Search

At present, the design of geosynthetic-reinforced soil structures is executed with reference to the tensile strength of the reinforcement obtained from in-air short-term tensile tests, decreasing this value by means of several factors. Among these, the creep effect resulting from in-air tensile creep tests reduces tensile strength the most. Consequently, this procedure does not take into account the effects of soil confinement and interaction on the tensile response of the reinforcements. This paper illustrates a new large-scale pullout prototype apparatus, with the capacity to investigate the behaviour of a geosynthetic reinforcement embedded in a compacted soil and subject to a tensile load kept constant over time. The apparatus allows the verification of how the soil can modify the prediction of the long-term behaviour of geosynthetics. Results in terms of confined tensile strains were analysed, and the comparison of those values with the strains obtained by in-air tensile creep tests has led to the conclusion that the creep reduction factor might be conservative. Moreover, the confined tensile strains were related to the apparent coefficients of friction to propose a new procedure capable of determining the design interaction parameter under long-term pullout load as a function of the allowable reinforcement strains.

To study the seismic behavior of reinforced concrete (RC) columns serviced in coastal areas and strengthened with textile-reinforced concrete (TRC) after a dry-wet cycle coupled with a sustained load, low cycle loading testing was performed with six RC columns strengthened with TRC and one control column without TRC. The sustained stress ratio of the control column during the dry-wet cycle was 0.4 while those of the others strengthened with TRC were 0, 0.2, 0.4 and 0.6. The results show that although chloride dry-wet cycles decrease the ductility and energy dissipation capacity of strengthened columns, compared with the control column, TRC could still delay stiffness degradation and increase the ductility of strengthened columns to a certain extent. Additionally, with an increase in the sustained stress ratio, the energy dissipation capacity of the specimens decreases. The peak loads of the specimens gradually decrease with increasing sustained stress ratio during loading in the forward direction, and the variation in the peak load is irregular during loading in the negative direction. Besides, a reasonable proportion of the vertical load could improve the seismic capacity of unstrengthened specimens under chloride corrosion.

Concrete-Filled Steel Tubular Structures (CFSTSs) have become popular among the structural engineering community due to significantly higher load carrying capacity compared to conventional reinforced-concrete structures. Much research has been conducted on understanding the behavior of CFSTSs under various loading conditions and design theories have been established to predict the load carrying capacities of such structures. However, existing models do not consider the effects of sustained early loads on concrete strength and elastic modulus development of CFSTSs. With the need for rapid construction, CFSTSs may be subjected to loading at an early stage before concrete is fully cured. Such early loading may incur negative effects on strength and elastic modulus development of concrete within the confined environment. This paper propose theoretical models based on the compressive packing model (CPM) to simulate strength and elastic modulus development of early-age concrete under sustained stress. Development of concrete properties at early age is described using Hydration kinetics, and maximum paste thickness in the CPM model is modified using energy conservation to simulate sustained loads. Early concrete strength and the elastic modulus development rules were investigated experimentally for sustained loads. Predictions from the proposed models are compared with conventional models from CEB-FIP Model Code. Results showed that when loaded at a very early stage, a relatively high stress to strength ratio will result in causing damage in concrete. Such damage significantly affects the strength and elastic modulus development. Compared with concrete loaded at 28 days, concrete loaded at early stages showed significant reduction in concrete strength and elastic modulus.

One of the solutions to improve the flexural behavior of Glass fiber reinforced polymer (GFRP) reinforced concrete (RC) beams is the addition of tensile longitudinal steel reinforcement. The numerous studies to date on hybrid GFRP/steel RC elements have mainly focused on the static and short-term responses, very little work has been done regarding the long-term performance. This paper presents experimental results of time-dependent deflections of cracked GFRP and hybrid GFRP/steel RC beams during a 330-day-period in natural climate conditions. Three hybrid GFRP/steel and one GFRP RC beams with dimensions 100×200×2000 mm were tested in four-point bending. Different steel reinforcement ratios were used to evaluate the effect of the steel reinforcement on the long-term behavior of the beams. Experimental results show that the immediate deflections are inversely proportional to the additional steel reinforcement. With the same initial instantaneous deflection, the total deflection increases when increasing the steel reinforcement ratio. Also, temperature (T) and relative humidity (RH) significantly affect the long-term deflection of the tested beams. The measured long-term deflections were found to be in good agreement with the theoretical values calculated from the proposed method. However, there was an overestimation when using ACI 440.1R-15 or CSA-S806-12 procedures.

Prestressed concrete beams are widely used in construction, while they may be attacked by the corrosion medium during the service. Past research works show that coupled effect of high stress and corrosion can significantly deteriorate the performance of prestressed concrete structures. This article presents an experimental investigation on long-term behavior of prestressed concrete beams under the coupled effect of sustained load and corrosion. During the accelerated corrosion process, six prestressed concrete beams were subjected to different levels of sustained loads, and time-dependent prestress loss and concrete stains were recorded and analyzed. It is observed that the local corrosion (i.e. pitting corrosion) of the steel strands was significantly affected by the sustained load level, and the coupled effect led to more serious damage on the beams than individual sustained load or corrosion. Bending loads were finally applied to investigate the influence of corrosion on flexural capacities of these beams. It was found that the flexural capacities and failure mode of the beams were significantly influenced by corrosion. Meanwhile, the corrosion has more significant effect on tension strength of strands rather than bond strength.

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG [Abstract] The purpose of this research is to analyse the long-term deformations of reinforced concrete beams made of recycled coarse aggregates (RCA), using four different replacement ratios, 0%, 20%, 50% and 100%. For said purpose, three different loading stages were carried out: firstly, loading and sustained load for 1300 days, secondly, unloading and recovery where the deformations were registered over a one year period, and lastly, testing the concrete beams up to failure in order to analyse the effect of recycled aggregates on pre-cracked concrete members. The results led to the analysis of deformations under sustained load, as well as those produced after removing the load and the performance of pre-cracked members, while identifying the effect of recycled coarse aggregates on concrete performance in terms of plastic deformation, recoverability and concrete stiffness, and also on structural design. The study has been funded by: “CLEAM: Clean, efficient and nice construction along its life cycle” funded by the Centre for the Technology and Industrial Development (CDTI) and led by the Group of Economical Interest CLEAM-CENIT, AIE comprising by the country’s largest construction companies (Acciona, Dragados, Ferrovial, FCC, Isolux Corsán, OHL and Sacyr) and some PYME (Informática 68, Quilosa and Martínez Segovia y asociados). “HACCURACEM: HACCURACEM project (BIA2017-85657-R), funded by the Ministry of Economy, Industry and Competitiveness, State Program for Research, Development and Innovation aimed at the challenges of Society, within the framework of the State Plan for Scientific and Technical Research and Innovation 2013-2016, Call 2017

This study examines the effects of load levels on corrosion-expansion cracking and corrosion characteristics of steel bars in reinforced concrete (RC) beams and their causes. Based on the study of the influence of stirrups and cathode positions on the corrosion characteristics of RC beams, a reasonable stirrup arrangement and cathode position for the beams to be studied were determined, and 13 beams were designed and made according to the stirrup arrangement. Under the same environment, corrosion tests of the beams under four sustained load levels were conducted under galvanostatic accelerated corrosion. The results showed that the load had an important influence on the generation and development of longitudinal and transverse corrosion cracks and the corrosion characteristics of various steel bars. The variation of the resistance and distribution of the protective layer due to the different load effects is the essential reason for the difference in the corrosion characteristics.

Reinforced-concrete (RC) structures are inevitably subjected to sustained loads during their service life. In the natural environment, the exposed surfaces of RC members often experience dry–wet cycles due to rainfall and tides. An important parameter in structural design is the the bond behaviour between steel bars and concrete, and this is affected by the coupled effect of sustained loads and dry–wet cycles. In this study, 32 RC beam specimens were subjected to different levels of sustained load (25%, 50% and 75% of ultimate load) and dry–wet cycles for a period of 120 d. The immediate slip and time-dependent slip (TDS) between the steel bar and concrete during testing were recorded and analysed. It was found that the TDS increased with an increase in the duration of sustained loading and greater sustained load levels resulted in larger TDSs. After long-term testing, the bond strength and bond stress–strain were investigated using a flexural-bond test. The results showed that bond strength increased due to the coupled effects of sustained load and dry–wet cycles. Based on the test results, an empirical model for steel bar–concrete TDS considering the effect of sustained load and dry–wet cycles was developed.

This article illustrates new research regarding long-term deterioration in the tensile strength of plain concrete samples due to sustained stress loading. Two different water-cement ratio mixtures were subjected to sustained load using a Pressure Tension (PT) apparatus. Both saturated and oven dry samples were tested. Long-term PT strength under different load levels was found to decrease linearly as a function of the logarithm of time, corroborating existing creep studies. Pressure tension testing was used to determine the original strength of samples, and the effects of sustained load on tensile strength were analyzed. These results were compared with previous data obtained by direct tension tests to demonstrate efficacy of pressure tension testing for long-term creep studies. In addition, based on the analysis of failed samples, an assumption for the mechanism of the damage process is proposed.

Recycled aggregate (RA) and recycled powder (RP) are the primary products of the reclamation of construction and demolition (C&D) wastes, and the question of how to use them to prepare recycled aggregate concrete (RAC) and recycled powder concrete (RPC) has been a hot topic in the construction industry in China. As concrete structures are frequently subjected to the effects of both applied loads and chloride attacks, it is necessary to examine their effects on both RAC and RPC, which have received little consideration in previous investigations. In this study, RAC and RPC were firstly prepared by using RA and RP, respectively, to replace natural coarse aggregate and cement by weight. For each type of recycled concrete, sustained load and repeated load tests were then conducted, followed by a chloride diffusion experiment after unloading. The results indicated that the chloride penetration increased with the loading degree as well as the repeated load cycles. As the RP used in this study has high fineness and activity, the chloride penetration of RPC was lower than that of natural concrete, while the opposite result was found in RAC. Besides, the correlation between the chloride diffusivity and the imposed loading damage was also established in this study.

Concrete-encased concrete-filled steel tube members (concrete-encased CFST) are innovative composite structural elements which are having gradually increasing utilization in high-rise buildings and bridge structures. In practice, the inner CFST component is generally erected first and thus subjected to constructional load before the outer reinforced concrete (RC) component is formed. Afterward, the whole composite cross-section is under long-term sustained load in the service stage. This paper studies the structural behaviour of such composite columns under combined preload and long-term sustained load through a series of experimental tests. Concrete-encased CFST columns under different loading conditions, together with their CFST counterparts, are tested and compared. The main parameters include preload ratio, loading age and sectional configuration. Influence of these parameters on the structural behaviour in terms of failure mode, bearing capacity, stiffness and ductility are evaluated. Finally, a simplified design method is recommended to account for the influences of combined preload and long-term sustained load on the ultimate strength of the concrete-encased CFST columns, where the combined effects can be reflected by a corresponding set of coefficients.

An overview of available solutions and historic experiences in the state-of-the-practice of prestressing technologies with non-conventional tendons is presented to determine whether mild prestressing with Glass Fiber Reinforced Polymer (GFRP) strands may represent a solution for some of the existing challenges. The experimental portion of this project explores the potential of GFRP strands coupled with traditional steel anchors as an efficient, easy-to-deploy, and non-corrosive pre-tensioning solution in applications that require limited levels of prestress. The strands are composed of seven twisted wires and have a nominal diameter of 15.2 mm. This study on GFRP strand prototypes covers: a) mechanical and geometrical characterization; b) jacking strength of the coupled strand-anchor under instantaneous and sustained load; c) load loss during anchor setting using various pulling techniques; and, d) some flexural properties and coilability. Results should only be considered indicative of the potential of the technology given also the limited material availability. Experimental outcomes are evaluated with reference to information available in archival literature with particular reference to GFRP reinforcing bars.

The detrimental influence of high levels of sustained load on the compressive strength of concrete has been acknowledged and investigated since the 1950s. Despite the potential significance of this phenomenon in many situations, current design codes still provide limited guidance on how to account for this effect at ultimate limit state. In addition, the practical case of a significant permanent stress followed by stress increases due to variable loads is not addressed in most codes of practice. The aim of this paper is to present a general but simple design approach to account for the influence of sustained loading on the uniaxial behavior of concrete in compression, both in terms of its strength and deformation capacity. This approach is based on a previously developed mechanical model accounting for nonlinear creep and material damage development on the long‐term response of concrete. On that basis, a design approach is formulated in a simple code‐like manner and used to investigate several practical design situations involving both sustained and variable actions. This approach is shown to be consistent and applicable to any general loading pattern, accurately reproducing the results of the general mechanical model and of available experimental results.

This paper presents an experimental study on concrete-filled steel tubular (CFST) beam-columns with square sections under combined sustained load and chloride corrosion. A total of 11 specimens (including 7 CFST members and 4 reference hollow steel tubes designed for comparison) were tested under 120-day sustained load in accelerated chloride corrosion environment and then loaded to failure. The main test parameters include load ratio (ratio between the long-term sustained load and the ultimate capacity of the specimen under short-term loading) and corrosion depth. The failure modes, full-range load-displacement relationship, load-strain relationship and ductility were measured and compared. Based on the experimental observation, mechanism of CFST beam-column members under the combined effects of sustained load and chloride corrosion was discussed and compared to the reference hollow steel tubes. Traditional design methods were modified for the consideration of combined sustained load and corrosion, which were calibrated and evaluated against the test results.

Steel corrosion is an important factor in the durability of reinforced concrete members. There are numerous studies of reinforced concrete compression members in corrosive environments, but most studies involve corrosion under no load. To investigate the comprehensive effects of sustained load and corrosion on the mechanical properties of reinforced concrete members, this study simultaneously subjects 20 axial compression members to both sustained load and corrosion. The stirrups of 10 members are ordinary steel bars (A-type members), and the stirrups of the other 10 members are epoxy-coated steel bars (B-type members). The results show that the ultimate bearing capacity of corroded members can be significantly improved by using epoxy-coated stirrups, especially when the corrosion degree is high. A sustained load can reduce the actual corrosion degree of A-type members and increase the actual corrosion degree of B-type members. Under the same corrosion degree, the ultimate bearing capacity of the columns with stirrups made of ordinary steel bars increases as the sustained load increases, whereas for the columns with stirrups made of epoxy-coated steel bars, the ultimate bearing capacity decreases slightly as the sustained load increases.

Steel corrosion is an important factor in the durability loss of reinforced concrete members. Many studies have examined reinforced concrete eccentric compression members in natural stress-...

In this paper, the procedure for the strength evaluation of the piping system installed on liquefied natural gas (LNG) carriers is discussed. A procedure that accounts for the ship&rsquo, s wave load and hull motion acceleration (as well as the deformation due to the thermal expansion and contraction experienced by the hull during seafaring operations) is presented. The load due to the temperature and self-weight of the piping installed on the deck is also considered. Various operating and load conditions of the LNG piping system are analyzed. Stress analysis is performed by combining various conditions of sustained, occasional, and expansion loads. Stress is assessed using finite element analysis based on beam elements that represent the behavior of the piping. The attributes of the piping system components (such as valves, expansion joints, and supports) are represented in the finite element model while CAESAR-II, a commercial software is used for finite element analysis. Component modeling, load assignment, and load combinations are presented to evaluate pipe stresses under various load conditions. An evaluation model is selected for the piping arrangement of LNG and the evaluated stress is compared with the allowable stress defined by the American Society of Mechanical Engineers (ASME).

The long-term performance of glass fiber-reinforced polymer (GFRP) bars under high levels of sustained load combined with real field conditioning has not yet been thoroughly investigated. ...

If reinforced concrete columns and walls in high-rise buildings are subjected to axial load for a sustained duration, long-term lateral deformation (or curvature) as well as axial shortening increase over time due to the creep and shrinkage of concrete. In the present study, sustained load tests were performed to evaluate the time-dependent axial shortening and lateral displacement occurring in eccentrically loaded columns. A test set-up using post-tensioning steel rods was introduced for the sustained loading with or without eccentricity. The test results showed that the lateral displacement as well as the axial shortening increased over the sustained duration but the increasing rate of the long-term deformations rapidly decreased with increasing time. The time-dependent axial shortening and lateral displacement increasing with time were predicted by using the age-adjusted effective modulus method based on the creep and shrinkage models of ACI 209. The predictions agreed well with the test results.

The life-cycle performance of concrete-filled steel tubular (CFST) columns is investigated through numerical modelling. A finite element analysis (FEA) model is established to simulate the performance of deteriorated CFST structures subject to lateral impact. The imposed effects cover a wide range of factors that may deteriorate the mechanical performance of CFST during its life-cycle, which include: construction issues (preload, residual stress, and imperfection), sustained load (axial load from upper structure), environmental aggressiveness (chloride corrosion) and extreme hazard (impact lateral load). The numerical model is stage-by-stage verified against experimental results and shows good accuracy. It is then employed to analyze the performance of CFST column under the coupled effects, including the failure modes, the full-range load-displacement relationship, and the residual compressive strength. Particularly, the effects of the coupled hazards on the loading capacity of the column are compared with the results when the hazards are separately and independently imposed, which highlights the importance of the life-cycle design for the composite structure. Finally, a parametric study is carried out to analyze the influence of salient parameters on the residual compressive strength of CFST subject to the coupled effects.

This paper presents an experimental investigation on substandard partially steel jacketed RC columns strengthened under sustained loads and loaded until failure. The aim of the study is to compare the compressive response of preloaded columns with respect to the tests without preload to obtain a more realistic prediction of load-carrying capacity of structural columns after retrofitting under serviceability loads. The influence of the preload level, the delayed contribution of the confining device during the loading process and the time dependent effects are highlighted for the tested specimens. Based on the experimental observations, an analysis-oriented model is proposed for the reproduction of the compressive response of steel jacketed columns taking into account the effects of the preload.

Reinforcement corrosion is a main cause of durability deterioration of reinforced concrete structures. In recent years, stainless steel bars have been gradually applied to engineering practice because of its good corrosion resistance. However, under harsh environments such as chloride ion exposure, stainless steel bars will rust. Therefore, scholars began to study the corrosion characteristics of stainless steel bars. Previous studies mainly focused on the corrosion of steel bars in the zero-strain state, but steel bars in actual structures usually bear a sustained load resulting in initial strains. In this study, the corrosion characteristics of S23043 duplex stainless steel bars (S23043) under different strain levels were investigated by using acceleration corrosion tests and tensile tests under constant strain conditions in a chloride environment. The results showed that corrosion degree of S23043 increased by about 20% as the strain levels increased from 0 to 1 × 10−3. As the corrosion degree increased, the strength of stainless steel bars decreased, and the ductility decreased significantly. Finally, the mechanical properties of the stainless steel bars after corrosion were compared with those of ordinary steel bars.

The creep of cracked fibre reinforced concrete is still being investigated for incorporation into design guidelines. While the mechanism responsible for the time-dependent crack opening of steel fibre reinforced concrete has been associated with the fibre pull-out, a combination of pull-out creep and fibre creep have been reported for macro-synthetic fibre reinforced concrete. However, these phenomena are yet to be fully understood. In macro-synthetic fibre reinforced concrete, two possibilities exist: simultaneous occurrence of pull-out creep and fibre lengthening occurring within the matrix under sustained loading, or pull-out creep followed by fibre lengthening due to creep. This study investigates these phenomena. Single synthetic macro fibres were embedded into 50 mm cube cement-mortar samples and subjected to 50% of the average maximum pull-out load obtained from the single fibre pull-out tests. All tests were conducted in a controlled climate room. X-ray computed tomography (CT) images of samples were taken at different time intervals to assess the phenomena responsible for the increased crack widening in macro-synthetic fibre reinforced concrete. The results obtained have shown that the phenomena associated with the increased crack widening of cracked macro-synthetic fibre reinforced concrete are a simultaneous interplay of fibre pull-out and lengthening within the matrix. It is significant to note that fibre lengthening is a prominent mechanism.

A simplified method was proposed to predict the fire resistance of flexure-dominated reinforced concrete (RC) beams strengthened with near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP). Based on the results of previous experimental observations and theoretical analyses, “steel yielding after FRP slipping” and “FRP fracture after steel yielding” were presumed to be the possible failure modes for NSM-FRP strengthening system under fire. An algorithm was thus developed to evaluate the fire resistance of the strengthening system. A series of pull-out tests were carried out to obtain the bond-slip relationships of NSM-FRP at different elevated temperatures for the proposed algorithm. Then this method was used to predict the fire resistance of nine RC beams strengthened with NSM-FRP, which were subjected to a sustained load and the ISO834 standard fire. The predictions showed good agreement with the test results, indicating the effectiveness of the proposed method. This study provides a helpful reference for the fire resistance design of flexure-dominated RC beans strengthened with NSM-FRP in engineering practice.

This paper compares the flexural behaviour of cracked partially bonded (in the mid-span, maximum moment zone) reinforced concrete beams subjected to (i) static sustained load and (ii) static sustained with cyclically repeating load. Information relating to surface strains and mid-span deflections were continuously recorded for a period of 90 days. The sustained load level represented that which produced the stabilized crack pattern. The amplitude of the superimposed cyclic load was considered to be a small fraction of the sustained load. The experimental outcome shows that under sustained load alone, the long-term mid-span deflection of reinforced concrete beams with artificially debonded reinforcement is substantially higher than that of normally bonded equivalent beams. For the cyclically exerted load addition there was no substantial difference between the observed ultimate deformations of bonded and debonded beams. Nonlinear finite element software (Midas FEA) was used to simulate these results and it was found that a numerical-experimental match can be achieved after applying necessary modifications to the distribution of shrinkage down through the beams’ cross-section.

The synthetic effects of crack width and environment actions on chloride diffusivity of reinforced concrete (RC) columns were experimentally studied after the alternative action of 300 cycles of freeze-thaw and 75 times of seawater immersion. Cracks and tension strain of concrete were induced by eccentric compression load which was sustained till the alternative action finished. The experimental results show that, when the crack width was less than 0.07 mm, the effect of cracks on chloride diffusivity was slight and it could be neglected. When the crack width was more than 0.07 mm, cracks accelerated the chloride diffusivity and the synthetic effects of crack and environment on chloride diffusivity was bigger than tension strain. Furthermore, chloride diffusion in one-dimensional and two-dimensional state was respectively considered. And the corresponding service life of RC columns were predicted by Monte-Carlo Method. It was concluded that crack width significantly expedited chloride diffusivity and shortened the service life of RC structures in cold coastal regions or deicing condition.

In this work, an experimental program comprising twenty-eight un-strengthened and NSM FRP strengthened RC beams was carried out to study their time-dependent behavior under sustained loading. At first, some beams were tested under short-term loading up to failure. Then, the others were subjected to long-term tests with different sequences. Each sequence included one or more of three stages: short-term test for two loading/unloading cycles up to service design load, sustained loading or aging for more than nine months, and short-term loading up to failure after removing the sustained loads. The studied parameters were CFRP area, tension steel reinforcement ratio, the ratio between the compression and tension steel ratio, concrete type, sustained load level, and loading history. Results in terms of the influence of the indicated parameters on the initial, time-dependent, and residual deflections, as well as residual flexural strength after removing the sustained load, are reported and discussed.

The development trend of glass fiber-reinforced polymer-reinforced concrete (GFRP-RC) beams partially or entirely replacing reinforced concrete beams has gained acceptance. However, related research on long-term fracture durability is insufficient. This paper aims to investigate the long-term fracture durability of GFRP-RC beams, 36 GFRP-RC beams under the alkaline corrosion environment and sustained load for eight years were subjected to three-point eccentric loading experiments. The load crack opening distance (P-CMOD) curve and the corresponding fracture energy were obtained. The results show that the bearing capacity and residual fracture energy retention (ER) of GFRP-RC beams with working cracks in the APS (alkaline + pre-cracks + sustained load) group were 18.71% and 26.38% lower, respectively than those in the AS (alkaline + sustained load) group. After analyzing the critical CMOD curves for each service period with failure probabilities of 60% and 85%, a long-term crack growth failure probability model for the service time t and the critical CMOD was proposed, which provide a non-destructive damage assessment method for GFRP-RC beams in actual service.

The corrosion of reinforcing steel bars (rebar) in reinforced concrete (RC) is the main cause of the degradation of interfacial bond behavior between rebar and concrete, which can further induce the degradation of structural performance. In this paper, the bond behavior between corroded rebar and concrete was experimentally studied focusing on the coupling effects of lateral confinement and sustained load for the first time. Continuous and uniform confining pressures were imposed on the corroded rebars via fiber reinforced polymer (FRP) and sustained loads were applied to the corroded rebar by a self-developed loading device. The effects of corrosion ratio, sustained load and lateral confinement on the bond behavior were tested and analyzed. The test results demonstrated that FRP lateral confinement could effectively restrain the corrosion rate of rebar and thus significantly improved the bond performance; moreover, the dual action of FRP lateral confinement and sustained load could significantly enhance the interfacial bond strength and ductility due to their great contribution to the prevention of corrosion expansion. However, without FRP lateral confinement, the interfacial bond strength and ductility would be weakened by sustained load.

Concrete is one of the most widely used materials in modern civil engineering construction. Concrete creep is traditionally defined to be a kind of time-dependent behavior exhibited by concrete under sustained constant loads. In this article, a new method is proposed to test the short-term creep of concrete and the deformation state after unloading process. The test results indicate that after loading, regardless of whether the constant load is sustained or not, the specimen exhibits a hysteretic recovery after unloading, which challenges the traditional definition of concrete creep. In addition, we first observed that the fundamental time-dependent strains of concrete, which can be separated from concrete creep under sustained load, could peak in several minutes and begin to fall. The long-term creep mainly results from unrecoverable deformation, which arises as the recoverable deformation is gradually transformed into unrecoverable deformation through micro-damage accumulation. The hysteretic recovery and time-dependent behaviors which were discovered reveal the fundamental viscoelastic origin of concrete creep.

This paper focuses on the developing rule of elastic modulus of Fly Ash Concrete (FAC) under sustained load. Mechanism of the action of sustained load on concrete is discussed and a new model for the elastic modulus of FAC is constructed based on the Triple-phase Spherical Model (TSM). An experiment with three concrete mix designs and five testing groups were conducted. The experimental compressive strength and elastic modulus at six different ages were used to calibrate the parameters in the model. Influences of two important parameters (loading age and loading level) in the model were analyzed. Test data confirmed the fact that elastic modulus for the loaded concrete have a small increment compared to the non-loaded, which is more evident for younger concrete, and calculation results showed that the newly constructed model can well predict the elastic modulus of concrete under sustained load. Besides, compared to the loading age, the loading level was proved to be a more important factor in deciding the development rule of elastic modulus for the loaded concrete.

RC beams strengthened with transverse prestressed unbonded carbon fiber–reinforced polymer (CFRP) straps have shown substantial increases in shear capacity when subjected to short-term stat...

[EN] Concrete-filled steel tubular (CFST) columns can generally be expected to better resist elevated temperatures as compared to unfilled steel hollow sections, whose evaluation after a fire is limited by the resulting deformation. A better understanding of the behaviour of CFST columns after a fire, which is dominated by the maximum temperature achieved by the concrete infill and plasticity of the steel, is required to properly estimate their residual strength and deformation and to adopt a reasonable strategy with minimum post-fire repair. In this paper, a fibre beam model for the simulation of the post-heating response of concrete-filled steel tubular (CFST) columns is presented. First, the model is validated against experimental results and subsequently it is employed to analyse the post-heating response of circular CFST columns under sustained loads. In reality, during a fire, the columns support load even during the cooling phase of a fire, so it is important to consider this loading condition when predicting the post-fire behaviour. The analysis presented in this paper comprises three stages: heating, cooling and post-fire (under sustained load) conditions. The model considers realistic features typical of the fire response of CFST columns, such as the existence of a gap conductance at the steel-concrete interface and the sliding and separation of the steel tube and the concrete. Based on the model, the response of CFST columns after heating is investigated via parametric analysis., The authors gratefully acknowledge the financial support given by Universitat Jaume I (Spain) E-2018-18 and by Generalitat Valenciana (Spain) BEST/2017/141 for the first author's stays as a visiting fellow at the School of Engineering of the University of Edinburgh.

Prestress loss of CFRP during prestressing and service stage is one of the critical technical issues in concrete structures rehabilitation. In this work, the prestress loss of reinforced concrete (RC) beams strengthened by a prestressed CFRP plate subjected to sustained loading and continuous wetting condition were experimentally studied. Eight RC beams strengthened by prestressed CFRP plate were prepared with different prestress levels of 20% and 40% respectively. The initial prestress loss was monitored up to 28 days and the loss ratios of all specimens were around 5%, which was dominated by slippage of the end anchorage system. Then, the specimens were sustainedly loaded at two levels under wet or dried environments for 170 days. The average prestress loss ratios during this period were all less than 2%, which indicates that the CFRP plate end anchorage system had good long-term performance. Moreover, the adhesive bonding was weakened by the moisture penetration, and the specimens with lower prestress levels were more susceptible to the wet condition. Nevertheless, regardless of the environmental exposure, the maximum crack widths of 40% prestressed specimens remained less than 0.2 mm even subjected to high sustained load level, which is within the limitation of the prestressed concrete structures.

Creep and damping, which are two important static and dynamic properties of concrete material, have similar mechanical characteristics and have not been studied integratedly. This paper aims to develop a new method to investigate the creep and damping properties of recycled aggregate concrete (RAC) from viscoelastic perspective. In this research, a fractional order derivative viscoelastic creep model of RAC under different load levels was proposed. Then, with the consideration of recovery coefficient and time-dependent behavior, the damping properties of RAC based on interconversion between viscoelastic functions were investigated. The creep tests of reinforced RAC beams, accompanied by loading and unloading tests, were conducted to study the viscoelastic properties, meanwhile, free-vibration tests at different ages were carried out to analyze the time-dependent damping behavior of RAC. Test results indicate that compared with three parameter model and generalized three parameter model, fractional three parameter model is a feasible viscoelastic model to characterize both creep and damping behavior of RAC. The creep compliance of RAC is higher than that of NAC, and increases with the increasing of load level. The damping property of RAC decreases with the increasing of load frequency and the decreasing of load level, meanwhile, the damping property of RAC is related to recoverable instantaneous deformation. In addition, the damping property of RAC decreases with the increasing age when it is not considering load, but increases when it is subjected to sustained load. The proposed damping model was validated with a database of RAC damping in published literatures.

Bond of steel reinforcement to concrete was extensively studied worldwide during the last thirty years. However, few studies approach the evaluation of the effects of elevated temperatures on such load transfer mechanism and, in those, the presence of a sustained load applied prior to temperature increase is widely neglected. The present work investigates local bond properties between an ordinary carbon steel and an ordinary concrete when subjected to elevated temperatures, accounting as parameters the clear cover (and, more in general, the bar position), the bonded length and, mostly important, the effects of heating and loading sequence. Through test results and comparison with existing literature data it is shown that, for both pull-out and splitting failure modes, the effects of a sustained load on the evaluation of bond strength at a given temperature cannot be ignored for load levels higher than approximately sixty percent of the bond strength at cold state. Conditions for which the influence of the bonded length and of the bar position can be neglected in such assessment are also discussed.

To study the fatigue properties of reinforced concrete (RC) beams strengthened with textile reinforced concrete (TRC), which consisted of a hybrid textile made from carbon and E-glass yarns and high performance concrete, after sustaining load corrosion, three RC beams without TRC and four RC beams with TRC were designed. Among these beams, one strengthened beam and one un-strengthened beam were not subjected to sustaining load corrosion, while the other five beams were immersed in a sodium chloride solution at a concentration of 5% and were subjected to different loads. After six months of dry-wet cycles, all of the beams were subjected to the bending fatigue test. The results showed that the distribution of fatigue cracks and the fatigue life of the strengthened beams were better than those of the un-strengthened beams regardless of their subjection to corrosion; however, corrosion was shown to weaken the bond between the TRC and the old concrete. The fatigue life of the beams was affected when they sustained loads because of the initial deformation that appeared. The existence of an initial deformation decreased the amplitude difference of the beams that suffered from corrosion. The development of deflection in the strengthened beams with corrosion could be divided into the following three stages: initial, stable, and rapid stages; the development of residual deflection exhibited an initially increasing and then decreasing tendency. The combination of sustained load and corrosion decreased the fatigue life of the beams more than either the sustained load or the corrosion alone.

In practice, reinforced concrete (RC) structures before and after strengthening usually is exposed to a wide variety of combined actions, which may cause various damage to the structures. The main objectives of this study are to investigate the effects of three types of pre- and post- strengthening damage in the marine environment on the flexural behaviors of carbon fiber-reinforced polymer (CFRP)-strengthened RC beams and to develop a design methodology for predicting their yielding load, debonding load and peak load with or without CFRP debonding. The pre-damage was induced by either sustained load only or by the combination of sustained load and corrosion. The post-damage was induced by combined sustained load and wetting/drying (WD) cycles. The failure mode, yielding load, debonding load and peak load were recorded and compared, and the related mechanism was discussed. Subsequently, by adopting nonlinear sectional analysis, the effect of non-uniform corrosion of steel reinforcement on the yielding load, the effect of WD cycles on the debonding load and the effective CFRP rupture strain were examined and several models were proposed accordingly. The yielding strength, debonding strength and peak strength of the beams can be predicted by the proposed models with sufficient accuracy. The reliability of the proposed models was further validated with a collected database from the existing literatures.

Changes in dimension of concrete due to early-age heat generation, long-term drying shrinkage, and creep under sustained load could lead to significant movements of the concrete structure and lock-up stresses if the movements are restrained. The lock-up stresses are often large enough to cause cracking and water leakage, thereby adversely affecting the serviceability and durability of the structure. Whilst the dimensional changes are quite variable because of their dependence on the environmental conditions and applied loads, they are all related to the concrete mix proportions. Generally, the dimensional stability of concrete can be improved by reducing its cement content and paste volume. Moreover, since the aggregate also shrinks, the dimensional stability can be improved by suppressing the shrinkage of aggregate as well. In this paper, strategies for improving the dimensional stability of concrete are formulated based on experimental research. Test results are presented to demonstrate their effectiveness and applicability.

The duration of load effect is a widespread phenomenon that timber structural members will experience certain decrease in strength under long-term service, which should be properly considered during the design of timber structures to ensure the lifetime structural safety. This paper presents a series of reliability-based investigations into the duration of load factor for the Chinese standard of timber structures. Firstly, the load models including dead load, live load and snow load were set up, and the main features of the load models were validated according to the requirements in Chinese load code. Then, the Foschi and Yao’s damage accumulation model was calibrated with long-term experimental data, and the calibrated duration of load model was validated by comparing the cumulative failure probability with different intensities and durations of sustained load. Based on the validated load models and the damage accumulation model, the short-term and long-term reliability was calculated using JC method and Monte Carlo simulation, respectively. Finally, recommended values of the duration of load factors were obtained under different loading conditions, including dead loads only, dead loads plus live loads, and dead loads plus snow loads. The presented research aims to provide technical basis for the determination of duration of load factors for the design of timber structures, therefore ensuring the structural safety of timber structures under long-term service.

The objective of the study is to examine the influence of high sustained loads and longitudinal reinforcement on the long-term deformation of concrete beams. First, the influence of high sustained loads on the creep of reinforced concrete beams was investigated using five beams with load levels (ratio of sustained load to failure load) of 0.3, 0.4, 0.5, 0.6, and 0.7. A load level of 0.3 is the service load for a comparative analysis, and load levels of higher than 0.3 are considered high loads. The influence of the longitudinal reinforcement on the creep of the reinforced concrete beams was then studied using four beams with a ratio of longitudinal reinforcement of 0.81%, 1.55%, 2.14%, and 2.72%, respectively. Furthermore, the influences of the load levels and longitudinal reinforcement on the curvature creep, creep of the concrete at the edge of the compressive zone, and the mid-span deflection creep were analysed. Moreover, a comparison between the test data and the existing calculation model of the deformation of a beam was conducted, and the existing model used to predict the long-term deflection of reinforced concrete beams with high sustained loads within a wide range of longitudinal reinforcement ratios was modified. Finally, post-creep tests of the beams were conducted and the calculated formulas of the total strain (the sum of the initial strain, creep strain, and increased strain after creep) of the concrete at the compression edge of the beams was obtained through a regression analysis.

This paper presents the results of an experimental study designed to investigate the performance of reinforced concrete (RC) beams strengthened with fibre-reinforced polymer (FRP) sheets subjected to wetting–drying cycles and a sustained load. Twenty-one RC beams were manufactured and externally bonded with carbon-FRP (CFRP) or glass-FRP (GFRP) sheets. Two U-shaped FRP anchorages were also applied on the ends of the beams to prevent end peeling failure. The FRP-strengthened beams were subjected to combined wetting–drying cycles in 5% NaCl solution and sustained loads for 0, 90, 180, and 360 days at load levels of 0%, 30%, and 60% of the ultimate loading capacity of the unstrengthened beam. During this period, the mid-span deflections and cracks of the beams were monitored, following which the beams were monotonically loaded to failure under a four-point bending. The failure patterns, mid-span deflections, and debonding loads were also examined. The test results showed that during sustained loading, the time-dependent and residual deflections of the FRP-strengthened RC beams increased with the increase in their periods of sustained loading and load levels. Intermediate crack-induced (IC) debonding of the CFRP-strengthened RC beams was induced by concrete separation and CFRP–concrete interface debonding, whereas that of the GFRP-strengthened RC beams was mainly caused by GFRP–concrete interface debonding. Combined wetting–drying cycles and a sustained load had an adverse effect on the yielding and debonding load of the FRP-strengthened RC beams, and the ductility of the GFRP-strengthened RC beams was higher than that of the CFRP-strengthened beams. In addition, a model considering the effects of wetting–drying cycles and sustained loads was proposed to predict the debonding load of FRP-strengthened RC beams, and it showed a reliable prediction.

Encasing recycled aggregate concrete (RAC) in circular steel tube can improve the mechanical behaviours of the RAC without considerably increasing the cost. The confinement effects, which significantly increase the strength and the ductility of the RAC, also influence the nonlinear creep behaviour of RAC, which has not been well investigated. This paper presented nonlinear creep tests on recycled concrete filled steel tubes (RACFST) for up to 500 days. The load level varied between 0.41 and 0.79. Two concrete strengths were included in the tests (i.e. 30 MPa and 50 MPa). All these members used 100% of the recycled coarse aggregates (RCA) and have the steel ratio (the ratio of the steel area over the concrete area) of 0.08. The tested time-dependent deformations were compared against those reported for the circular CFST members to investigate how the full use of RCA would influence the nonlinear creep behaviour of the composite sections. The confinement effects were also carefully monitored during the long-term tests. It was found that the nonlinear creep can increase the creep coefficient of the RACFST members by 52%–115%, which needs to be carefully considered in the design. Confined by steel tubes, the encased RAC could stand for up to 500 days without creep failure under the stress level of 0.78–0.98fcm, where fcm is the uniaxial strength of the RAC. The use of recycled aggregates can considerably increase the nonlinear creep effects by up to 22%, and such influence increased with the sustained loading level to the maximum value at the stress level of approximately 0.68 and then started to decrease.

Post-installed adhesive anchors play an important role in modern construction. As with many structural products, adhesive anchors must undergo rigorous testing to verify serviceability and ultimate performance characteristics under job-site-relevant conditions before being used in real projects. The check of long-term performance under sustained load is based on extrapolation from experimental short-term creep data approximating the creep mechanism of the entire anchor system with a power law model as detailed, for example, in ACI 355.4 and ETAG 001. These normative guidelines are critically reviewed from a scientific as well as practical point of view. Based on the presented evidence, specific recommendations for anchor testing and assessment guidelines are proposed.

Slim floor systems represent an economical and competitive solution for building applications that combine the advantages of concrete floors, prefabricated steel sections and a shallow depth. The distinctive feature of this form of construction relies on the fact that the lower steel flange is wider than the top flange to provide a continuous support along the beam length for the slab formwork. The latter is usually specified in the form of either profiled sheeting or cellular slabs. In this structural typology, the steel section is embedded within the thickness of the slab. This paper presents an experimental study aimed at evaluating the service behaviour of slim floor beams induced by the time-dependent behaviour of the concrete and at establishing the possible influence of creep effects on their ultimate response. For this purpose, two slim floor samples were prepared in a simply-supported configuration. Their long-term deflections and deformations were monitored over time for about ten months, after which the specimens were tested to failure. The specimens possessed identical concrete and steel geometries. The shear connection was provided by transverse steel reinforcing bars installed within regularly spaced holes incorporated in the steel web. The samples were cast unpropped so that the self-weight of the wet concrete was carried by the steel member. The only difference between the two specimens consisted in the loading history specified over time. In particular, one slim floor sample was kept unloaded for the entire duration of the test to monitor the influence of shrinkage effects while the second specimen was subjected to a sustained load to evaluate the effects of both creep and shrinkage. The experimental data reported in this study provides insight into their long-term and ultimate response, and valuable benchmarking data for the calibration of numerical models and design procedures related to the serviceability and ultimate limit states of slim floor systems.

An experimental study was conducted to evaluate the effect of the gross-to-cracked section moment of inertia ratio Ig:Icr on the long-term deflection multiplier for glass fiber–reinforced p...

Due to creep and shrinkage of the concrete core, concrete-filled steel tubular (CFST) arches continue to deform in the long-term under sustained loads. This paper presents analytical investigations of the effects of geometric nonlinearity on the long-term in-plane structural performance and stability of three-pinned CFST circular arches under a sustained uniform radial load. Non-linear long-term analysis is conducted and compared with its linear counterpart. It is found that the linear analysis predicts long-term increases of deformations of the CFST arches, but does not predict any long-term changes of the internal actions. However, non-linear analysis predicts not only more significant long-term increases of deformations, but also significant long-term increases of internal actions under the same sustained load. As a result, a three-pinned CFST arch satisfying the serviceability limit state predicted by the linear analysis may violate the serviceability requirement when its geometric nonlinearity is considered. It is also shown that the geometric nonlinearity greatly reduces the long-term in-plane stability of three-pinned CFST arches under the sustained load. A three-pinned CFST arch satisfying the stability limit state predicted by linear analysis in the long-term may lose its stability because of its geometric nonlinearity. Hence, non-linear analysis is needed for correctly predicting the long-term structural behaviour and stability of three-pinned CFST arches under the sustained load. The non-linear long-term behaviour and stability of three-pinned CFST arches are compared with those of two-pinned counterparts. The linear and non-linear analyses for the long-term behaviour and stability are validated by the finite element method.