Crack prevention structure for concrete structures

Abstract

To provide a cracking prevention structure for concrete structure body that suppress temperature cracking due to external restriction to prevent a structure body surface from cracking.SOLUTION: A cracking prevention structure for concrete structure body has a reinforcing bar structure 10 for temperature cracking suppression embedded in a temperature stress predominant region 9 where temperature stress due to heat of hydration becomes larger inside main reinforcing bars 6, 6 arranged at the surface part of the concrete structure body 3 than at the surface part so as to prevent cracking occurring to the surface of the concrete structure 3, wherein the reinforcing bar structure for temperature cracking suppression is formed by arranging a plurality of reinforcing bars 12, 12, ..., for temperature cracking suppression, directed crossing a longitudinal section of the concrete structure body, at intervals in a longitudinal direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a crack prevention structure for concrete structures that prevents cracks from occurring on the surface of concrete structures that are erected with the lower ends of wall-type bridge abutments, large-section box culverts, etc. restrained by restraining bodies such as base slabs or footings. [Background technology]

[0002] In reinforced concrete structures such as wall-type bridge abutments, piers, and large-section box culverts, multiple main reinforcements are arranged vertically on the surface to counteract the cross-sectional force (bending moment), and the structure is designed so that the concrete bears the compressive force and the steel bars (main reinforcements) bear the tensile force.

[0003] The main reinforcement bars are made of large diameter steel to resist cross-sectional forces, and are generally spaced 125 mm apart and connected to multiple distribution reinforcement bars spaced apart in the vertical direction.

[0004] These distribution bars are used to ensure a certain level of accuracy when pouring concrete, and are made of a smaller diameter than the main bars - for example, if the main bars are D25, then D19 or D16 bars, which are two to three stages smaller, are used, and are generally spaced at 250 mm intervals.

[0005] On the other hand, in this type of concrete structure, since strength is developed through the hydration reaction of cement, there is a risk of cracks (hereinafter referred to as temperature cracks) occurring due to heat generated by the hydration reaction (heat of hydration).

[0006] There are two types of thermal cracks: internally restrained cracks, which occur due to the temperature difference between the inside and outside of the cross section of a structure, and externally restrained cracks, which occur when the bottom end is restrained by the ground or an already poured concrete base slab or footing (hereinafter referred to as the restraining body).

[0007] Externally restrained cracks occur when poured concrete expands as its temperature rises due to the heat of hydration, and then contracts as its temperature drops. The bottom end of the concrete is restrained by a restraining body, preventing it from shrinking freely, causing tensile stress (temperature stress) in the concrete. This temperature stress exceeds the tensile strength of the concrete.

[0008] With this type of externally restrained crack, the temperature stress is greater the further inside the concrete structure, and the crack propagates from the inside, penetrating the thickness of the concrete structure and becoming apparent on the surface, which can significantly reduce the durability and watertightness of the structure.

[0009] Generally, surface cracks with a width of 0.2 mm or more are considered to be harmful to structures, and it is required to keep surface cracks to less than 0.2 mm.

[0010] The width of surface cracks depends on the stress of reinforcing bars such as deformed bars that are perpendicular to the longitudinal section of the structure and the distance from the concrete surface to the reinforcing bars (hereinafter referred to as the cover). Therefore, in order to reduce the width of surface cracks, it is necessary to reduce the stress of the reinforcing bars and the cover.

[0011] However, if the cover is made too thin, corrosive factors such as carbon dioxide, salt, and water can easily penetrate, causing corrosion of the reinforcing bars and reducing durability, so in general, the cover must be a certain thickness (approximately 50 to 100 mm). In particular, for marine structures and the like that are at risk of salt damage, the cover may be made 100 mm or more.

[0012] In other words, when the cover becomes large, in order to reduce the width of surface cracks, it is necessary to reduce the stress of the reinforcing bars in the direction perpendicular to the longitudinal section of the structure, and the reinforcing bar ratio must be increased.

[0013] However, the distribution bars are made of thin reinforcing bars, which are customarily placed at 250 mm intervals. The reinforcing bar ratio in the longitudinal section of the distribution bars is low, so they cannot be expected to withstand temperature stress. If cracks do occur, the crack width will become large, reaching 0.3 to 0.5 mm or more.

[0014] For this reason, in the past, it was necessary to suppress thermal cracking by placing a large amount of reinforcing bars in a direction perpendicular to the longitudinal section of the concrete structure, in addition to the distribution bars set at the time of design.

[0015] In recent years, the results of tests and trial construction have also shown that when the reinforcement ratio of a structure is approximately 0.3% or more, surface cracks of 0.15 mm or more do not occur.

[0016] In response to this, structures have been developed that incorporate reinforcing bars placed parallel to the distribution bars spaced apart in the vertical direction of the structure, integrating the structure with the reinforced concrete structure, and width stop bars connecting the main reinforcement bars and distribution bars on both sides of the structure support reinforcing bars oriented in a direction perpendicular to the longitudinal section of the structure, thereby suppressing the width of cracks that occur on the surface of the structure (see, for example, non-patent document 1). [Prior art documents] [Non-patent literature]

[0017] [Non-Patent Document 1] "Guide to Assurance of Quality in Concrete Structures" Yamaguchi Prefecture Civil Engineering and Architecture Department, April 2020 Summary of the Invention [Problem to be solved by the invention]

[0018] However, in the above-mentioned conventional technology, it is not possible to expect that the distribution bars alone will be able to bear the temperature stress, and in order to suppress the width of cracks that occur on the surface of a concrete structure to less than 0.2 mm, a large amount of reinforcing bars must be added in addition to the distribution bars, which requires a lot of effort to arrange these reinforcing bars and increases costs. This problem is particularly noticeable in structures with large cross sections, with wall thicknesses exceeding 1.5 m, where a single reinforcing bar may be required in amounts more than five times the amount of reinforcing bar required for the structure.

[0019] Furthermore, the above-mentioned conventional techniques only serve to suppress the width of cracks that occur on the surface of a concrete structure, but do not prevent cracks from occurring on the surface of the concrete structure itself, and therefore do not provide a solution to the fundamental problem of temperature cracking.

[0020] In view of the above-mentioned conventional problems, the present invention has been made with the objective of providing a crack prevention structure for concrete structures that suppresses temperature cracks caused by external restraints and prevents cracks on the surface of the structure. [Means for solving the problem]

[0021] The feature of the invention described in claim 1 for solving the above-mentioned conventional problems is that in a crack prevention structure for a concrete structure that prevents cracks from occurring on the surface of a concrete structure constructed on a restraint body, a thermal crack prevention rebar structure is embedded in a temperature stress dominant region in which the temperature stress caused by heat of hydration inside the main reinforcement arranged on the surface part of the concrete structure is larger than that of the surface part, and the thermal crack prevention rebar structure is composed of a plurality of thermal crack prevention rebars arranged at intervals in the vertical direction, oriented in a direction intersecting the vertical section of the concrete structure, the thermal crack prevention rebar structure includes a plurality of vertical assembly reinforcements arranged at intervals in a direction perpendicular to the vertical section on the restraint body, the thermal crack prevention rebars are supported by the plurality of assembly reinforcements, The assembly The lower end of the restraining body is supported in a state in which it is placed on the surface of the restraining body or in a state in which it is floating above the surface, and is erected in a state independent of the restraining body.

[0022] The invention described in claim 2 is characterized in that, in addition to the configuration of claim 1, the concrete structure is composed of multiple stages of joints constructed in sequence on the restraint body, the main reinforcement is continuously arranged across the multiple stages of joints, and the thermal crack suppression steel structure is embedded within the temperature stress dominated area inside the main reinforcement for each joint.

[0023] The invention as set forth in claim 3 is characterized in that, in addition to the configuration as set forth in claim 1, the temperature stress dominated region width is 0.4 to 0.85 times the thickness of the concrete structure.

[0024] The invention as set forth in claim 4 is characterized in that, in addition to the configuration as set forth in claim 2, the temperature stress dominated region width is 0.4 to 0.85 times the thickness of the concrete structure.

[0025] The feature of the invention described in claim 5 is that, in addition to the configuration of claim 3, the thermal crack suppression steel bar structure is arranged so that the steel bar ratio based on the temperature stress dominant zone width is 0.2 to 0.6%.

[0026] The feature of the invention described in claim 6 is that, in addition to the configuration of claim 4, the thermal crack suppression steel bars in the thermal crack suppression steel structure are arranged so that the steel bar ratio based on the temperature stress dominant zone width is 0.2 to 0.6%. Effect of the Invention

[0027] The thermal crack prevention structure for a concrete structure according to the present invention has the configuration described in claim 1, and thus can suppress the progression of thermal cracks caused by external constraints to the surface of the concrete structure by resisting temperature stress with a thermal crack prevention rebar structure arranged inside the concrete, thereby preventing cracks on the surface. Also, a thermal crack prevention rebar structure independent of the original rebar structure can be easily assembled inside the concrete structure.

[0028] Furthermore, in the present invention, by providing the configuration described in claim 2, when a concrete structure is constructed in stages, the thermal crack suppression steel structure arranged at each joint resists temperature stress, thereby suppressing the progression of thermal cracks that occur at each joint using the lower joint as a restraint to the surface, and preventing cracks on the surface.

[0029] Furthermore, in the present invention, by providing the configurations recited in claims 3 and 4, the reinforced bar structure for suppressing thermal cracking can be installed in a position suitable for suppressing thermal cracking.

[0030] Furthermore, in the present invention, by providing the configurations recited in claims 5 and 6, it is possible to prevent cracks on the surface and reduce the amount of reinforcing bars in the entire structure. [Brief description of the drawings]

[0031] [Figure 1] 1 is a vertical cross-sectional view showing an example of a crack prevention structure for a concrete structure according to the present invention. [Diagram 2] 4 is a graph showing an example of the relationship between the age of the concrete structure and the temperature due to heat of hydration in a cross section of the concrete structure. [Diagram 3] 13 is a graph showing an example of temperature distribution in a cross section of the above abutment body. [Figure 4] 4 is a graph showing an example of the relationship between material age and temperature stress in a cross section of the above concrete structure. [Diagram 5] 13 is a graph showing an example of temperature stress distribution in a cross section of the above abutment body. [Figure 6] FIG. 4 is a vertical cross-sectional view showing another embodiment of the crack prevention structure for a concrete structure according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Next, an embodiment of the crack prevention structure for a concrete structure according to the present invention will be described with reference to the examples shown in Figs.

[0033] In this embodiment, a wall-type abutment that supports both ends of a bridge will be described as an example, and the reference numeral 1 in the drawing indicates the wall-type abutment. In this embodiment, the up-down direction of the paper in Fig. 1 is defined as the vertical direction, the left-right direction as the wall thickness direction, and the direction perpendicular to the longitudinal section as the width direction.

[0034] The wall-type abutment 1 is constructed by constructing a reinforced concrete abutment body 3, which is a concrete structure, on a reinforced concrete footing 2, which serves as a restraint body, and is integrated with the footing 2 while being restrained at its lower end.

[0035] The footing 2 is made of concrete and has a rectangular shape with a certain thickness, and a number of reinforcing bars 4, 4... oriented in a direction perpendicular to the longitudinal section are embedded at intervals around the circumference of the section at a certain distance from the surface to form a reinforced concrete structure. Note that the reference symbol 5 in the figure denotes a reinforcing bar oriented in a direction intersecting with the reinforcing bar 4.

[0036] The abutment body 3 is made of concrete and has a rectangular parallelepiped shape with a wall thickness of 1000 mm or more, and is erected while being restrained on a footing 2 (restraint body).

[0037] In this abutment body 3, multiple main reinforcements 6, 6 facing in the height direction of the abutment body 3 are buried at intervals in the width direction of the abutment body 3 a certain distance inward from both surfaces, and multiple distribution reinforcements 7, 7... facing in a direction perpendicular to the longitudinal section of the abutment body 3 are placed at intervals in the height direction along the main reinforcements 6, 6 on the outside of the main reinforcements 6, 6, forming a reinforced concrete structure. Note that the reference numeral 8 in the drawing denotes width stoppers installed between adjacent distribution reinforcements 7, 7 in the thickness direction.

[0038] In addition, a steel bar structure 10 for suppressing thermal cracks is embedded in a temperature stress dominated region 9 formed inside the main reinforcements 6, 6 arranged on the surface of the abutment main body 3, forming a structure for preventing cracks on the surface of the abutment main body 3 (hereinafter referred to as a crack prevention structure).

[0039] As shown in Figure 1, the main reinforcements 6, 6 are embedded in the footing 2 at their lower ends and protrude upward from the upper end of the footing 2, and are embedded across the footing 2 and the abutment body 3 (concrete structure), so that the abutment body 3 is restrained by the footing 2.

[0040] The main reinforcements 6, 6 are made of reinforcing bars (D22 or more) such as large-diameter deformed reinforcing bars, and are designed to resist the tensile stress acting in the vertical direction of the abutment body 3.

[0041] The distance between the main reinforcements 6,6 and both surfaces of the abutment body 3 in the thickness direction (hereinafter referred to as "cover") is generally set to about 50mm to 100mm. Note that the cover is not limited to the above-mentioned 50mm to 100mm, but is determined based on various conditions such as the environment, climate, and size of the place where the concrete structure is constructed, and may be 100mm or more.

[0042] The distribution bars 7, 7... are made of deformed bars or other reinforcing bars (approximately D13 to D19) that are smaller in diameter than the main bars 6, 6. For example, if the main bars are D25, then D19 or D16 bars with two to three stages are used, and are generally arranged at intervals of 250 mm.

[0043] Each of the reinforcing bars 7, 7... is arranged to cross a number of main reinforcements 6, 6 arranged at intervals in the width direction, and by fastening each of the main reinforcements 6, 6 to the reinforcing bars, the accuracy of installation of the main reinforcements 6 when pouring concrete is ensured.

[0044] Based on a temperature stress distribution analysis performed in advance, the temperature stress dominated area 9 is defined as an area located on the inside of the main reinforcements 6,6 in the wall thickness direction, with both ends in the wall thickness direction at a position where the temperature stress is 40 to 70% (50% in this embodiment) of the maximum temperature stress, or an area inside the main reinforcements 6,6 that is 0.4 to 0.85 times the thickness of the abutment body 3 (concrete structure), and a steel bar structure 10 for suppressing temperature cracks is formed within this area.

[0045] As shown in Figure 2, the temperature inside the abutment body 3 rises rapidly immediately after the concrete is poured due to heat generated by the hydration reaction (hereinafter referred to as hydration heat), and after reaching a peak, the temperature gradually decreases to reach equilibrium with the outside air temperature over a period of about 1 to 4 weeks.

[0046] In addition, as shown in Figure 3, the temperature in the vertical cross section due to heat of hydration is lower on the surface side closer to the outside air and becomes higher toward the center.

[0047] Therefore, when this abutment body 3 expands due to the rise in temperature caused by the heat of hydration and then contracts due to a decrease in temperature, the lower end is restrained by the footing 2 (restraint) and is prevented from contracting freely. As shown in Figure 4, the heat accumulated in the abutment body 3 (concrete structure) due to the heat of hydration in the initial stage is released from its surface, causing the temperature to decrease, resulting in a tensile force (temperature stress) in the width direction of the abutment body 3 (concrete structure).

[0048] In addition, since the temperature rise due to heat of hydration of the abutment body 3 is greater toward the center and the concrete also expands significantly, the temperature stress acting on the abutment body 3 is small on both sides in the thickness direction and greatest approximately in the center, as shown in Figure 5.

[0049] In addition, since the lower end of this abutment body 3 is restrained by the footing 2, the temperature stress is also greater at the bottom and decreases toward the top, with the temperature stress being significantly reduced in the upper half.

[0050] That is, in the abutment body 3, a portion in which temperature stress prevails up to a predetermined height is formed inside the main reinforcements 6, 6, and within this temperature stress prevails region 9, a reinforcement structure 10 for suppressing thermal cracks is arranged.

[0051] The thermal crack suppression steel structure 10 is provided with a plurality of assembly bars 11, 11 oriented in the height direction and spaced apart on the footing 2 in the width direction, i.e., in the direction perpendicular to the longitudinal section, and a plurality of thermal crack suppression steel bars 12, 12... oriented in the width direction and spaced apart vertically, supported by the assembly bars 11, 11, on both sides of the temperature stress dominant region 9, and is designed to counter temperature stress with the thermal crack suppression steel bars 12, 12....

[0052] The assembly reinforcements 11, 11 are mainly made of thin-diameter deformed reinforcing bars (approximately D13, D16) and are tied to width stop bars 8, 8... erected between the main reinforcements 6, 6 arranged on both sides of the abutment body 3, and are supported vertically at predetermined positions.

[0053] The lower ends of these assembly reinforcements 11, 11 are supported in a state in which they are placed on the surface of the footing 2 or float above the surface, and are erected in an independent state from the footing 2 (restraint body).

[0054] Each thermal crack suppression steel bar 12, 12... is made of steel bars such as deformed steel bars, and is tied to vertical assembly bars 11, 11 arranged at intervals in the width direction, i.e., in the direction perpendicular to the longitudinal section, and is supported in the direction perpendicular to the longitudinal section.

[0055] Furthermore, it is sufficient that each thermal crack suppression steel bar 12, 12... is positioned at a position where the top row is subjected to a thermal stress in the vertical direction that is approximately half the tensile strength or more, and it is not necessary for it to be positioned over almost the entire vertical area of ​​the abutment body 3 like the distribution bars 7, 7....

[0056] The number and diameter of the thermal crack suppression rebars 12, 12... are set so that the rebar ratio p based on the width of the thermal stress dominated region 9 is 0.2 to 0.6%. In other words, the rebar ratio p = (total cross-sectional area of ​​the thermal crack suppression rebars 12, 12...) / (width 0.4 to 0.85 times the thickness of the abutment main body 3 × spacing between the thermal crack suppression rebars 12, 12) = 0.2 to 0.6%.

[0057] Furthermore, although not specifically shown, the thermal crack suppression steel structure 10 may be provided with assembly bars installed between each adjacent thermal crack suppression steel bar 12, 12... in the thickness direction, so that the entire structure forms a reinforced cage structure.

[0058] In the crack prevention structure of a concrete structure configured in this manner, as described above, when the concrete that constitutes the abutment main body 3 expands as the temperature rises due to the heat of hydration and then contracts as the temperature drops, the lower end is restrained by the footing 2 (restraint), preventing free contraction, and a tensile force (temperature stress) is generated in the width direction of the abutment main body 3 (concrete structure).Cracks attempt to propagate from the position where the temperature stress is maximum toward the upper part and both sides of the width direction of the abutment main body 3.

[0059] In contrast, in the present invention, a thermal crack suppression steel structure 10 independent of the footing 2 is provided within the temperature stress dominant area 9, and the thermal crack suppression steel bars 12, 12... directed in the width direction, i.e., in the direction perpendicular to the longitudinal section, oppose the temperature stress acting within the temperature stress dominant area 9, thereby preventing thermal cracks occurring in the center from progressing upward and toward the surface, and preventing cracks from occurring on the surface of the abutment main body 3.

[0060] In addition, since the thermal crack suppression steel bars 12, 12 can be spaced sufficiently far from the surface of the abutment body 3 (concrete structure) (the distance equivalent to the relationship between the distribution bars 7, 7 and the cover), the reinforcement ratio of the steel bars facing in a direction perpendicular to the longitudinal section can be significantly reduced compared to the reinforcing bars that were previously required to suppress surface cracks, and the amount of reinforcement in the entire structure can be significantly reduced.

[0061] In the above-mentioned embodiment, the thermal crack suppression steel bars 12, 12... are respectively arranged at both ends of the width direction of the thermal stress dominant area, but they may also be arranged further inward, in the central area where the thermal stress is greater.

[0062] In addition, in the above-mentioned embodiment, a wall-type bridge abutment 1 has been used as an example, but the concrete structure to which the present invention can be applied is not limited to this, and can also be applied to, for example, a side wall portion of a box culvert that uses the bottom slab as a restraint body.

[0063] Furthermore, as shown in Fig. 6, when structures such as abutments, piers, box culverts, etc. have a certain height or more and need to be constructed by pouring concrete upward in stages (hereinafter referred to as a divided construction structure 20), when pouring concrete for the upper joint 22a, the joint 22a is restrained by the lower joint 22a, so it is preferable to provide a thermal crack suppression rebar structure 10 for each joint 22a, 22a.... Note that the same symbols are used to describe the same configurations as in the above-mentioned embodiment.

[0064] The divided construction structure 20 has a divided construction structure main body 22 as a concrete structure constructed by a plurality of stages of construction joints 22a, 22a... which are constructed in sequence on a footing 21 serving as a restraint body.

[0065] In order to counter the bending moment acting on the entire divided construction structure main body 22, the main reinforcements 6, 6 protrude upward from the upper end of the footing 21 with their lower ends embedded in the footing 21, and are arranged continuously across the entire vertical direction of the divided construction structure main body 22, spanning multiple stages of joints 22a, 22a...

[0066] Therefore, during construction, the joints 22a, 22a... of each stage are restrained such that the lowermost joint 22a is restrained by the footing 21, and the joints 22a, 22a... of each stage are restrained by the joint 22a of the stage below it.

[0067] Therefore, by providing a temperature crack suppression steel structure 10 independent of the lower joint 22a within the temperature stress dominant region 9 inside the main reinforcement bars 6, 6 for each joint 22a, 22a..., the temperature crack suppression steel bars 12, 12... directed in the width direction, i.e., in the direction perpendicular to the longitudinal section, oppose the temperature stress acting within the temperature stress dominant region 9 of each joint 22a, 22a..., thereby preventing temperature cracks occurring from the center from progressing upward and toward the surface of each joint 22a, 22a..., and preventing the occurrence of cracks in the surface portion of the main body 22 of the divided construction structure.

[0068] Furthermore, since the lowest joint 22a is constructed on the footing 21, which is a restraining body, and has the greatest restraining force, the vertical spacing p1 of the thermal crack suppression steel bars 12, 12 in the other joints 22a, 22a... may be made smaller than the vertical spacing p2 of the thermal crack suppression steel bars 12, 12 in the other joints 22a, 22a..., and the reinforcement ratio may be made larger. [Explanation of symbols]

[0069] 1 Wall abutment 2 Footing (restraint) 3. Abutment body (concrete structure) 4. Steel Bars 5. Distribution Reinforcement 6 Main reinforcement 7. Distribution Reinforcement 8 Width stopper 9 Temperature stress dominated region 10. Reinforced concrete structure for preventing thermal cracking 11 Assembly bar 12 Reinforcement bars for preventing thermal cracking 20 Split construction structure 21 Footing (Restraint) 22 Main body of divided construction structure

Claims

1. A crack prevention structure for a concrete structure that prevents cracks from occurring on the surface of a concrete structure constructed on a restraint body, A thermal crack suppression rebar structure is embedded in a thermal stress dominant region inside the main reinforcement arranged on the surface of the concrete structure, where the thermal stress caused by heat of hydration is larger than that of the surface region, and the thermal crack suppression rebar structure is formed by arranging a plurality of thermal crack suppression rebars at intervals in the vertical direction, the rebars being oriented in a direction intersecting the vertical cross section of the concrete structure; The thermal crack suppression steel structure includes a plurality of vertical assembly bars arranged on the restraint body at intervals in a direction perpendicular to the longitudinal section, and the thermal crack suppression steel bars are supported by the plurality of assembly bars; A crack prevention structure for a concrete structure, characterized in that the assembly reinforcement is supported in a state where its lower end is placed on the surface of the restraint body or floats above the surface, and is erected independently of the restraint body.

2. The concrete structure is composed of a plurality of stages of joints constructed in sequence on the restraint body, and the main reinforcement is continuously arranged across the plurality of stages of joints, 2. A crack prevention structure for a concrete structure as described in claim 1, wherein the thermal crack suppression steel structure is embedded within the temperature stress dominated area inside the main reinforcement for each of the joints.

3. 2. A crack prevention structure for a concrete structure according to claim 1, wherein the width of the temperature stress dominant region is 0.4 to 0.85 times the thickness of the concrete structure.

4. 3. A crack prevention structure for a concrete structure according to claim 2, wherein the width of the temperature stress dominant region is 0.4 to 0.85 times the thickness of the concrete structure.

5. A crack prevention structure for a concrete structure as described in claim 3, wherein the thermal crack suppression steel bars are arranged so that the steel bar ratio based on the temperature stress dominant area width is 0.2 to 0.6%.

6. A crack prevention structure for a concrete structure as described in claim 4, wherein the thermal crack suppression steel bars are arranged so that the steel ratio based on the temperature stress dominant area width is 0.2 to 0.6%.

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