Steel rebar buckling is an important failure mode in reinforced concrete (RC) columns. Fiber-reinforced polymer (FRP) composites provide an effective mean that can restrain or delay longitudinal ...rebar buckling. To accurately predict the strength and ductility of FRP-strengthened rectangular RC columns, it is necessary to accurately evaluate the interactions between FRP confinement and steel reinforcement buckling behavior at as any loading levels as possible. This paper proposed a composite beam model and a tension-bending beam model to evaluate the lateral support stiffness of FRP-wrapped concrete cover on steel reinforcement in the corner regions and flat sides of rectangular columns. Then, the whole stress-strain curves for reinforcing bar buckling behavior considering the lateral support of FRP jackets can be obtained and validated with the existing test database. For FRP-strengthened rectangular RC columns under monotonic axial compression, the overall load-bearing capacity of RC columns can be determined by summarizing axial loads sustained by FRP-confined corresponding plain concrete (PC) columns and the load contribution of steel rebars considering FRP lateral support. The accuracy of the proposed method is demonstrated by comparing theoretical predictions with test results. This calculation method can accurately predict the strength and ductility of FRP-wrapped rectangular columns within a 20% variation margin, which provides a reliable basis for the design of concrete structures.
•A composite beam model and a tension-bending beam model are developed to evaluate the spring stiffness.•A method is established to predict the overall behavior of FRP-confined rectangular RC columns considering bar buckling.•The factors affecting rebar buckling and the performance of FRP-confined rectangular RC columns are analyzed.
Adding short steel fibers into slag-based geopolymer mortar and concrete is an effective method to enhance their mechanical properties. The fracture properties of steel fiber-reinforced slag-based ...geopolymer concrete/mortar (SGC/SGM) and unreinforced control samples were compared through three-point bending (TPB) tests. The influences of steel fiber volume contents (1.0%, 1.5% and 2.0%) on the fracture properties of SGC and SGM were studied. Load-midspan deflection (
) curves and load-crack mouth opening displacement (
-CMOD) curves of the tested beams were recorded. The compressive and splitting tensile strengths were also tested. The fracture energy, flexural strength parameters, and fracture toughness of steel fiber-reinforced SGC and SGM were calculated and analyzed. The softening curves of steel fiber-reinforced SGC and SGM were determined using inverse analysis. The experimental results show that the splitting tensile strength, fracture energy, and fracture toughness are significantly enhanced with fiber incorporation. A strong correlation between the equivalent and residual flexural strengths is also observed. In addition, the trilinear strain-softening curves obtained by inverse analysis predict well of the load-displacement curves recorded from TPB tests.
With the rapid development of sequencing technologies towards higher throughput and lower cost, sequence data are generated at an unprecedentedly explosive rate. To provide an efficient and ...easy-to-use platform for managing huge sequence data, here we present Genome Sequence Archive （GSA; http：//bigd.big.ac.cn/gsa or http：//gsa.big.ac.cn）, a data repository for archiving raw sequence data. In compliance with data standards and structures of the International Nucleotide Sequence Database Collaboration （INSDC）, GSA adopts four data objects （BioProject, BioSample, Experiment, and Run） for data organization, accepts raw sequence reads produced by a variety of sequencing platforms, stores both sequence reads and metadata submitted from all over the world, and makes all these data publicly available to worldwide scientific communities. In the era of big data, GSA is not only an important complement to existing INSDC members by alleviating the increasing burdens of handling sequence data deluge, but also takes the significant responsibility for global big data archive and provides free unrestricted access to all publicly available data in support of research activities throughout the world.
•The seismic behaviour of LRS FRP-wrapped non-ductile RC columns was experimentally investigated.•The advantage of LRS FRP-wrapped RC columns in seismic retrofit was demonstrated.•The influence of ...FRP confinement stiffness on seismic performance was studied.•The accuracy of cyclic compression model model of LRS FRP-strengthened concrete was verified.
This paper presents experimental studies and numerical simulations of circular reinforced concrete (RC) columns wrapped with Polyethylene Naphthalate fiber reinforced polymer (PEN FRP, a kind of large-rupture-strain FRP) and conventional CFRP. A total of seven columns were designed and fabricated, including a control column, three columns strengthened with CFRP, and three columns strengthened with PEN FRP. All columns were subjected to cyclic lateral load and a constant high axial load. The effectiveness of PEN FRP strengthening was systematically analyzed and discussed regarding the failure mode, displacement ductility, lateral cyclic load–displacement curve, stiffness degradation and energy dissipation capacity. The test results indicated that the control column exhibited inferior ductility and peak lateral strength, while FRP-strengthened columns possess better energy dissipation capacity and ductility. Further study found that PEN FRP was preferred to CFRP with similar tensile stiffness in seismic strengthening. Finally, based on the cyclic compression model of LRS FRP-strengthened concrete, the improved tensile model of FRP-strengthened concrete, and the cyclic stress–strain model of reinforcing steel, numerical simulations for seismic performance were carried out in OpenSees. It was found that the numerical simulation results of all FRP-strengthened RC columns were in excellent agreement with the test results. To better understand the effect of FRP confinement stiffness on the seismic performance of circular columns, parametric analysis was performed. Results showed that the numerical simulations need to consider the buckling of longitudinal reinforcement for accurately predicting the seismic behavior of FRP-strengthened columns of lower confinement stiffness.
•The seismic behaviour of LRS FRP-strengthened shear critical columns was experimentally investigated.•The advantage of LRS FRP-strengthened RC columns in seismic retrofit was ...demonstrated.•Progressive failure and superior ductility were observed for shear critical columns strengthened with LRS FRP.•An axial-shear-flexure approach was developed to predict the envelope curves of LRS FRP-strengthened columns.
This paper presents an experimental study on the seismic behaviour of shear critical square columns strengthened by large rupture strain (LRS) FRP under reversed cyclic lateral load together with constant axial load. The experimental program included 2 control columns, 6 LRS FRP-strengthened columns, and 2 CFRP-strengthened columns. The tested specimens’ failure modes, hysteretic responses, ductility, and cumulative energy dissipation were carefully analyzed. The influences of axial load ratio, FRP layers, and FRP type on the seismic performance of these columns were investigated. Compared with the control column, the ductility ratio and energy dissipation of the column wrapped by 3-layer LRS FRP at a low axial load ratio were raised by 491.7 % and 6498.3 %, respectively. The advantage of LRS FRP over CFRP in seismic retrofitting for shear critical RC columns was demonstrated through comparative analysis for the first time. Although the tensile stiffness of 1-layer PET FRP is much smaller than 1-layer CFRP, the ductility ratio and cumulative energy dissipation of the 1-layer PET FRP-strengthened column under a low axial load ratio were still slightly larger than that of the column wrapped by 1-layer CFRP. The CFRP-strengthened column under a high axial load ratio showed sudden explosive failure, while the LRS FRP-strengthened columns exhibited progressive failure. In addition, the axial flexure shear interaction (ASFI) method considering buckling of longitudinal reinforcement in FRP-confined concrete was developed and verified by comparing test results and predicted envelope curves.
•Axial compressive behavior of PET FRP/CFRP-confined SSC columns was investigated.•The type and thickness of FRP jackets and the type of mixing water of SSC were the variables of this study.•A ...modified analysis-oriented model with a new dilation equation was proposed for the PET FRP-confined SSC.
Since fiber-reinforced polymer (FRP) composites possess excellent corrosion resistance, they have great potential of use with seawater sea-sand concrete (SSC) for the construction of marine and coastal infrastructure. The existing research on SSC columns confined with FRP jackets, however, is rather limited. Most of the limited research has focused on the axial compressive behavior of SSC columns confined by conventional FRP jackets (made from carbon and glass FRP composites). This paper presents results from the first-ever experimental study on the behavior of axially loaded circular SSC columns confined by polyethylene terephthalate (PET) FRP jackets, which are composed of a new and more promising type of FRP composites with a bilinear stress-strain response and a large rupture strain. Parameters varied between the tests presented in this paper included the type and thickness of FRP jackets and the type of mixing water used for casting the SSC. A modified analysis-oriented model was developed in which a new dilation equation was proposed for the PET FRP-confined SSC. The reliability and accuracy of the analysis-oriented model were then examined by comparisons between the predicted results and the test results of the FRP-confined SSC.
Large rupture strain fiber-reinforced polymers (LRS FRPs) (e.g., with an elongation larger than 5%) provide ideal confinement for seismic retrofit of concrete columns. Most of existing ...design-oriented models for FRP-confined concrete define the slope of axial stress-strain curve (i.e., axial stiffness) as a function of FRP’s rupture strain. Consequently, concrete confined with FRP of the same jacket stiffness but different rupture strains usually lead to different axial stiffnesses, which is contrary to the experimental observations. For LRS FRP-confined concrete a slight deviation in the axial stiffness might cause significant error in predicting its ultimate state. Therefore, a design-oriented model is developed here to define axial stiffness using the jacket stiffness rather than its rupture strain. 26 LRS FRP-confined circular concrete cylinders are tested and added to a database of 36 existing specimens. Then an existing lateral-to-axial strain (dilation) relationship is upgraded based on the above database and a database on 113 conventional FRP-confined concrete cylinders. A three-fold approach is employed to define the full-range stress-strain relation of LRS FRP-confined concrete as a function of jacket stiffness. The proposed stiffness-based design-oriented model predicts both softening and hardening behaviors and is applicable to both conventional and LRS FRP-confined concrete columns.
•Multi-scale analyses of nano-CaCO3 modified UHP-ECC were conducted.•The inherent relations of multi-scale performances of UHP-ECC was clarified.
This paper investigated the effects of nano-CaCO3 ...(NC) with different contents (i.e., 0–4% by mass of cement) on hydration, micro-structure and mechanical properties of ultra-high performance engineered cementitious composites (UHP-ECC) and clarified the inherent relations of multi-scale performances of UHP-ECC. At the micro-scale, the heat evolution analysis was firstly conducted and the results showed that NC accelerated the heat of hydration of UHP-ECC matrices due to its nucleation and dilution effect. Then, the phase developments of NC modified UHP-ECC matrices were discussed using the thermal gravimetric analysis/differential thermal gravimetric analysis, which proved the filler effect of NC. Mercury intrusion tests (MIP) indicated that not only the porosity but also the critical pore size decreased with the increase of NC content up to 3%, beyond which the porosity increased due to the agglomerate of NC. Finally, the morphologies of ultra-high molecular weight polyethylene fiber surfaces and fiber-matrix interfaces were observed by scanning electron microscope. At the meso-scale, single fiber pullout tests were conducted and verified that the frictional stress between fiber and matrix reached the maximum value with NC content of 3%, which was in accordance with the results of MIP tests. At the macro-scale, the optimal NC dosage to enhance the compressive, tensile and flexural properties of UHP-ECC was found to be 3%, which was strongly supported by the results obtained at micro- and meso-scale. Hence, intense linkages between the micro-, meso- and macro-scale performances of UHP-ECC were exhibited, and the micro- and meso-scale properties induced a decisive impact on the mechanical properties of UHP-ECC at composites level.
This paper presents an experimental study on the dynamic compressive behaviors of concrete confined with unidirectional natural flax fiber reinforced polymers (FFRPs) under an axial impact load. A ...total of 80 specimens were prepared and tested using a 75 mm diameter split Hopkinson pressure bar (SHPB) at different strain rates varying from 50 to 200 s−1. The experimental results showed that the failure modes and dynamic compressive mechanical properties (i.e., dynamic compressive strength, critical compressive strain, and energy absorption capacity) of FFRP-confined concrete were sensitive to the strain rate. The unconfined concrete specimens were crushed into small pieces at relatively low strain rates, whereas the FFRP-confined concrete specimens failed with FRP rupture and partial damage to the core concrete at relatively high strain rates. This indicates that the confinement of FFRP jackets can alleviate concrete damage and improve impact resistance. Compared with unconfined concrete, the application of the FFRP jacket remarkably improves the compressive strength, critical strain, and toughness, which indicates outstanding impact resistance. Increasing the confinement stiffness of the FFRP contributed to increasing the compressive strength, critical strain, and toughness. Based on the experimental results, the confinement mechanism of external flax FRP jackets on concrete was discussed, and a new dynamic strength model was proposed to predict the dynamic compressive strength of FFRP-confined concrete within the investigated strain rate range.
•A modified concrete damaged plasticity model is proposed considering the deformation and mechanical characteristics of LRS FRP-confined concrete.•The 3D finite element analysis of longitudinal ...reinforcement buckling in LRS FRP-confined RC columns is realized.•The effect of longitudinal reinforcement buckling on member is illustrated by the FEA results.•The factors affecting the buckling of longitudinal reinforcement are studied, and an empirical formula for determining the failure of members by the buckling of longitudinal reinforcement is proposed.
Large rupture strain (LRS) FRPs have great potential to be used for seismic retrofit of RC columns because of its high rupture strain (i.e., > 5%) that provides better confinement effects. However, in FRP-confined RC columns, the longitudinal reinforcements may buckle before the LRS FRP rupture, especially in the case of large stirrup spacing. This study aims to present a 3D finite element (FE) model for simulating the buckling effect of longitudinal reinforcements in LRS FRP-confined RC columns. A modified concrete damaged plasticity model (CDPM) is proposed to consider the deformation and mechanical characteristics of LRS FRP-confined concrete. Removal of failed concrete elements is considered to facilitate the buckling of longitudinal reinforcements in LRS FRP-confined RC columns. The FE analysis accuracy was verified by comparisons with test results. The factors affecting the buckling of longitudinal reinforcements were discussed through an extensive parametric study. Empirical formulas for the stress and strain values at which the longitudinal reinforcement starts to soften were developed. An empirical formula for the ultimate axial strain of LRS FRP-confined RC columns determined by longitudinal reinforcement buckling was also obtained by regression analysis.
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