Mechanical Properties of Steel-Continuous Fiber Reinforced Bar (SFCB)

    时间: 2019-07-26

    Mechanical Properties of Steel-Continuous Fiber Reinforced Bar (SFCB)

    Research on Test Method

    Experimental Study on Test Method and Mechanical Properties of Steel-Fiber Composite Bars (SFCB)

    LO Yunbia1 WU Gang1,2  Wu Zhishen2,3   Wang Yanhua1  Wang Yan4

    (1.Collcge ofCivil Engincering Southeast Universiy, Nanjing 210096, China; 2.Intemaional Isiute for Urban SystemsEngincering, Nanjing 210096, China; 3.Department of Urban & Cil Engincering, Ibaraki University, Hiachi 316-8511,Japan; 4.Beijing Texida technology R & D Co.Lid , Beijing 000 , China)


    Abstract: This paper studies the mechanical properties of the steel-fiber composite bar (SFCB) under

    axial tension. The fabrication method of SFCB is introduced and a special testing device is designed to carry out

    the experiment. The failure mode of specimens is reproduced as the expected resul. The designed test method is

    validated by comparisions of measured data from experiments with thcoretical ones.

    KeyWords: Stefibere composite bars, Axial tension, Failure mode, , experimental study

    Reinforcement bar is the main structure and reinforcement material of civil building structure. However, it is easy to corrode, which seriously damages the performance and service life of the structure. In addition, as steel bar is an elastic-plastic material, the bearing capacity of the reinforced structure is no longer improved after yielding, which leads to the large deformation and poor recovery performance of reinforced concrete structures under strong earthquake. Wu Zhishen and Wu Gang 1,2 put forward the method of wrapping ordinary steel bar with continuous fiber composite material, and compounded a steel-continuous fiber composite bar (SFCB) with good corrosion resistance. Experiments show that this new type of composite bar not only has good corrosion resistance {1}, but also has stable post-yield stiffness (secondary stiffness) and ideal restorability. SFCB in Keli prefecture has developed a new type of concrete seismic structure with controllable damage and good restorability.

    However, there is no standard and effective testing method for the mechanical properties of steel-continuous fiber composite bars, which directly limits the promotion and application of this new reinforcing material in engineering (Fig. 1 is a steel-continuous fiber composite bars manufactured by a manufacturer, and its corrosion resistance is mainly used as reinforcing bolts in geotechnical engineering such as slope support recently). In order to further systematically study the mechanical properties of steel-continuous fiber composite bars (SFCB), this paper introduces the manufacturing method of SFCB, designs the SFCB unidirectional tension test device, and completes the unidirectional tension test of SFCB.

    1. General situation of test

    1.1 specimen preparation

    Figure 2 is a sketch of SFCB. Table 1 lists the basic mechanical properties of the raw materials used in the sample states, in which CFRP and basalt fibre sheets are used for CFRP, and steel cores are used for CFRP. The material is made of_10 ribbed steel bars. The preparation steps of the specimens are as follows: steel surface decontamination 1 > fiber cloth brush resin 1 > fiber cloth wrapping 1 > fiber bundle winding 1 > curing. The concrete process is as follows: 1) First, the rust on the surface of steel bar is gently removed with sandpaper, and then the fabric is dipped in acetone to clean it; 2) As shown in Figure 2, the cut fibre cloth is laid flat, one end is bonded with steel bar by 502 binder at five points for temporary fixing, then the epoxy resin is evenly coated, and then rolled and extruded with rollers to ensure that the resin of the fibre bundle is saturated and full. (3) Compressing the other end of the fabric with long steel bars, slowly rotating the steel bar from one end bonded with the steel bar at five points and applying the true tension force multiplied with the steel bar in order to realize the compact and uniform encapsulation of the steel bar by the fiber cloth; 4) extruding the excess resin along the axis of the steel bar with rubber gloves, and finally using basalt fiber bundles impregnated with epoxy resin to form 45 degrees between the ribs and teeth along the axis of the steel bar. The steel-continuous fibre composite bar is made by circumferential winding.

    In order to carry out tensile test on the testing machine, anchorage treatment at both ends of the SFCB is also needed after the SFCB is still formed. As fiber is a typical anisotropic material, its transverse and longitudinal strength is relatively small, only about 1:20. Therefore, the traditional splint anchorage is not suitable for SFCB. Otherwise, because of its low transverse strength, the outer cladding of steel bar will fire prematurely in the inner zone of the anchor. In order to ensure the constant anchorage performance of the anchorage system, the strength of reinforcing bars and fibers in SFCB can be fully exerted during the test. In this test, the SFCB is anchored by the bonded anchorage in Caizhou (shown in Figure 3). The multi-component mixed adhesive prepared by the bonding medium of Caizhou Beijing Texida Technology Development Co., Ltd. has high strength and good bonding effect, which can meet the requirements of the test research. Requirement.

    The following points should be taken into account in the test: 1) Because of the unidirectional tension test, the eccentricity between the axis of the bar and the center of the instrument will have a great influence on the test results, a special middle nut is designed to ensure that the tension force and the axis of the bar are in the true line; 2) In order to prevent the stress concentration at the end of the anchor when the bar is extended into the anchor, the stress concentration at the end of the anchor will occur. In case of early failure, increase the number of fiber layers in this section to strengthen the protection, as shown in Figure 4.

    Finally, the SFCB specimens are shown in Fig. 5 and Fig. 6.

    1.2 Test Device

    The bonded anchor with external screw teeth at both ends of the ButSFCB specimens can be easily connected to the MTS electro-hydraulic servo fatigue testing machine through nuts, fixed steel plates and bolts (as shown in Figure 7). Two sets of test data measurement and acquisition systems are used in this test. One is the measurement system of MTS test machine. The load and displacement between two loading ends of the test sample are measured and collected by the sensor equipment of the test machine itself. The second set of test data acquisition system is shown in Fig. 8, in which the tension force of composite tendons is measured by force sensor, the tensile strain of tendons is measured by LVDT bilateral electronic extensometer (which can eliminate the influence of the bending of tendons on the measurement of tensile strain compared with unilateral electronic extensometer). At the same time, the resistance strain gauge is affixed on both sides of the central part of the SFCB specimens to check with it. The test instruments are connected to TDS303 static strain tester for data acquisition.

    The reason why two sets of test and measurement systems are used is to measure the whole process curve of uniaxial tensile test of specimens. In order to protect the electronic extensometer, it is necessary to unload the fiber before it breaks, and the strain gauge will also have fire effect when the fiber breaks, so that the whole process curve of the test can not be measured by using only the second system. If only the first measurement system is used, the errors will be larger when the load and displacement are small at the beginning of the test. In the small deformation stage, the second set of measurement system data is used. After the large deformation and the removal of the electronic extensometer, the first set of measurement system data is used. In addition, the two measuring systems can be calibrated each other to obtain the ideal curve of the whole test process.

    1.3 Test process and results

    The loading mode controlled by displacement is adopted in the test process. SFCB specimens were installed on MTS test machine through nuts, steel plates and bolts, and uniaxial static loading was carried out at a loading speed of 0.5 mm/min. At the initial stage of loading, the inner core of steel bar and the outer cladding of fiber bear the load together. When the tensile strain is about 0.002, the yield phenomenon occurs, which shows that the stress level of SFCB decreases with the increase of strain, but still increases steadily. That is to say, SFCB shows higher post-yield stiffness (secondary stiffness). At this time, the inner core of steel bar has yielded, and can not bear higher load. The increased load is mainly borne by the fiber outer cladding. With the increase of load, the peak load-carrying capacity is reached when the fiber outer cladding in the middle of the specimen fails. With the fiber breaking, the load-carrying capacity decreases rapidly. At this time, the external load is borne by the inner core of the yielding steel bar, and the load-carrying capacity of SFCB does not increase basically. The failure mode of the specimen is also ideal. First, the steel bar in the middle of the specimen yields, then the fiber fracture near the steel bar yields, and finally the steel bar in the vicinity of the fiber fracture is broken.

    The main phenomena during the experiment are as follows:

    1) It shows obvious and stable secondary stiffness, from steel yield, fiber breakage to steel bar breakage, the failure process has obvious stages.

    2) The failure location is relatively concentrated. Fiber fracture, steel bar yield and fracture occur in a relatively concentrated section, while the other sections do not occur in the basic fiber damage and fiber reinforcement core pricking, as shown in Figure 9.

    3) As shown in Fig. 10, there is no obvious deformation and crack in the anchorage glue at the opening of the anchorage hole, and there is no gel detachment and slippage at the interface between the anchorage glue and the reinforcement, which shows that the anchorage effect is ideal.

    2. Calculation of stress-strain curve of SFCB

    Based on the stress-strain relationship of steel bar and fiber composite material and the composite rule of material, the stress-strain relationship of SFCB can be obtained. As shown in Figure 11, the reinforcement adopts the double fold linear elastic-plastic constitutive model, and the fiber composite adopts the linear elastic constitutive model. The strain interval I from initial tension to steel yield is SFCB, the tensile stress is_1 modulus of elasticity is E1, and the strain of SFCB is. The expression is shown in equation (1), (2).

    In formula,

    Es, As and y are the elastic modulus, cross section area and yield strain of steel bars respectively.

    Ep and AF are the elastic modulus sites and cross-sectional areas of the continuous fiber coatings respectively.

    A is the total cross-sectional area of SFCB, A = As + AF.

    Secondly, the strain interval II of SFCB is from steel bar yield to continuous fiber cladding fracture. Its tensile stress is_ll and elastic modulus is Ell. The expression is shown in formula (3), (4).

    In the formula, y e y is the yield stress and strain of steel bars, and E F is the fracture strain of continuous fiber coatings.

    Finally, the strain interval III of SFCB from the breaking of continuous fiber cladding layer to the breaking of steel bar is_m, and the elastic modulus is Em. The reinforcing effect of steel bar is not taken into account, and the stress of SFCB is calculated without taking into account the cross-sectional area of the continuous fiber cladding layer. The expression is shown in formula (5), (6).

    In the formula, Exmax is the fracture strain of steel bar.

    The experimental results are compared with the results of J theory. Fig. 12 is the stress-strain relationship of one of the specimens. Table 2 is the concrete test results of the specimens.

    From the stress-strain curve of SFCB, it can be seen that the yield point and pre-yield stiffness of SFCB in the test results are in good agreement with the theoretical calculation, which shows that the test method designed in this paper can better measure the mechanical properties of SFCB. However, for the stiffness and ultimate strain after yielding, there are still some errors between the experimental value and the theoretical calculation value. The experimental value of the stiffness after yielding is higher than the theoretical calculation value, while the experimental value of the ultimate strain is lower than the theoretical calculation value. As for the phenomenon that the secondary stiffness value of the test results is higher than that of the theoretical calculation, and whether there exists composite effect, it will be further discussed in the further experiment and theoretical research.

    3 conclusion

    Aiming at the characteristics of SFCB, a new reinforcing material, a unidirectional tension test device is designed, which effectively solves the problems of the center of the steel bar test and the failure of the steel bar caused by the stress concentration in the anchor zone. A series of unidirectional tensile tests of SFCB have been completed. The test results show that the failure characteristics of SFCB are ideal. The test results are in good agreement with the theoretical calculation. It proves that this test method can measure the mechanical properties of SFCB well. It lays a foundation for further research on the mechanical properties of SFCB.


    [1] Wu Zhi-espionage, Wu Gang, Lu Zhitao. Steel-continuous fibre instrument with their stable secondary stiffness

    Coagulation Upper Seismic Structure [P]. National Invention Patent: 20061006

    [2] Luo Yunbiao, Wu Gang. Lingzhisheng, Tensile Mechanical Properties of Wang Yanhua Steel-Continuous Fiber Reinforcement


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