Ultimate Axial Strain Research Articles

Experimental study and model evaluation on BFRP tube-confined RAC cylinders under monotonic axial compression

Using concrete waste as recycled aggregate (RA) in engineering structures can generate great ecological and economic benefits. To enhance the compressive properties of RA concrete (RAC), basalt fiber reinforced polymer (BFRP) tubes were used to provide lateral confinement in this study. Monotonic axial compressive tests were carried out on BFRP tube-confined RAC using 36 cylindrical specimens. Detailed test results were presented, including the failure mode, stress-strain curve, compressive strength, axial strain, and dilation property. The influences of the RA replacement ratio and the number of BFRP layers were analyzed. The test results indicated that BFRP tube-confined specimens exhibited strain hardening behavior in all of their stress-strain curves even if only a single layer of BFRP was used, and the failure was due to BFRP rupture along the hoop direction. The ultimate strength and deformability of RAC were significantly enhanced using BFRP confinement, with the confinement effect becoming more pronounced with increasing RA replacement ratios and BFRP layers. Compared to the unconfined specimens, the maximum enhancements in the ultimate strength and axial strain reached 2.8 times and 18.1 times, respectively, after BFRP confinement. Three typical theoretical models of FRP-confined concrete were employed to predict the ultimate conditions and axial stress-strain curves of the specimens. Finally, an improved model was formulated based on the test data to predict the ultimate conditions of BFRP tube-confined RAC cylinders. The proposed model was proven to predict the axial stress-strain curves, ultimate strength, ultimate axial strain with reasonable accuracy.

Axial stress-strain behavior of pre-stressed CFRP confined concrete columns

Bonded carbon fiber reinforced polymer (CFRP) confined concrete presents issues such as stress hysteresis and easy detachment at the interface between CFRP and concrete. Applying prestress to CFRP can improve this situation. To investigate the stress-strain behavior of prestressed CFRP strengthened plain concrete columns (PC) under axial compression, a total of 1 unreinforced concrete column, 9 bonded CFRP strengthened concrete columns, and 27 prestressed CFRP strengthened concrete columns were designed. Including CFRP layers and the level of prestress were taken as the study factors, and axial compression tests were conducted on each specimen to analyze their axial compression performance. The test results indicated that applying prestress to CFRP results in the core concrete being constantly subjected to triaxial compression, with initial transverse compressive strain and longitudinal tensile strain, thereby increasing the initial stiffness of the core concrete. In contrast to bonded CFRP-strengthened columns, the ultimate axial stress, ultimate axial strain, and initial stiffness of prestressed CFRP-strengthened columns increased with the level of CFRP prestress, with a more pronounced improvement observed in columns wrapped with a higher number of layers of CFRP. For a 4-layer CFRP strengthened column prestressed at 0.3, the ultimate axial stress and ultimate axial strain increased by up to 13 % and 20 %, respectively, compared to bonded 4-layer CFRP strengthened columns, with the initial stiffness increasing by up to 30 %. On the other hand, applying prestress to CFRP can significantly enhance the effective utilization of CFRP, but it is negatively correlated with the number of CFRP layers. Compared to bonded 1-layer CFRP strengthened columns, the effective utilization rate increased from 0.63 to 0.91, a 44 % increase, when prestressing CFRP at 0.3. Finally, based on existing relevant research, a peak stress and strain model considering the ratio of confinement stiffness to strain ratio was proposed. Based on the fundamental assumptions of curve analysis, the design-oriented axial stress-strain model suitable for prestressed CFRP strengthened concrete columns was established, with good match between theoretical and experimental curves.

Effectiveness of TRC jackets for the strengthening of concrete columns after elevated temperatures: Experimental and analytical investigation

Textile reinforced concrete (TRC) materials with high strength and excellent heat resistance might considerably enhance the performance of structures under thermal conditions. This study examined thirty concrete columns, each reinforced with varying TRC configurations, and subjected them to different high-temperature scenarios. Axial compression experiments were subsequently carried out to assess the impact of TRC on concrete's performance at elevated temperatures. Key variables in these tests included different textile layers (0–4 layers), and a temperature range extending from room temperature to 800 °C. The application of TRC was found to facilitate more gradual failure progress during compression tests. In contrast to the unconfined specimens, whose stress-strain curves exhibited a parabolic pattern, those reinforced with TRC displayed a hardening segment at temperatures up to 400 °C. The presence of TRC jackets significantly improved the compressive strength, the ultimate axial strain, and the energy dissipation, both at room temperature and after high-temperature treatments. Furthermore, this paper proposed an innovative model for estimating the residual strength of TRC-confined cylinders after being exposed to various temperatures. This model accounted for the impact of high temperature on the residual bearing capacity of unconfined cylinders, the modulus of elasticity for fibers, and the confinement effectiveness coefficient.

Predicting ultimate strength and axial strain of circular concrete columns having AFRP wraps and tube-encased confinement

The current study performs an in-depth analysis into the reinforcement of circular concrete columns (CCC) using aramid fiber-reinforced polymers (AFRP), focusing on two distinct techniques: wet-layup wrapping and tube-encasement with manufactured shells. The need for improved forecasting models for AFRP-wrapped concrete structures arises from observed inconsistencies and limited accuracy in existing models, which often fail to fully account for key parameters such as confining stiffness and composite strain. Addressing this gap, our study begins with a comprehensive review of prior experimental research, emphasizing the impact of various parameters on the performance of AFRP-wrapped CCC under monotonic axial compressive loading. To build a robust foundation for this investigation, we compiled an extensive database of experimental outcomes from existing literature, encompassing 180 concrete cylinders wrapped by AFRP. Our analysis meticulously evaluates nine different models designed to predict the ultimate compressive strength and axial strain of circular columns reinforced with AFRP. These models are scrutinized for their accuracy, revealing notable discrepancies, particularly when key parameters are overlooked. In response, we introduce new equations that offer more precise predictions compared to previously proposed models. These novel equations are derived from an extensive statistical analysis, ensuring their reliability and robustness. Prominently, our proposed model for compressive strength demonstrated superior performance with a coefficient of determination (R2 = 0.87), a root means square error (RMSE) of 0.29, and a mean absolute error (MAE) of 22.91. Similarly, the proposed strain model showed a high level of forecasting accuracy, with a coefficient of determination (R2 = 0.82), a root mean square error (RMSE) of 2.79, and a mean absolute error (MAE) of 208.69. To validate our findings, we employed the Taylor diagram and probability density function, which provide a detailed comparison of the forecasting accuracy of our proposed models against existing ones. The outcomes of this study not only enhance the understanding of AFRP-wrapped concrete columns but also offer valuable insights for the development of more effective reinforcement techniques. Our findings are expected to contribute significantly to the advancement of structural engineering practices, particularly in the application of AFRP for strengthening concrete structures.

Compressive behavior of concrete-filled FRP tubes with FRP solid waste as recycled aggregates: Experimental study and analytical modelling

The disposal and recycling of FRP (Fiber-Reinforced Polymer) solid waste pose significant challenges globally, with a recycling rate of less than 10 %. Current methods of handling FRP solid waste (FSW), including landfill disposal, incineration, chemical treatment, and thermal cracking, face limitations in achieving large-scale recycling and resource utilization. In civil engineering, FRP is widely employed for retrofitting structures and holds promise for new constructions. A new approach has emerged for recycling FSW by using it as aggregates in concrete, offering lower costs and reduced environmental impact. Research on using FSW aggregates in plain concrete shows that optimizing replacement ratios and aggregate shapes can enhance split tensile strength and toughness without significantly lowering compressive strength. To further improve the mechanical performance, this study investigated concrete-filled filament-wound FRP tubes with FSW aggregates (FSW-CFFTs) under axial compression. 35 specimens were tested to evaluate the influence of granular FSW aggregates (i.e., replacement ratio = 0 %, 20 %, 40 %, 60 %, and 80 %) and FRP thickness (i.e., 0 mm, 3 mm, and 5 mm) on their axial compression behavior. The experimental study revealed that: incorporation of FSW aggregates in unconfined concrete cylinders led to a notable decrease in Young's modulus and compressive strength, while concurrently increasing the peak compressive strain; as the replacement ratio of FSW aggregates increased, there was a tendency for the peak stress of FSW-CFFTs to decrease, while their ultimate axial strain tended to increase; compared FSW-CFFTs with unconfined concrete cylinders, the FRP confinement helped to mitigate the reduction ratio in axial compressive strength induced by the inclusion of FSW aggregates in the concrete, particularly at higher FSW aggregate replacement ratios; specimens with a thicker FRP tube exhibited higher secondary stiffness, along with higher strength and larger ultimate compressive strain. The modeling work indicated that accurately predicting axial stress-strain curves requires consideration of the bi-axial stress state of filament-wound FRP tubes.

Constitutive behavior of CFRP-confined normal- and high-strength geopolymer concrete: Experiments and modelling

The constitutive behavior of geopolymer concrete (GPC), which is essential to the design of GPC structures, has not been well explored, especially in multi-axial stress state. This study therefore performs an investigation on the constitutive behavior of normal- and high-strength GPC confined by carbon fiber-reinforced polymer (CFRP) jacket. A total of 36 CFRP-confined GPC specimens are fabricated, which cover six GPC mixtures and three thickness of CFRP jacket. A dataset comprising of 24 specimens is employed to develop the models for ultimate condition and axial stress-strain curve. Two testing datasets, one consisting of the rest 12 specimens and the other composed of six specimens collected from the literature, are exploited to test the models. A linear function of actual confinement ratio is competent for modelling the compressive strength, whereas a nonlinear function is capable of characterizing the ultimate axial strain. The average absolute error of predicted compressive strengths and ultimate axial strains is less than 8.0 % and 15.0 % for both testing datasets. A single four-parameter function originally proposed for OPCC can be extended to represent the axial stress-strain curve of CFRP-confined GPC by formulating expressions for the parameters. The correlation coefficient between predicted and measured curves is more than 0.96.

Confinement mechanism and modeling of basalt TRM confined concrete: Effect of mortar matrix grade

Textile reinforced mortar (TRM) confinement significantly enhances the compressive strength and ultimate axial strain of concrete. This study mainly investigated the influence of mortar strength on the stress-strain behavior of TRM-confined concrete to advance understanding of its confinement mechanism. We conducted 48 compression tests on concrete columns, comparing those without jackets to those confined in basalt TRM (BTRM). Variables included the number of textile layers (0 to 4) and mortar matrix strengths (from low-grade ‘M1′ to high-grade ‘M3′). Results indicated that a critical confinement stiffness ratio of 0.024 is essential for strain hardening behavior with marked strain capacity enhancement. Low-grade mortar diminished the effective confinement stiffness of BTRM jackets, manifesting as either a steeper descending plateau in the strain-softening stress-strain curve or a more gradual ascending plateau in the strain-hardening curve. High-strength mortar significantly reduced both the hoop rupture strain of BTRM jackets and the ultimate axial strain of the confined concrete. This study integrated the identified BTRM confinement mechanism into an existing stress-strain model for FRP-confined concrete, enhancing its predictive accuracy for BTRM-confined concrete. Specially, a coefficient, termed km, was introduced to quantify the mortar grade's impact on confinement stiffness in the confining pressure equation. Predictive results closely matched experimental data, especially in replicating plateau characteristics of the stress-strain curve.

Particle gradation effects on creep characteristics and the underlying mechanism in calcareous sand

Calcareous sand, a foundation material of marine origin, is an indispensable component in marine construction and has found extensive applications in building foundations and airport runways. The complex nature of the marine environment and the unique characteristics of calcareous sand present significant challenges for ensuring the stability and durability of engineering structures. To address these challenges, multistage loading triaxial creep tests were conducted on calcareous sands with different particle gradations to analyze their creep characteristics and underlying mechanisms. The result shows that the amount of ultimate axial creep strain is positively correlated with deviatoric stress but negatively correlated with the coefficient of uniformity (Cu). The creep process demonstrates a significant creep structure effect, characterized by creep rate lag and creep failure lag. The volumetric creep strain-time relationship at different creep stress ratios can be classified into three types. When the creep stress ratio is less than the stress ratio at the dilative point, the sample is first compacted followed by dilatancy; when the creep stress ratio is equal to the stress ratio at the dilative point, only compaction is obtained; as the creep stress ratio becomes larger than the stress ratio at the dilative point, pure dilatancy is found. The particle crushing modes and creep mechanisms of calcareous sand are also examined through meso-observations. It is found that the particle crushing modes can be categorized into three distinct types: grinding, angular breakage, and whole particle breakage. The particle gradation not only impacts the primary particle crushing mode but also governs the movement of particles and fragments, thereby influencing the occurrence of creep deformation. Specifically, a smaller Cu value corresponds to a higher fragmentation rate of the specimen, an increased number of overall crushing and angular fractures, a larger space for fragment migration after crushing, and eventually a greater creep deformation. Finally, a modified Burgers model is proposed based on the test results, which can effectively capture the creep deformation in the presence of creep structure effects and creep failure stages.

Enhancing performance of concrete with glass fiber-reinforced polymer short rebars as coarse aggregates by confinement

Cutting glass fiber-reinforced polymer (GFRP) waste into aggregate-size reinforcing bars (short rebars) as coarse aggregates in concrete shows to be an effective approach for recycling waste GFRP composites. The resulting concrete with such short rebars however exhibits a reduction in the compressive strength comparing to the corresponding normal concrete, making it unable to be directly used for structural elements. The concept of using the confining jacket is therefore employed for such concrete to improve its compressive strength and toughness and to be used in structural elements. A total of 48 cylindrical specimens were prepared and tested under axial compression with the replacement ratio of coarse aggregates by the GFRP short rebars and the stiffness of the confining jacket as the two test parameters. The test results indicate that incorporating short GFRP rebars results in a reduction in compressive performance by up to 57.2%, and the use of CFRP jacketing significantly enhances the performance of the concrete with or without GFRP short rebars. Such enhancement tends to be more significant when the stiffness of the confining jackets or the replacement ratio increases. The reduction in the compressive strength due to the incorporation of 25% recycled GFRP short rebars is from 57.2% to 30.7%, when the number of layers of the confining CFRP jacket increases from 0 to 3 piles. Comparisons were made between the test results and the predictions from two widely-used stress-strain models for confined concrete, showing that both models overestimate the ultimate axial strain and Teng et al.'s model generally produces the compressive strength slightly higher than the corresponding test results as well as the predictions from Jiang and Teng's model.

Integrated behavioural analysis of FRP-confined circular columns using FEM and machine learning

This study investigates the structural behaviour of double-skin columns, introducing novel double-skin double filled tubular (DSDFT) columns, which utilise double steel tubes and concrete to enhance the load-carrying capacity and ductility beyond conventional double-skin hollow tubular (DSHT) columns, employing a combination of finite element model (FEM) and machine learning (ML) techniques. A total of 48 columns (DSHT+DSDFT) were created to examine the impact of various parameters, such as double steel tube configurations, thickness of fibre-reinforced polymer (FRP) layer, type of FRP material, and steel tube diameter, on the load-carrying capacity and ductility of the columns. The results were validated against the experimental findings to ensure their accuracy. Key findings highlight the advantages of the DSDFT configuration. Compared to the DSHT columns, the DSDFT columns exhibited remarkable 19.54 % to 101.21 % increases in the load-carrying capacity, demonstrating improved ductility and load-bearing capabilities. Thicker FRP layers enhanced the load-carrying capacity up to 15 %, however at the expense of the reduced axial strain. It was also observed that glass FRP wrapping displayed 25 % superior ultimate axial strain than aramid FRP wrapping. Four different ML models were assessed to predict the axial load-carrying capacity of the columns, with long short-term memory (LSTM) and bidirectional LSTM models emerging as superior choices indicating exceptional predictive capabilities. This interdisciplinary approach offers valuable insights into designing and optimising confined column systems. It sheds light on both double-tube and single-tube configurations, propelling advancements in structural engineering practices for new constructions and retrofitting. Further, it lays out a blueprint for maximising the performance of the confined columns under the axial compression.

Evaluation of Ultimate Axial Strain Models for Fiber-Reinforced Polymer-Confined Concrete Cylinders

Fiber-reinforced polymer (FRP) composite has been widely applied to reinforce and rehabilitate concrete columns in civil engineering. It is necessary to deeply understand the ultimate deformation (i.e., the ultimate axial strain εcc ) of FRP-confined concrete columns. A large database including different types of FRP (conventional FRP and large-rupture-strain FRP), concrete compressive strengths (19.4 MPa-169.7 MPa), and cylinder sizes (51 mm-508 mm), as well as various concrete types, is collected in this study. For this reason, several existing ultimate axial strain modes for FRP-confined cylinders have been evaluated via some statistical parameters based on the database. The verification results illustrate that the prediction accuracy of axial strain models is not good enough due to the intrinsic difficulty in predicting deformation and variability of data source, although the available models with better performance can achieve the average ratio between experimental and predicted strain value ( εccexpe/εccpred ) below 10%.

Durability assessment of hybrid double-skin tubular columns (DSTCs) under simulated marine environments

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs), consisting of an outer FRP tube, an inner steel tube, and concrete infill between them, are a relatively new structural system. Despite DSTCs being proven to have enhanced structural capabilities over traditional concrete columns, their durability in aggressive environments remains unclear. This study presents a durability assessment of hybrid DSTCs with a glass fiber-reinforced polymer (GFRP) tube. Hybrid DSTCs were immersed in artificial seawater at different temperatures (i.e., room temperature, 40 °C, and 60 °C) for a duration of up to 540 days. The time-dependent behaviors, including axial load-strain (axial and hoop) curves, compressive strength, and ultimate axial and hoop strain, are systematically investigated at different aging times. Test results reveal that the compressive strength of DSTCs exhibits a slight reduction at room temperature and 40 °C, while an obvious reduction is observed at 60 °C. After 540 days, the compressive strengths for hybrid DSTCs with 3-mm-thick and 6-mm-thick GFRP tubes reduce to 79% and 93%, respectively. A decrease in strength is notable for DSTCs with a 3-mm-thick GFRP tube at 60 ℃. The degradation of GFRP tubes is also observed via scanning electron microscope (SEM) analysis, with greater degradation observed in 3 mm-thick GFRP tubes compared to the 6 mm-thick counterparts.

Parametric investigation of rectangular CFRP-confined concrete columns reinforced by inner elliptical steel tubes using finite element and machine learning models

Nowadays, due to the structural advantages gained by combining three different materials' properties, columns made of carbon-fiber reinforced polymer (CFRP)-confined concrete with inner steel tube have received researchers' interest. This article presents the nonlinear finite element analysis and multiple machine learning (ML) model-based study on the behavior of round corner rectangular CFRP-confined concrete short columns reinforced by the inner high-strength elliptical steel tube under the axial load. The reliability of the proposed nonlinear finite element model was verified against the existing experimental investigations. The effects of the parameters such as the concrete grade, thickness of reinforcing steel tube, cross-sectional size of inner steel tube, and thickness of CFRP on the behavior of the columns are comprehended in this study. Furthermore, multiple ML models were proposed to predict the ultimate axial load, ultimate axial strain, and lateral strain of the test specimens. The reliability of the proposed ML models was evaluated by six distinct performance metrics. From the parametric investigation, it was found that concrete with lower compressive strength gained more strength enhancement because of confinement between CFRP and the inner steel tube than high-strength concrete relative to its unconfined compressive strength. The proposed ML models of extreme gradient boosting and random forest provided the best-fit results than the artificial neural network and Gaussian process regression models in predicting the axial load and axial and lateral strains of the columns.

Compressive behavior of FRP-confined 3D printed ultra-high performance concrete cylinders

Three-dimensional (3D) concrete printing technology has attracted increasing applications due to its merits such as labor-saving. Due to difficulties in implementing reinforcements in 3D printed concrete (3DPC), 3DPC structures are commonly designed to predominantly resist compressive loadings. This paper proposes to further enhance the compressive performance of 3D printed ultra-high performance concrete (3DPU) elements by fiber-reinforced polymer (FRP) wrapping. Axial compression tests on FRP-confined 3D printed UHPC (FC3DPU) and unconfined 3D printed UHPC (UC3DPU) cylinders were conducted. The key variables include the loading directions (i.e., X, Y, Z directions) of the 3DPU cylinders and the FRP confinement thickness (i.e., one and two layers). Test results show that FRP wrapping can substantially enhance the strength and deformation capacity of 3DPU. Furthermore, the compressive strengths of the UC3DPU and FC3DPU in the X-direction are the highest, while they are the lowest in the Z-direction. The actual confinement ratio threshold for sufficient confinement of FC3DPU is 0.1. Two existing models of FRP-confined concrete were assessed, and results show that models of Liao et al. and Teng et al. have a comparable accuracy in predicting the ultimate axial stresses and strains of FC3DPU. Microscopic analysis reveals that 3DPU has more large defects (i.e., equivalent diameter (Eq) > 2 mm) in interlayers but less small defects (Eq < 2 mm) than the cast counterparts.

Compressive behaviour of FRP-confined rubberised alkali-activated concrete

This paper presents experimental and numerical assessments into the compressive stress-strain behaviour of circular FRP-confined rubberised alkali-activated concrete with unidirectional sheets incorporating fibres in the hoop direction. The parameters investigated in the experimental programme include three different rubber contents of 0%, 30% and 60% volumetric replacement of the total natural aggregates and three confinement levels of 1–3 aramid FRP layers. The numerical assessment is performed in ABAQUS/CAE and a new strain hardening-softening function is developed for the compressive behaviour in the Concrete Damaged Plasticity material model, which captures the passive confinement imparted by the FRP jacket on the rubberised concrete. The numerical results are validated against the experimental results and a parametric study involving 240 models covering a wide range of parameters, including varying FRP materials (basalt, glass, aramid, and carbon), reference concrete grades, rubber replacement ratios, and confinement levels, is performed. The experimental results show an increase in the confinement effectiveness, i.e., confined-to-unconfined strength, by 66.7% and 103.4% for specimens with 0% and 60% crumb rubber replacement ratio, respectively, as the number of FRP layers increase from 1 to 3. Test specimens with 0% and 60% crumb rubber replacement ratio indicate an increase in the ultimate axial strain by 228.2% and 76.2%, respectively, as the number of FRP layers increase from 1 to 3. The observed hoop rupture strain reduces by 32.8% and 21.3% for specimens with 0% and 60% crumb rubber replacement ratio as the number of FRP layers increase from 1 to 3, showing a trend of reduction in the hoop rupture strain with higher FRP confinement layers and rubber content. The numerical results show that the enhancement in compressive strength is linearly proportional to the confinement ratio and is only marginally influenced by the FRP jacket stiffness at a given confinement ratio. The ultimate axial strain enhancement is also linearly proportional to the confinement ratio but is dependent on the FRP jacket stiffness. The compressive stress-strain curve for sufficiently confined specimens is also shown to be well represented by a bi-linear curve with hardening. Based on the findings, a design-oriented model is introduced for sufficiently confined specimens and is shown to be in close agreement with the experimental results.

Design-oriented stress-strain model for FRP-confined engineered cementitious composites

Engineered cementitious composites (ECC) is known for its enhanced tensile performance compared with normal concrete. Ductile strain hardening behavior, multiple cracking beahvior as well as large tensile strain capacity can be achieved for ECC under tensile loadings. For the compressive performance, using lateral fiber-reinforced polymer (FRP) confinement is an effective approach to improve the compressive strength and strain. However, the research work on design models of FRP-confined ECC, especially on the stress-strain relationship, is limited at the current stage. To address this aspect, this study focuses on developing the design-oriented stress-strain model for FRP-confined ECC under axial compression. A test database on FRP-confined ECC was firstly assembled. Existing design equations on FRP-confined concrete were evaluated and found not be able to provide satisfactory predictions for FRP-confined ECC. New design equations on ultimate conditions, including the ultimate compressive strength and ultimate axial strain, were then proposed and verified with the test results. Finally, the design-oriented stress-strain model for FRP-confined ECC was developed, which consists of the formulated form of a stress-strain model for FRP-confined normal concrete and the new design equations on ultimate conditions proposed for FRP-confined ECC. Predictions of stress-strain curve show close agreements with test results, indicating the good performance of the developed design-oriented stress-strain model.

Behaviour of FRP-ECC-HSC composite stub columns under axial compression: Experimental and mathematical approach

The composite stub column composed of fiber-reinforced polymer (FRP), Eco engineered cementitious composite (ECC), and high-strength concrete (HSC) core is experimentally investigated in this study. The ductile performance of HSC-FRP is enhanced by proving an ECC ring of 30 mm thickness as sandwich layer between HSC and FRP. During experimentation the premature failure of the FRP-HSC column is observed at the initial phase, which exhibits brittle nature of the column. The material properties of FRP, ECC, and HSC were characterized. The performance of composite columns was observed with four FEH (FRP-ECC-HSC) columns, four HE (HSC-ECC) columns, and two FH (FRP-HSC) columns. The parameters such as uniform hoop strain distribution, ultimate load-carrying capacity, axial and hoop strain, and confining efficiency of the FRP were investigated to understand the behaviour of composite columns. From results, the ultimate load-carrying capacity of FH columns was 15.56 % higher than the FEH columns. The FEH columns showed the enhanced confining efficiency of FRP and 21% increase in uniform hoop strain distribution than the FH columns. The hoop strain capacity of FEH columns were increased twice the times of FH columns, which indicates the increased ductile behaviour of the FEH columns. In addition, experimental results of composite columns were highly correlated with the existing theoretical equation from literatures

Cyclic Compressive Behavior and Load-Strain Model of FRP-ECC-HSC Composite Columns

An innovative composite column, which consists of a high strength concrete (HSC) core, engineered cementitious composites (ECC) ring, and fiber-reinforced polymer (FRP) tube, has recently been developed and subjected to monotonic axial compression testing by the authors. In this study, the cyclic compressive behavior of this proposed FRP-ECC-HSC composite column was examined. Test parameters, including HSC core strength, FRP tube thickness, and ECC ring thickness, were investigated. Typical failure modes, dilation behavior, and axial load versus axial strain behavior were discussed and analyzed. It was found that the FRP-ECC-HSC composite columns exhibited improved deformability, compared with the counterpart traditional FRP-confined HSC columns, with the ultimate axial compressive strain increased by 0.7%–69.1% for the tested specimens. Meanwhile, the ultimate axial strain for cyclically loaded specimens is larger than that for monotonically loaded specimens in general, indicating a delayed column failure. Cyclic axial load-axial strain models, including the envelope model, unloading and reloading models, plastic strain equation, and stress deterioration equation, were proposed to predict the cyclic compressive behavior of the tested specimens. The proposed model was verified with the test results and exhibited good performance.

Assessment of cyclic deformation and critical stress amplitude of jointed rocks via cyclic triaxial testing

Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60° were prepared, and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes. The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz. At each level of confining pressure, the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed. Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude (i.e. critical dynamic deviatoric stress) beyond which the jointed rock specimens yielded. The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress. At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress, minor cumulative residual axial strains were observed, resulting in hysteretic damping. However, for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress, the plastic strains increased promptly, and the resilient moduli degraded rapidly during the initial loading cycles. Cyclic triaxial test results showed that at higher confining pressures, the ultimate residual axial strain attained by the jointed rock specimen decreased, the steady-state dissipated energy density and steady-state damping ratio per load cycle decreased, while steady-state resilient moduli increased.

Effect of compression casting method on the axial compressive behavior of FRP-confined seawater sea-sand concrete columns

The application of the novel compression casting method is beneficial to improve the compressive strength and durability of concrete. However, the compressed concrete fails rapidly after the peak load and exhibits stronger brittleness than the normal concrete, which could impair the seismic performance of the columns. As one of the passive confining measures, wrapping fiber-reinforced polymer (FRP) could be a practical technique to improve the ductility of such brittle columns. In this regard, this study experimentally investigated the axial compression behavior of both the partially and fully FRP-confined compression-cast seawater sea-sand concrete (SSC) columns. The test results indicated that the compression casting method enhanced the compressive strength but weakened the axial deformation capacity of the unconfined and FRP-confined SSC specimens. The compressive strength of unconfined concrete was improved by 38.5% due to compression casting. The measured average FRP rupture strain of the FRP-confined compression-cast SSC specimens was 6.3% smaller than that of FRP-confined normal SSC specimens due to the sudden appearance of the main oblique cracks, indicating poorer FRP utilization efficiency. Meanwhile, the average ultimate axial strain enhancement ratio of FRP-confined SSC specimens decreased from 12.04 to 7.83 after compression casting. The FRP hoop strain of the fully FRP-wrapped columns distributed more uniformly compared to the partially FRP-wrapped columns as the mean value of FRP efficiency factor increased from 0.807 to 0.928. With increasing FRP strip spacing, the post-peak segment of the stress–strain curve transferred from strain-hardening to strain-softening regardless of the confining stiffness. Finally, several existing stress–strain models were employed to examine their accuracy in anticipating the ultimate conditions of the FRP-confined compression-cast SSC columns.