The present invention relates to a cementitious memb reinforced by a non-woven mat of long steel fibers.

U.S. Patent 3,429,094 to Romualdi, U.S. Patents 3,986,885; 4,366,255 and 4,513,040 to Lan ard, U.S. Patent

4,617,219 to Schupak and U.S. Patent 2,677,955 to Constantine disclose metal fiber reinforced cementitious composites.

While these composites have been commercially successful, there are several areas in which improvement woul desirable. The fiber-filling step can be time-consuming and therefore expensive. The quantity of fiber required to devel improved resistance to thermal and mechanical shock is high. practice, the complete strength of the fiber is not always utilized.

Summary of the Invention

In accordance with the present invention, a metal fi reinforced structural member or composite is provided wherein member comprises a non-woven, pre-formed mat of metal fibers which is infiltrated by and encased in a hardened cementitiou composition; the mat is characterized in that the fibers are to .060 inches in effective diameter, greater than 3 inches i length and have an aspect ratio (length/diameter) of about 40 1000 or they are continuous, and the density of the mat expre as a percentage of the fiber occupied volume to the total vol of the mat is about 1 to 10%.

The fiber mat-reinforced composite of this invention advantageous for several reasons. First, for a comparable vo

of fibers, the composites of the invention provide higher fiexural strength. Consequently, for comparable flexural strength, the volume of fibers in the composite can be reduced The fiber mat reinforced composites of the invention exhibit higher energy absorption capacity than composites reinforced with discrete fibers, and their energy absorption efficiency (energy absorption capacity per unit volume of fibe is unexpectedly higher. The superior performance of the mat reinforcement over the discrete fibers is related to the bondi of the mat fibers in the composite. With discrete fibers havi relatively short embedment length (e.g., 1 in.) fiber pullout the primary failure mode. For example, see U.S. Patent 3,986,885, Col. 5, lines 2-8 and U.S. Patents 3,429,094, Col 4 lines 5-10. Once a crack forms in the member under flexural load, the energy absorption capacity of the specimen reflects energy required to propagate and enlarge this single crack. Energy absorption is dictated by fiber pullout resistance.

Composites reinforced with the metal fiber mats in accordance with this invention represent a departure from the prior art. The mode of failure in flexure is distinctly different. Here, multiple cracking occurs in the composites. Ultimate failure occurs through fiber breakage in high tensile stress zones of one or more of the crack planes. The energy absorption capacity of the mat reinforced composites reflects total energy required to initiate, enlarge, and propagate all the cracks and to break a portion of the fibers in one or more the cracks. In the fiber mat reinforced composites described herein, the yield strength of the steel is more fully utilized Contrary to what one skilled in the art might have expected ba on short fiber reinforced composites, energy absorption is not dictated by fiber pull out resistance. A further advantage of reinforcing concrete composites in accordance with this invent as opposed to using individual fibers is that a known amount o fiber can be placed into the composite at a known cost.

Brief Description of the Drawing

Fig. 1 is a photograph of a steel fiber mat useful i accordance with the invention.

Fig. 2 is a perspective view of a reinforced concret structure in accordance with the present invention.

Fig. 3 is a cross-sectional view of fiber mat reinfo structural member in accordance with the invention.

Fig. 4 is a photograph of a cracked steel fiber reinforced comparative composite. Fig. 5 is a photograph of a cracked long steel fiber composite in accordance with the invention.

Definition

The term "non-woven" as used herein with respect to metal fiber mats means that the fibers forming the mat are no systematically woven. The mat is held together by random entanglement of the fibers.

The term "effective diameter" is used herein as it i used in the art, namely, to mean the diameter of a circle the area of which is equal to the cross-sectional area of the ste fiber.

Detailed Description of the Invention

The fibers forming the reinforcing mat used in the present invention are greater than 3 inches long, preferably greater than 6 inches long and still more preferably about 7 12 inches long. The fibers have an aspect ratio of about 400 1000.

Fiber mats useful in accordance with the invention commercially available from Ribtec, Ribbon Technology

Corporation, Gahanna, Ohio under the tradename SIMCON. One suc mat is shown in Fig. 1.

Metal fiber mats useful in the present invention can b prepared by the methods and apparatus described in U.S. Patents 4,813,472 and 4,930,565 to Ribbon Technology Corporation. Thes patents disclose the production of metal filamentary materials ranging from a size less than one inch up to semicontinuous fibers. To-prepare metal fiber mats, the aforesaid methods may be used to prepare fibers. The fibers are drawn from the molte metal using a melt overflow or melt extraction technique simila to that described in these patents and related patents. Other methods may be used to prepare long fibers. For example, slit sheet processes and milling processes may be used to prepare lo fibers which in turn are air layed and compressed into mats. T fibers are preferably about 7 to 12 inches long and more preferably about 9 inches long. The fibers are blown into a ch where they are air layed on a conveyor and compressed into a ma By controlling the rate of the conveyor and the extent of compression of the mat, the density of the mat can be controlle to produce mats in the range of 1.5 to 6.0% density.

The optimum fiber length may vary with each particula application and the nature of the fiber, e.g., its diameter, steel composition and the method by which it is manufactured. length may be selected at which the force required to pull the fiber from the concrete exceeds the force required to form a n crack in the concrete, i.e., the minimum fiber length should b sufficient to prevent pullout.

Typically, the fibers are steel fibers such as carbon steel, stainless steel or manganese steel. Stainless steel fibers are preferred for refractory applications. The fibers commonly range in effective diameter from about .004 to .060 i and more preferably form about .010 to .025 inch. Smaller diameter fibers are shown to provide higher energy absorption capacity in Examples 1 and 2 below. The fibers typically are

circular in cross-section but have thickness and width dimensions. For example in the examples they range from abou .02 to .05 in width and from about .005 to .015 inch in thickness. A fiber having a circular cross section is also useful, but noncircular fibers are more commonly available an often less expensive. Depending upon the strength of the fib they may be corrugated. However, for smaller diameter wires commonly used (e.g., .010 or .020 inch) corrugation is genera not desirable under tensile stress, the corrugation tends to pulled out of the wire. As the corrugation is pulled out of wire, the wire strength does not reinforce the concrete and t concrete cracks and fails.

The amount of fiber in the mat and the composite may range from about 1 to 10% by volume. In order to incorporate more than about 10% fiber into a composite, the mat must be compressed to an extent that it cannot readily be infiltrated with a cementitious mixture. Typical composites in accordanc with the invention are prepared from mats which contain about to 6% by volume fiber. The fibers may be randomly oriented in the composite oriented to maximize the strength of the composite in a selec direction. For example, the mat fibers may be oriented paral to the direction in which the structural member will encounte its principal tensile stress. In many applications, due to t geometry of the structural member, the fibers will assume so degree of orientation. For example, typically, the fibers a about 7 to 12 inches long. In making a panel 2 inches thick, fibers will be oriented generally perpendicular to the thick or Z direction of the panel and generally parallel to the X- plane of the panel. Within the X-Y planes, the fibers may as a parallel or a random alignment.

Any cementitious composition which will infiltrate fiber mat may be used in the present invention including hydraulic and polymer cements. Mortar and concrete composit

are useful. Representative examples of useful cements include Portland cement, calcium aluminate cement, magnesium phosphate cement, and other inorganic cements. Useful aggregates may ran up to about 30 mesh (0.023 inch) so they are not strained from the composition as they impregnate the mat. Examples of aggregates include sand and small gravels. Refractory concrete are used in making refractory shapes such as plunging bells, injection lances and ladle lip rings.

A superplasticizing agent may be added to the slurry o the cementitious material to better enable it to infiltrate the fibers and fill the mold. A superplasticizing agent is not required but is preferred. Without the superplasticizer, more water must be added to the slurry to infiltrate the mat. Superplasticizing agents are known and have been used in flowin concrete and water-reduced, high strength concrete. See for example "Superplasticized Concrete", ACI Journal, May, 1977, pp N6-N11 and "Flowing Concrete", Concrete Constr., Jan., 1979 (pp 25-27). The most common superplasticizers are sulfonated melamine formaldehyde and sulfonated naphthalene formaldehyde. The superplasticizers used in the present invention are those which enable the aqueous cementitious slurry to fully infiltrat the packed fibers. Of those plasticizers that are commercially available. Mighty 150, a sulfonated naphthalene formaldehyde available from ICI is preferred. Composites in accordance with the invention are useful in making a variety of structural members including panels, beams, columns, pavement slabs and refractory shapes. A typica embodiment of a reinforced structure in accordance with the invention is shown in FIG. 2. The embodiment there shown is of panel 10 provided with a non-woven metal fiber reinforcing mat which is completely embedded in a cementitious composition 13. The face 14 of the panel 10 generally has fibers of the reinforcing layer incorporated therein and clearly visible. In this embodiment, the fibers are randomly oriented. The ends 18

of fibers forming the mat 12 can be seen from the sides 20 an of panel 10.

In one embodiment of the invention, the mat fiber reinforced panel may be incorporated in a sandwich constructi where a pair of mat fiber reinforced panels sandwich a layer large aggregate concrete or a layer of concrete reinforced by discrete metal fibers. Such a structural member can be prepa by placing a fiber mat in a form and infiltrating that mat wi concrete slurry containing an aggregate which will not infilt the mat, e.g., a stone aggregate greater than 35 mesh. The l aggregate will be "screened" from the slurry and collect on t top surface of the mat. A second mat is placed in the form overlying the first and sandwiching the layer of large aggreg therebetween. This second mat is infiltrated with a cement slurry not containing the larger aggregate resulting in a sla which the large area surfaces are mat reinforced and the core larger aggregate concrete. Alternatively, metal fibers of th type referred to in the Romualdi and Lankard patents may be m with the concrete in an amount of 1 or 2% and poured between long fiber mat reinforced concrete layers to form a sandwich construction.

FIG. 3 illustrates, in a cross-sectional view, the sandwich structure 30 described above. This structure compri two layers 22 and 24 which are formed from a hydraulic cement matrix with or without aggregate fillers and reinforced with woven elements 23 and 25 and which are separated from each ot by a layer of concrete 26. The thickness of the two reinforc outer layers 22 and 24 can be the same or different and the relative thickness of the two outer layers versus the thickne of the inner mortar or concrete layer can be varied over a wi range depending upon the particular application for which the resulting structure is to be employed. The inner layer of mo or concrete 26 can be reinforced, if desired, by means such a discrete metal fibers and the like to impart additional

structural strength to the sandwich structures of the invention The particular embodiment illustrated in FIG. 3 has two outer reinforcing layers enclosing a concrete core. However, as will be apparent to one skilled in the art, sandwich structures in accordance with the invention can be provided in which there ar a plurality of non-woven mat reinforced layers each of which is separated form its neighbor by a layer of concrete.

The reinforced structures of the invention can be prepared conveniently in a straightforward manner. In one preparation, the reinforcing member such as that illustrated in FIG. 1 is placed in a tray or mold the internal surface of whic optionally may be previously treated with a conventional mold release agent. An appropriate amount of cementitious compositi necessary to completely infiltrate and encapsulate the reinforcing member is then deposited on the latter. Means such as vibration, ultrasonic stimulation, and the like, can be employed in order to ensure thorough permeation of the reinforcing member by the cementitious composition. The upper surface of the mix can then be screeded if desired in order to ensure a planar surface of the desired finish. Thereafter, the impregnated reinforcing material is caused to cure by any conventional means.

The invention is illustrated in more detail by the following non-limiting examples.

Example 1

Two 304 stainless steel fiber mats were provided. Mat is 2 inch thick x 18 inch wide x 38 inch long. The weight of M A is 8.2 lb. These dimensions and the weight yield a fiber volume of 2.3 percent. The individual fibers comprising this have a length of 8.81 inch, a width of around 0.04 inch,- and thickness of about 0.011 inch (average of measurements on 10 fibers randomly selected from the mat) .

Mat B measures 2-1/2 to 3 inch thick x 20 inch wide x 38-1/2 inch long. The mat weighs 24.2 lb. These dimensions a weight yield a fiber volume percent of 4.5. However, when compressed to a 2 inch thickness, the fiber volume is 5.7 percent. The mats were individually placed in a plywood form a 2 inch x 20 inch x 40 inch long panel. Mat B is compressed a final thickness of about 2 inch by the sides of the form. A joints in the form are silicone sealed to contain the infiltrating slurry. The fiber mats are infiltrated with a fine-grained refractory concrete slurry (Wahl Refractories SIFCA Compositio Approximately 180 lb. of the slurry was used at a water conten of 18.5 percent to fill the form.

Following the slurry infiltration step, the slurry wa cured for 24 hours at 85 to 90° F. Following this curing perio the panels were removed from the mold and given a further four days cure at 80 ^* F at 100 percent relative humidity in a fog ro Following the fog room curing period, each panel was cut into inch wide beams using a diamond saw. Specimens 2 inch x 4 inc 20 inch were then oven dried at 230 ^* F prior to testing the flexural strength properties.

For comparison, a composite reinforced by discrete fibers was prepared using 1.0 inch long stainless steel fibers These specimens were prepared using the procedure described in U.S. Patent No. 4,366,255. The fiber content of the compariso composite is 14 volume percent. The composite specimens were cured and dried in the same manner as the composites prepared using the stainless steel fiber mats.

Flexural strength testing was done on two of the 2 i x 4 inch x 20 inch beams prepared form the various composites. Testing was done using third-point loading and a 12 inch span. Load-deflection data were recorded during the flexural streng testing. Testing was done in accordance with ASTM C1018, The Standard Test Method For Flexural Toughness and First-Crack

Strength of Fiber-Reinforced Concrete (using beam with third- point loading). The results are shown in Table 1. Table 1 compares the ultimate flexural strength and a measure of the flexural toughness index for the three composites investigated here. The comparison composite with a fiber loading of 14 vol percent had an ultimate flexural strength of 6440 psi. The composite prepared with Mat B (fiber loading = 5.7 volume percent) had 40 percent of the fiber loading of the convention composite yet exhibited an ultimate flexural strength 85 perce that of the conventional composite (5470 psi vs. 6440 psi).

Table 1- Flexural Strength Properties of Compr^ites Reinforced with Type 304

Stainless Steel Fibers in Mat Form and in Discrete 1.0 Inch Long Fiber Form

(a) Total area measured under the load-deflection curve to a deflection of 0.35 in.

(b) Relative to the composite yielding the highest area under the load-deflection curve (Mat B ⁾

Load-deflection data were recorded during the flexura strength testing of the composites. To permit a comparison between the composites evaluated here, a total area under the load-deflection curve to a maximum deflection of 0.35 inch was calculated. These results are also presented in Table 1.

A comparison of the areas under the load-deflection curves between the comparison composite at 14 volume percent fiber and Mat B at 5.7 volume percent fiber establish the superior energy absorbing capacity provided by the long fiber mode of reinforcement. The superior performance of the mat reinforcement over the discrete fibers is related to the bondi of the long mat fibers in the composite. In the comparison composite, the relatively short embedded length of the fibers (about 1.0 inch) results in fiber pullout as the primary failu mode. Once a crack forms in a comparison specimen under a flexural load, the energy absorption capacity of the specimen reflects the energy required to propagate and enlarge this sin crack. Energy absorption is dictated by fiber pullout resistance. The cracked sample is shown in Fig. 4. In the composites reinforced with the stainless steel fiber mats, multiple cracking occurs in the specimen subjected flexural loading. In the specimens containing the mat reinforcement, at least 50 cracks have developed in that porti of the specimens subjected to high tensile stresses. The ener absorption capacity of this composite reflects the energy required to initiate, enlarge, and propagate all of these crac and to break a portion of the 304 stainless steel fibers. Ultimate failure in the mat reinforced composite is breaking o fibers in one or more of these crack planes. This cracked sam is shown in Fig. 5. The cracks have been blackened for illustration.

The improvement which the invention provides in energ absorption efficiency as calculated by dividing the energy absorption capacity by the volume percent fiber loading is sho in Table 1A

Table 1A Energy Absorption Efficiency

Four carbon steel (manganese) fiber mats were used i preparation of panels. The mats measured approximately 40 in 20 inch x 2 to 3 inch thick. The mats are identified as foll

Mat C - 1% v/o Effective Diameter 0.010 inch.

MN Steel Mat D - 2% v/o Effective Diameter 0.013 inch MN Steel

Mat E - 4% v/o Effective Diameter 0.021 inch

MN Steel Mat F - 4% v/o Effective Diameter 0.013 inch MN Steel

Ten fibers from each mat were selected at random fo characterization. The characterization included measurements weight, length, width, and thickness. The results of the characterization measurements are presented in Table 2. The fiber loading values differ from the intended values of 1.0, and 4.0 volume percent. The reported values above were calcu based on fiber mat weight and dimensions.

Table 2. Characterization Measurements of Ribtec Carbon Steel (Manganese) Fibers Incorporated Into Fiber "Mats" on August 2, 1991

I

The length of the individual fibers is about 9-1/2 i Fiber width varied from 0.022 inch to 0.039 inch with thickne varying from 0.006 inch to 0.011 inch.

Each of fiber mats C-F was enclosed in a 20 inch x 4 inch x 2 inch wooden mold with the open face (filling port) b the 2 inch x 40 inch dimension. A calcium aluminate cement-b slurry (SIFCA Slurry manufactured by Wahl Refractories, Fremo Ohio) was used to infiltrate these panels. Once infiltration achieved, the panels were cured one day in the mold and then placed in a 74 ^* F/100%RH environment until two individual spec (roughly 2 inch x 4 inch x 20 inch) were sawcut from each pan

For comparison purposes, three 2 inch x 4 inch x 14 beam specimens were also prepared with the comparison composi containing 14 volume percent 1 inch long 304 stainless steel fibers prepared in Example 1. These specimens were also cure 74 ^* F/100%RH until sawcutting of the panels was complete.

Following the sawcutting step, all of the specimens oven dried at 230' F before flexural strength testing.

The criteria used for evaluating the reinforcement efficiency of these composites was flexural strength and flex load-deflection behavior. Flexural strength testing was done two beams for each fiber loading using third-point loading an 12 inch span. Table 3 compares the ultimate flexural strengt a measure of the flexural toughness for the composites investigated here.

Load-deflection data were recorded during the flexur strength testing of the composites. To permit a comparison between the composites evaluated here, a total area under the load-deflection curve to a maximum deflection of 0.35 inch, w calculated.

The mode of failure of the mat specimens in flexure through a breaking of the fibers at the point of highest tens stress in the specimen (bottom surface of the beam specimens the flexural strength test). The mat composites do not fail

Table 3. Ultimate Flexural Strength and Energy Absorption Capacity of Manganese Steel Mat Reinforced Composite Containing 1.2 to 3.6 Volume Percent Fibers

Oϊ I

catastrophically inasmuch as the fibers in 2ones away from th highest tensile stress zone do not fail at the same strain capacity. Fibers on the compression surface of the beams (to side) remain intact even at extremely high deflections. The mode of failure of the comparison composite with

1.0 inch fibers is principally fiber pullout.

The average ultimate flexural strength of the compar composite (14 volume percent 1 inch fibers) is 6440 psi. Manganese carbon steel Mats E and F at fiber loadings of 3.3 3.6 volume percent respectively provided an ultimate flexural strength of 4800 psi in the composite (roughly 75 percent tha the comparison composite). Manganese steel fiber Mat D at a loading of 1.7 volume percent had an average ultimate flexura strength of 3200 psi or 50 percent that of the comparison composite. Manganese steel Mat C at a fiber loading of 1.2 v percent had an average ultimate flexural strength of 1950 psi around 30 percent that of the comparison composite.

The efficiency of the mat concept regarding the achievement of flexural strength in composites is clearly demonstrated by these results. Mat F at a reinforcement leve only 25 percent that of the Comparison composite had an ultim flexural strength 75 percent that of the Comparison. Mat D a reinforcement level only 12 percent that of the Comparison provided an ultimate flexural strength of 50 percent that of Comparison.

The reinforcement efficiency of the mat of reinforce over the standard discrete fiber approach is even more clearl demonstrated by the ability of the mats to enhance energy absorption capacity of these composites and by the significan enhancement in energy absorption efficiency shown in the long fiber mats. Mat F with a fiber loading only 25 percent that the Comparison had an energy absorption capacity 15 percent greater than the Comparison. Energy absorption efficiency is shown in Table 3A.

Table 3A Ener Absor tion Efficienc

A comparison of the energy absorption capacity of the composites reinforced with Mat D and Mat E reveal the important effect of fiber diameter on this property in the mat reinforced composites. Mat D provided a fiber loading roughly half that o Mat E, yet provided an equivalent energy absorption capacity in the composite. The only intended difference in the two mats is smaller fiber diameter in Mat D (0.013 inch) relative to Mat E (0.021 inch). The influence of fiber diameter is also seen in the comparison of the energy absorption capacity of the composites prepared with Mat E and Mat F. Here, the fiber loading is roug equivalent (3.3 and 3.6 volume percent) but the energy absorpti capacity in the finer diameter steel Mat F is 35 percent greate than that in the composite containing the coarser fiber Mat E. The ultimate flexural strength of these two mat reinforced composites is equivalent (around 4800 psi).

The reason for the effect of fiber diameter on energy absorption capacity was revealed by an inspection of the beam specimens following flexural strength testing. In the composi containing the larger diameter fiber (Mat E), considerably few cracks were formed relative to the specimens containing the fi

fiber (equivalent diameter = 0.013 inch). For example, in the coarse fiber composite (Mat E) the number of cracks formed was around five while in the fine fiber composite (Mat D) over 20 cracks were formed. Additionally, in the composite prepared w Mat F (3.6 volume percent of the 0.013 inch diameter fiber) ultimate failure occurred through a shear plane oriented rough at a 45 degree angle to the long dimension of the beam. Multi cracking also occurred around this diagonal crack.

Having described the invention in detail and by refer to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing fr the scope of the invention defined in the appended claims.

What is claimed is: