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.

Thermal desorption spectroscopy (TDS) and sustained-load tests were used to investigate hydrogen embrittlement (HE), hydrogen profiles and the impact of the baking delay of cadmium-plated 4340 steel-notched bars. The results show a tremendous concentration of hydrogen in the cadmium coatings (200 to 1500 ppma) that does not contribute to HE. Hydrogen in manganese sulfide (MnS) traps was also found not to contribute to HE. The total hydrogen concentration measured by TDS is thus not suitable to determine an embrittlement threshold. Subtracting hydrogen in cadmium and MnS traps from total hydrogen allowed finding a true critical hydrogen concentration threshold of 0.6 ppma (0.01 ppmm). No impact of the baking delay was found.

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

The mechanical performance of fibre reinforced concrete presents aspects still under investigation, mostly those regarding the long-term behaviour. Even if creep and shrinkage are two well-known phenomena that characterize concrete, in case of FRCs, and in particular of macro-synthetic Fiber reinforced concretes (MSFRCs), there are no reliable models for predicting their long term-behaviour, because of the interaction between concrete, fibre creep and bond. In addition, temperature is a further variable to control since it affects the material performance. In this perspective, the present paper shows the experimental results of a large campaign of creep tests performed on macro synthetic fibre reinforced concrete specimens. The material tested had a compressive strength of about 55MPa and it is reinforced with 8kg/m3 of polypropylene crimped fibres. The experimental investigation is carried out by performing creep compression tests on cylinders and direct tensile test on notched cylinders. In addition, the tensile behaviour of the single fibre under sustained load is analysed. The tests were conducted in a humidity and temperature controlled chamber. Furthermore, the temperature was increased from 20°C to 30°C after a time of 50days of testing in order to understand how this condition modifies the creep deformations evolution of the material. The paper shows also the initial calibration of a numerical model based on the Lattice Discrete Particle Model (LDPM) theory. The LDPM is one of the most validated theories able to reflect the actual coarse aggregate distribution of a quasi-brittle material, i.e. concrete. Currently this theory has been extended to include the fibres reinforcement. The aim of the big study presented would be to elaborate a predictive model for the MSFRCs accounting also for concrete and polymers long term behaviour.

Glass fibre-reinforced polymer (GFRP) bars present multiple advantages over conventional steel reinforcement including high strength-to-weight ratios and corrosion resistance. Whereas many studies are reported in the literature on the behaviour of GFRP-reinforced concrete members, the performance of GFRP-prestressed concrete beams has been the focus of only a few research works. Zawam et al. (Zawam M, Soudki K, West JS (2017) Effect of prestressing level on the time-dependent behaviour of GFRP prestressed concrete beams. J Composites Constr 21(4) (2017)) demonstrated the potential of GFRP for prestressing applications with satisfactory creep performance under sustained loading. Very limited information available regarding the unloading behaviour of concrete beams after being subjected to sustained loading. Such information would be beneficial for example in cases of concrete bridges during rehabilitation projects. Similarly, very few studies are available that describe the behaviour of GFRP-prestressed concrete beams. This study addresses both of these gaps through an investigation of the unloading behaviour of GFRP-prestressed/reinforced concrete beams after being subjected to different levels of sustained loading for a period of 300 days.

This investigation consists of two parts (outdoor and laboratory experiments). The first part described RC slabs strengthened with prestressed and non-prestressed CFRP strips; these slabs were loaded and exposed to an outdoor environment for four years. It was found that the elevated temperatures reached in the adhesive did not significantly affect the service behaviour of non-prestressed EB CFRPs, but could lead to the failure of the prestressed system. In the second part of the discussion, laboratory experiments are presented. Failure tests were performed to determine the ultimate strengths of the slabs. The results demonstrate that long-term exposure (thermal cycles and sustained load) does not affect the load-carrying capacity of the strengthened RC elements. Furthermore, an additional slab with an identical strengthening configuration was tested after being subjected to thermal cycles under a heavy sustained load under laboratory conditions, allowing us to better understand and verify the results obtained during the long-term experiments.

In the last decades, fibre reinforced concretes became widely adopted in structural applications. Nevertheless, some elements of their mechanical behaviour are not fully understood. For instance, the effect of environmental conditions on the short and long-term behaviour of these materials has been studied to a limited extent only. In this perspective, the present paper presents the results of a large experimental campaign involving flexural tests on Macro Synthetic Fibre Reinforced Concrete (MSFRC) specimens under short- and long-term loads. Two different polypropylene Fibres were used, with dosages of 8kg/m3 and 10kg/m3. The effect of temperature on the short-term behaviour of these materials was investigated by performing three-point bending tests at 20 °C and 40 °C in cracked and uncracked conditions. The effect of temperature variations on long-term deformations was studied by means of four-point bending tests on pre-cracked notched beams at increasing temperatures, from 20 °C to 40°C. The paper presents the test results as well as analyses the effective number of fibres crossing the cracks.

GFRP bars are regarded as an alternative to steel reinforcement in marine and aggressive environments. However, there are some shortfalls to the use of GFRP reinforced members in flexure, which the addition of fibres can redress. This paper is concerned with the effect of synthetic fibres on the cracking behaviour of GFRP reinforced members. A number of FRC beams reinforced with GFRP bars were tested in flexure, considering different synthetic fibre contents and GFRP bar diameters. The flexural loads applied were representative of service conditions and were sustained for 90 days. The short- and long-term cracking behaviour was analysed in terms of crack spacing, distribution and development in pure bending sections. It was concluded that synthetic fibres increased the cracking moment capacity by up to 20% and reduced the crack width and crack spacing by up to 63% and 31%, respectively.

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.

During the last decades, Near Surface Mounted (NSM) Fibre Reinforced Polymer (FRP) strengthening system has been widely accepted as an efficient methodology in structural rehabilitation. The bond behaviour of the FRP-concrete joint has resulted to be governing the overall performance of the strengthening system. Moreover, due to the susceptibility of the materials used in the NSM strengthening system in front of high temperatures and sustained loads, their performance under service conditions can be affected. Although the short-term bond behaviour has been largely studied in the literature, there is still a lack of experimental data to better understanding the long-term performance. The paper aims to experimentally study the evolution of bond damage of the concrete-FRP bonded joint of an NSM FRP strengthened concrete element, and how it affects to the slip between the strengthening system and the concrete For this purpose, a total of eight NSM pull-out specimens with two different bonded lengths (150 and 225 mm) and two different groove thicknesses (7.5 and 10 mm) have been loaded with two different levels of sustained load (15 and 30% of the ultimate load) at room conditions up to 40 days. Specimens were previously characterized under a short-term test. Slip at the loaded end was measured to capture the evolution of slip during time. Results showed that the slip caused by creep increased with the sustained load level, whilst it decreased with the bonded length.

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-...

To investigate the residual bending strength of a corroded H-shaped steel beam in an underground coal mining environment, the law governing the degradation of the mechanical properties of corroded steel was first investigated through tensile testing. Subsequently, a four-point bending test was conducted on corroded H-shaped steel beams. The influence of the corrosion rate and sustained loading ratio on the residual bending performance of a corroded H-beam was investigated. The results reveal that the uniform corrosion and uneven corrosion of the steel occurred simultaneously. Additionally, pits with a small size appeared on the steel surface and the number of these pits increased with the corrosion time. Four different fracture modes were observed after the tensile test, and the yield strength and ultimate strength of the corroded steel decreased as the corrosion rate increased. In the bending test, the failure mode of the corroded H-shaped steel beam was not changed by the corrosion. The bearing capacity, stiffness, and ductility of the corroded H-shaped steel beams decreased with the increase in the corrosion rate, and the sustained loading further decreased the bearing capacity. Finally, a simple method for assessing the yield load and ultimate load of corroded H-shaped steel beams is proposed.

The research work presents the experimental long-term behaviour of reinforced concrete (RC) flat slab specimens at 1:2 scale, subjected to uniformly distributed sustained load in successive phases up to 90 days. The sustained load was applied in two stages, in the first stage, it was 3 kN/m2 for 30 days, whereas in the second stage, it was increased from 3 kN/m2 to 6 kN/m2 for the next 60 days. Age-dependent strength of cement mortar and concrete as used in RC flat slabs has been determined using the method as per the Indian code of practice. The contribution of creep and shrinkage deflection in total deflection for the column strip, middle strip, and cantilever portion has been observed in the range of 34% to 42% at the end of 90 days. After the first 30 days, the mid panel deflection of the flat slabs was larger as compared to column strip and cantilever deflection. At 90 days, the cantilever portion experienced the highest deflection as compared to middle strip and column strip deflection. The measured deflection has also been compared with the IS code procedure and it is found that the code underestimates the long-term deflection of the flat slab. Further, the finite element modelling using ABAQUS has also been carried out on the RC flat slab to validate the experimental results of the same slab within the elastic range. The present study gives an idea to the designer for fixing the thickness of the slab by considering the long-term deflection criteria in the design of the RC flat slab based on its behaviour under sustained loading.

The creep of concrete promotes strains over time in structural members kept under sustained load. It causes the stress decrease on the concrete and the steel stress increase in reinforced concrete members. The moisture content and temperature influence significantly such phenomenon. The creep strains model of the NBR 6118/2014 [1] is, applicable, solely, to those cases of constant stress magnitudes. Reinforced concrete members exhibit variations on the stress magnitudes and, in this way, requires the use of an alternative model for the prediction of the creep strains as the so known the State Model. This report refers itself to temperature influence analysis upon creep strains of reinforced concrete structural members. The results have revealed that temperature speeds up the creep effects and, in this way, the steel yielding caused by the stress increase on the reinforcement bars occurs at earlier ages. resumo: A Fluência do concreto promove deformações com o tempo em membros estruturais mantidos carregados permanentemente. Ela causa a redução das tensões no concreto e o aumento das tensões no aço em membros de concreto armado. A umidade e a temperatura influenciam significativamente a Fluência. O modelo de deformações por Fluência da NBR 6118/2014 [1] é, aplicável, apenas, aos casos de tensões constantes. Em membros de concreto armado as tensões variam exigindo o uso de modelo alternativo de cálculo de suas deformações a exemplo do modelo de estado. Este relato refere-se à análise da influência da temperatura na Fluência de membros estruturais de concreto armado. Os resultados revelaram que a temperatura acelera os efeitos de fluência e, assim, o escoamento do aço mediante o aumento de tensões nas barras da armadura ocorre precocemente.

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. ...

The considerable increment in use of Fabric Reinforced Matrix (FRM) strengthening systems in recent years has led to the need of investigating their durability when exposed to long-term actions. Single lap shear tests were performed on concrete and masonry specimens reinforced with different types of fabrics embedded in an inorganic matrix. A new test setup was developed with the aim of applying a sustained load for up to six months simultaneously to considerable amounts of specimens under the same controlled temperature and relative humidity conditions. The paper will describe the proposed setup and the instrumentation. It will also describe the results in terms of measured displacements for clay and natural stone masonry as well as concrete reinforced with fabrics made of different materials. The experimental campaign is part of an ongoing extensive assessment plan for the analysis of the durability of fabric reinforced inorganic matrix composites.

This paper studied the mechanical properties of textile reinforced concrete (TRC) under chloride wet-dry and freeze-thaw cycles, considering the number of cycles, the coupling effect of bending sustained load and corrosion environment, and adding short-cut fibers into fine grained concrete. The results showed that as the number of chloride wet-dry cycles increased, the average interfacial bonding strength between the fiber yarn and fine grained concrete decreased; while the bending bearing capability of the TRC thin plates showed no coincident trend. However, with the increase of the number of chloride freeze-thaw cycles, strengths were both reduced. Adding short-cut fibers improved the average interfacial bonding strength between the fiber yarn and fine grained concrete, as well as the bending bearing capability of the TRC thin plates under chloride wet-dry cycles. Moreover, the mechanical performance of the TRC thin plates has not been significantly improved under chloride freeze-thaw cycles.

With the continuous development of production technology, the performance of glass-fiber-reinforced polymer (GFRP) bars is also changing, and some design codes are no longer applicable to new materials based on previous research results. In this study, a series of durability tests were carried out on a new generation of GFRP bars in laboratory-simulated seawater and a concrete environment under different temperatures and sustained loads. The durability performance of GFRP bars was investigated by analysing the residual tensile properties. The degradation mechanism of GFRP bars was also analysed by scanning electronic microscopy (SEM). Furthermore, the long-term performance of GFRP bars exposed to concrete pore solution under different stress levels was predicted using Arrhenius theory. The research results show that the degradation rate of GFRP bars was increased significantly at a 40% stress level. By comparing the test results, design limits, and other scholars&rsquo, research results, it is demonstrated that the GFRP bars used in this test have a good durability performance. It is found that the main degradation mechanism of the GFRP bars is the debonding at the fiber-matrix interface. In the range test, the effects of a 20% stress level on the degradation of GFRP bars were not obvious. However, the long-term performance prediction results show that when the exposure time was long enough, the degradation processes were accelerated by a 20% stress level.

Textile reinforced concrete (TRC) has good bearing capacity, crack resistance and corrosion resistance and it is suitable for repairing and reinforcing concrete structures in harsh marine environments. The four-point bending method was used to analyze the influence of the salt concentration, the damage degree and the coupled effect of the environment and load on the bending performance of TRC-strengthened beams with a secondary load. The results showed that as the salt concentration increased, the crack width and mid-span deflection of the beam quickly increased, and its bearing capacity decreased. As the damage degree increased, the early-stage crack development and mid-span deflection of the beam were less affected and the ultimate bearing capacity significantly decreased. In addition, the coupled effect of the environment and load on the beams with a secondary load was significant. As the sustained load increased, the ultimate bearing capacity of the strengthened beam decreased, and cracks developed faster in the later stage. In addition, the mid-span deflection of the beam decreased at the same load level because of the influence of the initial deflection due to the sustained load corrosion.

The hypothesis of contact maturing advocates static fatigue at contacts between sand grains as the key cause contributing to time effects in sand. The focus of the paper is on a model of an individual grain subjected to a sustained load applied through two steel plates. The grain is characterized by the roughness of its surface. The distinct element method is used to construct a model on two scales: the grain scale and the contact scale. Assemblies of bonded sub-particles are used to model both the grain and the contact region. The bond model includes the stress corrosion process, which simulates decaying strength of bonds and fracture. Two components of the time-dependent grain deflection under sustained loads are the displacement owed to sub-critical fracturing of asperities on the grain surface at the contacts and the creep of the core mineral in the grain. The model demonstrates what may be difficult capturing in physical testing. The simulated nominal contact evolves as the number of contact points increases due to sub-critical fracturing of asperities, leaving the contact firmer. The contact evolution process, as observed in simulations, is consistent with the contact maturing hypothesis.

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.

The research work reported in this paper involves investigation of the mechanical and durability performance of unstressed and stressed Tokyo Rope carbon-fiber composite cables for prestressing applications. This research is critical in order to establish the critical (allowable) stress and safety factors for the use of carbon-fiber composite cable tendons for prestressed precast-concrete members. The carbon-fiber composite cable specimens were exposed to simultaneous high alkali environment and sustained loading at different elevated exposure temperatures (22℃ and 60℃) for 3000, 5000, and 7000 h. The high alkali environment (12.8 pH) simulated the concrete pore solution and the elevated temperature was used to accelerate the aging process. The applied sustained stress on the carbon-fiber composite cable strands was equivalent to 40% and 65% of their guaranteed tensile strength. This was achieved through testing 171 carbon-fiber composite cable specimens subjected to stress levels of 0%, 40%, and 65% of their guaranteed strength, under tensile load. Also, 136 carbon-fiber composite cable specimens were tested to investigate the transverse shear strength. In addition, the durability characteristics of the constituent materials of the carbon-fiber composite cable strands were assessed to understand the long-term behavior of these materials. The results showed the effect of sustained stress on the degradation of carbon-fiber composite cable strands. Under sustained stress of 40% and 65%, the reductions in tensile strength were 10.6% and 12.3%, respectively. Scanning electron microscope results on epoxy resin indicated that no degradation is detected since the surface remains smooth without any pitting or loose material.

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.

Recently, in responding to the resource shortage and steel corrosion, the combination of seawater and sea sand concrete (SWSSC) and fibre reinforced polymer (FRP) bars was proposed to replace the traditional reinforced concrete structures in civil engineering. For FRP bars in the real service condition of concrete structure, the synergistic effect of sustained load and corrosion environment needs to be considered. This paper presents an investigation on the durability of basalt- and glass-fibre reinforced polymer bars exposed to a SWSSC environment under different sustained stress levels. Scanning electron microscopy was employed to study the degradation mechanism of FRP 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 issue of the freeze–thaw has significantly affected the safety of reinforced concrete (RC) structures. Strengthening is an effective method to improve the safety performance of RC structures. To investigate the freeze–thaw resistance of RC beams strengthened with textile-reinforced concrete (TRC), the crack pattern, failure mode, load–deflection behavior, and bearing capacity of TRC-strengthened RC beams under a salt freeze–thaw environment were studied. The test parameters included the number of freeze–thaw cycles, installation method (cast-in-place versus precast), and sustained loading. The test results showed that the failure mode of the TRC-strengthened beam changes from flexural failure to shear failure due to freeze–thaw cycles. The installation method also affected the failure mode of the beams. The beam prepared using the precast method exhibited debonding failure, whereas the beam prepared using the cast-in-place method exhibited shear failure. When the freeze–thaw cycles exceeded a certain number or the sustained load surpassed a certain value, the crack control ability of the TRC weakened. The freeze–thaw cycle changed the load–deflection curve of the TRC-strengthened beam from a typical three-stage form to an approximate line. The ultimate load of the TRC-strengthened beam decreased with an increase in the number of freeze–thaw cycles. The ultimate load was also affected by the sustained load: the greater the sustained load, the greater the decrease in the ultimate load.