JP3355112B2 - Steel fiber for concrete reinforcement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel fiber for reinforcing concrete mainly used for shotcrete, which is incorporated for the purpose of reinforcing the strength and toughness of concrete.

[0002]

2. Description of the Related Art Steel fiber reinforced concrete (hereinafter referred to as "SFRC") in which steel fibers are mixed and uniformly dispersed in concrete used as a general construction material is compared with ordinary concrete in which steel fibers are not mixed. Since crack resistance, tensile strength, shear strength, etc. are large, and the toughness of concrete after cracking is also large, for the conventional civil engineering field, tunnel wrapping, primary lining spraying concrete of tunnel by NATM method, and It has been widely used in roadside slope spray concrete.

[0003] Steel fibers used in shotcrete are:
Because the SFRC is blown from the tip of the cylindrical nozzle having a diameter of 10 to 13 cm by high-pressure compressed air, the diameter is 0.4 to 0.6 mm and the length is 20 to 30 m so that the nozzle is not blocked.
m is used, and about 1% by volume percentage (about 80 kg in weight) of steel fiber is mixed per m ^{3 of} concrete. Further, if the coarse aggregate used for spraying SFRC has a particle size of about half the length of the steel fiber, it is necessary to use SF.
Since the performance of RC is best exhibited, fine gravel or crushed stone having a maximum size of the coarse aggregate of 10 to 15 mm is used.

[0004] The required performance of this type of steel fiber is that the tensile strength per steel fiber is high in order to enhance the reinforcing effect, and that the steel fiber is moderately mixed so that it does not bend or break when mixed in concrete and mixed. Hardness, good adhesion to concrete, and good dispersibility without the formation of fiber balls when mixed and mixed with concrete. Various shapes and dimensions have been devised according to the law. A typical method for producing a steel fiber is as follows. (1) A thin plate shearing method in which a cold-rolled thin steel plate having a thickness of about 0.5 mm is finely sheared with a rotary blade (Japanese Patent Laid-Open No. 52-29689).
JP-A-57-176362). (Less than,
"Prior art 1"). (2) A steel wire cutting method for cutting a cold-rolled and drawn round steel wire to a predetermined length (Japanese Patent Application Laid-Open Nos. 60-195043, 60-235751, and 4-310553).
Publication (hereinafter referred to as “prior art 2”).

[0005] The steel fibers produced by these prior art methods have specific properties depending on the production method, fiber length, and the material of the raw material of the steel fibers, and when SFRC is used. It has been found that their strength properties and deformability differ considerably. The difference between these properties is that when the steel fiber is subjected to an external force, the adhesive properties between the steel fiber and the concrete are directly dominant factors. Various ideas have been devised on the shape of the steel fiber in order to further increase the bonding strength with concrete. As this method,
There are the following (a) to (c). (A) A cross-section processed into an irregular shape along the axis of a steel fiber. (B) A steel sheet having a wavy deformed portion formed by using an uneven die when a thin steel sheet is thinly sheared. (C) A steel fiber in which the end portion is bent to enhance pull-out resistance by an anchoring action at the end portion.

Here, it should be noted that when the steel fiber is processed, when an external force acts on the SFRC, the tensile strength of the steel fiber and the adhesion strength to the concrete are balanced, and the steel fiber is broken. An object of the present invention is to determine the material and shape of the steel fiber so that the concrete is less likely to be broken and the material is more tenacious by sharing the external force acting on the concrete while maintaining a high bonding strength without performing the bonding. Heretofore, for a short steel fiber having a steel fiber length of about 30 mm, the optimum shape has been empirically determined through abundant practical examples and the like. Further, the strength of the concrete used was sometimes as low as 210 to 240 kgf / cm ² in design standard strength, and no problem occurred in SFRC performance. However, the use of SFRC has been diversified in recent years, and concrete used for ultra-large-diameter tunnels and the like has a strength of 50%.
A high strength of 0 to 600 kgf / cm ² , which has never been considered before, and a high toughness in SFRC are required. As described above, almost no study has been made on the optimum shape and the like of the steel fiber by the thin plate shearing method which provides high strength and high toughness SFRC.

[0007]

The steel fiber produced by the thin plate shearing method having a tensile strength of steel fiber of 60 to 100 kgf / mm ² and a length of about 30 mm is the following. If the deformed portion having a waveform in which the bearing area coefficient, which is an index indicating the adhesion characteristic, is set in the range of 0.005 to 0.015 is distributed over the entire length of the steel fiber, the SFR
It has been found that when an external force acts on C, the steel fiber resists the external force by pulling out while maintaining a high adhesive strength without breaking.

[0008] bearing area coefficient = overhang area of deformed part (=
Bearing area) / (length of one pitch × perimeter of steel fiber) Here, the bearing area coefficient refers to a surface area of a steel fiber of one pitch length consisting of a deformed portion of a waveform and an unprocessed axis portion. Of the bearing area (= the width of the steel fiber x the height of the peak of the wavy deformed portion).

By the way, the above relationship is for concrete strength corresponding to 210~240kgf / cm ² and an ordinary concrete strength at design strength, concrete strength is 500 kgf / cm ² or more high strength SFRC
When an external force acts on the steel fiber, the bonding strength between the steel fiber and concrete may increase, and in the steel fiber by the conventional thin plate shearing method, a phenomenon occurs in which the steel fiber breaks and as a result, the toughness of SFRC is greatly reduced. Have been. And length 30
When a high-strength spraying SFRC using steel fibers of about mm was subjected to an external force, no study has been made so far to resist the external force by a thin plate shearing method.

Accordingly, an object of the present invention is to provide a concrete reinforcing steel fiber used for shotcrete which can solve the above-mentioned problems.

[0011]

According to the first aspect of the present invention,
In a steel fiber for concrete reinforcement produced by thinly shearing a high-tensile steel sheet, the steel fiber is 20 to 30 m.
m, a width of 0.6-0.8 mm, 60
Has a tensile strength of 100 kgf / mm ² ,
%, And the steel fiber is provided with corrugated deformed portions at predetermined intervals over its entire length, and the concrete to which the steel fiber is applied has a compressive strength of 5 %.
It is characterized by being high-strength concrete of 00 kgf / cm ² or more .

[0012]

As a method for solving the above-mentioned problem, the tensile strength of the steel fiber is set to be as high as 100 kgf / mm ² or more like the steel fiber by the steel wire cutting method (prior art 2) so that the steel fiber is not broken. There is a method of controlling by controlling the material of the high-strength thin steel plate which is a raw material of the steel fiber, the number of irregularly shaped portions to be processed into the axial portion of the steel fiber, and the like.

[0014] The method of increasing the tensile strength of the steel fiber increases the cost of the high-strength thin steel sheet as a raw material,
Further, when the tensile strength is 100 kgf / mm ² or more, the hardness of the steel sheet becomes larger than that of the conventional thin steel sheet, and it becomes difficult to perform shearing with a rotary blade. The method of determining the optimum shape of the steel fiber is a preferable method in which a large amount of steel fiber can be produced at low cost.

Concrete strength containing steel fiber is 50
One of the main causes of breakage of conventional steel fibers at 0 kgf / cm ² is that when cracks occur when the concrete has high strength, large strain energy accumulated inside the concrete is released at once, The steel fiber is unable to withstand this and breaks down individually. For this reason, the material of the steel fiber is changed to a material with much better elongation than the conventional thin steel sheet, and a large release energy when a crack occurs is absorbed and softened by the elongation of the steel fiber located on the cracked surface to weaken it. And SFR by resisting external force both in the bond strength between the deformed part processed into the axis part of the steel fiber and the concrete and in the elongation of the steel fiber.
A method of increasing the strength and toughness as C is conceivable.

In order to confirm this idea, a large number of steel fibers were trial-produced using nine types of thin steel sheets having different elongations as shown in Table 1, and the steel fibers were used to form a high-strength mortar specimen 9 as shown in FIG. 8 was embedded with one embedded length being half of the entire length of the steel fiber, and a steel fiber pull-out test was performed by pulling out the steel fiber with a tensile tester, and the absorbed energy per steel fiber was compared.

[0017]

[Table 1]

FIG. 2 is a graph schematically showing a relationship between a pulling load P in a steel fiber pulling test and a sliding displacement curve in which the steel fiber comes out of the mortar matrix. The area of the portion surrounded by the hatch corresponds to the absorbed energy per steel fiber. The larger the absorption energy is, the larger the absorption energy at the time of forming the SFRC, that is, the toughness is. Here, in the eleventh curve in which the slipping out of the steel fiber is not excellent, the steel fiber breaks when the drawing load exceeds the breaking load _SF Pmax of the steel fiber, and the absorbed energy is naturally the curve 10 when the steel fiber sticks out.
Smaller than. In addition, when the adhesion detachment is excellent, there are as many peaks as the number of deformed portions embedded in the mortar in the pulling load-sliding displacement curve as shown in FIG. Is equivalent to the resistance due to the adhesion of

In a steel fiber drawing test using a mortar specimen, the mortar has a compressive strength of 600 to 610 kgf / c.
m ^2, and the size of the mortar specimen was a cube of 40 × 40 × 40 mm. In the exam,
After constraining the end of the surface from which the steel fiber is exposed, the end of the steel fiber that is exposed is pinched by a chuck of a tensile tester having a maximum capacity of 500 kgf, and the steel fiber is pulled upward, whereby a pulling load-sliding displacement. The curve was measured, and the absorbed energy per steel fiber was calculated from the obtained curve.

As shown in Table 1, the test steel fibers used for the test were prepared from a thin steel plate having an elongation in the range of 1.9 to 35.4% by a thin plate shearing method. The three irregularly shaped portions were arranged only on one side of the steel fiber so that the adhesion and detachment were excellent, and the other side was formed in a straight shape without the irregularly shaped portion because it was sandwiched by a chuck of a tensile tester. In addition, in order to make the influence of the adhesion between the deformed portion and the mortar matrix the same condition, the bearing steel area coefficient of the deformed portion is about 0.007, and one pitch length from the deformed portion to the deformed portion is 5 mm. Further, the shape of the steel fiber was determined so that the circumference of the steel fiber in the cross section orthogonal to the material axis of the steel fiber was the same. Elongation of 1.9% among the prototype steel fibers shown in Table 1.
No. 1 steel fiber corresponds to the material of the conventional product applied to normal concrete strength.

FIG. 3 is a graph showing the relationship between the absorbed energy and the elongation of the steel fiber obtained from the average value of the drawing test in which six steel fibers were produced for one type of steel fiber. The vertical axis represents the dimensionless absorption energy ratio obtained by dividing the average value of the absorption energy of each prototype steel fiber by the average value of the absorption energy obtained from the steel fiber of No. 1 shown in Table 1. In FIG.
●: Elongation of steel fiber is within the range of the present invention, ○: Elongation of steel fiber is out of the range of the present invention. From FIG. 3, the elongation is 10-2.
The absorbed energy of the steel fiber in the range of 5% is about 2 to the absorbed energy of the steel fiber in the elongation other than this range.
It was found to be 2.2 times as high, and the idea was confirmed. The result shown in FIG. 3 can be explained by a typical pulling load-sliding displacement curve in each region of different elongation obtained as a result of the test shown in FIG. In FIG. 4, curve 12 is for steel fibers with elongation less than 10%, curve 13 is for steel fibers with elongation in the range of 10 to 25%, and curve 14 is greater than 25% elongation. For steel fibers. If the elongation is less than 10%, the mechanism by which the steel fiber resists external force depends mostly on the adhesive force between the deformed portion of the steel fiber and the matrix. If the entire steel fiber slips, it will escape and the absorbed energy will be small. In addition, the growth is 25
%, The elongation is so large that the steel fiber is in a deep drawing state like pulling rubber before the deformed part comes out, so the cross-sectional area decreases and the steel fiber breaks. The absorbed energy is reduced. On the other hand, the growth is 1
Steel fibers in the range of 0 to 25% resist external force by both the resistance due to the elongation of the steel fibers and the adhesive force due to the deformed portion for each deformed portion, and are absorbed as shown in FIG. The energy is much higher.

The present invention has been made as a result of earnest research based on the above preliminary test. In a 20 to 30 mm long spraying steel fiber produced by a thin plate shearing method, a high tensile strength which is a raw material of the steel fiber is used. Elongation of 10 to 2 among thin steel sheet materials
By setting it to 5%, even when high strength SFRC is used,
It has been found that a high performance spraying steel fiber excellent in both strength and toughness can be obtained.

Fiber length of 20 to 3 in which corrugated deformed portions are arranged on the entire surface of the steel fiber axis produced by the thin plate shearing method.
0 mm, desirably 30 mm, and the width of the steel fiber is 0.6-0.
In order to select the optimum combination for the material and shape of the steel fiber to be used for the spray fiber of 8 mm, preferably 0.7 mm, for the sprayed concrete, a number of steel fiber specimens were prepared and the Japan Concrete Institute, "Research on Steel Fiber Reinforced Concrete" SF based on "Bending strength and bending toughness test method of fiber reinforced concrete (draft)" by "Subcommittee"
A number of RC bending tests were performed. As a result, a shape in which the bearing area coefficient is 0.005 to 0.015, and the length per pitch of the wavy deformed portion is 5 mm over the entire surface of the steel fiber axis within a range in which the adhesion and pull-out of the steel fiber is excellent, If the material of the steel fiber has a tensile strength of 60 to 100 kgf / mm ² and an elongation of 10 to 25%, the concrete strength is 500 k.
Even with SFRC of gf / cm ² or more, the steel fibers did not break while maintaining high adhesion to concrete, and SFRC characteristics excellent in strength and toughness were obtained. As a result, the bending strength of SFRC and the SFR after bending cracking
The flexural toughness coefficient (toughness), which is an index indicating the energy absorption capacity of C, also increases, and it is found that SFRC that maximizes the reinforcing effect of steel fibers can be obtained even with high-strength concrete by using steel fibers in the above range. It is a thing.

[0024]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 5 is a cross-sectional view in the axial direction of the steel fiber by the thin plate shearing method according to the present invention. As shown in the drawing, the steel fiber 8 includes an axis portion 1 and a deformed portion 2 processed into a corrugated shape. The deformed portion 2 is provided over the entire length 1 of the steel fiber 8, and the linear portion 1 and the deformed portion 2 have a pitch length q from the deformed portion 2 to the adjacent deformed portion 2,
They appear alternately.

Table 2 shows that the compressive strength on the 28th of material age is 500k.
FIG. 3 shows the performance of SFRC when steel fibers according to the present invention and steel fibers manufactured from conventional thin steel sheets are used when steel fibers are mixed into high-strength concrete of gf / cm ² or more.

[0026]

[Table 2]

Table 2 also shows the mechanical properties of the high-strength thin steel sheet used in the production of the steel fiber, the dimensions of the steel fiber, and the total number of deformed portions arranged on one steel fiber. In the SFRC performance test shown in Table 2, the bending strength, bending toughness coefficient (toughness), diameter 10 cm, and height 20 were measured using a small rectangular beam bending test specimen having a cross section of 10 × 10 cm and a length of 40 cm.
The compressive strength of each of the test pieces was measured using a cylindrical test piece of cm.
The small beam bending test is based on the two-point concentrated load bending test method (the distance between fulcrums of 30) proposed by the Japan Concrete Institute.
cm, loading point interval 10 cm). The composition of steel fiber reinforced concrete is as follows: water cement ratio 35%, fine aggregate ratio 60%, unit water amount 175 kg / m ³ , unit cement amount 5
00kg / m ^3, a steel fiber incorporation amount 79kg / m ³ (1.0% by volume percentage), the maximum size of the coarse aggregate used was 15m
m. After the SFRC was cast, the test specimens were cured at 20 ° C. in standard water, and a performance test was performed on a 28-day material age.

In Table 2, Formulations Nos. 1 to 5 are based on the steel fiber of the present invention, and Formulations Nos. 6 to 10 are those using a steel fiber for comparison. Formulations Nos. 1-3 were made by changing the width of the steel fiber without changing the material of the thin steel sheet and the total number of deformed parts. It is a change in elongation. The composition Nos. 6 to 8 based on the comparative steel fiber were obtained by changing the width of the steel fiber without changing the material of the steel fiber and the total number of deformed portions. No. 9
Is the steel fiber when the elongation of the thin steel sheet is slightly better than the conventional product,
Formulation No. 10 shows a steel fiber having an elongation of steel fiber larger than that of the present invention.

The SFRC using the steel fiber according to the present invention is:
For high-strength concrete with a compressive strength of 500 kgf / cm ² or more in design standard strength, after bending cracking, the steel fiber maintains a high bond strength without breaking, and thus has a small drop in proof stress. The flexural toughness coefficient, which indicates the resistance and the toughness of SFRC, is at most 3 compared to the steel fiber having a smaller elongation than the product of the present invention.
The performance was twice as high.

On the other hand, Comparative Examples Nos. 6 and 7, which have the same number of deformed parts as the product of the present invention and have a low elongation of 2.1%, cannot withstand a large strain releasing energy at the time of cracking. In turn, the steel fibers broke one after another and the bending toughness coefficient was low.

In order to prevent the steel fiber from breaking, the total number of the deformed portions was set to three and the width of the steel fiber was further increased. With the steel fiber of No. 8, the steel fiber did not break when cracking occurred. In addition, the bending strength was lower than that of the product of the present invention, and the increase in the flexural toughness coefficient was small, because the adhesive force with concrete was reduced.

In the case of the composition No. 9 using a steel fiber having a slightly higher elongation of 5%, the steel fiber does not break, but the release energy at the time of the occurrence of cracking is smaller than that of the product of the present invention. The rate of absorption by elongation was low, and the increase in the flexural toughness coefficient was lower than that according to the present invention.

In the composition No. 10 using a steel fiber having an elongation of 35.4%, which is larger than that of the present invention, the steel fiber does not break when a crack occurs, but the steel fiber tends to come out when the crack progresses. Before this, the steel fiber axis portion was in a deep drawing state, the cross-sectional area was reduced, and the steel fiber was broken, so that the value of the bending toughness coefficient was smaller than that of the product of the present invention.

FIG. 6 shows the relationship between the bending load P in the bending toughness test using a small beam specimen of 10 × 10 × 40 cm and the deflection δ at the central point of the specimen.
FIG. 9 is a graph showing the case of No. 2 and the case of No. 7 with comparative steel fibers. S by the steel fiber of the present invention
In the case of FRC, the large strain release energy at the time of crack generation is absorbed by the large elongation of the steel fiber at the crack surface and weakened, and at the time of crack growth, the steel fiber elongation and the high adhesive force between the deformed part of the steel fiber and concrete. In order to resist the external force in both, as shown in the drawing, after the occurrence of cracks, it shows a so-called strain hardening phenomenon in which the yield strength further increases, and the steel fiber is pulled out gradually while maintaining a high adhesion without breaking. High toughness performance can be obtained. On the other hand, in the case of using a steel fiber having a small elongation, most of the steel fiber on the cracked surface breaks one after another at the time of bending cracking, so that the proof stress rapidly decreases and the bending toughness coefficient decreases.

As described above, the preferred shape and material of the steel fiber according to the present invention are as follows: a steel fiber axis is provided with a corrugated deformed portion over the entire length of the steel fiber;
0.6-0.8mm, tensile strength of steel fiber is 60-100
kgf / mm ² and the elongation of the steel fiber are preferably in the range of 10 to 25%. If the width of the steel fiber is less than 0.6 mm, the number of steel fibers in the concrete increases and the value of the bending strength increases, but when cracks occur, some of the steel fibers on the cracked surface break and the toughness decreases. Not preferred. When the width of the steel fiber exceeds 0.8 mm, the steel fiber does not break at the time of cracking, but the effect of reinforcing the entire concrete surface cannot be sufficiently exhibited due to the decrease in the number of steel fibers in the concrete, Both the bending strength and the bending toughness coefficient decrease. If the elongation of the steel fiber is less than 10%, it is difficult to absorb the large strain release energy at the time of the occurrence of cracking only by the elongation of the steel fiber, and the majority of the release energy will be borne by the adhesive force between the steel fiber and concrete. If the steel fiber breaks, the toughness as SFRC also decreases. Decreasing the number of deformed portions or increasing the width of the steel fiber so as not to break the steel fiber reduces the adhesion strength to concrete, and has little effect on improving the yield strength and toughness. Further, when the elongation of the steel fiber exceeds 25%, the tensile strength of the thin steel sheet becomes lower than 60 kgf / mm ² , and when the steel fiber is used, the steel fiber is easily bent during the concrete kneading, and the reinforcing effect as SFRC is reduced. This is undesirably lower.

[0036]

As described above, according to the present invention,
In this case, the following useful effects are obtained. As described in the embodiment, the present invention provides a thin plate shearing method.
10-25% increase compared to conventional spraying steel fiber
The concrete compressive strength is 500
kgf / cm^TwoIn the above high-strength concrete,
SFRC with significantly higher strength and toughness than before
Steel fiber can be provided. Further, the steel fiber according to the present invention has a design standard strength of 21.
0-240kgf / cm ^TwoAnd normal concrete strength
On the other hand, a mechanism that absorbs strain energy when cracks occur
Flexural toughness due to superior cannism
Tensile SFR higher than the conventional one with a coefficient of 30 to 50%
C. Mixing steel fibers according to the invention into concrete
SFRC when external force acts on concrete
To maximize the performance required for
Providing highly productive and highly practical steel fibers
Can be.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a drawing test of a steel fiber using a mortar specimen.

FIG. 2 is a graph showing the concept of absorbed energy per steel fiber in a steel fiber drawing test.

FIG. 3 is a graph showing the relationship between the elongation of a steel fiber obtained from a steel fiber drawing test and the absorbed energy ratio obtained by making the energy absorbed by the steel fiber dimensionless.

FIG. 4 is a graph showing a typical example of a drawing load-sliding displacement curve of a steel fiber in each region having different elongation in a steel fiber drawing test.

FIG. 5 is an axial cross-sectional view of the steel fiber according to the embodiment of the present invention.

FIG. 6 is a graph showing a bending relationship between a bending load and a center point of a beam of a small SFRC beam test specimen of 10 × 10 × 40 cm.

[Explanation of symbols]

1: axis portion 2: deformed portion 3: bending load of SFRC using steel fiber according to the present invention-
Deflection curve 4: Bending load of SFRC using steel fiber for comparison-
Deflection curve 5: Schematic diagram showing small beam bending toughness test 8: Steel fiber 9: Mortar specimen 10: Curve when steel fiber adheres and pulls out 11: Curve when steel fiber breaks 12: Elongation 10% Curve for steel fibers less than 13: Curve for steel fibers with elongation in the range of 10 to 25% 14: Curve for steel fibers with elongation greater than 25% P in FIG. 1: P: drawing load _SF Pmax: steel fiber Breaking load of p: Pitch length where the axial part and deformed part alternately appear 1: Total length of the steel fiber P in Fig. 6: Bending load δ: Deflection at the center point of the beam specimen

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Nobuyuki Nakamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Tomiyasu Kaneko 35-1 Negishi-cho, Kohoku-ku, Yokohama, Kanagawa Prefecture, Ltd. JP-A-7-10619 (JP, A) JP-A-52-31123 (JP, A) JP-A-63-130846 (JP, A) JP-A-52-79533 (JP, A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C04B 14/48 D01F 9/08 E04C 5/03 E21D 11/10

Claims (1)

(57) [Claims] 1. A concrete reinforcing steel fiber produced by thinly shearing a high-tensile steel sheet, wherein the steel fiber comprises:
Has a length of 20 to 30 mm, a width of 0.6 to 0.8 mm, have a tensile strength of 60~100kgf / mm ^2, has an elongation of 10% to 25%, the steel The fiber is provided with corrugated sections at predetermined intervals over the entire length of the fiber.
High-strength concrete with a compressive strength of 500 kgf / cm ² or more
A steel fiber for concrete reinforcement, which is a cleat .