Four-point Bending Load Research Articles

Rheology, mechanical properties and pore structure of sprayed ultra-high performance concrete (SUHPC) with viscosity-enhancing agent

• Rheological properties of SUHPC with 50 mm sprayed thickness were investigated. • Comparison between mechanical properties of SUHPC and poured UHPC. • 3D pore structures of SUHPC and UHPC were clearly observed by CT test. • Deflection-hardening curves of SUHPC and UHPC were measured and analyzed. A sprayed ultra-high performance concrete (SUHPC) with suitable sprayed thickness due to the addition of a viscosity-enhancing agent based on the designed UHPC has been developed in this research. The internal relations among the high viscosity, spraying process, mechanical properties and pore structure of SUHPC were revealed and discussed. Appropriate air bubbles and high viscosity slurry greatly improved the pumping and spraying of fresh SUHPC, the sprayed thickness could be up to 50 mm when the relative thixotropy of SUHPC matrix was 5 times based on poured UHPC. For the harden SUHPC, the strength and toughness of it were both slightly decreased than that of UHPC containing the same fiber content (1.5 %), including compressive strength was 109.2 MPa, decreased 20 % compared with UHPC, and exhibited the deflection-hardening behavior (excellent toughness) under four-point bending loading. Additionally, the porosity of SUHPC was increased than UHPC containing the same fiber content (1.5 %), both mainly distributed among 3–20 nm in micro-pores, but air bubbles would be easily trapped into fresh SUHPC due to the spraying process and high viscosity, transformed into macro pores in hardened SUHPC matrix.

Effect of Fiber Wrapping on Bending Behavior of Reinforced Concrete Filled Pultruded GFRP Composite Hybrid Beams.

The application of pultruded fiber reinforced polymer (FRP) composites in civil engineering is increasing as a high-performance structural element or reinforcing material for rehabilitation purposes. The advantageous aspects of the pultrusion production technique and the weaknesses arising from the 0° fiber orientation in the drawing direction should be considered. In this direction, it is thought that the structural performance of the profiles produced by the pultrusion technique can be increased with 90° windings by using different fiber types. This paper presents experimental studies on the effect of FRP composite wrapping on the flexure performance of reinforced concrete (RC) filled pultruded glass-FRP (GFRP) profile hybrid beams with damage analysis. The hybrid beams are wrapped fully and partially with Glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) composites. Hybrid beam specimens with 0° to 90° fiber orientations were tested under three- and four-point bending loads. Based on the experimental load–displacement relationship results, initial stiffness, ductility, and energy dissipation capacity were compared. The experimental findings revealed that the maximum load-carrying capacities of beams produced with pultrude profiles increased by 24% with glass wrapping and 64.4% with carbon wrapping due to the change in the damages. A detailed damage analysis is provided. Similarly, significant increases were observed in structural performance ratios such as initial stiffness and ductility ratio.

Flexural behavior of composite beams with concrete filled steel tube flanges and honeycombed steel webs

Honeycombed beams are widely used because they are conducive to the layout of pipelines and circuits, and the traditional honeycombed beams are generally H-shaped steel beams and concrete-encased upper flange composite beams. In this paper, a composite beam with concrete-filled steel tube as both upper and lower flanges and steel plate with circular openings as web is proposed. The effective constraint of steel tube on concrete strengthens the deformation ability of concrete. In order to study the flexural behavior of composite beams with concrete filled steel tube flanges and honeycombed steel webs (SHC), 25 simply-supported SHC beams were designed to carry out the four-point bending load simulation analysis. By reasonable modeling method. The main parameters of SHC concluding the compression strength of concrete (fck), yield strength of steel (fyfk), height-to-span ratio (H/L), ratio of space to height of web (s/hw), thickness of steel tube flange (t1), thickness of web (t2) and ratio of hole to height of web (a/hw) were discussed, and the whole loading process of the beam was analyzed. The results showed that the SHC specimens could be divided into elastic stage, elastic-plastic stage and plastic stage under four-point bending load. Steel tube in the tensile zone yielded firstly before specimens failed, and the failure mode of the specimens was that the concrete at the loading point was crushed. Though the steel tube in lower flange at the support, loading point and mid-span yielded, the web did not buckle. SHC could meet the requirements of Chinese code in the normal use stage, and the cross-section deformation basically conformed to the plane assumption. Finally, based on the unified strength theory, a prediction formula of ultimate bending moment is established, which is in good agreement with experiment and can lay the foundation for the popularization and application of this kind of beams.

Behavior of steel fiber reinforced concrete beams with CFRP wrapped lap splice bars

This research employed an experimental and numerical investigation of a new method for over-lap splicing rebar employing an epoxy resin-coated carbon fiber-reinforced polymer (CFRP) sheet to enhance the behavior of the splice tension bars. The normal concrete strength with Steel fiber mix were prepared for casting seven specimens for this purpose, and tested under monotonic, four-point bending load conditions. three of the specimens were made of CFRP over-lap splice in addition to three traditional lap-splice beams, with one specimen as a reference with full length continuous bar. In addition, three ratios of lapping length were studied, they are; 33%, 66%, and 100% for each type of splice. The used percentage of steel fiber (SF) was equal to 0.5% by mix volume. It was noticed a significant enhancement in the behavior and flexural strength with the use of CFRP over-lap splice as compared with the traditional lap splice. Moreover, this approach was simulated numerically by using Abaqus software. The simulation resulted according to the adopted criteria showed good agreement to the results of the work that was done in the lab.

Research into effect of hybrid steel fibers on the V-shaped RC folded plate thickness

Folded plates have been widely used in some structures, such as industrial buildings, hangar, storage, swimming pools etc., due to their natural stiffness and higher load bearing capacity as well as their economic advantage and aesthetic appearance. The thickness of the reinforced concrete (RC) folded plate is an important parameter in terms of its economic cost and load bearing capacity. The works performed on numerical solution of RC folded plates are enough in the literature although there is no any work related to its thickness effect on flexural behavior. In this work, the behavior of V-shaped RC folded plates having various thicknesses (60, 70 and 80 mm) produced from the self-compacting concrete (SCC) with/out steel fiber are experimentally investigated and developed the empirical formula. After 90-day curing period, full-scale V-shaped RC folded plates are subjected to four-point bending load. The load bearing capacity, load–displacement behavior, toughness and ductility values as well as cracking patterns of all V-shaped RC folded plate specimens are found. It is also seen from experimental results that the use of hybrid fiber reinforced SCC in production of V-shaped RC folded plates provides superior flexural behavior. It is addressed that the thickness of the V-shaped RC folded plates can be effectively reduced and its flexural performance is improved by using hybrid steel fibers. Also, a design method for the V-shaped RC folded plates with hybrid steel fibers is also proposed for estimating the nominal moment bearing capacity of V-shaped RC folded plates with some assumptions based on the ACI 544 procedure.

Study on stress measurement for steel bars inside RC beams based on self-magnetic flux leakage effect

The self-magnetic flux leakage (SMFL) effect has been widely studied because it not only can identify early damage evaluation of ferromagnetic materials but is highly sensitive to the stress state. To promote the application of the SMFL effect in the health detection of reinforced concrete (RC) bridge, the whole process failure experiment of the RC beams under four-point bending loading was carried out and a method for detecting and quantitatively evaluating reinforcement stress in RC beams was established. A three-axis automatic displacement device based on metal magnetic memory was used to collect the magnetic signals of the RC beams under different loads, the magnetic signals under different values (LOVs) and multiple scanning paths were analyzed. Based on the magnetic signals under different stress states, a dimensionless parameter Aσ was defined to quantitatively calculate the steel bars stress of RC beams. The experimental results show that in the pure bending section of RC beam, the steel bars stress, and the dimensionless parameter Aσ emerged with a linear relationship. Furthermore, the effect of parameter lift-off values (LOV) on the test results was discussed.

Experimental study on the behavior of RC beams with artificially corroded bars

Due to the adverse effects of local corrosion of bars on the behavior of reinforced concrete structures, this problem has been investigated by many experimental studies. Reducing the cross-sectional area of steel bars due to corrosion diminishes the bearing capacity. On the other hand, weakening the interaction characteristics between bar and concrete due to the increase of steel corrosion significantly reduces the performance of reinforced concrete as a composite material. In this study, the effect of artificially localized corrosion of reinforcement on the structural behavior of reinforced concrete beams was investigated. The studied parameters include the length and location of the localized corroded section. A total of 5 RC beams were constructed and subjected to four-point bending load to investigate their performance. The results of these experiments showed that localized corrosion and increasing its length reduced the yield displacement and increased ultimate mid-span deflection of reinforced concrete beams. Local corrosion of steel bars had little effect on the change of elastic stiffness. However the stiffness corresponds to maximum strength due to artificial corrosion of reinforcement decreased significantly. Localized corrosion in the beam caused a significant increase in displacement corresponds to maximum strength. The specimens with steel corrosion had shear and shear-flexure failure pattern while the reference specimen showed flexural failure pattern.

Study on Flexural Behavior of Self-Compacting Concrete Beams with Recycled Aggregates

Currently, numerous studies have focused on the differences in the properties of self-compacting concrete with recycled aggregate (RASCC) and normal concrete (NC), while less attention has been paid to the application of RASCC in reinforced concrete structures. In this paper, four-point bending loading tests were performed on seven RASCC beams and four NC beams, considering the parameter of reinforcement ratio, and the flexural properties were analyzed and compared. The results showed that the failure form, moment–deflection curves, and flexural capacity of the RASCC beams were similar to those of the NC beams. However, the cracking moment and the crack width of the RASCC beams were significantly smaller than that of the NC beams. With an increase in the longitudinal reinforcement ratio, the cracking resistance and flexural capacity of the RASCC beams increased significantly. The cracking moment and flexural capacity could be calculated using the method of the Chinese code GB50010-2010. However, compared with the test values, the predicted deflection was slightly less safe, while the maximum crack width calculation was slightly more conservative. Therefore, the current code formula was revised according to the test results.

Flexural behavior of delta and bi-delta cold-formed steel beams: experimental investigation and numerical analysis

Cold-formed steel (CFS) structural members retain their positions in the lightweight construction industry. This is due to the significant advantages of CFS. The optimization of these CFS elements will allow the construction of economical buildings with increased load capacities and solutions for stable and economical construction will be obtained. The main aim of this research was to evaluate the effectiveness of these new CFS sections with the estimation of remarkable instabilities and failure modes. This article deals with an experimental study on the behavior of CFS beams of open delta and bi-delta form stressed by four-point bending loads. These cross section shapes are often used in floors as main and secondary beams. The section properties are based on the effective width method designated by the Eurocode 3 standard. A nonlinear finite element (FE) analysis using the ABAQUS program is performed and the comparison between the experimental, numerical and theoretical results is done. Finally, the results showed above all that the breaking loads of the delta and bi-delta beams corresponded to the modes of local buckling and crushing of the web.

Using Fibers instead of Stirrups for Shear in Ultra-High Performance Concrete T-beams

ABSTRACT Transverse shear reinforcement is usually used to resist the induced shear stresses in conventional concrete beams. Ultra-high performance concrete (UHPC) with fibres has high strength, post-peak strength, and strain softening. Such features can impact the behavior of the beam under loading. This paper presents the results of experimental tests performed on simply supported UHPC T-beams with basalt or steel fibres, with or without stirrups, subjected to four-point bending loads. The volume fraction of basalt fibres, the shear span, and the presence of stirrups are adopted as parameters. The possibility of achieving equivalent performance by using basalt or steel fibres, as shear micro-reinforcement, instead of using traditional steel stirrups is discussed from viewpoint of the beam structural behavior due to shear stress. The results show that the shear resistance increases with increasing fibre content and decreases with increasing shear span. The addition of basalt or steel fibres alters the fracture from brittle to ductile. Also, it is possible to partially replace the stirrups by using 1.0 % or more steel fibres or 0.5-1.0 % basalt fibres in the UHPC matrix. However, it is preferable to use a considerable ratio of shear rebars to avoid the dominance of shear failure.

Flexural behaviour of full-scale precast recycled concrete sandwich panels with BFRP connectors

In prefabricated buildings, the exterior walls play an important role in the mechanical stability and thermal insulation of the structures. In this study, a new type of precast sandwich wall panel was developed by using recycled aggregate (RA) and basalt-fibre reinforced polymer (BFRP) shear connectors. A series of double shear tests were designed and conducted to evaluate the effect of the insertion angle of the BFRP connectors on their shear performance. Based on the test results, six full-scale precast sandwich panels, including two conventional panels with glass-fibre reinforced polymer (GFRP) needle connectors, were cast and tested under four-point bending loads. The tested parameters included the distribution of BFRP connectors and the RA content. In addition, the flexural performance of the new sandwich wall panel was compared with that of conventional non-composite panels. The results showed that the proposed precast sandwich wall panels exhibited better load bearing capacity and ductility. Smaller insertion angles of the BFRP shear connectors led to greater stiffness and better shear transfer capacity because they closely connected two concrete wythes in the sandwich panels. The new recycled concrete sandwich panel is also a more environmentally friendly alternative to conventional panels for use in the exterior walls of prefabricated buildings.

Experimental investigation on interlaminar tension strength of curved composite laminates subjected to four-point bending load

The present work aims at experimentally quantifying the effect of the process, environmental, and geometric conditions on the interlaminar tension stress (ILTS) performance of curved composite laminates. First, an innovative experimental matrix is established considering the material and process variability, environmental conditions, lay-up, thickness, radius-to-thickness ratio, defects, and thermal cycles. Subsequently, four-point bending tests are performed on the L-shaped laminated specimens. The effects of each condition on ILTS are obtained, with an interpretation of the corresponding mechanism. The results demonstrated that the material and process variability for the ILTS is about 24%. The stress of the hygrothermally aged specimens was reduced by 34.84%, but the performance was improved by 64.63% at −55 °C. Moreover, it is worth noting that, among all the design factors, the initial defects resulted in a significant stress reduction of up to 63.94%. Finally, the design allowable values under different environmental conditions were accurately calculated by the proposed method. The experimental results have been compared with corresponding observations reported in the literature, exhibiting a good agreement. Consequently, the conclusions of this study are highly useful for civil aircraft design applications.

Usage of EB-CFRP for Improved Flexural Capacity of Unbonded Post-Tensioned Concrete Members Exposed to Partially Damaged Strands

The study presents the performance of flexural strengthening of concrete members exposed to partially unbonded prestressing with a particular emphasis on the amount (0, 14.2, and 28.5%) of cut strands-symmetrical and asymmetrical damage. In addition to examining the influence of cut strands on the remaining capacity of post-tensioned unbonded members and the effectiveness of carbon fiber reinforced polymer laminates restoration, The investigated results on rectangular members subjected to a four-point static bending load based on the composition of the laminate affected the stress of the CFRP, the failure mode, and flexural strength and deflection are covered in this study. The experimental results revealed that the usage of CFRP laminates has a considerable impact on strand strain. In addition to that, the flexural stiffness of strengthened members becomes increasingly significant within the serviceability phases as the damaged strand ratios increase. The EB-CFRP laminates increased the flexural capacity by approximately 13%, which corresponds to strand damage of 14.28% and about 9.5% for 28.57% of strand damage, which represents one of the unique findings in this field. Additionally, semi-empirical equations for forecasting the actual strain of unbonded tendons were presented. The suggested equations are simple to solve and produce precise results. Doi: 10.28991/CEJ-2022-08-06-014 Full Text: PDF

Experimental investigation of four-point bending of thin walled open section steel beam loaded and set in the shear center

Thin walled cold formed structures were, and still are, very popular structural elements used in mechanical engineering. Modern technology and the progress in materials engineering allow to fabricate various shapes of thin walled cold formed components. Therefore, combinations between technology possibilities, material properties, loads and engineering requirements are wide and unlimited for thin walled components. The aim of this paper is to perform an experimental study of cold formed C-channel steel beams under four-points bending loads based on the global and the local buckling phenomena. A test rig was developed with a specially designed support system to subject thin-walled steel beams to a four-point bending load, where the support and loads were applied to the shear center of the open section steel beam. It is shown that it is not possible to completely eliminate the torque load on a thin-walled beam with an open section where load and support are applied in the shear center because investigated beams weren’t ideal made. Thin walled open section beams are very sensitive for boundary conditions and geometrical accuracy. The force of gravity is also working. The presented research methodology can be improved and do test with other open section thin walled beams.

Study of fracture processes in sandstone subjected to four-point bending by means of 4D X-ray computed micro-tomography

<p class="Abstract">High-resolution X-ray computed micro-tomography (CT) is a powerful technique for studying the processes of crack propagation in non-homogenous quasi-brittle materials such as rocks. To obtain all the significant information about the deformation behaviour and fracture characteristics of the studied rocks, the use of a highly specialised loading device suitable for the integration into existing tomographic setups is crucial. Since no adequate commercial solution is currently available, a completely newly-designed loading device with a four-point bending setup and vertically-oriented scanned samples was used. This design of the loading procedure, coupled with the high stiffness of the loading frame, allows the loading process to be interrupted at any time and for CT scanning to be performed without the risk of the sudden destruction of the scanned sample.</p>This article deals with the use of the 4D CT for the visualisation of crack initiation and propagation in clastic sedimentary rocks. Two types of quartz-rich sandstones of Czech provenance were used for tomographic observations during the four-point bending loading performed on chevron notched test specimens. It was found that the crack begins to propagate from the moment that ca. 80 % of the maximum loading force is applied.

Failure Analysis of Hat-Stringer-Stiffened Aircraft Composite Panels under Four-Point Bending Loading.

Hat-stringer-stiffened composite panels have been widely used in aircrafts. Accurate failure analysis of them is important for the safety and integrity of the fuselage. During the service period, these panels will bear not only the lateral force caused by the bending of fuselage, but also the radial pressure caused by the internal and external differential pressure during the take-off and landing of the aircraft. However, the latter case lacks investigation. Therefore, experimental and numerical studies for the static and fatigue failure of hat-stringer-stiffened composite panels under four-point bending loading have been performed in this work. To accurately predict the fatigue failure, a novel theoretical model has been proposed based on the fatigue damage theory. In addition, a user-defined subroutine USDFLD is developed for the implementation of the proposed theoretical model in Abaqus. Experimental results show that the main failure modes are the delamination of the skin and debonding between the girder flange and the skin. The experimental average value of the initial debonding load and displacement in static tests are 897.3 N and 10.8 mm, respectively. Predictions exhibit good agreement with experimental results with relative errors within 10%. Experimental average fatigue failure life of the specimens is 33,085 cycles, which is also close to the prediction with relative errors within 10%. This indicates the reliability and applicability of the established theoretical model and numerical method for predicting the failure of hat-shaped girder structures.

Experimental study of corroded RC beams: dissipation and equivalent viscous damping ratio identification

Corrosion of steel reinforcement is one of the most common durability issues for reinforced concrete (RC) structures, especially when located in marine areas. The study of the mechanical consequences of this pathology is of great importance to assess their performance. Experimental campaigns have been carried out on corroded structures. So far, assessing quasi-static behavior or predicting the dynamic response of structures have been based exclusively on quasi-static loadings. In particular, the dissipated energy and the damping capacity, which are important features to study when assessing the seismic performance of corroded RC members, are evaluated based on quasi-static loadings. The objective of this study is to characterize these two significant engineering demand parameters (EDP) using both quasi-static and dynamic tests. To reach this goal, an experimental campaign is conducted on large-scale RC beams. The corroded and non-corroded RC beams are subjected to a four-point bending loading and to dynamic loading keeping the same loadings and boundary conditions. The dynamic loading is applied on the specimen by means of the AZALEE shaking table. In this paper, a brief description of the experimental campaign is made. Then, results showing the influence of the corrosion rate on the dissipation capacity of RC structures are exposed. Especially, the consistency between quasi-static and dynamic results is assessed and discussed.

The Effects of Stresses and Interfaces on Texture Transformation in Silver Thin Films.

Thin metal films are critical elements in nano- and micro-fabricated technologies. The texture orientation of thin films has a significant effect on applied devices. For Face-Centered Cubic (FCC) metal thin films, when the critical thickness is reached, the texture orientation can transform from (111) to (100) based on the model related to the balance between interfacial energy and strain energy. This research focused on the texture transformation of thin films under two conditions: (1) with or without an adhesion layer in the thin film and (2) with or without initial stress applied through a four-point bending load. In the experiment, two samples (silicon/silver and silicon/titanium/silver) were used to apply different initial stress/strain values and different annealing times. After annealing, an X-ray Diffractometer (XRD) was used to ascertain the preferred orientation of the thin films and the percentage of (111) and (100). Finally, Electron Back-Scattered Diffraction (EBSD) was used to observe the grain size of the thin films. The results showed that, regardless of the existence of an adhesion layer, texture transformation occurred, and this was relatively significant with Ti adhesion layers. Further, the initial stress was found to be small compared to the internal stress; thus, the initial stress imposed in the tests in this research was not significantly influenced by the texture transformation.

An Experimental and Analytical Study on the Bending Performance of CFRP-Reinforced Glulam Beams

The fiber-reinforced polymer is one kind of composite material made of synthetic fiber and resin, which has attracted research interests for the reinforcement of timber elements. In this study, 18 glued-laminated (glulam) beams, unreinforced or reinforced with internally embedded carbon fiber–reinforced polymer (CFRP) sheets, were tested under four-point bending loads. For the reinforced glulam beams, the influences of the strengthening ratio, the modulus of elasticity of the CFRP, and the CFRP arrangement on their bending performance were experimentally investigated. Subsequently, a finite element model developed was verified with the experimental results; furthermore, a general theoretical model considering the typical tensile failure mode was employed to predict the bending–resisting capacities of the reinforced glulam beams. It is found that the reinforced glulam beams are featured with relatively ductile bending failure, compared to the brittle tensile failure of the unreinforced ones. Besides, the compressive properties of the uppermost grain of the glulam can be fully utilized in the CFRP-reinforced beams. For the beams with a 0.040% strengthening ratio, the bending–resisting capacity and the maximum deflection can be enhanced approximately by 6.51 and 12.02%, respectively. The difference between the experimental results and the numerical results and that between the experimental results and analytical results are within 20 and 10%, respectively.

Reinforced Concrete Beams Strengthened in Flexure with Near-Surface Mounted 7075 Aluminum Alloy Bars

The flexural behavior of reinforced concrete (RC) beams strengthened with near-surface mounted (NSM) 7075 aluminum alloy (AA) bars was investigated through experimental testing. In particular, 10 RC beams strengthened with NSM AA bars and one control beam were tested under four-point bending loading. The experimental variables included NSM reinforcement ratio, bonded length of NSM reinforcement, end anchoring of NSM reinforcement, and type of bonding agent. The test results were evaluated in terms of the load-deflection curves, failure modes, ductility, and strains in NSM reinforcements, and the effects of different test parameters on the effectiveness of the proposed strengthening approach were discussed. In addition, analytical models were developed to predict the load-bearing capacities of strengthened beams failed by intermediate crack debonding and concrete cover separation, respectively. The RC beams strengthened with NSM AA bars exhibited a significant improvement in maximum load ranging from 35% to 120% over the reference specimen.