Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. Pat. No. 6,986,812 entitled SLURRY FEED APPARATUS FOR FIBER-REINFORCED STRUCTURAL CEMENTITIOUS PANEL PRODUCTION and co-pending application U.S. Ser. No. 10/665,541 entitled EMBEDMENT DEVICE FOR FIBER-ENHANCED SLURRY, filed concurrently herewith and herein incorporated by reference. 
   FIELD OF THE INVENTION 
   This invention relates to a continuous process and related apparatus for producing structural panels using a settable slurry, and more specifically, to a process for manufacturing reinforced cementitious panels, referred to herein as structural cementitious panels (SCP) (also known as structural cement panels), in which discrete fibers are combined with a quick-setting slurry for providing flexural strength and toughness. The invention also relates to a SCP panel produced according to the present process. 
   Cementitious panels have been used in the construction industry to form the interior and exterior walls of residential and/or commercial structures. The advantages of such panels include resistance to moisture compared to standard gypsum-based wallboard. However, a drawback of such conventional panels is that they do not have sufficient structural strength to the extent that such panels may be comparable to, if not stronger than, structural plywood or oriented strand board (OSB). 
   Typically, the present state-of-the-art cementitious panels include at least one hardened cement or plaster composite layer between layers of a reinforcing or stabilizing material. In some instances, the reinforcing or stabilizing material is continuous fiberglass mesh or the equivalent, while in other instances, short, discrete fibers are used in the cementitious core as reinforcing material. In the former case, the mesh is usually applied from a roll in sheet fashion upon or between layers of settable slurry. Examples of production techniques used in conventional cementitious panels are provided in U.S. Pat. Nos. 4,420,295; 4,504,335 and 6,176,920, the contents of which are incorporated by reference herein. Further, other gypsum-cement compositions are disclosed generally in U.S. Pat. Nos. 5,685,903; 5,858,083 and 5,958,131. 
   One drawback of conventional processes for producing cementitious panels that utilize building up of multiple layers of slurry and discrete fibers to obtain desired panel thickness is that the discrete fibers introduced in the slurry in a mat or web form, are not properly and uniformly distributed in the slurry, and as such, the reinforcing properties that essentially result due to interaction between fibers and matrix vary through the thickness of the board, depending on the thickness of each board layer and a number of other variables. When insufficient penetration of the slurry through the fiber network occurs, poor bonding and interaction between the fibers and the matrix results, leading to low panel strength development. Also, in extreme cases when distinct layering of slurry and fibers occurs, improper bonding and inefficient distribution of fibers causes inefficient utilization of fibers, eventually leading to extremely poor panel strength development. 
   Another drawback of conventional processes for producing cementitious panels is that the resulting products are too costly and as such are not competitive with outdoor/structural plywood or oriented strand board (OSB). 
   One source of the relatively high cost of conventional cementitious panels is due to production line downtime caused by premature setting of the slurry, especially in particles or clumps which impair the appearance of the resulting board, and interfere with the efficiency of production equipment. Significant buildups of prematurely set slurry on production equipment require shutdowns of the production line, thus increasing the ultimate board cost. 
   Thus, there is a need for a process and/or a related apparatus for producing fiber-reinforced cementitious panels which results in a board with structural properties comparable to structural plywood and OSB which reduces production line downtime due to prematurely set slurry particles. There is also a need for a process and/or a related apparatus for producing such structural cementitious panels which more efficiently uses component materials to reduce production costs over conventional production processes. 
   Furthermore, the above-described need for cementitious structural panels, also referred to as SCP&#39;s, that are configured to behave in the construction environment similar to plywood and OSB, means that the panels are nailable and can be cut or worked using conventional saws and other conventional carpentry tools. Further, the SCP panels should meet building code standards for shear resistance, load capacity, water-induced expansion and resistance to combustion, as measured by recognized tests, such as ASTM E72, ASTM 661, ASTM C 1185 and ASTM E136 or equivalent, as applied to structural plywood sheets. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The above-listed needs are met or exceeded by the present invention that features a multi-layer process for producing structural cementitious panels (SCP&#39;s or SCP panels), and SCP&#39;s produced by such a process. After one of an initial deposition of loosely distributed, chopped fibers or a layer of slurry upon a moving web, fibers are deposited upon the slurry layer. An embedment device mixes the recently deposited fibers into the slurry, after which additional layers of slurry, then chopped fibers are added, followed by more embedment. The process is repeated for each layer of the board, as desired. Upon completion, the board has a more evenly distributed fiber component, which results in relatively strong panels without the need for thick mats of reinforcing fibers, as are taught in prior art production techniques for cementitious panels. 
   More specifically, the invention relates to a multi-layer process for producing structural cementitious panels, including: (a.) providing a moving web; (b.) one of depositing a first layer of loose fibers and (c.) depositing a layer of settable slurry upon the web; (d.) depositing a second layer of loose fibers upon the slurry; (e.) embedding said second layer of fibers into the slurry; and (f.) repeating the slurry deposition of step (c.) through step (d.) until the desired number of layers of settable fiber-enhanced slurry in the panel is obtained. Also provided is a structural cementitious panel (SCP) produced by the present process, and an apparatus suitable for producing structural cementitious panels according to the present process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic elevational view of an apparatus which is suitable for performing the present process; 
       FIG. 2  is a perspective view of a slurry feed station of the type used in the present process; 
       FIG. 3  is a fragmentary overhead plan view of an embedment device suitable for use with the present process; 
       FIG. 4  is a fragmentary vertical section of a structural cementitious panel produced according to the present procedure; and 
       FIG. 5  is a diagrammatic elevational view of an alternate apparatus used to practice an alternate process to that embodied in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , a structural panel production line is diagrammatically shown and is generally designated  10 . The production line  10  includes a support frame or forming table  12  having a plurality of legs  13  or other supports. Included on the support frame  12  is a moving carrier  14 , such as an endless rubber-like conveyor belt with a smooth, water-impervious surface, however porous surfaces are contemplated. As is well known in the art, the support frame  12  may be made of at least one table-like segment, which may include designated legs  13 . The support frame  12  also includes a main drive roll  16  at a distal end  18  of the frame, and an idler roll  20  at a proximal end  22  of the frame. Also, at least one belt tracking and/or tensioning device  24  is preferably provided for maintaining a desired tension and positioning of the carrier  14  upon the rolls  16 ,  20 . 
   Also, in the preferred embodiment, a web  26  of kraft paper, release paper, and/or other webs of support material designed for supporting a slurry prior to setting, as is well known in the art, may be provided and laid upon the carrier  14  to protect it and/or keep it clean. However, it is also contemplated that the panels produced by the present line  10  are formed directly upon the carrier  14 . In the latter situation, at least one belt washing unit  28  is provided. The carrier  14  is moved along the support frame  12  by a combination of motors, pulleys, belts or chains which drive the main drive roll  16  as is known in the art. It is contemplated that the speed of the carrier  14  may vary to suit the application. 
   In the present invention, structural cementitious panel production is initiated by one of depositing a layer of loose, chopped fibers  30  or a layer of slurry upon the web  26 . An advantage of depositing the fibers  30  before the first deposition of slurry is that fibers will be embedded near the outer surface of the resulting panel. A variety of fiber depositing and chopping devices are contemplated by the present line  10 , however the preferred system employs at least one rack  31  holding several spools  32  of fiberglass cord, from each of which a cord  34  of fiber is fed to a chopping station or apparatus, also referred to as a chopper  36 . 
   The chopper  36  includes a rotating bladed roll  38  from which project radially extending blades  40  extending transversely across the width of the carrier  14 , and which is disposed in close, contacting, rotating relationship with an anvil roll  42 . In the preferred embodiment, the bladed roll  38  and the anvil roll  42  are disposed in relatively close relationship such that the rotation of the bladed roll  38  also rotates the anvil roll  42 , however the reverse is also contemplated. Also, the anvil roll  42  is preferably covered with a resilient support material against which the blades  40  chop the cords  34  into segments. The spacing of the blades  40  on the roll  38  determines the length of the chopped fibers. As is seen in  FIG. 1 , the chopper  36  is disposed above the carrier  14  near the proximal end  22  to maximize the productive use of the length of the production line  10 . As the fiber cords  34  are chopped, the fibers  30  fall loosely upon the carrier web  26 . 
   Next, a slurry feed station, or a slurry feeder  44  receives a supply of slurry  46  from a remote mixing location  47  such as a hopper, bin or the like. It is also contemplated that the process may begin with the initial deposition of slurry upon the carrier  14 . While a variety of settable slurries are contemplated, the present process is particularly designed for producing structural cementitious panels. As such, the slurry is preferably comprised of varying amounts of Portland cement, gypsum, aggregate, water, accelerators, plasticizers, foaming agents, fillers and/or other ingredients well known in the art, and described in the patents listed above which have been incorporated by reference. The relative amounts of these ingredients, including the elimination of some of the above or the addition of others, may vary to suit the application. 
   While various configurations of slurry feeders  44  are contemplated which evenly deposit a thin layer of slurry  46  upon the moving carrier  14 , the preferred slurry feeder  44  includes a main metering roll  48  disposed transversely to the direction of travel of the carrier  14 . A companion or back up roll  50  is disposed in close parallel, rotational relationship to the metering roll  48  to form a nip  52  therebetween. A pair of sidewalls  54 , preferably of non-stick material such as Teflon® brand material or the like, prevents slurry  46  poured into the nip  52  from escaping out the sides of the feeder  44 . 
   An important feature of the present invention is that the feeder  44  deposits an even, relatively thin layer of the slurry  46  upon the moving carrier  14  or the carrier web  26 . Suitable layer thicknesses range from about 0.05 inch to 0.20 inch. However, with four layers preferred in the preferred structural panel produced by the present process, and a suitable building panel being approximately 0.5 inch, an especially preferred slurry layer thickness is approximately 0.125 inch. 
   Referring now to  FIGS. 1 and 2 , to achieve a slurry layer thickness as described above, several features are provided to the slurry feeder  44 . First, to ensure a uniform disposition of the slurry  46  across the entire web  26 , the slurry is delivered to the feeder  44  through a hose  56  located in a laterally reciprocating, cable driven, fluid powered dispenser  58  of the type well known in the art. Slurry flowing from the hose  56  is thus poured into the feeder  44  in a laterally reciprocating motion to fill a reservoir  59  defined by the rolls  48 ,  50  and the sidewalls  54 . Rotation of the metering roll  48  thus draws a layer of the slurry  46  from the reservoir. 
   Next, a thickness monitoring or thickness control roll  60  is disposed slightly above and/or slightly downstream of a vertical centerline of the main metering roll  48  to regulate the thickness of the slurry  46  drawn from the feeder reservoir  59  upon an outer surface  62  of the main metering roll  48 . Another related feature of the thickness control roll  60  is that it allows handling of slurries with different and constantly changing viscosities. The main metering roll  48  is driven in the same direction of travel as the direction of movement of the carrier  14  and the carrier web  26 , and the main metering roll  48 , the backup roll  50  and the thickness monitoring roll  60  are all rotatably driven in the same direction, which minimizes the opportunities for premature setting of slurry on the respective moving outer surfaces. As the slurry  46  on the outer surface  62  moves toward the carrier web  26 , a transverse stripping wire  64  located between the main metering roll  48  and the carrier web  26  ensures that the slurry  46  is completely deposited upon the carrier web and does not proceed back up toward the nip  52  and the feeder reservoir  59 . The stripping wire  64  also helps keep the main metering roll  48  free of prematurely setting slurry and maintains a relatively uniform curtain of slurry. 
   A second chopper station or apparatus  66 , preferably identical to the chopper  36 , is disposed downstream of the feeder  44  to deposit a second layer of fibers  68  upon the slurry  46 . In the preferred embodiment, the chopper apparatus  66  is fed cords  34  from the same rack  31  that feeds the chopper  36 . However, it is contemplated that separate racks  31  could be supplied to each individual chopper, depending on the application. 
   Referring now to  FIGS. 1 and 3 , next, an embedment device, generally designated  70  is disposed in operational relationship to the slurry  46  and the moving carrier  14  of the production line  10  to embed the fibers  68  into the slurry  46 . While a variety of embedment devices are contemplated, including, but not limited to vibrators, sheep&#39;s foot rollers and the like, in the preferred embodiment, the embedment device  70  includes at least a pair of generally parallel shafts  72  mounted transversely to the direction of travel ‘T’ of the carrier web  26  on the frame  12 . Each shaft  72  is provided with a plurality of relatively large diameter disks  74  which are axially separated from each other on the shaft by small diameter disks  76 . 
   During SCP panel production, the shafts  72  and the disks  74 ,  76  rotate together about the longitudinal axis of the shaft. As is well known in the art, either one or both of the shafts  72  may be powered, and if only one is powered, the other may be driven by belts, chains, gear drives or other known power transmission technologies to maintain a corresponding direction and speed to the driving roll. The respective disks  74 ,  76  of the adjacent, preferably parallel shafts  72  are intermeshed with each other for creating a “kneading” or “massaging” action in the slurry, which embeds the fibers  68  previously deposited thereon. In addition, the close, intermeshed and rotating relationship of the disks  74 ,  76  prevents the buildup of slurry  46  on the disks, and in effect creates a “self-cleaning” action which significantly reduces production line downtime due to premature setting of clumps of slurry. 
   The intermeshed relationship of the disks  74 ,  76  on the shafts  72  includes a closely adjacent disposition of opposing peripheries of the small diameter spacer disks  76  and the relatively large diameter main disks  74 , which also facilitates the self-cleaning action. As the disks  74 ,  76  rotate relative to each other in close proximity (but preferably in the same direction), it is difficult for particles of slurry to become caught in the apparatus and prematurely set. By providing two sets of disks  74  which are laterally offset relative to each other, the slurry  46  is subjected to multiple acts of disruption, creating a “kneading” action which further embeds the fibers  68  in the slurry  46 . 
   Once the fibers  68  have been embedded, or in other words, as the moving carrier web  26  passes the embedment device  70 , a first layer  77  of the SCP panel is complete. In the preferred embodiment, the height or thickness of the first layer  77  is in the approximate range of 0.05-0.20 inches. This range has been found to provide the desired strength and rigidity when combined with like layers in a SCP panel. However, other thicknesses are contemplated depending on the application. 
   To build a structural cementitious panel of desired thickness, additional layers are needed. To that end, a second slurry feeder  78 , which is substantially identical to the feeder  44 , is provided in operational relationship to the moving carrier  14 , and is disposed for deposition of an additional layer  80  of the slurry  46  upon the existing layer  77 . 
   Next, an additional chopper  82 , substantially identical to the choppers  36  and  66 , is provided in operational relationship to the frame  12  to deposit a third layer of fibers  84  provided from a rack (not shown) constructed and disposed relative to the frame  12  in similar fashion to the rack  31 . The fibers  84  are deposited upon the slurry layer  80  and are embedded using a second embedment device  86 . Similar in construction and arrangement to the embedment device  70 , the second embedment device  86  is mounted slightly higher relative to the moving carrier web  14  so that the first layer  77  is not disturbed. In this manner, the second layer  80  of slurry and embedded fibers is created. 
   Referring now to  FIGS. 1 and 4 , with each successive layer of settable slurry and fibers, an additional slurry feeder station  44 ,  78  followed by a fiber chopper  36 ,  66 ,  82  and an embedment device  70 ,  86  is provided on the production line  10 . In the preferred embodiment, four total layers  77 ,  80 ,  88 ,  90  are provided to form the SCP panel  92 . Upon the disposition of the four layers of fiber-embedded settable slurry as described above, a forming device  94  ( FIG. 1 ) is preferably provided to the frame  12  to shape an upper surface  96  of the panel  92 . Such forming devices  94  are known in the settable slurry/board production art, and typically are spring-loaded or vibrating plates which conform the height and shape of the multi-layered panel to suit the desired dimensional characteristics. An important feature of the present invention is that the panel  92  consists of multiple layers  77 ,  80 ,  88 ,  90  which upon setting, form an integral, fiber-reinforced mass. Provided that the presence and placement of fibers in each layer are controlled by and maintained within certain desired parameters as is disclosed and described below, it will be virtually impossible to delaminate the panel  92  produced by the present process. 
   At this point, the layers of slurry have begun to set, and the respective panels  92  are separated from each other by a cutting device  98 , which in the preferred embodiment is a water jet cutter. Other cutting devices, including moving blades, are considered suitable for this operation, provided that they can create suitably sharp edges in the present panel composition. The cutting device  98  is disposed relative to the line  10  and the frame  12  so that panels are produced having a desired length, which may be different from the representation shown in  FIG. 1 . Since the speed of the carrier web  14  is relatively slow, the cutting device  98  may be mounted to cut perpendicularly to the direction of travel of the web  14 . With faster production speeds, such cutting devices are known to be mounted to the production line  10  on an angle to the direction of web travel. Upon cutting, the separated panels  92  are stacked for further handling, packaging, storage and/or shipment as is well known in the art. 
   Referring now to  FIGS. 4 and 5 , an alternate embodiment to the production line  10  is generally designated  100 . The line  100  shares many components with the line  10 , and these shared components have been designated with identical reference numbers. The main difference between the line  100  and the line  10  is that in the line  100 , upon creation of the SCP panels  92 , an underside  102  or bottom face of the panel will be smoother than the upper side or top face  96 , even after being engaged by the forming device  94 . In some cases, depending on the application of the panel  92 , it may be preferable to have a smooth face and a relatively rough face. However, in other applications, it may be desirable to have a board in which both faces  96 ,  102  are smooth. Since the smooth texture is generated by the contact of the slurry with the smooth carrier  14  or the carrier web  26 , to obtain a SCP panel with both faces or sides smooth, both upper and lower faces  96 ,  102  need to be formed against the carrier  14  or the release web  26 . 
   To that end, the production line  100  includes sufficient fiber chopping stations  36 ,  66 ,  82 , slurry feeder stations  44 ,  78  and embedment devices  70 ,  86  to produce at least three layers  77 ,  80  and  88 . Additional layers may be created by repetition of stations as described above in relation to the production line  10 . However, in the production line  100 , in the production of the last layer of the SCP panel, an upper deck  106  is provided having a reverse rotating web  108  looped about main rolls  110 ,  112  (one of which is driven) which deposits a layer of slurry and fibers  114  with a smooth outer surface upon the moving, multi-layered slurry  46 . 
   More particularly, the upper deck  106  includes an upper fiber deposition station  116  similar to the fiber deposition station  36 , an upper slurry feeder station  118  similar to the feeder station  44 , a second upper fiber deposition station  120  similar to the chopping station  66  and an embedment device  122  similar to the embedment device  70  for depositing the covering layer  114  in inverted position upon the moving slurry  46 . Thus, the resulting SCP panel  124  has smooth upper and lower surfaces  96 ,  102 . 
   Another feature of the present invention is that the resulting SCP panel  92 ,  124  is constructed so that the fibers  30 ,  68 ,  84  are uniformly distributed throughout the panel. This has been found to enable the production of relatively stronger panels with relatively less, more efficient use of fibers. The percentage of fibers relative to the volume of slurry in each layer preferably constitutes approximately in the range of 1.5% to 3% by volume of the slurry layers  77 ,  80 ,  88 ,  90 ,  114 . 
   In quantitative terms, the influence of the number of fiber and slurry layers, the volume fraction of fibers in the panel, and the thickness of each slurry layer, and fiber strand diameter on fiber embedment efficiency has been investigated and established as part of this invention. In the analysis, the following parameters were identified:
         ν T =Total composite volume   ν S =Total panel slurry volume   ν f =Total panel fiber volume   ν f,l =Total fiber volume/layer   ν T,l =Total composite volume/layer   ν s,l =Total slurry volume/layer   N l =Total number of slurry layers; Total number of fiber layers   V f =Total panel fiber volume fraction   d f =Equivalent diameter of individual fiber strand   l f =Length of individual fiber strand   t=Panel thickness   t l =Total thickness of individual layer including slurry and fibers   t s,l =Thickness of individual slurry layer   n f,l , n f1,l , n f2,l =Total number of fibers in a fiber layer   s f,l   P , S f1,l   P , S f2,l   P =Total projected surface area of fibers contained in a fiber layer   S f,l   P , S f1,l   P , S f2,l   P =Projected fiber surface area fraction for a fiber layer.       

   Projected Fiber Surface Area Fraction, S f,l   P    
   Assume a panel composed of equal number of slurry and fiber layers. Let the number of these layers be equal to N l , and the fiber volume fraction in the panel be equal to V f .
 
Total composite volume=Total slurry volume+Total fiber volume
 
ν T =ν s +ν f   (1)
 
Total composite volume/layer=Total slurry volume/layer+Total fiber volume/layer
 
                     v   T       N   l       =         v   s       N   l       +       v   f       N   l                 (   2   )                 v     T   ,   l       =       v     s   ,   l       +     v     f   ,   l                 (   3   )               
where, ν T,l =ν l   /N   l ; ν s,l =ν s   /N   l ; ν f,t =ν f   /N   l    
   Assuming that all fiber layers contain equal amount of fibers, the total fiber volume/layer, ν f,l  is equal to 
                   v     f   ,   l       =         v   T     *     V   f         N   l               (   4   )               
Assuming fibers to have cylindrical shape, total number of fiber strands/layer, n f,l  is equal to:
 
                   n     f   ,   l       =             v   T     *     V   f         N   l             π   ⁢           ⁢     d   2       4     *     l   f         =       4   ⁢           ⁢     v   T     ⁢     V   f         π   ⁢           ⁢     d   f   2     ⁢     l   f     ⁢     N   l                   (   5   )               
where, d f  is the equivalent fiber strand diameter.
 
   The projected surface area of a cylindrical fiber is equal to the product of its length and diameter. Therefore, the total projected surface area of all fibers contained in a fiber layer is equal to 
                   s     f   ,   l     P     =         n     f   ,   l       *     d   f     *     l   f       =       4   ⁢     v   T     ⁢     V   f           N   l     ⁢   π   ⁢           ⁢     d   f                   (   6   )               
Projected fiber surface area fraction, S f,l   P  is defined as follows:
 
                       S     f   ,   l     P     =         Projected   ⁢           ⁢   surface   ⁢           ⁢   area   ⁢           ⁢   of   ⁢           ⁢     fibers   /   layer       ,     s     f   ,   l     P           Projected   ⁢           ⁢   surface   ⁢           ⁢   area   ⁢           ⁢   of   ⁢           ⁢   the   ⁢           ⁢   slurry   ⁢           ⁢   layer     ,     s     f   ,   l     P           ⁢     
     ⁢       S     f   ,   l     P     =           4   ⁢     v   T     ⁢     V   f           N   l     ⁢   π   ⁢           ⁢     d   f             v     s   ,   l         t     s   ,   l           =           4   ⁢     v   T     ⁢     V   f           N   l     ⁢   π   ⁢           ⁢     d   f               v   T     t     ⁢     (     =         v     s   ,   l         t     s   ,   l         =       v     T   ,   l         t   l           )         =       4   ⁢     V   f     ⁢   t       π   ⁢           ⁢     N   l     ⁢           ⁢     d   f                 ⁢                   (   7   )               
where, t s,l  and ν s,l  are the thickness and volume of the individual slurry layer, respectively.
 
   Thus, the projected fiber surface area fraction, S f,l   P  can be written as: 
                   S     f   ,   l     P     =       4   ⁢     V   f     ⁢   t       π   ⁢           ⁢     N   l     ⁢           ⁢     d   f                 (   8   )               
The projected fiber surface area fraction, S f,l   P  can also be derived in the following form from Equation 7 as follows:
 
                         S     f   ,   l     P     =       ⁢           4   ⁢     v   T     ⁢     V   f           N   l     ⁢   π   ⁢           ⁢     d   f             v     s   ,   l         t     s   ,   l           =         4   ⁢     v   T     ⁢     V   f           N   l     ⁢   π   ⁢           ⁢     d   f                 (     1   -     V   f       )     *     v   T         N   l       *     1     t     s   ,   l                           =       ⁢         4   ⁢     V   f     *     t     s   ,   l           π   ⁢           ⁢       d   f     ⁡     (     1   -     V   f       )           ⁢           =       4   ⁢     V   f     *     t   l         π   ⁢           ⁢     d   f                         (   9   )               
where, t s,l  is the thickness of distinct slurry layer and t l  is the thickness of the individual layer including slurry and fibers.
 
   Thus, the projected fiber surface area fraction, S f,l   P  can also be written as: 
                   S     f   ,   l     P     =       4   ⁢     V   f     *     t     s   ,   l           π   ⁢           ⁢       d   f     ⁡     (     1   -     V   f       )                   (   10   )               
Equations 8 and 10 depict dependence of the parameter projected fiber surface area fraction, S f,l   P  on several other variables in addition to the variable total fiber volume fraction, V f .
 
   In summary, the projected fiber surface area fraction, S f,l   P  of a layer of fiber network being deposited over a distinct slurry layer is given by the following mathematical relationship: 
                   S     f   ,   l     P     =         4   ⁢     V   f     ⁢   t       π   ⁢           ⁢     N   l     ⁢           ⁢     d   f         =       4   ⁢     V   f     *     t     s   ,   l           π   ⁢           ⁢       d   f     ⁡     (     1   -     V   f       )                                     
where, V f  is the total panel fiber volume fraction, t is the total panel thickness, d f  is the diameter of the fiber strand, N l  is the total number of fiber layers and t s,l  is the thickness of the distinct slurry layer being used. A discussion analyzing contribution of these variables on the parameter projected fiber surface area fraction, S f,l   P  is given below:
         The projected fiber surface area fraction, S f,l   P  is inversely proportional to the total number of fiber layers, N l . Accordingly, for a given fiber diameter, panel thickness and fiber volume fraction, an increase in the total number of fiber layers, N l , lowers the projected fiber surface area fraction, S f,l   P  and vice-versa.   The projected fiber surface area fraction, S f,l   P  is directly proportional to the thickness of the distinct slurry layer thickness, t s,l . Accordingly, for a given fiber strand diameter and fiber volume fraction, an increase in the distinct slurry layer thickness, t s,l , increases the projected fiber surface area fraction, S f,l   P  and vice-versa.   The projected fiber surface area fraction, S f,l   P  is inversely proportional to the fiber strand diameter, d f . Accordingly, for a given panel thickness, fiber volume fraction and total number of fiber layers, an increase in the fiber strand diameter, d f , lowers the projected fiber surface area fraction, S f,l   P  and vice-versa.   The projected fiber surface area fraction, S f,l   P  is directly proportional to volume fraction of the fiber, V f . Accordingly, for a given fiber panel thickness, fiber strand diameter and total number of fiber layers, the projected fiber surface area fraction, S f,l   P  increases in proportion to increase in the fiber volume fraction, V f  and vice-versa.   The projected fiber surface area fraction, S f,l   P  is directly proportional to the total panel thickness, t. Accordingly, for a given fiber strand diameter, fiber volume fraction and total number of fiber layers, increase in the total panel thickness, t, increases the projected fiber surface area fraction, S f,l   P  and vice-versa.       
   Experimental observations confirm that the embedment efficiency of a layer of fiber network laid over a cementitious slurry layer is a function of the parameter “projected fiber surface area fraction”. It has been found that the smaller the projected fiber surface area fraction, the easier it is to embed the fiber layer into the slurry layer. The reason for good fiber embedment efficiency can be explained by the fact that the extent of open area or porosity in a layer of fiber network increases with decreases in the projected fiber surface area fraction. With more open area available, the slurry penetration through the layer of fiber network is augmented, which translates into enhanced fiber embedment efficiency. 
   Accordingly, to achieve good fiber embedment efficiency, the objective function becomes keeping the fiber surface area fraction below a certain critical value. It is noteworthy that by varying one or more variables appearing in the Equations 8 and 10, the projected fiber surface area fraction can be tailored to achieve good fiber embedment efficiency. 
   Different variables that affect the magnitude of projected fiber surface area fraction are identified and approaches have been suggested to tailor the magnitude of “projected fiber surface area fraction” to achieve good fiber embedment efficiency. These approaches involve varying one or more of the following variables to keep projected fiber surface area fraction below a critical threshold value: number of distinct fiber and slurry layers, thickness of distinct slurry layers and diameter of fiber strand. 
   Based on this fundamental work, the preferred magnitudes of the projected fiber surface area fraction, S f,l   P  have been discovered to be as follows: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Preferred projected fiber surface area fraction, S f,l   p   
               &lt;0.65 
             
             
                 
               Most preferred projected fiber surface area fraction, S f,l   p   
               &lt;0.45 
             
             
                 
                 
             
           
        
       
     
   
   For a design panel fiber volume fraction, V f , achievement of the aforementioned preferred magnitudes of projected fiber surface area fraction can be made possible by tailoring one or more of the following variables—total number of distinct fiber layers, thickness of distinct slurry layers and fiber strand diameter. In particular, the desirable ranges for these variables that lead to the preferred magnitudes of projected fiber surface area fraction are as follows: 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               Thickness of Distinct Slurry Layers, t s,l   
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Preferred thickness of distinct slurry layers, t s,l   
               ≦0.20 inches 
             
             
               More Preferred thickness of distinct slurry layers, t s,l   
               ≦0.12 inches 
             
             
               Most preferred thickness of distinct slurry layers, t s,l   
               ≦0.08 inches 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Number of Distinct Fiber Layers, N l   
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Preferred number of distinct fiber layers, N l   
               ≧4 
             
             
                 
               Most preferred number of distinct fiber layers, N l   
               ≧6 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Fiber Strand Diameter, d f   
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Preferred fiber strand diameter, d f   
               ≧30 tex 
             
             
                 
               Most preferred fiber strand diameter, d f   
               ≧70 tex 
             
             
                 
                 
             
           
        
       
     
   
   While a particular embodiment of the multi-layer process for producing high strength fiber-reinforced structural cement panels has been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.

Technology Category: 4