Method for estimating tensile toughness of steel fiber reinforced concrete and method for manufacturing steel fiber reinforced concrete members

Abstract

To propose a method for estimating tensile toughness of steel fiber reinforced concrete that enables quantitative evaluation of the tensile toughness of steel fiber reinforced concrete based on limited information obtained at a design stage, and a method for manufacturing concrete members that satisfy required quality with respect to the tensile toughness.SOLUTION: An embodiment of the present invention includes: a process of calculating an average tensile toughness property value Ft for design basis strength by substituting the design basis strength Fc into an Equation 1; and a process of estimating a tensile toughness property value Ftd for steel fiber reinforced concrete for structural design by substituting the average tensile toughness property value Ft, a coefficient of variation of compressive strength Vc and the coefficient of variation of tensile toughness property value Vt into Equation 2, where Ft=α×Fc+β...Equation 1, Ftd=Ft×(1+1.73×Vc) / (1+1.73×Vt)...Equation 2.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for estimating the tensile toughness of steel fiber reinforced concrete and a method for manufacturing a steel fiber reinforced concrete member. [Background technology]

[0002] Steel fiber reinforced concrete is expected to have improved toughness due to the incorporation of steel fibers into the concrete. In addition, by forming both ends of the steel fibers into a hook shape, it is possible to obtain high toughness with a small amount of steel fiber mixed in, thereby improving material costs and workability. The toughness of steel fiber reinforced concrete varies depending on the characteristics and amount of steel fiber mixed in, the strength properties of the concrete, and the adhesive properties between the steel fiber and concrete. If the toughness of steel fiber reinforced concrete can be properly understood, it will be possible to design concrete structures more rationally. In general-purpose concrete, mechanical properties other than compressive strength (such as Young's modulus) that are required for structural calculations are evaluated in relation to compressive strength. Therefore, in quality control of general concrete, by confirming that the compressive strength meets the required quality, it is possible to evaluate that the structure satisfies the specified performance. On the other hand, in the case of steel fiber reinforced concrete, while the compressive strength is almost the same as that of general-purpose concrete, the tensile toughness improves significantly with the amount of steel fiber mixed in, so it is not appropriate to evaluate tensile toughness based only on compressive strength. Therefore, if the effect of improved toughness is to be incorporated into structural calculations, it is necessary to confirm not only the compressive strength but also the toughness through testing.

[0003] Patent Document 1 discloses a method for estimating the crack initiation load based on the acoustic emission generated when a load is applied to a specimen made of high-strength fiber-reinforced concrete in accordance with JIS A 1113 (Test method for splitting tensile strength of concrete). However, the estimation method in Patent Document 1 does not estimate tensile toughness, which evaluates behavior after cracks occur. As a test method for evaluating the toughness of fiber reinforced concrete, a three-point bending load test may be performed using a rectangular column-shaped specimen made from the same steel fiber reinforced concrete as the member, in accordance with JCI-S-002-2003 (Test method for load-displacement curve of fiber reinforced concrete using notched beams). At the design stage, it is assumed that the manufacturing plant of the steel fiber reinforced concrete, detailed mix conditions, and tensile toughness test results of the steel fiber reinforced concrete itself to be applied are not clear. Therefore, it is desirable to be able to evaluate the tensile toughness based on the limited information obtained at the design stage. However, a design method for reflecting the toughness of steel fiber reinforced concrete has not been established. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-223761 A Summary of the Invention [Problem to be solved by the invention]

[0005] An objective of the present invention is to propose a method for estimating the tensile toughness of steel fiber reinforced concrete that makes it possible to quantitatively evaluate the tensile toughness of steel fiber reinforced concrete based on the limited information available at the design stage, and a method for manufacturing steel fiber reinforced concrete members that meet the required quality regarding tensile toughness. [Means for solving the problem]

[0006] In this specification, the terms used to describe the material properties of concrete are defined as follows. Tensile toughness is a material property that indicates the relationship between the tensile load and deformation properties of concrete. The tensile toughness characteristic value is defined as the maximum value of tensile stress after cracks occur, or the average value of tensile stress up to a crack width of X mm (X is any value between 0.5 and 2.0). The method for estimating the tensile toughness of steel fiber reinforced concrete of the present invention includes a step of substituting the design standard strength Fc of the steel fiber reinforced concrete into equation 1 to calculate an average tensile toughness characteristic value Ft for the design standard strength, and a step of substituting the tensile toughness characteristic value Ft, the coefficient of variation Vc of the compressive strength, and the coefficient of variation Vt of the tensile toughness characteristic value Ft into equation 2 to estimate the tensile toughness characteristic value Ftd of the steel fiber reinforced concrete to be used in structural design. Ft = α×Fc + β Equation 1 Ftd = Ft×( 1 + 1.73×Vc ) / (1 +1.73×Vt ) ··· Formula 2 Here, Ft is the average tensile toughness characteristic value for Fc [N / mm 2 ] Fc: Design strength of steel fiber reinforced concrete [N / mm 2 ] α: Coefficient determined by the type of steel fiber and its volume fraction Vsf β: Constant determined by the type of steel fiber and its volume fraction Vsf Ftd: Tensile toughness characteristic value of steel fiber reinforced concrete used in structural design [N / mm 2 ] Vc: Coefficient of variation of compressive strength (=0.10) Vt: Coefficient of variation of tensile toughness characteristic value (=0.20 to 0.30)

[0007] According to this method for estimating the tensile toughness of steel fiber reinforced concrete, it is possible to estimate the tensile toughness of steel fiber reinforced concrete without requiring extensive testing, by using the type and amount of steel fiber (volume ratio Vsf≦1%) and the compressive strength of steel fiber reinforced concrete (design standard strength Fc≦80N / mm 2 ) it is possible to quantitatively evaluate the tensile toughness of steel fiber reinforced concrete.

[0008] The coefficient of variation of the compressive strength and the coefficient of variation of the tensile toughness characteristic value may be extracted from a material database. The material database includes a compressive strength measured by performing a concrete compressive strength test in accordance with JIS A 1108 on a cylindrical specimen of steel fiber reinforced concrete, a tensile toughness characteristic value measured by performing a three-point bending test in accordance with JCI-S-002-2003 on a prismatic specimen of steel fiber reinforced concrete, a regression equation between the compressive strength and the tensile toughness characteristic, and the coefficient of variation of the compressive strength and the coefficient of variation of the tensile toughness characteristic value. According to this method for estimating the tensile toughness of steel fiber reinforced concrete, the coefficient of variation of the compressive strength and the coefficient of variation of the tensile toughness characteristic value are calculated based on the results of the compressive strength test on a cylindrical specimen and the results of the bending test using a rectangular specimen, so that the tensile toughness can be evaluated according to the degree of variation in the compressive strength and tensile toughness characteristic value.

[0009] In addition, the manufacturing method of a steel fiber reinforced concrete member of the present invention includes the steps of setting the mix proportion of the steel fiber reinforced concrete, extracting the coefficient of variation Vc of compressive strength and the coefficient of variation Vt of tensile toughness value, substituting the design standard strength Fc of the steel fiber reinforced concrete into equation 1 to calculate the average tensile toughness value Ft for the design standard strength Fc, substituting the tensile toughness value Ft, the coefficient of variation Vc of compressive strength and the coefficient of variation Vt of tensile toughness value into equation 2 to estimate the tensile toughness value Ftd of the steel fiber reinforced concrete to be used in structural design, and manufacturing a concrete member from steel fiber reinforced concrete having a mix proportion for which it has been confirmed that the tensile toughness value Ftd satisfies the required quality of the steel fiber reinforced concrete to be used in structural design. Ft = α×Fc + β Equation 1 Ftd = Ft×( 1 + 1.73×Vc ) / (1 +1.73×Vt ) ··· Formula 2 Here, Ft is the average tensile toughness characteristic value for Fc [N / mm 2 ] Fc: Design strength of steel fiber reinforced concrete [N / mm 2 ] α: Coefficient determined by the type of steel fiber and its volume fraction Vsf β: Constant determined by the type of steel fiber and its volume fraction Vsf Ftd: Tensile toughness characteristic value of steel fiber reinforced concrete used in structural design [N / mm 2 ] Vc: Coefficient of variation of compressive strength (=0.10) Vt: Coefficient of variation of tensile toughness characteristic value (= 0.20 to 0.30)

[0010] According to this method for manufacturing a concrete member, a steel fiber reinforced concrete member is manufactured after confirming that the tensile toughness (tensile toughness characteristic value) satisfies the required quality, so that a high-quality steel fiber reinforced concrete member can be manufactured. In addition, since there is no need to conduct large-scale tests, the tensile toughness can be easily estimated. Furthermore, if the tensile toughness of the test-mixed steel fiber reinforced concrete does not satisfy the required quality, the mix proportions of the steel fiber reinforced concrete are reconsidered to manufacture a steel fiber reinforced concrete with tensile toughness that satisfies the required quality. Even when reconsidering, the tensile toughness characteristic value Ftd can be estimated using Equation 2, so the effort required for correcting the mix proportion can be reduced. Effect of the Invention

[0011] According to the method for estimating the tensile toughness of steel fiber reinforced concrete of the present invention, the tensile toughness can be estimated with high accuracy without conducting large-scale tests. Also, according to the method for manufacturing a steel fiber reinforced concrete member of the present invention, a steel fiber reinforced concrete member that satisfies the required quality regarding tensile toughness can be manufactured. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 shows an example of steel fibers. [Diagram 2] 4 is a flowchart showing the steps of a method for manufacturing a concrete member. [Diagram 3] Graphs (a) and (b) show examples of the tensile toughness (tension softening curve) of steel fiber reinforced concrete. [Figure 4] 1 is a graph showing the relationship between two characteristic values ​​related to tensile toughness. [Diagram 5] 1 is a graph showing the relationship between compressive strength and tensile toughness characteristic value (maximum tensile stress value). [Figure 6] 1 is a graph showing the relationship between compressive strength and tensile toughness characteristic value (maximum tensile stress value) for each type of aggregate. [Figure 7] 1 is a graph showing the results of a comparison of tensile toughness (tension softening curve) between mixes with different coarse aggregate contents. [Figure 8] 1 is a graph showing the relationship between compressive strength and tensile toughness characteristic value (maximum tensile stress value) for each target air content. [Figure 9] 1 is a graph showing the quality fluctuation of the tensile toughness characteristic value (maximum tensile stress value) in actual production. [Figure 10] FIG. 1 is a conceptual diagram regarding the fluctuation of compressive strength and tensile toughness characteristic values ​​(maximum tensile stress value). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention relates to a method for estimating the tensile toughness of steel fiber reinforced concrete, and a method for manufacturing a steel fiber reinforced concrete member using the method for estimating tensile toughness. Steel fiber reinforced concrete has excellent toughness, and if its performance can be properly evaluated, more rational design of concrete structures becomes possible. In this embodiment, based on the limited information obtained at the design stage, toughness performance (tensile toughness) that can be reflected in the design is estimated, taking into account the safety factor against quality fluctuations during the manufacturing of steel fiber reinforced concrete, and after confirming whether the required quality regarding tensile toughness is satisfied, a steel fiber reinforced concrete member is manufactured. Specifically, the steel fiber volume mixing ratio Vsf is in the range of 0.5 to 1.0%, and the design strength Fc is 30 to 60 N / mm 2For steel fiber reinforced concrete within the range, the average tensile toughness is evaluated for the design standard strength Fc based on the relational equation between the compressive strength and tensile toughness of steel fiber reinforced concrete obtained in advance for each type of steel fiber and volumetric mixing rate Vsf. Furthermore, the tensile toughness of steel fiber reinforced concrete used in structural design is evaluated based on the relationship between the compressive strength and tensile toughness coefficient of variation of the steel fiber reinforced concrete at the time of manufacture obtained in advance and a safety factor for that. The steel fiber reinforced concrete to be evaluated in this embodiment has a compressive strength of 80 N / mm 2 The concrete is mixed with steel fibers having hook shapes at both ends at a ratio of 1% or less of the concrete volume. An example of steel fibers 1 is shown in Fig. 1. In this embodiment, as shown in Fig. 1, steel fibers 1 having hooks 11 at both ends are used.

[0014] The steps of the method for manufacturing a concrete member of this embodiment (method for estimating the tensile toughness of steel fiber reinforced concrete) are shown in Figure 2. As shown in Figure 2, the method for manufacturing a concrete member of this embodiment includes a mix setting process S1, an average tensile toughness calculation process S2, a coefficient of variation extraction process S3, a tensile toughness estimation process S4 for use in structural design, and a member manufacturing process S5. The mix setting step S1 is a process for setting a mix ratio of a concrete having a steel fiber volume ratio Vsf (the ratio of the volume of steel fiber to the volume of concrete) of 0.5 to 1.0% and a design strength Fc of 30 to 60 N / mm 2 This is the process of setting the mix ratio of steel fiber reinforced concrete within the range. A trial mix is ​​carried out in advance to set the mix ratio of steel fiber reinforced concrete that is thought to satisfy the required performance such as potential workability (fluidity, separation resistance, etc.), mechanical properties (compressive strength, tensile toughness, etc.), and durability.

[0015] The average tensile toughness calculation step S2 is a step of substituting the design strength Fc into Equation 1 to calculate an average tensile toughness characteristic value Ft for the design strength Fc. Ft = α×Fc + β Equation 1 Here, Ft is the average tensile toughness characteristic value for Fc [N / mm2 ] Fc: Design standard strength [N / mm 2 ] α: Coefficient determined by the type of steel fiber and its volume fraction Vsf β: Constant determined by the type of steel fiber and its volume fraction Vsf

[0016] The variation coefficient extraction step S3 is a step of extracting the variation coefficient Vc of the compressive strength and the variation coefficient Vt of the tensile toughness characteristic value. The variation coefficient Vc of the compressive strength and the variation coefficient Vt of the tensile toughness characteristic value are extracted from a material database. The material database includes a compressive strength measured by performing a concrete compressive strength test in accordance with JIS A 1108 on a cylindrical specimen of steel fiber reinforced concrete, a tensile toughness characteristic value measured by performing a three-point bending test in accordance with JCI-S-002-2003 on a rectangular specimen of fiber reinforced concrete, a regression equation between the compressive strength and the tensile toughness characteristic, and a variation coefficient calculated from the compressive strength and a variation coefficient calculated from the tensile toughness characteristic value.

[0017] The tensile toughness estimation process S4 for structural design is a process of substituting the average tensile toughness characteristic value Ft, the coefficient of variation of the compressive strength Vc, and the coefficient of variation of the tensile toughness characteristic value Vt into Equation 2 to estimate the tensile toughness characteristic value Ftd of the steel fiber reinforced concrete to be used in the structural design. Ftd = Ft×( 1 + 1.73×Vc ) / (1 +1.73×Vt ) ··· Formula 2 Where, Ftd: Tensile toughness characteristic value reflected in design [N / mm 2 ] Ft: Design standard strength Fc [N / mm 2 Average tensile toughness characteristic value [N / mm 2 ] Vc: Coefficient of variation of compressive strength (=0.10) Vt: Coefficient of variation of tensile toughness characteristic value (=0.20 to 0.30)

[0018] The member manufacturing process S5 is a process for manufacturing a concrete member. The concrete member is manufactured from steel fiber reinforced concrete according to a mix that has been confirmed to have a tensile toughness characteristic value Ftd that satisfies the required quality of steel fiber reinforced concrete used in structural design.

[0019] According to the manufacturing method of concrete members of this embodiment, steel fiber reinforced concrete members are manufactured after confirming that the tensile toughness (tensile toughness characteristic value) satisfies the required quality, so that high-quality steel fiber reinforced concrete members can be manufactured. In addition, the type and amount of steel fiber specified in the design documents (volume ratio Vsf≦1%), the compressive strength of steel fiber reinforced concrete (design standard strength Fc≦80N / mm 2 ) it is possible to quantitatively evaluate the tensile toughness of steel fiber reinforced concrete. If the tensile toughness does not satisfy the required quality, the mix proportions of the steel fiber reinforced concrete may be reconsidered to produce a steel fiber reinforced concrete having tensile toughness that satisfies the required quality. In addition, the coefficient of variation of the compressive strength and the coefficient of variation of the tensile toughness characteristic value were calculated based on the results of the compressive strength test on cylindrical specimens and the results of the bending test using rectangular prism specimens, so that the tensile toughness can be evaluated according to the degree of variation in the compressive strength and tensile toughness characteristic values.

[0020] The results of experiments carried out on the method for estimating tensile toughness according to this embodiment are shown below. Table 1 shows the materials used in the steel fiber reinforced concrete used in the experiment. For the fine aggregate, three types of mixed sand (S1, S2, S4) and natural sand (S3) were used that meet the regulations of the Architectural Institute of Japan's "Standard Specifications for Building Construction and Commentary JASS 5 Reinforced Concrete Construction." For the coarse aggregate, three types of crushed stone (G1, G2, G3) that meet the regulations of Crushed Stone 2005 of JIS A 5005 (Crushed Stone and Crushed Sand for Concrete) were used. For the fine aggregate S2 and the coarse aggregates G2 and G3, different manufacturing lots (S2a-S2d, G2a-G2c, and G3a-G3c) were used. Fine aggregate S2 is a mixture of natural sand and crushed sand, coarse aggregate G1 is a mixture of two types of crushed stone 2005, and coarse aggregate G3 is a mixture of crushed stone 2015 and crushed stone 1505.Since the surface dry density and mixing ratio of these materials before mixing were known, the surface dry density after mixing was calculated to three decimal places based on these.

[0021] [Table 1]

[0022] Since the purpose of this experiment was to confirm the effect of the concrete properties (concrete strength properties and bond properties with steel fibers) on the tensile toughness of steel fiber reinforced concrete, only one type of steel fiber was used. In general concrete, bleeding causes water films and voids to form under the horizontal reinforcing bars and coarse aggregate, which reduces the adhesive strength between the concrete and the horizontal reinforcing bars and coarse aggregate. In addition, by actively using chemical admixtures, the unit water content can be reduced to 175-185 kg / m 3 Therefore, the difference in unit water volume between mixes is small, but it is expected that bleeding and even differences in adhesion properties with steel fibers will occur depending on the type of aggregate and the composition ratio of fine and coarse aggregate. Also, air in concrete not only affects the strength properties of the concrete, but if it remains below the steel fibers, it is also expected to affect the adhesion properties with the steel fibers in the same way as bleeding. Considering the above, in this experiment, the steel fiber volume mixing ratio Vsf (0.5-1.0%) and compressive strength (38-81N / mm 2 ), aggregate type, unit coarse aggregate absolute volume Vg, and air content were used as variation factors.

[0023] Table 2 shows the mix conditions for the steel fiber reinforced concrete used in this experiment. The volumetric mix ratio Vsf of steel fiber was set to two levels: 0.5% and 1.0%. The combinations of fine and coarse aggregate were set to five patterns: "S1·G1", "S2·G2", "S2·G3", "S3·G3", and "S4·G3". In order to be able to evaluate the differences in trends due to the combinations, data was collected for each volumetric mix ratio Vsf in each pattern, or for two or more types of water-cement ratio W / C for any volumetric mix ratio Vsf. The target air content at the end of mixing is set at 2.5, 4.5 or 6.5%, and adjustment is basically made within a range of ±0.5%. However, since fine adjustment is difficult when manufacturing using an actual mixer, it is set at ±1.0%. The target air content at the end of mixing differs for each mix, but the calculated air content for each mix is ​​4.5% (the total volume of C, W, S, and G calculated from the unit amounts in Table 2 is 955 L / m 3 (so that

[0024] [Table 2]

[0025] For each mix, three cylindrical specimens measuring φ100×200mm or φ150×300mm were prepared, and after sealing and curing at 20℃ until the material was aged 28 days, a compressive strength test was conducted in accordance with JIS A 1108 (Method of testing compressive strength of concrete). In addition, three or four rectangular specimens measuring □150×530mm were prepared, and after sealing and curing at 20℃ until the material was aged 28 days, a notched beam three-point bending load test was conducted in accordance with JCI-S-002-2003. The tension softening curve was calculated using the load-crack opening displacement relationship obtained from the above, in accordance with the Annex of JCI-S-001-2003 (Method of estimating the tension softening curve of concrete).

[0026] Figure 3 shows examples of tension softening curves obtained in this experiment ((a) is mix 10, (b) is mix 24). As shown in Figure 3, the tensile stress suddenly drops after cracks occur and becomes a minimum (crack width range 0.02-0.10 mm), then rises again and becomes a maximum (crack width range 0.3-1.3 mm), and then gradually drops. The thin solid lines in Figure 3 are the tension softening curves of each specimen. The thick solid lines are curves calculated by calculating the average value of tensile stress for each crack width of each specimen. In addition, the dashed lines show simplified curves connecting the points for crack widths of 0 mm to 0.5 mm and the points of minimum and maximum after crack occurrence described above. Here, we show the results of the study in which the maximum value of tensile stress after crack occurrence (hereinafter referred to as the tensile stress maximum value) is used as the characteristic value of tensile toughness.

[0027] In addition, Figure 4 is a graph showing the relationship between the tensile toughness characteristic value (maximum value of tensile stress after crack occurrence) and the average value of tensile stress up to a crack width of 1.0 mm, and it can be seen that there is a high correlation between the maximum value and the average value. When the average value of tensile stress up to crack widths of "0.5 mm", "1.5 mm", and "2.0 mm" in the simplified curve described above was also used as an index, a high correlation was observed between the two indices as in Figure 4 (example of average value of tensile stress up to a crack width of 1.0 mm), so it can be said that the influence of various mix conditions has a similar tendency to the case of the maximum value of tensile stress. Therefore, even if the characteristic value of tensile toughness is the average value of tensile stress up to a crack width of 0.5, 1.0, 1.5, and 2.0 mm, it is considered that it is possible to evaluate it in the same way as when the maximum value of tensile stress is used as an index of tensile toughness.

[0028] Figure 5 shows the relationship between compressive strength and maximum tensile stress. In the figure, the dashed and solid lines show regression lines passing through the origin for all data when the volumetric mixing ratio Vsf is 0.5% and 1.0%. Since the coefficient of determination of the regression line was relatively large, it is believed that the maximum tensile stress can be roughly evaluated using a linear equation for compressive strength for each volumetric mixing ratio Vsf. In addition, in the analysis results for this test result, β in formula 1 is zero, and the coefficient α is 0.050 when the volumetric mixing rate Vsf is 0.5%, and is 0.063 when Vsf is 1.0%.

[0029] Figure 6 shows the relationship between compressive strength and maximum tensile stress for each type of aggregate. As in Figure 5, the figure also shows a regression line passing through the origin. For all five combinations of fine and coarse aggregate, the slope of the regression line was within ±0.002 (from 0.050) of the slope of the regression line in Figure 5 when the volumetric mix ratio Vsf was 0.5%, and within ±0.001 (from 0.063) when the volumetric mix ratio Vsf was 1.0%, indicating that the effect of aggregate type on the relationship between compressive strength and maximum tensile stress was small.

[0030] Figure 7 shows a comparison of the tension softening curves between mixes 2 and 4, where only the unit coarse aggregate absolute volume Vg was changed. As shown in Figure 7, there was no clear effect of changing the unit coarse aggregate absolute volume Vg on the tension softening curve, and it is inferred that the effect of the coarse aggregate amount on the relationship between compressive strength and maximum tensile stress value is small. Figure 8 shows the relationship between compressive strength and maximum tensile stress for each air content (2.5%, 4.5%, and 6.5%) (target at the time of mixing). As in Figure 5, the figure shows a regression line passing through the origin for each air content. For each air content, the slope of the regression line is within ±0.003 (from 0.050) of the slope of the regression line in Figure 5 when the volumetric mixing rate Vsf is 0.5%, and within ±0.002 (from 0.063) when the volumetric mixing rate Vsf is 1.0%. In other words, within the scope of this experiment, the effect of the air content on the relationship between compressive strength and maximum tensile stress was relatively small.

[0031] From the above results, it can be said that the average value of the maximum tensile stress, which is used as an index of tensile toughness, can be roughly evaluated based on the compressive strength (design strength Fc) using Equation 1 (according to Figure 5, when Vsf = 0.5%, (α, β) = (0.050, 0), and when Vsf = 1.0%, (α, β) = (0.063, 0)) which was calculated in advance for each type of steel fiber and volumetric mixing rate Vsf. In addition, since the effects of aggregate type, unit coarse aggregate absolute volume Vg, and air content were relatively small, it is believed that there is no problem even if they are not taken into consideration at the design stage.

[0032] Next, we confirm the quality variation of the maximum tensile stress value of steel fiber reinforced concrete in actual production, and based on that, we present the results of our study on the evaluation of the maximum tensile stress value, assuming that it will be reflected in the design. Table 3 shows the mix conditions for the steel fiber reinforced concrete in actual production. Design standard strength 60N / mm 2 Add the structural strength correction value to the mixture control strength of 66.2N / mm 2 The standard deviation, which corresponds to a coefficient of variation of 10%, is multiplied by 1.73 to obtain a mixed strength of 77.7 N / mm 2 The volumetric mixing ratio Vsf is 1.0%, W / C is 31%, and the unit coarse aggregate absolute volume Vg is 250 L / m 3 The target air content was set at 4.5%, but the air content was controlled at 5% or less to avoid an excessively high value.

[0033] [Table 3]

[0034] The steel fiber reinforced concrete was manufactured on four separate days, and three square column specimens measuring 150 x 530 mm were taken from each of two different batches on each manufacturing date (10 / 19, 10 / 24, 10 / 27, 10 / 31), and a total of eight notched beam three-point bending load tests were conducted. The quality fluctuation of the maximum tensile stress value in the tension softening curve obtained from the load-crack opening displacement relationship obtained from the test was confirmed.

[0035] Table 4 shows the quality control results of the steel fiber reinforced concrete in actual production. The slump, air content, and compressive strength of the standard cured specimens at 28 days, which were the targets of strength control, all met the prescribed ranges. The average compressive strength was 77.7 N / mm2, the mix strength shown in Table 3. 2 This is because the relationship between the compressive strength and the cement-water ratio was reduced by 4-8% and reflected in the mix design. Design standard strength 60N / mm 2 The coefficient of variation of compressive strength measured at the factory for normal mixes without steel fibers was approximately 5%, and similar results were obtained for steel fiber reinforced concrete.

[0036] [Table 4]

[0037] Figure 9 shows the quality variation of the maximum tensile stress value of steel fiber reinforced concrete in actual production. The horizontal axis of Figure 9 is the production date and batch number. The coefficient of variation of the maximum tensile stress value was larger than the coefficient of variation of the compressive strength mentioned above. Therefore, it is considered that the maximum tensile stress value reflected in the design should be set not as an average value corresponding to the design reference strength Fc, but by taking into consideration the difference in the coefficient of variation of the two. For example, the average value of the tensile stress up to a crack width of 1.0 mm in the actual production was about 0.9 times the maximum tensile stress value as shown in Figure 4, and the coefficient of variation was 12.7%, which was equivalent to the coefficient of variation of the maximum tensile stress value in Figure 9. Therefore, even if the characteristic value of tensile toughness is the average value of the tensile stress up to crack widths of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, it is considered that it can be evaluated in the same way as the maximum tensile stress value.

[0038] Figure 10 shows a conceptual diagram of the variation in compressive strength and maximum tensile stress value for evaluating the maximum tensile stress value to be reflected in the design. To make it easier to visualize, Figure 10 shows values ​​corresponding to the actual manufacturing data. The coefficient of variation for compressive strength was set to 10% (of the mix control strength) adopted in the mix design, and the coefficient of variation for maximum tensile stress value was set to 20%, which allows for a sufficient margin (1.5 times or more) against the coefficient of variation for the actual measurement results in Figure 9, just as in the case of compressive strength. The normal deviation was set to 1.73, which is generally adopted in mix design. The average strength of structural concrete was 71.5 (=60+1.73×(60+6.2)×10%) N / mm, which is the mode of the normal distribution. 2 The average value of the maximum tensile stress is calculated as 4.50 (=0.063 × 71.5) N / mm from the corresponding regression equation in Figure 5. 2 This value roughly corresponds to the average value of the actual measurement results in Figure 9. The maximum tensile stress value reflected in the design must be evaluated on the safe side (smaller) by reducing this value according to the quality fluctuations in the actual manufacturing. Specifically, assuming a defect rate equal to or less than the compressive strength, it is set at 3.34 (=4.50 / (1+1.73×20%)) N / mm 2 It was decided.

[0039] At the design stage, when the manufacturing plant of steel fiber reinforced concrete and detailed mix conditions are not specified, the structural strength correction value must be set under some assumptions to evaluate the maximum tensile stress value. 2 Assuming this, and calculating based on formulas 1 and 2, the maximum tensile stress value to be reflected in the structural design is 0.055 (= (1 + 1.73 × 10%) × 0.063 / (1 + 1.73 × 20%) times the design strength Fc, which is 3.30 N / mm 2 The error is small compared to the above calculation results, which used the actual value for the structural strength correction value. The maximum tensile stress value calculated in this way and reflected in the design is the minimum value of 3.9 N / mm in Figure 9. 2It is sufficiently on the safe side (smaller) than the above. In addition, in Figure 5, the tensile stress maximum values ​​at a volume mixing ratio Vsf of 1.0% are all 0.058 times or more the compressive strength, and considering that in actual production, the strength of structural concrete is designed to be sufficiently greater than the design reference strength Fc, it is believed that a sufficient safety factor can be expected as a material characteristic value by setting the tensile stress maximum value reflected in the design based on formulas 1 and 2. Regarding the margin for the actual measured value of the coefficient of variation in formula 2, the compressive strength is also about 2 times, and since there are some differences depending on the factory, it is desirable to set the coefficient of variation of the tensile stress maximum value at about 20 to 30%.

[0040] Although an embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and each of the above-described components can be appropriately modified without departing from the spirit of the present invention. [Explanation of symbols]

[0041] 1 Steel Fiber 11. Hook S1 Mixture setting process S2 Average tensile toughness calculation process S3 Coefficient of variation extraction process S4 Tensile toughness estimation process for structural design S5 Parts manufacturing process

Claims

1. A method for estimating the tensile toughness of steel fiber reinforced concrete, comprising the steps of: A step of calculating an average tensile toughness characteristic value Ft for the design standard strength Fc of the steel fiber reinforced concrete by substituting the design standard strength Fc into Equation 1; and substituting the average tensile toughness characteristic value Ft, the coefficient of variation of compressive strength Vc, and the coefficient of variation Vt of the tensile toughness characteristic value into Equation 2 to estimate the tensile toughness characteristic value Ftd of the steel fiber reinforced concrete to be used in structural design. Ft = α×Fc + β ...Formula 1 Ftd = Ft×( 1 + 1.73×Vc ) / (1 +1.73×Vt ) ・・・Formula 2 Where, Ft: average tensile toughness characteristic value for Fc [N / mm 2 ] Fc: Design strength of steel fiber reinforced concrete [N / mm 2 ] α: Coefficient determined by the type of steel fiber and its volume fraction Vsf β: Constant determined by the type of steel fiber and its volume fraction Vsf Ftd: Tensile toughness characteristic value of steel fiber reinforced concrete used in structural design [N / mm 2 ] Vc: Coefficient of variation of compressive strength (=0.10) Vt: Coefficient of variation of tensile toughness characteristic value ( = 0.20 to 0.30)

2. The coefficient of variation of the compressive strength and the coefficient of variation of the tensile toughness characteristic value are extracted from a material database, The material database includes: The compressive strength measured by conducting a compressive strength test of concrete in accordance with JIS A 1108 on a cylindrical specimen of steel fiber reinforced concrete; Tensile toughness characteristic values ​​measured by performing a three-point bending test in accordance with JCI-S-002-2003 on a rectangular specimen of steel fiber reinforced concrete; A regression equation between the compressive strength and the tensile toughness property; 2. The method for estimating the tensile toughness of steel fiber reinforced concrete according to claim 1, further comprising: a coefficient of variation calculated from the compressive strength; and a coefficient of variation calculated from the tensile toughness characteristic value.

3. A method for manufacturing a steel fiber reinforced concrete member, comprising: Setting a mix for steel fiber reinforced concrete; A step of extracting a coefficient of variation Vc of compressive strength and a coefficient of variation Vt of tensile toughness characteristic value; A step of calculating an average tensile toughness characteristic value Ft for the design standard strength Fc of the steel fiber reinforced concrete by substituting the design standard strength Fc into Equation 1; A step of estimating the tensile toughness characteristic value Ftd of the steel fiber reinforced concrete used in structural design by substituting the average tensile toughness characteristic value Ft, the coefficient of variation Vc of the compressive strength, and the coefficient of variation Vt of the tensile toughness characteristic value into Equation 2; and manufacturing a concrete member from steel fiber reinforced concrete having a mix that has been confirmed to have a tensile toughness characteristic value Ftd that satisfies the required quality of steel fiber reinforced concrete used in structural design. Ft = α×Fc + β ...Formula 1 Ftd = Ft×( 1 + 1.73×Vc ) / (1 +1.73×Vt ) ・・・Formula 2 Where, Ft: average tensile toughness characteristic value for Fc [N / mm 2 ] Fc: Design strength of steel fiber reinforced concrete [N / mm 2 ] α: Coefficient determined by the type of steel fiber and its volume fraction Vsf β: Constant determined by the type of steel fiber and its volume fraction Vsf Ftd: Tensile toughness characteristic value of steel fiber reinforced concrete used in structural design [N / mm 2 ] Vc: Coefficient of variation of compressive strength (=0.10) Vt: Coefficient of variation of tensile toughness characteristic value (= 0.20 to 0.30)

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