Repair method and structure for column bases

Abstract

This prevents the concrete on the top surface of the foundation from being damaged when the base of the column tilts. [Solution] In a structure in which a reinforced concrete column 2 is integrally joined to a reinforced concrete foundation 3, a repair method is provided for repairing the column base 4 of the column. The repair method comprises a first step of removing the concrete 5 on the surface of the column base to a predetermined first depth, a second step of removing the concrete on the upper surface of the foundation connected to the column base to a predetermined second depth, and a third step of pouring ultra-high-strength fiber-reinforced concrete 40 into the concrete removed portions on the surface of the column base and the upper surface of the foundation.

Description

[Technical Field]

[0001] This disclosure relates to a method for repairing the base of a column and to a structure thereof. [Background technology]

[0002] In reinforced concrete structures, the reinforced concrete foundation and the reinforced concrete columns are structurally joined together to transmit stress from the columns to the foundation. In such structures, the bending moment is designed to concentrate at the bottom of the lowest column, so damage to the column base is most severe. For this reason, even in small to medium-sized earthquakes, cracks are likely to form in the concrete surface of the column base, and if rainwater or condensation reaches the reinforcing steel through these cracks, the steel rusts. As the reinforcing steel rusts, it thickens, which enlarges the cracks, making it easier for rainwater to penetrate and further rusting the steel. Eventually, the concrete cover on the surface may peel off, exposing the steel. When this happens, it becomes difficult to maintain the original performance of the reinforced concrete column, and repairs become necessary.

[0003] In typical repairs, the surface concrete is removed, and the rust on the exposed reinforcing steel is scraped off. This reduces the cross-sectional area of ​​the reinforcing steel, so additional reinforcement is welded on to compensate for the reduced area. Alternatively, the rusted portion of the reinforcing steel may be cut out entirely, and new reinforcing steel may be welded on to replace it. Finally, concrete is poured again into the area where the concrete was removed, completing the repair.

[0004] This common repair method has the disadvantage of being time-consuming because it requires repairing the reinforcing steel itself, such as adding or replacing it. Also, if reinforcement is done with concrete of similar strength, it is likely to follow a similar course and end up in the same condition. Therefore, a technique has been proposed in which ultra-high-strength fiber-reinforced concrete (UFC) is poured into the concrete removal area, replacing the existing ordinary concrete with UFC (see, for example, Patent Document 1).

[0005] UFC (Ultra-Fiber Concrete) is stronger than ordinary concrete and contains metal fibers, resulting in superior tensile strength. Therefore, after removing rust from the reinforcing steel, there is no need to reinforce the steel itself; simply pouring UFC will achieve performance equal to or better than before the repair. The decrease in member strength due to the reduction in the cross-sectional area of ​​the reinforcing steel can be compensated for by the increase in strength provided by the UFC. As a result, reinforcement of the reinforcing steel after rust removal becomes unnecessary, simplifying the repair work. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 3790088 specification [Patent Document 2] Patent No. 4713218 Specification [Overview of the project] [Problems that the invention aims to solve]

[0007] However, if the conventional concrete at the base of the column is replaced with UFC, when the column base tilts during an earthquake or other event, the conventional concrete below the area replaced with UFC on the upper surface of the foundation connected to the column base may be crushed and collapsed by the higher-strength UFC at the column base. If this happens, the foundation will no longer be able to support the column base, and the load-bearing capacity of the column will decrease.

[0008] Therefore, this disclosure was devised in view of these circumstances, and its purpose is to provide a method for repairing a column base and a structure that can prevent the concrete on the upper surface of the foundation from being destroyed when the column base tilts. [Means for solving the problem]

[0009] According to one aspect of this disclosure, A method for repairing the base of a column in a structure in which a reinforced concrete column is integrally joined to a reinforced concrete foundation, The first step is to remove the concrete from the surface of the column base to a predetermined first depth, A second step involves removing the concrete from the upper surface of the foundation connected to the column base to a predetermined second depth, A third step involves pouring ultra-high-strength fiber-reinforced concrete into the concrete removal area between the surface of the column base and the upper surface of the foundation, A method for repairing the base of a column is provided, characterized by comprising the following:

[0010] Preferably, the column base has main reinforcement bars embedded in the concrete, The first depth is equal to or less than the distance from the surface of the column base to the inner end of the main reinforcement bar of the column.

[0011] Preferably, the foundation has upper foundation reinforcement embedded in the concrete, The second depth is equal to or less than the distance from the top surface of the foundation to the bottom end of the upper reinforcement bar of the foundation.

[0012] Preferably, in the second step, the concrete is removed from the surface of the column base to a position horizontally separated by a predetermined foundation removal width.

[0013] Preferably, the foundation has upper foundation reinforcement embedded in the concrete, The width of the foundation removal is three times or more the distance from the top surface of the foundation to the top end of the reinforcement bar at the top of the foundation.

[0014] Preferably, when a horizontal load is applied to the upper end of the column from a predetermined direction, and M is the minimum bending moment required to separate the UFC layer around the base of the column from the concrete below, the minimum width of the foundation removal is such that the product of the adhesive force between the UFC layer around the base of the column and the concrete on the side of the column center in the predetermined direction and the distance from the center of gravity of the UFC layer to the center of rotation is equal to the minimum bending moment M.

[0015] Preferably, the second depth is equal to or greater than the depth x calculated from the following formula (1). T = √{x , , , , , , , , , , , , , [Figure 3] , [Figure 2] , [Figure 1] , [Figure 7] ,

[0019] , [Figure 6] , , [Figure 5] , , [Figure 4] ,

[0018] , +(3x) 2} × (B + bx × 2) × K ···(1) However, T is the initial bearing capacity of the column, B is the width of the column in the horizontal direction, [[ID=1B]]bx is the foundation removal width in the same direction as the width B of the column, K is the tensile strength of the concrete of the foundation.

[0016] [[ID=1S]] Preferably, after the first step and the second step and before the third step, the repair method includes a fourth step of removing rust from the reinforcing bars exposed by removing the concrete. <000009S>[[ID=2S]] According to another aspect of the present disclosure, a structure repaired by the repair method of the column base is provided.

Advantages of the Invention

[0018] According to the present disclosure, it is possible to prevent the concrete on the upper surface of the foundation from being destroyed when the column base is tilted.

Brief Description of the Drawings

[0019] [Figure 1] It is a rear view showing the structure according to the present embodiment transparently. [Figure 2] It is a sectional view taken along the line II-II of FIG. 1. [Figure 3] It is a sectional view taken along the line III-III of FIG. 1. [Figure 4] It is an enlarged view of part IV of FIG. 1. [Figure 5] It is a partial rear sectional view for explaining the repair method of the present embodiment. [Figure 6] It is a partial rear sectional view for explaining the repair method of the present embodiment. I [Figure 7] It is a partial rear sectional view for explaining the repair method of the present embodiment. [Figure 8] This is a cross-sectional view corresponding to line III-III in Figure 1, showing the foundation after repair. [Figure 9] This is a partial rear cross-sectional view showing the first modified example. [Figure 10] This is a partial rear cross-sectional view showing a second modified example. [Figure 11] This is a schematic rear cross-sectional view showing the column base and foundation after repairs. [Figure 12] This is a cross-sectional view taken along line XII-XII in Figure 11. [Figure 13] This figure shows the test results. [Figure 14] This figure shows the test results. [Modes for carrying out the invention]

[0020] The embodiments of this disclosure will be described below with reference to the attached drawings. It should be noted that this disclosure is not limited to the embodiments described below.

[0021] Figure 1 is a transparent rear view of the structure according to this embodiment. For convenience, the front, rear, left, right, up, and down directions of the structure are defined as shown in the figure. The front, rear and left, right directions are horizontal and perpendicular to each other. The up and down direction is vertical and perpendicular to the front, rear and left, right directions. Figure 2 is a cross-sectional view taken along line II-II of Figure 1. Figure 3 is a cross-sectional view taken along line III-III of Figure 1. Figure 4 is an enlarged view of section IV of Figure 1.

[0022] Structure 1 is a reinforced concrete structure in which a reinforced concrete column 2 is integrally joined to a reinforced concrete foundation 3. The lower end of the column 2, which is the joint with the foundation 3, is the column base 4, and this embodiment relates to a method for repairing this column base 4.

[0023] Figures 1 to 4 show structure 1 before repair. For convenience, concrete 5 is shown as transparent.

[0024] Column 2 has multiple main reinforcement bars 7 embedded in concrete 5 near its surface 6 and extending vertically, and multiple stirrups 8 extending horizontally and wrapped around the outside of the multiple main reinforcement bars 7. Figure 2 shows a cross-section of column 2 (specifically the column base 4) perpendicular to the center (also called the column center) C of column 2. In this cross-sectional view, column 2 is approximately square, and the front-to-back width and left-to-right width of column 2 are approximately equal. However, the cross-sectional shape of column 2 is arbitrary and may be rectangular or circular, etc. The center C of column 2 passes through the center of the cross-section of column 2 and extends vertically. Column 2 also extends vertically. The cross-sectional shape of column 2 is constant.

[0025] In the plan view shown in Figure 2, the main reinforcement bars 7 are arranged in a roughly square shape at intervals along the roughly square surface 6 of the column 2. The stirrups 8 have a roughly square shape and are in contact with or close to multiple main reinforcement bars 7. In this embodiment, the hooks 9 and 10 at both ends of the stirrups 8 are hooked onto the main reinforcement bar 7 at the front right corner.

[0026] Here, in the plan view shown in Figure 2, the side farther from the column center C is called the outside, and the side closer to the column center C is called the inside. The concrete 5 that exists between the outer end of the stirrup 8, i.e., the outer end 11, and the surface 6 of the column 2 is called the column cover concrete 5A. The column cover concrete 5A covers the column main reinforcement 7 and stirrup 8 from the outside. The thickness of the column cover concrete 5A is indicated by tA in the figure. There is no reinforcement within the column cover concrete 5A.

[0027] Furthermore, the concrete 5 located inside the column cover concrete 5A is called core concrete 5C.

[0028] As shown in Figure 1, the main column reinforcement bars 7 extend to near the inner bottom of the foundation 3, and the stirrups 8 are wrapped around the main column reinforcement bars 7 even within the foundation 3. In this way, the column 2 is firmly anchored to the foundation 3. In this embodiment, as shown by the dashed line a in the figure, the column 2 is assumed to extend to near the inner bottom of the foundation 3 while maintaining the same cross-sectional shape. The lower ends of the multiple main column reinforcement bars 7 are connected by connectors 7A, but they may also be bent at a right angle toward the outside of the column 2 and anchored therein.

[0029] Foundation 3 has cage reinforcement bars 14 embedded inside. The cage reinforcement bars 14 extend throughout almost the entire interior of foundation 3 to reinforce foundation 3, and surround the main column reinforcement bars 7 and stirrups 8 from the periphery. The center of foundation 3 (also called the foundation center) is coaxial with the column center C. Figure 3 shows a cross-section of foundation 3 perpendicular to the foundation center, i.e., the column center C. In this cross-sectional view, foundation 3 is a roughly square larger than column 2, and the front-to-back width and left-to-right width of foundation 3 are approximately equal. However, the cross-sectional shape of foundation 3 is arbitrary and may be rectangular, circular, etc.

[0030] Furthermore, in the rear view shown in Figure 1, the base 3 is a rectangular prism shape in which the width from left to right is greater than the height from right to left. Although not shown in the illustration, the front-to-back width of the base 3 is also greater than the height from right to left.

[0031] As shown in Figure 1, the cage reinforcement 14 has multiple upper reinforcement bars 15 and multiple lower reinforcement bars 16. The upper reinforcement bars 15 extend in the left-right direction and are bent downwards at both ends, forming a U-shape with the bottom open. The lower reinforcement bars 16 extend in the left-right direction and are bent upwards at both ends, forming a U-shape with the top open. These upper reinforcement bars 15 and lower reinforcement bars 16 are combined from above and below and fixed to each other by binding wire or the like (not shown). Multiple sets of these upper reinforcement bars 15 and lower reinforcement bars 16 are arranged at intervals in the front-to-back direction.

[0032] Furthermore, the cage reinforcement 14 has a first outer reinforcement 17 that is wrapped around the outside of multiple sets of upper reinforcement 15 and lower reinforcement 16. The first outer reinforcement 17 is formed in an endless ring shape of a rectangle that extends in the front-to-back and up-to-down directions and completes a full circle, and in a plan view, it is positioned perpendicular to the sets of upper reinforcement 15 and lower reinforcement 16. Multiple first outer reinforcement 17s are arranged at intervals in the left-to-right direction and are fixed to the sets of upper reinforcement 15 and lower reinforcement 16 by binding wire or the like (not shown).

[0033] Furthermore, the cage reinforcement 14 has a second outer reinforcement 18 that is wrapped around the outside of the set of multiple upper reinforcement 15 and lower reinforcement 16 and the first outer reinforcement 17. The second outer reinforcement 18 is formed in an endless ring shape of a rectangle (approximately square) that extends in the front-to-back and left-to-right directions and completes one full circle. Multiple second outer reinforcement 18s are arranged at intervals in the vertical direction and are fixed to the set of upper reinforcement 15 and lower reinforcement 16 and the first outer reinforcement 17 by binding wire or the like (not shown).

[0034] Thus, the cage reinforcement 14 is formed as a whole into a cage shape by multiple upper reinforcement bars 15, lower reinforcement bars 16, a first outer reinforcement bar 17, and a second outer reinforcement bar 18. Each of these reinforcement bars surrounds the main column reinforcement bars 7 and stirrups 8 of the column base 4, and is positioned so as not to interfere with the main column reinforcement bars 7 and stirrups 8.

[0035] In this embodiment, the upper and lower reinforcement bars 15 and 16, the first lateral reinforcement bar 17, and the second lateral reinforcement bar 18 are arranged sequentially outward, but the arrangement method is not limited to this and is arbitrary. For example, the first lateral reinforcement bar 17 may be placed outside the second lateral reinforcement bar 18, or the upper and lower reinforcement bar 15 and 16 may be placed furthest outward.

[0036] As shown in Figure 3, of the multiple sets of upper and lower reinforcement bars 15 and 16, most are positioned outside the stirrups 8, but some (two sets in this embodiment) are inserted inside the stirrups 8, between the main reinforcement bars 7 of the column.

[0037] Furthermore, while most of the multiple first outer reinforcements 17 are located on the outside of the stirrups 8, some of them (two in this embodiment) are inserted between the main column reinforcements 7 on the inside of the stirrups 8.

[0038] As shown in Figure 4, the upper reinforcement bar 15 has an upper end portion 19 that extends in the left-right direction. The first outer reinforcement bar 17 has an upper end portion 20 that extends in the front-back direction and is positioned above the upper end portion 19. The upper end portion 19 of the upper reinforcement bar 15 and the upper end portion 20 of the first outer reinforcement bar 17 form the foundation upper reinforcement bar 21. The foundation upper reinforcement bar 21 is the reinforcement bar or reinforcement bar assembly closest to the top surface 22 of the foundation 3.

[0039] The concrete 5 that exists between the upper end 23 of the foundation top reinforcement 21 (specifically, the upper end 20 of the upper portion 20 of the first outer reinforcement 17) and the upper surface 22 of the foundation 3 is called the foundation cover concrete 5B. The foundation cover concrete 5B covers the foundation top reinforcement 21 from above. The thickness of the foundation cover concrete 5B is indicated by tB in the figure. There are no reinforcing bars within the foundation cover concrete 5B, except for the portion through which the column 2 passes.

[0040] As mentioned earlier, in the structures described above, the damage is most severe at the base of the columns. In this case, the concrete on the surface of the column base cracks, allowing water to penetrate inside and causing the reinforcing steel to rust. Therefore, repairs become necessary.

[0041] Generally, the process involves removing the surface concrete, removing rust from the reinforcing steel, reinforcing the steel to compensate for the reduction in cross-sectional area, and then pouring concrete again into the area where the concrete was removed.

[0042] However, this method requires considerable effort to reinforce the rebar. Therefore, as an alternative, instead of reinforcing the rebar, it is conceivable to pour ultra-high-strength fiber-reinforced concrete (UFC) into the concrete removal area, and compensate for the decrease in strength due to the rebar with the increase in strength due to the UFC.

[0043] However, if this is done, when the column base tilts during an earthquake or other event, the concrete below the area replaced with UFC on the upper surface of the foundation connected to the column base may be crushed and collapsed by the higher-strength UFC of the column base. In this case, the foundation will not be able to support the column base, resulting in a failure mode different from that assumed during the design phase, and consequently, the load-bearing capacity of the column will decrease.

[0044] Therefore, in this embodiment, the concrete on the upper surface of the foundation connected to the column base is also replaced with UFC. This point will be explained below.

[0045] The method for repairing the column base 4 in this embodiment generally comprises the following first to third steps. (1) A first step of removing the concrete 5 from the surface of the column base 4 to a predetermined first depth. (2) A second step in which the concrete 5 on the upper surface of the foundation 3 connected to the column base 4 is removed to a predetermined second depth. (3) The third step involves pouring UFC into the concrete removal area between the surface of the column base 4 and the upper surface of the foundation 3.

[0046] Furthermore, the method for repairing the column base 4 in this embodiment also includes the following fourth step. (4) After the first and second steps and before the third step, a fourth step is to remove the rust from the reinforcing bars exposed by the removal of the concrete 5.

[0047] Figures 5 to 7 are partial rear cross-sectional views illustrating the repair method of this embodiment. Figure 5 shows the condition before repair.

[0048] Figure 6 shows the state after completing the first and second steps. As shown in the figure, the concrete 5 on the surface of the column base 4 has been removed to a predetermined first depth t1. Also, the concrete 5 on the upper surface of the foundation 3 connected to the column base 4 has been removed to a predetermined second depth t2. Note that the first and second steps do not necessarily have to be performed in this order. They may be performed simultaneously or in the reverse order. The concrete 5 is removed using known chipping tools or water jets.

[0049] The first depth t1 is equal to or less than the distance t3 between the surface 6 of the column base 4 and the inner end 12 of the column main reinforcement 7 (also called the first maximum depth) (t1 ≤ t3). This avoids the need to remove the core concrete 5C (see Figure 2) inside the column main reinforcement 7, thus preventing a significant impact on the original strength of the column 2.

[0050] In the illustrated example, the first depth t1 is made smaller than the first maximum depth t3 (t1 < t3). Taking the surface 6 of the column base portion 4 as a reference, if the distance to the outer end 11 of the stirrup 8 is t4 and the distance to the inner end 25 of the stirrup 8 (that is, the distance to the outer end 26 of the column main reinforcement 7) is t5, the first depth t1 in the illustrated example is made larger than t4 and smaller than t5 (t4 < t1 < t5). Thereby, the most rust-prone outer end 11 of the stirrup 8, which is the reinforcement closest to the surface 6 of the column base portion 4, and its periphery can be exposed, and the rust on the exposed portion can be effectively removed.

[0051] The removal range of the concrete 5 in the column base portion 4 is determined by observing the deterioration condition at the actual site. In the illustrated example, the height position of the upper end 28 of the concrete removal portion 27 of the column base portion 4 is determined to be at a position a predetermined height H1 above the upper surface 22 of the foundation 3. Also, the height position of the lower end 29 of the concrete removal portion 27 of the column base portion 4 is determined to be at a position a predetermined height H2 below the upper surface 22 of the foundation of the foundation 3. The height position of this lower end 29 is at a position a second depth t2 below the upper surface 22 of the foundation 3 (H2 = t2). That is, the height position of the lower end of the concrete removal portion 27 of the column base portion 4 is equal to the height position of the lower end 31 of the concrete removal portion 30 of the foundation 3.

[0052] Fig. 6 shows a state where the concrete 5 has been removed on the right side surface of the column base portion 4. Actually, the concrete 5 is similarly removed on the entire surface (front surface, rear surface, right side surface, and left side surface) of the column base portion 4.

[0053] As shown in Fig. 6, the concrete 5 on the upper surface portion of the foundation 3 has been removed by a predetermined second depth t2. The second depth t2 is equal to or smaller than the distance (referred to as the second maximum depth) t6 from the upper surface 22 of the foundation 3 to the lower end 32 of the upper foundation reinforcement 21 (that is, the lower end of the upper end portion 19 of the upper reinforcement 15) (t2 ≤ t6). By doing so, only the minimum amount of concrete 5 necessary for repair needs to be removed, and it is possible to prevent significantly affecting the strength of the original foundation 3.

[0054] In the illustrated example, the second depth t2 is made smaller than the second maximum depth t6 (t2 < t6). Taking the upper surface 22 of the foundation 3 as a reference, the distance from the upper surface 22 of the foundation 3 to the upper end of the upper foundation bars 21 (i.e., the upper end 23 of the upper end portion 20 of the first outer bar 17) is t7, and the distance from the upper surface 22 of the foundation 3 to the lower end 33 of the upper end portion 20 of the first outer bar 17 (i.e., the upper end 34 of the upper end portion 19 of the upper foundation bar 15) is t8. The second depth t2 in the illustrated example is made larger than t7 and smaller than t8 (t7 < t2 < t8).

[0055] The concrete removal portion 30 of the foundation 3 is continuous with the concrete removal portion 27 of the column base portion 4, and as shown in the figure, a concrete removal portion having an L-shaped cross section is formed by both of them.

[0056] In the horizontal or left - right direction, the position of the inner end 35 (the end closer to the column center C) of the concrete removal portion 30 of the foundation 3 is made equal to the position of the surface 6 of the column base portion 4, as shown by the virtual line a.

[0057] Also, in the foundation 3, the concrete 5 is removed from the surface 6 of the column base portion 4 to a position that is separated by a predetermined foundation removal width bx in the horizontal or left - right direction. That is, in the horizontal or left - right direction, the position of the outer end 36 (the end farther from the column center C) of the concrete removal portion 30 of the foundation 3 is made equal to the position that is separated by the foundation removal width bx outward from the surface 6 of the column base portion 4.

[0058] Although it will be described in detail later, it is desirable that the foundation removal width bx is not less than three times the distance t7 between the upper surface 22 of the foundation 3 and the upper end 23 of the upper foundation bars 21. As shown in FIG. 4, the distance t7 is equal to the thickness tB of the foundation cover concrete 5B (t7 = tB). Therefore, this distance t7 is referred to as the foundation cover thickness tB. bx ≧ 3×tB.

[0059] FIG. 6 shows a state where the concrete 5 of the upper surface portion of the foundation 3 connected to the right - hand side surface portion of the column base portion 4 is removed. However, in actuality, the concrete 5 of the upper surface portion of the foundation 3 connected to the entire surface of the column base portion 4 (the front surface portion, the rear surface portion, the right - hand side surface portion, and the left - hand side surface portion) is similarly removed.

[0060] After the concrete 5 is removed from the surface of the column base 4 and the top surface of the foundation 3, the rust on the reinforcing bars exposed by the removal of the concrete 5 is removed. In the illustrated example, the rust is removed from the stirrups 8 exposed by the removal of the concrete 5, or the rusted stirrups 8 are replaced. The rust is removed using abrasive tools such as a file or an electric grinder. The cross-sectional area and strength of the stirrups 8 are reduced somewhat by the removal of the rust. However, the spread of rust is reliably suppressed thereafter. Conventional reinforcement (such as adding reinforcing bars) may be performed on the reinforcing bars themselves after rust removal, but this may not be necessary if the removed area is small.

[0061] Next, as shown in Figure 7, UFC40 is poured into the concrete removal sections 27 and 30 of the column base 4 and foundation 3. This replaces the existing concrete 5 that was originally present in the concrete removal sections 27 and 30 with UFC40. The pouring of UFC40 completes the repair work. The pouring of UFC40 is carried out using formwork.

[0062] As is well known, UFC40 contains metal fibers and other materials, and has higher tensile strength, toughness, compressive strength, and durability compared to ordinary general-purpose concrete, concrete 5. Therefore, the strength of reinforcing steel that has been reduced by rust removal can be adequately compensated for by UFC40.

[0063] Figure 8 is a cross-sectional view equivalent to section III-III in Figure 1, showing the foundation 3 after repair. The UFC40 is provided only on the surface of the column base 4 and the upper surface of the foundation 3 connected thereto. The core concrete 5C located inside the column main reinforcement 7 is not replaced by the UFC40. Therefore, the cross-sectional shape of the UFC40 is generally an annular shape that encircles the surface of the column base 4, specifically a hollow, approximately square annular shape.

[0064] Reference 2 describes a technique for forming the upper surface of the foundation with high-strength concrete. However, this technique is used for newly constructed structures, and high-strength concrete is poured over the entire cross-section of the column base, including the core concrete area both inside and outside the foundation. The cross-sectional shape of the high-strength concrete inside and outside the foundation is solid, not hollow and annular. Therefore, this embodiment differs significantly from Reference 2 in terms of this cross-sectional shape. Furthermore, pouring high-strength concrete in such a solid shape is impossible for repairs to existing structures. Consequently, the information in Reference 2 cannot be applied to Reference 1.

[0065] As described above, according to this embodiment, not only the surface of the column base 4 but also the concrete 5 on the upper surface of the foundation 3 connected to the column base 4 is replaced with UFC40. Therefore, even if the column base 4 tilts during an earthquake or the like, the surrounding UFC40 will not be crushed. This is because the UFC40 of the column base 4 and the UFC40 of the foundation 3 have the same strength. Thus, it is possible to prevent the concrete on the upper surface of the foundation 3 from being destroyed when the column base 4 tilts.

[0066] Furthermore, according to this embodiment, when the column base 4 tilts due to a major earthquake or the like after repairs, it is possible to prevent the concrete on the upper surface of the foundation 3 from being destroyed before a large deformation occurs in the column base 4 that would result in a decrease in load-bearing capacity, and the column member can perform as designed.

[0067] Next, a modified example of the basic embodiment will be described. Parts similar to those in the basic embodiment will be omitted from the explanation, and the differences will be the main focus of the description below.

[0068] Figure 9 shows the first modified example. In this first modified example, the first depth t1 and second depth t2 from which concrete is removed at the column base 4 and foundation 3 are changed from the basic embodiment, specifically increased. Consequently, the thickness of the UFC 40 is also increased.

[0069] The first depth t1 can be arbitrarily set within the range up to the first maximum depth t3. Therefore, it is possible to increase the first depth t1 from the basic embodiment.

[0070] Similarly, the second depth t2 can also be arbitrarily set within the range up to the second maximum depth t6. Therefore, it is also possible to increase the second depth t2 from the basic embodiment.

[0071] In this modified example, the first depth t1 of this modified example is greater than t5 and less than t3 (t5 < t1 < t3). As a result, it is possible to expose the entire hoop bar 8, which is the reinforcement closest to the surface 6 of the column base portion 4, and the most rust-prone outer end 26 of the column main reinforcement 7, which is the reinforcement next closest, and its periphery, and it is possible to effectively remove the rust on the exposed portion.

[0072] Also, the second depth t2 of this modified example is greater than t8 and less than t6 (t8 < t2 < t6).

[0073] This modified example is effective, for example, in the case of severe deterioration where the degree of deterioration and cracking of the column base portion 4 is large and the internal reinforcement is significantly rusted.

[0074] FIG. 10 shows a second modified example. In this second modified example as well, the first depth t1 and the second depth t2 are changed from the basic embodiment, but in this modified example, they are decreased. Along with this, the thickness of the UFC40 is also decreased.

[0075] In the case of this modified example, the first depth t1 is made equal to t4 (t1 = t4). As a result, only the column cover concrete 5A (see FIG. 5) of the column base portion 4 is removed and replaced with the UFC40.

[0076] Also, the second depth t2 is made equal to t7 (t2 = t7). As a result, only the foundation cover concrete 5B (see FIG. 5) of the foundation 3 is removed and replaced with the UFC40.

[0077] This modified example is effective, for example, in the case of mild deterioration where the degree of deterioration and cracking of the column base portion 4 is small, cracks have occurred in the concrete 5 on the surface portion of the column base portion 4, but the internal reinforcing bars are not rusted. In this case, if possible, the fourth step of removing the rust of the reinforcing bars can be omitted.

[0078] Alternatively, the first depth t1 may be made smaller than t4 (t1 < t4). Also, the second depth t2 may be made smaller than t7 (t2 < t7).

[0079] Of course, in addition to these modified examples, examples in which the first depth t1 and the second depth t2 are changed are possible. For example, an example in which the first depth t1 is made relatively large and the second depth t2 is made relatively small is possible.

[0080] By the way, regarding the removal range of the concrete 5 of the foundation 3, it is also possible to determine or calculate the foundation removal width bx and the second depth t2 as follows.

[0081] Figs. 11 and 12 are rear cross-sectional views and plan cross-sectional views schematically showing the repaired column base portion 4 and the foundation 3 in a test body for confirming the effects of the present disclosure. Since these figures show the state after replacing the concrete 5 with UFC40, the installation range of UFC40 corresponds to the removal range of the concrete 5.

[0082] B1 is the front-back width of the column base portion 4, and B2 is the left-right width of the column base portion 4. b is the width as a variable used for determining the foundation removal width bx. x is the depth as a variable used for determining the second depth t2. First, the method for determining the foundation removal width bx will be described.

[0083] As mentioned above, it is preferable that the foundation removal width bx be at least three times the thickness of the foundation cover concrete 5B (foundation cover thickness) tB (=t7) (bx≧3×tB). Experiments have shown that this maintains and secures the load-bearing capacity of column 2 to the same level as or better than when it was new. To determine the minimum required values ​​for the removal width and depth of the foundation cover concrete 5B (foundation removal width bx and second depth t2), FEM analysis may be performed, or the calculation method shown below may be used.

[0084] Here, as shown in Figure 11, when a horizontal load F is applied to the upper end of column 2 of the test specimen, the maximum value of that horizontal load F is the load-bearing capacity of column 2. After repair, it is necessary to obtain a load-bearing capacity of column 2 that is equal to or greater than the load-bearing capacity of column 2 when it was new (i.e., the initial load-bearing capacity).

[0085] As shown in Figure 11, when a leftward horizontal load F, for example as shown, is applied to the upper end of column 2, the UFC40 layer around the column base 4 will attempt to peel away from the surface of the concrete 5 below and move to the left. To counteract this, the product of the area of ​​the UFC40 layer in plan view (Figure 12) and the adhesive strength between the concrete 5 and the UFC40 must be greater than the horizontal load F. As a result, the column base 4 will tilt to the left, and a bending moment will be generated in the UFC40 layer on the upper surface of the foundation 3 to the right of the column center C, causing it to peel away from the concrete 5 due to tensile stress. To resist this peeling, the range of UFC40 to the right of the column center C must be large enough to resist the minimum bending moment M that would cause the UFC40 layer to peel away from the concrete 5. Specifically, in order to resist the peeling described above, as shown in Figure 12, the product of the adhesive force (planar area × adhesive strength) of the UFC40 layer around the column base 4 to the right of the column center C and the distance Lg from the centroid G of the UFC40 layer to the rotation center Cg (passing through the column center C) must be greater than or equal to the minimum bending moment M described above.

[0086] The greater the resistance to this peeling, the greater the value of the horizontal load F that can be applied. Therefore, based on the above considerations, a width b is determined such that the horizontal load F, which is equal to the initial load-bearing capacity of column 2, and the resistance force are just balanced, and this width b is set as the minimum value of the foundation removal width bx.

[0087] In other words, when a horizontal load F is applied to the upper end of column 2 from a predetermined direction (right side), if M is the minimum bending moment required to separate the UFC 40 layer around the column base 4 from the concrete 5 below, then the minimum width bx of the foundation removal is such that the product of the adhesive force between the UFC 40 layer and the concrete 5 around the column base 4 on the predetermined direction side (right side) from the column center C, and the distance Lg from the centroid G of the UFC 40 layer to the rotation center Cg is equal to the minimum bending moment M.

[0088] Based on past research data, the adhesive strength when concrete 5 and UFC40 are bonded together is preferably in the range of 2 to 5 MPa, and a minimum value of 2 MPa can be used for calculations. Alternatively, a value confirmed through experiments or other means may be used for this adhesive strength.

[0089] As shown in Figure 12, the UFC 40 layer around the column base 4 to the right of the column center C or rotation center Cg is U-shaped in plan view, and its plan view area has the following dimensions. (B1 + b × 2) × (B2 / 2 + b) - B1 × B2 / 2

[0090] B1 is the width of column 2 in a horizontal first direction, and B2 is the width of column 2 in a horizontal second direction perpendicular to the first direction. In this embodiment, the horizontal first direction refers to the front-to-back direction. Therefore, B1 is the width of column 2 in the front-to-back direction. Similarly, the horizontal second direction perpendicular to the first direction refers to the left-to-right direction. Therefore, B2 is the width of column 2 in the left-to-right direction.

[0091] B1 + b × 2 is the front-to-back width of the UFC40 layer on foundation 3 in the right half of the region from column center C. (B2 / 2 + b) is the left-to-right width of the UFC40 layer on foundation 3 in the right half of the region from column center C. B1 × B2 / 2 is the cross-sectional area of ​​column 2 in the right half of the region from column center C.

[0092] In this embodiment, the width b of the UFC40 layer and the foundation removal width bx are equal in the front-to-back and left-to-right directions. However, they may be different.

[0093] Next, we will explain how to determine the second depth t2, which corresponds to the thickness of the UFC40 layer on foundation 3.

[0094] The second depth t2 is preferably greater than or equal to the depth x calculated from the following equation (1). T=√{x 2 +(3x) 2} × (B + bx × 2) × K ···(1) however, T is the initial load-bearing capacity of column 2. B is the width of column 2 in the horizontal direction. bx is the width of the foundation to be removed in the same direction as the width B of column 2. K is the tensile strength of the concrete 5 in the foundation 3.

[0095] This equation (1) is derived based on the following idea: As shown in Figure 11, a leftward horizontal load F is applied to the upper end of column 2, for example, as shown in the figure. As this horizontal load F is increased, eventually the UFC 40 layer on the foundation 3 will delaminate from the concrete 5. At this time, as shown by the dashed line Y in Figure 11, the concrete 5 layer located next to the UFC 40 layer will fracture diagonally due to tension. The dashed line Y shows the fracture surface at this time.

[0096] The greater the resistance to this breakage, the larger the value of the applicable horizontal load F. Therefore, the depth x at which the horizontal load F equal to the initial bearing capacity of the column 2 and the resistance are exactly balanced is obtained from Equation (1). And this depth x is set as the minimum value of the second depth t2. By setting the second depth t2 to be not less than the minimum value x, as in the case of a normal column, instead of breakage occurring at the upper surface of the foundation where the virtual line Y becomes the break surface, large deformation accompanied by a decrease in bearing capacity occurs at the column base, and the bearing capacity is thus determined. Also, even if breakage occurs at the upper surface of the foundation, the bearing capacity is not less than the initial bearing capacity T.

[0097] Here, for the sake of convenience, B is taken as the width B1 of the column 2 in the front-rear direction. Similarly, the foundation removal width bx is taken as the foundation removal width bx in the front-rear direction. The foundation removal width bx is obtained from the method described above and is assumed to be known.

[0098] As shown in FIG. 11, the virtual line or break surface Y extends obliquely upward and outward from the lower end of the UFC40 layer to the upper surface 22 on the foundation 3. And it has been experimentally found that the width of the break surface Y in the left-right direction is approximately three times (3x) the thickness x of the UFC40 layer. Therefore, as shown in FIG. 11, when assuming a right triangle surrounded by one side along the outer end surface of the UFC40 layer, one side along the upper surface 22 on the foundation 3, and one side along the break surface Y, the length of one side along the break surface Y is √{x 2 +(3x) 2}, which is the meaning of √{x [[ID=I3]] 2 +(3x) 2} in Equation (1). [[ID=I7]]

[0099] B + bx × 2 in Equation (1) is B1 + bx × 2 here and represents the length of the break surface Y in the front-rear direction as shown in FIG. 12. Therefore, the product of √{x 2 +(3x) 2} and B1 + bx × 2 represents the area of the break surface Y. In Equation (1), multiplying this area by the concrete tensile strength K is assumed to be equal to the initial bearing capacity T of the column.

[0100] By setting the second depth t2 to be greater than or equal to the depth x obtained from equation (1), it is possible to obtain a load-bearing capacity of column 2 that is greater than or equal to the initial load-bearing capacity T, thereby satisfying the requirement.

[0101] Furthermore, depending on whether the column width B is defined as the width B1 in the front-to-back direction or the width B2 in the left-to-right direction, it may be possible to obtain two different values ​​for the second depth t2. In this case, the final value of the second depth t2 can be determined based on these two values. For example, for safety reasons, the larger value may be used as the final value.

[0102] From equation (1), if a depth x smaller than t7 is found, the second depth t2 can be made smaller than t7. Conversely, if a depth x larger than t7 is found, instead of making the second depth t2 smaller than t7, the foundation removal width bx may be increased. In this case, only the foundation cover concrete 5B above the foundation top reinforcement 21 needs to be removed, making the concrete removal work easier.

[0103] Figures 13 and 14 show the test results for equation (1), where (A) is a four-view drawing from the east, west, north, and south directions, and (B) is a plan view from above. The test was a positive and negative alternating horizontal loading test in which a horizontal load F, changing direction alternately and in opposite directions, was applied to the upper end of the column 2 of the test specimen. After the test, the degree of cracks 41 that occurred in the column base 4 and foundation 3 was evaluated. In the figures, labels 42 and 43 are washers and nuts used to fix the foundation 3 to the ground. In this test, x = 60 mm was calculated from equation (1).

[0104] Figures 13(A) and (B) show the appearance of crack 41 when the thickness of the UFC 40 layer on the foundation 3, i.e., the second depth t2, is 50 mm, which is less than x = 60 mm. Figures 14(A) and (B) show the appearance of crack 41 when the second depth t2 is 100 mm, which is greater than x = 60 mm.

[0105] As shown in Figures 13(A) and (B), when t2 = 50 mm, the surface of the column base 4 is not severely damaged, but the area around the UFC 40 layer of the foundation 3 peels off, reducing its load-bearing capacity, indicating that the second depth t2 is insufficient. On the other hand, as shown in Figures 14(A) and (B), when t2 = 100 mm, cracks 41 are concentrated at the column base 4, while there are fewer cracks in the UFC 40 layer of the foundation 3 and its surroundings, indicating that the required second depth t2 is secured. From these results, it can be confirmed that x = 60 mm obtained from equation (1) is a reasonable value.

[0106] Although embodiments of the present disclosure have been described in detail above, various other embodiments and modifications of the present disclosure are conceivable. For example, the structure may not be a simple structure like those in the embodiments described above, but a more complex structure or building structure that also includes beams.

[0107] The configurations of each embodiment and each variation described above can be combined in part or in whole, as long as there is no particular contradiction. The embodiments of this disclosure are not limited to those described above, but include any variations, applications, and equivalents that are encompassed within the spirit of this disclosure as defined by the claims. Therefore, this disclosure should not be constrained and may be applied to any other art that falls within the scope of the spirit of this disclosure. [Explanation of symbols]

[0108] 1 structure 2 pillars 3 Basics 4 Column base 5. Concrete 6 surface 7 Column main reinforcement 12 Inner edge 21 Foundation top reinforcement 22 Top side 23 Top 27,30 Concrete removal section 32 Bottom end 40. Ultra-high-strength fiber-reinforced concrete (UFC) bx Base removal width

Claims

1. A method for repairing the base of a column in a structure in which a reinforced concrete column is integrally joined to a reinforced concrete foundation, The first step is to remove the concrete from the surface of the column base to a predetermined first depth, A second step involves removing the concrete from the upper surface of the foundation connected to the column base to a predetermined second depth, A third step involves pouring ultra-high-strength fiber-reinforced concrete into the concrete removal area between the surface of the column base and the upper surface of the foundation, A method for repairing the base of a column, characterized by comprising the following:

2. The column base has main reinforcement bars embedded in the concrete, The first depth is equal to or less than the distance from the surface of the column base to the inner end of the main reinforcement bar of the column. A method for repairing the base of a column according to claim 1.

3. The aforementioned foundation has upper foundation reinforcement embedded in the concrete, The second depth is equal to or less than the distance from the top surface of the foundation to the bottom end of the upper reinforcement bar of the foundation. A method for repairing the base of a column according to claim 1.

4. In the second step, the concrete is removed from the surface of the column base to a position horizontally separated by a predetermined foundation removal width. A method for repairing the base of a column according to claim 1.

5. The aforementioned foundation has upper foundation reinforcement embedded in the concrete, The width of the foundation removal is three times or more the distance from the top surface of the foundation to the top end of the reinforcement bar at the top of the foundation. The method for repairing the base of a column according to claim 4.

6. When a load is applied to the column, if M is the bending moment that attempts to separate the UFC layer around the base of the column from the concrete below, then the minimum width of the foundation removal is such that the product of the adhesive force between the UFC layer and the concrete on the side where tensile stress acts on the UFC layer from the center of the column, and the distance from the center of gravity of the UFC layer to the center of rotation, is equal to the bending moment M. The method for repairing the base of a column according to claim 4.

7. The second depth is greater than or equal to the depth x calculated from the following equation (1). The method for repairing the base of a column according to claim 4. T=√{x ２ +(3x) ２ }×(B+bx×2)×K ・・・(1) however, T is the initial load-bearing capacity of the column. B is the width of the column in the horizontal direction. bx is the width of the foundation removal in the same direction as the width B of the column, K is the tensile strength of the concrete of the foundation.

8. After the first and second steps, and before the third step, a fourth step is to remove rust from the reinforcing bars exposed by the removal of the concrete. A method for repairing the base of a column according to claim 1.

9. A structure repaired by the method for repairing the base of a column according to any one of claims 1 to 8.

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