US20090193749A1

US20090193749A1 - Internally trussed monolithic structural members

Info

Publication number: US20090193749A1
Application number: US12/151,188
Authority: US
Inventors: Michael P. Gembol
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-02-05
Filing date: 2008-05-05
Publication date: 2009-08-06

Abstract

A prefabricated, monolithic, structural building panel of box-like configuration includes face, side and end members, all constructed of fiber reinforced concrete, the panel having a plurality of internal truss members, also constructed of fiber reinforced concrete, extending angularly between the panel face members. Forms for the casting of the structural components making up the panel are fabricated from a light weight, insulative material that is left in place within the panel after casting to provide heat and sound insulation.

Description

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 61/063,637 filed on Feb. 5, 2008, and the benefit of U.S. Provisional Patent Application No. 61/069,734 filed on Mar. 17, 2008.

BACKGROUND OF THE INVENTION

1. Technical Field
This invention relates generally to internally trussed, structural members constructed of glass fiber reinforced concrete and to methods for fabricating those structural members.
More specifically, this invention relates to light weight, unitary, structural members such as building panels and beams that are fabricated entirely of fiber reinforced concrete with a plurality of integral truss members of the same fiber reinforced concrete extending between faces of the structural member to divide the spaces between those faces into compartments.
This invention further relates to methods for manufacturing the structural members and for incorporating insulating materials that fill the compartments therein.
2. Description of Related Art
It is common in the industry to employ either hollow core or solid concrete building panels to form walls, floors and roofs of structures. Solid concrete panels with steel reinforcement are ordinarily used in weight-bearing applications as well as in those requiring substantial structural strength to withstand high wind loading and the like. Such solid panels are extremely heavy, are poorly insulating, and are expensive and energy intensive in their production and transportation from the manufacturing facility to the job site.
Prefabricated hollow core panels in which part of the interior concrete is replaced by void space or insulating fill material are often presented as an alternative to solid panels, and examples of such panels are described in U.S. Pat. Nos. 5,966,896 to Tylman, U.S. Pat. No. 6,718,712 to Heath, U.S. Pat. No. 6,898,908 to Messenger et al, and to U.S. Pat. No. 6,955,014 to LeJeune et al. Each of the insulated or hollow core panels described in the patents have inherent disadvantages as do all other such panels known to the inventor. Among common disadvantages in the manufacturing of such panels are problems in maintaining structural strength of the panel after attaching an outer panel face to its mating inner face through a void space or fill material, difficulties in maintaining the fill or insulating material in proper position during casting, and the requirement for use of metal reinforcing or to employ special expensive cements in order to obtain sufficient strength in tension. Prior art panels and other structural members commonly require multiple complex elements, and a plurality of different materials, have unnecessary weight, use connecting means that transmit heat energy through the panels, and require complicated fabrication procedures. Also, many such panels lack structural or shear strength along one or more axes.
The invention described in this application provides light weight, high strength, monolithic structural members and a simple and novel method for their fabrication.

SUMMARY OF THE INVENTION

A monolithic structural member, such as a beam or a panel, includes a pair of generally parallel face members that are spaced apart and connected by means of truss members extending between the two faces. A pair of side panels, together with a pair of end panels, complete the structural member. All structural components, face members, trusses, and side and end panels, are fabricated entirely of fiber reinforced concrete to form a unitary monolithic unit. In a preferred embodiment, the truss members extend angularly between the face members and, optionally, between the end panels as well. The components of the structural member are shaped by pouring a slurry of fiber reinforced concrete around a form array that is constructed of a shaped, light weight insulative material, preferably a foamed plastic, and after casting, the forms are left in place to provide heat and sound insulation by filling the compartments or void spaces that would remain after stripping away a conventional form.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially cut-away oblique view of the structural member of this invention configured as a building panel with angular internal trusses;

FIG. 2 is another embodiment of the building panel illustrated in FIG. 1;

FIG. 3 is an elevational view showing one preferred embodiment of the truss member of the FIG. 2 panel;

FIG. 4 presents a detail view of the juncture between an internal truss member and a panel face;

FIG. 5 is a cross sectional view of a form assembly for the fabrication of the structural member shown in FIG. 1;

FIG. 6 is a side view of a face surface of an element of the form assembly shown in FIG. 5;

FIG. 7 is an oblique, partially cut-away view of a second building panel embodiment having vertical ribs extending between panel faces;

FIG. 8 is a sectional view taken along the lines 8-8′ of FIG. 7;

FIG. 9 is a cut-away, elevational view of a third building panel embodiment having a honeycomb structure extending between panel faces;

FIG. 10 is an oblique view showing the honeycomb structure of the FIG. 9 panel in a truss configuration;

FIG. 11 is an oblique, sectional view of the structural member of this invention configured as an internally trussed beam;

FIG. 12 is a cross sectional view of the beam of FIG. 11;

FIG. 13 depicts an arrangement of structural members of this invention in which the beam of FIG. 11 is employed as a column supporting a plurality of those panel members illustrated in FIGS. 1 and 2;

FIG. 14 depicts another arrangement of the trussed beam structural members of FIGS. 11 and 12; and

FIG. 15 is an oblique, cut-away view of the internal truss for a structural member that is formed by practice of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

This invention comprises a high strength, light weight, internally trussed, and monolithic structural member, such as a beam or building panel, that is cast from fiber reinforced concrete, and further comprises a method for the fabrication of that panel. Portland cement concrete is a relatively brittle material having substantial strength in compression but little strength in tension or shear and thus does not have sufficient strength to be employed in this invention. It has been found that a fiber reinforced concrete mixed to form a castable slurry and containing a sufficient quantity of high-strength reinforcing fibers to impart a significant degree of tensile strength to the resulting composite is suitable for use in the manufacture of the herein described structural members.
A first embodiment of this invention, comprising a building panel, is shown in FIGS. 1 and 2 at 10. Panel 10 includes an upper face member 12 and a spaced apart lower face member 14, which is disposed generally parallel to face 12. A plurality of truss members 16 extend angularly between the face panels and provide both compressive and shear strength to the panel. A pair of side members 18 and 19, together with a pair of end members (not shown in this view), complete the panel. All structural components making up the panel, face members, trusses, side and end panels, are fabricated of fiber reinforced concrete to form a monolithic composite having no mechanical or adhesive bonds.
Although truss member 16 may be a simple rectangular plane, it is preferred that it be configured as is illustrated in FIGS. 2 and 3. As is best shown in FIG. 3, the truss member is formed in a generally planar configuration having a length equal to the interior length of panel 10 and a width equal to a diagonal distance between the panel face members 12 and 14. A series of adjacent triangular cut-outs, or voids 21, divide the truss member into a plurality of ribs 23 that extend, preferably angularly, between an upper truss beam 25 and a lower truss beam 26. That configuration results in an overall reduction in the weight of panel 10 without significant sacrifice of strength. It is to be noted that, as is illustrated in the detail view of FIG. 4, upper truss beam 25 may actually comprise a portion of the top face member 12 and the lower truss beam 26 may actually comprise a portion of lower face member 14 resulting in a monolithic structure without mechanical or adhesive connections or bonds between the truss ribs and face panels.
A unique feature of this invention is the use of a shaped, light weight insulative material to construct a form assembly, or structured array, for the casting of the structural components making up a structural member such as panel 10. After casting, the form assembly is left in place, thereby simplifying the construction of the forms, avoiding the step of stripping the forms from the cast composite, and providing heat and sound insulation by filling some, or all, of the compartments or void spaces that would remain after conventional casting in which the forms are stripped. The insulative material from which the form arrays are constructed need have only sufficient compressive strength to support the weight of the fiber reinforced concrete making up the panel, which allows the use of a variety of materials for the fabrication of the forms. Preferred form materials include foamed plastics such as those of polystyrene, polyurethane, and isocyanurate, and light weight mineral aggregates such as perlite or vermiculite in a concrete or other adhesive matrix. Selection of a particular form material is governed, at least in part, by the properties that are desired or required in the finished panel. Relevant properties may include the panel weight, the level of thermal insulation or sound absorption desired, and the fireproofing and toxicity requirements of a particular installation.
FIG. 5 shows at 30 a cross sectional view of a partial form assembly for fabricating the panel embodiment of FIGS. 1 and 2. Assembly 30 includes a plurality of light weight billets 32 of regular triangular cross section, each billet having a base 34 and sides 35. Adjacent billets are arranged one to another, a first billet positioned with the base 34 at the bottom and the next billet with base 34 at the top. Adjacent billets are joined and rigidly positioned substantially parallel one relative to another by means of spacers 37 to form a generally rectangular, panel-like assembly. It is preferred that the a cross section through a billet be shaped as an isosceles triangle having a base angle 39 that varies broadly from about 30 to about 75 degrees depending upon the truss configuration that is desired, but it is preferred that base angle 39 fall within the range of about 45 to 60 degrees
FIG. 6 shows a preferred arrangement of spacers 37 as they are fixed to and join adjacent billets 32. In this view, billet 32 is oriented with its base 34 flat and the surface of side 35 slanted away from the viewer. Although spacers 37 may be of any shape and arrangement, so long as they are of uniform thickness, and so may be used to join adjacent billets to produce a form assembly, substantial advantages are obtained by employing triangular spacers of uniform size and shape. It is preferred that spacers 37 be of equilateral shape to obtain maximum truss strength but, less desirably, spacers 37 may be shaped as an isosceles triangle having a base angle 41 suitably not smaller than about 45 degrees. The spacers 37 are sized relative to the billet side 35 as is shown, and arranged in the manner illustrated in FIG. 6. It is important that the bases of spacers 37 fit flush and level with the billet base 34. If there is a gap, then a viscous slurry of fiber reinforced concrete will have a difficult time flowing under it, leaving a void at a critical location and weakening the structural member. To ensure that flush fit, it is necessary to shape or bevel the base of spacers 37 to obtain a mating vertical face 43 on adjacent spacers as is best shown in FIG. 5.
The thickness of spacers 37 and their placement on the billet faces determines the dimensions of each individual truss member or rib 23, best shown in FIG. 3. Rib dimensions can vary over a fairly wide range depending upon the panel strength required and the panel weight desired. A typical high strength, light weight building panel having exterior dimensions of 8 feet in length, 4 feet in width, and having a thickness of 8 inches might typically employ spacers that are ½ inch in thickness and spaced about 1 inch apart to form a channel, and after casting, a rib 23 of those same dimensions.
Fabrication of a structural member, such as the building panel that has been described, may be accomplished by use of a simple, open-topped, box mold sized to the desired length and width of the structural member and having a depth equal to the member thickness. A layer of fiber reinforced concrete is then poured into the mold box and is leveled, as by vibration, to form a first, or bottom, panel face. Mold assembly 30 is then placed atop the panel face layer and additional fiber reinforced concrete slurry is poured into the mold to fill the channels between billets and spacers and to form side and end panels from the spaces at the ends and sides of the mold assembly as well as to cover the top of the mold assembly to form a second, or top, panel face. It does not matter whether the entire panel is poured in one step or if the various segments are poured sequentially so long as the delay between sequential pours is sufficiently brief as to preclude any significant degree of curing of the first poured batch It is often advantageous to subject the mold box to vibration during the casting process as that tends to increase the homogeneity of the resulting casting and reduces the possibility of voids remaining around the mold assembly. The resulting structure is then allowed to cure, resulting in a single piece, monolithic panel having the mold assembly encased therein.
Structural members of this invention, such as the described building panels, require far less concrete than do concrete structural members of conventional design and construction while equaling and often exceeding the strength of those conventional members. In preferred embodiments of this invention, the total volume taken up by the mold assembly greatly exceeds the volume of the fiber reinforced concrete making up the remainder of the structural member. Because of the very low specific gravity of foamed plastics and like materials from which the mold assembly is made as compared to concrete, there is a corresponding and generally equivalent reduction in weight of the resulting structural member. In most cases, the volume of the mold array will exceed 75% of the total volume of the structural member, reducing the weight of the resulting structural member by approximately the same amount as compared to a structural member of conventional construction. In many applications, the volume of the mold array may exceed 85% of the total volume of the structural member while at the same time equaling or exceeding the strength and performance of the conventional structure that it replaces. Important savings are thereby realized in materials cost through lessened concrete use, and substantial savings in transportation costs are also realized in the shipment of fabricated structural members from a production site to a use site.
Because the fiber reinforced concrete used in this invention is impervious to corrosion, the reinforcing fibers may be laid at, or even concentrated at, the panel surface to provide high tensile strength at those panel areas that are most highly stressed under flexural loading. Ordinary concrete, on the other hand, has insufficient tensile strength to withstand flexure stress while steel reinforcement cannot be placed near the surface of a concrete member because of its vulnerability to corrosion.
Turning now to FIGS. 7 and 8, there is illustrated at 50 another embodiment of a building panel constructed in accordance with this invention. The panel has a plurality of longitudinal ribs 52 that extend perpendicularly between a pair of spaced-apart face panels 54 and 55 to enclose a generally rectangular area 57. A pair of side panels 59 and 60 together with a pair of end panels 62 and 63 complete the panel. All of the structural components making up the panel are cast from fiber reinforced concrete. Conventional Portland cement concrete is not suitable as it does not have sufficient strength in tension or in shear to be employed in this invention. It has been found that a fiber reinforced concrete mixed to form a castable slurry and containing a sufficient quantity of high strength reinforcing fibers to impart a significant degree of tensile strength to the resulting composite is, however, suitable for use.
In a preferred embodiment of this invention, areas 57 comprise generally rectangular billets fabricated of a light weight insulative material, preferably a foamed plastic. Fabrication of the panels 50 may be accomplished in the same general fashion as described in the fabrication of panel 10, employing an open-topped box mold sized to the desired panel length and width and having a depth equal to the panel thickness. A layer of fiber reinforced concrete is put down on the mold floor to form a first, or bottom, panel face. Billets 57 are then placed atop the concrete layer in a regular parallel pattern with the spaces between billets defining the dimensions of the side and end panels and the thickness of the vertical ribs 52. Additional fiber reinforced concrete slurry is then poured into the mold to fill the spaces between and at the ends and sides of the billets and to form a second, or top, panel face. The resulting structure is then allowed to cure, resulting in a unitary and monolithic panel having the insulating mold billet members 57 encased therein.
Ribs 52 act as internal trusses along the longitudinal axis of the panel and impart great rigidity to the panel across its axial span. The panel is able to bear large facial and axial loads as well as high longitudinal shear loads but, unlike panel 10, is unable to bear large shear loads across the panel width, or perpendicular to the panel longitudinal axis.
In a preferred embodiment, ribs 52 are configured as shown in FIG. 3. In this embodiment, triangular spacers 21 are fixed between adjacent billets to form a unitary mold assembly having channels extending between an upper and a lower part of the rib. That modification reduces the weight of each vertical rib by as much as 75% without a significant sacrifice of strength. It also significantly reduces the thermal conductivity between the upper and lower panel faces as well as binding the billets together into a unitary form assembly that maintains rib spacing and uniformity.
Referring now to FIG. 9, there is illustrated at 70 another building panel embodiment having a honeycomb structure with ribs 72 that extend between the panel faces to define a plurality of hexagonal cells 73. As in the previously described embodiments, all structural components of panel 70 are cast from fiber reinforced concrete. Ribs 72 are shown in an oblique view in FIG. 10. Each rib segment includes an angular truss member 75 that extends diagonally from one corner of a rib segment to another. The hexagonal cells 73 are defined by shaped billets of a light weight insulative material, such as a foamed plastic, and are bonded together by triangular spacers that are placed apart in a pattern that defines truss members 75. The resulting structural array then comprises a unitary form assembly which, after filling with a slurry of fiber reinforced concrete, results in the illustrated structure having the form assembly encased therein.
This same general approach taken in the fabrication of panels is equally applicable to the construction of internally trussed beams and columns as is illustrated in FIG. 11. That Figure shows an oblique sectional view of a column 80 that is constructed according to this invention while FIG. 12 comprises a planar cross section view of that same column. Column 80 includes four column face members 82 having a plurality of regularly spaced truss members 84 extending diagonally between opposing corners of the column. Areas 86, between the interior of the column face members and the truss members, comprise billets of a light weight, insulative foam material. Billets 86 are bonded together through triangular spaces that are placed apart in a regular pattern in the manner previously described to create a structural array which comprises a unitary form assembly. That form assembly, after being filled with a slurry of fiber reinforced concrete, results in the illustrated beam structure which also encases the form assembly.
FIG. 13 depicts an arrangement of structural members fabricated according to this invention. In this illustration, beam 80 is employed as a column that supports a plurality of building panels 10 through a variety of connection methods. For example, an indent 91 sized to fit the end of building panel 10 may be formed in one face of beam 80 during casting to physically secure the panel to the column. In analogous fashion, a tongue 93 may be formed along a beam face during casting and a mating groove 95 may be formed along a panel edge during cast to again secure the panel to the casting. A variety of other techniques for securing a panel edge or end to a column or beam may also be used as is generally illustrated at 97. Additionally, FIG. 14 illustrates at 100 another possible arrangement for the connection of beams 80 to form a supporting framework for a building or other structure.
FIG. 15 is a cut-away view showing the arrangement of the internal truss system for the building panel 10 of this invention that is best illustrated in FIG. 2. It depicts the interconnected structure of ribs 23 as they would appear were the face, side, and end panels to be removed from panel 10 and the forms stripped away as well leaving the truss ribs as the only structural members shown. The extraordinary complexity of the truss ribs array shown requires only a few simple steps to complete, using as few as two form components and a few minutes of fabrication time, resulting in a light weight, immensely strong but competitively priced product.
As can readily be appreciated from this view, the internal truss system that is obtained according to this invention provides high strength and rigidity along every axis of the resulting structural member.
A further illustration of a building panel of this invention and of its fabrication is set out in the following example.

EXAMPLE

A full-size, structural building panel conforming to the invention embodiment of FIGS. 1-4 was fabricated. The panel measured 10 feet in height, 4 feet in width, and 8 inches in thickness and was fabricated from fiber reinforced concrete having a compressive strength of approximately 8,000 psi. All face, end and side panels and all internal truss panels were one-half inch in thickness and the finished panel weighed 510 pounds. That compares to a weight of at least 4,100 pounds for a solid concrete panel of the same dimensions. The panel was found to have a thermal insulation rating of R-24. It was then tested to simulate a uniform 150 mph wind loading using a standard test protocol resulting in a total deflection at the center of the panel of 0.028 inches, which is less than one-tenth of the allowable deflection. It was concluded that the physical properties of the test panel allowed its use as a structural wall panel or as a floor panel having strength sufficient to support both concentrated loads, as from furniture and fixtures, as well as distributed loads such as building occupants. Panels of lesser weight and strength for use as non-structural partition walls may be constructed in the same fashion. For example, reducing the total panel thickness from 8 inches to 4 inches and reducing the thickness of the face, end and side panels and all internal truss panels from one-half to one-quarter inch in thickness would result in a panel weight of approximately 255 pounds.
The fibers that may be employed to reinforce the concrete used to manufacture the building panel of this invention include all of those that are stable in the strongly alkaline concrete environment and that impart sufficient strength to the composite. Exemplary fibers that are suitable for use in this invention include alkali resistant (zirconia) glass, corrosion resistant metal fibers, graphite, and the like. Of these various fibers, zirconia glass fibers are presently preferred. The fiber level, or concentration in the concrete, will ordinarily be in the general range of 1% to 5% by volume, but that can vary depending upon the desired characteristics of the resulting concrete.
Panel surfaces, both exterior and interior, may be textured and/or pigmented to provide a decorative finished appearance to the panel surfaces. That capability allows the modular construction of finished commercial and residential spaces when both exterior and exterior surfaces are so treated.
Many other variations in the designs and fabrication techniques that are set out in this disclosure will become apparent to others skilled in this art, and such variations are specifically included within the scope of this invention.

Claims

1. An internally trussed, unitary, structural member comprising:

a first and a second face member, each said member consisting essentially of fiber reinforced, Portland cement concrete, said face members being generally planar and parallel one to the other;

a first and a second edge member, each said member consisting essentially of fiber reinforced, Portland cement concrete, said edge members extending between the face members at a border thereof to define an enclosed space; and

a plurality of truss members extending between said first and second face members, each said truss member consisting essentially of fiber reinforced Portland cement concrete.

2. The structural member of claim 1 including truss members that extend between said first and second edge members.

3. The structural member of claim 1 wherein the combined volume of said face members, said edge members, and said truss members is less than 25% of the volume of said enclosed space.

4. The structural member of claim 1 wherein the volume that is not occupied by said face members, edge members, and truss members comprises a light weight, solid material shaped to form a mold assembly that defines said truss members.

5. A unitary mold assembly for fabrication of an internally trussed structural member manufactured entirely of fiber reinforced, Portland cement concrete comprising:

a plurality of form billets arranged in a planar and parallel relationship to create a form assembly, all of said billets having the same shape and size, each said billet connected to the next adjacent billet through a plurality of spacer means, said spacer means arranged on the billet surfaces to define a plurality of channels, each said channel extending from one side of said form assembly to its opposing side.

6. A method for fabricating a structural member comprising:

providing an open-topped box mold sized to the desired length and width of the structural member, said mold having a depth equal to the thickness of said structural member;

pouring a slurry of fiber reinforced Portland cement concrete into said box mold and leveling said slurry in the box mold bottom to form a first member face;

placing a form assembly atop the first member face, said form assembly comprising a plurality of form billets, all of said billets having the same shape and size, each said billet connected to the next adjacent billet through a plurality of spacer means, said spacer means arranged on the billet surfaces to define a plurality of channels, each said channel extending from one side of said form assembly to its opposing side;

adding additional concrete slurry into said mold box to fill the channels between billets, to cover the mold assembly, and to form a second member face; and

allowing the concrete to cure to thereby produce a single piece, unitary structure having the mold assembly encased therein.

7. The structural member of claim 1 wherein the fibers employed to reinforce said Portland cement concrete are stable in a strongly alkaline environment.

8. The structural member of claim 7 wherein the fibers employed to reinforce said Portland cement concrete are selected from the group consisting of zirconia glass, corrosion resistant metal fibers, and graphite and wherein the fiber concentration in the Portland cement concrete is in the range of 1% to 5% by volume.

9. The structural member of claim 1 wherein said truss members are disposed parallel one to another, and extend between said face members.

10. The structural member of claim 9 wherein each said truss member includes an upper truss beam and a lower truss beam, each of said truss beams extending the length of said truss member, said upper and lower truss beams connected by a plurality of rib members that extend between said upper and lower truss beams.

11. The structural member of claim 10 wherein alternate ones of said rib members are disposed parallel one to another and extend diagonally between said truss beams to form a triangular rib lattice.

12. The structural member of claim 10 wherein said upper and lower truss beams are integral with said upper and lower face members respectively.

13. The mold assembly of claim 5 wherein said billets and said spacer means comprise a foamed plastic.

14. The mold assembly of claim 13 wherein said foamed plastic is selected from the group consisting of polystyrene, polyurethane, and isocyanurate.

15. The mold assembly of claim 5 wherein said billets and spacer means are fabricated of a light weight mineral aggregate in an adhesive matrix.

16. The mold assembly of claim 15 wherein said light weight mineral aggregate is selected from the group consisting of perlite, vermiculite, and mixtures thereof.

17. The mold assembly of claim 5 in which the mold assembly has a top side and a bottom side and wherein said billets are triangular in cross-section, each billet having a base and two sides, said billets disposed such that a first billet is positioned with its base plane and generally parallel with the mold assembly top side and the adjacent billet positioned with its base plane and generally parallel with the mold assembly bottom side.

18. The mold assembly of claim 17 wherein the cross section of each said billet is in the shape of an isosceles triangle having a base angle ranging from about 30° to about 75°.

19. The mold assembly of claim 18 wherein said base angle ranges from 45° to 60°.

20. The mold assembly of claim 17 wherein said spacer means are triangular is shape, are of uniform size and thickness, and are equally spaced along said billets.

21. The mold assembly of claim 20 wherein the bases of adjacent triangular spacers are shaped such that they fit flush and level with the bases of adjacent billets.

22. The method of claim 6 wherein said form billets are triangular in cross section, each billet having a base and two sides that meet at an apex, said billets being disposed such that a first billet is positioned with its base plane and generally parallel with the bottom of said box mold, and the next adjacent billet positioned with its apex plane and generally parallel with the top of said box mold.

23. The method of claim 22 wherein said spacer means are triangular in shape, are of uniform size and thickness, and are equally spaced along said billets to form channels of uniform cross section.

24. The method of claim 23 wherein the total volume of said mold assembly is more than half of the total volume of said structural member.

25. The method of claim 23 wherein the total volume of said mold assembly is more than three-quarters of the total volume of said structural member.

26. The method of claim 6 wherein the fibers employed to reinforce said Portland cement concrete are stable in a strongly alkaline environment.

27. The method of claim 6 wherein the fibers employed to reinforce said Portland cement concrete are selected from the group consisting of zirconia glass, corrosion resistant metal fibers, graphite, and mixtures thereof, and wherein the fiber concentration in the Portland cement concrete is in the range of 1% to 5% by volume.