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Four rubber concrete steel pipe (RuCFST) elements, one concrete steel pipe (CFST) element and one empty element were tested under pure bending conditions. The main parameters are shear ratio (λ) from 3 to 5 and rubber replacement ratio (r) from 10% to 20%. A bending moment-strain curve, a bending moment-deflection curve, and a bending moment-curvature curve are obtained. The mode of destruction of concrete with a rubber core was analyzed. The results show that the type of failure of the RuCFST members is bend failure. Cracks in rubber concrete are distributed evenly and sparingly, and filling the core concrete with rubber prevents the development of cracks. The shear-to-span ratio had little effect on the behavior of the test specimens. The rubber replacement rate has little effect on the ability to withstand a bending moment, but has a certain effect on the bending stiffness of the specimen. After filling with rubber concrete, compared with samples from an empty steel pipe, the bending ability and bending stiffness are improved.
Due to their good seismic performance and high bearing capacity, traditional reinforced concrete tubular structures (CFST) are widely used in modern engineering practice1,2,3. As a new type of rubber concrete, rubber particles are used to partially replace natural aggregates. Rubber Concrete Filled Steel Pipe (RuCFST) structures are formed by filling steel pipes with rubber concrete to increase the ductility and energy efficiency of composite structures4. It not only takes advantage of the excellent performance of CFST members, but also makes efficient use of rubber waste, which meets the development needs of a green circular economy5,6.
In the past few years, the behavior of traditional CFST members under axial load7,8, axial load-moment interaction9,10,11 and pure bending12,13,14 has been intensively studied. The results show that the bending capacity, stiffness, ductility and energy dissipation capacity of CFST columns and beams are improved by internal concrete filling and show good fracture ductility.
Currently, some researchers have studied the behavior and performance of RuCFST columns under combined axial loads. Liu and Liang15 performed several experiments on short RuCFST columns, and compared with CFST columns, the bearing capacity and stiffness decreased with increasing rubber substitution degree and rubber particle size, while ductility increased. Duarte4,16 tested several short RuCFST columns and showed that the RuCFST columns were more ductile with increasing rubber content. Liang17 and Gao18 also reported similar results on the properties of smooth and thin-walled RuCFST plugs. Gu et al.19 and Jiang et al.20 studied the bearing capacity of RuCFST elements at high temperature. The results showed that the addition of rubber increased the ductility of the structure. As the temperature rises, the bearing capacity initially decreases slightly. Patel21 analyzed the compressive and flexural behavior of short CFST beams and columns with round ends under axial and uniaxial loading. Computational modeling and parametric analysis demonstrate that fiber-based simulation strategies can accurately examine the performance of short RCFSTs. Flexibility increases with aspect ratio, strength of steel and concrete, and decreases with depth to thickness ratio. In general, short RuCFST columns behave similarly to CFST columns and are more ductile than CFST columns.
It can be seen from the above review that RuCFST columns improve after the proper use of rubber additives in the base concrete of CFST columns. Since there is no axial load, the net bending occurs at one end of the column beam. In fact, the bending characteristics of RuCFST are independent of the axial load characteristics22. In practical engineering, RuCFST structures are often subjected to bending moment loads. The study of its pure bending properties helps to determine the deformation and failure modes of RuCFST elements under seismic action23. For RuCFST structures, it is necessary to study the pure bending properties of the RuCFST elements.
In this regard, six samples were tested to study the mechanical properties of purely curved steel square pipe elements. The rest of this article is organized as follows. First, six square-section specimens with or without rubber filling were tested. Observe the failure mode of each sample for test results. Second, the performance of RuCFST elements in pure bending was analyzed, and the effect of a shear-to-span ratio of 3-5 and a rubber replacement ratio of 10-20% on the structural properties of RuCFST was discussed. Finally, the differences in load-bearing capacity and bending stiffness between RuCFST elements and traditional CFST elements are compared.
Six CFST specimens were completed, four filled with rubberized concrete, one filled with normal concrete, and the sixth was empty. The effects of rubber change rate (r) and span shear ratio (λ) are discussed. The main parameters of the sample are given in Table 1. The letter t denotes the pipe thickness, B is the length of the side of the sample, L is the height of the sample, Mue is the measured bending capacity, Kie is the initial bending stiffness, Kse is the bending stiffness in service. scene.
The RuCFST specimen was fabricated from four steel plates welded in pairs to form a hollow square steel tube, which was then filled with concrete. A 10 mm thick steel plate is welded to each end of the specimen. The mechanical properties of the steel are shown in Table 2. According to the Chinese standard GB/T228-201024, the tensile strength (fu) and yield strength (fy) of a steel pipe are determined by a standard tensile test method. The test results are 260 MPa and 350 MPa respectively. The modulus of elasticity (Es) is 176 GPa, and the Poisson’s ratio (ν) of steel is 0.3.
During testing, the cubic compressive strength (fcu) of the reference concrete on day 28 was calculated at 40 MPa. Ratios 3, 4 and 5 were chosen based on previous reference 25 as this may reveal any problems with shift transmission. Two rubber replacement rates of 10% and 20% replace sand in the concrete mix. In this study, conventional tire rubber powder from Tianyu Cement Plant (Tianyu brand in China) was used. The particle size of rubber is 1-2 mm. Table 3 shows the ratio of rubber concrete and mixtures. For each type of rubber concrete, three cubes with a side of 150 mm were cast and cured under test conditions prescribed by the standards. The sand used in the mixture is siliceous sand and the coarse aggregate is carbonate rock in Shenyang City, Northeast China. The 28-day cubic compressive strength (fcu), prismatic compressive strength (fc’) and modulus of elasticity (Ec) for various rubber replacement ratios (10% and 20%) are shown in Table 3. Implement the GB50081-201926 standard.
All test specimens are tested with a hydraulic cylinder with a force of 600 kN. During loading, two concentrated forces are applied symmetrically to the four-point bending test stand and then distributed over the specimen. Deformation is measured by five strain gauges on each sample surface. Deviation is observed using three displacement sensors shown in Figures 1 and 2. 1 and 2.
The test used a preload system. Load at a speed of 2kN/s, then pause at a load of up to 10kN, check whether the tool and load cell are in normal working condition. Within the elastic band, each load increment applies to less than one tenth of the predicted peak load. When the steel pipe wears out, the applied load is less than one-fifteenth of the predicted peak load. Hold for about two minutes after applying each load level during the loading phase. As the sample approaches failure, the rate of continuous loading slows down. When the axial load reaches less than 50% of the ultimate load or obvious damage is found on the specimen, the loading is terminated.
The destruction of all test specimens showed good ductility. No obvious tensile cracks were found in the tensile zone of the steel pipe of the test piece. Typical types of damage to steel pipes are shown in fig. 3. Taking sample SB1 as an example, at the initial stage of loading when the bending moment is less than 18 kN m, sample SB1 is in the elastic stage without obvious deformation, and the rate of increase in the measured bending moment is greater than the rate of increase in curvature. Subsequently, the steel pipe in the tensile zone is deformable and passes into the elastic-plastic stage. When the bending moment reaches about 26 kNm, the compression zone of the medium-span steel begins to expand. Edema develops gradually as the load increases. The load-deflection curve does not decrease until the load reaches its peak point.
After the experiment was completed, sample SB1 (RuCFST) and sample SB5 (CFST) were cut to more clearly observe the failure mode of the base concrete, as shown in Fig. 4. It can be seen from Figure 4 that the cracks in sample SB1 are distributed evenly and sparsely in the base concrete, and the distance between them is from 10 to 15 cm. The distance between cracks in sample SB5 is from 5 to 8 cm, the cracks are irregular and obvious. In addition, the cracks in sample SB5 extend about 90° from the tension zone to the compression zone and develop up to about 3/4 of the section height. The main concrete cracks in sample SB1 are smaller and less frequent than in sample SB5. Replacing sand with rubber can, to a certain extent, prevent the development of cracks in concrete.
On fig. 5 shows the distribution of deflection along the length of each specimen. The solid line is the deflection curve of the test piece and the dotted line is the sinusoidal half wave. From fig. Figure 5 shows that the rod deflection curve is in good agreement with the sinusoidal half-wave curve at initial loading. As the load increases, the deflection curve deviates slightly from the sinusoidal half-wave curve. As a rule, during loading, the deflection curves of all samples at each measurement point are a symmetrical half-sinusoidal curve.
Since the deflection of RuCFST elements in pure bending follows a sinusoidal half-wave curve, the bending equation can be expressed as:
When the maximum fiber strain is 0.01, considering actual application conditions, the corresponding bending moment is determined as the element’s ultimate bending moment capacity27. The measured bending moment capacity (Mue) thus determined is shown in Table 1. According to the measured bending moment capacity (Mue) and the formula (3) for calculating the curvature (φ), the M-φ curve in Figure 6 can be plotted. For M = 0.2Mue28, the initial stiffness Kie is considered as the corresponding shear bending stiffness. When M = 0.6Mue, the bending stiffness (Kse) of the working stage was set to the corresponding secant bending stiffness.
It can be seen from the bending moment curvature curve that the bending moment and curvature increase significantly linearly in the elastic stage. The rate of growth of the bending moment is clearly higher than that of the curvature. When the bending moment M is 0.2Mue, the specimen reaches the elastic limit stage. As the load increases, the sample undergoes plastic deformation and passes into the elastoplastic stage. With a bending moment M equal to 0.7-0.8 Mue, the steel pipe will be deformed in the tension zone and in the compression zone alternately. At the same time, the M-f curve of the sample begins to manifest itself as an inflection point and grows non-linearly, which enhances the combined effect of the steel pipe and the rubber concrete core. When M is equal to Mue, the specimen enters the plastic hardening stage, with the deflection and curvature of the specimen rapidly increasing, while the bending moment increases slowly.
On fig. 7 shows curves of bending moment (M) versus strain (ε) for each sample. The upper part of the mid-span section of the sample is under compression, and the lower part is under tension. Strain gauges marked “1″ and “2″ are located at the top of the test piece, strain gauges marked “3″ are located in the middle of the specimen, and strain gauges marked “4″ and “5″. ” are located under the test sample. The lower part of the sample is shown in Fig. 2. From Fig. 7 it can be seen that at the initial stage of loading, the longitudinal deformations in the tension zone and in the compression zone of the element are very close, and the deformations are approximately linear. In the middle part, there is a slight increase longitudinal deformation, but the magnitude of this increase is small.Subsequently, the rubber concrete in the tension zone cracked.Because the steel pipe in the tension zone only needs to withstand the force, and the rubber concrete and steel pipe in the compression zone bear the load together, the deformation in the tension zone of the element is greater than the deformation in the As the load increases, the deformations exceed the yield strength of the steel, and the steel pipe enters the elastoplastic stage.The rate of increase in the strain of the sample was significantly higher than the bending moment, and the plastic zone began to develop to the full cross section.
The M-um curves for each sample are shown in Figure 8. On fig. 8, all M-um curves follow the same trend as the traditional CFST members22,27. In each case, the M-um curves show an elastic response in the initial phase, followed by an inelastic behavior with decreasing stiffness, until the maximum allowable bending moment is gradually reached. However, due to different test parameters, the M-um curves are slightly different. The deflection moment for shear-to-span ratios from 3 to 5 is shown in fig. 8a. The allowable bending capacity of sample SB2 (shear factor λ = 4) is 6.57% lower than that of sample SB1 (λ = 5), and the ability to bending moment of sample SB3 (λ = 3) is greater than that of sample SB2 (λ = 4) 3.76%. Generally speaking, as the shear-to-span ratio increases, the trend of the change in the allowable moment is not obvious. The M-um curve does not appear to be related to the shear-to-span ratio. This is consistent with what Lu and Kennedy25 observed for CFST beams with shear-to-span ratios ranging from 1.03 to 5.05. A possible reason for CFST members is that at different span shear ratios, the force transmission mechanism between the concrete core and steel pipes is almost the same, which is not as obvious as for reinforced concrete members25.
From fig. 8b shows that the bearing capacity of samples SB4 (r = 10%) and SB1 (r = 20%) is slightly higher or lower than that of the traditional sample CFST SB5 (r = 0), and increased by 3.15 percent and decreased by 1 .57 percent. However, the initial bending stiffness (Kie) of samples SB4 and SB1 is significantly higher than that of sample SB5, which are 19.03% and 18.11%, respectively. The bending stiffness (Kse) of samples SB4 and SB1 in the operating phase is 8.16% and 7.53% higher than that of sample SB5, respectively. They show that the rate of rubber substitution has little effect on the bending ability, but has a large effect on the bending stiffness of the RuCFST specimens. This may be due to the fact that the plasticity of rubber concrete in RuCFST samples is higher than the plasticity of natural concrete in conventional CFST samples. In general, cracking and cracking in natural concrete begin to propagate earlier than in rubberized concrete29. From the typical failure mode of the base concrete (Fig. 4), the cracks of sample SB5 (natural concrete) are larger and denser than those of sample SB1 (rubber concrete). This may contribute to the higher restraint provided by the steel pipes for the SB1 Reinforced Concrete sample compared to the SB5 Natural Concrete sample. The Durate16 study also came to similar conclusions.
From fig. 8c shows that the RuCFST element has better bending ability and ductility than the hollow steel pipe element. The bending strength of sample SB1 from RuCFST (r=20%) is 68.90% higher than that of sample SB6 from empty steel pipe, and the initial bending stiffness (Kie) and bending stiffness at the stage of operation (Kse) of sample SB1 are 40.52% respectively. , which is higher than sample SB6, was 16.88% higher. The combined action of the steel pipe and the rubberized concrete core increases the flexural capacity and stiffness of the composite element. RuCFST elements exhibit good ductility specimens when subjected to pure bending loads.
The resulting bending moments were compared with bending moments specified in current design standards such as Japanese rules AIJ (2008) 30, British rules BS5400 (2005) 31, European rules EC4 (2005) 32 and Chinese rules GB50936 (2014) 33. bending moment (Muc) to the experimental bending moment (Mue) is given in Table 4 and presented in fig. 9. The calculated values of AIJ (2008), BS5400 (2005) and GB50936 (2014) are 19%, 13.2% and 19.4% lower than the average experimental values, respectively. The bending moment calculated by EC4 (2005) is 7% below the average test value, which is the closest.
The mechanical properties of RuCFST elements under pure bending are experimentally investigated. Based on the research, the following conclusions can be drawn.
The tested members of RuCFST exhibited behavior similar to traditional CFST patterns. With the exception of the empty steel pipe specimens, the RuCFST and CFST specimens have good ductility due to the filling of rubber concrete and concrete.
The shear to span ratio varied from 3 to 5 with little effect on the tested moment and bending stiffness. The rate of rubber replacement has practically no effect on the resistance of the sample to bending moment, but it has a certain effect on the bending stiffness of the sample. The initial flexural stiffness of specimen SB1 with a rubber replacement ratio of 10% is 19.03% higher than that of the traditional specimen CFST SB5. Eurocode EC4 (2005) allows an accurate evaluation of the ultimate bending capacity of RuCFST elements. The addition of rubber to the base concrete improves the brittleness of the concrete, giving the Confucian elements good toughness.
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Post time: Jan-05-2023