Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~90~87
CE~ENT BOARD HAVING REINFORCED EDGES
This invention relates to the continuous production of a reinforced
cementitious panel. More particularly, it relates to-a method and an
apparatus for casting a cementitious slurry in the form of a thin, inde-
finitely long panel whose faces and longitudinal edges are reinforced bya network of fibers which is submerged just below the cementitious surface.
Still more particularly, this invention relates to a bare cement board
whose faces and longitudinal edges are reinforced by a sub-surface net-
work of fibers.
Cement board, a thin, reinforced concrete panel, has become increas-
ingly popular during the past two decades as a durable substrate for cer-
amic tile in bath rooms, shower rooms, and other areas where the walls are
subject to frequent splashing of water and high humidity. There is a
growing interest in the use of cement boards on the exterior of buildings
as in the construction of curtain walls. Having such uses, a covering for
the surface of the concrete is neither needed nor desired. ~ecause the
boards are often attached at the margins to the building framework with
nails or screws, however, it is highly desirable that the longitudinal
edges of the boards be fully and uniformly filled and that they be rein-
forced at least as well as the faces of the boards. The border regionsof the faces adjacent to the edges must not be thicker than the field
regions thereof lest the wall turn out to be wavy rather than flat.
1~0~8~7
--2--
Reinforced panels having cores formed of a cementitious composition
are presently known. U.S. Pat. No. 1,439,954 discloses a wallboard having
a core of gypsum or Portland cement and a mesh material such as cotton
gauze, wire cloth, perforated paper or perforated cloth applied to both
faces of the core while the cementitious material is still in the plastic
state.
U.S. Pat. No. 3,284,980 (Dinkel) discloses a pre-cast, lightweight
concrete panel having a cellular core, a thin, high density layer on
each face, and a layer of fiber mesh embedded in each of the high density
layers. Each panel is case separately in forms in a step-wise procedure
beginning with a thin layer of dense concrete mix, laying the mesh
thereupon, pouring the lightweight concrete mix over the mesh to form
the core, laying a second layer of mesh over the core mix, and pouring
another layer of dense concrete mix over the second mesh layer.
Clear, in U.S. Pat. No. 4,203,799, discloses a continuous
method for the production of the panels disclosed by Dinkel. In said
method, a continuous web of glass fiber mesh is passed through a cemen-
titious slurry, the slurry-laden mesh is laid on a plurality of moving
carrier sheets, a lightweight concrete mix is deposited on the mesh as
it moves along with the carrier sheets, a second continuous web of mesh
is passed through a cementitious slurry and laid over the lightweight
concrete core mix. The elongated sheet of concrete travels to a cutter
station where the sheet is cut into individual panels.
Schupack, in U.S. Pat. No. 4,159,361, discloses a cold formable
cementitious panel in which fabric reinforcing layers are encapsulated
by the cementitious core. The layers of reinforcing fabric and cemen-
titious material of the Schupack panel are laid and deposited on a
vibrating forming table from a fabrication train which reciprocates
longitudinally over the table. The cementitious core mix is smoothed by
a laterally oscillating screed.
British Patent Application No. 2 053 779 A discloses a method for
the continuous production of a building board which comprises advancing
a pervious fabric on a lower support surface, depositing a sl`urry of
1~905~37
cementitious material such as gypsum plaster on said advancing fabric,
contacting the exposed face of the slurry with a second fabric, passing
the fabric faced slurry under a second support surface, and advancing
the fabric faced slurry between the two support surfaces while vibrating
said surfaces. The vibration is said to cause the slurry to penetrate
through the fabric to form a thin, continuous film on the outer faces of
the fabric.
The problem common to all methods of production of fiber mesh
reinforced cementitious panels is the forming and reinforcing of smooth
uniform longitudinal edges. Schupack teaches the utilization of more
tightly woven reinforcing fabric at the margins of the panel but the
fabric does not wrap around upright edges of the panel. The problem is
particularly difficult when the economies of continuous production are
desired. Glass fiber mesh, the reinforcing fabric of choice in most
instances, is bent easily but its resiliency causes it to spring back to
its original shape when the bending force is removed.
In a method for the continuous production of a fiber reinforced
cement board, Galer teaches in U.S. Patent No. 4,450,022 that the edges
of a moving carrier sheet are bent upright as a concrete mix is directed
onto a fiber network carried by the carrier sheet. The trough-like
sheet thus becomes a form for the continuous ribbon of concrete. After
the mix is spread across and under the lower network and a second net-
work is submerged in the upper surface of the mix, the upright edges of
the carrier sheet are turned onto the upper surface. The fiber networks
are, however, not wrapped around the edges of the cement board. Con-
sistently uniform filling of the edge portions of the cement board has
remained a problem until the time of the invention disclosed and claimed
in this application even when the improved method of concrete mix dis-
tribution taught by Galer in U.S. Patent No. 4,504,335 is employed.
Trimming of the irregular edges has been necessary to have a commercially
acceptable product.
Altenhofer et al, in U.S. Patent No. 4,504,533, points to diffi-
culties that are encountered in making a gypsum board in which a first
1:~90587
composite web of an impermeable non-woven fiberglass felt and a woven
fiberglass mat covers the lower face of the gypsum core and is wrapped
around the longitudinal edges of the gypsum core so that the border
regions of the composite web lie on the upper face of the core. The
extension of both the non-woven felt and the fiberglass mat, as a com-
posite web, around the longitudinal edges causes problems in the scoring
of the composite web which is necessary for the wrapping around and
folding process. Further problems arise when a second composite web,
placed on the upper surface of the board and overlapping the borders
of the first composite web, is adhesively bonded to the first web.
Ridges and undulations form on the overlapping border regions, according
to Altenhofer et al. These are said to be undesirable because they
cause poor adhesion and detract from the desired smooth surface of the
gypsum board. To solve the problems, Altenhofer et al teaches the use
of composite webs in which the fiberglass mat component is absent from
the longitudinal border regions. In the use of such a composite on the
lower face of the gypsum core only the layer of non-woven felt needs to
be scored, folded, and wrapped around. Cutting away the mat from the
border regions of the upper composite web permits improved adhesive
bonding between the upper and lower webs. The product is a gypsum board
having a woven fiberglass mat embedded in the upper and lower faces of
the core and a non-woven fiberglass felt extending across the lower
face, around the longitudinal edges, and partially inward from the edges
while the upper face is covered by another non-woven felt which is glued
to the folded-in lower felt.
Thus, there still remains a need for a bare cement board fully
reinforced by a submerged network of fibers under both faces and both
longitudinal edges, said edges being uniform and smoothly surfaced and
said board having a substantially uniform thickness.
It is an object of this invention, therefore, to provide a flat,
bare cement board having smooth, uniform longitudinal edges which are
reinforced by a ~Y~ mesh of glass fibers immediately below the edge
surfaces.
1~90587
It is a related object of this invention to provide a bare cement
board having a~ven mesh of glass fibers immediately below each face
thereof, the mesh in one face continuing under the surface of both longi-
tudinal edges 7 with the option of having the two meshes in an abutting
S or an overlapping relation along the longitudinal margins of the opposite
face.
It is another related object of this invention to provide a cement
board having reinforcing ~ov-on glass fibers embedded in the faces and
longitudinal edges thereof and whose marginal regions along said edges
do not protrude above the plane of the field of the board.
It is another object of this invention to provide such a cement
board having longitudinal marginal regions which taper slightly on one
face.
It is another object of this invention to provide a method for
continuously forming smooth, uniform and reinforced longitudinal edges
on a cement board.
It is yet another object of this invention to provide an apparatus
for forming such longitudinal edges on a cement board.
It is a further object of this invention to provide a bare cement
board having a significantly stronger longitudinal ed~e so that the
board will have an increased resistance to shattering when nailed along
the margin to the framework of a building.
It is a still further object of this invention to provide a cost-
saving method for continuously producing a cement board having fully
formed uniform edges.
These and other objects of this invention which will become apparent
from the attached drawings and the following description are achieved by:
continuously towing on an endless conveyor belt an
indefinitely long, non-adherent carrier sheet over a
forming table which ls upstream from the conveyor belt,
said sheet being wider than the cement board being made;
forming a continuous trough by bending outer portions
of the sheet upright;
continuously laying an indefinitely long ~70'7C~ mesh of
~- 35 glass fibers into the trough, the mesh being wider than the
trough;
~.~9l~S87
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continuously depositing a hydrating cementitious mix
on the mesh and distributing the mix laterally to fill the
trough to a substantially uniform depth;
towing the filled trough in an abutting relationship
with and between a pair of fixedly spaced apart, indefinitely
long edger rails which rest longitudinally on the conveyor
; belt in slidable engagement therewith;
folding upright portions of the carrier sheet and outer
portions of the mesh inward and over the mix; and
pressing the folded-over carrier sheet down onto the
surface of the mix and the mesh into the mix.
U.S. Patent 4,450,022 describes an apparatus and a method for
creating a gap between the carrier sheet and the bottom mesh as they
move over a forming table so that the concrete mix can penetrate the
voids of the mesh and form a layer of concrete between the sheet and
the mesh. U.S. Patent 4,504,335 describes a method for submerging
a woven glass fiber mesh in the top surface of the concrete mix while
the mix is moving over the forming table; the mesh is towed into the
nip between the advancing mix and a cylindrical screeding roller which
rotates counter to the direction of travel of the mix so that the
roller presses the mesh into the surface of the mix and cleans itself
of adhering mix by wiping the mix onto the upper surface of the mesh
and into the voids thereof. U.S. Patent 4,488,909 describes a concrete
mix which is preferred for the high speed continuous production of
the cement board of this invention.
For a ready understanding of the apparatus and method used in
the production of the cement board of this invention, they are illus-
trated in the attached drawings and described herein in association
with portions of the production line described in the '022 and '335
patents.
sp ~
(
~9(~S~7
Turning now to the drawings:
FIG. 1 is a fragmentary perspective view of the forming end of a
cement board production line employing the apparatus of the invention.
FIG. Z is a sectional view of the production line taken along
line 2-2 of FIG. 1.
FIG. 3 is a diagrammatic side view, partially broken away, of
another embodiment of the inventive apparatus.
FIG. 4 is a sectional view of the production line of FIG. 5, taken
along the line 4-4.
FIG. 5 is a diagrammatic plan view of the production line of FIG. 3.
FIG. 6 is a cross-section of the cement board of this invention.
In FIG. 1, the forming table 10 and the conveyor belt 12 constitute
the support for the carrier sheet 14 and the woven glass fiber mesh 16.
Mounted transversely above the forming table 10 are the mortar distri-
bution belt 18 and the stationary plow 20 whose blades 20a, 20b, 20c,
and 20d contact the surfaee of the distribution belt 18 in scraping
relationship. The guide flanges 22 are mounted on the table 10 just
upstream from the mortar screeding roller 24 which is adjustable up
and down so that the nip between it and the carrier sheet 14 may be
set to the desired thickness of the panel to be manufactured. The
roller 24 is journalled and driven by conventional means not shown.
The carrier sheet 14 is wider than the cement board being formed so
that the sheet may be made into a continuous trough. The creaser wheels
26 are optional; they may be used to score longitudinal lines along side
each lateral margin of the carrier sheet 14 to facilitate the bending of
the sheet to form the upright walls 28 as the sheet is towed between the
guide flanges 22. The mesh 16 is also wider than the desired board and,
therefore, wider than the trough formed by the bent carrier sheet; it
may be of the same or narrower width as the flat carrier sheet but not
wider. The mesh 16 is fed into the trough under the hold-down roller 30
but because it is not scored and is rather resilient it does not conform
precisely to the corners of the trough but rather curves from the bottom
of the trough to the walls 28, leaving the spaces 32, as shown in FIG.
2.
1290587
The longitudinal edger rails 34 extend downstream from the forming
table 10 in slidable contact with the conveyor belt 12. The posts 36
are mounted on the rails 34 and the rods 38 are slidably mounted within
the rings 40, as shown more clearly in FIG. 4. The distance between the
rails 34 is adjusted and maintained by sliding the rings 40 a]ong the
rods 38 and tightening the set screws 42 at the selected points. As
shown in FIG. 3, several sets of the posts 36 and the rods 38 are
spaced apart along the rails 34 to prevent lateral movement of the rails
independently of each other and thus assure a constant cement board
width. The rails may move laterally in tandem in response to occasional
shifting of the conveyor belt as it travels around the drive and take-up
pulleys but, since the distance between them is constant, the upright
walls 28 of the carrier sheet are not allowed to fall away and let the
concrete mix spread haphazardly. The edger rails 34 are continuous
lengths of a lightweight material such as aluminum and, in a preferred
embodiment of this invention, the rails are hollow in order to further
ligh~en their weight and allow them to, in effect, float on the conveyor
belt with negligible wear. The posts and rods are also made of light-
weight material to achieve that effect. Preferably, the rails are
rectangular in cross-section and about 1.5 inches wide and about 0.75
inch thick, their weight being distributed across their width as the
conveyor belt glides beneath them.
The spatulas 44 are mounted in pairs on the rods 38, as shown in
detail in FIG. 4. Only three pairs of spatulas are shown in FIG. 3 but
it is to be understood that as many as eight or more pairs of spatulas
may be spaced apart downstream from the roller 24. The first pair of
spatulas are preferably spaced from about four to about eight feet (1.2
to 2.5 meters) downstream from said roller and the space between conse-
cutive pairs is preferably from about five to about ten feet (1.5 to 3
meters). Each spatula is pivotably fastened to a bracket 46 by a screw
47. The bracket extends tangentially from a collar 48 which in turn is
rotatably mounted on a rod 38 inboard from a ring 40 and is locked in
place by a set screw 50. The blade tip 52 of each spatula is preferably
cut back at an angle of about 20 or less as shown in FIG. 5 so that each
spatula may be canted toward the respective rail 34 by pivoting it on the
~9(~5~
bracket 46 and thus cause its tip 52 to be aligned at a substantially
right angle with its respective rail. The outboard edge of the tip is
thus caused to press down more heavily than the inboard edge on the fold-
ed strip 54 of the carrier sheet 14. In this manner, the margins of the
cement are tapered to the desired degree. An angle of from about 5 to
about 201 is preferred, 5 bein~ particularly preferred. In the event
that a spatula having a squared-off tip is used or that further biasing
is needed, a rubber band 56 or other restraining means connects a peg 58
on the spatula blade to a set screw 42 as shown or to a ring 40. The
spatula blade is made of a resilient material such as a chrome plated
spring steel which is not readily corroded by contact with a hydraulic
cement mixture. The blade is thin, e.g. about 20 gauge, and is about
nine to twelve inches (23 to 30 cm) long. The folded strip 54 is pre-
ferably about 1.5 inches wide and the spatula blade may be as wide as
the strip 54 but no wider because scraping of the concrete mix adjacent
the strip is to be avoided.
An alternative means for mounting the spatulas on the rails 34 is
a carrier having a foot insertable in the hollow end of a rail 34, an
upright leg attached at an angle to the foot and extending above the
horizontal plane of the foot, and a shaft attached to the leg at a
right angle to the vertical plane passing through the foot so as to
extend inboard when the foot is inserted in the rail. The first pair
of spatula carriers are mountable in the upstream end of hollow rails
34; succeeding pairs may be inserted in hollow rail segments mounted
atop the rails 34. Individual carriers may be right-handed or left-
handed or they may be made reversible by making the feet bidirectional.
The spatulas are mounted on the carrier shafts in the same way as on
the rods 38.
Also shown in FIGS. 1, 3, and 5 are the air jets 60 connected to
the valves 62 which are mounted on the forming table 10 and are con-
nected to a source of compressed air. In FIGS. 3 and 5, the fingers 64,
used only when it is desired to fold the margins of the lower mesh 16 to
lie under the top mesh 66, are mounted on the table 10 and extend in
over the guide flanges 22 to urge the upstanding margins of the bottom
mesh 16 inward and downward so that said margins may be further bent
down as they pass under the roller 24.
1~9()~87
--10--
The finished cement board 70 is shown in cross-section in FIG. 6
to reveal the core 72 which extends through the bottom mesh 16 even as
said mesh bends up and around to overlap the top mesh 66 which lies just
beneath the upper surface of the board. Thus, the concrete mix in the
cement board is an autogenous binder for the lapping meshes 16 and 66
at the margins 76 of the upper surface of the board. As shown, the
edges 74 and the margins 76 are smooth because of the smoothing effect
of the carrier sheet strips 54 being pressed onto the mix by the rails
34 and the spatulas 44. The smooth margins 76 are preferred when the
cement boards are fastened side-by-side on a partition and joint tape is
adhesively applied to the margins before joint compound is applied. If
it is desired that the entire field of the upper surface of the board be
nubby, the strips 54 may be peeled off, along creases made by the spatulas,
before final set of the concrete mix has occurred. The strips 54 will
then remove a thin layer of the mix from the margins and leave a roughened
surface. If the creaser wheels 26 are used, all but the bottom of the
carrier sheet 14 may be removed before or after final set.
Although FIG. 6 shows the folded bottom mesh 16 overlying the woven
top mesh 66 along the margins, the board of this invention may be made
20 so that the mesh 16 lies under the top mesh 66 when the fingers 64 are
employed to bend the upstanding portions of the mesh 16 inward and
downward before they reach the roller 24.
Moreover, although the continuous manufacture of the cement board
having the top mesh 66 is further described as follows, it will be under-
stood that said mesh i5 not essential to this invention.
The creased carrier sheet 14 and the woven mesh 16 are passed
manually beneath the distribution belt 18, between the flanges 22, under
the screeding roller 24 and onto the conveyor belt 12 so that when the
conveyor drive means (conventional, not shown) is actuated, a mesh lined
trough having the upright walls 28 is towed in the machine direction
indicated by the arrow MD. Concrete mix is fed onto the belt 18 from a
continuous mixer shown as the box CM and is scraped onto the mesh 16 by
the plow blades 20a, b, c, and d. The streams of concrete mix thus
formed spread and merge as the roller 24 dams their movement. The
spreading mix penetrates the curved mesh 16 and moves into the spaces
32. The top mesh 66 is dragged between the roller 24 and the dammed
mix while the roller rotates counter to the MD. The roller constantly
0587
picks up a coat of concrete mix which squeezes through the voids of the
woven top mesh 66 at the nip and then it wipes the mix onto the obverse
face of the top mesh 66 to aid in the impregnation thereof. If the top
mesh is slightly narrower than the cylindrical roller 24, a ring of the
concrete mix clings to the unwiped edges of the cylinder. Said mix is
thrown by centrifugal force alongside the upright walls 28 of the paper
trough. If the walls 28 show a tendency to bend over prematurely, they
may be held upright by the force of air directed against the walls by
the air jets 60. Unwanted splatters of the mix on the walls 28 may be
cleaned off by such air, also.
As the trough of concrete mix approaches the first pair of flexed
spatulas 44a, the margins of the mesh 16 and the walls 28 of the trough
are tucked under the spatulas 44a to initiate the folding over of the
continuously approaching carrier sheet 14 and mesh 16. It is preferred
to fold the bottom mesh over onto the concrete mix which already covers
the top mesh 66 and use the pressure of the flexed spatula blades to
press the strips 54 down onto the folded over mesh 16 to urge the woven
glass fibers into the mix. Folding of the margins of the mesh 16 onto the
body of the mix before the top mesh 66 is applied is another way to pro-
duce the reinforced-edge cement board of this invention. To do so, the
fingers 64 of FIGS. 3 and 5 are placed so as to urge the margins of the
mesh 16 inward and downward and the concrete mix ringing the edges of the
roller 24 is thrown onto the bent-over margins. The weight of the mix
further bends the margins down before the top mesh 66 is applied. The
folded-over mesh 16 is thus embedded near the upper surface of the board
along with the mesh 66 as they emerge from under the roller 24 but the
mesh 16 still tends to rise up because of its resilience; the spatulas 44
are still necessary to press the margins of the mesh 16 down as the concrete
mix sets.
The pressure of the flexed spatula blades on the strips 54 is varied
according to the consistency of the concrete mix and the stiffness of the
mesh. A range of from about 1 to about 4 psi (gauge) is preferred. The
smallest pressure is applied by the first pair of spatulas 44a and the
pressure is increased in increments as the strips 54 pass under the
succeeding pairs of flexed spatulas 44b, 44c, etc.
~f~0587
-12-
The placing of the spatulas 44 downstream from the mixer CM is deter-
mined by the line speed at which the board is manufactured and the rate
of hydration of the cement which, in turn, is a function the cement formu-
lation and the temperature of the concrete mix. A rapid hardening, high
early strength cement such as that described in the aforementioned U.S.
Patent No. 4,488,909 is preferred in the production of the cement board
of this invention. The high temperature concrete mix described in the
'909 patent is preferred, also. Although U.S. 4,504,335 describes the
mix as a relatively stiff, immobile mortar, a particularly preferred
mix for the purposes of this invention has a consistency su-h that a
dimple made in the mix just after it has been deposited on the belt 12
will disappear by the time the mix arrives at the roller 24, i.e., about
4 seconds. It has been found that when such a self-leveling mortar is
used the bottom mesh 16 may be well embedded in the mortar even though
the means for creating a gap between the carrier sheet and the bottom
mesh described in U.S. 4,450,022 is not used. An example of such a
mortar is one in which the cement powder consists of 68.1% Type III
portland cement, 17.79% high alumina cement, 5.69% landplaster, 0.57%
hydrated lime, and 7.84% fly ash. A lower cost cement powder may be
used if a fine high alumina cement (about 6000 cm2/g Blaine) is employed
at about a 12.5% level with concomitant changes in the amounts of the
other cementitious solids for an optimized formulation. The mortar also
contains blast furnace slag in an amount equal to, on a dried basis, the
weight of the cement powder. The self-leveling property of the mortar
is enhanced and prolonged by one part of Lomar D superplasticizer and
about 0.5 part of an 8% aqueous solution of citric acid per hundred
parts by weight of the cement~powder. The water to cement powder ratio
is about 0.35 by weight, including the water introduced with wet slag,
the superplasticizer and citric acid solution. Foam and expanded poly-
styrene beads are also introduced into the continuous mixer along with
the other solids and liquids so as to make a cement board having a
density of from about 74 to about 80 pounds per cubic foot.
1~90587
-13-
The embedding of the folded-over mesh 16 must, of course, take
place before the initial set of the concrete has occurred but the mix
cannot be so soupy at the first spatula pair that the mesh will rise up
again after passing under a spatula. A convenient and satifactory way
to measure the extent of hydration of the cement at various points along
the line is to place a sample from the mixer in a calorimeter connected
to a recording chart so as to plot the rise in temperature against
elapsed time. The total temperature rise up to the equilibrium temper-
ature is noted. The distance between the roller 24 and the selected
spatula position is measured and that distance is divided by the line
speed to give the travel time for the concrete mix from the roller 24 to
the selected position. A time factor for the travel of the mix from the
mixer CM to the roller 24 must be added. This factor can be determined
by measuring the travel time of a spot of pigment such as iron oxide
placed in the mix at the mouth of the mixer. A plot of the age of the
concrete mix on the time-temperature curve gives the temperature rise at
the selected spatula position. The ratio of the incremental temperature
rise against the total temperature rise is an indication of the extent
of hydration at the selected position. For example, a concrete mix
prepared according to the '909 patent reached the equilibrium temper-
ature in 12.5 minutes, which is within the range of set time disclosed
in said patent, and the total temperature rise was 27F (from 103F to
130F). At a line speed of 32 feet per minute (1 foot = 0.3 meter), the
extent of hydration, as a percentage of the hydration which has occurred
at the equilibrium temperature, at the locations of four pairs of the
spatulas 44, spaced at 7 feet, 17 feet, 26 feet, and 35 feet from the
roller 24, was 15%, 22%, 26%, and 32%, respectively, The travel time
for the concrete mix from the mixer to the roller 24 was estimated to be
about 12 seconds. The spatulas may be used to press the mesh 16 into
the upper longitudinal margins of the concrete ribbon and to form, in
co-operation with the edger rails 34, smooth reinforced edges along the
ribbon while the extent of hydration, as so expressed, is in the range
of from about 10 to about 35%. It is preferable that the spatulas 44a
are placed to press down lightly upon the strips 54 as the hydration
reaches a stage equal to from about 10 to about 18% of the hydration
which will have occurred at the equilibrium temperature.
1'~90~
-14-
The importance of penetration of the mesh by the cementitious
slurry is pointed out in U.S. 4,450,022, mentioned hereinabove,
wherein a momentary gap is created between the bottom mesh and the
carrier sheet to permit such penetration. The criticality of the
open, porous nature of the reinforcing f:Lber network, whether it be
woven mesh or a non-woven, pervious fabr:Lc, is emphasized. The
embedment of the reinforcing fibers just beneath the surfaces of the
slurry occurs according to this invention whether the fibers are in
the form of a woven or a nonwoven mesh.
The woven mesh is preferably composed of glass fibers but nylon,
metal, and aramid resin fibers may also be used. The mesh size and
the fiber diameter are selected according to the strength desired in
the board and the size of the aggregate in the concrete mix. A mesh
having a thread count per inch of from 4 x 4 to 18 x 14 or 10 x 20
is acceptable for most purposes. A mesh having a tighter weave along
the margains may be used to further strengthen the edges and margins
of the board.
In the manufacture of a 36 inch (1 inch = 2.54 cm) wide x ~ inch
thick cement board of this invention, for example, the mesh 16 was
38.5 inches wide, the mesh 66 was 35.75 inches wide, the thread count
of each was 10 x 10, and the carrier sheet 14 was 40 inches wide. The
edge of the mesh 66 was inset 118 inch from each longitudinal edge
of the board and there was a 7/8 inch overlap of the folded-over por-
tion of the mesh 16 above the mesh 66 at each longitudinal margin of
the board.
The cement board of this invention is an improved tile backer
board for the construction of bathrooms, particularly shower enclosures,
locker rooms, swimming pool rooms and other units which are subject
to high humidity and splashing water. Reinforcement of the edges and mar-
g~ns of the board makes attachment of the board to the framework of
a room with nails or screws more secure. Use of the edge-reinforced
boards in the construction of exterior curtain walls is also con-
templated.
Ten samples of ~ inch thick cement board of this invention were
tested to learn how much force would be necessary to pull a nail
laterally through the reinforced edge of the board. To do so, a 1/8
inch hole centered 3/8 inch from the edge of the board is drilled in
the margin of the board and the board is clamped in place. A l/8 inch
s p :
1;~905137
-15-
diameter pin simulating a nail is passed through the hole and pulled
laterally by a Tinius-Olsen machine attached to both ends of the pin
and the force necessary to pull the pin laterally through the edge
of the board is recorded. The average force required in the ten tests
was 96 pounds (427 newtons). When the same test was performed on glass
fiber reinforced cement boards of approximately the same age but not
having the reinforced edges, the force required to pull the pin out
laterally was generally on the order of about 40 pounds (178 newtons).
The invention has thus far been described in terms of a wallboard
having a hydraulic cementitious core. A wallboard having a non-hydrau-
lic but, nevertheless, hydrated cementitious core is also regarded
as part of the subject matter of this invention. Thus, a gypsum wall-
board without the usual paper covering but strengthened by a woven
mesh of reinforcing fibers embedded in the core at the top, bottom
and longitudinal edge surfaces may be made by substituting a slurry
of calcium sulfate hemihydrate for the concrete mix in the process
described above.
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