Background of the Invention Fiber reinforced concrete ("FRC") is a new construction material obtained by dispersing short fibers having small cross sectional square measure in concrete. Non-reinforced cement or concrete shows low tensile strength and is easily broken when exposed to stress. In contrast, the fiber reinforced concrete is known to have improved toughness and ductility since the fibers resist the initiation and propagation of crack caused by impact or heat. Various FRCs made of steel, polyolefin, carbon, nylon, aramid or glass fibers have been suggested for such purpose.

In an article entitled"Flexural Characteristics of Steel Fiber and Polyethylene Fiber Hybrid-Reinforced Conc, Tete," Kobayashi and Cho describe a fiber-reinforced concrete made by dispersing discontinuous steel and polyethylene fibers in a randomly oriented state into the concrete to provide it with both strength and toughness (K Kobayashi and R. Cho, Composites, Vol. 13 (Butterworth & Co. Ltd. 1982), pp. 164-168).

Tn World Patent Application WO 98/27022, J. Seewald discloses a high strength concrete of enhanced ductility comprising 30-200 kgf/m2 of inorganic

(e. g. , steel) fibers (approximately 0.4-2. 6 percent volume) along with a small amount of organic fibers of a low elasticity modulus.

Further, Korean Patent Publication No. 2001-80383 discloses a reinforced concrete having improved toughness and ductility, by employing fiber system comprising (A) a first component comprising fibers having a Young's modulus of at least 30 GigaPascals and having a width to thickness ratio of I 0-2Q0 and an average length of 5-50 mm (and more preferably 5-25 mm) ; and (B) a second component comprising fibers having a length to diameter ratio of 25-125 (diameter may be equivalent diameter, See ACI 544. l R-5), an average length of 10-100 mm ; the volume ratio of component A to B being at leasl 1 : 2.

However, portland cement, a major component of concrete is strongly alkalized up to pH 12.5 to 13 when exposed to water. Accordingly, unless fibers to be incorporated to FRC are resistant to alkali, the physical properties of the fibers may not be sufficiently manifested due to abrupt degradation of fiber in alkaline environment. Further, even though the fibers are resistant to alkali, the fibers having high ratio of surface area to weight may face to abrupt surface degradation when exposed to strong alkaline environment.

In addition, the self-aggregation of the fibers may prevent FRC from containing fibers in a large amount, and further the aggregation of fiber may serve a weak point of FRC.

Accordingly, there has been requested to develop reinforcing components for improving toughness and ductility of FRC, which is resistant to alkali and self-aggregation. The present inventors found that the fibers coated with thermoset resin resist to the degradation in alkaline environment

and the self-aggregation, and accomplished the present invention relating to a FRC having improved toughness and ductility, comprising the fibers coated with thermoset resin.

Summary of the Invention Accordingly, it is an object of the present invention to provide a FRC composition having improved toughness and ductility, and the preparation method thereof.

In accordance with the aspect or the present invention, there is provided a FRC comprising the liber coated with thermoset resin and the preparation method thereof.

Detailed Description of the Invention The term"concrete"refers to a composition containing a cement binder, usually with fine and course aggregates. As used hereinafter, however, the term means and refers to any cementetious material, such as cement, mortar cement, and masonry, into which fibers may be incorporated for purpose of reinforcing the material.

The present invention may employ conventional fibers without limited to particular fibers made of particular material. The fiber employed in the present invention may be glass fiber, acryl fiber, aramid fiber, carbon fiber, synthetic fibers such as nylon, polyester, polyethylene, polypropylene and polyvinyl alcohol (PVA) fiber, man-made fibers, or natural fibers such as sisal,

elephant grass and Kraft pulp, and preferably PVA fiber.

Further, the fiber having fineness of at least 200 denier may not improve toughness and ductility of concrete due to low potentiality to resist propagation of crack caused by static or dynamic load. Nevertheless, the fiber having fineness of 2 to 50 denier can effectively improve the homogeneity of FRC as well as toughness and ductility by splitting crack and absorbing fracture energy.

The thermoset resin used in the present invention may comprise conventional thermoset resin including epoxy, vinyl ester, unsaturated polyesters, phenolic resins or a mixture thereof.

The present invention further provides the method for preparing FRC comprising following steps: (1) coating the fibers with thermoset resin; (2) stapling the coated fibers ; and (3) mixing the stapled fiber coated with thermoset resin with concrete composition.

(1) the step of coating the fibers with thermoset resin The method for coating the fiber with thermoset resin is partially modified from the method for sizing warp threads for weaving or the method of prepreging for forming composite material.

The fiber of the present invention, preferably polyvinyl alcohol fiber is dipped in the thermoset resin, and the excessive thermoset resin is removed by passing the fiber through a squeezing roller under an appropriated pressure, tor example 10 to 15 kg/cm. The thermoset resin absorbed into the fiber is stiffened in a heating chamber. The production rate of the fibers coated

with the thermoset resin depends on the kind of the thermoset resin and hardening agent used, the temperature of heating chamber and the heating time.

One of the most important factors in coating the fiber with the thermoset resin is the amount of the thermoset resin used. When the thermoset resin is used in a small amount as in sizing the warp threads, i. e. , 2 to 5 weight percent, the fiber is not sufficiently mixed with concrete to avoid self-aggregation and thus the FRC cannot contain the fiber in a large amount.

! n contrast, when the thermoset resin is used in an excessive amount as in prcprcging for composite materials, i. c., 40 to 45 weight percent, the fiber may lose the ability to absorb the fracture energy. Accordingly, the thermoset resin is preferably used in the present invention in an amount of 7 to 20 weight percent, more preferably 10 to 15 weight percent based on the weight of fiber.

Further, the fiber absorbing the thermoset resin should be heated at a temperature that the thermoset resin is sufficiently stiffened. For instance, vinyl ester is stiffened by treated at a temperature of about 130'C for about 2 minutes. The complete stiffening of the thermoset resin provides the FRC with resistance to alkaline environment.

(2) the step of stapling the coated fibers The fiber coated with the thermoset resin can be stapled by conventional methods for making stapled fiber, and preferably a cutter with high degree of hardness may be used for stapling the coated fiber of the present invention.

The length of the stapled fiber of the present invention may be 10 to 30

mm, preferably 20 mm.

(3) the step of mixing the stapled fiber with concrete composition The stapled fiber coated with the thermoset resin of the present invention may be mixed with concrete composition by a known process to obtain FRC comprising the fiber coated with the thermoset resin. The stapled fiber coated with the thermoset resin of the present invention shows enhanced workability and miscibility in mixing with the concrete composition.

In a conventional FRC comprising the fibers which are not coated with the thermoset resin, it is difficult to incorporate the fiber in an amount of 1 weight % and the physical properties of the FRC may be deteriorated due to the self-aggregation of the fiber. However, the Fiber of the present invention can be incorporated in an amount of at least 1 weight % and the physical properties of the FRC such as toughness, ductility and ability of absorbing fracture energy are continuously enhanced in proportion to the amount of fiber incorporated, since the fibers of the present invention may prevent the self- aggregation.

Accordingly, the FRC comprising the fiber coated with the thermoset resin of the present invention has improved toughness and ductility owing to the resistance of the fiber coated with the thermoset resin to degradation in alkaline environment and self-aggregation.

The present invention is further described in the following Examples which are given for the purpose of illustration only, and are not intended to limit the scope of the invention.

Example 1: The preparation of the fiber coated with thermoset resin Polyvinyl alcohol fiber (the fineness of monofilament : 2 denier, the fineness of whole fibers : 200 denier, Kuray, Japan) was dipped in 15 weight % of vinyl ester (Korea Chemical, Co. ) based on the weight of the fiber for 10 seconds, followed by passed through a squeezing roller under a pressure of 10 kg/cm. Then the vinyl ester was stiffened in heating chamber at 13U C to obtain polyvinyl alcohol fiber coated with vinyl ester.

Example 2: The preparation of FRC comprising the fiber coated with thcrmosct rcsin Polyvinyl alcohol fiber obtained in Example 1 was cut to 20 mm by a diamond wheel cutter. Cut fiber was mixed with concrete (portland concrete, Hanil Cement) in the amount of 1 weight % based on the weight of FRC, and water was added thereto, followed by solidifying at room temperature for 28 days, to obtain the FRC comprising the fiber coated with thermoset resin of the present invention.

Experimental Example 1 : The test on the ph present invention Polyvinyl alcohol fiber (the fineness of mono filament : 5 denier, 200 filaments, Kuray, Japan) was coated with 15 weight % of vinyl ester based on the weight of fiber by the method of Example 1. Coated fiber was mixed with concrete in the amount of 0.75 weight % based on the weight of whole FRC by the method identical to Example 2 to obtain the FRC of the present invention. A control FRC was identically prepared to the present FRC except for coating polyvinyl alcohol fiber with vinyl ester. Flexural, tensile, compressive and shear strength of the present and control FRC were measured

by using a universal test machine (Heung-Jin Precision Machine), and the result is shown in Table 1. The compressive strength (KSF 2405) was measured using a cylindrical specimen of 10 cm in diameter and 20 cm in length at the cross head speed of 0.03 mm/sec. The tensile strength (KS F 2423) was indirectly tested by loading a cylindrical specimen of 10 cm in diameter and 20 cm in length until the specimen is broken at a right angle to the loading direction. Further, tlexural strength (KS F 2408) was measured by using a beam-shaped specimen of 10 cm in width and thickness and 40 cm in length.

[Tablc l] Physical properties Present FRC Control FRC Flexural Strength (kg/cm2) 3.5 2.3 Tensile Strength (kÇem2) 150 120 Compressive Strength (kg/cm2) 980 800 Shear Strength (kg/cm2) 345 312

According to the result, the present FRC shows superior physical properties, i. e., flexural, tensile, compressive and shear strength to the control FRC'.

Experimental Example 2: The test on the physical properties of FRC according to the amount of fiber to be incorporated Poly vinyl alcohol fiber (the fineness of mono filament : 5 denier, 200 filaments, Kuray, Japan) was coated with 15 weight % of vinyl ester based on the weight of Fiber by the method of Example 1. Coated fiber was mixed with concrete in the amount shown in table 2 based on the weight of whole

FRC by the method identical to Example 2 to obtain the present FRC. A control FRC was identically prepared to the present FRC except for coating polyvinyl alcohol fiber with vinyl ester. Flexural, tensile, compressive and shear strength of the present and control FRC were measured by using a universal test machine (Heung-Jin Precision Machine), and the result is shown in Table 2. table 2J Present FRC Control FRC Amount of fiber incorporated (wt/wt %) 0.37 0.75 1.10 1.50 1.80 0.37 0.75 1.10 1.50 1.80 Flexura1 Strength (kg/cm2) 3. l 3.5 3.5 3.7 3.8 2.0 2. 3 1. 9 1. 8 1.6 Tensile Strength (kg/cm2) 143 150 157 159 160 109 120 123 118 107 Compressive Strength 953 980 980 983 982 750 800 743 710 680 (kg/cm2) Shear Strength (kg/cm2) 340 345 342 350 340 313 312 290 270 230

According to the result, the physical properties of the present FRC was improved up to 1. 80 wt/wt % of coated fiber being incorporated. However, those of the control FRC were deteriorated when fiber was added in an amount of more than 0. 75 wt/wt %, and it was inferred to be due to the aggregation of fiber. Accordingly, it is believed that the present FRC efficiently prevents the self-aggregation of Fiber in the FRC.

Experimental Example 3: The test on the physical properties of the

FRC according to the amount of thermoset resin used in coating process.

Polyvinyl alcohol fiber (the fineness of monofilamcnt : 5 denier, 200 filaments, Kuray, Japan) was coated with vinyl ester in the amount shown in table 3 based on the weight of fiber by the method of Example 1. Coated fiber was mixed with concrete in the amount of 0.75 weight % based on the weight of whole FRC by the method identical to Example 2 to obtain the present FRC. Flexural, tensile, compressive and shear strength of the present FRC were measured by using a universal test machine (Heung-Jin Precision Machine), and the result is shown in Table 3.

[Tablc 3] Amount of vinyl ester (wt/wt%) 5 10 15 20 25 Flextural Strength (kg/cm2) 2.8 3.1 3.5 3.3 3.2 Tensile Strength (kg/cm2) 131 149 150 120 109 Compressive Strength (kg/cm2) 780 865 980 981 954 Shear Strength (kg/cm2) 307 332 345 308 287 According to the rcsult, the physical properties of the FRC were dctcrioratcd when the vinyl cstcr was uscd in the amount of morc than 20 weight % based on the weight of polyvinyl alcohol fiber.

Experimental Example 4 : The test on the physical properties of FRC according to the fineness of monofilament Polyvinyl alcohol fiber (200 filaments, Kuray, Japan) having the fineness of monofilament shown in table 4 was coated with 15 weight % of vinyl ester based on the weight of fiber by the method of Example 1. Coated fiber was mixed with concrete in the amount of 0.75 weight % based on the weight of whole FRC by the method identical to Example 2 to obtain the present. Tensile and compressive strength of the present FRC were measured by using a universal test machine (Hcung-Jin Precision Machine), and the result is shown in Table 4.

[Table 4] Fineness of monofilament (denier) 2 50 200 300 Tensile Strength (kg/cm2) 157 150 130 121 Compressive Strength (kg/cm') 980 970 910 897

According to the result, the tensile and compressive strength of the present FRC were dropped when the fiber having the fineness of monofilament of more than 200 denier was employed.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.