Sustained Load Research Articles

Performance of bolted steel laminated bamboo lumber connections under axial cyclic loading

Bolted connections are well known for their simplistic design and installation, and have been used in bare frame structures of various types of materials including steel and timber. Compared with monotonic loading, cyclic loading is more dangerous. Bamboo has excellent toughness and deformation recovery ability but a quantitative description of the mechanical performance of bolted steel laminated bamboo lumber (LBL) connections under cyclic loading has not yet been determined. As a result, tension and compression tests were carried out separately under axial cyclic loading. Obtained experimental results indicated that the tensile specimens suffer brittle failure because of the premature cracking of LBL due to cyclic loading; the bolts remained almost straight after failure. On the other hand, cyclic compressive specimens showed considerable ductile behavior as the bolts underwent plastic deformation since LBL sustained considerable load prior to failure due. Key mechanical connection parameters such as elastic stiffness, yield and ultimate displacement, yield and ultimate load, ductility ratio obtained from the skeleton curves were carefully analyzed to study the influence of bolt number on various properties. The increase in elastic stiffness from two to four bolts is about 60 % for the cyclic tension specimens, whilst that was about 25 % for the cyclic compression specimens. Compared with the cyclic tensile specimens, cyclic compressive specimens have higher ductility ratio, which is mostly greater than 2. In addition, theoretical models for both loading and unloading curves have been proposed, which showed good agreement with test results.

Non-linear mechanical behaviour of thermoplastic elastomeric materials and its vulcanizate under tension/tension fatigue deformation by fourier transform rheological studies

Fourier transform (FT) rheology opens up novel frontiers in understanding non-linear mechanical behaviours of polymeric materials under sustained long-term dynamic load. The prediction of the exact point of appearance of a crack in a sample under dynamic mechanical strains of large amplitude and the fatigue analysis has been made possible to such degrees of precision, previously not possible through conventional rheological and mechanical analysis. In this work, the fatigue behaviour of a thermoplastic elastomeric material (TPE) and a thermoplastic vulcanizate (TPV) from thermoplastic polyurethane (TPU) and epichlorohydrin− ethylene oxide−allyl glycidyl ether (GECO) rubber is investigated by Fourier transform studies under oscillatory strain-controlled tensional test to understand the linear and non-linear mechanical behaviour. Fatigue analysis is performed through the fingerprint harmonics of the material’s stress response, i.e., the second and third harmonics, ( I 2/1 and I 3/1) to establish the stress nonlinearity, asymmetry, and the formation of macrocracks. Furthermore, these higher harmonics are fitted with the Neo-Hooke and Mooney-Rivlin model to fundamentally understand the mode of fatigue failure, and a close agreement between theoretical fitting and experimental outcomes is established. Strain-life curves were utilized for the thorough investigation of the fatigue behaviour of the specimens and more than 9-fold enhancement in the strain life of the TPV is observed over TPU at a strain amplitude of ∼1.05 indicating an increasing ductility. Finally, the necessity of FT rheology was further emphasised as it is observed that I 2/1 and I 3/1 harmonics are more sensitive towards fatigue response as compared to the storage modulus and the complex modulus. Henceforth, the FT rheology analysis has enabled the current study to efficiently establish the ductility of each individual specimens and for prediction of the dynamic service lifetime of the developed materials.

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.

Durability of partially cured GFRP reinforcing bars in alkaline environments with or without sustained tensile load

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).

Kenaf/glass fiber‐reinforced polymer composites: Pioneering sustainable materials with enhanced mechanical and tribological properties

AbstractHybrid kenaf/glass fiber reinforced polymer composites have emerged as promising structural materials, garnering significant attention due to their unique blend of natural kenaf fibers and synthetic glass fibers. However, despite their potential, there remains a gap in the comprehensive understanding of their quasi‐static mechanical behavior, creep resistance, and fatigue performance. This paper addresses this gap by presenting recent advancements in studying these key properties of hybrid composites. Studies reveal that the combination of kenaf and glass fibers results in enhanced tensile, flexural, and impact strengths compared to individual fiber composites. Additionally, the hybridization offers improved creep resistance, with the glass fibers reinforcing the polymer matrix against deformation under sustained loads. Furthermore, investigations into fatigue properties demonstrate the resilience of hybrid composites to cyclic loading, contributing to prolonged service life in high‐stress environments. By elucidating the interplay between kenaf and glass fibers, this review underscores the potential of hybrid composites in various structural applications. The synergistic effects between natural and synthetic fibers offer a balance between sustainability, performance, and durability, making hybrid kenaf/glass fiber reinforced polymer composites a compelling choice for industries seeking lightweight, high‐performance materials in which aligns with the sustainable development goals (SDGs) especially on Goal 12.Highlights In composite engineering, combining glass and kenaf fibers could cut production costs, yield high‐performance materials, and promote green technology. Substituting part of the glass fiber with kenaf can enhance the strength‐to‐weight ratio and promote greater biodegradability in current synthetic composites. Quasi‐mechanical properties of hybrid kenaf/glass‐based composites was enhanced by optimal stacking sequences, filler addition, and fiber treatment. Failures due to fatigue and creep can be reduced by hybridizing kenaf/glass fiber composites can prevent in polymer composite due to enhance elastic modulus. Enhanced tribological performance of hybrid kenaf/glass‐based composites due to less damage in microstructure via good interlocking of kenaf and glass in matrix.

Experimental behavior of prestressed concrete beams under simultaneous sustained loading and corrosion

Experimental behavior of prestressed concrete beams under simultaneous sustained loading and corrosion

Flexural behavior of pre-damaged RC beams strengthened with UHPFRC-CFRP grid layer

Ultra-high performance fiber reinforced concrete (UHPFRC) using recycled tyre fibers and carbon fiber-reinforced polymer (CFRP) grid were combined to strengthen the pre-damaged reinforced concrete (RC) beams caused by coupling action of sustained loads and marine environment. The flexural tests were conducted to investigate failure modes, characteristic loads, deflection features, and strain behavior of the strengthened RC beams with different configurations. The results showed that they exhibited the typical bending failure without interfacial debonding, and their cracking, yield, and ultimate loads increased by 64.7 %−152.9 %, 105.7 % to 192 %, and 65.4–131.2 % compared with that of the RC beam (B0). By increasing the thickness of the UHPFRC layer and adopting the Y-type UHPFRC (using recycled tyre fibers), the stiffness, ductility, and synergistic performance among the main materials would be further improved, while the increase of the CFRP grid layers would decrease the ductility. Furthermore, analytical models considering the coupling action of sustained loads and marine environment were developed to calculate the characteristic loads, manifesting reasonable accuracy. The proposed UHPFRC-CFRP grid strengthening method has excellent mechanical and durability performance, cost-effectiveness, and environmental benefits, which can serve as a valuable reference for the innovation and development of structural strengthening technology.

Residual strength and time to failure of bonded wooden lap joints at high temperatures and superimposed constant loading

ABSTRACT The study focused on the effect of high temperatures on wooden bond lines of single lap tensile shear specimens according to EN 302-1 made from beech wood at superimposed sustained mechanical loading. The quasi-duration of load (DOL) tests encompassed temperature levels of 160–232°C with specimens loaded at 30% of their mean shear strength at ambient temperature for 40 min. The investigations comprised one phenol resorcinol formaldehyde (PRF), two melamine urea formaldehyde (MUF) and three one-component polyurethane (1C-PUR) adhesives as well as solid wood reference specimens. Complementary ramp load tests were performed with specimens heated in unloaded state up to 250°C. The investigations revealed three highly differentiated adhesive DOL behaviours although showing a well comparable pure temperature-bound strength reduction effect at 200°C. PRF and one MUF showed little to no additional influence of applied load, while for two 1C-PUR brands a drastic impact on survival times and residual strength was revealed. One MUF and a special 1C-PUR forwarded results between both extremes. The investigations provide evidence that the assessment of the temperature behaviour of structural adhesives can be biased when relying exclusively on ramp load tests with specimens heated in unloaded state, and thus necessitates a superimposed load as present in realistic fire scenarios.

Evolution of Long-Term Load Reduction Using Borrowed Soil

AbstractThe effectiveness of load-reduction techniques often diminishes due to creep behavior observed in geomaterials, as loess backfill is used, the load reduction rate of high-filled cut-and-cover tunnels (HFCCTs) after creep will decrease by 10.83%, posing a threat to the long-term stability of deeply buried structures such as HFCCTs. Therefore, a geotechnical solution is crucial to ensuring sustained effectiveness in load-reduction strategies over time. This study utilizes a finite-difference method to examine three promising measures for mitigating creep effects. Our analysis focuses on the time-dependent changes in earth pressure atop the cut-and-cover tunnel (CCT) and the internal distribution of cross-sectional forces, including bending moment, shear force, axial force, and displacement. Results indicate that the creep behavior of load-reduction materials significantly influences the internal force distribution. Furthermore, sustained load reduction is achieved when utilizing low-creep materials like dry sandy gravel as backfill soil, which needs to be borrowed from other sites. Additionally, integrating concrete wedges with load-reduction techniques facilitates a more uniform stress distribution atop CCTs.

Comprehensive Review on the Flexure Behaviour of Corroded Reinforcement Concrete Beams Under Sustained Loads

This study presents a comprehensive review of the flexural behaviour of corroded Reinforced Concrete (RC) beams subjected to sustained loads. The investigation synthesizes extant literature to elucidate the complex interaction between concurrent stress and steel corrosion in RC members. Emphasis is placed on the critical necessity of conducting corrosion tests under continuous stress conditions to accurately simulate in-situ structural behaviour. The review encompasses previous numerical studies on deteriorated RC beam performance and provides a critical analysis of historical loading regimes designed to mitigate corrosion in loaded RC beams. Notably, the literature reveals conflicting findings regarding the influence of loading on corrosion rates and crack propagation, highlighting areas necessitating further research. The review also considers the effects of creep, shrinkage, and stress, with particular emphasis on long-term deflection characteristics. This comprehensive analysis aims to consolidate current knowledge and identify critical research gaps in understanding the flexural behaviour of corroded RC beams under sustained loads, thereby providing a foundation for future investigations in this domain.

Long-term loading effect on vibration performance of CLT floors: An 896-day monitoring study

Timber, a viscoelastic material, undergoes deformation over time when exposed to sustained loads, a process known as creep. Its rising popularity as a construction material, especially for timber floors, is notable. However, the influence of creep on the dynamic characteristics of timber floors, such as their natural frequency and vibration response, is not well studied. This research focused on how long-term loading (creep) affects the vibration behaviours of a cross-laminated timber (CLT) floor. A full-scale CLT floor was constructed in a lab and subjected to long-term loading using sandbags. Over 896 days, the centroid point deflection and environmental conditions (temperature and relative humidity) were monitored. Human-induced vibration tests were carried out at the beginning, throughout, and at the end of this period. The vibration response, measured in terms of the Vibration Dose Value (VDV), was assessed at various stages of long-term loading. The findings showed a moderate positive correlation between the creep deflection and environmental conditions. The fundamental frequency slightly increased over time due to creep, and a general decrease in VDV was observed as the creep advanced.

Glass Fiber-Reinforced Polymer Bars under Sustained Load and Alkaline Conditions

Glass Fiber-Reinforced Polymer Bars under Sustained Load and Alkaline Conditions

Mechanical evaluation of a threaded interference interlocking mechanism for angle-stable intramedullary nailing.

To assess the fatigue and load-to-failure mechanical characteristics of an intramedullary nail with a threaded interference design (TID) in comparison to a commercially available veterinary angle-stable nail with a Morse taper bolt design (I-Loc) of an equivalent size. 10 single interlocking screw/bolt constructs of TID and I-Loc implants were assembled using steel pipe segments and placed through 50,000 cycles of simulated, physiologic axial or torsional loading. Entry torque, postfatigue extraction torque, and 10th, 25,000th, and 50,000th cycle torsional toggle were assessed. Each construct was then loaded to failure in the same respective direction as fatigue testing. Four complete constructs of each design were then assessed using a synthetic bone analog with a 50-mm central defect via nondestructive torsional and axial loading followed by axial load to failure. All constructs were angle stable at all time points and withstood fatigue loading. Median insertional torque, extraction torque-to-insertion torque ratio, and torsional yield load were 33%, 33%, and 72.5% lower, respectively, for the TID interlocking screws. No differences in torsional peak load, torsional stiffness, axial yield load, axial stiffness, or axial peak load were identified. No differences in complete construct angle stability, torsional stiffness, axial peak load, axial stiffness, or axial yield load were identified. The TID had an inferior torsional yield load when compared to I-Loc implants but generated angle stability and sustained simulated physiologic fatigue loading. The TID may be a suitable mechanism for generating angle stability in interlocking nails.

Bond properties of GFRP bar embedded in marine concrete subjected to sustained loads

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 (Pu)) 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 Pu. 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.

Durability of glass FRP bar‐reinforced seawater–sea sand concrete beams under sustained loading in a subtropical coastal environment

AbstractThe replacement of conventional bars with fiber‐reinforced polymer (FRP) bars in seawater–sea sand concrete (SSC) structures is a promising method for sustainable construction. To promote the engineering application of the aforementioned structures, the primary objective of this study was to investigate the concerns regarding the durability of SSC beams reinforced with GFRP bars. For this purpose, three‐point bending tests were conducted on GFRP bar‐reinforced SSC beams to examine the coupled effects of sustained loading and exposure to a subtropical coastal environment on the flexural behavior of exposed SSC beams. The test results revealed that SSC beams reinforced with GFRP bars exhibited excellent durability in a subtropical coastal environment. The coupled effect of sustained loading and exposure to the subtropical coastal environment had an insignificant negative influence on the ultimate flexural capacity of GFRP bar‐reinforced SSC beams at 540 days of age. The failure mechanism of the exposed GFRP bar‐reinforced SSC beams was concrete crushing due to overreinforcement. Finally, the flexural capacity of the exposed beams was predicted by a function of the degradation in the tensile strength of the GFRP bars.Highlights The coupled effects of loading and environmental exposure on GFRP‐SSC beams were examined. The failure mechanisms of the exposed GFRP‐SSC beams was investigated. The long‐term flexural capacity of the GFRP‐SSC beams was predicted.

Design of Glass Handrails

Glass handrail loading and design in the United States lags behind best practice in other parts of the world. Improvements are possible for the design load, residual capacity and damage-event loading, each of which could be based on occupancy. Neither static loading nor impactor testing accounts for the dynamic effects of sustained loading to handrails during crowd load events. Alternate configurations for improved robustness at similar size and cost are presented.

Mechanical Characteristics of Sandwich Structures with 3D-Printed Bio-Inspired Gyroid Structure Core and Carbon Fiber-Reinforced Polymer Laminate Face-Sheet.

The gyroid structure is a bio-inspired structure that was discovered in butterfly wings. The geometric design of the gyroid structure in butterfly wings offers a unique combination of strength and flexibility. This study investigated sandwich panels consisting of a 3D-printed gyroid structure core and carbon fiber-reinforced polymer (CFRP) facing skin. A filament fused fabrication 3D printer machine was used to print the gyroid cores with three different relative densities, namely 10%, 15%, and 20%. Polylactic acid (PLA) was used as the printing material for the gyroid. The gyroid structure was then sandwiched and joined by an epoxy resin between CFRP laminates. Polyurethane foam (PUF) was filled into the gyroid core to fill the cavity on the core for another set of samples. Flexural and compression tests were performed on the samples to investigate the mechanical behavior of the sandwiches. Moreover, the two-parameter Weibull distribution was used to evaluate the results statistically. As a result, the sandwich-specific facing stress and core shear strength from the three-point bending test of the composites increased with the increase in sandwich density. Core density controls the flexural characteristics of the sandwich. Adding PUF improves the deflection at the maximum stress and the sustained load after fracture of the sandwich. Compression strength, modulus, and energy absorbed by gyroid core sandwiches and their specific properties are higher than the PUF-filled gyroid core sandwiches at equal sandwich density.

Investigation of long-term degradation of the interface in the anchorage zone of GFRP-RC beams

In the long-term service process of Glass Fiber Reinforced Polymer – Reinforced Concrete (GFRP-RC) structures, degradation of GFRP bars and concrete materials can occur, leading to a partial loss of the internal bonding performance of the structure. In this paper, a long-term study on the interface performance of GFRP-RC beams was conducted over an 8-year period to investigate the long-term degradation characteristics of the interface materials and the long-term Effective Anchorage Length (EAL) of GFRP bars. Firstly, a comprehensive analysis from a review perspective was conducted on the long-term degradation mechanisms of concrete and GFRP bars that considers the chemical behavior between microscopic molecules, the two-dimensional (2D) structure observed through micro electron microscopy, and the three-dimensional (3D) structure revealed by micro-X-ray imaging. Secondly, a new EAL calculation model is derived, and the long-term variation parameters of EAL under different conditions are determined. It was observed that the influence of alkaline corrosion and sustained loading is relatively small compared to the influence of initial structural defects on EAL. Finally, a comparison with four international standards revealed that the anchorage length design provided by JSCE-97 and ACI 440–1R-15 is overly conservative for GFRP bars, with values approximately 2–3 times the test results. On the other hand, CSA-S806–12 and CSA S6–19 standards are relatively close to the test values. Considering both design rationality and long-term corrosion resistance, it is concluded that the CSA-S806–12 specification is more reasonable.

A refined model for the flexural behavior of reinforced UHPC members under sustained loading

The long-term behavior of reinforced Ultra-High Performance Concrete (UHPC) members is significantly influenced by UHPC’s creep properties. To predict the long-term performance of structural members, it is essential to have both a material-level creep model and a constitutive model for structural analysis. This paper presents a refined model for the long-term performance of UHPC, incorporating critical nonlinear deformation mechanisms: instantaneous damage, plastic strain, creep strain, and time-dependent damage. The constitutive model for instantaneous damage and plastic strain mechanisms is based on experimental data from short-term cyclic loading tests on UHPC specimens. Additionally, the creep strain components and time-dependent damage evolution laws are derived from well-established linear and nonlinear creep models, validated for cementitious materials through extensive research. The model parameters were meticulously calibrated using creep damage test results from UHPC. The validation process included linear and nonlinear creep tests on UHPC cylinders under sustained stress levels ranging from 0.2fc to 0.66fc, creep failure tests on UHPC cylinders under sustained stress levels from 0.85fc to 0.95fc, and long-term bending tests of reinforced UHPC beams under sustained load. In the linear and nonlinear creep tests, comparison between the calculated ultimate creep coefficients and the experimental results demonstrates the model’s validity under low and moderate stress levels. In the creep failure test, a comparative analysis of strain evolution and creep lifetime between experimental and model results confirms the model’s validity under high sustained stress levels. Similarly, during the long-term bending test, simulated time-dependent displacement and compressive strain show good agreement with the experimental data, confirming the model’s accuracy in predicting the long-term performance of reinforced UHPC members. Furthermore, to streamline the constitutive model, the instantaneous damage effect was simplified and ultimately neglected. A comparison of simulation results between the original model and the simplified version demonstrates that disregarding the instantaneous damage effect is a safe approach for practical engineering applications.

A State-of-the-Art Review on Deformation Performance of Concrete Beams Prestressed with FRP Tendons Under Sustained Loading

A State-of-the-Art Review on Deformation Performance of Concrete Beams Prestressed with FRP Tendons Under Sustained Loading