Search

The deterioration of concrete structures is mainly due to the combined action of the environment and external load. In this study, 32 reinforced concrete columns were prepared to evaluate the coupling actions on the properties of reinforced concrete structures. The durability, bearing capacity, and failure mode of reinforced concrete columns were investigated under the combined action of freeze–thaw (F–T) cycles, sustained load, and salt corrosion (water or composite salt solution). Results show that the mass fluctuation of reinforced concrete columns under a sustained load was more obvious during F-T cycles. During the early F-T cycles, the sustained load was beneficial to the F-T resistance of the reinforced concrete columns. With the increase in F-T cycles, the damage to the columns with a sustained load gradually aggravated. In the composite salt solution, the damage to the reinforced concrete columns was postponed, and its durability showed a two-stage evolution. After 100 F-T cycles, the mass loss and relative dynamic modulus of elasticity (RDME) deterioration of the columns with a sustained load sped up significantly. The combined action of salt corrosion, load, and F-T cycles has the most significant influence on the bearing capacity, stiffness deterioration, and crack development of reinforced concrete columns. [ABSTRACT FROM AUTHOR]

The paper presents an experimental study for the evaluation of the flexural response and failure mode of a prestressed concrete (PC) beam subjected to simultaneous sustained loads and corrosion. The obtained results are judged and discussed also through a comparison with the experimental outcomes on a reference sound PC beam, and a companion specimen subjected to artificial corrosion first, and then tested in bending. The three specimens are characterized by a 200 × 300 mm rectangular cross section, a total length of 3700 mm, and a clear span of 2700 mm. The value of the sustained load, applied with a simultaneous accelerated corrosion process of the strands, was chosen, based on the result of the uncorroded reference beam, to achieve a scenario that can occur in a real structure in situ. The flexural response of the tested element was monitored over a period of 70 days, up to failure, and showed to be highly dependent on the localization of the corrosion phenomena affecting the strands, especially when coinciding with the maximum bending moment position. The obtained results are finally compared with the ones obtained on a PC beam with the same geometry and material properties, first subjected to corrosion and then tested in bending. The differences in corrosion morphology and location and in the failure mode of the strand confirm the importance of accounting for the combined effect of reinforcement corrosion and loading when assessing the structural performance of PC beams. [ABSTRACT FROM AUTHOR]

This paper presents the results of creep rupture tests conducted on two different sizes of glass fiber-reinforced polymer (GFRP) bars from two different manufacturers under high alkaline conditions at room temperature (approximately 23 and 60°C [73.4 and 140°F]). Regular tensile tests were also conducted on the bars at the two temperatures to provide insight into the effects of high temperature on their long-term performance. The results show that the larger bar performed slightly better at room temperature but significantly better at the elevated temperature. The larger-sized bars also lost less tensile strength at the elevated temperature. It was observed that temperature had a greater effect on the long-term performance of GFRP bars than alkalinity. The current design code limits on the allowable stresses were evaluated against the test results and found to be overly conservative. [ABSTRACT FROM AUTHOR]

This study presents a novel construction method for prefabricated wall elements by integrating a framework made of thin-walled sheet steel profiles into an optimized thermally insulating lightweight aggregate concrete (LAC) building envelope. The load-bearing function of the framework is provided by cold-formed Sigma-profiles, which are spot-welded to non-load-bearing U-profiles at the vertical ends. The LAC shapes the wall and stabilizes the thin-walled steel profiles against buckling, but has no further load-bearing function, thus allowing the reduction of its necessary compressive strength and subsequently minimizing its density. As a result, the LAC exhibits strength and density values well beyond existing standards, providing highly competitive thermal conductivity values that meet today's energy requirements without the need for additional insulation materials. Tailored composite specimens verify the stabilization of load-bearing sheet steel profiles by the LAC, which not only prevents buckling but also increases the load-bearing capacity of the overall system. The feasibility of this approach is validated by the production of two prototypes, each comprising a full-sized wall, in two different precast plants using distinct process technologies. [ABSTRACT FROM AUTHOR]

Using carbon fiber-reinforced composite (CFRP) grid stirrups to replace traditional steel stirrups in reinforced concrete (RC) beams can effectively improve the corrosion resistance of structures in marine environments. In this study, the durability of RC beams with CFRP grid stirrups under the coupled effects of sustained loads and immersion in seawater was studied. The sustained loads were set at 24% and 48% of the ultimate bearing capacity of the RC beam, and the specimens were immersed in artificial seawater at room temperature for 90 days, 180 days, and 360 days, respectively. The crack load and shear capacity of the beam were measured at the scheduled time. The initial stiffness of the beam decreased as the immersion time increased, and the higher the sustained load was, the lower the initial stiffness of the conditioned beam. The contribution of the CFRP grid stirrup to the shear load capacity after the cracking load was reached was more significant than that before. The effects of the 48% sustained load on the ultimate tensile strain of the CFRP stirrup were more significant than those for the 24% sustained load. Both immersion and sustained loading improved the cracking load and shear capacity of the beam, and the contribution of sustained loading to the shear capacity decreased as the immersion time and the sustained load level increased. Equations were proposed for predicting the shear strength of RC beams with CFRP grid stirrups under sustained loading and seawater immersion. [ABSTRACT FROM AUTHOR]

Cast-in fasteners, like headed-studs, are important elements widely used in safety-critical applications of the building industry. They allow the connection of structural components through transfer of stresses from load-bearing elements to concrete. Their tensile strength in an unconfined configuration with wide supports depends on the concrete's mechanical properties. Despite numerous studies performed on the short-term behaviour of cast-in anchors, little information is available on their sustained-load behaviour and the effect of the loading rate on their load capacity. The present research aims at studying these two effects by performing an experimental investigation consisting of sustained load tensile tests on cast-in headed studs. Firstly, short-term tests at different loading rates were performed. Secondly, long-term sustained load tests were performed at different load levels with respect to the ultimate load capacity. Two sets of anchors installed in two different concretes were tested. The first set consisted of 10 anchors tested to failure at different loading rates while sustained load tests were executed on additional 11 anchors. The second consisted of 14 anchors, 3 of which were tested to failure to determine their ultimate capacity and the remaining anchors were subjected to sustained loads at different load levels. The corresponding displacements and time-to-failure were continuously measured throughout the long-term tests. The results were used to construct time-to-failure curves where the load level is plotted against the time-to-failure in a semi logarithmic scale. The lifetime prediction of the anchors was assessed by applying a new model based on a sigmoid function. The predicted sustained load values for a 50-year service life are noticeably lower than the short-term capacity but remain larger than the characteristic load calculated according to standards for the design of cast-in anchors. [ABSTRACT FROM AUTHOR]

The steel corrosion of composite beams with corrugated steel webs (CBCSWs) is prone to occur in a chloride environment, and the load can change the rate of steel corrosion, thereby affecting the degradation of the mechanical properties of CBCSWs. In this paper, the flexural behavior of CBCSWs under the coupled effect of chloride ion erosion and sustained load was studied through an accelerated corrosion test and bending failure test. The results showed that, during the accelerated corrosion test, the deflection at the mid-span of the corroded CBCSWs increased more and faster than that of the uncorroded CBCSWs, and the stress of the externally prestressed tendons of the CBCSWs did not change significantly. During the loading failure process, the relative slip between the web and the concrete plates of the corroded CBCSWs had not been fully developed. The ultimate load and ultimate deflection of the corroded CBCSWs were decreased by 41.1% and 17.9%, respectively, compared to those of the CBCSWs before corrosion. The quasi-plane section assumption was still approximately applicable to the corroded CBCSWs. Compared with the uncorroded CBCSWs, the shear lag effect of the top plate of the corroded CBCSWs was more obvious. The externally prestressed tendons of the corroded CBCSWs could not give full play to their performance during the process of loading failure. [ABSTRACT FROM AUTHOR]

This study seeks to identify bond strength and bond-slip behavior of deformed reinforcing bars under high sustained loads. ASTM A944 beam-end test specimens were subjected to sustained load levels ranging from 71 to 100% of their ultimate capacity. The sustained loading tests were conducted until bond failure or at least 20 days. Three of the specimens failed under sustained load at load levels as low as 80% of the control specimens. Subsequent loading of the specimens that survived the high sustained load showed that the residual ultimate capacity was not reduced by the application of sustained load. The slip creep under sustained load was on average 124% of the initial slip, and over 45% of the slip creep occurred in the first day. The effects of concrete cover depth and bond length were examined. With the available data, a simple time-dependent model for determining the bond-slip under high sustained loading was suggested. [ABSTRACT FROM AUTHOR]

Recycled construction and demolition (C&D) wastes have been pointed out as a feasible alternative to traditional backfill materials of geosynthetic-reinforced structures, but the current knowledge about the interface behaviour between these unconventional (recycled) materials and the reinforcement is still limited, particularly as far as the time-dependent response is concerned. In this study, a series of large-scale direct shear tests was conducted using an innovative multistage method to evaluate the influence of shear creep loading on the direct shear response of the interfaces between a fine-grained C&D material and two different geosynthetic reinforcements (high-strength geotextile and geogrid). The peak and large-displacement interface shear strength parameters obtained from tests involving sustained loading were compared with those from conventional interface tests. Test results have shown that the shear creep deformation of the interfaces increased with the magnitude of sustained loading. The test specimens experienced additional vertical contraction during the creep stage, which tended to increase with the applied normal stress. For the recycled C&D material–geotextile interface, the sustained loading induced a reduction in the apparent cohesion and a slight increase in the friction angle, when compared to the values estimated from conventional tests. In turn, for the geogrid interface, the apparent cohesion values increased, whereas the friction angle did not significantly change upon shear creep loading. [ABSTRACT FROM AUTHOR]

The steel corrosion of composite beams with corrugated steel webs (CBCSWs) is prone to occur in a chloride environment, and the load can change the rate of steel corrosion, thereby affecting the degradation of the mechanical properties of CBCSWs. In this paper, the flexural behavior of CBCSWs under the coupled effect of chloride ion erosion and sustained load was studied through an accelerated corrosion test and bending failure test. The results showed that, during the accelerated corrosion test, the deflection at the mid-span of the corroded CBCSWs increased more and faster than that of the uncorroded CBCSWs, and the stress of the externally prestressed tendons of the CBCSWs did not change significantly. During the loading failure process, the relative slip between the web and the concrete plates of the corroded CBCSWs had not been fully developed. The ultimate load and ultimate deflection of the corroded CBCSWs were decreased by 41.1% and 17.9%, respectively, compared to those of the CBCSWs before corrosion. The quasi-plane section assumption was still approximately applicable to the corroded CBCSWs. Compared with the uncorroded CBCSWs, the shear lag effect of the top plate of the corroded CBCSWs was more obvious. The externally prestressed tendons of the corroded CBCSWs could not give full play to their performance during the process of loading failure.

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As part of a comprehensive study on the effects of climate change, tensile tests were performed on glass fiber-reinforced polymer (GFRP) and carbon FRP (CFRP) sheets under different temperature loadings to simulate field conditions. Specimens were loaded under two scenarios—that is, sustained temperature (ST) for 15 minutes followed by monotonic load until failure, and sustained deformation or sustained load (SD/SL) while the temperature was increased until the specimen failed. The elevated temperature caused a sharp decrease in the mechanical properties of the FRP as the temperature approached the glass-transition temperature (Tg) of epoxy. Beyond Tg, almost 40% and 50% residual tensile strength were retained by the GFRP and CFRP sheets, respectively. Elevated temperatures also shifted the failure pattern from brittle to softer failure due to epoxy softening. An equation is proposed for FRP properties subjected to elevated temperatures. [ABSTRACT FROM AUTHOR]

Recycled aggregate concrete (RAC) structures situated in cold regions are required to withstand the combined effects of mechanical load and environmental factors throughout their operational lifespan. The influence of sustained load introduces complexity to the degradation mechanism of RAC. This research examines the frost resistance of RAC across varying levels of recycled coarse aggregate (RCA) replacement (0, 50 %, 100 %) and sustained compressive loading (0, 0.3 f c , 0.5 f c). The frost resistance was quantified by the indices of macroscopic morphology change, mass loss, relative dynamic modulus of elasticity, and mechanical properties. Results show that 0.3 f c can effectively improve the frost resistance of RAC, a trend that becomes more pronounced with increasing replacement rates of RCA. Excessive stress (0.5 f c) was found to be unfavorable to the frost resistance of RAC and increase the compressive strength variability of RAC after freeze-thaw cycles. Analysis of meso/micro-structure characteristics using optical and scanning electron microscopes, along with the mercury intrusion porosimetry, reveals that optimal compressive loading (0.3 f c) enhances mortar integrity, reduces porosity, and improves compactness, thereby bolstering RAC's frost resistance. • Achieve and monitor the coupling effect of sustained compressive load and freeze-thaw cycles. • Construct a compressive strength prediction model for damage RAC. • Analyze the meso/micro-structure characteristics of damaged RAC. [ABSTRACT FROM AUTHOR]

Corrosion of steel reinforcing bars (rebars) in reinforced concrete structures can be addressed by using corrosion-resistant materials like Fiber-Reinforced Polymers (FRP). However, in France, FRP rebars are primarily restricted to temporary applications due to lack of comprehensive durability data. While numerous studies have explored FRP durability in alkaline environments, understanding the combined effect of alkali ions and sustained loading remains limited. Furthermore, the impact of incomplete matrix curing on the long-term performance of FRP rebars is limited. Accordingly, this paper investigates the durability of Glass FRP (GFRP) rebars exposed to alkali environments at varying temperatures. A batch of partially cured rebars was selected for this study, with part of them undergoing post-curing to investigate the effect of curing state on the ageing behaviour. Additionally, some partially cured specimens were subjected to combined sustained tensile loading at 20 % and 40 % of ultimate strength. Mechanical, physico-chemical and microstructural characterizations were conducted after various exposure durations. Results revealed that incomplete curing minimally impacted long-term tensile properties but significantly affected the interlaminar shear strength of aged rebars. Moreover, applying a combined load had little influence on residual properties at 20°C but led to rapid reduction in GFRP residual tensile strength at higher ageing temperatures (40 and 60°C). • Incomplete curing of GFRPs did not affect residual tensile properties after 90 days of direct alkaline aging. • For partially cured GFRP rebars, both post-curing and degradation occur during aging. • Sustained load significantly impacts the durability of GFRPs in alkaline environments. • This load effect exhibits high dependence on temperature. • Partial curing of the GFRPs may amplify the effect of load on long term performance. [ABSTRACT FROM AUTHOR]

Glass fiber-reinforced polymer (GFRP) and carbon fiberreinforced polymer (CFRP) sheets bonded to concrete were investigated to evaluate bond behavior, including effective bond lengths under two conditions: sustained temperature (ST) or sustained load with increasing temperature (SL). Tests were conducted using either double shear or flexural shear. Effective bond lengths of 70 and 100 mm (2.8 and 3.9 in.) were inferred for GFRP and CFRP sheets, respectively, from the double-shear tests. The effective bond lengths were then used for elevated temperature specimens, which showed significant reductions in the bond strength beyond the glass-transition temperature (55°C [131°F]). For 60°C (140°F) ST condition, up to 73% and 52% bond strength reductions were observed in double-shear and flexure specimens, respectively. The elevated temperatures also shifted the failure mode from thin concrete shearing to adhesive failure. Due to adhesive failure, the SL specimens with 30% and 50% sustained loads failed around temperatures of 125 and 110°C (257 and 230°F), respectively, regardless of the fiber type. [ABSTRACT FROM AUTHOR]

The effects of reinforcement corrosion and sustained axial load on mechanical performance of reinforced concrete (RC) columns were investigated in the present study. Three different degrees of reinforcement corrosion were achieved by controlling the durations of accelerated corrosion test (i.e., 16 days, 31 days, and 63 days). Three levels of sustained axial load (i.e., 0%, 30%, and 60% of the ultimate bearing capacity) were concentrically applied on column specimens. The impressing current and the sustained load were applied on column specimens simultaneously, mimicking the degradation of RC columns in real structures. Results indicated that transverse stirrups yielded higher corrosion degree than that of the longitudinal rebar under identical duration of accelerated corrosion test. The application of sustained axial load improved the performance of corroded RC columns in terms of the reinforcement corrosion, the ultimate axial load, as well as the stiffness. Additionally, more longitudinal cracks along the main rebar were exhibited for column specimens subjected to sustained axial load. For both loaded and unloaded column specimens, corrosion of reinforcing steels exacerbated the mechanical deterioration of RC columns, lowering the ultimate load carrying capacity and the axial deformation compared to the uncorroded columns. [ABSTRACT FROM AUTHOR]

High levels of sustained load can lead to time-dependent failure of reinforced concrete (RC) members. This in turn may lead to collapse of all or part of a building. Design errors, construction errors, and material deterioration may lead to concrete elements being subjected to high levels of sustained loads well exceeding typical service loads. Plain concrete can experience compressive failure when subjected to a high sustained stress (over 75% of its short-term strength). However, there is a lack of knowledge about the time-dependent strength and stiffness characteristics of RC members under high sustained loads. This paper presents the results of experimental testing of simply supported shear-controlled RC beams under high sustained loads. Two series of beams, consisting of 4 and 5 beams, were tested at concrete ages of 67 to 543 days to represent in-service concrete structures. The applied sustained loads ranged from 82% to 98% of the short-term capacity and lasted for 24 to 52 days. Test results indicated that high sustained load may eventually lead to failure (collapse); however, the level of load needs to be very close (~98%) to the short-term capacity. Under sustained load, all specimens experienced increased deflection with over half of the deflection increase occurring in the first 24 h. The sustained load increased the deflection at shear failure by 190% on average. The increase in the beam deflection may allow for load redistribution in redundant structural systems. A sharp increase in deflection due to tertiary creep occurred in a short time (~2 min) before failure, indicating little warning of the impending failure. [ABSTRACT FROM AUTHOR]

Frost damage is an important deterioration parameter for reinforced concrete (RC) structures in cold and humid environment, which usually acts simultaneously with sustained load caused by self‐weight, immobile weight, and so forth. This article conducts a thermo‐hydro‐mechanical simulation on the structural behavior of RC beams under the coupled effect of frost damage and sustained load. This simulation approach starts from the microscale material‐based frost damage model of multicomponent substance to the macroscale structural‐based performance evaluation of RC beam. On the basis of this approach, the damage accumulation of RC beam model under both frost action and sustained load is simulated. Afterward, flexural bending load is applied to investigate the structural performance of the RC beam which has suffered coupled damage. Laboratory experiments also show a satisfactory agreement with numerical simulation. Parametric study is also conducted to further evaluate the influence by different parameters on the structural deterioration, for example, reinforcement arrangement, water to cement ratio, and so on. Finally, the equivalent frost‐damaged strength with respect to the damaged region is adopted to calculate the flexural capacity according to the design code, and a simple method is proposed to verify or predict the properties of RC beam under the coupled effect of frost damage and sustained load. [ABSTRACT FROM AUTHOR]

Long-term marine erosion and sustained loading will lead to the deterioration of the interface mechanical properties between smart-functional engineered cementitious composite (ECC) and steel rebar. To synthetically investigate the mechanical properties and self-sensing properties of interface between smart-functional ECC and steel rebar subjected to sustained load and chloride ion erosion, the coupling test of sustained load and chloride ion erosion and the self-sensing test were carried out, in which the test variables included ECC type, electrochemical corrosion time, preloading sustained load, and different erosion environments. The results showed that the smart-functional ECC type, electrochemical corrosion time and preloading sustained load had significant effects on the corrosion rate of rebar, interface bond strength and slip. The smart-functional ECC of TN type had the best corrosion resistance of steel rebar. Two failure modes of pull-out split failure and pull-out shear failure were found. An important finding is that the failure mode has significant influence on the self-sensing performance of the interface. Among them, the specimens with pull-out split failure mode had a good self-sensing performance, while the specimens with pull-out shear failure mode had a poor self-sensing performance. Another important finding is that the general development trend of the interface bond strength-time curve and the self-sensing property-time curve of specimens with pull-split failure mode was basically the same, and the two curves had a very good consistency. This finding provided a feasible way to monitor the mechanical properties of the interface by self-sensing properties. This study can provide a more comprehensive understanding and guidance of mechanical and self-sensing properties of interface between smart-functional ECC and rebar suffered from sustained load and chloride ion erosion. [Display omitted] • The feasibility of the self-sensing performance of interface between rebar and ECC based on the smart-functional ECC is studied. • The degradation law of mechanical properties of rebar-to-ECC interface under the coupling action of sustained load and chloride ion erosion is revealed. • The correlation and variation of the interface bond strength-time curve and the self-sensing property-time curve are revealed. [ABSTRACT FROM AUTHOR]

In this work, a total of 20 beam type concrete specimens were prepared to investigate bond behavior of glass fiber reinforced polymer (GFRP) bar embedded in marine concrete. The influence of sustained loads (load level: 0.25, 0.45 and 0.65 ultimate load (P u)) and stirrup on bond properties was investigated. After sustained load period, flexural bond test was conducted to determine bond stress and slip. Test results indicate that bond strength of GFRP bars shows decreased trend with increasing sustained loading. Compared with specimen not subjected to sustained load, bond strength significantly decreased under 0.25, 0.45 and 0.65 P u. Also, sustained load deteriorates the bond stiffness and bond toughness. What' s more, differences in failure modes and bond strength of steel bar and GFRP bars in flexural bonding tests were analyzed and compared. All GFRP bar specimens subjected to sustained load exhibit bars pull-out failure. In contrast to steel bar, the influence of stirrups on GFRP bar-marine concrete bond strength is relatively limited (specimens with stirrups exhibited only 1.8 % increase over specimens without stirrups), due to GFRP bars lower rib heights and weaker mechanical interlocking with concrete. Characterization of different stages of bond-slip curves of GFRP bar beam specimen subjected to sustained loads was carried out and bond-slip curves of GFRP shows two peaks. According bond strength and slip test data, bond stress-slip mathematical relationship model between GFRP bar and marine concrete under sustained load has been fitted. • Bond strength of GFRP bars shows decreasing trend with increasing sustained load. • Influence of stirrups on bond strength of GFRP - marine concrete is relatively limited. • Bond stress-slip mathematical relationship model under sustained load has been fitted. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) bar reinforced ultra high-performance engineered cementitious composite (UHP-ECC) structures (referred to as FRP-UHP-ECC structures) have been demonstrated to have excellent mechanical properties, while their durability in the alkaline environment has never been explored, particularly when being subjected to tensile loadings. To this end, this article aims to investigate the effects of sustained loads on the durability performance of Glass FRP (GFRP) bars embedded in UHP-ECCs while immersed in an alkaline solution. The test results revealed that the effect of elevated temperatures was exacerbated when specimens were subjected to even a low sustained load (20 % of ultimate strength). Furthermore, UHP-ECC covers considerably postponed the degradation of GFRP bars. Nevertheless, the protective effects of UHP-ECC covers were nearly entirely compromised when the specimens were subjected to sustained loads. The findings of this study provide useful knowledge for a comprehensive evaluation of the durability of GFRP bars under UHP-ECC environments. • The effect of elevated temperatures on GFRP bar degradation was exacerbated under sustained loads. • This study revealed that even lower-level sustained loads (i.e., 20 %) led to evidently more pronounced degradation of UHPECC-embedded GFRP bars compared to those without sustained loads. • UHPECC covers effectively postponed the degradation of GFRP bars, but their protective effects were nearly completely compromised under sustained loads. • SEM and X-CT analysis revealed that the degradation of GFRP bars may be associated with the development of porosity or connected porosity after exposure. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Architecture & Civil Engineering is the property of Chang'an Daxue Zazhishe and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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. [ABSTRACT FROM AUTHOR]

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-free states and have drawn conclusions about durability from these data. To study the influence of different stress levels on the corrosion and eccentric compression performance of reinforced concrete columns, this study applied an accelerated corrosion test to reinforced concrete specimens with different preloading levels and then conducted eccentric compression axial force capacity tests and analyses. The results showed that for an ordinary reinforcement stirrup member, an increase in the level of preloading reduced the mass loss of the reinforcing bars, and there was a corresponding decrease in the loss of eccentric compression capacity and stiffness. For members with an epoxy-coated reinforcement stirrup, an increase in the preloading level increased the mass loss of the reinforcing bar, and the ultimate axial force capacity of the member under eccentric compression was reduced accordingly. [ABSTRACT FROM AUTHOR]

Corrosion-resistant glass fiber-reinforced polymer (GFRP) bars have been growing in popularity as an alternative to steel reinforcement. This study investigated newly developed GFRP bent bars for strength reduction of the bends in relation to the straight portion, as well as their long-term behavior under sustained load. Two test methods were used to determine the bend strength, consisting of 24 stirrups in the standardized testing procedure, and 10 bent bars in the shape of a “J” in a modified direct tension pullout test. The bend strength ranged from 35 to 55% of the straight portion. Additionally, sustained load tests were carried out on eight stirrups to estimate the stress level the bars could sustain for a 100-year service life. This stress, extrapolated from a logarithmic creep equation developed from regression analysis of the test results, was found to be close to 50% of the instantaneous strength. [ABSTRACT FROM AUTHOR]

High levels of sustained load can lead to time-dependent failure of reinforced concrete (RC) members. This in turn may lead to collapse of all or part of a building. Design errors, construction errors, and material deterioration may lead to concrete elements being subjected to high levels of sustained loads well exceeding typical service loads. Plain concrete can experience compressive failure when subjected to a high sustained stress (over 75% of its short-term strength). However, there is a lack of knowledge about the time-dependent strength and stiffness characteristics of RC members under high sustained loads. This paper presents the results of experimental testing of simply supported shear-controlled RC beams under high sustained loads. Two series of beams, consisting of 4 and 5 beams, were tested at concrete ages of 67 to 543 days to represent in-service concrete structures. The applied sustained loads ranged from 82% to 98% of the short-term capacity and lasted for 24 to 52 days. Test results indicated that high sustained load may eventually lead to failure (collapse); however, the level of load needs to be very close (~98%) to the short-term capacity. Under sustained load, all specimens experienced increased deflection with over half of the deflection increase occurring in the first 24 h. The sustained load increased the deflection at shear failure by 190% on average. The increase in the beam deflection may allow for load redistribution in redundant structural systems. A sharp increase in deflection due to tertiary creep occurred in a short time (~2 min) before failure, indicating little warning of the impending failure.

Abstract 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.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In recent decades, concrete-filled fiber-reinforced polymer tube (CFFT) columns have gained increasing popularity in bridge construction as an alternative to conventional reinforced concrete columns. CFFT columns have excellent structural performance, which is attributed to the superior properties of the fiber-reinforced polymer (FRP) tubes. Furthermore, using FRP tubes eases the construction of CFFT columns. However, one obstacle hindering the greater acceptance of FRP as a common construction material in civil infrastructure application is the susceptibility of FRP to degradation during long-term exposure to a severe environment. The purpose of this study is to investigate the durability of CFFT columns subjected to seawater corrosion, which is the scenario for seashore bridges. CFFT stubs were immersed in simulated seawater with two different elevated temperatures for up to 450 days. Sustained axial loads were also applied to the stubs to simulate the real-life service load. Compression tests and hoop tensile tests were carried out on both pre- and post-conditioned specimens. [ABSTRACT FROM AUTHOR]

This study presented an experimental investigation on the degradation of mechanical performance of reinforced concrete (RC) columns with the reinforcements corroded under sustained loads. A total of ten RC column specimens were tested. The effects of different levels of sustained load (0%, 30%, and 60% of the designed ultimate bearing capacity Nu) and reinforcement corrosion (0%, 5%, 10%, and 20%) on the failure modes, ultimate bearing capacity, and axial load–axial deformation relationship were analyzed. The results showed that the coupling adverse effects due to the reinforcement corrosion and sustained load considerably exacerbate the mechanical deterioration of RC columns and turn the failure mode into a much more brittle manner. Compared with the control specimen L-0-0, the ultimate bearing capacity of the specimen L-4-20 could be reduced as much as about 42%. Based on the test results, an improved model was proposed to estimate the ultimate bearing capacity of corroded RC columns, in which the effects of the corrosion of both longitudinal reinforcements and stirrups and the corrosion-induced spalling of concrete cover were taken into consideration. The close agreements between the analytical predictions and test results prove the applicability of the model. [ABSTRACT FROM AUTHOR]

In this study, the chloride ion diffusion and structural performance of beams reinforced with textile-reinforced concrete (TRC) were evaluated. The parameters investigated were chloride concentration, sustained load and number of textile layers. The results demonstrate that the content and diffusion coefficient of chloride increased with increasing chloride concentration. Higher chloride concentrations accelerated the crack propagation and deflection changes and caused the reduction of the load-carrying capacity of the beams. The sustained load promoted the chloride transport of the TRC, increasing the chloride ion content and diffusion coefficient and causing substantial damage to the microstructure of the TRC. In addition, the performance (such as cracking resistance, deflection and flexural capacity) of beams with a large sustained load ratio decreased to a less extent than did the performance of the unloaded beams. The content and diffusion coefficient of chloride in the unstrengthened beams were obviously larger than those in the strengthened beams, but increasing the textile layers number had little influence on these factors. In addition, for the unstrengthened beams, the cracks and deflections developed rapidly, and the load decreased greatly, especially the cracking load. Finally, in accordance with Fick's second law of diffusion, a chloride diffusion equation in TRC layers under new boundary conditions was proposed. [ABSTRACT FROM AUTHOR]

The electro-mechanical impedance (EMI) technique has been applied successfully to detect minor damage in engineering structures including reinforced concrete (RC). However, in the presence of temperature variations, it can cause false alarms in structural health monitoring (SHM) applications. This paper has developed an innovative approach that integrates the EMI methodology with multilevel hierarchical machine learning techniques and the use of fiber Bragg grating (FBG) temperature and strain sensors to evaluate the mechanical performance of RC beams strengthened with near surface mounted (NSM)-fiber reinforced polymer (FRP) under sustained load and varied temperatures. This problem is a real challenge since the bond behavior at the concrete–FRP interface plays a key role in the performance of this type of structure, and additionally, its failure occurs in a brittle and sudden way. The method was validated in a specimen tested over a period of 1.5 years under different conditions of sustained load and temperature. The analysis of the experimental results in an especially complex problem with the proposed approach demonstrated its effectiveness as an SHM method in a combined EMI–FBG framework.

Although some extended studies about the short-term behavior of NSM FRP strengthened beams have been carried out, there is a lack of knowledge about the behavior of this kind of strengthening under sustained loads and high service temperatures. Electromechanical impedance method formulated from measurements obtained from PZT patches gives the ability for monitoring the performance and changes experienced by these strengthened beams at a local level, which is a key aspect considering its possible premature debonding failure modes. This paper presents an experimental testing program aimed at investigating the long-term performance of a concrete beam strengthened with a NSM CFRP laminate. Long term performance under different levels of sustained loading and temperature conditions is correlated with EMI signatures processed using Linear Mixed-effects models. These models are very powerful to process datasets that have a multilevel or hierarchical structure as those yielded by our tests. Results have demonstrated the potential of these techniques as health monitoring methodology under different conditions in an especially complex problem such as NSM-FRP strengthened concrete structures.

Motivated by tunnel accidents in the recent past, several investigations into the sustained load behavior of adhesive anchors have been initiated. Nevertheless, the reliable lifetime prediction of bonded anchor systems based on a relatively short testing period still represents an unsolved challenge due to the complex nonlinear viscoelastic behavior of concrete and adhesives alike. This contribution summarizes the results of a comprehensive experimental investigation and systematically carried out time-to-failure analysis performed on bonded anchors under sustained tensile load. Two different adhesive materials that find widespread application in the building industry were used, one epoxy and one vinylester based. Performed experiments include full material characterizations of concrete and the adhesives, bonded anchor pull-out tests at different loading rates, and time-to-failure sustained load tests. All anchor tests are performed in a confined configuration with close support. After a thorough review of available experimental data and analysis methods in the literature, the experimental data are presented with the main goal to (i) provide guidance for the analysis of load versus time-to-failure test data, and (ii) to derive a set of recommendations for efficient time-to-failure tests having in mind the needs associated with different analysis techniques. Finally, a new approach based on a sigmoid function, previously used only for concrete, is for the first time applied to bonded anchors systems and compared to the established regression models. [ABSTRACT FROM AUTHOR]

• The similarity between natural corrosion and electrified corrosion was considered in this paper. • Stirrup and cathode position had important influences on the corrosion characteristics of RC beams. • The influence of different sustained load levels on the corrosion characteristics of RC beams was studied. • The corrosion of stirrups was the most serious, followed by tensile longitudinal bars and compressive longitudinal bars. 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. [ABSTRACT FROM AUTHOR]