler provides sufficient support to stabilize the spine, while hydrogel fillers offer little support.

Experimental and numerical study on nonlinear basic and drying creep of normal strength concrete under uniaxial compression

The material behavior of concrete is highly nonlinear and time-dependent. In particular, the evolution of deformation due to creep is crucially dependent on the acting stress and the environmental conditions. Numerous experimental campaigns for determining the complex material behavior of concrete in various loading conditions and environmental conditions can be found in the literature. However, the experimental investigation of the time-dependent deformation due to high sustained loading, i.e., nonlinear creep, has received less attention. Especially, the evaluation of the impact of different environmental conditions on nonlinear creep is still pending. In this contribution an experimental study on basic and drying creep under moderate and high uniaxial compression is presented. The experimental results indicate that the nonlinearity of the deformation due to creep in high uniaxial compression is dependent on the environmental conditions. Additionally, the capability of selected constitutive models for representing nonlinear creep is assessed in a numerical study based on the novel experimental results.

Influence of sustained loading on resistance and deformation capacity of reinforced concrete members in compression

AbstractAs acknowledged explicitly or implicitly in most of current design codes, a high level of sustained loading has a detrimental effect on the concrete compressive strength. However, its beneficial effect in terms of deformation capacity is neglected in most cases. This latter topic and its practical consequences are addressed in the present paper on the basis of an experimental and theoretical investigation. In particular, the development of nonlinear creep strains in structural concrete members is investigated, clarifying the internal redistribution of forces between concrete and the reinforcement. The results of an experimental program with refined measurements are presented to better understand the phenomenon and to verify the predictions of a mechanical model developed by the authors on the time‐dependent response of concrete. The experimental program consists of 14 prismatic specimens with different reinforcement ratios tested under a wide range of uniaxial stress rates. The results allow clarifying the material response and validating the mechanical model, both in terms of strength reduction and enhancement of deformation capacity. Finally, the results of parametric analyses, performed considering different concrete ages, reinforcement ratios, and materials properties are presented to evaluate the practical implications of the findings.

Mass transport properties of recycled aggregate concrete under coupling the action of chloride salt attack and uniaxial tensile loading

Water and chloride permeability are two important parameters for the evaluation of recycled aggregate concrete (RAC) durability. In service, the tensile loading plays an extremely important role in the generation and propagation of microcracks inside the RAC. Thus, aiming at understanding the mass transport properties of RAC under the coupled action of chloride salt attack and uniaxial sustained tensile loading, an improved experimental set-up was self-designed. And then a series of tests involving the effects of different uniaxial sustained tensile stress levels (10%, 30% and 50% of ultimate tensile stress, namely λt = 0.1, 0.3 and 0.5) and different replacement ratios of recycled coarse aggregate (RCA) (0, 30%, 50% and 100% mass replacement of normal coarse aggregate, namely R0, R30, R50 and R100) on the transport properties of RAC were conducted, and their coupling effects were further analyzed. Generally, the results indicate that both the sorptivity and chloride diffusion coefficient of RAC increase with the increase of RCA replacement ratio and tensile stress level. The synergistic effect of RCAs and tensile stress significantly accelerates the process of water and chloride invasion into concrete. The initial sorptivity of R100 specimens at λt = 0.5 was about 4.44 times that of R0 specimens without loading. Moreover, the chloride diffusion coefficient exhibits a linear relationship with the RCA content, while it has a quadratic relationship with the tensile stress level.

Investigations on the influence of load width on web compression buckling strength of wide-flange steel members

A comprehensive study on web-compression buckling strength of wide-flange steel members subjected to concentric transverse loads spread over lengths longer than member section depth is presented in this paper. Calculation methodologies previously developed for web-compression-buckling strength of wide-flange steel members were established for use in beam-to-column moment connections, where the load width is relatively very narrow compared to the depth of the section, therefore does not include load width as a parameter in strength equations. More recently developed calculation procedures that account for load widths are typically limited to member section depth and are merely based on numerical models, and lack experimental evidence that validates these models. This study presents experimental research on steel wide-flange members subjected to transverse compression loading by having the load length as the primary variable. Experiments were conducted on specimens with measured imperfection levels and provided results on the buckling strength, displacement, and strain variation of the web portion during testing. The experimental program also investigated the potential of creep under sustained loading on web compression buckling strength of wide flange members at load levels near buckling strength. Benchmark numerical models using the finite element method were developed using the test results to obtain insights into the observed behavior and were employed in conducting numerical parametric studies to investigate the sensitivity of results due to load width and the angle of the applied load for various cross sections. The results obtained from the experimental and numerical studies were compared with the calculation methods provided in the North American and European design codes to evaluate their accuracy. The parametric study results were also used to develop an analytical calculation procedure for estimating the web-compression-buckling strength of steel wide-flange shapes subjected to vertical and inclined loading.

Revamping Optimization of a Pressure Piping System Using Artificial Neural Networks

The paper proposes a new methodology for revamping design and optimization of a process piping system. Starting from ASME B31.3 Process Piping prescriptions for stress analysis, a nonlinear model is built to express the relationship between stress distribution generated by expansion and sustained loads (pressure, weight) and the geometry and routing of the pipeline, focusing on geometric parameters of expansion loops. The number of design variables affecting stress distribution over the pipe, together with the constraints to be respected, would make it hard to formulate an optimization procedure based on deterministic methods. This problem is overcome by applying a Feed Forward Neural Network, backpropagation trained, which makes it possible to interpolate a non-linear and multidimensional relation over a domain enclosed within the boundaries of a training set. Prediction of code stresses is obtained through the fitting of an artificial neural network for each examined loadcases. Network parameters are tuned offline, starting from a set of data obtained by finite element numerical simulation. As a result, an optimal geometry for expansion loops is found, allowing to revamp pipe routing by halving loops number and keeping code stress within the allowable limits.

Omicron Variant Generates a Higher and More Sustained Viral Load in Nasopharynx and Saliva Than the Delta Variant of SARS-CoV-2.

The Omicron variant of SARS-CoV-2 spreads more easily than earlier variants, possibly as a result of a higher viral load in the upper respiratory tract and oral cavity. Hence, we investigated whether the Omicron variant generates a higher viral load than that of the Delta variant in saliva and nasopharynx. Both specimens were collected from 52 Omicron and 17 Delta cases at two time points one week apart and analyzed by qRT-PCR. Viral load was measured as 10 log RNA genome copies per 1000 human cells according to the WHO reference standard. We found that Omicron cases carried a higher viral load and had more sustained viral shedding compared to the Delta cases, especially in the nasopharynx.

Creep damage coupling model of concrete based on the statistical damage theory

The creep behavior is one of the key issues in the design and construction of hydraulic concrete structures. However, it is an enormous challenge to accurately predict the development of creep deformation in concrete structures. Since the creep deformation behavior of concrete under long-term load action originates from the interaction between creep and damage, a new creep damage coupling model was developed by introducing the damage variable into the creep constitutive equation. This model utilized the statistical damage theory framework and assumed that the damage variable follows the random distribution described by the Weibull function. Then Weibull distribution parameters F0 and m were derived to determine the evolution equation of the damage variable D. Considering stress under complex stress conditions may change, the stress-strain correlation of concrete under complex stress conditions was deduced using the elastic creep theory. Moreover, the results of uniaxial compressive concrete creep tests with the corresponding numerical simulations under different levels of sustained loads, and the accuracy and applicability of the creep damage coupling model were verified. Consequently, the nonlinear creep properties of hydraulic concrete were analyzed by combining it with acoustic emission techniques.

SARS-CoV-2 Variant-Specific Infectivity and Immune Profiles Are Detectable in a Humanized Lung Mouse Model.

Small animal models that accurately model pathogenesis of SARS-CoV-2 variants are required for ongoing research efforts. We modified our human immune system mouse model to support replication of SARS-CoV-2 by implantation of human lung tissue into the mice to create TKO-BLT-Lung (L) mice and compared infection with two different variants in a humanized lung model. Infection of TKO-BLT-L mice with SARS-CoV-2 recapitulated the higher infectivity of the B.1.1.7 variant with more animals becoming infected and higher sustained viral loads compared to mice challenged with an early B lineage (614D) virus. Viral lesions were observed in lung organoids but no differences were detected between the viral variants as expected. Partially overlapping but distinct immune profiles were also observed between the variants with a greater Th1 profile in VIDO-01 and greater Th2 profile in B.1.1.7 infection. Overall, the TKO-BLT-L mouse supported SARS-CoV-2 infection, recapitulated key known similarities and differences in infectivity and pathogenesis as well as revealing previously unreported differences in immune responses between the two viral variants. Thus, the TKO-BLT-L model may serve as a useful animal model to study the immunopathobiology of newly emerging variants in the context of genuine human lung tissue and immune cells.

Numerical simulation of the compression crushing energy of carbon fiber-epoxy woven composite structures

The crashworthiness of composites is investigated in an experimental campaign and simulated using the explicit Finite Element solver ESI-VPS. Two damage models, derived from Ladevèze and Waas-Pineda, are implemented in modified forms, to work with woven-fabrics. The former uses conventional continuum damage formulations, while the latter introduces a traction-separation law to simulate cracks with prescribed fracture energies. The Waas-Pineda approach aims to simplify the material card definition and make the calibration more straightforward. Characterization tests are carried out on specimens manufactured with plain weave carbon fiber-epoxy tape. Notched coupons are analysed, as these tests are representative of the conditions observed during crushing of a composite structure. Innovative procedures to calibrate the damage models are introduced to remove, where possible, the need for iterative tuning. Once implemented, the calibrated material cards are validated against quasi-static crushing of corrugated specimens with two different layups. In the Waas-Pineda model, mesh objectivity is obtained only after correction of the modal fracture energies, instead, the Ladevèze model formulation is already consistent at different scale lengths. In addition, a sensitivity analysis shows a modest influence of friction coefficient and interlaminar fracture toughness on sustained load. Results demonstrate that it is possible to obtain adequate results from both damage models when the proposed calibration strategy is used, but validation on test results is still advised to ensure the mesh discretization is suitable for the selected problems.

Investigating the Deflection and Strain of Reinforced Green Concrete Beams Made With Partial Replacement of RCA under Sustained Loading

Artificial intelligence (AI) and statistical methods are used in various fields and have played a vital role in investigating the deflection and strain of reinforced green concrete beams made with partial replacement of recycled concrete aggregates under sustained loading. The methods used to assess structural contributors are time-saving and cost-effective compared to experimental evaluation. This study investigated the numerical modeling of reinforced concrete beams produced by replacing 50% of coarse natural aggregates with demolished vintage concrete under sustained loading. Multivariate regression analysis was used to determine the mathematical equations for long-term deflection and stress from experimental data of 6, 9, and 12 months of loading. Three software suites were used for the regression analysis, namely NCSS, Matlab, and Microsoft Excel. Six beams were cast using demolished concrete as 50% of coarse aggregates to test and validate the regression equations, where three of them were examined for two months of sustained loading and the other three for three months. The regression results were in accordance with the experimental observations with a maximum error of 10.34%. Therefore, the provided regression equations for deflection and pressure could be used to estimate the parameters of reinforced concrete beams.

Time-Dependent Behavior of Reinforced Concrete Columns Subjected to High Sustained Loads

Time-Dependent Behavior of Reinforced Concrete Columns Subjected to High Sustained Loads

Nonlinear analysis of reinforced concrete slabs under high-cyclic fatigue loading

Infrastructures are frequently vulnerable to sustained cyclic loads and structural vibration. The accumulated cyclic stresses will induce fatigue in the structures and contribute to their inadequate service lifespan. Consequently, analyzing the present structural health status by the structural stiffness measurement is crucial. This study investigated the fatigue performance and dynamic progressive damage behavior of reinforced concrete (RC) slabs under high-cyclic fatigue loadings using nonlinear finite element (FE) analysis. A new model was recommended for predicting the concrete's residual strength. The accuracy of the suggested model and the FE simulation was validated by comparing the predicted natural frequencies, mode shapes, residual strength, and crack characteristics of specimens with the experimental results. Finally, a novel model for determining the dynamic stiffness of RC slabs was developed. Results showed that the cumulative degradation of the natural frequency, stiffness, and damage development of steel rebars and concrete increased as the fatigue loading cycle increased, indicating that the dynamic response and fatigue damage for RC slabs in the later stages of high-cyclic fatigue loading were more severe. Additionally, the RC slab's natural frequency was reduced rapidly during the initial steps of fatigue and, after that, slowed noticeably. Validation of the dynamic stiffness model demonstrated its capability in predicting the stiffness of fatigued and damaged RC slabs. The results provide practical insights for analyzing the dynamic behavior of existing structures by considering the nonlinear progressive damage and may improve the efficiency of structural damage detection.

Time-dependent modelling of steel-tube confined concrete columns considering confining and bonding effects

Steel tube confined reinforced concrete (STCRC) columns have been applied in high-rise buildings with a promising future due to its high load-bearing capacity and excellent seismic behaviour. Different from traditional reinforced concrete (RC) and concrete-filled steel tubular (CFST) columns, the concrete of STCRC columns is subjected to triaxial time-dependent varying stresses even at low sustained loads, and the stresses of the tube and concrete are non-uniformly distributed along the column length due to the interfacial bonding effects. In this context, available long-term research outcomes for RC or CFST columns cannot be implemented to STCRC columns directly. Hence, a confined concrete creep model for STCRC columns was determined based on the collected 56 creep tests on sealed confined concrete under multiaxial stress states. Subsequently, accounting for the time-dependent varying confinement effects and the influence of the non-uniform interfacial stress distribution, a refined numerical analysis method was established for the long-term behaviour prediction. A parametric study was then conducted with particular attention to the difference in the long-term behaviour of STCRC, RC and CFST columns and the determination of the key parameters. Results indicate that the uniaxial creep model (i.e. Eurocode 2) can be extended to the creep prediction of confined concrete with the effective Poisson's ratio of 0.187. Due to the longitudinal bonding effects, the slenderness ratio would significantly influence the long-term deformation of STCRC columns. The influence of the steel tube on the time effects is less noticeable when compared with the concrete strength, loading age and slenderness ratio.

Flexural creep of carbon-TRC beams

This work aims to investigate the behavior of textile reinforced concrete (TRC) beams subjected to sustained loads. Four-point bending tests were carried out in members with I-sections and made with textiles with various coating conditions. The testing protocol included pre-cracking, 3-month creep, 10-day recovery and post-creep monotonic up to failure. Deflections and crack widths were measured during each stage and an analytical model was used to estimate the individual responses of textile and matrix-reinforcement bond to sustained loads. The results indicate that the creep deformation of styrene-butadiene rubber coated textile is by far greater than its sand-coated counterpart and textiles made of acrylate and epoxy. It is also seen that the increases in textile compliance were not accompanied by increases in crack width, as a consequence of an apparent bond improvement at early ages. Finally, no loss of capacity was observed in post-creep tests.

Experiences of recently HIV-diagnosed gay and bisexual migrants in Australia: Implications for sexual health programmes and health promotion.

Gay and bisexual migrants from low- and middle-income countries living in high-income countries are disproportionately diagnosed with HIV. Most research focuses on preventing HIV acquisition among HIV-negative migrant gay and bisexual men (GBM). This study is uniquely positioned to report on migrant GBM's experiences and needs at and after an HIV diagnosis. Semi-structured interviews were conducted with 24 migrant GBM diagnosed at sexual health clinics in Australia from 2017 onwards. Interviews were analysed using a codebook thematic analysis. Due to the stigma of HIV and homosexuality in their countries of origin, about half of participants had poor HIV knowledge prior to diagnosis. Absorbing diagnosis information was consequently difficult, and feelings of shame, hopelessness, lost sexual opportunities and infectiousness were common. However, many were thankful for the comprehensive clinical support they received and believed that over time life would 'normalise' with sustained undetectable viral load. None reported that their clinician stigmatised them, but the anticipation of stigma nonetheless infused their experiences after diagnosis. Many were selective about HIV disclosure, and some mentioned that clinic systems posed a risk to confidentiality. Non-permanent residents were concerned about the impacts of HIV status on future visa applications. We recommend that newly HIV-diagnosed migrant GBM receive referral to legal and culturally appropriate migration services to help absorb what a diagnosis might mean for their health and visa status. We also recommend sexual health clinics continue to assess confidentiality in their systems. Health promotion initiatives should highlight to migrant GBM that high-HIV caseload sexual health clinicians provide confidential and comprehensive care.