Strength Of Concrete Core Research Articles

Experimental investigation and analytical model for the shear behavior of composite beams with prefabricated U‐shaped RPC permanent formworks

AbstractThis study aimed to investigate the shear performance of composite beams with prefabricated U‐shaped reactive powder concrete (RPC) permanent formworks with normal strength concrete poured inside. Three‐point bending tests were performed on eight composite beams with prefabricated U‐shaped RPC‐reinforced formworks and one reinforce concrete beam. Additional parameters, such as the stirrup spacing, shear span ratio, and inner core concrete strength, were evaluated. The test results indicated that the prefabricated U‐shaped RPC permanent formworks increased the shear capacity of the composite beams. Under the same configuration, the bending cracking load, stirrup yield load, and peak load of the composite beams increased by 51.54%, 88.02%, and 78.38%, respectively. An analytical model was proposed to predict the shear strength of composite beams with prefabricated U‐shaped RPC formwork and validated using the experimental results.

Compressive behavior of manufactured sand recycled coarse aggregate concrete-filled steel tubular short columns

To study the mechanical properties of manufactured sand recycled aggregate concrete-filled steel tubular (RACFST) columns, a total of 81 square short RACFST column specimens with different types of manufactured sand and natural sand were created and subjected to axial compression tests. The variables included sand type, core concrete strength, slenderness ratio, and steel tube thickness. The failure mode, axial load-displacement curve, axial compressive capacity, and deformation capacity of the specimens were analyzed in detail. The results indicate that the failure mode of the specimen is generally waist drum-type failure and shear-type failure, and both of them are local buckling. The load-displacement curve of the manufactured sand specimen is similar to that of the natural sand specimen, but the steel tube of the manufactured sand specimen yields later than that of the natural sand specimen. Generally, the limestone manufactured sand specimen has the highest axial compressive capacity and the best ductility ratio, while the pebble manufactured sand specimen has an axial compressive capacity similar to the natural sand specimen but a lower ductility ratio. The effects of concrete compressive strength, slenderness ratio, and steel tube thickness on specimens with manufactured sand are similar to those with natural sand. Based on the results of this experimental study, a formula for calculating the axial compressive capacity of square RACFST columns is provided.

Reliability Analysis of Axial Compressive Strength of Concrete-Filled Steel Tube (CFST) under Coupled Corrosion and Load Effects

This paper presents a finite element analysis (FEA) of and reliability study on concrete-filled steel tube (CFST) members under the combined effects of corrosion and compressive loading. First, a stochastic-based FE model is established through the proposed secondary development program based on ABAQUS 2021 software. The model could account for the uncertainties of material, geometric, and corrosion effect on CFST members. The reliability of the built model was validated through experimental data of corroded CFST members under compression loading. Subsequently, the compressive performance of CFST under a combination of corrosion and loading was further investigated by numerical parameter analysis. A total of 1800 models were created to clarify the coupling mechanism among the core concrete strength, the steel tube strength, the steel ratio, and the maximum strength of the CFST member. Three theoretical formulas presented in classical design standards were used to calculate the axial compressive strength of the corroded CFST, and the uncertainty parameters μkp and δkp were also obtained for the discussed design formulas. Finally, the First Order and Second Moment (FOSM) method was employed to estimate the reliability indices β across different standards. The calculations revealed that the reliability indices β according to European standard ranges from 2.93 to 5.52, with some results falling below the target reliability index βT of 3.65. In addition, the multi-parameter coupling effects on reliability index β were investigated, and the main influencing factors were obtained. By leveraging the reliability analysis, reasonable design requirements can be proposed for CFST members under the coupling effects of corrosion and external load, which provides a design basis for the CFST member.

Mechanism and bearing load-capacity of compound stirrup UHPC-encased CFST columns under axial compression

To augment the synergistic performance between the outer reinforced concrete and the core concrete-filled steel tube (CFST) in standard reinforced concrete encased CFST columns, 14 columns underwent axial compression testing. Of these, 12 columns were encased with compound stirrup Ultra-High Performance Concrete (UHPC), one with compound stirrup high-strength cement-based material, and one with ordinary reinforced concrete. The experiment investigated the effects of compound stirrup forms, stirrup spacing, steel fiber content, steel tube wall thickness, outer concrete types, and core concrete strength on the final failure mode, load-displacement curve, strain characteristics, and development pattern of the specimens. Results indicated improved deformation coordination between the outer compound stirrup UHPC and the core CFST in compound stirrup UHPC encased CFST columns. The yield strength of both the compound stirrup and the steel tube was fully developed within the tested parameter range. A calculation method considering the compound stirrups, fiber constrained outer concrete, and the CFST’s contribution ratio was used to predict the ultimate bearing capacity of reinforced concrete encased CFST columns.

Eccentric compression capacity of bolt-welded joints in concrete-encased CFST

The new bolt-welded joint in concrete-encased Concrete-Filled Steel Tube (CFST) without cutting flange demonstrates great mechanical properties and durability, which is also easily constructed at high altitudes or in long-span arch bridges. It is crucial to study the mechanical properties of composite member made of the bolt-welded joint and outer concrete. In this study, a parametric analysis on bearing behavior of bolt-welded joints in concreted-encased CFST under eccentric compression was explored, based on experiments and a validated finite element model. Furthermore, a simplified calculation formula for the eccentric bearing capacity of the bolt-welded joint in concreted-encased CFST was proposed. The results indicated that the bearing capacity increased with the increase of steel tube strength, core concrete strength, reinforcement strength, outer concrete strength, steel ratio, and reinforcement ratio. Increasing the steel ratio and the steel tube diameter-section ratio can enhance the ductility of the member. A rise in stirrup spacing had a detrimental impact on both bearing capacity and ductility. An excessive amount of eccentricity diminished the bearing capacity of the member and alter its failure mode. The proposed simplified calculation formula for eccentric bearing capacity was in strong agreement with both experimental and numerical results.

Numerical analysis of the local damage of RC beams under close‐in explosions based on the calibrated Karagozian & Case (K&C) concrete model

AbstractLimited research has been carried out to investigate the local damage features of reinforced concrete (RC) beams under close‐in explosions. To better capture the local damage features of rectangular cross‐sectional RC beams, a numerical study on the RC beams subjected to close‐in explosions based on the calibrated Karagozian & Case (K&C) concrete model was conducted in LSDYNA. First, a systematic calibration of the K&C model for concrete was performed in the single‐element numerical analysis. Calibration of the K&C model consists of parameter determination for the new strength surfaces, modification of the damage function to better feature the tensile behavior of concrete, elimination of the element size effect by introducing an element size factor and determination of the scaled damage factors. The new strength surfaces are verified and determined through both the approximation calculation method and the parameter fitting method. And the damage function was modified by reducing the termination of λ to better characterize the brittleness of concrete and feature the tensile failure. A comparison between the numerical results and the corresponding experimental results demonstrates that the calibrated K&C model and established FEM model of RC beams subjected to close‐in explosions can well characterize the local damage features of RC beams. On this basis, a further parameter analysis on the factors influencing the local damage response of RC beams was conducted by considering reinforcement details (stirrup spacing and longitudinal reinforcement diameter) and charge details (charge mass and aspect ratio). Results show that the close‐in explosions destroyed the concrete corners in the blast surface with shear stress first and the side shear failure of concrete corners weakens the concrete core strength and contributes to the greater loss of concrete core. Rear spalling damage was initially caused by the bond failure between the tensile longitudinal reinforcement and the surrounding concrete. Furthermore, increasing stirrup spacing, tensile longitudinal reinforcement diameter, and charge mass contribute to the increase in local damage sizes.

Experimental and simulation study on seismic performance of steel‐PVA hybrid fiber cementitious composites‐encased CFST

AbstractSix hybrid steel and PVA fiber cementitious composite (S&PFCC)‐encased concrete‐filled steel tube (CFST) columns were designed and fabricated, and the effects of axial compression ratio, core concrete strength grade and steel tube thickness on the seismic performance of composite columns were studied. The test results show that the increase of core concrete strength can obviously improve the bearing capacity and energy dissipation capacity of composite column and can restrain the strain growth rate of steel tube after yielding. Low axial compression ratio and thicker steel tube thickness can improve the overall seismic performance of composite columns. Three calculation methods are modified, and the modified An method can predict the lateral bearing capacity of composite column well. Finally, based on the OpenSees finite element software, the seismic performance of the CFST column wrapped in S&PFCC is simulated to verify the test, and the numerical study shows that the seismic performance of the composite column can be optimized by increasing the steel fiber content, while PVA fiber has a weak effect in the elastic stage, and can effectively improve the ductility of the composite column in the plastic stage.

Seismic performance and damage analysis of steel-PVA hybrid fiber cementitious composites encased CFST columns

Using axial compression ratio, core concrete strength, steel tube thickness, and SPC strength as research variables, cyclic loading tests were conducted on 12 steel-polyvinyl alcohol (PVA) hybrid fiber cementitious composites (SPC) encased concrete-filled steel tube（CFST）columns to study the seismic performance of specimens under earthquake action, including failure mode, strength, stiffness, ductility, energy dissipation, and second-order effects. According to the study's findings, fiber bridging enabled SPC-encased CFST columns to effectively prevent cracks and maintain high integrity even after failing. The axial compression ratio, core concrete strength, and lateral bearing capacity were positively correlated, with a maximum bearing capacity increase of 46.9%, but their deformation capacity had decreased to varying degrees. The thickness and SPC strength of the steel tube exhibited excellent performance in terms of deformation capacity and energy consumption, with a maximum ductility coefficient of 6.21. In the overall statistical analysis of research variables, it was found that composite columns with core concrete strength grades higher than outer SPC strength had better seismic performance compared to other specimens. This article proposed an improved Park-Ang damage model that effectively evaluates the damage status of SPC-encased CFST columns. The model complies with the boundary conditions of the composite column, and the damage development trend reflected is consistent with experimental phenomena.

Experimental study on axial compression properties of concrete filled steel tubular stub columns in varying ambient temperature

To better understand the mechanical property issues of in-service concrete-filled steel tube (CFST) bridges susceptible to natural environmental temperatures, 13 CFST stub columns were assessed under axial compression with different ambient temperatures and steel ratios, with the strength of the core concrete as the main parameters. The influence of ambient temperature on the peak strain and joint elastic modulus of the CFST stubs was investigated. The influence mechanism of steel content and core concrete strength on the residual toughness of the CFST at different ambient temperatures was revealed. In this study, we proposed a modified constitutive equation for CFST, taking into account the ambient temperature. We demonstrated that the peak strain, residual toughness ratio, and elastic modulus of the CFST stubs significantly decreased when the temperature decreased. The calculated values of the modified constitutive equation for CFST, taking into account the ambient temperature, were in good agreement with the test results and could be used to predict the trends in stress properties of the CFST stubs at different temperatures. The results could serve as a critical reference for the design and calculation of CFST structures in very cold regions.

Axial compressive bearing capacity of high-strength concrete-filled Q690 square steel tubular stub column

Concrete-filled high-strength steel tubular (CFHSST) column is a kind of composite structure that uses high-strength steel instead of ordinary steel to strengthen the transverse restraint of concrete. In this work, in order to investigated the axial bearing capacity of CFHSST square column, a series of 22 CFHSST square stub columns were prepared by using two kinds of Q690 high-strength steel plates with different thickness of 3 mm and 6 mm with the strength of 741–749 MPa and 819–871 MPa, respectively. The static axial compression tests were carried out to understand the confinement effect of high-strength steel tube on concrete of different strength under static axial pressure and the influence of local buckling on the specimen. The axial compressive performances of CFHSST square stub columns are analyzed, including the failure mode, the relationship between load and deformation, and the stress-strain relationship. The test results show that the core concrete strength and area are major factors which have affected on bearing capacity of CFHSST square stub columns. Furthermore, width-to-thickness ratio exhibits significantly improving the failure mode and ductility of CFHSST square stub columns, while width-to-thickness ratio, the ratio of steel or the coefficient of hoop determine confinement effect. Q690 steel tube has good confinement effects, but the confinement effects on low strength concrete is small, especially for the specimen with relatively small width-to-thickness ratio. Finally, the calculation method of the ultimate bearing capacity of CFHSST square stub columns under axial compression is proposed, which can effectively evaluate the ultimate bearing capacity of CFHSST square stub columns.

Effect of ultra‐high performance concrete repair layer thickness on the behavior of concrete columns

AbstractThe use of ultra‐high‐performance fiber reinforced concrete (UHPFRC) to repair or rehabilitate old concrete structures has been shown to be appropriate due to its properties, such as higher compressive strength, ductility, and durability than conventional concretes. One of the studied strengthening techniques is the reinforced concrete columns jacketing by UHPFRC. Some recent research evaluated some parameters, such as the repair thickness and the volumetric ratio of stirrups. However, other important parameters, such as UHPFRC jacketing thickness in different column cross‐sections and the concrete core compressive strength, need to be addressed. In addition, the analytical models presented in the literature are restricted to only one concrete grade. The present study aims to evaluate the behavior of short reinforced concrete columns strengthened with UHPFRC subjected to centered compression. Numerical analyses of three‐dimensional models were performed using the finite element method, validated, and calibrated using experimental results. The influence of the concrete core compressive strength, the UHPFRC jacketing thickness, and the shape of the cross‐section were evaluated. Numerical simulations concluded that the lower concrete core compressive strength columns showed the best relative increase performance. Moreover, the concrete core strength was inversely proportional to the resistance capacity of the repair system. In addition, the increase in repair thickness contributes to the axial strength of the column. Furthermore, the cross‐section influences the distribution of stresses and strains, showing the best performance for circular sections. Finally, regardless of cross‐section type, the proposed equations accurately predicted the numerical results.

Cyclic tests and finite element analysis on hybrid fiber–cement-based composite encased CFST columns

Six hybrid steel–polyvinyl alcohol (PVA) hybrid fiber–cement (HFC)-based composite encased concrete-filled steel tube (CFST) columns were tested for their seismic performance. The impacts of core concrete strength, steel tube thickness, and axial ratio on the seismic performance of the specimens were studied. According to the experimental findings, strengthening the core concrete can enhance the column's lateral bearing capacity and energy dissipation capabilities while also slowing the rate at which the steel tube's strain increases after yielding. Furthermore, HFC-encased CFST columns with a lower axial ratio and a thicker steel tube demonstrated superior seismic performance. An's equation of computation could better forecast the lateral bearing capacity of HFC-encased CFST columns when compared to other calculation methods. Finally, an experimentally validated finite element model of HFC-encased CFST columns was developed. Numerical studies showed that the seismic performance of composite columns can be significantly improved by increasing the strength of the HFC and the volume concentration of the steel fiber, and that PVA primarily acts in the plastic stage.

Mechanical properties of CFST column with bolt-welded joints under eccentric compression

With the increase of arch bridge span and difficulty in welding construction, the traditional connection method of arch rib steel tube cannot satisfy the requirements of construction safety and economy. In this paper, a new bolt-welded connection method for arch rib of long-span Concrete Filled Steel Tubular (CFST) arch bridge is proposed, which can avoid the stress redistribution of joints of arch rib caused by cutting the joint flange in the traditional connection method. In order to study the mechanical properties of the bolt-welded joint of CFST, the eccentric compression scale tests were carried out, and the finite element model is validated based on the experiment. The parameter analysis was performed and the calculation formula of eccentric bearing capacity of bolt-welded joint of CFST is proposed. The results show that the bolt-welded joint can significantly improve the stiffness, stability and effectively avoid buckling at the joint. With the increase of steel tube strength, core concrete strength and steel ratio, the bearing capacity of bolt-welded joints of CFST significantly improved, and the bearing capacity of bolt-welded joints under small eccentric is better than welded joints. The bolt-welded area ratio is recommended not to exceed 0.3. The calculation results of the proposed formula are consistent with the experimental and simulated results.

Numerical simulation and load-bearing capacity of concrete-filled steel tubes strengthened with CFRP grid-reinforced ECC under axial compression

A numerical study on the compressive behavior and load-bearing capacity of the concrete-filled steel tube (CFST), strengthened with a carbon fiber-reinforced polymer (CFRP) grid-reinforced engineered cementitious composite (ECC), is presented. The numerical simulation, which was based on a previous test performed by the authors, indicated that an increase in the number of CFRP grid layers results in a significant increase in the bearing load on the concrete core. The confinement is provided to the concrete core by the strengthening layer only after the yielding of the steel tube, which occurs because of the contact pressure between the steel tube and ECC. Parameter studies that considered the effect of the number of CFRP grid layers and the strength of the CFRP, compressive strength of the ECC and concrete core, diameter-to-thickness ratio, and yield strength of the steel tube were conducted. The increase in fcu and decrease in Ds/t weakened the function of the confinement of the strengthening layer, and the influence of fcu was more significant. Furthermore, an analysis model that considered the nonlinear confining behavior and stress history for predicting the load-bearing capacity of the CFST columns strengthened with CFRP grid-reinforced ECC was established. The stress increment iteration method was employed to determine the varying confining pressure during loading. The proposed model showed high accuracy for predicting the load-bearing capacities of the strengthened columns.

Unified Model for Axial Bearing Capacity of Concrete-Filled Steel Tubular Circular Columns Based on Hoek–Brown Failure Criterion

Concrete-filled steel tubular (CFST) composite columns can overcome the brittleness of concrete and improve the plastic deformation ability of concrete, thus improving its strength and deformation ability. At present, most of the published models for predicting the axial bearing capacity of CFST columns are empirical models based on the nonlinear fitting of experimental data, which has some limitations on the application of the models. Therefore, to establish a new unified theoretical model, a new ultimate compressive strength of core concrete was established by the Hoek–Brown failure criterion, and the conversion formula between the cube and cylinder compressive strength was also established in this paper. At the same time, the strength-reduction coefficient influenced by the slenderness ratio was also established. The newly established unified model can predict the axial bearing capacity of CFST columns with different steel types, concrete types, slenderness ratios, diameter-to-thickness ratios, and cross-sectional dimensions. At the same time, the newly established unified model can be applied to a wider range of test parameters. To determine the parameters in the proposed model and assess the models, a total of 798 test data were collected. Based on the test database, the existing models and the proposed model were evaluated. The results show that the proposed model has very high accuracy in predicting the test results of CFST short and long columns, and the average value (AV) and integral absolute error (IAE) are 1.012 and 0.094, respectively. In addition, the model proposed in this paper also has high accuracy in predicting the axial bearing capacity of CFST columns under high temperatures.

Experimental study and numerical analysis of the interfacial bonding performance of T‐shaped concrete‐filled steel tubes

SummaryThe effects of core concrete strength (C30, C40, and C50), steel tube length (600, 1000, and 1400 mm), and steel tube wall thickness (3, 4, and 5 mm) on the bonding performance of T‐shaped concrete‐filled steel tubes (CFSTs) were systematically investigated using push‐out test. Via the test, the patterns of the failure modes, load‐slip curves, stress‐slip curves, and distribution law of longitudinal strain were observed for the specimens, and the bond‐slip constitutive model for T‐shaped CFSTs was established. The test results indicate that the bond stress of T‐shaped CFSTs increases with the increase in concrete strength and tube wall thickness, while the steel tube length has less influence on the bond stress. In addition, non‐linear spring elements were used to simulate the interfacial bonding behavior based on the bond‐slip equation, which is in good agreement with the experimental curve results.

Axial compression behaviour of circular and square UHPC-filled stainless steel tube columns

To improve the durability and mechanical behaviour, an ultra-high performance concrete (UHPC)-filled duplex stainless steel tube stub column is proposed in this study. To explore the axial compression performance of UHPC-filled duplex stainless steel tube (UFSST) stub columns, a total of 12 UFSST, 6 confined concrete-filled duplex stainless steel tube (CFSST) and 6 hollow duplex stainless steel tube (HSST) stub columns were subjected to axial compression. The experimental parameters included the section type, the tube thickness and the core concrete strength. For the UFSST stub columns, when the constraining index (ξ) exceeds 0.82 and 1.52 for circular and square stub columns, respectively, the failure mode changes from shear-type failure to waist-drum failure. The concrete contribution rate (CCR) is positively correlated with the concrete strength and negatively correlated with the tube thickness, whereas the variation law for the strength index (SI) is the opposite. The circular UFSST stub columns have a higher CCR than the square columns. A test database, including UFSST and UHPC-filled steel tube (UFST) stub columns, was established to evaluate the applicability of existing design codes. Furthermore, a modified ultimate load prediction formula with higher accuracy is proposed and verified.

Size effect on the compressive behavior of BFRP RC circular columns: Experiments and a size-dependent stress-strain model

To investigate the size effect on compressive performance of circular concrete columns reinforced with basalt fiber-reinforced polymer (BFRP) bars and spirals, nine geometrically similar columns were fabricated and tested under concentric compression. The variables tested were cross-sectional diameters (200, 400, and 600 mm) and transverse reinforcement ratios (1.2%, 2.2%, and 3.3%). The results evidently indicated the size effect on the confined strength, corresponding strain and deterioration rate (in the post-peak branch) of concrete core confined by BFRP spirals. Specifically, with the cross-sectional diameter changing from 200 mm to 600 mm, the maximum decreases in the confined strength and corresponding strain were 14.1% and 31.9%, respectively, while the maximum increase in the deterioration rate was 318%. In addition, the size effect on confined strength could be weakened with the increase of transverse reinforcement ratio. Increasing the transverse reinforcement ratio could enhance the load-carrying capacity as well as effectively increase the confinement strength and ductility. Finally, a size-dependent stress–strain model, which considers the size effect of confined strength, corresponding strain and deterioration rate, was developed for evaluating the confined behavior of FRP RC columns.

The behavior of steel-concrete-steel sandwich composite beams with box-profile shear connectors: Experimental and numerical

This study investigated the flexural behavior of new box-profile (BP) shear connectors in composite steel–concrete-steel (SCS) structures. To this end, nine beam specimens were fabricated and tested to evaluate the flexural and shear behaviors of BP shear connectors. The effects of geometric parameters, including the concrete core thickness, steel faceplate thickness, and shear connector thickness and width, on the flexural strength of BP shear connectors were then analyzed and reported. According to the results, increasing the shear connector width increased the rigidity of the beam, leading to a stress concentration in the slot welds. Therefore, the slot welds failed, leading to the detachment of the faceplate from the core. Increasing the strength of the concrete core above 40 MPa not only did not improve structural behavior but also led to BP shear connectors failing earlier than the concrete core, with the beam experiencing shear failure. Beams with 25≤hcontc≤30 and 3.33≤hconbc≤5 showed higher composite behavior. Finally, the beams were numerically modeled and validated using the nonlinear finite element method (FEM). A sensitivity analysis was conducted, and the concrete core strength and shear connector arrangement contributions were discussed.

Experimental study on axial compression of hybrid fiber cement-based composite column with circular cross section

In this study, hybrid fiber cement-based composite materials are used to make the confinement layer of the composite column, and plain concrete is poured inside to form the composite column. Considering the thickness of the confinement layer, the strength of the mixed fiber cement-based composite material of the confinement layer, and the strength of the inner core concrete, two sets of eighteen composite column specimens were designed and fabricated for axial compression tests to study the axial compression performance of the composite columns. The results showed that: The main factor that affects the occurrence time of cracks in composite columns is the strength of the cement-based composite material of the confinement layer. Through the analysis of the load–strain curve, it is found that the influence of the thickness of the confinement layer and the strength of the core concrete on the peak load of the composite column depends on the strength of the confinement layer: When the strength of the confinement layer is greater, the greater the thickness of the confinement layer and the smaller the strength level of the core concrete, the greater the peak load; when the strength of the confinement layer is smaller, the greater the thickness of the confinement layer and the smaller the strength level of the core concrete, the smaller the peak load. The peak strain is mainly related to the thickness of the constrained layer and the strength of the cement-based composite material of the confinement layer; When the strength of the confinement layer is smaller, the greater the thickness of the confinement layer and the smaller the strength of the core concrete, the better the ductility; when the strength of the confinement layer is greater, the specimen with the thickness of the confinement layer of 62.5 mm has better ductility; Based on statistical principles, the bearing capacity prediction formula and peak stress prediction formula of the composite column are established, and the calculated values are in good agreement with the experimental values.