Concrete Core Research Articles

Flexural performance of FRP-SWSSC-steel composite beams: Experimental and analytical investigation

The steel-concrete-fiber-reinforced polymer (FRP) composite structure was proposed in this paper, where FRP and steel were designed in the tensile region and compression region of concrete-filled steel tube (CFST) beams in the form of sheets, bars, and tubes, respectively, with the expectation of improving the performance of the structure. Epoxy mortar was used to modify the interfacial properties and part of the normal concrete (NC) was replaced by seawater sea sand concrete (SWSSC). A total of 10 FRP-SWSSC-steel composite beams were fabricated and subjected to four-point bending tests, and they exhibited the failure modes of local buckling. Reinforcing the tensile region significantly improved the flexural performance of the composite beams since the steel tube buckling and the concrete cracking were limited, and the flexural capacity of the composite beams was increased by 18.3%− 28.6% by designing FRP sheets or steel bars in the tensile region. Reinforcing the compression region had little effect since the existence of the in-built tube divided the monolithic concrete, and the confinement on the core concrete was not fully utilized. Based on research experience with CFST beams, a flexural strength model for FRP-SWSSC-steel composite beams was established.

Experimental study of post-fire bond behavior of concrete-filled stiffened steel tubes: A crucial aspect for composite structures

The slip between steel and concrete components and the bond strength between the two materials are important factors in load transfer under fire and cooling conditions. This study investigates the effect of the post-heating interaction of concrete and steel on the performance of composite members considering the bond between the two materials. Unidirectional push-out tests were conducted on circular concrete-filled stiffened steel tube (CFSST) columns after experiencing heating and subsequent cooling. The parameters under study were the diameter-to-thickness ratio of the tube (63.5, 31.75, and 21.17), the number of shear connectors (0, 4, and 6), and the target temperature (25, 250, 500, and 750 °C). The results showed that shear connectors effectively prevent concrete from sliding against the steel surface, facilitating load transfer from concrete to the steel tube. This increased interlocking between steel and concrete surfaces, leading to a substantial improvement in bond stiffness by 1–284% and bond strength by 2–590% for specimens with 4 and 6 shear studs. Moreover, the reduction in the diameter-to-thickness ratio enhanced interlocking between the steel tube and concrete core, leading to an increase in bond stiffness by 4–201% and bond strength by 10–216%. The application of thermal loading and the subsequent cooling induced macro-changes in the geometry of both the concrete core and steel tube, resulting in significant improvements in the bond properties of CFSST columns. In the specimens without shear connectors, slip between the concrete and steel occurred without any obstacle, while in the specimens with shear connectors, failure occurred in the form of the local crippling of the tube and the crushing of concrete around the shear stud together with the outward local buckling of the steel tube around the shear stud. Ultimately, a model to capture the bond stress-slip relationship of CFSST columns after thermal loading was proposed and then evaluated against the experimental data of the present study, which showed a good correlation between the experiment and prediction results.

CFRP-steel composite beams with seawater sea sand concrete cores subjected to bending

A redesign was attempted for the concrete-filled double-skin tube subjected to bending, and by adjusting the core tube position and applying carbon fiber-reinforced polymer (CFRP) sheets or steel bars in the tensile region, it was hoped to delay the cracking of the concrete in the tensile region. Epoxy mortar was used to modify the interface. Seawater and sea sand has been successfully applied from reinforced concrete beams to concrete-filled steel tube beams, benefiting from the corrosion-resistant properties of CFRP and epoxy mortar. The four-point bending test parameters included the type of core tube, the number of CFRP layers, and the diameter of the steel bars. The beams with FRP sheets exhibited better flexural capacity and residual stiffness, while the brittle fracture of FRP led to major crack in concrete. The beams with steel bars exhibited better initial stiffness and the cracks in concrete were more uniform in width. When the layers of CFRP or the diameter of the steel bars was increased, the flexural capacity of the composite beams increased by 11.0%-13.6% and 7.0%-9.1%, respectively. The enhancements provided by the core tubes were small, and the design of the composite beams should be considered from various perspectives. The moment-curvature relationships, ultimate states, and flexural stiffness were defined by considering concrete cracking, FRP fracturing, and steel buckling. Several formulas for calculating flexural stiffness in the code were evaluated in combination with the test data.

Design Method for Local Buckling Resistance of Double Steel Plate–Concrete Composite Walls with Stiffening Ribs and Tie Plates

An ordinary double steel plate–concrete composite wall (ODSC wall) is composed of core concrete, the faceplates, and shear connectors such as studs, etc. Based on an ODSC wall, a new type of stiffened double steel plate–concrete composite wall (SDSC wall) is conceived by incorporating additional stiffeners and tie plates on the internal surface, which aims to improve the local stability of the faceplates. In the authors’ previous study, a series of axial compression tests were conducted on the SDSC walls. The SDSC walls in the test showed better mechanical performance, as the presence of stiffeners changed the buckling deformation mode and significantly improved the corresponding local buckling stress and ultimate strength. In this paper, a comprehensive summary of the prior research on SDSC walls is provided, and the effect of the constructive parameters on the local stability is discussed. The results reveal that the modified formula of the critical stress can degrade to the Euler formula when the stiffener-to-stud spacing ratio (i.e., a/B ratio) approaches infinity. What is more, the analysis model is also applicable for DSC walls with enclosed side plates, and the proposed formula can predict the buckling stress of the SDSC walls with different a/B ratios. In addition, according to the analysis of the numerical simulation, a design approach for SDSC walls to prevent local buckling is provided, which is applicable in practical engineering applications.

Experimental and numerical research on the uniaxial behavior of the stiffened circular concrete-filled steel tube stub columns

In large-span bridges or high-rise buildings, CFST columns with large width-to-thickness ratio are often adopted, which require proper stiffening measures to improve the mechanical behaviors. This paper presents a comparative study of circular CFST stub columns with different stiffening methods, including longitudinal stiffeners (cross-shaped, T-shaped and ordinary type), additional reinforcement and headed studs. Axial compression tests have been conducted to investigate the effects of different stiffening methods on the failure mode, ultimate strength and ductility of the circular CFST stub columns. Finite element models are established to simulate the columns’ behaviors and conduct parametric studies. Based on test and simulation results, an axial bearing capacity prediction model for circular CFST stub columns stiffened by longitudinal stiffeners is proposed. It is demonstrated that the failure modes of the steel tube and core concrete are changed by the longitudinal stiffeners. The longitudinal steel stiffeners as well as reinforcing cage can effectively improve the axial bearing capacity and ductility of the columns. The latter is more efficient in promoting the bearing capacity while the former is more efficient in improving the ductility and residual strength. A combination of the additional stirrups and the ordinary stiffeners will be a more economical choice. In addition, the proposed formula gives better prediction accuracy compared to the equations in various design codes.

Cyclic behaviour of circular CFT-SG columns under axial tension-compression: Novel FE modelling and design methods

The ultra-high concrete-filled steel tubular (CFT) power transmission tower is popularly applied in engineering practice, which may be under a typical cyclic axial tension-compression loading condition due to the dynamic wind load. Nonetheless, the CFT columns of the tower may be accompanied by significant gap defects due to the influence of structural feature, material property, and service environment. Hence, to figure out the performance of circular CFT columns with spherical-cap gaps (CFT-SG) subjected to cyclic axial tension-compression, this paper investigates a constitutive relationship for the gapped core concrete, which is verified by several existing experiments. A novel numerical modelling approach for simulating the irregular cross-section of the circular CFT-SG columns under cyclic axial load was following established based on the triangulation method and code programming. The influence of important parameters such as gap ratio, material strength, and diameter to thickness ratio on the hysteresis characteristics, load-bearing capacity, elastic stiffness, energy dissipation and ductility of circular CFT-SG columns under cyclic axial tension-compression force is studied. Moreover, the restoring force model of the circular CFT-SG columns under cyclic axial tension-compression is finally presented. Research results declare that the compressive bearing capacity, stiffness, ductility, and energy dissipation of circular CFT-SG columns under cyclic axial tension-compression are gradually weakened with the increment in the gap ratio. The numerical modelling approach and restoring force model proposed in this article can accurately evaluate the hysteresis performance of circular CFT-SG columns subjected to cyclic axial tension-compression.

Experimental and numerical investigations of circular concrete filled steel double-skin and double-tube columns exposed to fire

Circular concrete-filled steel double-tube (CFSDT) columns have shown excellent ductility and load-carrying capacity. However, research on the fire resistance of CFSDT columns is still limited. This paper presents the fire tests, finite element (FE) modeling, behavior, and design of circular CFSDT columns and concrete-filled steel double-skin (CFSDS) columns exposed to fire. The fire resistance tests of one CFSDS and six CFSDT columns are described. Analysis is given on the effects of the material strength, the thickness of the inner steel tube, and the load ratio on the fire resistance of the specimens. Finite element models are developed that simulate the fire behavior of filled composite columns with double tubes and are verified by fire test results. The verified FE models are employed to undertake parametric studies on the structural responses of CFSDS and CFSDT columns under fire exposure. A design model is proposed for calculating the fire resistances of CFSDT columns. The results show that the load ratio and the column slenderness ratio have the most significant effect on the fire resistance of CFSDT columns; the cross-sectional parameters remarkably affect the fire resistance; and the material strength has a moderate impact on the fire resistance of CFSDT columns. Compared with CFSDS specimens, the existence of the core concrete significantly increases the fire resistance of CFSDT columns. The proposed design model for fire resistance can yield a safe prediction.

Application of various microscopy techniques to study early-age and longer-term behaviour of super sulphated cement microstructure.

Super sulphated cement (SSC) is a very promising substitute for traditional construction materials (i.e. Portland cement), due to its enhanced durability and particularly low environmental impact. This paper explores the microstructure and certain properties of SSC, focusing on the particular complexities of its microstructure and the difficulties of microanalysis of its hydrates. To do so, SSC paste samples were first cast to identify hydration products using X-ray diffraction, then observed at early age using confocal laser scanning microscopy (CLSM) and at early and late age using scanning electron microscopy. In addition, concrete cores impregnated with fluorescein in order to highlight porosity, cracking and aggregates debonding were observed under UV light using optical microscopy (OM), showing a complete absence of cracking and aggregate debonding. Both microscopy techniques (CLSM and UV light OM) have been applied to this type of binder for the first time. The results show that SSC microstructure is characterised by a sophisticated intergrowth of various phases, including ettringite and amorphous calcium-(alumina)-silicate hydrate gels. Finally, Monte-Carlo simulation of electron-matter has been provided for a better understanding of EDS analysis.

Finite element analysis on composite steel-concrete shear walls with centered and staggered openings

Present paper presents a comparative study made on the seismic performance of composite steel-concrete shear walls with rectangular openings, by performing numerical analyses using ATENA 3D software. Two specimens were designed at a reduced scale 1:3 having similar arrangements in the cross-section and the same geometry. First element was conceived with centered openings while the second one was designed with staggered openings. Steel connectors disposed as horizontal stiffeners were used to design and to assure the full connection between the structural steel profiles embedded in the edges and the concrete core. The openings that were considered on each story of the walls, were having the dimension of a common architectural door. A part of the results recorded from the experimental tests were used to calibrate the numerical models. This study aims to establish the seismic performance and to investigate the influence of the openings and the geometric position on the overall seismic behavior of the composite shear walls. Key parameters which describe the seismic performance of structural elements such: bearing capacity, deformation capacity, stiffness degradation, cracks and strain development are compared between the specimens and discussed. The results showed that different arrangements of the openings could significantly modify or increase the seismic performance of the composite structural walls.

Seismic column-to-footing connections reinforced with steel/GFRP bars and GFRP spirals

Hybrid reinforcement is explored for column-to-footing connections in seismic regions constructed with grouted duct connections as an accelerated bridge construction method. Two 38% scale precast concrete column-to-footing specimens were built; the first column was reinforced with hybrid reinforcement using a combination of mild steel and glass fiber reinforced polymer (GFRP) longitudinal bars, and the second with all-steel longitudinal bars. Double layers of GFRP spirals were used to provide improved concrete confinement for both columns; GFRP spirals exert higher levels of confining pressure on the column concrete core as deformation increases due to their elastic nature compared to steel spirals that yield. An equation was developed for the cross sectional area and pitch of a GFRP spiral to provide the same confining pressure as a steel spiral. The purpose of the cyclic load tests was to show that the column reinforced with hybrid reinforcement could reduce residual displacements using the elastic nature of GFRP bars. Pull-out tests of GFRP bars inside grouted ducts were carried out to determine the minimum embedment depth to achieve GFRP bar tensile strength. The seismic performance of both specimens was examined under quasi-static cyclic loads. The hybrid column reached a drift ratio of 9.0% drift ratio when a GFRP longitudinal bar fractured in compression; the all-steel column reached a drift ratio of 11.0% when two longitudinal steel bars fractured in tension. The elastic nature of the GFRP longitudinal bars provided substantial self-centering ability and reduced the residual displacement of the hybrid column compared to the all-steel column, thus enhancing its seismic resilience.

Seismic Response of Embankment Dams with Asphalt–Concrete Cores Based on the Three-Dimensional Time-Domain Inversion of Ground Motions

Seismic Response of Embankment Dams with Asphalt–Concrete Cores Based on the Three-Dimensional Time-Domain Inversion of Ground Motions

Damage analysis of axially compressed concrete-filled steel tube stub columns based on acoustic emission

Concrete-filled steel tube (CFST) widely applied in engineering structures due to its superior behavior requires monitoring and assessment for damage level. Here, the acoustic emission (AE) technique was used to monitor the fracture process of concrete core in CFST stub columns subjected to axial compression. Test and analyzed results show that the damage process of CFST specimens can be divided into four stages: compaction, elastic-plastic stage, strengthening and secondary strengthening. In the elastic-plastic stage, the evolutionary features of the AE event rate, cumulative energy, Ib-value and crack classification are capable of providing an early warning for cracked concrete core. Full crack propagation can be identified by the rapid increase in the proportion of shear cracks near the inflection point of load, which is impermissible in engineering structures. According to the analyses of the AE event rate and signal intensity in the elastic-plastic stage, the confinement of steel tube with thicker wall thickness or higher strength is delayed, which implies that this confinement is suggested to be triggered early. It is indicated that the AE technique has the potential to monitor and evaluate the damage process of CFST stub columns under axial compression, which can provide additional insight into the failure mechanism and assist in the scheme of repairs.

Influence of difference in deformation modulus between asphalt concrete core and transition layer on core behavior and difference threshold determination

Asphalt concrete core is the main impermeable structure of asphalt concrete core dams (ACCDs). The deformation modulus difference between the core and transition layer is directly related to the core behavior, especially the arch effect. This paper mainly studies the threshold of the deformation modulus difference between the transition and asphalt concrete core and the core arc safety. A database of 54 ACCD cases from domestic and abroad is collected. The thickness and material characteristics of the transition and core are statistically analyzed, and the range of deformation modulus ratio η between them is determined. A numerical model is established, and the impact of different interface models on the core behavior is compared and analyzed. By comparing measured data and numerical calculation results, an appropriate interface simulation method is selected to study the influence of deformation modulus ratio η on the core performance. On this basis, the threshold ηmax is determined using a cusp catastrophe model. A new valley shape characterization parameter δ is proposed, and the arch safety area under different δ is studied based on ηmax for reference in ACCDs design and construction. Finally, engineering measures to enhance the arch safety of the core are discussed.

Analysis on Mechanical Properties of FRP Constrained Concrete Core Reinforcement Under Cyclic Load

This article aims to analyse the mechanical properties of FRP confined concrete core rods under cyclic loading. By observing the behaviour characteristics of FRP confined concrete core rods under different cyclic loads, the influence of FRP confinement on concrete cyclic loading is analysed. Based on theoretical analysis, this article establishes a corresponding mechanical model and analyses the response mechanism of FRP constraints to concrete cyclic loads through simulation calculations. Through experimental research and theoretical analysis, it is possible to fully understand the performance of new composite structural materials. The research results of this article indicate that the confinement of FRP bars can significantly improve the compressive strength of concrete specimens. At a constraint ratio of 20%, the compressive strength of the specimen increased by 30% compared to the unconstrained ratio; At 40%, the compressive strength increased by 50%, therefore, FRP confinement can significantly improve the ductility of concrete and reduce fatigue damage under cyclic loading. Through this research method, important evidence can be provided for the application of concrete in practical engineering.

Post-fire compressive behavior of CFRP stirrups reinforced CFST columns: Experimental investigation and calculation models

This paper investigates the post-fire compressive behavior of CFST columns reinforced with CFRP strip stirrups. CFRP materials possess higher tensile strength than steel, making them promising candidates to enhance the ductility of high strength concrete when properly confined. However, direct exposure to fire leads to burning and rapid loss of tensile capacity in CFRP materials. To mitigate this, the study explores embedding CFRP stirrups in concrete to provide additional confinement besides the steel tube which prevents fire penetration through concrete cracks and protects the CFRP stirrups. Through experimental tests and comparisons with steel stirrups reinforced counterparts, it was observed that the CFRP stirrups, even after heating, still provided confinement to the concrete core as the overlapping joints of the strips remained intact. The failure mode observed in the CFRP stirrups was the rupture of FRP, rather than debonding of overlapping joints. Additionally, the use of CFRP stirrups led to reduced concrete temperatures and significantly higher unit enhancement in residual load-bearing capacity for the CFST columns compared to steel stirrups. Practical calculation models were developed to estimate the historical maximum temperatures and residual load-bearing capacity of the CFRP stirrups reinforced CFST columns, regardless of whether high strength or normal strength concrete was used. The calculated values demonstrated good agreement with experimental results. This study provides valuable insights into the performance of CFRP stirrups reinforced CFST columns under post-fire conditions, highlighting their potential as effective fire-resistant and structurally efficient solutions in civil engineering applications.

Experimental study on compressive strength of in situ concrete tested with arc pressure method

The innovative arc pressure testing method (APTM) is adopted in this study to obtain the strength of in situ concrete and reduce damage to the concrete structure. This method is related to the double shear plane of the concrete core. A specific APTM apparatus is used to apply loads in order to obtain the approximate pure shear stress and shear strength of the concrete core. Factors affecting the strength of concrete, such as aggregate type and concrete moisture, were found not to have an impact of the applicability of APTM. The compressive strength range of the tested cube concrete samples is 20–60 MPa. The reliability and repeatability of APTM are superior to in situ testing techniques such as Schmidt hammer (SRH) and pull-off testing methods. The experimental results indicate that APTM is suitable for in situ testing of concrete compressive strength of prefabricated buildings and beam–column joints in dense steel structures. Compared with other testing methods, its accuracy is much higher and the damage to the structure is minimised.

Evaluation of vibration properties of an 18-story mass timber–concrete hybrid building by on-site vibration tests

Timber–concrete hybrid structural systems are a practical option to provide tall mass timber buildings with a lateral load-resisting system. This paper discusses the dynamic behavior of an 18-story timber–concrete hybrid building based on the vibration properties evaluated by on-site vibration tests. First, microtremor measurements and human-powered excitation tests were carried out and the obtained vibration data were analyzed using a stochastic subspace identification method to derive natural frequencies, damping ratios, and mode shapes. Then, a finite-element (FE) model was developed based on detailed structural design information, and its eigenvalues and eigenvectors were compared with the test results. The vibration test results showed various mode shapes, including in-plane deformation of the floor diaphragm composed of cross-laminated timber (CLT) panels. The damping ratios in all the modes were scattered between 1 and 3%, and no frequency dependency was observed. The modal properties of the FE model agreed well with the test results by considering the additional stiffness of non-structural components. In order to simulate the in-plane deformation of the CLT floor diaphragm, detailed modeling of the connection between each CLT floor panel and the connection between CLT floor panels and concrete cores is recommended. The findings provide practitioners with an insight into dynamic properties and FE modeling methods of tall timber–concrete hybrid buildings.

Overtopping Failure Process and Core Wall Fracture Mechanism of a New Concrete Core Wall Dam

Concrete core wall dams have emerged as a cost-effective alternative to the reconstruction of old dams. However, the flooding mechanism and flood line of these dams differ significantly from traditional earth and rockfill dams due to the concrete core wall functioning as reinforcement. This study presents model tests that simulate the overtopping failure of earth and rockfill dams with concrete core walls of various thicknesses. The hydrologic curve and two-dimensional evolution of the breach are analyzed, and mechanical analysis examines the relationship between core wall thickness and free face during core wall failure. Results indicate that the presence of the concrete core wall shifts the overtopping failure mode to the scour pit failure mode. The scour pit failure mode occurs when upstream water scours the downstream core wall to form scour holes and free faces. Continuous scouring increases the depth of the free face, ultimately causing the moment between the two sides of the core wall to exceed the bending moment of the core wall, resulting in fracture. The study provides a theoretical basis for the design of core walls for this new type of dam.

Experimental study on the seismic enhancement of brick masonry spandrels using a single-sided composite reinforced mortar coating

AbstractThe paper reports on four cyclic tests of brick masonry spandrels in reference state and strengthened state. The tests were carried out on full-scale, H-shaped masonry panels to investigate the coupling role of the spandrel in connecting two piers in a historical masonry wall subjected to in-plane lateral actions. Two 250-mm-thick solid-brick masonry samples were tested—one single-leaf, the other two-leaves masonry. Both samples were tested before and after strengthening by applying a Composite Reinforced Mortar (CRM) coating on one surface. The CRM coating consisted of a lime mortar coating (nominal thickness = 30 mm), reinforced with mesh made of Glass Fibre-Reinforced Polymer (GFRP) that was anchored to the wall by GFRP transverse connectors. For the double-leaf masonry, there were additional transverse connectors made of steel bars in concrete cores (artificial diatones) to prevent masonry leaves separation and to improve the bonding of the CRM coating to the masonry. The test responses are compared in terms of crack pattern, failure mode, resistance, displacement and energy dissipation capacity. The tests showed the effectiveness of the proposed CRM system, which increased the spandrel resistance by 33% and 125% in the single—and double-leaf masonry, respectively, with the ultimate drift being 3.2% (one order of magnitude greater than for the unstrengthened reference samples). Data on energy dissipation and the equivalent viscous damping are also collected and compared. Importantly, the presence of the reinforcing mesh and the composite action of the coating and the wall changed the damage evolution and response mechanism, which resulted in a much better seismic response.

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.