Axial Stress-strain Curves Research Articles

A novel procedure to determine the hoop mechanical properties of pipe materials using ring elongation testing technique

In this study, experiments including axial tensile tests and ring elongation tests, as well as analytical analyses were developed to determine the mechanical properties of ASTM A106 Grade B seamless carbon steel (A106-B) pipe material. Ring elongation tests were performed on the ring specimens to determine the mechanical properties of the A106-B pipe material in the hoop direction. Whereas axial tensile tests were carried out on standard axial tensile specimens to determine the axial stress-strain behavior of the A106-B pipe material. The axial stress-strain curve of the A106-B pipe material was used as a reference to validate the ring elongation test results. Besides, Analytical analyses using curved and straight beam formulas were developed. Directly from the ring elongation tests experimental results, both the curved and straight beam formulas accurately predicted the young's modulus and hoop yield strength of the A106-B pipe material. Moreover, the A106-B pipe material's plastic stress-strain behavior in the hoop direction was also successfully determined directly from the ring elongation test results with high accuracy.

A constitutive model of expansive concrete and steel fiber-reinforced expansive concrete under tri-axial compression

Expansive concrete (EC) or steel fiber-reinforced expansive concrete (SF-EC) have been adopted for enhancing the performance of concrete-filled steel tube (CFST) columns, while the investigations on confinement mechanism and theoretical model for EC and SF-EC are limited. This study aims to develop a constitutive model for EC and SF-EC under tri-axial compression. In the first part, the results of tri-axial compression tests on 52 concrete cubes, were summarized and collected to form a database. In the second part, the failure criterions of concrete cubes, and performances of four existing stress–strain models were assessed and discussed using the database. The results of tri-axial compression with equal lateral confining stresses generally complied well with the compressive meridians of Ottosen failure criterion, whereas the Lode angles were around 56°–58° for the true tri-axial compression results. However, the existing models substantially underestimated the axial strains at peak stresses by at least 40%, and thus the predicted stress–strain curves deviated considerably from the test results with much steeper initial ascending slopes. To this end, a modified constitutive model was proposed with Montoya et al.’s model as the base form and the key parameters updated as per the result database. The proposed model had good performances in predicting the peak axial stresses and the corresponding strains (average error at 8% and 19%, respectively), as well as the full axial stress–strain curves.

Study of Strain-rate Effect in Two-dimensional Biaxial Test on Granular Material using Discrete Element Method

Numerical study to investigate the effect of various strain rates on global friction angle in the sand has been performed. Granular material behavior is influenced by several factors, among others: pack configuration, grain macro and micro roughness, confinement pressure, loading rate, etc. Sand is a granular material composed of discrete particles that the most refined microscopic techniques are needed to study its mechanical properties. In Indonesia, research related to the Discrete Element Method is still very limited. The two-dimensional discrete element method is capable to calculate the motion and interparticle contacts of large number of small particles, and each particle is modeled as a rigid circular element. The study started with the validation of the DEM model using YADE. Particles with a local friction angle of 35° are arranged in a closed rectangular box (frictionless wall). The number of particles used in this model validation simulation is 1000 sphere-type particles with monodisperse particle gradations. The simulation was done by a drained biaxial test with confinement stress of 100 kPa, thereafter varying strain rate are applied. Based on the deviatoric stress–axial strain curve from YADE, the result can be depicted on the Mohr circle to obtain the value of the global friction angle. It is found that the different value of strain rate affects its global friction angle. Increasing the value of the strain rate can increase the material global friction angle, which increases the strain rate from 1% to 5%, 10%, 25%, and 50% will increase its global friction angle by 5%, 5%, 14%, and 18%, respectively.

Compressive performance of SWSSC-filled CFRP-stainless steel composite tube columns

The structure of concrete confined by carbon fiber-reinforced polymer (CFRP) and stainless steel tubes was investigated. The columns, which take advantage of the ductility and corrosion resistance of stainless steel, are composed of stainless steel tube, core seawater and sea sand concrete (SWSSC) and outer FRP. Forty circular SWSSC-filled FRP-stainless steel composite tube columns (SCFSSCTs) were manufactured and subjected to axial compression tests. After fracturing of the FRP, the failure modes of the specimens mainly showed extrusion expansion, with a small amount of shear failure. The increase in the thickness of the stainless steel and the number of FRP layers resulted in a significant improvement in the load capacity and deformation capacity of the structure, which was more evident in the specimens with higher steel contents. With the increase in the number of FRP layers, the increases in ultimate stress and ultimate strain were 40.4% − 130.0% and 118.6% − 282.4%, respectively, compared with the column with single stainless steel confinement. The stress–strain curves for the SCFSSCTs were similar to those of structures reinforced by FRP and carbon steel and were divided into four stages. Models were proposed to predict the full axial stress–strain curves of SCFSSCTs, including the ultimate point and the peak point.

Investigation of mechanical properties and elastoplastic numerical calculation model of frozen soil containing ice lenses

As the natural foundation of engineering structures in cold regions, the frozen soil has a vital impact on the deformation of structures during construction and operation stage. The particularity of frozen soil is mainly reflected in its sensitivity to temperature and cryostructure. In this research, a series of uniaxial compressive tests were conducted on four structural types of frozen soil under different temperatures. The test results showed that the axial stress-strain curves of frozen soil containing compaction stage, elastic deformation stage, strain hardening at pre-peak, and failure at post-peak. The unconfined compression strength (UCS) increases as the temperature decreases, the failure strain increases with decreasing temperature when temperature is higher than −3 °C and decreases with decreasing temperature when temperature is lower than −3 °C. Compared with homogeneous frozen soil, the failure surface of heterogeneous frozen soil is formed along the ice-frozen soil interface and the cracks at the tip of ice lenses. Subsequently, the damage variable considering the influence of ice lens and initial defects was deduced based on equivalent strain principle and continuous damage mechanics. An elastoplastic model of heterogeneous frozen soil containing ice lensed is established by using associated flow criterion and Mises strength criterion, and the model was redeveloped in COMSOL Multiphysics. The rationality of the proposed model was verified, and exhibits that the proposed model can capture the mechanical properties of heterogeneous frozen soil.

Compressive behavior of FRP-confined ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHPFRC)

Due to its superior mechanical properties and durability, ultra-high performance concrete (UHPC) or ultra-high performance fiber reinforced concrete (UHPFRC) has emerged as a promising alternative to conventional materials in both strengthening of deficient structures and building of new structures. Contrast to the ductile tensile behavior of UHPFRC, the compressive behavior of both UHPC and UHPFRC is much more brittle. To tackle such demerit, the use of fiber-reinforced polymer (FRP) to provide confinement is an efficient method. This study aims to investigate the stress–strain behavior of FRP-confined UHPC/UHPFRC through axial compression tests of 40 FRP-confined UHPC/UHPFRC cylinders, with the confinement mechanism being carefully analyzed and explained. The test variables included the type and thickness of FRP jacket, the compressive strength of UHPC/UHPFRC, the volume fraction of steel fibers, and the specimen size. The test results showed that FRP-confined UHPC/UHPFRC exhibited a large deformation capacity and generally a bilinear axial stress–strain curve. Axial stress reductions or fluctuations occurred after the first ascending branch for the UHPC specimens; however, such axial stress reductions or fluctuations became less obvious after steel fibers were added. Finally, existing models for the ultimate condition of FRP-confined UHPC/UHPFRC were evaluated using the test results in the present study.

An Artificial Network-Based Prediction of Key Reference Zones on Axial Stress–Strain Curves of FRP-Confined Concrete

The accurate prediction of reference points on the axial stress–axial strain relationship of fiber-reinforced polymer (FRP)-confined concrete is vital to pre-design structures made with this system. This study uses an artificial neural network (ANN) for predicting hoop rupture strain (εh,rup) and transition zone, namely transition strain (εc1) and stress (f’c1), on axial stress–strain curves of FRP-confined concrete. These are key parameters for estimating a transition zone of stress–strain curves. In this study, accompanied with these parameters, ultimate condition parameters, including compressive strength and ultimate axial strain, were predicted using a comprehensive database. Various combinations of input data and ANN parameters were used to increase the accuracy of the predictions. A sensitivity analysis and a model validation assessment were performed to evaluate the predictability of the developed models. At the end, a comparison between the proposed models in this study and existing ANN and design-oriented models was presented. It is shown that the accuracy of the developed ANN models in this study is higher or comparable to that of existing ANN models. Additionally, the developed models in this study to predict f’c1 and εc1 exhibit a higher accuracy compared to existing design-oriented models. These results indicate that the proposed ANN models capture the lateral confinement effect on ultimate and transition zones of FRP-confined concrete with a more robust performance compared to existing models.

Tri-axial compressive behavior of expansive concrete and steel fiber-reinforced expansive concrete

Concrete is widely used with external confining devises such as fiber-reinforced polymer (FRP) tubes and steel tubes. Expansive concrete and steel fiber-reinforced expansive concrete has been proposed to prevent concrete shrinkage, and even to provide some pre-stresses in the confining devices so as to enhance confinement efficiency. However, the confinement mechanisms on expansive concrete and steel fiber-reinforced expansive concrete have not been fully understood yet. In this study, tri-axial compression tests were conducted to 52 concrete cubes with different expansive agent contents and steel fiber additions, in which 12 concrete cubes were under true tri-axial compression with different lateral confining pressures. It is found that concrete cubes under true tri-axial compression typically exhibited oblique shear failure, and their strength enhancements were limited by the magnitudes of minor principal stress. Furthermore, the effects of expansive agent content and steel fiber volume fraction on the axial stress-strain relationship were investigated. Results showed that the CaO-based expansive agent addition should be at least more than 4% to make a difference for the axial stress-strain curves. Concrete cubes with 8% expansive agent addition exhibited a steeper initial ascending slope, but the strength enhancements diminished with increasing confining pressures. Steel fiber addition could significantly enhance the peak axial stress (by 10–20%), and the enhancement effect became more prominent with increasing confining pressures. Therefore, the combined use of expansive agent and steel fiber has synergistic and complementary effects.

Confinement of concrete with FRCM: Influence of bond aspects under cyclic axial loading

Bond aspects may significantly influence the effectiveness of Fiber Reinforced Cementitious Matrix (FRCM) jackets as confining system. Among them, this paper focuses on the following variables: the overlapping length, the equivalent thickness of the fibers (directly related with the fabric weight), fibers coating presence (pre-impregnated with epoxy resin or fluid cement paste), and lastly, the reinforcement configuration (continuous or discontinuous layers). For this scope, an experimental campaign was carried out on cylindric specimens, subject to cyclic axial loading, strengthened with two layers of carbon-based FRCM (CFRCM). Failure modes, axial stress–strain curve, strength and stiffness deterioration and, lastly, plastic strains were analyzed. Experimental results indicate a clear effect of the investigated parameters on the overall effectiveness of the confining jackets: the best results were obtained when carbon fibers are epoxy-coated, recalling that for making the application feasible, a relatively large mesh size and a low degree of coating impregnation should be used for the textiles. Lastly, new relationships to predict the main parameters that define the cyclic behavior of confined concrete are proposed and compared with those found in literature for other confining systems (based on fiber-reinforced polymer FRP and transverse steel).

Fracture Closure Empirical Model and Theoretical Damage Model of Rock under Compression.

The rock or rock mass in engineering often contains joints, fractures, voids, and other defects, which are the root cause of local or overall failure. In response to most of the current constitutive models that fail to simulate the nonlinear fracture compaction deformation in the whole process of rock failure, especially brittle rocks, a piecewise constitutive model was proposed to represent the global constitutive relation of rocks in this study, which was composed of the fracture compaction empirical model and the damage statistical constitutive model. The fracture empirical compaction model was determined by fitting the expressions of fracture closure curves of various rocks, while the rock damage evolution equation was derived underpinned by the fracture growth. According to the effective stress concept and strain equivalence hypothesis, the rock damage constitutive model was deduced. The model parameters of the fracture compaction empirical model and damage statistical constitutive model were all calculated by the geometrical characteristics of the global axial stress-strain curve to guarantee that the models are continuous and smooth at the curve intersection, which is also simple and ready to program. Finally, the uniaxial compression test data and the triaxial compression test data of different rocks in previous studies were employed to validate the models, and the determination coefficient was used to measure the accuracy. The results showed great consistency between the model curves and test data, especially in the pre-peak stage.

Mechanical Properties on Various FRP-Reinforced Concrete in Cold Regions

The evaluation of frost resistance varies with different reinforcement methods, but it is a hot research topic for concrete reinforced with Fiber-Reinforced plastic (FRP). Freezing and thawing tests of FRP-reinforced concrete prisms and cylinders are presented to simulate beams and piers of buildings in cold climates. To evaluate the specimens’ frost resistance, tests with various reinforcement techniques, morphological analysis, weight tests, and relative dynamic modulus of elasticity tests were used. Examined also were the variations in stress–strain curves for axial compression tests and load–displacement curves for bending tests following various freeze–thaw cycles. The findings indicated that after 100 freeze–thaw cycles, the weight of unreinforced concrete cylinders decreased by 9.7%, and its compressive strength decreased by 27.6%. On the other hand, CFRP-reinforced concrete cylinders (Carbon-Fiber-Reinforced Plastics) and GFRP (Glass-Fiber-Reinforced Plastics) gained 1.1% and 1.58% in weight, respectively, while the compressive strength decreased by 7.4% and 8%. After 100 freeze–thaw cycles, the weights of concrete prisms with reinforcement, without reinforcement, and with CFRP reinforcement decreased by 12.13%, 8.7%, and 9.6%, respectively, and their bending strength was reduced by 20%, 42%, and 53%, respectively. The frost resistance of the two FRP-reinforced concrete types had significant differences under freeze–thaw cycles because the prismatic specimens were not fully wrapped with FRP materials. Finally, finite element software ABAQUS was used to simulate the freeze–thaw cycle test of the two specimens. Calculated values were compared to experimental results for the load–displacement curve and the axial stress–strain curve under bending load. The comparison of peak displacement produced a maximum error of 8.6%, and the FRP-reinforced concrete model validity was verified.

Predicting ultimate condition and transition point on axial stress–strain curve of FRP-confined concrete using a meta-heuristic algorithm

Accurately predicting key reference points on the axial stress-strain curve of fiber-reinforced polymer (FRP)-confined concrete is of great importance for the pre-design and modeling of structures manufactured with this composite system. This paper presents a detailed study on the development of accurate and practical expressions for predicting the ultimate condition and transition point, as key reference points, on axial stress-strain curves of FRP-confined concrete using generic programming (GP). A comprehensive data tuning and cross-validation analysis was firstly performed to develop prediction models. Afterwards, the accuracy and performance of the developed empirical expressions were examined by sensitivity analysis, parametric analysis and model validation. Finally, a comparison was made between the performance of these proposed expressions and that of the existing best-performing expressions in the literature using statistical analysis. Based on the sensitivity and parametric analysis of the database, it is shown that: compressive strength ( f’ cc ) and axial transition strain ( ɛ c1 ) are more sensitive to FRP lateral stiffness ( K l ); ultimate axial strain ( ɛ cu ) is more sensitive to K l -to-unconfined compressive strength ( f’ co ) ratio and fiber ultimate tensile strain ( ɛ fu ); hoop rupture strain ( ɛ h,rup ) is more sensitive to fiber elastic modulus ( E f ); and axial transition strength ( f’ c1 ) is more sensitive to f’ co . It is also shown that the proposed expressions provided more accurate predictions of the ultimate condition and transition point on the axial stress-strain curve of FRP-confined concrete than the existing expressions. This was achieved by using a larger number of datasets and accurately capturing the effects of the most influential input parameters in the proposed expressions.

Basalt FRP-confined slag-based alkali-activated seawater coral aggregate concrete: Concept and axial stress-strain behavior

This paper presented an experimental investigation on the axial stress–strain behavior of cement-based seawater coral aggregate concrete (CAC) and slag-based alkali-activated seawater coral aggregate concrete (AACAC) confined by basalt fiber‐reinforced polymer (BFRP) sheet, with different BFRP confinement stiffnesses (i.e., 0, 1, 2, 4, and 6 plies) being considered. The experimental results indicated that the FRP confinement was an effective method in enhancing the loading capacity and deformation of cylinder specimens. As the FRP jacket confinement (layer) increased, the slope of the axial stress–strain curves at the ascending branch, axial compressive strength (fcc), and ultimate axial strain (εcc) corresponding to fcc of BFRP-confined CAC and AACAC were gradually enhanced. Compared with the un-confined specimens, the axial compressive strength and ultimate axial strain of the CAC specimens with 6-layer FRP confinement increased by 1.94 and 12.35 times, respectively, while those of the AACAC specimens increased by 2.40 and 10.94 times, respectively. Finally, the expressions for the ultimate strength and ultimate strain of BFRP-confined CAC and AACAC were proposed, and were compared with tested results of FRP-confined CAC available in the literature to verify the reliability of the proposed models.

Influence of the oval hole on rock mechanical mechanism under uniaxial and biaxial compression: insights from the combined finite-discrete element method

To investigate the effects of the oval hole on the strength and the failure mechanisms of rock materials, a series of uniaxial and biaxial compression numerical simulations were carried out using the combined finite-discrete element method (FDEM). Based on the numerical simulation results, the effects of dip angle (β), major–minor ratio (ω) and position of the oval hole on rock strength characteristics crack initiation and development process and final failure modes were analysed in detail. The numerical simulation results indicate that: (1) Under uniaxial compression, the ultimate failure modes of rock containing oval holes can be mainly classified into single inclined plane shear failure, X-shaped tensile-shear composite failure and Y-shaped tensile-shear composite failure. Under biaxial compression, the ultimate failure mode is single-inclined plane shear failure dominated by shear cracks. Whether it is uniaxial compression or biaxial compression, the crack initiation position of the main fracture surface is near the end of the major axis of the oval hole. (2) Under uniaxial compression, the axial stress–strain curve of rock containing the oval hole is smooth before and after the peak, but under biaxial compression, there is obvious fluctuation in the post-peak stage due to crack initiation and propagation. In both uniaxial and biaxial compression, the pre-peak elastic stage of the axial stress–strain curve of the rock containing the oval hole is significantly shortened and the peak stress is significantly reduced compared with the intact rock, which indicates that the bearing capacity of the rock is seriously reduced by the existence of oval hole. (3) When the position and major–minor axis ratio of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength increase with the increase of the dip angle of the oval hole. When the position and dip angle of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength decrease with the increase of the major–minor axis ratio of the oval hole. (4) At different dip angles, the brittleness index Bi of rock containing oval holes decreases with the increase of confining pressure, and the brittleness degree shows a gradual weakening trend. At the same time, when the dip angle increases, the decreasing amplitude of brittleness index Bi decreases with the increase of confining pressure.

Experimental and numerical investigation on dynamic deformation and ignition response of NEPE-based propellants

An experimental and numerical study has been conducted to investigate the effects of strain rates on the mechanical responses and ignition mechanism of NEPE-based propellants (Nitrate Ester Plasticized Polyether). The modified split Hopkinson pressure bar (SHPB) equipped with a high-speed camera was used to capture the mechanical-chemical feature images during the whole impact process. The macro-mesoscopic thermomechanical model was employed to describe the viscous shear flow hotspot formation at the mesoscale. The results showed that the mechanical and ignition responses of the NEPE-based propellants are heavily dependent on strain rates. It is observed that the impact processes can be summarized into four stages: homogeneous deformation, shear flowing, initiation of local shear region and ultimate fracture (or ignition). Selected image sequence and SEM images indicate that viscous shear flow is important to the ignition of propellants. The calculated axial stress-strain curves indicate that propellant exhibits a remarkable rate-dependence and viscous effect. The predicted features of hot spot-ignition evolution are consistent with the experimental phenomenon and prove the critical strain rate of ignition is 4500 s−1.

Effects of lateral cyclic loading on compression behaviours of granular waste rocks for backfilling to reduce environmental pollution.

Granular waste rocks filled in goaf can replace coal seams to support roofs, thus reducing the extent of overlying strata movement, and thus reducing the environment damage caused by coal mining. To better to control the compression-induced deformation (CID) of waste rocks for backfill (WRBs), a loose material, it is feasible to apply lateral cyclic loads on granular waste rocks in advance. In order to study the effect of lateral cyclic loading on granular waste rocks, by utilising pre-lateral cyclic loading and axial loading, the deformation of granular waste rocks under load was tested on a compression simulation experimental platform for solid backfill materials. Furthermore, the changes in displacement, stress and mass of granular waste rocks during lateral cyclic loading were attained. The test results showed that (1) the loading stress progressively increased with the growth of the loading displacement, in which the rising process was divided into three stages according to the rate of loading. (2) With the increase in number of loading cycles, the rate of increase of stress in stage I increased and the stress reached increasingly higher levels; at the same time, stage II was gradually shortened. Eventually, only a single stage prevailed as the stress rapidly reached its preset maximum value, and the final displacement of the samples gradually declined. (3) The axial stress-strain curves of the samples exhibited a quasi-exponential relationship. Through lateral cyclic loading, the relative density of WRBs was significantly improved, and their deformation resistance was strengthened. (4) During backfill mining, lateral cyclic loads were applied to granular waste rocks, which improved the bearing capacity thereof and reduced strata movement and surface subsidence. This is beneficial to the protection of the surface environment and surrounding buildings.

Strength and deformation performances of silt-based foamed concrete under triaxial shear loading

Previous triaxial experiments have paid little attention to the shear strength and solid materials‒voids interaction of foamed concrete. Therefore, this study conducted comprehensive triaxial shear tests to investigate the strength and deformation characteristics of the silt-based foamed concrete, in which the silt was added to replace a portion of the cement. The results indicate that the deviatoric stress‒axial strain curves can be divided into the elastic stage, softening or hardening stage, and strength stability stage. In addition, the increasing confining pressure can gradually transform the type of stress‒strain curves from softening type to hardening type. The solid skeleton around the shear plane is broken into particle materials and then moves to fill the pores, while some residual solid skeletons still retain the characteristics of cementing binder. The volumetric strain of foamed concrete in triaxial shear loading presents a contraction feature. Moreover, as the confining pressure increased by 0.1 MPa, the peak strength and residual strength are observed to respectively rise on average by 0.075 MPa and 0.16 MPa. The cemented solid skeleton in foamed concrete primarily presents the cohesion in shear strength, which could be several hundred kPa. Furthermore, an equation is employed to describe the degradation of cohesion caused by the fracture of the cemented binder.

A micro–macro method for evaluating progressive and direct tensile fractures in brittle rocks

A micro–macro method for evaluating the behaviors of direct tensile fractures during progressive loadings in brittle rocks is proposed in this study. The method consists of the suggested equation of the stress intensity factor of the mode-I crack that considers crack initiation, growth, and coalescence subjected to triaxial tensile loadings and the expression of axial strain relating to the extended length of the wing crack. The direct tensile correlation of stress and strain for depicting the complete initial elasticity, strain hardening, strain softening, and fracture stages is also studied. The reasonability of the presented method is proved by contrasting published results of experiment. Furthermore, the sensitivities of the density, inclination angle and size of the initial crack on the axial stress–strain curve, axial stress–crack length curve, tensile strength, crack initiation stress, and elastic modulus are determined. The tensile peak stress, tensile stress at crack initiation, and tensile elastic modulus descend with the increment of the inclination angle or size of the initial crack. The tensile peak stress initially descends and then remains constant, finally reaching a critical value with the increment of the density of the initial crack. The tensile elastic modulus descends with the increment of the density of the initial crack. The calculated results have a great significance for the safety evaluation of surrounding rocks in deep-buried underground engineering.

Investigation of Anisotropic Behavior and Statistical Evolution of Acoustic Emission Energy during the Deformation of Layered Sandstone

To further understand the anisotropic behavior of layered rock and the precursor characteristics of rock mass instability, a series of uniaxial compression experiments using a loading system and an acoustic emission system was conducted on sandstone specimens. The influence of bedding on the mechanical parameters and failure modes and the statistical evolution of the acoustic emission energy were successively discussed. The results of axial stress-strain curves and crack propagation modes showed that the existence of a bedding plane increased the anisotropy of the rocks, and the magnitude of the bedding inclination also exerted certain influence on this anisotropy. Furthermore, we used the least squares method and the maximum likelihood method to analyze the b value and power-law exponent, respectively. The results of statistical evolution of acoustic emission energy showed that the b value, the effective power-law exponent, and the optimal exponent could be used as monitoring indexes for the rock mass stability. With the progress of the experiment, the following phenomena pertaining to acoustic emission activities occurred, which may indicate imminent danger of collapse: (1) the crackling noises increased significantly; (2) the variation of the b value exhibited a significant downward trend; and (3) the effective power-law exponent and the optimal exponent changed in different stages and gradually decreased as the final failure was approached. The findings in this paper may provide a theoretical basis for predicting the collapse and instability of rock mass structures.

Stress-strain behavior and design-oriented model for FRP spiral strip-confined concrete

Partial fiber-reinforced polymer (FRP) wrapping has become an attractive strengthening technique for reinforced concrete (RC) columns. Apart from FRP full wrapping strengthening technique, FRP partial wrapping with FRP spiral strips leads to some other advantages such as better fire resistance. However, existing study on FRP spiral strip is limited and the confinement mechanism of FRP spiral strip wrapped concrete columns remains unclear. To this end, axial compression tests on 28 concrete columns wrapped with FRP spiral strips are conducted and the test results are presented in this paper. Results show that the ultimate axial stress and the slope of the post-peak linear branch of the axial stress–strain curve are significantly affected by the proposed confinement effectiveness factor. The desired confinement effectiveness factors for specimens can be achieved by varying the key confinement parameters (i.e., spiral angle, width and layers of FRP strips). It is found that the increases in FRP strip width and thickness lead to enhancements in both ultimate axial stress and strain of the FRP spiral strip-confined concrete, whereas a decrease in the spiral angle has a more pronounced effect on the ultimate axial stress enhancement than the ultimate axial strain enhancement. A new design-oriented stress–strain model is proposed for FRP spiral strip-confined concrete based on the new test database assembled in this study. The predictions corroborate reasonably well with the test results, demonstrating the accuracy and robustness of the proposed model.