JP5930799B2 - Concrete joint structure - Google Patents

Description

  The present invention is a structure of a joint part (joint part of the two concretes) formed by placing new ready-mixed concrete on existing hardened concrete, and in particular, newly placed concrete The present invention relates to a special joint structure in which cracks are unlikely to occur in the vicinity of the joint part.

  When building a concrete structure, there are cases where it is inevitable that a new ready-mixed concrete is handed over existing hardened concrete due to work schedule restrictions. In this case, when the newly placed concrete is hardened, it receives an external restraining force from the joint surface with the existing concrete, and cracks are likely to occur near the joint portion of the new concrete. This type of crack usually does not immediately have an adverse effect on the strength of the enclosure. However, when the joint portion comes into contact with water, water enters the hardened concrete body from the cracked portion. The infiltrated water becomes a factor that causes water leakage, and also a factor that lowers the durability of the reinforced concrete.

  In addition, the jointing force between the newly installed concrete and the existing concrete (hereinafter sometimes referred to as “new and old concrete”) tends to be insufficient at the joint portion, which may cause a problem in that the shear strength decreases. Therefore, a method of applying an adhesive to the surface of existing concrete in advance before placing new concrete is often employed. In this case, an adhesive layer is interposed in the joint portion, and the external restraint force generated in the new concrete is slightly relaxed by this adhesive layer. However, it is insufficient as a measure for completely preventing the occurrence of cracks.

  On the other hand, since the joint part tends to be a weak point with respect to the strength and durability of the concrete structure, there has also been proposed a method in which a mortar reinforced with fiber as a joint material is interposed between the old and new concrete (Patent Document) 1). However, even if such a method of interposing a high-strength mortar is employed, it is not sufficient as a measure for preventing the occurrence of cracks in the vicinity of the joint portion.

JP 2001-322857 A JP 2009-35885 A JP 2009-84095 A JP 2003-192421 A

  The present invention provides a joint structure in which cracking is prevented from occurring in a joint part formed by placing new ready-mixed concrete on existing concrete, and has excellent water-stopping performance and high joint strength. The purpose is to do.

  The purpose of the above is to provide a clay mineral joint material layer sandwiched between opposing embedded formwork (sometimes called "permanent formwork") in a joint part where new concrete is placed on top of existing concrete. This is achieved by a concrete joint structure in which the two concretes are fastened by a plurality of metal studs that are interposed between the two concretes and are embedded across the two concretes. As the embedded formwork, a cement-based precast product (for example, steel fiber, stainless fiber, organic fiber, carbon fiber, etc. is mixed to increase the tensile strength, and silica fume, It is more preferable to apply a cementitious material mixed with fly ash to increase the compressive strength. In addition, the thickness of the embedded formwork (per one side) is 10 mm or more, and the clay mineral-based jointing material layer occupies the width of the new concrete at the upper end position of the embedded formwork (referred to as the “width of joint part”). More preferably, the width is 30% or more.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to prevent the crack of new concrete by the external restraining force in the joint part produced between old and new concrete. Moreover, even if water permeates from the joining interface between the materials generated in the joint portion, water leakage into the housing is prevented by the water absorption and swelling action of the clay mineral-based joint material layer. In addition, the joint strength between the old and new concrete is sufficiently secured. Therefore, this invention exhibits the outstanding water stop effect especially in the concrete structure exposed to the environment where a joint part gets wet with water.

The figure which shows typically the cross-section of the joint part seen in a conventional general concrete structure. The figure which shows typically the cross-section of the joint part at the time of interposing a cement-type joint material between old and new concrete. The figure which shows typically an example of the cross-sectional structure of the splicing part of this invention. The figure which shows typically the cross section of the splicing structure which simulated this invention used for experiment. The figure which shows typically the cross section of the joint structure of the comparative example used for experiment. The figure which shows typically the external appearance after curing of the test body which has the joint structure of FIG. The figure which shows typically the external appearance after the curing of the test body which has the splicing structure of FIG.

  FIG. 1 schematically shows a cross-sectional structure of a joint portion found in a conventional general concrete structure. Fresh concrete is placed on the already hardened existing concrete 1 to form a hardened body of the new concrete 2. That is, the new concrete 2 is handed over the existing concrete 1 to construct one concrete structure. In the present specification, the joint portion between the existing concrete 1 and the new concrete 2 is referred to as a “joining portion”, and is indicated by reference numeral 10 in the drawing. In the joint portion 10, when the new concrete 2 is dried and contracted, an external restraining force is applied from the existing concrete 1, so that the surface 20 near the joint portion of the new concrete 2 is likely to crack. As described above, this type of crack usually does not cause an immediate strength problem, but if water enters from the crack, it may cause a problem of water leakage or concrete deterioration.

  Moreover, in order to raise the joining force of the old and new concrete (the existing concrete 1 and the new concrete 2 concrete) in the joint part 10, an adhesive agent may be applied between the said both concrete. Although it is considered that the external restraint force is slightly relieved by the adhesive layer, it is not a countermeasure against the cracks.

  FIG. 2 schematically shows a cross-sectional structure of the joint portion when a cement-based joint material is interposed between the old and new concrete. A cement-based joint material layer 6 is interposed in the joint portion 10 between the existing concrete 1 and the new concrete 2. By applying a high-strength cement-based material such as fiber reinforced mortar as the cement-based joint material, the joint portion 10 that tends to be a weak point in the structure is reinforced. However, in this case as well, the external restraint force due to the drying shrinkage of the new concrete 2 is generated, and the tendency that the surface 20 near the joint portion is likely to be cracked is not eliminated.

  FIG. 3 schematically illustrates a cross-sectional structure of the joint portion of the present invention. In the joint portion 10, a clay mineral-based joint material layer 4 sandwiched between the buried molds 3 is provided on the existing concrete 1, and fresh concrete is placed thereon to form a hardened body of the new concrete 2. ing. Moreover, the said both concrete is fastened by the some metal stud 5 laid over both new and old concrete. The embedded form 3 and the metal stud 5 serve to increase the joint strength of the joint 10. In particular, it is effective to use a precast product of a cement-based material having a bending strength higher than that of new concrete as an embedded formwork. The clay mineral-based joint material layer 4 serves as a “buffer material” that absorbs strain caused by drying shrinkage of the new concrete 2 and releases an external binding force from the existing concrete 1. For this reason, the crack resulting from drying shrinkage does not arise in the joint surface vicinity surface 20 of the new concrete 2. The clay mineral-based joint material layer 4 has a function of a “water-stop material” that absorbs water that has entered from the gap between the embedded mold 3 and the concrete 1 or 2 to swell and stop the water. Therefore, the joint structure of the present invention imparts remarkable waterproof performance to the joint part of the concrete structure by the function of the clay mineral-based joint material layer 4 as the “buffer material” and the “waterproof material”. Is.

  The existing concrete and the new concrete are targeted for various concretes, including concrete that has been applied to the concrete placing method based on the conventional casting method.

  Although a non-cement-based material can be applied to the embedded formwork, it is desirable to use a cement-based material with high compressive strength when used as a member that bears the strength of the structure. In particular, it is effective to use a cement-based material precast product that has higher bending strength and tensile strength than new concrete. Specific examples include ultra-high-strength fiber-reinforced cement composites using steel fibers (Patent Document 3), crack-dispersed cement composites using organic fibers (Patent Document 4), high-performance AE water reduction Cement-based material (AQ foam) that imparts excellent durability by providing a low water cement ratio with an agent and mechanically superior mechanical strength by adding fibers as a shrinkage reducing agent, γ A suitable material is a cement-based material in which 20-85 parts by mass of belite is blended with 20 to 85 parts by mass of forcibly forcibly carbonated to increase strength and densify the structure.

  In particular, it is extremely advantageous to employ a material exhibiting a tensile strain of 1% or more as the above-mentioned crack dispersion type cement-based material. That is, when using a buried form made of such a material, it is possible to avoid material breakage of the buried form by generating a fine crack when a local strain is applied to the joint portion due to an earthquake or the like. it can. Further, even if water enters from the fine cracks, the water is stopped by the clay mineral-based joint material layer filled inside the buried mold.

  In addition, there is no example in which the embedded form made of the material as described above is applied to the joint portion together with the water-absorbing clay mineral, and the present invention has a durability that causes a problem in the joint portion by combining these materials. It is the one that has improved waterproofness at once.

  The thickness of the embedded mold (shown as t in FIG. 3) is desirably 10 mm or more. If it is thinner than that, it tends to be insufficient in strength when used as a member that bears the strength of the structure. However, if it is excessively thick, the desired action of the clay mineral joint material layer described later is reduced, so that the range satisfying the definition of “the width of the clay mineral joint material layer occupying the width of the joint portion” described later is satisfied. It is desirable to do.

The clay mineral-based joint material layer is a mixture of water containing a water-absorbing clay mineral and water. Examples of the clay mineral include bentonite and sepiolite. The proportion of the clay mineral in the solid content is preferably 50 to 100% by mass. Solids other than clay minerals include aggregates such as sand and cement. When bentonite ore is used, it itself functions as an aggregate. In the composition of the clay mineral joint material layer, the water content represented by the following formula (1) is preferably 15 to 25%, more preferably 5 to 30%.
[Moisture content ratio] = ([Mass of water] / [Mass of solid content]) × 100 (1)
Examples of compounding ingredients other than the solid content and water include aggregates such as sand, cement and the like. However, the blending ratio is preferably 50 parts by mass or less with respect to 100 parts by mass of the solid content.

As described above, the clay mineral-based joint material layer exhibits the functions of the “buffer material” and the “water blocking material”. Among these, the function as a buffer material is more effective as the clay mineral-based joint material layer is wider and thicker.
As a result of various studies, the width of the clay mineral-based joint material occupying the width of the new concrete at the upper end position of the buried formwork (referred to as “width of joint”; corresponding to W ₀ in the example of FIG. 3) it is more preferred third example corresponds to W ₁₎ is 30% or more. That is, it is more preferable to satisfy the following formula (2).
(W ₁ / W ₀ ) × 100 ≧ 30 (2)
As the width of the joining portion, the value of the portion where the interval between the embedded form frames facing each other is the narrowest is adopted. Further, when the height of the upper ends of the embedded molds on both sides is different in the portion where the distance between the opposing embedded molds is the narrowest, the width of the new concrete at the upper end position of the higher embedded mold may be adopted. .
In addition, the average thickness of the clay mineral-based joint material layer (corresponding to t ₁ in the example of FIG. 3) is preferably in the range of 2 to 30 mm.

A metal stud bears the fastening force of both the concrete by being buried over both old and new concrete. A steel stud bolt having a diameter of about 6 to 51 mm can be suitably used. Headed ones are more effective. The depth of the metal stud embedded in the concrete may be about 50 to 150 mm in the existing concrete and about 50 to 150 mm in the new concrete. The installation density of the metal studs is preferably about ^{2 to} 9 per 1 m ² of the area of the upper surface of the clay mineral-based joint material layer (including the presence of the metal studs). Also, it is desirable to disperse them as evenly as possible. For example, it is more effective to manage the distance from an arbitrary position on the upper surface of the clay mineral-based joint material layer to the nearest metal stud center position to be 50 mm or more.

A procedure for constructing the joint structure of the present invention will be briefly exemplified.
1. Care is taken in the same manner as in the case of the normal joining method on the surface of the existing concrete.
2. A metal stud is driven into a predetermined part of the existing concrete.
3. Install buried formwork. The spacing between the embedded molds facing each other is maintained by a separator or the like in accordance with normal mold installation.
4). Reinforcing reinforcing bars for new concrete.
5. Install a new formwork for concrete.
6). A clay mineral joint material is injected between the opposing embedded molds to form a clay mineral joint layer having a predetermined thickness. It can inject | pour by spraying from a nozzle.
7). Placing new concrete.

  In order to confirm the cracking prevention effect of the new concrete by the joint structure using the clay mineral joint material, the steel plate is used instead of the existing concrete, and the thermal expansion of the steel plate is used to sandwich the joint part. However, an experiment to intentionally expand the strain difference between the two materials was performed as follows.

FIG. 4 schematically shows a cross section of a splicing structure simulating the present invention. Instead of the existing concrete, an iron plate 7 having a thickness t: 10 mm was used. As the metal stud 5, a steel stud with a head (10 mm diameter) was prepared. A metal stud 5 was attached to a predetermined position of the iron plate 7 by welding. The installation density of the studs was 9 per 1 m ² of the upper surface area of the clay mineral-based joint material layer 4 (including the presence of studs). A 30 mm thick plate-like body made of ultra high strength fiber reinforced mortar (trade name; Saxem) using steel fibers was used as the embedded form 3. The interval W ₁ between the opposing embedded molds was 150 mm. The interval was maintained by Sepa. A formwork (not shown) for placing the new concrete 2 is provided, and then a clay mineral joint material having the following composition is injected between the buried formwork 3 by spraying, and the average thickness t ₁ : 10 mm clay mineral-based joint material layer 4 was formed. Next, a new concrete 2 having the following composition was placed under an environment controlled at 20 ° C. The embedding depth of the metal stud in the newly installed concrete was 50 mm.

[Composition of clay mineral joint material]
Solid content: 90 parts by weight of bentonite, 10 parts by weight of sand Components other than the above-mentioned solids: Water Moisture ratio represented by the formula (1) 21%

[Composition of new concrete]
Cement used: Ordinary Portland cement Admixture: AE water reducing agent Coarse aggregate Maximum particle size: 20mm
Water cement ratio: 55%
Fine aggregate rate: 45%
Unit water volume: 168 kg / m ³
Unit cement amount: 306kg / m ³
Unit fine aggregate amount: 822kg / m ³
Unit coarse aggregate amount: 1004 kg / m ³
Unit admixture amount: 0.765 kg / m ³

  After placing the new concrete 2, the formwork around the new concrete 2 was removed at the age of 28 days to obtain a test specimen. The specimen was cured in a temperature pattern in which the ambient environmental temperature was increased from 20 ° C. to 60 ° C. at 20 ° C./h, maintained at 60 ° C. for 2 hours, and decreased to 20 ° C. at 20 ° C./h. Due to this temperature pattern, the new concrete 2 expands when the temperature rises and contracts when the temperature drops, so that the relative difference in volume change rate between the steel plate 7 and the concrete 2 simulates the constraint that occurs in actual jointing. ing.

  For comparison, a specimen having a cross-sectional structure shown in FIG. 5 was prepared and cured with the above temperature pattern. The experimental conditions in this case are the same as those described above except that the new concrete 2 is directly placed on the iron plate 7.

  The appearance of the test specimen after curing as seen from the direction A in FIGS. 4 and 5 is schematically shown in FIGS. 6 and 7, respectively. In the specimen using the joint structure using the clay mineral joint layer 4 and the metal stud 5 sandwiched between the opposing buried molds 3, cracks in the new concrete 2 at the joint are observed. None (Figure 6). On the other hand, in the test body in which the new concrete 2 is directly placed on the iron plate 7, the surface near the joint portion (indicated by reference numeral 20 in FIG. 5) of the new concrete 2 is caused by the external binding force from the joint surface. Many cracks (indicated by reference numeral 8 in FIG. 7) were observed.

DESCRIPTION OF SYMBOLS 1 Existing concrete 2 New concrete 3 Embedded form 4 Clay mineral type joint material layer 5 Metal stud 6 Cement system joint material layer 7 Iron plate 8 Crack 10 Joint part 20 Surface near joint part

Claims (3)

  In a joint where new concrete is cast on top of the existing concrete, a clay mineral-based joint material layer sandwiched between opposing embedding forms is interposed between the two concretes, and is buried across both concretes. A concrete joint structure in which both concretes are fastened by a plurality of metal studs.   The concrete casting structure according to claim 1, wherein the embedded form is a precast product of a cement-based material having higher bending strength and tensile strength than new concrete.   The thickness of the buried formwork (per side) is 10 mm or more, and the width of the clay mineral-based joint material layer in the width of the new concrete at the upper end position of the buried formwork (referred to as “width of joint”) The concrete joint structure according to claim 1, wherein is 30% or more.

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