Sustained Load Level Research Articles

Flexure Behaviour of Corroded Reinforcement Concrete Beams Under Sustained Loads

Abstract The long-term behaviour of concrete beams with corroded reinforcement is a critical aspect that demands thorough comprehension to ensure the serviceability and durability of reinforced concrete structures. This paper aims to investigate the long-term performance of reinforced concrete (RC) beams with corroded reinforcement numerically. Two experimental datasets from existing literature were selected to build a nonlinear finite element model. The selected beams were tested under four-points sustained bending load and three bending point flexural test with deferent level of corrosion. A parametric study was then conducted taking into account concrete compressive strength, reinforcement yield strength, ambient temperature, relative humidity and sustained load level to get a better understanding on the factors that effecting the long-term behaviour of concrete beams with corroded reinforcement. Results demonstrated that, Diana FEA has an excellent ability to predicating the long-term behaviour of RC beams with corroded reinforcement. The analysis of the parametric study revealed that increasing relative humidity from 70% to 90% resulted in a 45% to 53% reduction in developed deflections. Whereas once the temperature rose from 20°C to 40°C the developed deflection increased from 13.9% to 17.2%. This outcome demonstrating the significant impact of environmental factors on the long-term behaviour of corroded reinforced concrete beams. Finaly, once the yiled stregth of the steel reinforcement increased from 380 Mpa to 550 Mpa, the developed deflection rate increased from 46% to 54%.

Experimental study on bond behavior of corroded reinforced concrete under coupling effect of fatigue load and elevated temperature

The bond properties of corroded reinforcing bars in concrete are mainly determined by the failure time and ultimate capacity of reinforced concrete (RC) structures. In this study, 98 cube specimens were used to study the corrosion effect on the bond failure mechanism of RC structures under transient high temperature. Firstly, an electrification accelerated corrosion test was carried out to fabricate the corroded specimen with various corrosion degrees (0 %, 5 %, 10 %, and 15 %). Secondly, the undamaged specimen was conducted by pull-out test at ambient temperature to obtain the initial bond strength, and then a transient temperature test under sustained load was also performed to analyze the effects of different load ratios, concrete strength, and corrosion degrees on the bond behavior. SEM (Scanning Electron Microscope) was conducted to verify the damage level by comparing the interfacial transition zone. Test results indicate that the coupling effect of corrosion, fatigue load, and high temperature increases the carbonization level of the specimens and reduces the concrete compressive strength after bonding damage. The ultrasonic pulse velocity decreases with increasing corrosion degree and sustained load level and decreases with increasing concrete strength grade. Under the coupling effect of corrosion, high temperature, and sustained load, the bonding failure time is significantly affected by the sustained load ratio, and the failure slip is determined by corrosion and temperature. Finally, an empirical slip-location constitutive model was proposed, incorporating the sustained load ratio, corrosion degree, and temperature. The accuracy of the predicted model agrees with the error requirement, and the constitutive model provides a theoretical basis for the collapse warning of corroded structures under fire.

Association between different levels of suppressed viral load and the risk of sexual transmission of HIV among serodiscordant couples on antiretroviral therapy: a protocol for a two-step systematic review and individual participant data meta-analysis

IntroductionHIV is a major global public health issue. The risk of sexual transmission of HIV in serodiscordant couples when the partner living with HIV maintains a suppressed viral load of...

Fracture properties of mixed mode I-II cracks in concrete after sustained loading

To investigate the fracture properties of mixed mode I-II cracks in concrete after sustained loading, the basic creep tests were conducted on concrete beams with five initial mode mixity ratios under three-point bending (TPB) loading or four-point shearing (FPS) loading, two sustained load levels, 80 % of the initial cracking load and the initial cracking load, for a period of 90 days. After creep tests, the specimens were unloaded and immediately loaded up to failure under the quasi-static TPB/FPS loading. Meanwhile, the entire crack propagation process was monitored using the digital image correlation technique. The numerical simulations based on the Norton-Bailey creep model were conducted to capture the time-dependent mechanical response of concrete under sustained loading. The experimental and numerical results showed that the pre-crack did not initiate in the creep tests unless the applied sustained loading exceeded the initial cracking load observed under quasi-static loading. It can be attributed to the viscoelastic characteristics of the concrete, which caused a decrease in the average stress around the pre-crack tip. A load increment was necessary to compensate for the reduced stress, resulting in an increase in the initial cracking load and the crack resistance of the structures. Moreover, sustained loading showed no significant effect on the peak load, fracture angle, initial cracking angle and crack propagation path. It suggests that sustained loading primarily induced a viscoelastic response of the concrete around the pre-crack tip rather than promoting extensive crack growth. Therefore, it is reasonable to approximate crack propagation paths of the concrete after sustained loading by considering the paths observed under quasi-static loading.

Fracture energy of concrete after sustained loading

The fracture energy of concrete after sustained loading is a necessary parameter to calculate the residual carrying capacity of concrete structures in service. This study investigated the fracture energy of concrete after sustained loading by combining experimental observations and theoretical analyses. Firstly, a series of creep fracture tests were conducted on 100 three-point bending beams under various sustained loads, i.e., 75 %, 80 %, 85 %, and 90 % of peak loads. After different loading time, i.e., 10 %, 30 %, 50 %, 70 %, and 90 % of creep fracture lifetimes, the static fracture tests were performed until failure. Then, based on the relationship between the work done by external forces and the energy consumed by the crack propagation of specimens, an analytical model for the fracture energy of concrete after sustained loading was developed, describing the quantitative relationship between the fracture energy of concrete and the crack opening displacement, and the loading time. Finally, the impact of the loading time and sustained load levels on the fracture energy was investigated. The results indicated that the fracture energy of concrete after sustained loading decayed rapidly, with the decay rate gradually decreasing until stability over the loading time. Additionally, specimens subjected to lower sustained load levels experienced a longer loading time for the same crack opening displacement, indicating a greater decay degree of the fracture energy. The analytical model proposed in this study can be employed for predicting and evaluating the crack propagation process and residual carrying capacity of concrete structures during their service life.

Time-dependent behavior of RC slab-column connections under high sustained loads

Reinforced concrete (RC) flat-plate slab-column connections may be exposed to high sustained stresses under gravity loads far exceeding the typical service-level stresses due to design errors, construction defects, or material deterioration. If the connections fail, then collapse of the structure could occur. Plain concrete can fail when exposed to sustained compressive stress in excess of 75% of its short-term strength. However, very little research has been conducted on the time-dependent strength and stiffness characteristics of RC slab-column connections under high sustained loads. This paper presents the experimental results of eleven aged slab-column connections under sustained concentric and eccentric gravity loading. The specimens were constructed at a 0.47-scale and had slab tensile reinforcement ratios of 0.64% and 1.0% without using shear reinforcement. Three specimens were loaded to failure in a short time, while eight specimens were tested for periods of approximately 45 days under high sustained loads (83% to 98% of their peak capacity). Test results showed that high sustained loads may lead to an eventual failure; however, the level of sustained load needs to be very close (∼95%) to the short-term capacity. Under the sustained loads, the deflection of specimens increased by 18% to 39% of the initial deflections. The deflection and steel strain creep rate approximately followed a logarithmic function, but there was a greater increase in deflection than steel strain. A sharp increase in deflection due to tertiary creep occurred only 2 to 3 min before failure, leaving little warning of the impending failure.

Long-term shear performance of concrete beam with carbon fiber-reinforced composite grid stirrups under sustained loadings and seawater immersion

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.

Determination of SIFs and application of a SIF-based fracture criterion for concrete-rock interface under sustained loading

This study proposes an effective method to determine the stress intensity factors (SIFs) of the interface crack considering the viscoelasticities of the bilateral materials and verifies the feasibility of the initial fracture toughness-based fracture criterion for the concrete-rock interface under sustained loading. Firstly, low-level sustained loading tests were conducted on the composite concrete-rock specimens with respect to two interface roughness degrees and two sustained load levels less than the initial cracking load. Then, an implicit creep equation was employed to characterise the viscoelasticities of bilateral materials and then calculate the mechanical responses of the concrete-rock interface in the low-level sustained loading test. The SIFs of the interface crack under sustained loading were determined by using the elastic strain energy density ahead of the crack tip. By averaging the SIFs over the radius of the core region of the crack tip, the representative SIFs under sustained loading were obtained. The test results indicated that the SIFs showed decreasing tendencies under sustained loading, and the initial fracture toughness can be considered as a material property of the concrete-rock interface. The initial fracture toughness-based fracture criterion was verified to be feasible to determine the initial cracking status of the concrete-rock interface under sustained loading. In addition, due to the reductions of the SIFs under sustained loading, additional loads were required to compensate the losses of the SIFs, leading to the enhancement of the initial cracking loads after experiencing the sustained loading.

Physical modelling of subsea structure removal via thermally induced pore pressure

Given the requirements of environmental agencies, the decommissioning of subsea structures, including removal, recycling, restructuring or re-adaptation, is inherent to deepwater oil exploration activities. During removal by lifting, the possible development of negative excess pore pressure (i.e., suction) in the soil beneath the structure significantly hampers the recovery of subsea infrastructure. This study presents a method for potentially reducing this suction by heating the fine soil under the foundation during uplift to produce excess thermally-induced positive pore pressure, which may reduce or eliminate suction. Reduced-scale physical modelling studies of mudmat/skirted like foundation structures showed that the negative pore pressure generated during pullout was in certain amount counterbalanced by the thermally-induced positive pore pressure, facilitating their removal. However, this process depends on the magnitude of the sustained loading with respect to the pullout resistance. Upward displacements take place just after heating, without further load increments and the displacement rate is dependent on the level of sustained load.

Numerical studies on creep behaviour of SRCFST columns with initial stress of steel tube

The long-term creep behaviour of steel-reinforced concrete-filled steel tubular (SRCFST) column under whole life cycle is a key issue in engineering design. Therefore, a finite element (FE) analysis based on ABAQUS software is conducted to study the creep performance of SRCFST columns considering the initial stress of steel tube. Based on the verified FE model, the whole process load-deformation curves, load distributions of each component and longitudinal strain versus time curves are obtained. The stress distributions of each component are analyzed at the feature points. The effects of main parameters on the creep behaviour are studied, including the sustained load level, initial stress ratio, compressive strength of concrete, steel ratio of steel tube and encased steel section. The studies show that the SRCFST member exhibits an elastoplastic behaviour subjected to the coupling effect of initial stress on steel tube and sustained load. However, the creep ratio undergoes a change from linear to nonlinear by increasing long-term load or initial stress ratio. Finally, a method for calculating the creep in SRCFST columns under combined loads is provided, and the long-term performance can be better predicted.

Tensile properties of carbon fabric-reinforced cementitious matrix (FRCM) at high temperatures

Despite many advantages of externally bonded FRP systems, the rapid loss of strength at high temperatures is a concern for fire scenarios. Fabric-reinforced cementitious matrix (FRCM) is deemed to perform superior at high temperatures. In this study, 84 carbon FRCM coupons were tested in tension at temperatures from 25 to 400 °C using both steady-state and transient-state test protocols. The main parameters considered in this study are the number of fabric layers and FRCM thickness, fabric orientation, target temperatures, and heating protocol. Results showed that at room temperature, the ultimate strength and cracked elastic modulus decreased by about 18% to 20% when the number of layers was increased from one to three. At 400 °C, the ultimate strength and cracked elastic modulus were reduced by up to 60 to 70% of their original values. The bidirectional FRCM specimens presented a slightly higher elastic modulus and strength than unidirectional specimens in most tests. In transient-state tests and under the same sustained load level, the failure temperature of 30 mm-thick specimens were approximately 100 °C higher than 20 mm-thick specimens. The test results showed that the carbon FRCM composites tested in this study were able to resist non-negligible tensile forces at elevated temperatures although they are significantly reduced.

Hydro-mechanical-damage model for the secondary creep of fiber reinforced mortar at high stress-to-strength ratio

Recent works have showed that the secondary creep of concrete under sustained high load level of load-to-strength ratio is likely due to a strong coupling between damage and drying shrinkage, which locally occurs in the fracture process zone. The scope of this work is to develop a simplified damage-poromechanical model for the secondary creep of concrete, which directly accounts for such coupling. It was simply assumed that microcracking affects the distribution of the moisture content by scaling the adsorption isotherm with damage. The proposed hydro-mechanical-damage model couplings has been implemented into a discrete lattice method based on dually coupled conduit elements and mechanical element. Notably, the drying shrinkage is accounted within the poromechanical framework of the partially saturated media. The hydro-mechanical-damage model can engender microcrack process zone which govers the secondary creep of concrete at high stress. The model has been validated on 2D experiments of secondary creep on FRC which considers the effect of water-to-cement ratio and aggregate inclusion Finally, the model is validated against experimental results on secondary creep fiber reinforced mortar (FRM) beam considering the effect of concrete heterogeneity.

Bond performance of the additional aluminum alloy ribs anchored CFRP bars in concrete under sustained loading

The novel additional aluminum alloy ribs (ARs) anchorage is a convenient and effective solution to improve the bond of FRP bars in concrete. Although the bond behavior between the ARs anchored FRP bars and concrete under the short-term loading has been recently investigated, its long-term behavior under sustained loading has yet been assessed. This paper aims to study the long-term bond behavior of both the unanchored and ARs anchored CFRP bars in concrete under sustained loading for a period of 380 days. 12 pull-out specimens were tested under sustained loading considering parameters including the bond length, sustained loading level, and the type of FRP bars. Results showed that the ARs anchored specimens effectively reduced the long-term slip by as much as 48.7%-52.5% compared to that of the referenced unanchored specimens due to an improved bond mechanism. In addition, an analytical model was proposed to predict the long-term end slip for both the unanchored and ARs anchored FRP bars in concrete.

Performance assessment of partially corrosion-damaged RC segment incorporating the spatial variability of steel corrosion

Steel corrosion is a dominant source of structural performance deterioration for reinforced concrete (RC) shield tunnels, which is always spatially nonuniform and discretely distributed in RC segmental linings. To accurately estimate the deteriorated performance of RC segmental linings under coupling actions of flexural and axial loads, the research on the mechanical behavior of RC segments has to be integrated with the effects of nonuniform steel corrosion and loading eccentricity. In this paper, the failure modes of corroded RC segmental specimens and their damaging evolution under ultimate loading were experimentally investigated, and the effects of spatial variability of steel corrosion and level of sustained load were discussed. A probabilistic model for simulating the spatial variability of steel corrosion within a partial length of longitudinal reinforcement was proposed. Finally, Monte Carlo simulation (MCS) incorporated with finite element (FE) analysis was performed to investigate the strength loss of corroded RC segmental specimens, emphasizing on the effects of spatial distribution of steel cross-section area loss, degree of steel corrosion and eccentricity of loading.

Time-Dependent Fracture Behavior of Rock–Concrete Interface Coupling Viscoelasticity and Cohesive Stress Relaxation

This study investigated the time-dependent crack propagation of the rock–concrete interface under sustained loading. First, sustained loading tests were performed on the composite rock–concrete beams under three-point bending with respect to three interface roughness degrees, i.e., natural, 4×4, and 7×7 interfaces, and three sustained load levels, i.e., 80% of the maximum load, the initial cracking load, and 97% of the maximum load. Then, by employing the Norton-Bailey model and the time-dependent fictitious crack model, the constitutive relationships of bilateral materials and the rock–concrete interface were established and the mechanical responses of the rock–concrete interface under sustained loading were simulated. In addition, a crack propagation criterion proposed in this study was applied in the numerical procedure to simulate the time-dependent crack propagation process under sustained loading. The criterion implied that the interface crack propagated when the difference between the average elastic strain energy densities near the crack tip caused by the external load and cohesive stress exceeded that at the initial fracture status under quasi-static loading. The results indicated that the crack propagation under sustained loading could be interpreted from the view of energy balance between the driving energy from the external load and the resistance energy from the cohesive stress. The time-dependent crack propagation under sustained loading experienced three stages, i.e., decelerated propagation, uniform propagation, and accelerated propagation. A linear relationship between the logarithms of the crack mouth opening rate in the uniform propagation period and the failure time was established to predict the failure time of the rock–concrete interface under sustained loading.

Long-term behaviour of recycled aggregate concrete beams prestressed with carbon fibre-reinforced polymer (CFRP) tendons

The utilisation of recycled aggregate concrete (RAC) is commonly recognised as one of the most effective means to achieve cleaner and sustainable development in civil engineering. To promote the application of RAC, prestressing technique can be employed to overcome the inherent disadvantages of RAC beams associated with poor cracking resistance and large deflections. However, the research on the prestressed RAC beam is quite limited, and no research has been performed on their long-term behaviour. This paper presents a finite element (FE) analysis for investigating the long-term behaviour of RAC beams prestressed with carbon fibre-reinforced polymer (CFRP) tendons considering the effects of concrete creep, concrete shrinkage, tendon relaxation as well as the cracking and tension stiffening of concrete. Based on the principle of superposition, the variations of the stress and strain with time are considered in the numerical analysis using step-by-step method. The FE model is calibrated with the experimental results obtained from the literature. Based on the validated model, a comprehensive parametric study is performed to investigate the effects of the replacement ratio of recycled coarse aggregate (RCA), concrete strength, reinforcement ratio, prestress level, and sustained load level on the long-term behaviour of prestressed RAC beams. The obtained results demonstrate that increasing the prestress load is an effective way to reduce the long-term deflection of RAC beams, and the use of RAC with low RCA replacement ratio is suggested. Besides, the main causes of the long-term deflection and axial shorting of the CFRP prestressed RAC beams are also studied. This research provides a pioneering and insightful study of the long-term behaviour of CFRP prestressed RAC beams. The obtained results demonstrate that the time-dependent effects should be well considered in the design before the prestressed RAC beam is used in practice.

Viscoelasticity-induced fracture behavior of rock-concrete interface after sustaining creep process

This paper explored the viscoelasticity-induced fracture behavior of rock-concrete interface after sustaining the creep process. Creep tests were firstly conducted on the composite rock-concrete specimens under three-point bending loading at the load levels of 50% and 75% of the maximum load. After 90-day sustained loading, composite specimens were unloaded and then reloaded under quasi-static loading conditions, and the digital image correlation technique was employed to quantify the crack profiles at the rock-concrete interface. The results indicate that the pre-set crack at the rock-concrete interface would not initiate further during the creep tests if the sustained load was below the initial cracking load. After sustaining a creep process, the initial cracking load increased obviously but the elastic strain energy near the pre-crack tip remained unchanged. The creep deformations during the creep tests made the characteristic lengths of the rock-concrete interfaces decreased largely, leading to the obvious enhancement of the interfacial brittleness. As a result, the interfacial nonlinear fracture behavior from the initial cracking state to the unstable fracture state would become weaker. In addition, the fully formed fracture process zone length and the corresponding crack length increased with the increase of the sustained load level, indicating that the boundary effect of the rock-concrete interface would decreased.

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

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

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

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

Carbonation Behavior of Engineered Cementitious Composites under Coupled Sustained Flexural Load and Accelerated Carbonation.

Engineered cementitious composites (ECCs) belong to a broad class of fibre-reinforced concrete. They incorporate synthetic polyvinyl alcohol (PVA) fibres, cement, fly ash and fine aggregates, and are designed to have a tensile strain capacity typically beyond 3%. This paper presents an investigation on the carbonation behaviour of engineered cementitious composites (ECCs) under coupled sustained flexural load and accelerated carbonation. The carbonation depth under a sustained stress level of 0, 0.075, 0.15, 0.3 and 0.6 relative to flexural strength was measured after 7, 14 and 28 days of accelerated carbonation. Thermogravimetric analysis, mercury intrusion porosimetry and microhardness measurements were carried out to show the coupled influence of sustained flexural load and accelerated carbonation on the changes of the mineral phases, porosity, pore size distribution and microhardness along the carbonation profile. A modified carbonation depth model that can be used to consider the coupled effect of flexural tensile stress and carbonation time was proposed. The results show that an exponential relationship can be observed between stress influence coefficient and flexural tensile stress level in the carbonation depth model of ECC, which is different when using plain concrete. Areas with a higher carbonation degree have greater microhardness, even under a large sustained load level, as the carbonation process refines the pore structure and the fibre bridges the crack effectively.