JP2021123905A - Evaluation method of ultimate flexure yield strength of steel fiber reinforced concrete member - Google Patents

Abstract

To provide an evaluation method of ultimate flexure yield strength of a steel fiber reinforced concrete member.SOLUTION: An evaluation method of ultimate flexure yield strength of a steel fiber reinforced concrete (SFRC) member in a construction method constructing a structure using the SFRC member is characterized in that measurement of the ultimate flexure yield strength of the SFRC member is calculated with a following stress block factor formula in accordance with compression strength of concrete used in the SFRC by using stress blocks in which SFRC stress is evenly distributed on a compression side and by adding cleavage strength of SFRC stress assuming it is evenly distributed on a tension side.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for evaluating the ultimate bending strength of a reinforced concrete member mixed with steel fibers.

The tensile strength of concrete is generally about 1/10 of the compressive strength. Reinforced concrete members are designed so that when compressive stress is applied, the concrete is stressed, and when tensile stress is applied, the reinforcing bar is stressed. That is, in the current design of reinforced concrete members, tensile strength is ignored. In order to expect tensile strength from concrete, it has been studied to mix fibers into concrete (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2003-306366

In recent years, steel fiber reinforced concrete (hereinafter, also referred to as SFRC) in which fibers mixed in concrete are used as steel fibers has been studied. It is expected that the use of SFRC improves the toughness performance of the member against seismic force, suppresses the occurrence of cracks, and suppresses damage such as peeling of the cover concrete covering the outside of the reinforcing bar. In the design of a reinforced concrete member, a stress block model is used to evaluate the ultimate bending strength of the member when the member is repeatedly loaded in one direction. However, an evaluation method using a stress block model for SFRC members has not yet been proposed.

An object of the present invention is to provide a method for evaluating the ultimate bending proof stress of an SFRC member.

In order to achieve the above object, the present invention is a method for evaluating the ultimate bending strength of a reinforced concrete member mixed with steel fibers in a construction method for constructing a structure using a reinforced concrete member (SFRC member) mixed with steel fibers. Therefore, for the ultimate bending strength of the SFRC member, a stress block in which the SFRC stress is uniformly distributed on the compression side is used, and this is added assuming that the SFRC split strength is uniformly distributed on the tension side. However, the stress block is mixed with steel fibers, which is characterized by being calculated using the following formula (1) or (2) of the stress block coefficient according to the compressive strength of the SFRC used for the SFRC. This is a method for evaluating the ultimate bending strength of reinforced concrete members.
Equation (1): In the case of σ _B ≤27.5 N / mm ² , β ₁ = 0.85-0.51 (σ _B -27.5) / 70
Equation (2): In the case of σ _B > 27.5 N / mm ² , β ₁ = 0.65
However,
σ _B : Compressive strength of SFRC

According to the present invention, the ultimate bending strength of the SFRC member can be calculated assuming that the SFRC stress block having a rectangular distribution is used on the compression side and the tensile strength of the SFRC is uniformly distributed on the tension side.

Further, the present invention may be configured such that the stress uniformly distributed in the SFRC member on the tension side is 25% of the SFRC split strength of the SFRC member.

According to the present invention, 25% of the split strength of SFRC is uniformly distributed on the tensile side of the test piece, and it can be applied to the design of SFRC members.

According to the present invention, the ultimate bending strength of the SFRC member can be evaluated.

It is a figure which shows the structure of the shear test apparatus which concerns on embodiment of this invention. It is a figure which shows the stress block model for evaluating a test piece. It is a figure which shows the comparison result between the maximum proof stress experimental value of a test piece, and each proof stress calculated value. It is a figure which shows the skeleton curve and the restoring force characteristic of a test body. It is a figure which shows the skeleton curve and the restoring force characteristic of a test body.

Hereinafter, embodiments of each step performed in the method for evaluating the ultimate bending proof stress of the SFRC member according to the present invention will be described with reference to the drawings. The concrete evaluation method according to the present proposal is a method for evaluating the ultimate bending strength of SFRC members.

As shown in FIG. 1, when the test body (member) of the reinforced concrete is experimentally deformed, for example, cracks and peeling occur at the cover portion of the cover concrete on the tension side. This is because, as described above, in general reinforced concrete, the tensile strength of concrete is about 1/10 of the compressive strength. If the concrete is cracked or the covered concrete is peeled off, the toughness performance is deteriorated, moisture or the like invades the inside of the member from the cracked portion, and the reinforcing bar is corroded, which may reduce the durability of the member.

When steel fibers with a length of several millimeters to several centimeters are mixed into concrete in order to increase the tensile strength of concrete, when tensile force is generated in concrete, the steel fibers dispersed in the concrete crack the concrete. It is possible to suppress the occurrence and prevent the cover concrete from peeling off.

The SFRC test piece is an experimental member for confirming structural performance, and is formed on the assumption that it is applied to a flat beam used for a short span frame of a super high-rise RC building, for example.

The test specimen names are concrete compressive strength (Fc36, Fc60), volume mixing ratio of steel fibers (Vf = 0.0vоl.%, 0.5vоl.%, 1.0vоl.%), Failure type at the time of planning (bending yield). By combining the leading type and shear fracture type), the symbol "N: Fc36, H: Fc60" + "Fracture type at the time of planning" is represented by the symbol "F: bending". In the case of yield-preceding type, S: in the case of shear fracture type "+ in the case of the symbol" 00: Vf = 0.0vоl.%, 05: Vf = 0.5vоl.% 10: In the case of Vf = 1.0 vоl.% ”. Using this, the test body name is described as, for example, NF00, NF05, NF10, or the like.

As shown in FIGS. 2 and 3, the test body C is, for example, reinforced concrete and is formed in the shape of a rod having a rectangular cross section. The test body C is formed to have the dimensions of the width (b) of the cross section, the height of the cross section = the entire beam (D), and the length (L). Reinforcing bars V are arranged at predetermined positions inside the test body C. The effective beam is the distance from the compression edge to the position of the center of gravity of the reinforcing bar V on the tension side when viewed from the cross-sectional direction. The concrete from the surface layer of the reinforcing bar V to the lower side of the outermost edge of the concrete cross section is a cover portion.

The test body C is set in the bending shear force device E. The bending shear force E applies a load so that a bending shear force is generated in the test section of the test piece C. The bending shear force applying device E is, for example, a device capable of applying an inversely symmetrical bending force to the test piece C.

In the bending shear force device E, the test body C is supported via the support points T1 and T2 at the lower two points when the positive force is applied, and through the support points T1'and T2'at the lower two points when the negative force is applied. The test body C has two predetermined positions + P, + {b / (2a + b)} P at the time of positive load, and -P,-{b / (2a + b)} at the upper two points at the time of negative load. A load of P is applied.

In the test body C, + P, + {b / (2a + b)} P is added from above when a positive force is applied, and -P,-{b / (2a + b)} P is added from above when a negative force is applied by the bending shear force device E. A bending shear force is generated in the test section. The vertical deformation that occurs in the test piece C is measured based on the displacement meter attached to the test piece C. The member angle generated in the test piece C is calculated based on the measurement result of the vertical deformation.

_{As shown in FIG. 4, the relationship between the beam shear force (Q b} ) generated in the test body C and the member angle (R = δ _V / L) is obtained based on the experimental result in which the test body C is loaded. Be done. Here, δ _V is a vertical displacement caused in the test piece C due to the load. _exp Q _bmax is the maximum proof stress experimental value.

_cal Q _b < _Mu > is a calculated value of ultimate bending strength. The calculated ultimate bending strength of the SFRC test piece was calculated based on the ACI standard concrete stress block model shown in FIG. 2 (a). The ACI standard concrete stress block model is described in Non-Patent Document 1 "American Concrete Institute: Building Code Requirements for Structural Concrete (ACI318-14) and Commentary, 2014". The stress distribution of concrete on the side receiving compressive force is generally a parabolic type distribution, but it is replaced with a rectangular distribution (uniform distribution) in the stress block model of concrete based on the ACI standard.

The calculated bending strength of the SFRC test piece is calculated based on the stress block model (see FIG. 2). This stress block model is based on Non-Patent Document 2 “Satoru Nagai, Tetsushi Kabata, Yoshikazu Takahashi, Makoto Maruta: Bending and Shear Properties of Beam Members Using High Tough Fiber Reinforced Cement Composite Material Part 2 Consideration of Experimental Results, Japan Architectural Institute of Japan Conference Academic Lecture Abstracts (Kanto), Structure IV, pp. 313-314, September 2001 ”. In FIG. 2, X _n : the position of the neutral axis, C _c : the compressive strength of the SFRC, C _s : the compressive strength of the reinforcing bar, β ₁ : the stress block coefficient, T _s : the tensile strength of the reinforcing _{bar, T c} : the tensile strength of the SFRC. , Σ _B : compressive strength of SFRC, σ _t : split strength of SFRC.

The stress distribution of concrete on the side receiving compressive force is generally a parabolic type distribution, but in order to simplify the calculation, a rectangle (stress block) equivalent to the parabolic type distribution is considered. According to the commonly used stress block model, the compressive force C ′ of concrete is calculated based on the _{following formula C ′ = 0.85 f c} ′ × 0.8 _{X n × b.} 0.8 is the stress block coefficient. However,'σ _B is the compressive strength of concrete.

In the calculation of the stress block of SFRC of the present embodiment, the stress block coefficient: β ₁ is set according to the compressive strength of concrete. The stress block coefficient is β ₁ = 0.85-0.51 (σ _B -27.5) / 70 _{when σ B} ≤ 27.5 N / mm ^2. When σ _B > 27.5 N / mm ² , β ₁ = 0.65. However, σ _B is the compressive strength of SFRC.

In this calculation, the strength on the tensile side of the SFRC is added (see FIG. 2B). It is considered that 25% of the split strength of SFRC is uniformly distributed on the tensile side of the SFRC member _{in addition to the tensile strength of the reinforcing bar: T s.}

Returning to Fig. 4, _cal Q _b <V _u > is the yield strength shown in Non-Patent Document 3 “Architectural Institute of Japan: Guarantee-type Seismic Design Guideline for Reinforced Concrete Buildings, Explanation, 1999”. This is the calculated ultimate shear strength calculated according to the formula. _cal Q _b <V _bu > is an adhesion splitting proof stress calculated value calculated based on the proof stress formula shown in Non-Patent Document 3. The calculated proof stress values are all shown in terms of beam shear force.

The maximum proof stress of each test piece C tended to increase as the amount of steel fibers increased. The rate of increase in maximum proof stress tended to be larger in the NS and HS series test specimens C than in the NF and HF series test specimens C.

Next, the maximum proof stress experimental value and the proof stress calculated value of the test body C formed by a plurality of types are compared. Among the bending yield leading type NF and HF series test bodies C, the ultimate bending strength of the test bodies C of NF05, NF10, HF05 and HF10 formed by SFRC is evaluated based on the following steps.

Deformation is given to the SFRC test piece C. Measure the deformation that occurs on the test piece. Based on the restoring force characteristics obtained from the experiment, the maximum proof stress experimental value of the test piece C is confirmed. By using a stress block in which the stress of SFRC is uniformly distributed on the compression side and adding this assuming that 25% of the split strength of SFRC is uniformly distributed on the tension side, a margin equivalent to that of the RC specimen is obtained. Can be evaluated at.

As described above, according to the concrete evaluation method, the ultimate bending strength of the SFRC test piece C is that the SFRC on the side receiving the compressive force splits the stress block of the concrete according to the ACI standard, and the SFRC splits the SFRC into the SFRC on the side receiving the tensile force. Assuming that 25% of the strength is uniformly distributed in the cross section on the tension side, it can be evaluated with a margin equivalent to that of the RC test piece. According to this evaluation method, the maximum yield strength of building members can be evaluated.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned one embodiment, and can be appropriately modified without departing from the spirit of the present invention. For example, the concrete evaluation method may be applied not only to steel fiber reinforced concrete but also to fiber reinforced concrete mixed with other fibers.

C Specimen E Bending Shear Forcer T1, T2, T1', T2' Support Point V Reinforcing Bar W Skeleton Curve X History Loop

Claims (2)

In the construction method of constructing a structure using a reinforced concrete member (SFRC member) mixed with steel fibers, there is a method of evaluating the ultimate bending strength of the reinforced concrete member mixed with steel fibers.
For the ultimate bending strength of the SFRC member, a stress block in which the SFRC stress is uniformly distributed on the compression side is used, and this is added assuming that the SFRC split strength is uniformly distributed on the tension side.
The stress block is calculated by using the following formula (1) or (2) of the stress block coefficient according to the compressive strength of the SFRC used for the SFRC.
A method for evaluating the ultimate bending strength of a reinforced concrete member mixed with steel fibers.
Equation (1): In the case of σ _B ≤27.5 N / mm ² , β ₁ = 0.85-0.51 (σ _B -27.5) / 70
Equation (2): In the case of σ _B > 27.5 N / mm ² , β ₁ = 0.65
However,
σ _B : Compressive strength of SFRC The stress uniformly distributed in the SFRC member on the tension side is 25% of the SFRC split strength of the SFRC member.
The method for evaluating the ultimate bending proof stress of a reinforced concrete member mixed with steel fibers according to claim 1.

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