JP6765485B2 - How to measure the cracking strength of ultra-high strength fiber reinforced concrete - Google Patents

Description

The present invention relates to a method for measuring the crack generation strength of ultra-high-strength fiber-reinforced concrete with high accuracy, and a specimen of ultra-high-strength fiber-reinforced concrete used in the method.

When the ultra-high strength fiber reinforced concrete receives a uniaxial tensile force, linear elasticity is established in the initial stage when the load is small, but linear elasticity is not established when the load is further increased and cracks occur. Therefore, in Non-Patent Document 1, the stress at the point where the linear elasticity is not established on the graph showing the relationship between the tensile stress (load) and the strain is defined as the crack generation strength, and the tensile strength which is the maximum stress after the crack occurs. Is distinguished from. Incidentally, Non-Patent Document 1 defines that the crack generation strength (design value) of ultra-high strength fiber reinforced concrete is 8 N / mm ² and the tensile strength (design value) is 8.8 N / mm ² .
The method for measuring the crack generation strength includes a direct tensile strength test, a split tensile strength test (FIG. 1, FIG. 3.2.4 of Non-Patent Document 1), and a bending strength test. Then, as a general rule, the measuring method is selected from these test methods in consideration of the state of tensile stress assumed for the structure to be measured. However, although the direct tensile strength test is general, the test method has not been standardized. Further, the split tensile strength test is preferably performed in accordance with JIS A 1113 "Concrete split tensile strength test method", which is a concrete test method.

However, when the split tensile strength test of concrete with a tensile strength of about 3 N / mm ² is used to measure the crack generation strength of ultra-high strength fiber reinforced concrete with a significantly high tensile strength of 8.8 N / mm ² , cracks occur. The variation in strength becomes large and the measurement accuracy decreases.

Japan Society of Civil Engineers Concrete Committee, edited by Ultra High Strength Fiber Reinforced Concrete Research Subcommittee, "Design and Construction Guidelines for Ultra High Strength Fiber Reinforced Concrete (Draft)", pp. 11-12, Japan Society of Civil Engineers, September 28, 2004 Issued daily

Therefore, an object of the present invention is to provide a method for measuring the crack generation strength of ultra-high-strength fiber-reinforced concrete with high accuracy, and a specimen of ultra-high-strength fiber-reinforced concrete used in the method.

As a result of various studies on means for reducing the variation in crack generation strength of ultra-high strength fiber reinforced concrete, the present inventor (i) has smooth both ends of a cylindrical specimen used as a specimen for measuring crack generation strength. , (Ii) The present invention was completed by finding that the variation in the measured value of the crack generation strength can be reduced if there is no gap between the upper and lower pressure plates of the testing machine and the test piece.
That is, the present invention is a method for measuring the crack generation strength of ultra-high-strength fiber-reinforced concrete having the following configurations, and a specimen of ultra-high-strength fiber-reinforced concrete.

[1] The load is loaded on a cylindrical specimen that has undergone the following steps (A) to (D), the relationship between the load and strain is obtained, and the load at the point where linear elasticity is not established in the relationship between the load and strain is supported. A method for measuring the cracking strength of ultra-high-strength fiber-reinforced concrete, in which the tensile stress to be applied is defined as the cracking strength.
(A) Thickness of the two end faces of the columnar specimen of ultra-high strength fiber reinforced concrete so that the average value of the height difference of the unevenness on the two end faces of the columnar specimen is 1.0 mm or less per one end face. Polishing step of a cylindrical specimen to be polished by (length) by 5 to 10 mm (B) A portion where the polished cylindrical specimen is placed sideways on a flat plate and the flat plate and the side surface of the cylindrical specimen come into contact with each other. In addition, the first confirmation work to confirm whether or not there is a gap, and when it is confirmed that there is no gap, the side surface of the cylindrical specimen located 180 ° rotated from the side surface of the cylindrical specimen. , Again, place it on the flat plate and perform the second confirmation work to confirm whether there is a gap in the part where the flat plate and the side surface of the cylindrical specimen come into contact, and in any of the confirmation operations. A step of selecting a measurement target, in which a cylindrical specimen that has been confirmed to have no gap is selected as a measurement target for crack generation strength (C) orthogonal to the surface connecting the two sides that have been confirmed to have no gap. The strain gauge is attached to the center of the two end faces of the selected cylindrical specimen. (D) The two sides confirmed that the gap is not formed are the upper and lower sides of the compression tester. The step of placing the cylindrical specimen on the compression tester so that it comes into contact with the pressure plate of the above, however, within the range of the load up to 50 kN.
(i) When the strains on the two end faces are opposite (asymmetric) between positive and negative (asymmetric)
(ii) If there is no strain on one end face
(iii) When any of the cases where the strain of one end face becomes 1.5 times or more the strain of the other end face occurs, the loading is interrupted and the load is removed, and then the above (B) to (D) The process is re-executed, and loading is performed again.
[2] When the characteristic value selected through the following steps (a) and (b) exceeds the threshold value, the loading is interrupted and the load is removed, and then the steps (B) to (D) are repeated. Then, the method for measuring the crack generation strength of the ultra-high strength fiber reinforced concrete according to the above [1], wherein the load is performed again.
(A) Within the range of strain up to 100 × ^10-6 , the proportional relationship between the load (explanatory variable) and strain (objective variable) is determined for each end face using the measured values of the load and the strain of the two end faces. Simple regression analysis step of obtaining the two simple regression equations to be represented (b) Comparison of the inclination values in the two simple regression equations and selecting the larger value as the characteristic value [3] ] The crack generation strength of the ultra-high strength fiber reinforced concrete according to the above [1] or [2], wherein the ultra-high strength fiber reinforced concrete having a crack generation strength of 5 N / mm ² or more is the target for measuring the crack generation strength. Measuring method.

The method for measuring the crack generation strength of the ultra-high strength fiber reinforced concrete of the present invention has high measurement efficiency and can reduce the variation in the measured value of the crack generation strength, so that the measurement accuracy is high.

It is a figure which shows an example of the split tensile test using a cylindrical specimen (reprinted FIG. 3.2.4 of page 11 of Non-Patent Document 1). It is a photograph which shows the state of the upper surface (driving surface) of a cylindrical specimen. It is a photograph which shows the gap between a flat plate and a side surface of a cylindrical specimen. It is a photograph shining light between the flat plate and the side surface of the cylindrical specimen. This is a photograph in which a feeler gauge is inserted between the flat plate and the side surface of the cylindrical specimen. It is a photograph showing the scraping line on the sandpaper generated when the cylindrical specimen is placed sideways on the sandpaper laid on the flat plate and moved back and forth along the length direction of the cylindrical specimen. It is a photograph which shows the state which there is no gap between a flat plate and a side surface of a cylindrical specimen. It is a photograph showing a state in which a line is drawn on the end face of the cylindrical specimen, which is confirmed to have no gap between the flat plate and the side surface of the cylindrical specimen. It is a photograph which shows the straight line which connects the side surface which passes through the center of the end face of the columnar specimen, and the gap is not confirmed, and the straight line which is orthogonal to the straight line. It is a photograph which shows the state which attached the strain gauge to the center of the end face of a cylindrical specimen. It is a photograph which shows the state which the cylindrical specimen is placed on the compression tester so that it comes into contact with the upper and lower pressure plates of a compression tester respectively. It is a graph which shows the relationship (linear elasticity) of a load and strain at the initial stage of loading. The "gauge" in the legend means a strain gauge. It is a graph which shows the relationship between the load and strain in the whole period of loading.

1. 1. Method for measuring crack generation strength of ultra-high-strength fiber-reinforced concrete The method for measuring crack-generating strength of ultra-high-strength fiber-reinforced concrete of the present invention is as described above: (A) polishing step of columnar specimen, (B) measurement target. The load and strain are obtained by loading on the columnar specimen after undergoing the sorting step, (C) the strain gauge pasting step, and (D) the columnar specimen mounting step, and the relationship between the load and strain. In this method, the tensile stress corresponding to the load at the point where the linear elasticity is not established is defined as the crack generation strength and measured. In the present invention, the ultra-high-strength fiber-reinforced concrete is a concept including an ultra-high-strength fiber-reinforced mortar in addition to concrete.
Hereinafter, the present invention will be described in detail with reference to FIGS. 2 to 11 in each of the above steps.

(A) Polishing Step of Cylindrical Specimen In this step, the average value of the height difference of the unevenness between the two end faces of the columnar specimen of ultra-high strength fiber reinforced concrete is 1.5 mm or less per one end face. It is a process.
Since the upper surface (driving surface) and lower surface (bottom surface) of the columnar specimen of ultra-high-strength fiber-reinforced concrete have irregularities, the load tends to be uneven, and the crack generation strength varies widely. Therefore, in order to smooth the end faces, the two end faces are polished. Considering that the steel fibers and excess pile on the upper surface may be burrs, especially the upper surface may have many air bubbles (Fig. 2), and the lower surface may be distorted near the bottom surface of the formwork. Then, the thickness (length) to be polished is preferably 5 to 10 mm per one end surface. Further, the average value of the height difference of the unevenness between the two polished end faces is preferably 1.5 mm or less per one end face. The average value of the height difference is more preferably 1.0 mm or less, still more preferably 0.5 mm or less per one end surface. The height difference can be measured using a caliper or the like. Further, as a polishing apparatus, a concrete end face forming machine can be mentioned.
The columnar specimen of ultra-high strength fiber reinforced concrete may be produced in accordance with JIS A 1113 “Concrete split tensile strength test method”. When manufacturing the specimen, the impact and vibration of the formwork surface using a mallet or the like, which is performed for the purpose of reducing the entrapped air on the formwork surface, should be minimized so that the fibers in the concrete do not settle. stop.

(B) Selection Step for Measurement Target In this step, the polished cylindrical specimen is placed sideways on a flat plate, and a portion where the flat plate and the side surface of the cylindrical specimen are in line contact (hereinafter referred to as "contact line"). The first confirmation work to confirm whether or not there is a gap in), and if it is confirmed that there is no gap, the side surface of the cylindrical specimen located 180 ° rotated from the side surface of the cylindrical specimen. , Again, the cylindrical specimen was placed on a flat plate and the second confirmation work was performed to confirm whether or not there was a gap in the contact line, and it was confirmed that no gap was generated in any of the confirmation operations. Is a step of selecting as a measurement target of the crack generation strength.
When the polished cylindrical specimen is placed sideways on a flat plate, a gap may be formed in the contact line (FIG. 3). The presence of this gap can be determined by shining light between the flat plate and the side surface of the cylindrical specimen (Fig. 4), inserting a feeler gauge (Fig. 5), or on sandpaper laid on the flat plate. This can be confirmed by the presence or absence of continuity of the scraping lines on the sandpaper that occur when the cylindrical specimen is placed sideways and moved back and forth along the length direction of the cylindrical specimen. That is, if there is no light leakage, the feeler gauge cannot be inserted, or the scraping lines on the sandpaper are continuous (FIG. 6), it can be confirmed that there are no gaps in the contact lines (FIG. 7).
When a gap is confirmed, the cylindrical specimen is rotated a little until a side surface where the gap is not confirmed is found, and the gap confirmation work (first confirmation work) is repeated for another side surface.
When a side surface in which no gap is confirmed is found, the gap confirmation work (second confirmation work) is performed on the side surface at a position where the cylindrical specimen is rotated 180 ° from the side surface. Then, in both the first and second confirmation operations, the first and second confirmation operations are repeated until a side surface on which no gap is confirmed is found. In a cylindrical specimen produced by using a mold having a seam in the length direction, the side surface portion corresponding to the seam of the specimen has a large gap and a large gap. Therefore, the side surface portion is preferably from the target of the confirmation work. except.
Then, in the first and second confirmation operations, when a side surface in which no gap is confirmed is found, the columnar specimen is selected as a measurement target of the crack generation strength, and for example, in the subsequent step (C). , In order to clearly display the position and direction to which the strain gauge is attached, a straight line connecting two side surfaces (FIG. 8) through the center of the end face of the cylindrical specimen and no gap is confirmed, and a straight line orthogonal to the straight line. Fill in the two straight lines (Fig. 9).

(C) Strain gauge affixing step In this step, a strain gauge is affixed to the center of the two end faces of the selected cylindrical specimen so as to be orthogonal to the surface connecting the two side surfaces confirmed that no gap is formed. (Fig. 10).

(D) Placement Step of Cylindrical Specimen In this step, the cylindrical specimen was subjected to a compression test so that the two sides confirmed that no gap was formed were in contact with the upper and lower pressure plates of the compression tester. This is a process of mounting on a machine (Fig. 11).
Next, in the present invention, two types of methods for further suppressing the variation in crack generation strength will be described in detail.

2. Suppression of variation in crack generation strength (Part 1)
After going through the steps (A) to (D), the load and strain are measured by loading on the cylindrical specimen, and the load is within a range of up to 50 kN.
(i) When the strains on the two end faces are opposite (asymmetric) between positive and negative (asymmetric)
(ii) If there is no strain on one end face
(iii) When any of the cases where the strain of one end face is 1.5 times or more the strain of the other end face occurs, the measured value of the crack generation strength becomes large, so it is preferable to load the load. After interrupting and unloading, the above steps (B) to (D) are repeated, and loading is performed again. Within the load range of up to 50 kN, repeated loading does not affect the obtained cracking strength value.
The ultra-high-strength fiber-reinforced concrete may be loaded onto a cylindrical specimen in accordance with JIS A 1113 “Concrete Split Tensile Strength Test Method”. In addition, it is preferable to continuously measure the load and strain because it is possible to clearly understand that the linear elasticity is not established due to the occurrence of cracks.

3. 3. Suppression of variation in crack generation strength (Part 2)
Further, in the present invention, in order to suppress the variation in the crack generation strength, for example, as shown in FIG. 12 (1), the characteristic values selected through the following steps (a) and (b) set the threshold value. When the load is exceeded, the loading is interrupted and the load is removed, and then the above steps (B) to (D) are repeated and the loading is performed again.
(A) Within the range of strain up to 100 × ^10-6 , the proportional relationship between the load (explanatory variable) and strain (objective variable) is determined for each end face using the measured values of the load and the strain of the two end faces. Simple regression analysis step to obtain two simple regression equations to represent (Fig. 12)
Here, the load is set as the explanatory variable (horizontal axis) and the strain is set as the objective variable (vertical axis) because the strain is measured while being loaded, and the graph is easier to see when the load is set on the horizontal axis.
(B) Selection step of characteristic value in which the slope values in the two simple regression equations are compared and the larger value is selected as the characteristic value. Here, the value with the larger slope is selected as the characteristic value. The reason for this is that, from experience, the larger the slope (the smaller the tensile Young's modulus), the lower the crack generation strength (that is, on the safe side) can be evaluated.
In the range of strain up to 100 × ^10-6 , repeated loading does not affect the obtained value of crack generation strength.

Next, the threshold value will be described.
(1) Concept of threshold The crack generation strength of ultra-high strength fiber reinforced concrete is the load (marginal stress) at the point where linear elasticity is not established in the relationship between load (tensile stress) and strain, as described above. ..
In the first place, concrete that has developed sufficient strength exhibits mechanical behavior close to that of an elastic body, so that the relationship between load and strain is linear, similar to steel materials.
Here, if the concrete is a completely elastic body, the behavior of strain with respect to compressive stress and tensile stress is the same. However, since concrete is not a perfect elastic body, the compressive strain and tensile strain are not the same for the same stress, and the tensile Young's modulus is smaller than the compressive Young's modulus (this means that it hardens and is sufficiently strong. It is generally confirmed in the developed concrete, except for the phenomenon at the young age.) For example, in high-strength concrete having a water-bonding material ratio of 19 to 25%, the tensile Young's modulus is about 0.8 times the compressive Young's modulus.
Therefore, in order to determine the threshold value used in the present invention, the following assumptions are made.
(I) compression Young's modulus of the ultra high strength fiber reinforced concrete was subjected to standard heat curing (FM) and ultra high strength fiber-reinforced concrete (FO) (Ec), respectively, assuming 50 kN / mm ² and 45 kN / mm ² To do. The FM and FO are part numbers of the ultra-strong advanced fiber reinforced concrete "Dactal" (registered trademark) described later.
(Ii) The tensile Young's modulus (Et) is assumed to be 0.8 times the compressive Young's modulus.
(Iii) It is assumed that the member safety factor (γ) is 1.3.

(C2) Determination of threshold value Based on these assumptions, the relational expression (1) between the compression Young's modulus (Ec) and the tensile Young's modulus (Et) can be derived.
Et = 0.8 × Ec / γ = 0.615 × Ec ・ ・ ・ (1)
On the other hand, the relational expression (2) between tensile stress (σ) and strain (ε) is
σ ＝ Et × ε ・ ・ ・ (2)
The relational expression (3) between the tensile stress (σ) and the load (P) is
σ = 2P / (π × d × L) ・ ・ ・ (3)
However, in Eq. (3), d represents the diameter of the end face of the cylindrical specimen, and L represents the distance (length) between the two polished end faces.
Therefore, the relational expression (4) between strain and load can be obtained from the above equations (1) to (3).
ε = 2 / (0.615 × Ec × π × d × L) × P ・ ・ ・ (4)
Wherein (4) the coefficient a (slope) 2 / (0.615 × Ec × π × d × L) of formula, Ec = 50kN / mm ² or ^{45kN / mm 2, d = 100mm} , and L = 200 mm Substituting, it becomes 1.04 for ultra-high strength fiber reinforced concrete (FM) and 1.17 for ultra-high strength fiber reinforced concrete (FO). Here, as described above, from experience, the larger the inclination, the lower the crack generation strength can be evaluated (that is, on the safe side), so the second decimal place is rounded up to 1.1 and 1.2, respectively. Is adopted as the threshold value.

3. 3. Determining the crack generation strength based on the relationship between load and strain And, as shown in (2) of FIG. 12, when the characteristic value selected through the steps (a) and (b) is equal to or less than the threshold value, the said. Using the measured values of the total loading period of the load and strain using a part of the load in the step (a), for example, a graph (FIG. 13) is created with the load on the vertical axis and the strain on the horizontal axis. The value of the load (P) at the point where the linear elasticity is not established is read from, and substituted into the above equation (3), and the crack generation strength which is the tensile stress (σ) corresponding to the load at the point where the linear elasticity is not established is obtained. To get.

2. Specimen of ultra-high-strength fiber reinforced concrete The specimen is a cylindrical specimen in which the average value of the height difference of the unevenness on the two end faces of the specimen is 1.5 mm or less per one end face. When the average value of the height difference of the unevenness is 1.5 mm or less, the variation in the crack generation strength can be reduced. As described above, the average value of the height difference is preferably 1.0 mm or less, more preferably 0.5 mm or less.
In addition, for the columnar specimen of ultra-high strength fiber reinforced concrete, except that one layer of concrete is cast at the time of casting, "Reference Material 2 How to make a specimen for strength test" described on page 80 of Non-Patent Document 1. Produce according to.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples.
1. 1. Superstrong Advanced Fiber Reinforced Concrete Using the materials used and the concrete mix is Ductal FM and Ductal FO (registered trademark, Pacific Ocean manufactured Cement Co.), and the unit quantity of water is Ductal FM 180 kg / ^{m 3,} Ductal FO is 185 kg / ^{m 3} And said.

2. Measurement of cracking strength of ultra-high-strength fiber reinforced concrete (1) Preparation of specimen using Ductal FM The specimen is 80 of Non-Patent Document 1 except that one layer of concrete was cast at the time of casting. A columnar specimen having a diameter of 100 mm and a length of 200 mm was prepared using a steel formwork according to "Reference Material 2 How to Make a Specimen for Strength Test" described on the page.
In addition, concrete was continuously poured into the formwork in one layer so as not to entrap entrapped air as much as possible, and vibration compaction was not performed. Further, the concrete was cured in the air for 24 hours, then demolded, and then steam-cured at 90 ° C. for 48 hours to prepare a cylindrical specimen (FIG. 2).

(2) Polishing of a cylindrical specimen Next, the two end faces of the specimen were polished by about 5 mm on both end faces using a concrete end face molding machine, and the average value of the height difference of the unevenness on the two end faces was calculated. One end surface was set to 1 mm.

(3) Selection of measurement target The polished cylindrical specimen was placed sideways, and light was applied from the opposite side to confirm whether or not there was a gap in the contact line (Fig. 4). After confirming that no gap is formed, the side surface of the cylindrical specimen located 180 ° rotated from the side surface of the cylindrical specimen is placed on the flat plate again, and no gap is generated in the contact line. It was confirmed by shining light on it. Then, the columnar specimens confirmed that no gap was generated in any of the confirmation operations were selected as the measurement target of the crack generation strength (FIG. 7).

(4) Attachment of strain gauge Further, two straight lines, a straight line connecting two side surfaces through the center of the end face of the cylindrical specimen and no gap is confirmed, and a straight line orthogonal to the straight line were drawn (FIG. 9). Then, the strain gauge 1 and the strain gauge 2 were attached in parallel with the orthogonal straight lines at the centers of the two end faces of the selected cylindrical specimen (FIG. 10).

(5) Placement of the cylindrical specimen The cylindrical specimen is placed in the compression tester (model number: ACA) so that the two sides confirmed that no gap is formed come into contact with the upper and lower pressure plates of the compression tester. It was placed on -100A, manufactured by Maekawa Testing Machine Mfg. Co., Ltd. (Fig. 11).

(6) Measurement of cracking strength In accordance with JIS A 1113 "Split tensile strength test method for concrete", the load is placed on a columnar specimen placed on a compression tester, the relationship between load and strain is determined, and the load is obtained. The crack generation strength was obtained from the tensile stress corresponding to the load at the point where the linear elasticity was not established in relation to the strain.
When the load is within the range of up to 50 kN, (i) the strains on the two end faces are opposite (asymmetric) to positive and negative, (ii) no strain occurs on one end face, or (iii). ) If the strain on one end face is 1.5 times or more the strain on the other end face, the loading is interrupted and the load is removed, and then the steps (3) to (5) above are repeated. , The load was carried out again (re-implementation 1).
Further, even when the characteristic value obtained in the measurement of the strain in the range of 100 × ^10-6 exceeds the threshold value (1.1) adopted in paragraph 0018, the loading is interrupted and the load is removed, and then the above ( The steps 3) to (5) were re-executed, and loading was performed again (re-implementation 2).
Table 1 shows the values of crack generation strength of each cylindrical specimen of Dactal FM. Table 1 also shows whether or not the re-implementation 1 or re-execution 2 has been carried out.

As a comparison, the tilt and crack generation strength of the simple regression equation were determined for the cylindrical specimen prepared in paragraph 0022 according to the method described in Non-Patent Document 1 (conventional method). Table 2 shows the obtained crack generation strength. Table 2 shows the presence or absence of any one of the following (i) to (iii) within the range of the load up to 50 kN.
(i) The strains on the two end faces are opposite (asymmetric) between positive and negative.
(ii) No strain was generated on one end face.
(iii) The strain on one end face is more than 1.5 times the strain on the other end face.
In addition, Table 2 also shows the value of the characteristic value obtained in the measurement of strain in the range of 100 × ^10-6 and the determination of the necessity of re-execution 2.

Next, Table 3 shows the average value, standard deviation, and coefficient of variation of the crack occurrence intensity calculated based on the data in Tables 1 and 2.

The inclination and crack generation strength of the simple regression equation were determined by the same test method as described above except that the ductal FO was used instead of the ductal FM. The measurement results of the crack generation strength are shown in Tables 4 and 5. Table 6 shows the average value, standard deviation, and coefficient of variation of the crack occurrence strength calculated based on the data in Tables 4 and 5.

(7) Variation in cracking strength As shown in Tables 3 and 6, the coefficient of variation of both Dactal FM and Dactal FO is as small as 1/7 to 2/3 as compared with Comparative Examples in all the examples. Therefore, in the method for measuring the crack generation strength of the present invention, the variation in the crack generation strength is remarkably small as compared with the conventional method.
From the above, the present invention can efficiently measure the crack generation strength of ultra-high strength fiber reinforced concrete with high accuracy.

Claims (3)

The load is placed on a columnar specimen that has undergone the following steps (A) to (D), the relationship between the load and strain is obtained, and the tensile stress corresponding to the load at which linear elasticity is not established in the relationship between the load and strain. Is a method for measuring the cracking strength of ultra-high-strength fiber-reinforced concrete, which is determined as the cracking strength.
(A) Thickness of the two end faces of the columnar specimen of ultra-high strength fiber reinforced concrete so that the average value of the height difference of the unevenness on the two end faces of the columnar specimen is 1.0 mm or less per one end face. Polishing step of a cylindrical specimen to be polished by (length) by 5 to 10 mm (B) A portion where the polished cylindrical specimen is placed sideways on a flat plate and the flat plate and the side surface of the cylindrical specimen come into contact with each other. In addition, the first confirmation work to confirm whether or not there is a gap, and when it is confirmed that there is no gap, the side surface of the cylindrical specimen located 180 ° rotated from the side surface of the cylindrical specimen. , Again, place it on the flat plate and perform the second confirmation work to confirm whether there is a gap in the part where the flat plate and the side surface of the cylindrical specimen come into contact, and in any of the confirmation operations. A step of selecting a measurement target, in which a cylindrical specimen for which it has been confirmed that no gap has not been formed is selected as a measurement target for the crack generation strength (C). A strain gauge affixing step (D) in which a strain gauge is affixed to the center of two end faces of the selected cylindrical specimen so that the gap is not formed, the upper and lower sides of the compression tester are The step of placing the cylindrical specimen on the compression tester so that it comes into contact with each of the pressure plates of the above, however, within the range of the load up to 50 kN.
(i) When the strains on the two end faces are opposite (asymmetric) between positive and negative (asymmetric)
(ii) If there is no strain on one end face
(iii) When any of the cases where the strain of one end face becomes 1.5 times or more the strain of the other end face occurs, the loading is interrupted and the load is removed, and then the above (B) to (D) The step is re-executed and the loading is performed again. When the characteristic value selected through the following steps (a) and (b) exceeds the threshold value, the loading is interrupted and the load is removed, and then the steps (B) to (D) are repeated. The method for measuring the crack generation strength of the ultra-high-strength fiber-reinforced concrete according to claim 1, wherein the load is carried out again.
(A) Within the range of strain up to 100 × ^10-6 , the proportional relationship between load (explanatory variable) and strain (objective variable) is determined for each end face using the measured values of load and strain of the two end faces. Simple regression analysis step of obtaining the two simple regression equations to be represented (b) Selection step of the characteristic value in which the slope values in the two simple regression equations are compared and the larger value is selected as the characteristic value. The method for measuring the crack generation strength of the ultra-high strength fiber reinforced concrete according to claim 1 or 2, wherein the ultra-high strength fiber reinforced concrete having a crack generation strength of 5 N / mm ² or more is the target for measuring the crack generation strength.