Cement composition and concrete molded body

Abstract

To provide a cement composition and a concrete molded body having a high compressive strength and improved bending characteristics, especially toughness.SOLUTION: A cement composition contains cement, water, fine aggregate, and reinforcing fiber, wherein the reinforcing fiber contains thick fiber and fine fiber having different fiber diameters, a fiber diameter DA of the thick fiber exceeds 0.35 mm, a fiber diameter DB of the fine fiber is 0.026 mm or more and 0.1 mm or less, and a ratio LB / DB of a fiber length LB to a fiber diameter DB of the fine fiber is 20 or more and 800 or less, a fiber diameter ratio DA / DB between the thick fiber and the fine fiber is 10 or more, and a blending ratio by volume of the thick fiber to the fine fiber is 50 / 50 to 95 / 5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a cement composition and a concrete molded body containing reinforcing fibers. [Background technology]

[0002] To strengthen concrete molded bodies obtained by hardening a cement composition, reinforcing fibers have been incorporated into the cement composition. For example, Patent Document 1 proposes a cement composition having a surface layer of organic fibers containing thick fibers with a fiber diameter of 20 μm or more and thin fibers with a fiber diameter of less than 20 μm to prevent spalling and spalling. Specifically, Patent Document 1 discloses a concrete structure using polypropylene fibers with a fiber diameter of 49 μm and a fiber length of 20 mm as the thick fibers and polypropylene fibers with a fiber diameter of 18 μm and a fiber length of 10 mm as the thin fibers. Patent Document 2 proposes a hydraulic molded body containing polyvinyl alcohol-based fibers with a fineness of 100 to 10,000 dtex and a length of 100 mm or less and polyolefin-based fibers with a fineness of 0.1 to 80 dtex and a length of 100 mm or less to enhance toughness and impact resistance. Patent Document 3 proposes a kneaded and molded hydraulic material body containing Group A organic short fibers having a fiber diameter of 200 μm or more and 2000 μm or less and a fiber length of 5 mm or more and 60 mm or less, and Group B organic short fibers having a fiber diameter of 10 μm or more and 150 μm or less and a fiber length of 4 mm or more and 20 mm or less, in a mixing ratio of A:B=70:30 to 10:90. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-91668 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-139360 [Patent Document 3] JP 2003-327462 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the cement compositions described in the cited documents 1 to 3 have problems such as high toughness but low compressive strength, or high compressive strength but low toughness.

[0005] In order to solve the above-mentioned problems of the prior art, the present invention provides a cement composition and a concrete molded product having high compressive strength and improved flexural properties, particularly toughness. [Means for solving the problem]

[0006] The present invention relates to a cement composition comprising cement, water, fine aggregate, and reinforcing fibers, wherein the reinforcing fibers include thick fibers and thin fibers having different fiber diameters, the thick fibers having a fiber diameter DA exceeding 0.35 mm, the thin fibers having a fiber diameter DB of 0.026 mm or more and 0.1 mm or less, the fiber length to fiber diameter ratio LB / DB of the thin fibers being 20 or more and 800 or less, the fiber diameter ratio DA / DB of the thick fibers to the thin fibers being 10 or more, and the blending ratio of the thick fibers to the thin fibers being 50 / 50 to 95 / 5 by volume.

[0007] The present invention also relates to a concrete molded article obtained by hardening the cement composition. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a cement composition and a concrete molded product having high compressive strength and improved flexural properties, particularly toughness. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a cross section of a thick fiber according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of a cross section of a thick fiber according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram of a cross section of a fine fiber according to an embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram of a cross section of a fine fiber according to an embodiment of the present invention. [Figure 5] FIG. 5 is a scanning electron microscope photograph of the fracture surface of the hardened cement paste of Example 1 after the bending test. [Figure 6] FIG. 6 is a schematic diagram illustrating the dimensions of the protrusions in the cross section of a fine fiber according to one example of the present invention. [Figure 7] FIG. 7 is a graph showing the relationship between the load and the opening displacement in the bending test. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems of the prior art, and as a result have found that in a cement composition containing thick fibers and thin fibers with different fiber diameters as reinforcing fibers, by adjusting the fiber diameters and ratio of the thick fibers to the thin fibers, the ratio of the fiber length to the fiber diameter of the thin fibers, and the blending ratio of the thick fibers to the thin fibers within predetermined ranges, the concrete molded body obtained by hardening the cement composition can maintain high compressive strength while improving its bending properties, particularly toughness.

[0011] The thick fibers have a fiber diameter DA exceeding 0.35 mm. If the fiber diameter DA of the thick fibers is 0.35 mm or less, this may lead to a decrease in toughness. From the viewpoint of toughness, the thick fibers preferably have a fiber diameter DA of 0.4 mm or more, more preferably 0.5 mm or more. The thick fibers are not particularly limited, but for example, in order to distribute (disperse) the thick fibers evenly throughout the concrete, the fiber diameter DA is preferably 1.5 mm or less, more preferably 1.2 mm or less. In the present invention, the fiber diameter refers to the diameter D (maximum across length) of the circumscribing circle of the fiber cross section. In addition, in the present invention, if there is no circumscribing circle of the fiber cross section, the fiber diameter refers to the maximum across length of the smallest encompassing circle.

[0012] The thick fibers are not particularly limited, but the fiber length LA is preferably 15 mm or more and 48 mm or less, and more preferably 20 mm or more and 45 mm or less. When LA is 15 mm or more, toughness is easily improved. When LA is 48 mm or less, the dispersion of the fibers in the cement composition is good.

[0013] The thick fibers are not particularly limited, but the ratio LA / DA of fiber length LA to fiber diameter DA is preferably 25 or more and 60 or less, and more preferably 28 or more and 50 or less. When LA / DA is 25 or more, toughness is likely to be improved. When LA / DA is 50 or less, compressive strength is unlikely to decrease.

[0014] The fine fibers have a fiber diameter DB of 0.026 mm or more and 0.1 mm or less. When DB is within the above range, toughness tends to be improved and the fibers tend to be well dispersed in the cement composition. DB is preferably 0.035 mm or more and 0.09 mm or less, and more preferably 0.040 mm or more and 0.08 mm or less.

[0015] The fine fibers have a ratio LB / DB of fiber length LB to fiber diameter DB of 20 or more and 800 or less. When LB / DB is 20 or more, toughness is likely to be improved. When LB / DB is 800 or less, the fibers are well dispersed in the cement composition, compressive strength is less likely to decrease, and toughness is likely to be improved. LB / DB is preferably 65 or more and 700 or less, and more preferably 70 or more and 650 or less.

[0016] The fine fibers are not particularly limited, but preferably have a fiber length LB of 1 mm or more and 20 mm or less, more preferably 2 mm or more and 18 mm or less. When LB is 1 mm or more, toughness is likely to be improved. When LB is 20 mm or less, dispersibility in the cement composition is improved.

[0017] The fiber diameter ratio DA / DB of the thick fibers to the thin fibers is 10 or more. When DA / DB is 10 or more, toughness is likely to be improved. DA / DB is preferably 12 or more and 50 or less, and more preferably 13 or more and 45 or less. When DA / DB is 50 or less, the proportional limit of bending (LOP) is less likely to decrease, and toughness is likely to be improved.

[0018] The fiber length ratio LA / LB of the thick fibers to the thin fibers is not particularly limited, but is preferably 1 or more and 40 or less, and more preferably 1.2 or more and 30 or less. When the fiber length of the thick fibers is equal to or longer than the fiber length of the thin fibers, the fibers are well dispersed in the cement composition, and toughness is likely to be improved.

[0019] The cross-sectional shape of the thick fibers is not particularly limited and may be, for example, circular, elliptical, flat, cross-shaped, or any other shape. However, from the viewpoint of increasing the contact area with the base material and improving toughness, it is preferable to include fibers having a flat or cross-shaped cross-sectional shape. Figure 1 is a schematic diagram of a fiber cross-section having a flat cross-sectional shape. Figure 2 is a schematic diagram of a fiber cross-section having a cross-shaped cross-sectional shape. In Figures 1 and 2, D indicates the fiber diameter. In the present invention, unless otherwise specified, the fiber cross-section refers to a cross section cut perpendicular to the longitudinal direction of the fiber, and the cross-sectional shape refers to the shape of the cross section cut perpendicular to the longitudinal direction of the fiber.

[0020] The cross-sectional shape of the fine fibers is not particularly limited and may be any of various shapes, such as circular, elliptical, flat, multilobal, and cross-shaped. However, fibers with irregular cross sections are preferred because they increase the contact area with the base material and improve toughness. In particular, the fine fibers preferably contain fibers with a cross-sectional shape that can be fibrillated, and more preferably contain fibers with a multilobal cross-sectional shape having three or more convex portions. The number of convex portions is preferably 3 to 16, more preferably 3 to 8, and even more preferably 3 to 5. Specific examples of multilobal shapes include trilobal shapes with three convex portions, tetralobal shapes with four convex portions, and octolobal shapes with eight convex portions. Having a multilobal cross-sectional shape with three or more convex portions increases the surface area in contact with the cement material and ensures space for the cement material to penetrate. Furthermore, it is preferable that the convex portions of the multilobal cross-sectional shape are formed radially from near the center of the fiber. The radial formation of the protrusions makes it easier for the cement material to penetrate between adjacent protrusions. Examples of multi-lobed cross-sectional shapes with radially formed protrusions include a four-lobed shape shown in Fig. 3 and an eight-lobed shape shown in Fig. 4. In Figs. 3 and 4, D indicates the fiber diameter. The protrusions may be continuous or discontinuous in the length direction of the fiber (fiber side surface), but considering the manufacturing process, it is preferable that the protrusions exist continuously on the fiber side surface.

[0021] The fine fibers preferably fibrillate when cracks or fissures occur in the hardened cement body due to stress from bending or impact. By forming the fine fibers in a shape that allows them to fibrillate, toughness is likely to be improved. While the reason for this is unclear, it is presumed that when a crack appears in a part of the hardened cement body due to an initial load, the fibers also crack, and as the crack progresses, the fibers fibrillate, increasing the energy required for fracture (fracture energy). This can be seen from the fact that, as shown in Figure 5, when the fracture surface of the hardened cement body after a bending test is observed with a scanning electron microscope, some fine fibers (e.g., tetralobal fibers with four convex portions) are split and fibrillated at the constricted portion of the convex portion toward the center of the fiber (hereinafter also referred to as the "root portion"), which is the boundary.

[0022] As described above, fibers having a multilobal cross-sectional shape with three or more convex portions can be used as fibrillizable fine fibers. The presence of convex portions increases the contact area with the base material, making it easier for the fibers to split at the base, resulting in easier fibrillation. In the fibers having a multilobal cross-sectional shape, it is preferable that at least one convex portion present in the fiber cross section has a generally curved tip, and the width of the base is smaller than the maximum width of the tip. More preferably, all convex portions present in the fiber cross section have generally curved tips, and the width of the base is smaller than the maximum width of the tip. Such a shape facilitates deformation from the base, making it easier for cement material to penetrate into the recesses between adjacent convex portions, and making the hardened cement body more susceptible to fibrillation when cracks or fissures occur due to stresses such as bending or impact.

[0023] The width of the base of a protrusion in the cross section of a fiber refers to the length Wb of the line connecting the two bases of the protrusions, as shown in Figure 6. The width Wb of the base of the protrusion is preferably 3 μm or more and 40 μm or less, more preferably 4 μm or more and 30 μm or less, and even more preferably 6 μm or more and 20 μm or less. When the width Wb of the base of the protrusions is within this range, cement particles and aggregates with small particle sizes can easily enter the recesses formed between adjacent protrusions, thereby enhancing the reinforcing effect. Furthermore, when the width Wb of the base of the protrusions is within this range, stresses such as bending and impact are applied to the hardened cement paste, which tends to cause fibrillation when cracks or fissures occur. The width Wb of the base of the protrusions can be determined by magnifying the cross section of the fiber bundle using an electron microscope or the like and averaging the values of any 10 fibers.

[0024] As shown in Figure 6, the maximum width of the tip of a protrusion in the fiber cross section refers to the maximum length Wt of a line drawn from the midpoint u of the line connecting the two bases of the protrusion to the tip (vertex t) of the protrusion, and then a perpendicular line drawn from that line to the outer shape of the protrusion. The maximum width Wt of the tip of the protrusion is preferably 4 μm to 48 μm, more preferably 6 μm to 36 μm, and even more preferably 8 μm to 30 μm. Within this range, cement particles and small-sized aggregates can easily enter the recesses formed between adjacent protrusions, enhancing the reinforcing effect. Furthermore, when stresses such as bending and impact are applied to the hardened cement paste, cracks or fissures form, making the protrusions more susceptible to stress and more likely to fibrillate. The maximum width Wt of the tip of the protrusion can be determined by magnifying the cross section of the fiber bundle using an electron microscope or the like and averaging the values of 10 random fibers.

[0025] In the convex portions, the ratio (Wt / Wb) of the maximum width Wt at the tip to the width Wb at the base is preferably 1.2 or more and 5.0 or less, more preferably 1.3 or more and 3.5 or less, and even more preferably 1.5 or more and 2.5 or less. When Wt / Wb satisfies the above range, the hardened cement paste tends to be more susceptible to fibrillation when cracks or fissures occur due to stress from bending or impact. The maximum width Wt at the tip of the convex portions and the width Wb at the base of the convex portions can be determined by magnifying the cross section of the fiber bundle using an electron microscope or the like and averaging the values of any 10 fibers.

[0026] As shown in Figure 6, the length of a convex portion refers to the length L of the line connecting the midpoint u of the line connecting the two bases of the convex portion to the tip (vertex t) of the convex portion. The length L of the convex portion is preferably 12 μm or more and 45 μm or less, more preferably 16 μm or more and 41 μm or less, and even more preferably 18 μm or more and 36 μm or less. If the length L of the convex portion is 12 μm or more, the convex portion is likely to deform from its base, and when stress such as bending or impact is applied to the hardened cement paste, cracks or fibrillation occurs, which tends to occur easily. If the length L of the convex portion is 45 μm or less, the cement material is well bonded or retained in the recesses formed between the convex portions. The length L of the convex portion can be determined by magnifying the cross section of the fiber bundle using an electron microscope or the like and averaging the values of any 10 fibers.

[0027] The ratio (L / Wb) of the length L of the protrusion to the width Wb of the base of the protrusion is preferably 1.2 or more and 4.0 or less, more preferably 1.5 or more and 3.5 or less, and even more preferably 1.8 or more and 3.0 or less. When L / Wb satisfies this range, the protrusions are likely to deform from their bases, and when stress such as bending or impact is applied to the hardened cement paste, cracks or fibrillation tends to occur.

[0028] The thick fibers and the thin fibers preferably have different cross-sectional shapes, which further increases the compressive strength and toughness of the cement composition. More preferably, the thick fibers include fibers having a flat or crisscross cross-sectional shape, and the thin fibers include fibers having a multi-lobed cross-sectional shape with three or more projections.

[0029] The thick fibers and thin fibers are not particularly limited, and suitable materials that can be used include polyolefin fibers, vinylon fibers, acrylic fibers, aramid fibers, carbon fibers, glass fibers, etc. Examples of polyolefin fibers include fibers made of polyolefin resins such as polypropylene, polyethylene, poly-4-methylpentene-1, copolymers thereof, and polymers modified with functional groups such as acidic groups.

[0030] From the viewpoint of alkali resistance and versatility, the thick fibers and the thin fibers preferably contain polyolefin-based fibers, preferably contain 50% by mass or more of polyolefin-based fibers, preferably contain 60% by mass or more of polyolefin-based fibers, preferably contain 75% by mass or more of polyolefin-based fibers, preferably contain 85% by mass or more of polyolefin-based fibers, and it is particularly preferable that they consist essentially of polyolefin-based fibers.

[0031] The polyolefin fiber is preferably a synthetic fiber containing 50% by mass or more of a polyolefin resin, more preferably a synthetic fiber containing 60% by mass or more of a polyolefin resin, even more preferably a synthetic fiber containing 75% by mass or more of a polyolefin resin, even more preferably a synthetic fiber containing 85% by mass or more of a polyolefin resin, and particularly preferably a synthetic fiber substantially consisting of a polyolefin resin. From the viewpoint of alkali resistance, the polyolefin resin is preferably one or more selected from the group consisting of polypropylene and polymethylpentene.

[0032] The polypropylene is not particularly limited, but polypropylene having an isotactic pentad fraction (IPF: mol%) of preferably 90% or more, more preferably 93% or more, and even more preferably 94% or more can be used because it can produce high-strength fibers in terms of stereoregularity. The IPF can be measured for n-heptane insoluble components in accordance with "Macromolecules" (Macromoleculer, Vol. 6, 925 (1973) and Macromoleculer, Vol. 8, 687 (1975)).

[0033] The polypropylene is not particularly limited, but preferably has a Q value (Mw / Mn) of less than 6, which has high extensibility and therefore produces high-strength fibers. A more preferred Q value is less than 5, and even more preferably 4 or less.

[0034] The thick fibers and the thin fibers may be single fibers or composite fibers. Examples of composite fibers include sheath-core composite fibers, eccentric sheath-core composite fibers, side-by-side composite fibers, splittable composite fibers, and islands-in-the-sea composite fibers. Both components, for example, the sheath component and the core component, are preferably polyolefin resins, more preferably polypropylene, polymethylpentene, or a mixture of polypropylene and polymethylpentene.

[0035] In the cement composition, the blending ratio of the thick fibers to the thin fibers is 50 / 50 to 95 / 5 by volume. When the blending ratio of the thick fibers to the thin fibers is within the above range, the compressive strength is less likely to decrease and the toughness is likely to improve. In the cement composition, the blending ratio of the thick fibers to the thin fibers is preferably 65 / 35 to 90 / 10 by volume.

[0036] The cement composition can contain an appropriate amount of the reinforcing fibers depending on the application. For example, the cement composition may contain 0.05% by volume to 2.0% by volume of the reinforcing fibers (total of thick fibers and thin fibers), or 0.1% by volume to 0.5% by volume. Here, the content of the reinforcing fibers is expressed as vol% (vol%) when the total volume of the other components in the cement composition excluding the reinforcing fibers is taken as 100% by volume. The reinforcing fibers improve the compressive strength and toughness of the concrete molded body, while also improving workability.

[0037] The cement composition can be obtained by mixing and stirring cement, water, fine aggregate, and reinforcing fibers. The stirring can be performed using a stirrer such as a pan mixer or an omnimixer. From the viewpoint of improving the miscibility between the materials and enhancing the dispersibility of the reinforcing fibers, the cement and fine aggregate may be first stirred and mixed, then water may be added and stirred and mixed, and then the reinforcing fibers may be added and stirred and mixed.

[0038] The cement composition may further contain coarse aggregate in addition to cement, water, fine aggregate, and reinforcing fibers. In this case, from the viewpoint of improving the miscibility between the materials and improving the dispersibility of the reinforcing fibers, the cement and fine aggregate may be first stirred and mixed, then water may be added and stirred and mixed, then the coarse aggregate may be added and stirred and mixed, and finally the reinforcing fibers may be added and stirred and mixed.

[0039] As the cement, various cements can be used, such as ordinary Portland cement, high-early-strength Portland cement, extra-high-early-strength Portland cement, moderate-heat Portland cement, low-heat Portland cement, sulfate-resistant Portland cement, etc. As the fine aggregate and coarse aggregate, silica sand, river sand, sea sand, beach sand, crushed stone, and various slags such as blast furnace slag, ferronickel slag, copper slag, and electric furnace slag can be used, and the particle size of the aggregate can be selected depending on the application of the cement composition and used as the fine aggregate or coarse aggregate.

[0040] The cement composition may contain admixtures as needed, such as air-entraining agents, air-entraining water-reducing agents, high-performance air-entraining water-reducing agents, superplasticizers, hardening accelerators, rust inhibitors, setting retarders, quick-setting admixtures, and shrinkage-reducing agents.

[0041] The cement composition of the present invention is hardened and used as a concrete molded product, and is suitable for use in, for example, mass concrete for dams, etc.; fluidized concrete for high-rise buildings and precast factory products; high-fluidity concrete and low-heat concrete for high-rise buildings and large structures; high-strength concrete for bridges, high-rise buildings, precast factory products; expansive concrete for building floors and walls, concrete products, and leak prevention; prestressed concrete for piles, poles, long-span bridges, and building beams; low-shrinkage concrete for thin members, walls, and slabs; polymer concrete for waterproof linings, pipes, and U-shaped gutters; fiber-reinforced concrete for tunnel lining concrete products and precast members; watertight concrete for areas where pressurized water acts, such as water tanks, pools, and groundwater; underwater concrete for undersea bridge pier foundations, revetments, and breakwaters; road pavement and street trees. It is useful as permeable and drainage concrete for things like foot protection and water storage basins, repair and deterioration prevention due to freeze-thaw, salt damage and alkali-aggregate reaction, and resin-impregnated concrete for waterproofing and water-stopping underground structures, shielding concrete for nuclear power plants, isotope storage facilities and medical irradiation rooms, lightweight concrete for reinforced concrete walls, floors, roofing materials, exterior walls and partitions, prepacked concrete for use in complex areas where construction with normal concrete is difficult, such as underwater concrete work and radiation shielding concrete work, shotcrete for primary lining and lining of tunnels, recycled concrete for backfill concrete and leveling concrete, ultra-hard mixed concrete for paving concrete, dams made using roller compacted concrete methods, roller compacted concrete pavements and products made using the immediate demolding method, and precast concrete for sleepers, blocks, protective fences and building walls.

[0042] A concrete molded body can be obtained by hardening the cement composition. Specifically, the cement composition is filled into a formwork of a predetermined shape, and after pouring, sufficient moisture is added to prevent the surface of the cement composition (concrete) from drying out. Moisture can be added by known methods, such as flood curing, sprinkling curing, wet sand curing, and spray curing, which supply moisture to the concrete surface while curing. Other methods include underwater curing, in which the concrete is cured in water after being removed from the formwork. Curing can be performed in an air environment with an air temperature of 5°C to 35°C, or in the case of underwater curing, curing can be performed in water adjusted to a temperature of 5°C to 35°C. When curing in air, it is preferable to perform curing in an atmosphere with as high a relative humidity as possible to prevent the concrete surface from drying out. The curing period varies depending on the curing method, but if the above-mentioned curing method is used to provide sufficient moisture to prevent the concrete surface from drying out and the concrete surface is cured while being kept moist, a concrete molded body with sufficient hydration reaction can be obtained by curing for 28 days or more at an temperature of 20 to 30°C and a humidity of 95% or more.

[0043] The concrete compacts are used in applications requiring strength, such as buildings and bridges, and have a compressive strength of 37.2 N / mm2 measured in accordance with JIS A 1108. 2 It is preferable that the resistance is 37.5N / mm or more. 2 More preferably, it is equal to or greater than this.

[0044] When the concrete molded body is used for applications requiring durability even after cracks have occurred, such as tunnel linings, roads, and bridges, it is preferable that the fracture energy at an opening displacement of 7.5 mm, measured based on the concrete fracture energy test method using a notched beam in JCI-S-001-2003 of the Japan Concrete Institute, exceeds 0.510 N / mm, more preferably 0.530 N / mm or more, and even more preferably 0.590 N / mm or more.

[0045] When the concrete molded body is used for applications requiring strength, such as buildings and bridges, it is preferable that the bending proportional limit strength (LOP) of the concrete molded body be 4000 N or more, and more preferably 4500 N or more, as measured based on the concrete fracture energy test method using a notched beam in JCI-S-001-2003 of the Japan Concrete Institute. [Example]

[0046] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples.

[0047] The measurement and evaluation methods used in the examples will be explained below.

[0048] (fiber diameter) The fiber cross section was observed with a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, model number "SU3500"), and the fiber diameter was calculated using two-dimensional image measurement software (manufactured by Scalar Corporation, Micro Measure).

[0049] (Compressive strength) The cement composition was filled into a cylindrical formwork (diameter 100 mm, height 200 mm), and the compressive strength of the obtained test specimen was measured in accordance with JIS A 1108.

[0050] (bending test) The cement composition was filled into a formwork (100 mm high, 100 mm wide, 400 mm long), and the proportional limit strength (LOP) and fracture energy of the obtained specimens were measured and calculated based on the fracture energy test method for concrete using a notched beam in JCI-S-001-2003 of the Japan Concrete Institute.

[0051] (reinforcing fiber) The thick and thin fibers shown in Table 1 below were used. For fine fibers 1 to 4, the following fibers were used. Fine fiber 1: The fiber cross section is tetralobal with four convex portions, the maximum width at the tip of the convex portion Wt is 14.2 μm, the width at the base Wb is 7.9 μm, Wt / Wb is 1.79, the length of the convex portion L is 18.7 μm, L / Wb is 2.4, the single fiber fineness is 5.4 dtex, and the fiber length is 15 mm. It is a single fiber made of polypropylene (propylene homopolymer: Q value 3.1). Fine fiber 2: The fiber cross section is tetralobal with four convex portions, the maximum width at the tip of the convex portion Wt is 9.1 μm, the width at the base Wb is 5.3 μm, Wt / Wb is 1.72, the length of the convex portion L is 11.3 μm, L / Wb is 2.1, the single fiber fineness is 2.2 dtex, and the fiber length is 15 mm. It is a single fiber made of polypropylene (propylene homopolymer: Q value 3.1). Fine fiber 3: The fiber cross section is tetralobal with four convex portions, the maximum width at the tip of the convex portion Wt is 24.3 μm, the width at the base Wb is 13.6 μm, Wt / Wb is 1.79, the length of the convex portion L is 31.3 μm, L / Wb is 2.30, the single fiber fineness is 15.0 dtex, and the fiber length is 12 mm. It is a single fiber made of polypropylene (propylene homopolymer: Q value 3.1). Fine fiber 4: A single fiber having the same shape as fiber 1, except that the fiber length of fine fiber 1 was 3 mm.

[0052] [Table 1]

[0053] Example 1 Cement (ordinary Portland cement, manufactured by Ube Mitsubishi Cement Co., Ltd.) was used at 417 kg / m 3 , water 179 kg / m 3 , fine aggregate (river sand, produced in Yodogawa, Osaka Prefecture, coarseness ratio 2.85 according to JIS A 1102 (2014) "Sieving test method for aggregates" 6.4) 858 kg / m 3 Coarse aggregate: crushed stone (produced in Ome, Tokyo, coarse grain ratio 8.21 according to JIS A 1102 (2014) "Sieving test method for aggregates" 6.4) 933 kg / m 3 , total reinforcement fiber 1.82 kg / m 3(0.2% by volume) was blended and thoroughly stirred to obtain a cement composition. The thick and thin fibers shown in Table 2 below were used as reinforcing fibers in the blending ratios shown in Table 2 below. The resulting cement composition was filled into a cylindrical formwork with a diameter of 10 cm and a height of 20 cm for compressive strength testing, and into a formwork with dimensions of 10 cm x 10 cm x 40 cm for bending testing. After casting, the composition was cured at 20°C for 24 hours, then demolded and cured in water at 20°C for 27 days to obtain a hardened cement body (concrete compact).

[0054] Examples 2 to 7 A concrete molded body was obtained in the same manner as in Example 1, except that thick fibers and thin fibers shown in Table 2 below were used as reinforcing fibers in the blending ratio shown in Table 2.

[0055] (Comparative Example 1) A concrete molded body was obtained in the same manner as in Example 1, except that no reinforcing fibers were added.

[0056] (Comparative Example 2) A concrete molded body was obtained in the same manner as in Example 1, except that only thick fibers shown in Table 2 below were used as reinforcing fibers.

[0057] (Comparative Example 3) A concrete molded body was obtained in the same manner as in Example 1, except that only the fine fibers shown in Table 2 below were used as reinforcing fibers.

[0058] (Comparative Examples 4 to 6) A concrete molded body was obtained in the same manner as in Example 1, except that thick fibers and thin fibers shown in Table 2 below were used as reinforcing fibers in the blending ratio shown in Table 2.

[0059] In the examples and comparative examples, the compressive strength, flexural proportional limit strength (LOP), and fracture energy of the concrete molded bodies were measured and calculated as described above, and the results are shown in Table 2 below. In Example 2, the fracture surface of the hardened cement paste after the bending test was observed with a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, model number "SU3500," 100x magnification), and the results are shown in Figure 5. Figure 7 shows the relationship between load and opening displacement in the bending test of the concrete molded bodies of Comparative Example 2 and Examples 1 to 3.

[0060] [Table 2]

[0061] As can be seen from Fig. 5, in Example 1, the four-lobed fine fibers having four projections are split and fibrillated at the constricted portion (base portion) as a boundary. In this way, when four-lobed fine fibers having four projections are used, when a crack is generated in a part of the hardened cement paste due to the initial load, a crack occurs in the fiber, and as the crack progresses, the fine fibers are fibrillated, which is presumably why the energy required for fracture (fracture energy) increases.

[0062] As can be seen from Figure 7, the load immediately after cracking is low in Comparative Example 2, which used only thick fibers as reinforcing fibers. This is presumably because, since the number of thick fibers (total surface area) is small, cracks tend to spread quickly once they occur, and the frictional force between the thick fibers and the base material per unit cross-sectional area is weak (adhesion area × frictional force per unit area / fiber cross-sectional area is small), so immediately after cracking in the bending test, the fibers tend to slip, resulting in a low bending load. In Examples 1 to 3, in which thick fibers having a predetermined fiber diameter and thin fibers having a predetermined fiber diameter and fiber diameter / fiber length were used as reinforcing fibers in a predetermined ratio, the load immediately after cracking and the load after the crack opening had expanded were both high, and toughness was improved. This is presumably because the thin fibers have a large number of fibers (total surface area), and immediately after cracking, the frictional force between the thin fibers and the base material per unit cross-sectional area is strong (adhesion area × frictional force per unit area / fiber cross-sectional area is large), making the thin fibers less likely to slip and increasing the bending load, and after the crack opening has expanded, the load is maintained by the thick fibers, increasing the bending load, and improving toughness both immediately after cracking and after it has opened.

[0063] As can be seen from a comparison between Example 2 and Example 6, Example 2, which uses fine fibers with a four-lobe shape that easily splits where a load is applied, has a higher fracture energy at an opening displacement of 7.5 mm and higher toughness after opening. It is presumed that because the fine fibers have a four-lobe shape, after a crack is generated in a part by the initial load, the crack gradually propagates in the fiber axis direction, resulting in a larger energy required for fracture. [Industrial Applicability]

[0064] The cement composition of the present invention is hardened and used as a concrete molded product, and is useful as, for example, fluidized concrete, high-fluidity concrete, low-heat concrete, high-strength concrete, expansive prestressed concrete, polymer concrete, fiber-reinforced concrete, watertight concrete, underwater concrete, permeable / drainable concrete, resin-impregnated concrete, shielding concrete, lightweight concrete, prepacked concrete, shotcrete, recycled concrete, paving concrete, ultra-dry mix concrete, and precast concrete.

Claims

1. A cement composition comprising cement, water, fine aggregate, and reinforcing fibers; The reinforcing fibers include thick fibers and thin fibers having different fiber diameters, The fiber diameter DA of the thick fiber exceeds 0.35 mm, The fiber diameter DB of the fine fibers is 0.026 mm or more and 0.1 mm or less, the ratio LB / DB of the fine fibers to the fiber diameter is 20 or more and 800 or less; a fiber diameter ratio DA / DB of the thick fibers to the thin fibers is 10 or more; the blending ratio of the thick fibers to the thin fibers is 50 / 50 to 95 / 5 by volume; The cement composition is characterized in that the fine fibers include multi-lobed fibers having a cross-sectional shape with three or more convex portions.

2. A cement composition comprising cement, water, fine aggregate, and reinforcing fibers; The reinforcing fibers include thick fibers and thin fibers having different fiber diameters, The fiber diameter DA of the thick fiber exceeds 0.35 mm, The fiber diameter DB of the fine fibers is 0.026 mm or more and 0.1 mm or less, the ratio LB / DB of the fine fibers to the fiber diameter is 20 or more and 800 or less; a fiber diameter ratio DA / DB of the thick fibers to the thin fibers is 10 or more; the blending ratio of the thick fibers to the thin fibers is 50 / 50 to 95 / 5 by volume; The thick fibers and thin fibers each contain a polyolefin fiber, The cement composition is characterized in that the thick fibers include fibers having a cross-shaped cross section.

3. A cement composition comprising cement, water, fine aggregate, and reinforcing fibers; The reinforcing fibers include thick fibers and thin fibers having different fiber diameters, The fiber diameter DA of the thick fiber exceeds 0.35 mm, The fiber diameter DB of the fine fibers is 0.026 mm or more and 0.1 mm or less, the ratio LB / DB of the fine fibers to the fiber diameter is 20 or more and 800 or less; the blending ratio of the thick fibers to the thin fibers is 50 / 50 to 95 / 5 by volume; The thick fibers and thin fibers each contain a polyolefin fiber, The thick fibers include fibers having a flat or cross-shaped cross section, A cement composition characterized by satisfying at least one of the following (1) and (2): (1) The ratio LA / DA of the fiber length LA to the fiber diameter DA of the thick fiber is 25 or more and 36 or less. (2) The fiber diameter ratio DA / DB of the thick fibers to the thin fibers is 25 or more and 50 or less.

4. The cement composition according to claim 1 , wherein the thick fibers and the thin fibers each comprise a polyolefin fiber.

5. 4. The cement composition according to claim 2, wherein the fine fibers include multi-lobed fibers having a cross-sectional shape with three or more projections.

6. The cement composition according to claim 1, wherein the thick fibers include fibers having a cross-sectional shape of a flattened or crisscross shape.

7. The cement composition according to any one of claims 1 to 4, wherein the thick fibers and the thin fibers include fibers having different cross-sectional shapes.

8. 8. The cement composition according to claim 1, wherein the fiber length ratio LA / LB of the thick fibers to the thin fibers is 1 or more and 40 or less.

9. The cement composition according to any one of claims 1 to 8, wherein the fiber length LB of the fine fibers is 1 mm or more and 20 mm or less.

10. The cement composition according to any one of claims 1 to 9, wherein the thick fibers have a fiber length LA of 15 mm or more and 48 mm or less.

11. 7. The cement composition according to claim 1, 2, 4 or 6, wherein the ratio LA / DA of the fiber length to the fiber diameter of the thick fibers is 25 or more and 60 or less.

12. The cement composition according to any one of claims 1 to 11, further comprising coarse aggregate.

13. A concrete molded body obtained by hardening the cement composition according to any one of claims 1 to 12.

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