Although fibers of the present invention are suitable for reinforcing various matrix materials, such as adhesives, asphalts, composites, plastics, rubbers, etc., and structures made from these, the fibers that will be described herein are especially suited for reinforcing hydratable cementitious compositions, such as ready-mix concrete, precast concrete, masonry concrete (mortar), shotcrete, bituminous concrete, gypsum compositions, gypsum- and/or Portland cement-based fireproofing compositions, and others.
A major purpose of the fibers of the present invention is to reinforce concrete, e.g., ready-mix, shotcrete, etc., and structures made from these. Such matrix materials pose numerous challenges for those who design reinforcing fibers.
Concrete is made using a hydratable cement binder, a fine aggregate (e.g., sand), and a coarse aggregate (e.g., small stones, gravel). A mortar is made using cement binder and fine aggregate. Concretes and mortars are hence brittle materials. If a mortar or concrete structure is subjected to stresses that exceed its maximum tensile strength, then cracks can be initiated and propagated therein. The ability of the cementitious structure to resist crack initiation and crack propagation can be understood with reference to the “strength” and “fracture toughness” of the material.
“Strength” relates to the ability of a cement or concrete structure to resist crack initiation. In other words, strength is proportional to the maximum load sustainable by the structure without cracking and is a measure of the minimum load or stress (e.g., the “critical stress intensity factor”) required to initiate cracking in that structure.
On the other hand, “fracture toughness” relates to the specific “fracture energy” of a cement or concrete structure. This concept refers to the ability of the structure to resist propagation—or widening—of an existing crack in the structure. This toughness property is proportional to the energy required to propagate or widen the crack (or cracks). This property can be determined by simultaneously measuring the load required to deform or “deflect” a fiber-reinforced concrete (FRC) beam specimen at an opened crack and the amount or extent of deflection. The fracture toughness is therefore determined by dividing the area under a load deflection curve (generated from plotting the load against deflection of the FRC specimen) by its cross-sectional area.
In the cement and concrete arts, fibers have been designed to increase the strength and fracture toughness in reinforcing materials. Numerous fiber materials have been used for these purposes, such as steel, synthetic polymers (e.g., polyolefins), carbon, nylon, aramid, and glass. The use of steel fibers for reinforcing concrete structures remains popular due to the inherent strength of the metal. However, one of the concerns in steel fiber product design is to increase fiber “pull out” resistance because this increases the ability of the fiber to defeat crack propagation. In this connection, U.S. Pat. No. 3,953,953 of Marsden disclosed fibers having “J”-shaped ends for resisting pull-out from concrete. However, stiff fibers having physical deformities may cause entanglement problems that render the fibers difficult to handle and to disperse uniformly within a wet concrete mix. More recent designs, involving the use of “crimped” or “wave-like” polymer fibers, may have similar complications, depending on the stiffness of the fiber material employed.
Polyolefin materials, such as polypropylene and polyethylene, have been used for reinforcing concrete and offer an economic advantage due to relative lower cost of the material. However, these polyolefinic materials, being hydrophobic in nature, resist the aqueous environment of wet concrete. Moreover, the higher amount of polyolefin fibers needed in concrete to approximate the strength and fracture toughness of steel fiber-reinforced concrete often leads to fiber clumping or “balling” and increased mixing time at the job site. This tendency to form fiber balls means that the desired fiber dosage is not achieved. The correct concentration of fibers is often not attained because the fiber balls are removed (when seen at the concrete surface) by workers intent on achieving a finished concrete surface. It is sometimes the case that locations within the cementitious structure are devoid of the reinforcing fibers entirely. The desired homogeneous fiber dispersion, consequently, is not obtained.
Methods for facilitating dispersion of fibers in concrete are known. For example, U.S. Pat. No. 4,961,790 of Smith et al. disclosed the use of a water-soluble bag for introducing a large number of fibers into a wet mix. U.S. Pat. No. 5,224,774 of Valle et al. disclosed the use of non-water-soluble packaging that mechanically disintegrated upon mixing to avoid clumping and to achieve uniform dispersal of fibers within the concrete mix.
The dispersal of reinforcing fibers could also be achieved through packaging of smaller discrete amounts of fibers. For example, U.S. Pat. No. 5,807,458 of Sanders disclosed fibers that were bundled using a circumferential perimeter wrap. According to this patent, the continuity of the peripheral wrapping could be disrupted by agitation within the wet concrete mix, and the fibers could be released and dispersed in the mix.
On the other hand, World Patent Application No. WO 00/49211 of Leon (published Aug. 24, 2000) disclosed fibers “packeted” together but separable when mixed in concrete. A plurality of fibers were separably-bound together, such as by tape adhered to cut ends of the fibers, thereby forming a “packet.” Within a wet cementitious mix, the packets could be made to break and/or dissolve apart to permit separation and dispersal of individual fibers within the mix.
The dispersal of reinforcing fibers may also be achieved by altering fibers during mixing. For example, U.S. Pat. No. 5,993,537 of Trottier et al. disclosed fibers that progressively fibrillated upon agitation of the wet concrete mix. The fibers presented a “low initial surface area” to facilitate introducing fibers into the wet mix, and, upon agitation and under the grinding effect of aggregates in the mix, underwent “fibrillation,” which is the separation of the initial low-surface-area fibrous material into smaller, individual fibrils. It was believed that homogeneous fiber distribution, at higher addition rates, could thereby be attained.
A novel approach was taught in U.S. Pat. No. 6,197,423 of Rieder et al., which disclosed mechanically-flattened fibers. For improved keying within concrete, fibers were flattened between opposed rollers to attain variable width and/or thickness dimensions and stress-fractures perceivable through microscope as discontinuities and irregular and random displacements of polymer on the surface of the individual fibers. This microscopic stress fracturing was believed to improve bonding between cement and fibers, and, because the stress-fractures were noncontinuous in nature, the fibers were softened to the point at which fiber-to-fiber entanglement (and hence fiber balling) was minimized or avoided. The mechanical-flattening method of Rieder et al. was different from the method disclosed in U.S. Pat. No. 5,298,071 of Vondran, wherein fibers were interground with cement clinker and retained cement particles embedded into the surface.
In this vein, the nature of the fiber surface has also been a frequent topic of research in fiber dispersion and bonding in concrete. For example, U.S. Pat. No. 5,753,368 of Hansen disclosed a list of wetting agents such as emulsifiers, detergents, and surfactants to render fiber surfaces more hydrophilic and thus more susceptible to mixing in wet concrete. On the other hand, U.S. Pat. No. 5,753,368 of Berke et al. taught that the bonding between concrete and fibers could be enhanced by employing particular glycol ether coatings instead of conventional wetting agents that tended to introduce unwanted air at the fiber/concrete interface.
Of course, as mentioned in U.S. Pat. Nos. 5,298,071 and 6,197,423 as discussed above, physical deformation of the fiber surface was also believed to improve the fiber-concrete bond. U.S. Pat. No. 4,297,414 of Matsumoto, as another example, taught the use of protrusions and ridges to enhance bond strength. Other surface treatments, such as the use of embossing wheels to impose patterns on the fiber, were also used for improving fiber-concrete bond. Fiber designers have even bent fibers into sinusoidal wave shapes to increase the ability of fibers to resist being pulled out from concrete. However, the present inventors realized that increased structural deformations in the fiber structure may actually enhance opportunities for unwanted fiber balling to occur.
Against this background, the present inventors see a need for novel polymeric synthetic reinforcing fibers having ease of dispersibility in concrete so as to avoid fiber balling and to achieve intended fiber dosage rates, while at the same time to provide strength and fracture toughness in matrix materials and particularly brittle materials such as concrete, mortar, shotcrete, gypsum fireproofing, and the like.