KR100969630B1 - A Reinforcing Structure For Improved Punching Shear Stress - Google Patents

Abstract

The present invention relates to a reinforcing structure for the punching shear of the slab-column joint, and in particular, the GFRP bar is concentrated on the slab-column joint to improve the resistance to corrosion, and the punching shear strength is lowered by the use of the GFRP bar. The present invention relates to a reinforcing structure for the punching shear of a slab-column joint, which is improved by the reinforcement of fiber reinforced concrete, which improves the punching shear strength and at the same time increases the ductility and provides excellent crack control.

Joint, GFRP Bar, Steel Fiber Reinforced Concrete

Description

A Reinforcing Structure For Improved Punching Shear Stress}

1 is a side cross-sectional view showing a prior art,

2 is a plan view showing a top surface of the slab in the present invention,

3 is a plan view showing a lower surface of the slab in the present invention,

4 is a side cross-sectional view of the present invention,

5 shows the installation of the slab test body,

6 is a plan view showing a state placed on each test body,

7 shows an embodiment of the use of the GFRP bar,

8 is a graph showing the results of the experimental example of the present invention,

Fig. 9 is a plan view showing the breaking pattern of each test body.

10 is a view of the surface of the GFRP bar after the test.

<Description of the symbols for the main parts of the drawings>

10: upper pillar 20: slab

30: lower pillar 40: general rebar

50: GFRP Bar 51: Sand

60: steel fiber reinforced concrete

The present invention relates to a reinforcing structure for the punching shear of the slab-column joint, and in particular, the GFRP bar is concentrated on the slab-column joint to improve the resistance to corrosion, and the punching shear strength is lowered by the use of the GFRP bar. The present invention relates to a reinforcing structure for the punching shear of a slab-column joint, which is improved by the reinforcement of fiber reinforced concrete, which improves the punching shear strength and at the same time increases the ductility and provides excellent crack control.

In general, in the case of reinforced concrete structures, it is common to include a slab that provides a constant area while forming the floor of each floor, and a pillar that supports the slab and transmits the self-weight of the structure and the working load generated in each floor to the foundation. Form. In the reinforced concrete structure, such a joint where the slab and the column meet, the shear force acts between the slab along the periphery of the pillar, there is a fear that shear failure occurs if the strength is not enough.

In this type of shear failure, the slab-column junction is excessively stressed around the column in such a structure that the slab is directly supported by the column without installing girders or beams, which causes the slab to form an inverted trapezoidal surface. Shear failure will occur. This punching shear failure is brittle and unlikely to other forms of failure, and therefore fatal to the stability of the slab-column joint.

In this structure, the method of installing the fingerboard and the pontoon around the column is used as a method of reinforcing the slab-column joint, but there is a problem that formwork for installing the fingerboard or the pontoon is cumbersome and not very effective in terms of reinforcement performance. .

Due to this problem, as shown in FIG. 1, a method of using a stub has been proposed. The method of the stub is to reinforce the shear force by winding a stub at a joint portion of a lower bar on a slab across a slab-column joint. It is not necessary to install the formwork for installing the above mentioned fingerboards and pontoons, but there is a problem in that the thickness of the coating can not be properly adjusted, and the resistance to corrosion decreases under construction conditions exposed to corrosion such as underwater structures. There is a problem of reducing the load capacity of the structure.

On the other hand, due to the high resistance to corrosion of fiber-reinforced polymer (FRP) recently, the demand for reinforcement of concrete structures is rapidly increasing. However, FRP is physically and mechanically different from conventional rebars, so the use of FRP stiffeners remains a challenging field. In fact Japan (JSCE 1997), Canada (CSA 2002) and USA (ACI 2006) Although only a few countries, such as Korea, have standards for the design and construction of concrete structures using FRP, studies on the practical use of FRP have been inadequate. FRP reinforced concrete flexural members have been extensively studied, while the study of punching shear of FRP reinforced two-way slabs is relatively poor.

In addition, when using the FRP bar to improve the resistance to corrosion at the slab-column joint, FRP bar has not been proposed to compensate for the relatively low strength compared to the general reinforcing bar.

 The present invention has been made to solve the above problems, the concentration of GFRP bar to the slab-column joint to improve the resistance to corrosion, and the strength of the punching shear is reduced by the use of GFRP bar steel fiber reinforced concrete By reinforcing, the punching shear strength is improved and at the same time the ductility is increased, and the reinforcing structure for the punching shear of the slab-column joint with excellent crack control is proposed.

In order to achieve the above object, the reinforcing structure for the punching shear of the slab-column junction of the present invention is a reinforcing structure for the punching shear of the slab-column junction, the general reinforcing bar is disposed on the lower surface of the slab; GFRP bar which is placed on the upper surface of the slab but concentrated at the junction of the slab and the pillar, and coated with sand on the outer periphery; Characterized in that the fiber reinforced concrete is reinforced to the joint.

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Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 2 is a plan view showing a slab top surface in the present invention, FIG. 3 is a plan view showing a slab bottom surface in the present invention, and FIG. 4 shows a side cross-sectional view of the present invention.

The reinforcing structure for the punching shear of the slab-column junction of the present invention is common in the slab-column structure, namely in the upper column 10, the slab 20, and the lower column 30, the lower surface of the slab 20 is general Reinforcement to the reinforcement 40, but as the crack occurs in the junction (J) due to the generation of the punching shear is absorbed moisture through the crack, the upper surface of the slab 20 to prevent corrosion by the absorbed moisture While reinforcing the GFRP bar 50 and at the same time the GFRP bar 50 concentrated in the junction (J) portion, the structure of reinforcing the stiffness degradation due to the reinforcement of the GFRP bar 50 by the steel fiber reinforced concrete (60) to be.

When the GFRP bar 50 is described, fiber-reinforced plastics (hereinafter referred to as "FRP"). In other names, in fiber-reinforced resins and reinforced plastics, as the reinforcing material, aromatics such as glass fiber, carbon fiber and kevlar Nylon fiber is used, and the glass fiber reinforced FRP is called glass fiber reinforced plastic (hereinafter, referred to as "GFRP"). The GFRP constituent material is composed of glass fiber, thermosetting resin and sand, and the thermosetting resin uses unsaturated polyester resin, epoxy resin, vinyl ester resin and the like.
Here, unsaturated polyester resins have moderate performances such as heat resistance and corrosion resistance, but material prices are more economical than other resins, and vinyl ester resins have excellent corrosion resistance and can be selectively used according to construction conditions.

In particular, when the unsaturated polyester resin is reinforced with the glass fiber, a material having great strength and impact resistance is obtained. The unsaturated polyester resin itself has a smaller strength than hard polyvinyl chloride and polymethyl methacrylate, but when reinforced with glass fiber, the strength increases as the content thereof increases.

Therefore, since the GFRP bar 50 has the same weight as the resistance to corrosion as well as the weight ratio stiffness is greater than that of ordinary reinforcing bars, the buckling strength is much stronger than that of the general reinforcing bars, and the weight for obtaining the same strength will be reduced than that of the general reinforcing bars. However, if the GFRP bar 50 is manufactured to have the same weight as a general rebar, the diameter thereof becomes too large, so there may be disadvantages in terms of concrete placing according to the concentration of the bar. In the present invention, the GFRP bar 50 is shown in FIG. 3. While having the same diameter as the general reinforcing bar 40 in the GFRP bar 50 at the same diameter is to reinforce the disadvantages in terms of rigidity compared to the general reinforcing bar 40 with the fiber-reinforced concrete (60).

The fiber reinforced concrete 60 is composed of steel fiber reinforced concrete (SFRC), which is a concrete made by dispersing and mixing steel fibers having a thickness of 0.01 to 05 mm and a length of 20 to 30 mm in concrete. In addition to the increase in impact resistance, acid resistance, alkali resistance is characterized by an increase.

In the present invention, the junction (J) portion, that is, as shown in FIG. 2, the length of one side of the column as the length plus the length corresponding to 1.5 times the height (h) of the slab 20 in both directions to the column width (c) Concentrate the GFRP bar 50 in the corresponding part while limiting the appropriate rebar ratio to 2 to 3%.

As described above, defining the periphery of the column into a rectangular section with a length equal to 1.5 times the height (h) of the slab 20 in both directions along the width of the column (c) suggests the relevant standards in the CSA and BS Standard. Among them, the CSA Standard specifies that the minimum amount of upper flexion bars should be concentrated within the slab width, which is 1.5 times the slab thickness from the column surface, and in BS Standard, the average flexural strength ratio required for the punching shear strength formula is calculated. Is determined based on the calculation from to 1.5 times the effective depth of the slab.

In addition, the GFRP bar 50 is coated with the sand 51 on the outer periphery to enhance the adhesion with concrete.

Hereinafter, the present invention will be described in detail through experimental examples.

1. Test body detail

5 shows the installation of the two-way slab specimens. The specimen consisted of a 2.3 m square slab 150 mm thick and a 225 mm square column 300 mm in length, above and below the center of the slab. The lower column of the test body was supported by the steel block. In addition, a concentrated load was applied at eight points outside the slab to simulate the uniform load on the specimen, and a steel distribution beam was installed below the slab for load application between the load points of 750 mm adjacent to each side of the slab. . And after installing hydraulic jacks on each of the four steel rods (steel rod) connected to each steel beam, each hydraulic jack is connected to one hydraulic pump to control the same load to all load points. The load, deflection and strain were recorded automatically while the monotonic loading was slowly applied. In addition, the shape and crack width of the cracks were recorded for each major loading stage. The working load was recorded through load cells installed in each hydraulic jack, and the slab deflection was measured by LVDT (Linear Voltage Differential Transformer) installed at eight load points. Four additional LVDTs were installed near the column surface at the bottom of the slab to monitor the relative deflection of the slab column and the onset of punching shear failure. Strain gages were buried along columns parallel to the column face in both directions of the slab to measure the strain of the slab upper rebar.

Figure 6 shows the reinforcement details of the upper and lower slabs. The main parameters of the experiment were the use of slab reinforcement materials, concentrated reinforcement to the slab adjacent to the column, and the use of steel fiber reinforced concrete. Test specimens are divided into S series using general rebar and GF series using GFRP bar, depending on the type of reinforcing material. The upper reinforcing bar has the shape of concentrated bar. In the name of the psalm, B (banded) means concentration. The number given to the specimen represents the reinforcement ratio from the column to the point 1.5 times the thickness of the slab. For example, SB1, GFB1, and GFB3 refer to general reinforcement slabs concentrated at 1% rebar ratio, GFRP reinforcement slabs concentrated at 1% rebar ratio, and GFRP reinforcement slabs concentrated at 3% rebar ratio. do. In addition, GFBF3 has the same reinforcement shape as GFB3, but indicates that steel fiber reinforced concrete (SFRC) is reinforced from the column surface to the point 1.5 times the thickness of the slab slab. The flexural rebar ratio was determined to induce fracture by punching shear while preventing flexural fracture of the test specimen. It should be noted that all specimens were designed to have similar flexural strength, allowing an objective comparison of the punching shear behavior of each specimen. The slab bottoms of all test specimens have the same reinforcement details using ordinary rebars as shown in FIG. The slab effective depth of all specimens is 110 mm.

SB1 and SB2 have the same back muscle details as GFB1 and GFB2, respectively. And the slab concrete strength used for each is also the same.

2 material properties

The standard compressive strength test was carried out using 150 ㅧ 300 mm for the slab general strength concrete and 100 ㅧ 200 mm for the high strength concrete of the column. The flexural strength was obtained through the three-point loading test. Table 1 shows the properties of the concrete used for the specimens. In this experiment, the steel fiber mixing ratio of SFRC to be placed on the slab adjacent to the column of GFBF3 was selected as 0.5% per volume. The steel fibers used were 30 mm long hooks with a diameter of 0.5 mm and a maximum tensile strength of 1,200 MPa. The characteristics of SFRC are summarized in Table 1. Table 2 summarizes the properties of ordinary rebar and GFRP bars used in the test specimen. For the GFRP bar was used GFRP bar surface-coated with sand to improve adhesion as shown in FIG.

TABLE 1

Specimen

(MPa)

(MPa)

(με)

(MPa) S series 37.2 2248 3.5 GF series (column) 80.3 2490 8.9 GF series (slab) 36.3 1936 4.4 SFRC 33.8 1620 3.9

TABLE 2

Designation Area (mm ² )

(GPa)

(GPa)

(MPa)

or

(MPa)

(%) GFRP # 5 198 48.2 N / A 683

= 1.58 Steel 10M 100 200 454 676 0.25 Steel 15M 200 200 445 588 0.23

3. Experimental Results

3.1 Load-deflection relationship

8 shows the relationship between the total shear force and the average deflection of the slab. The total load was obtained by adding the loads at the eight loading points and the dead load (about 21 kN). Deflection is an average of the values measured at eight loading points. Table 3 summarizes the total shear forces and deflections at initial cracking, initial yield of stiffeners and at maximum load. Since the GFRP bar has no yield section due to the characteristics of the material, the load and deflection of the GFRP bar under the allowable working stress of 0.0045 was considered as the yield state and the load and deflection were recorded.

In addition, Table 3 summarizes the rigidity of all test specimens. In the load-deflection relationship curve, the slope of the straight line between the initial crack point and the initial yield point indicates the post cracking stiffness. As can be seen from Table 3 and Figure 8, the stiffness of the test specimen is directly affected by the elastic modulus of the rebar and the reinforcement ratio around the column. All slabs showed the same behavior until cracking occurred, but after cracking, the GF series had significantly lower rigidity than S series. However, the slopes of the load-deflection relationship curves of GFBF3 and GFB3 are almost the same until the breakdown of GFB3, so the use of SFRC does not affect the stiffness enhancement. In general, the ductility of the specimen can be quantified by the ratio of the deflection at the maximum load to the deflection at the initial yield of the slab flexural bars. In other words, as shown in Table 3, SFRC significantly improves the ductility of the slab. In the case of GFBF3 with limited SFRC only in the area around the column, there was a 63% increase in member ductility compared to GFB3. However, other experimental variables, such as the reinforcement ratio around the column or the type of reinforcement, did not have a significant effect on the ductility of the specimen.

As can be seen in Figure 8, all the test specimens were brittle fracture occurred by the punching shear.

TABLE 3

Specimen

(kN)

(kN)

(kN)

(kN)

(mm)

(mm)

(mm) Ductility (

) SB1 56 203 301 0.75 9.82 16.95 1.73 SB2 58 211 317 0.80 8.93 15.44 1.73 GFB1 81 163 222 0.72 14.16 26.15 1.85 GFB2 101 186 246 1.37 14.37 23.39 1.63 GFB3 87 166 248 1.23 11.74 20.93 1.78 GFBF3 95 169 330 1.36 11.13 32.43 2.91

Although all specimens were designed to have the same flexural strength, the GFRP bars showed lower punching shear strengths than those of ordinary bars. This is due to the small compression of the concrete due to the low modulus of GFRP bars. In addition, the low modulus of GFRP reduces the effective cross-sectional secondary moment, resulting in a relatively larger deflection.

In particular, as shown in Table 3 and FIG. 8, when compared with the punching shear strength, that is, the ultimate strength, it is shown that GFB3 and GFB2, which are reinforced with a larger reinforcement ratio, exhibit greater punching shear strength than GFB1. However, it is worth noting that GFB3 has a more dense concentration of reinforcement (larger reinforcement ratio) than GFB2, but the two specimens exhibit similar extreme loads. In other words, reinforcement of GFRP bars above a certain reinforcement ratio (3%) is inefficient in improving the punching shear strength (ultimate strength) of slabs, and there is a problem in the firm casting even when placing concrete. .

In addition, the addition of steel fiber (GFBF3) was found to be more effective in improving the punching shear strength than the concentrated barley of the reinforcement. GFBF3 showed significantly higher punching shear strength than GFB3, even better than SB2, which is concentrated with regular rebar. This suggests that the use of SFRC may be the solution to the problems that may arise when using GFRP bars in place of conventional bars.

3.2 Crack and Fracture Aspects

Figure 9 shows the crack pattern of the GF series at the maximum load. For all GF series, the initial crack occurred at the slab-column junction in a direction perpendicular to the medial GFRP bar in the upper slab reinforcement, followed by a similar crack in the direction perpendicular to the outer GFRP bar. Subsequently, radial cracks developed near the column, advancing to the edge of the slab. At the same time, a columnar crack was formed near the column connecting the radial crack.

In addition, as shown in FIG. 9, GFBF3 exhibited excellent crack control effects such as small crack width and even distribution of cracks due to SFRC in the vicinity of the pillar. This is because steel fibers incorporated into concrete inhibit crack propagation and increase bridging of cracks.

In addition, according to the strain of the flexural reinforcement, anchorage failure does not appear. As shown in FIG. 10, when the surface state of the GFRP bar was examined after the completion of the experiment, the phenomenon of the concrete peeling along the surface of the GFRP bar was not observed even after the destruction. In other words, it can be confirmed that GFRP bars treated with sand show excellent adhesion and are effective in preventing settlement failure.

As shown in Figure 9 all the slabs showed a typical punching failure pattern. The contour of the punching cone in punching failure is shown by the thick line in FIG. 9. All specimens showed a sloped fracture that extends from the slab compression of the column surface to the slab tension at some distance from the column. Shear failure surface was affected by the concentration of flexural reinforcement bars near the column. In other words, as the concentration is concentrated, the shear fracture surface is farther from the column, and the punching angle is smaller, indicating a good shear fracture surface. However, GFB2 and GFB3 show almost similar punching angles, so it is not necessary to concentrate the rebar ratio more than 3%.

Based on the test results, the following results were obtained through the experiments of two-way slabs reinforced with GFRP and general rebar.

First of all, the use of steel fibers has an excellent effect on increasing punching shear strength and crack control. Moreover, the change of reinforcement and concentrated barley showed no significant effect on improving the ductility of the members, while the ductility was significantly improved through steel fibers. The use of SFRC will help solve problems that may arise when using GFRP bars in place of existing rebars.

As the reinforcing ratio of the GFRP bar increases, the punching shear strength increases, and it can be seen that 2 to 3% shows almost the same punching shear strength in the punching shear strength, and it is reasonable to limit 3% to the upper reinforcing bar ratio. The punching shear fracture surface also occurred farther away from the column surface as the rebar ratio increased, but the punching shear surface occurred at almost the same distance in the rebar ratio 2 to 3%.

The reinforcing structure for the punching shear of the slab-column joint according to the present invention increases the resistance to corrosion due to cracking by focusing GFRP bars on the joints as the upper part of the slab, and decreases the punching shear strength by the use of the GFRP bars. Reinforcement with reinforced concrete has advantages in punching shear strength as well as crack control.

In addition, the present invention has the advantage of preventing the brittle fracture, the construction of the closed room of the concrete by presenting the upper limit of the reinforcing ratio in the concentrated GFRP bar.

In addition, there is an advantage that can prevent adhesion failure by sand coating on the outer periphery of the GFRP.

In the detailed description of the present invention described above with reference to the preferred embodiment of the present invention, those skilled in the art or those skilled in the art having ordinary knowledge of the present invention described in the claims It will be understood that various modifications and changes can be made in the present invention without departing from the spirit and scope of the invention.

Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (4)

delete delete delete In the reinforcement structure for the punching shear of the slab-column junction, A general reinforcing bar arranged on the lower surface of the slab; GFRP bar which is placed on the upper surface of the slab but concentrated at the junction of the slab and the pillar, and coated with sand on the outer periphery; Reinforcing structure for the punching shear of the slab-column joint, characterized in that composed of fiber reinforced concrete reinforced to the joint.

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