Load-deflection Curves Research Articles

Prediction of the complete flexural load–deflection curve of self-compacting concrete reinforced with hook-end steel fiber

The flexural performance is vital for evaluating the characteristics of steel fiber reinforced self-compacting concrete (SFR-SCC). Although the tested load–deflection curve is commonly used to predict the flexural performance, little research has been done on the tested curve prediction. In this study, the complete flexural load–deflection curves of SFR-SCC beam specimens were obtained by a four-point bending test. The hook-end steel fiber with a length about 25 mm, 30 mm or 35 mm was respectively added for SFR-SCC at volume fraction of 0.4 %, 0.8 %, 1.2 % or 1.4 %. The characteristic points of tested curves are statistically analyzed to build the prediction formulas for the initial cracking strength, the flexural strength, and the first and second residual flexural strengths. The formulas for predicting the complete flexural load–deflection curve of SFR-SCC are proposed, and verified by comparing with the test curves. Meanwhile, the distribution of steel fiber in concrete matrix of beam specimen was measured to explain the mechanisms of flexural performance of SFR-SCC.

Experimental and numerical investigation of load carrying behavior of hollow beam using textile reinforced concrete

The paper presents an experimental and numerical investigation aimed at determining the load-carrying behavior of a new type of structure, which is a textile reinforced concrete hollow beam. This new material consists of fine-grained concrete reinforced with textile made from non-metallic fibers. In addition, numerical simulations were also conducted. The simulation results showed a similarity to the experimental results in terms of the failure mode, load-deflection curves.

Experimental investigation of a hybrid steel truss reinforced concrete beam for aerospace shelters

This paper illustrates experimentally the significance of using hybrid steel truss reinforced concrete beam (HSTRCB) in the construction of aerospace shelters. A finite element analysis was carried out in a previous stage to compare the flexural behaviour of HSTRCB with steel truss beams (without concrete casting) and traditional reinforced concrete beams. The Finite element simulations showed a significant improvement in the flexural behaviour of the HSTRCB. In this paper two beams were tested by 4-point bending test. The first is standard reinforced concrete beam. The other is HSTRCB with steel angles as top and bottom reinforcement. Stirrups were replaced by vertical steel plates. Flexural behaviour was represented by the load-deflection curves. Results showed that the adhesion strength between steel truss members and concrete should be enhanced. Failure occurred due to slippage between steel truss elements and concrete. Moreover, HSTRCB showed a noticeable improvement in the ductile behaviour for the zone after the ultimate capacity.

Torsion Improvement of Reinforced Self-Compacting Concrete Beams Using Epoxy Injection and CFRP

Few researchers have investigated the internal torsional reinforcement of box timbers, so little research has been conducted on this particular fortification method. The primary objective is to find out the possibility of adding a certain percentage of RCA to the NC mixtures, as well as verifying the success achieved in repairing the cracks that occurred as a result of torsion with CFRP or injecting with epoxy, which has not been addressed in previous research and literature reviews. This study reinforces reinforced SCC box columns subjected to complete torsion with CFRP sheets and epoxy resin injections. Four reinforced SCC specimens (the first beam with 0%, the second beam with 33.3%, the third beam with 67.7%, and the fourth beam with 100% RCA by weight) were subjected to pure torsion until failure. The dimensions and reinforcement of every specimen are identical. In addition, the applied torque-twist angle relationship at the midspan and end span was investigated. Bending experiments were performed to establish load-deflection curves and assess failure modes. After structural rehabilitation, all beams exhibited increased rigidity values, according to the results. Epoxy resin and CFRP sheet contributed to the specimens' increased ultimate load. The ultimate strength of RCA beams strengthened with CFRP and injected with epoxy increased. The specimens' flexural strength was considerably enhanced by the combination of surface roughness and fracture injection, and the effectiveness of using RCA was very good; it could be replaced with NCA in concrete mixtures, according to the ratio and need. Doi: 10.28991/CEJ-2023-09-11-05 Full Text: PDF

Behavior of circular concrete-filled steel tubular columns with different end moment ratios: Modelling and discussion

This paper develops a nonlinear model for the analysis of circular CFST columns under equal and unequal end moments using fiber beam element (FBE) method. An iterative process adopting Newton’s method combined with secant method was proposed to improve the calculation efficiency. A test database was collected to assess the accuracy of the proposed model regarding the prediction of axial load-bearing capacity, deflection curves and load–deflection curves. The verified model was then further used to investigate the behavior of CFST columns under different end moments. It was found that the normalized sectional locations of maximum deflection of CFST columns are only significantly affected by the end moment ratio, whereas those of critical sections are also influenced by the axial load. The specimen with a larger end moment ratio, steel strength, L/D ratio and/or a smaller e1/D ratio, D/t ratio experiences more significant second order effect. The bending stiffness of CFST columns is hardly affected by the end moment ratio. Moreover, the current codes, i.e., EC4 and AISC 360, may underestimate the capacities of CFST columns under unequal end moments.

Follow effect of low cost glass fibre reinforced polymer (GFRP) on the performance of concentrically loaded concrete column

(FRP) plays a major role in the strengthening of existing structures due to the age of the structure or natural calamities like earthquakes, floods, cyclones, etc. For reducing the size of structural members, the FRP wrapping assists to achieve the performance of the structure. Concrete is widely used due to its advantages and FRP is added to improve its quality in terms of strength. A study has been conducted on 21 cylinders based on their slenderness ratio. The slenderness ratios in the columns were 8, 16, and 24. At thicknesses of 5 mm and 7 mm, two types of wrap materials (UDCGFRP) and (WRGFRP)] were employed. Up to the point of failure, the columns were subjected to monotonic axial compressive force. The column’s yield loads, and ultimate load, were deduced from the load-deflection curves. The overall, uni-directional cloth provided the most effective confinement and led to a highly desirable failure mechanism, which was a gradual process.

Experimental study on mechanical properties of locally composite steel and steel fiber reinforced concrete beams

Addressing the construction challenges of steel reinforced concrete structures, steel fibers are employed as replacements for longitudinal rebars and stirrups. Additionally, considering the tensile and compressive properties of steel fiber reinforced concrete, the locally composite steel and steel fiber reinforced concrete (LCSSFRC) structures are proposed. Four-point bending tests were carried out on 18 LCSSFRC beams. Different steel fiber volume fractions (ρsf), specimen lengths (l), concrete cover thicknesses for upper and lower parts of steel compression flanges (Css and Csv), and specifications of shaped steel (Is) were studied. Considering the crack development, bearing capacity, ductility, and final damage pattern, the failure modes of 18 specimens were divided into interface failure, bending-torsion failure, and bending failure. Two types of load deflection curve models were established based on the relationship between failure modes and load deflection curves. Furthermore, the sudden load drop was closely related to the development of bonding cracks. Due to the “bridging effect”, specimens showed increasing trends in bearing capacity and ductility with the increase of ρsf. And it is worth noting that there existed a coupling effect of Csv and l on both the bearing capacity and ductility. Increasing ρsf, Csv, and Is was effective for inhibiting the stiffness degradation.

Experimental and theoretical study on shear behavior of HRB600 reinforced concrete beams after high temperature exposure

To investigate the variation pattern of shear performance in HRB600 RC beams after exposure to high temperatures (i.e., 200 °C, 400 °C, 600 °C, and 800 °C), 4 plain concrete beams were tested for temperature field. Additionally, 15 RC beams with shear span-to-depth (a/d) ratios of 1.2, 2, and 3 were subjected to four-point bending tests at room temperature and after high temperatures. The test results analysis focused on the effects of temperature and shear span-to-depth ratio on the failure pattern, concrete strain, load–deflection curve, and shear bearing capacity of the RC beams. The results demonstrated that increasing the shear span-to-depth ratio from 1.2 to 3 resulted in approximately 61.6 %, 51.8 %, 57.9 %, 53.0 %, and 55.5 % decreases in the ultimate bearing capacity of RC beams at 25 °C, 200 °C, 400 °C, 600 °C, and 800 °C, respectively. The load capacity of the test beam only decreased when the exposure temperature exceeded 600 °C, with the beam (a/d = 2) exposed at 800 °C exhibiting the most significant decrease of 32.4 % compared to room temperature. Finally, a method for determining the material parameters for the structure after exposure to high temperature is proposed. This method considers the sectional temperature field and is applied to the shear resistance calculation model at room temperature. The results indicate a strong agreement between the experimental and calculated values for both heated and unheated beams.

Mechanical properties of alkali-activated slag based SIFCON incorporating waste steel fibers and waste glass

This experimental study focuses on sustainable slurry infiltrated fiber concretes (SIFCONs) by utilizing alkali-activated ground granulated blast furnace slag (GGBS) and waste fibers. The waste fibers were high-strength steel fibers recovered from scrap tires. Additionally, waste glass was employed as a substitute for commercial sodium silicate activator, contributing to the reutilization of a waste material. Such an approach stands out for its use of various waste materials to achieve a high-performance concrete primarily composed of recycled materials. Literature survey indicate that there are no studies available on concrete primarily composed of waste or recycled materials. In this study, the molarities of the activators were selected as 8 M and 14 M based on a prior research. Due to the fiber geometry and the associated bulking effect, a maximum waste fiber content of 5 % was adopted, with increments of 1 %. Load–deflection curves of the specimens were obtained in the bending test. Various mechanical properties, including compressive strength, splitting tensile strength, flexural strength, and fracture energy, were determined for the mixtures. The mechanical properties of the alkali-activated mixtures were improved compared to those of the reference concretes. Based on the mixture proportions and materials employed, alkali-activated mixtures in the study achieved compressive strengths exceeding 120 MPa and flexural strengths approaching 30 MPa. Furthermore, significant enhancements in fracture energies were observed. The strengths of the mixtures with waste glass, however, were lower compared to the other mixtures. Nonetheless, in summary, the study shows the promising potential of waste steel fibers and waste glass for the production of alkali-activated slag SIFCON. This approach not only enhances cost-effectiveness, sustainability, and mechanical properties but also reflects a significant step towards achieving a concrete composition that relies solely on waste materials.

Unbonded Pre-Tensioned CF-Laminates Mechanically Anchored to HSC Beams as a Sustainable Repair Solution for Detachment of Bonded CF-Laminates

The present article outlines a Finite Element Model (FEM) that was created and validated by comparing it to prior experimental investigations to estimate the flexural performance of HSC beams strengthened with exterior bonded, unbonded, and unbonded pre-tensioned Carbon Fibre Reinforced Polymer (CFRP) sheets in several patterns. Nonlinear analysis was performed on three-point-loaded beams using ANSYS software, incorporating the constitutive characteristics of various components (concrete, CFRP, and steel). The comparison of FE-models and experimental data, namely for load-deflection curves, crack patterns, and failure modes, revealed that the developed numerical FE-models and experimental outcomes are in good accord. There has been numerous prior research on the behavior of beams strengthened with externally bonded CFRP sheets, but few on those reinforced with externally unbonded CFRP laminates, and even fewer on HSC beams reinforced with externally unbonded pre-tensioned CFRP laminates. Therefore, the major contribution of this article is to investigate the flexural behavior of HSC beams strengthened utilizing externally unbonded pre-tensioned CFRP laminates. The analysis revealed that the bending performance of RC-beams strengthened using external unbonded pre-tensioned CFRP-laminates is quite similar to that of bonded CFRP-strengthened beams, indicating a high potential for tackling the durability issues caused by detachment of bonded CFRP-strips in such structural elements. The existence of a fully wrapped CF sheet forced the beam to develop diagonal shear cracks in the region between the wrapped CF sheet and beam supports while also enhancing the flexural cracked zone at mid-span to change from smeared to discrete fractures. The flexural fractures spread over a deeper and wider area of the beam as a result of the incorporation of a half-wrap CF laminate. Externally unbonded CFRP-sheets pre-tensioned with 45% of the CFRP ultimate strength utilizing various patterns (straight and U-wrap) performed similarly to bonded CFRP-sheets, with a slight boost in load capacity of around 4.5% and notable reduces in deflection ranging from 9.7% to 16.24%. Using exterior unbonded CFRP laminates to strengthen RC-beams resulted in a flexural capacity increase ranging from 22.3% for NC beams to 71.6% for HSC beams.

Flexural behavior of novel profiled steel-UHTCC assembled composite bridge decks

Ultra-high toughness cementitious composite (UHTCC) has been noticed in the application of composite bridge deck (CBD) structures to avoid tensile cracks, benefiting from its excellent crack resistance behavior and characteristic of tensile strain hardening. In this study, a novel profiled steel-UHTCC assembled CBDs was proposed by adopting duplicate profiled steel parts that integrated the Perfobond rib (PBL) shear connector onto their top flange plates, which will improve fabrication convenience and economic efficiency. The flexural behavior of the assembled CBDs was investigated. Four specimens assembled respectively with T-shaped, Z-shaped, B-shaped, and H-shaped profiled steel parts were designed and fabricated. The failure modes, load-deflection curves, strain distributions, and slippage situations of four specimens were monitored and discussed. Then, the theoretical analyses on the shear connection degrees and ultimate flexural capacities of the assembled CBDs were performed, and a theoretical formula was proposed for calculating the ultimate flexural capacity. Finite element (FE) models of the assembled CBDs were established via ABAQUS software and validated against the test results. Ten groups of FE examples were designed for the detailed study of several key design parameters including shear span length, PBL hole diameter, thickness of UHTCC slab, thickness of steel web, and normalized PBL hole numbers. Finally, the proposed theoretical formula was validated using experimental and simulated results, demonstrating the validity of the proposed formula.

Exploring the creep and oxidation behaviors of four types of FeCrAl alloys through small punch test at 600 °C: Experiments and simulations

Four types of iron-chromium-aluminum (FeCrAl) alloys, composed of different elements, were used to investigate the creep performance and high-temperature oxidation resistance. The microstructures in the undeformed regions after the small punch creep test (SPCT) remained consistent with the as-received (AR) FeCrAl alloys, while the grain aspect ratio (GAR) decreased and grains elongated in the deformed regions. Amongst the four alloys, Q1 exhibited the worst oxidation resistance, whereas Q4 showed superior oxidation resistance due to the addition of Si. Precipitates were observed in the deformed regions after SPCT but not in undeformed regions, indicating that deformation had a positive effect on element segregation. Creep models including Norton power law, Monkman-Grant relationship, Larson-Miller Parameters, and Fracture-Time law were obtained. Calculation results from these creep models revealed that under low loads Q4 had lower minimum creep rate δm and longer creep fracture time tr compared to other alloys; however, Q3 demonstrated better performance in terms of δm and tr under high loads. This suggests a strong correlation between creep performance and loads. Additionally, finite element method (FEM) was employed to explore the effects of minor variation (±0.005 mm) in specimen thickness on SPT curves and SPCT curves. FEM results indicated that this slight variation alone did not account for discrepancies observed in load-deflection curves; deviations of ±6.1% for δm and ±6.3% for tr were found corresponding to specimens with 0.5 ± 0.005 mm thickness.

Experimental study on mechanical behavior of steel truss-reinforced concrete box girders

This paper proposes a new design concept for a steel truss-reinforced concrete box girder which incorporates a steel truss instead of longitudinal bars and stirrups. A comprehensive assessment of the flexural and shear behavior of the proposed steel truss-reinforced concrete box girders was conducted through the testing of twelve girders until failure. All test specimens had the same concrete depth and width of 400 mm and 300 mm, but the length of concrete in the shear and flexural specimens were 3300 mm and 3100 mm, respectively. Moreover, the reinforcing steel truss configuration and member sizes were different. The effects of the angle steel size of the lower chord, vertical webs spacing, shear span ratio and presence of diagonal webs on the cracking, yield and ultimate loads, crack patterns, failure modes, vertical load-deflection curves and strain distribution of these steel truss-reinforced concrete box girders were studied. The test results showed that the flexural capacity of the steel truss-reinforced concrete box girder increases with the increase of angle steel size of the lower chord. Moreover, the spacing of vertical webs and presence of diagonal webs have little effect on the flexural capacity of steel truss-reinforced concrete box girders tested. With the decrease of the shear span ratio and vertical webs spacing, the shear capacity of the steel truss-reinforced concrete box girder increases. Finally, simplified formulae for calculating the flexural and shear capacities of steel truss-reinforced concrete box girders were proposed, showing good agreement with the experimental results.

External precast concrete nib beam-to-column connection under elevated temperatures: Experimental and finite element analysis

The precast concrete nib is one of the most widely used beam-to-column connections to address the architectural drawback of the ordinary precast concrete corbel. However, precast concrete nib beam-to-column connections are not classified as fully rigid or pin but exhibit semirigid properties. Semirigid connections are at risk of deflection at lower loads and cracking during early fire events. In this study, full-scale fire tests were carried out to determine the thermal and structural behaviour of the external monolithic and precast concrete nib beam-to-column connections subjected to 120 min standard cellulose fires. A finite element simulation using ANSYS validated the experimental results based on the internal temperature distributions, load–deflection curves, and stress visualization. Results show that the tension reinforcement of the concrete nib specimen at high temperature was below the critical temperature of 300 °C during the 120 min of heating, compared to the monolithic specimen, which reached the critical temperature of 300 °C at 105 min. The crack patterns and failure modes of monolithic and precast concrete nib specimens at high temperatures were more visible than at ambient temperature. Based on the ductility factor and Monforton’s fixity factor, the concrete nib specimen at high temperatures was categorized as structures with limited ductility and classified in Zone I (pin connection), respectively. This study concluded that external precast concrete nib beam-to-column connections were more vulnerable at high temperatures than monolithic connections.

Cryogenic impact fracture behavior of a high-Mn austenitic steel using electron backscatter diffraction and neutron Bragg-edge transmission imaging

A unique impact fracture behavior is found in a high-Mn austenitic steel (24Mn–4Cr-0.4C-0.3Cu) in this work. The steel exhibits concurrent twinning-induced plasticity (TWIP) effect and the transformation-induced plasticity (TRIP) effect. By analyzing the load-deflection curves recorded during Charpy impact testing, the resistance to crack initiation and propagation is quantified from the absorbed energy. The high-Mn steel demonstrates good resistance to crack initiation at 273 K and 77 K. However, as the temperature decreases from 273 K to 77 K, there is an accelerated transition from stable crack growth to unstable crack growth during impact, resulting in the deterioration of resistance to crack propagation. The plastic deformation of the impact-tested samples, especially in the region close to the crack-path profile was quantitatively analyzed using neutron Bragg-edge transmission (BET) imaging. The deformation zones, divided by using the width of the 200 Bragg edge, exhibit good agreement with the impact absorbed energy characteristics obtained from dynamic load-deflection curves. Moreover, the unstable growth transition point was roughly determined on the impact-tested sample. Then, the electron backscatter diffraction (EBSD) technique is employed to examine the deformation microstructure along the crack-path in the impact-tested samples. The results revealed the dual roles of TRIP effect in impact toughness of the high-Mn steel. On one hand, the TRIP effect plays a positive role in improving resistance to crack initiation and propagation. On the other hand, the excessive accumulation of brittle ε/α՛-martensite caused by the enhanced TRIP effect at 77 K leads to quasi-cleavage fracture, thereby playing a negative role. Finally, we discussed the prominent toughening mechanisms associated with the TWIP and TRIP effects, which greatly impact the impact fracture behavior.

Nonlinear buckling and postbuckling analysis of cylindrical and sinusoid FG-GPLRC panels under axial compressive load

This paper presents and analyzes the nonlinear buckling responses of cylindrical and sinusoid Functionally graded graphene platelet reinforced composite (FG-GPLRC) panels under axial compressive load. The higher-order shear deformation theory is applied to establish the governing equations of structures. The stress function is approximated using the like-Galerkin method, and the Galerkin method is applied to solve the governing equations. The critical buckling loads and postbuckling load-deflection curves are expressed in explicit form. The effects of panel types, geometrical parameters, imperfection, and foundation on the critical buckling loads and postbuckling curves of cylindrical and sinusoid FG-GPLRC panels are discussed. Numerical results also evaluate and compare the buckling responses of cylindrical and sinusoid panels.

Nonlinear buckling and postbuckling analysis of cylindrical and sinusoid FG-GPLRC panels under axial compressive load

This paper presents and analyzes the nonlinear buckling responses of cylindrical and sinusoid Functionally graded graphene platelet reinforced composite (FG-GPLRC) panels under axial compressive load. The higher-order shear deformation theory is applied to establish the governing equations of structures. The stress function is approximated using the like-Galerkin method, and the Galerkin method is applied to solve the governing equations. The critical buckling loads and postbuckling load-deflection curves are expressed in explicit form. The effects of panel types, geometrical parameters, imperfection, and foundation on the critical buckling loads and postbuckling curves of cylindrical and sinusoid FG-GPLRC panels are discussed. Numerical results also evaluate and compare the buckling responses of cylindrical and sinusoid panels.

Theoretical and Numerical Analyses of Steel-timber Composite Beams with LVL Slabs

Recently conducted studies have shown that significant benefits are to be gained by joining steel beams and timber slabs. Steel-timber composite beams present a sustainable solution for the construction industry because of their high strength and stiffness, and lower carbon footprint and self-weight than steel-concrete composite beams. The behaviour of steel-timber composite beams is still being investigated to reduce knowledge gaps. This paper presents theoretical and numerical analyses of steel-timber composite beams consisting of steel girders and laminated veneer lumber slabs. The elastic and plastic resistance to bending were estimated analytically based on the elastic analysis and the rigid-plastic theory. The impact of the composite action, the LVL slab thickness, the cross-section of a steel girder and the steel grade on resistance to bending was evaluated. The load-deflection curve of the composite beam was obtained using a 2D finite element model, in which timber failure was captured using the Hashin damage model. The results of the numerical simulation were in good agreement with the ones of the theoretical analyses.

An Accurate Solution for Buckling and Nonlinear Analysis of a Sandwich Beam with Functionally Graded Material by Considering Zigzag Displacements

In this paper, critical buckling analysis and nonlinear load-deflection curve for sandwich beam with functionally graded material are presented based on refined zigzag theory (RZT). By using the variational principle, the equilibrium equations in buckling analysis are given based on the RZT formulations as well as the nonlinear strain-displacement relations. The solutions are also derived for eigenvalue problems in critical buckling load calculations and for load-deflection relations with initial geometric imperfection. The solutions are presented analytically, and the mathematical properties during the derivation process have been proven in order to keep the mathematical rigor. The present analytical RZT critical buckling loads are validated by the RZT FEM, which is the finite element solutions of the sandwich beam meshed by the beam elements based on RZT. These solutions are also compared by commercial software ANSYS, resulting that this approach can obtain an accurate critical buckling load. Various parameters such as aspect ratio, thickness ratio and modulus ratio are considered to investigate their effects on the critical buckling loads. The present results are compared to the beams with higher-order shear deformation theory (HSDT). From the comparisons of the RZT and the HSDT results, it is seen that both theories approach to CBT for slender beam. The results show that the HSDT overestimates the stiffness in the load-deflection curve. It is shown that the RZT exhibits the zigzag displacements at high accuracy, resulting in accurate calculation in critical buckling loads, mode shapes and nonlinear load-deflection curves than HSDT. The superiority of the RZT solutions is presented especially for the case of FGM sandwich beam with soft middle layer.

Geometrically Nonlinear Buckling of FG-CNTRC Plates Stiffened by FG-CNTRC Stiffeners Subjected to Combined Loads Using Nonlinear Reddy’s HSDT

The nonlinear buckling and postbuckling of functionally graded carbon nanotube reinforcement composite (FG-CNTRC) plates resting on the nonlinear effect of elastic foundation under combined load including external pressure and axial compression loads with uniformly distributed temperature rises are fully investigated in this paper. The FG-CNTRC plates are reinforced by FG-CNTRC stiffeners and the plate-foundation interaction is modeled using a nonlinear model. The higher-order shear deformation plate theory with the geometrical nonlinearities of von Kármán is applied to establish the basic formulations. Additionally, by using the anisotropic higher-order shear deformation beam theory, a new smeared stiffener technique is successfully developed for FG-CNTRC stiffeners. Galerkin’s method is used to achieve the equilibrium equation system in the nonlinear algebraic forms, and the critical load and postbuckling load–deflection curve expression are explicitly determined. The numerical investigations discuss the influences of FG-CNTRC stiffeners, hardening/softening nonlinear elastic foundation, uniformly distributed temperature rises, geometrical and material features on nonlinear stability of stiffened FG-CNTRC plates.