A prestressed lightweight panel is disclosed having a face sheet under tension in the longitudinal direction and reinforcing members attached to the rear side of the face sheet for locking in that tension. The panel may be rectangular and may include side rails and end rails along the edges of the face sheet, a plurality of cross-ribs for bracing or reinforcing the face sheet and a plurality of coped reinforcing sections between the cross braces. Compression resistant members, such as a honeycomb filling extend in voids between certain of the reinforcing members and the face sheet to further reduce face sheet deflection under load. The panel has a number of applications, such as for concrete forms.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
This invention relates to prestressed structural panels. More particularly, 
the present invention relates to a substantially planar structure having a 
face sheet in tension in at least one direction and a plurality of braces 
for maintaining the tension in the face sheet for use in a concrete form 
panel, among others. 
2. Related Art 
It is known to provide structural panels having a relatively thin face 
sheet that is bolstered or reinforced by one or more reinforcing members 
fixed to the rear side of the face sheet by means such as welding. Such 
panels are used in many applications, such as building walls, roofs, 
portable bridges, ships' hulls and so forth. In addition, a significant 
use for such panels is found in concrete forming systems, particularly in 
residential applications. 
An application in concrete forming systems is found in U.S. Pat. No. 
4,744,541, issued to Carlson et al. on May 17, 1988. Carlson et al. 
discloses a concrete form having a face sheet reinforced by a plurality of 
different types of reinforcement members. The face sheet has a face side 
that is presented to the poured concrete in the completed form, and a rear 
side to which the reinforcing members are fixed or attached by means such 
as welding. Such forms have established their value in the marketplace 
where they save significant amounts of labor compared with older competing 
forming systems, notably custom made wood forms. In addition, such forms 
typically lead to a superior finished concrete structure. 
Such forms, however, have at least two related shortcomings. First, they 
are quite heavy, with a typical 3'.times.8' (0.9 m.times.2.5 m) panel 
weighing about 90 lbs. (41 kg.). Usually a single worker manipulates these 
form panels and moves them around on the job site. A lighter weight form 
panel could be expected to lead to increase worker productivity and fewer 
claims for job related injuries. Second, the pressure developed on the 
form panels from the hydrostatic head of the substantially fluid poured 
concrete and the expansion of the concrete upon setting distort the forms, 
causing the face sheet to bulge outwardly from the concrete in between the 
reinforcing members. One of the manifestations of this deformation is 
known as pillowing and results in a series of bulges which become 
increasingly pronounced from the top to the bottom of the form panel. The 
effect is frequently visible in the finished concrete product and detracts 
from the aesthetics of a formed concrete wall. Naturally, when the wall 
will be covered with an outer facade, such as stucco, or brick, the 
pillowing effect is not as important. Increasingly, however, architects 
are designing buildings with exposed concrete facades, sometimes molded to 
resemble brick or cut stone. In these applications the pillowing effect is 
substantially unacceptable. 
Typically, solutions to these two problems are antithetical to one another. 
Decreasing the weight of the form reduces material costs, shipping costs, 
and may increase labor productivity, but leads to increased pillowing and 
decreased form life. In contrast, the pillowing effect can be virtually 
eliminated, but at the expense of considerably increasing the weight of 
the form panel. The increased weight of such a heavily reinforced form 
panel increases the cost of materials and shipping to prohibitive levels, 
as well as making it difficult at best for a single worker to handle the 
panel, leading to declines in productivity. 
Accordingly, there is a need for a structural panel, such as a concrete 
form panel, that is both lightweight and strong enough to withstand the 
hydrostatic head pressure and the pressure caused by expansion of the 
setting concrete, of poured concrete, or other forces, and reduce 
pillowing between the reinforcing members. 
SUMMARY OF THE INVENTION 
It is, therefore, the primary object of the present invention to provide a 
structural panel that is lightweight and that retains the strength of much 
heavier panels of similar size. 
It is a further object of the present invention to provide a structural 
panel, such as a concrete form panel, that resists the hydrostatic and 
expansion pressure of poured concrete, or other forces, to reduce or 
eliminate the pillowing effect between the reinforcement members, or other 
distortion. 
It is a further object of the present invention to provide a panel that is 
lighter weight than comparable panels in the prior art. 
It is a further object of the present invention to provide a panel that has 
an improved weight-to-strength ratio over heavier panels of the prior art. 
These and other objects of the present invention are achieved by providing 
a substantially planar structure comprising a polygonal sheet having a 
x-axis and a y-axis. The polygonal sheet has a face side and a rear side 
and the sheet is under tension in at least one direction. A brace means 
for maintaining the tension in the sheet is fixed to the rear side of the 
sheet. The brace is typically aligned in a direction parallel to the 
primary direction of the tension forces in the sheet, or face sheet. For 
purposes of clarity, the tension will be oriented along the y-axis. The 
structure further comprises anti-bowing means for maintaining the sheet in 
a substantially flat condition by balancing the tension forces in the 
sheet and the counteracting compression forces in the reinforcing members. 
Alternatively, if desired, the anti-bowing means may be tensioned to 
provide a substantially flat face side on the face sheet. The anti-bowing 
means further comprises a spine reinforcing member disposed parallel to 
the y-axis and fixed to the reinforcing member or to the rear side of the 
face sheet itself. The tension forces in the anti-bowing means are in a 
direction parallel to the y-axis. 
In brief summary, the face sheet is in tension along the y-axis. Fixed to 
the rear side of the face sheet is at least one reinforcing member in 
compression in a direction parallel to the y-axis. Fixed to the 
reinforcing member (on the rear side of the face sheet) is the anti-bowing 
means or spine reinforcing member, which is in tension along its 
longitudinal axis or centerline, with both the anti-bowing means and the 
tension forces in it oriented in a direction parallel to the y-axis. These 
and other forces are locked into the substantially planar polygonal 
structure during manufacture by fixing the components together while these 
forces are applied to the respective members in a process described below. 
In no case do the stress and strain distort the respective members beyond 
the yield point or elastic limit of the material. Accordingly, each of the 
component elements of the planar structure would return to their original 
dimensions if the stress forces were removed. It is the characteristic 
urge of the materials to return to their original dimensions or rebound 
that locks these forces into the panel. The components cannot, however, 
rebound because some are locked in longitudinal tension and these forces 
are opposed and balanced by compression forces locked into other members 
or components, so that the various opposing forces are in equilibrium. 
A preferred material for all these structural elements is aluminum, and the 
preferred means for joining the members is welding. 
In an embodiment specifically designed for use as a concrete form panel, 
the invention comprises a rectangular panel having a face sheet under 
tension fixed to at least one brace means for maintaining the tension in 
the sheet. The brace means comprises a pair of side rails disposed along 
opposite long sides adjacent to the respective long edges of the face 
sheet, which are fixed to the rear side of the face sheet. An end rail is 
disposed along each of the two short sides of the panel adjacent to the 
respective short edges of the face sheet. A plurality of cross braces or 
cross-ribs is disposed throughout the length of the panel and parallel to 
the end rails and is fixed to the rear side of the face sheet and to the 
side rails by welding or other means. 
Then the face sheet is put in tension in a longitudinal direction by 
pulling on it. A plurality of coped reinforcing members is disposed 
between adjacent cross braces, or cross-ribs, forming a line along most of 
a longitudinal centerline of the panel and additional cross braces are 
disposed between each end rail and the adjacent cross-rib. These 
cross-ribs are in compression because they are inserted while the face 
sheet is stretched and they are welded into place without any gaps between 
the coped reinforcing sections and the cross-ribs prior to releasing the 
tension on the face sheet and side rails. 
A spine reinforcing member may then be fixed to the coped reinforcing 
members by welding while the spine reinforcing member is under tension 
along the y-axis. The spine reinforcing member maintains the face sheet in 
a substantially flat state. A further reinforcing member or stiffening 
member in compression may be applied along a direction parallel to the 
y-axis, such as by routing it through the cross-ribs and perpendicular to 
them and welding it in place against the rear side of the face sheet or to 
reinforcing members at two or more joints while the face sheet is under 
compression. Releasing the external tension from the face sheet puts the 
stiffening members in compression. Such additional stiffening members may 
be tubular. 
A process for manufacturing the prestressed panel comprises deforming a 
flat polygonal face sheet having a face side and a rear side so that the 
face side is moderately concave; about the x-axis bending at least one 
reinforcement member to conform to the shape of a cross section of the 
face sheet along the line where the reinforcement member will be placed on 
the face sheet; fixing the reinforcing member to the rear side of the face 
sheet to form a panel; pulling on the panel and the reinforcing member; 
and maintaining the stress and strain created in the face sheet and the 
reinforcing member. The process further comprises fixing at least one 
coped reinforcing member to the rear side of the face sheet while the face 
sheet is in tension. Alternatively, the coped reinforcing member may be 
fixed to the reinforcing members while the face sheet is under tension. 
Then the tension is released and a prestressed lightweight panel suitable 
for many applications is complete. 
In an alternative embodiment, the face sheet is placed under tension and 
the side rails are welded to the face sheet. Then the tension is released. 
Partial rebounding of the face sheet leaves the face sheet in tension and 
the side rails in partial compression, greatly strengthening the finished 
panel. Then other reinforcing members can be added as desired or as later 
described herein. In this embodiment it is not necessary to bend the sheet 
metal face prior to welding. 
In another alternative embodiment, the process further comprises the 
additional steps of pulling on at least one spine reinforcing member and 
fixing the spine reinforcing member to the face sheet at not less than two 
points and then releasing the tension on the spine reinforcing member. 
In the case of a rectangular shaped face sheet, a crown is formed into the 
face sheet about a transverse centerline or x-axis and a secondary reverse 
crown is formed about a longitudinal centerline. The crown and the 
secondary reverse crown are tapered from the respective transverse 
centerline and longitudinal centerline to the respective short edges and 
long edges of the rectangular face sheet. 
These and other objects and advantages of this invention will become 
apparent from the following description taken in connection with the 
accompanying drawings wherein is set forth by way of illustration and 
example, an embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As required by the statutes and case law, a detailed embodiment of the 
present invention is disclosed herein. It is to be understood, however, 
that the disclosed embodiment is merely exemplary of the invention, which 
may be embodied in various forms suited for every specific functions. 
Therefore, specific structural and functional details disclosed herein are 
not to be interpreted as limiting, but merely as a basis for the claims 
and as a representative basis for teaching one skilled in the art to 
variously employ the present invention in virtually any appropriately 
detailed structure. 
Referring to FIG. 7, there is shown a rear plan view of the completed 
prestressed lightweight panel 10. A brief description of the parts and the 
finished panel 10 will precede the disclosure of the process of 
manufacturing it. The prestressed lightweight panel 10 includes the face 
sheet 12 having a rear side 14 and a face side 16 (see FIG. 8). In use, 
the face side 16 of the face sheet 12 is subjected to the working load of 
whatever use is made of the panel. For example, if the prestressed 
lightweight panel 10 is employed in forms to receive poured concrete, the 
concrete will bear against the face side 16 of the face sheet 12. All 
reinforcing members are fixed to the rear side 14 of the face sheet 12. 
The reinforcing members comprise additional components of the prestressed 
lightweight panel 10. 
Still referring to FIG. 7, the prestressed lightweight panel 10 further 
comprises the side rails 18, the end rails 20, a plurality of cross-ribs 
22, which may number six in the preferred embodiment of a prestressed 
lightweight panel 10 having a face sheet 3 feet.times.8 feet (0.9 
m.times.2.5 m). The prestressed lightweight panel 10 further comprises a 
plurality of coped reinforcing members 24 which form a column along the 
longitudinal centerline of the prestressed lightweight panel 10 (i.e., the 
y-axis, see FIG. 2). Also included is a spine reinforcing member 26. 
Finally, the prestressed lightweight panel 10 may further comprise one or 
more stiffening members 28. Each of these components will be described 
more thoroughly in conjunction with the process of manufacture. 
Referring to FIG. 2, there is shown a rear plan view of the face sheet 12 
illustrating the y-axis and the x-axis. The y-axis lies along the the 
longitudinal centerline of the rectangular face sheet 12, and naturally, 
also along the longitudinal centerline of the finished prestressed 
lightweight panel 10. The x-axis lies perpendicular to the y-axis and in 
the same plane, consisting of a line lying along the transverse centerline 
of the face sheet 12 and the finished prestressed lightweight panel 10. 
The x-axis and y-axis are useful in describing the directions of various 
forces that are applied to the components during manufacture of the 
prestressed lightweight panel 10 and that are locked into the finished 
product. These internal forces account for the improved performance of the 
prestressed lightweight panel 10 relative to unstressed panels. In use, 
the prestressed lightweight panel 10 designed for use in pouring concrete 
into forms would typically be fastened to adjacent panels and would be 
standing vertically on the narrow or short side of the rectangular form. 
In use, therefore, the panel would normally be oriented such that the 
y-axis is vertical and the x-axis is horizontal. 
The process of manufacturing the prestressed lightweight form panel 10 
includes sequential steps that are illustrated sequentially in the drawing 
Figures. Referring now to FIG. 1, there is shown the face sheet 12 with 
the rear side 14 or back visible. The face sheet 12 is deformed by bending 
it to form a crown having its highest point along the x-axis. The 
deviation from a straight line caused by this crown 30 can be seen by 
comparing the path of the dotted straight line 32, which indicates the 
shape of the long edge 34 of the face sheet 12 prior to formation of the 
crown 30. Similarly, a reverse secondary crown 36 lies along the y-axis 
with the face sheet 12 being bent upward from the secondary reverse crown 
36 as viewed in FIG. 1 by about 1/8 inch (0.31 cm), that is, the long 
edges of the face sheet are bent upward as illustrated in FIG. 1 by 1/8 
inch (0.31 cm). In the preferred embodiment, the taper from the crown 30 
to the short edges 40 of the face sheet 12 is uniform about the x-axis and 
the taper of the secondary reverse crown 36 is uniform from the y-axis to 
each of the long edges 34 of the face sheet 12. These two crowns peaking 
in two opposite directions cause formation of certain complex curves of 
moderate dimensions. 
In the preferred embodiment, the face sheet is 3'.times.8'.times.38 gauge 
(0.080 inches thick) (0.9 m.times.2.5 m.times.0.20 cm) aluminum sheeting 
such as 5086 aluminum alloy. A considerable amount of welding onto the 
face sheet 12 takes place during the process. 
Much of the welding, particularly the initial welding, is performed on a 
temperature controlled table to quickly conduct excess heat away from the 
weld to improve the quality of the weld and its strength and to minimize 
warping. The temperature controlled table (not shown) includes a form 
having the crown and secondary reverse crown formed into the table top and 
copper sheathing or cladding covering the form to provide a heat sink. 
Temperature controlled water is run through a series of pipes in contact 
with the copper top of the table and through a rubber bladder around the 
perimeter of the bed or table top. The temperature and flow rate of the 
water are controlled to prevent warping of the face sheet 12. 
The copper and temperature controlled table top also include a number of 
holes that are connected to a source of vacuum which allow vacuum to be 
pulled against the face sheet 12. The force generated by air pressure 
pulling against the vacuum is sufficient to conform the face sheet to the 
shape of the table top and reduces heat build up. The face sheet is not 
otherwise subjected to stress. The vacuum is maintained through the stage 
of manufacture illustrated in FIG. 3, that is, until the side rails 18, 
end rails 20, and cross-ribs 22 are welded in place on the rear side 14 of 
the face sheet 12. 
Referring to FIG. 2, the two end rails 20 and the two side rails 18 are 
laid down along the edges of the face sheet 12 as illustrated. All four of 
the rails 18, 20 are then clamped to the face sheet 12 and the table top 
with sufficient force to bend them to conform with the curvatures 
resulting from the crown 30 and the secondary reverse crown 36. The rails 
18, 20 are aluminum bar stock. The end rails are approximately 
3/8".times.2.times.3' (0.95 cm.times.5 cm.times.2.4 m). Then the side 
rails are approximately 3/8".times.2".times.8'. The rails 18, 20 are 
welded to the face sheet 12 with weld beads that will not interfere with 
the placement of subsequent reinforcing members. 
Referring to FIG. 3, six cross-ribs 22 are equally spaced throughout the 
length of the face sheet 12 such that the length of the space between an 
end rail 20 and the adjacent cross-rib 22 is the same as the length of the 
spaces between adjacent cross-ribs 22 and all have longitudinal axes that 
are oriented parallel with the x-axis. That is, the cross-ribs 22 are 
longitudinally oriented parallel to the end rails 20 and perpendicular to 
the side rails 18, and lie substantially in the same plane as the rails 
18, 20. The cross-ribs 22 are also made of aluminum and have a slightly 
arcuate cross section as best seen in FIG. 8. The cross-ribs 22 are bent 
sheet metal in the preferred embodiment, although greater strength could 
be achieved by using heavier gauge metal or solid bars. The cross-ribs 22 
may be bent or bowed into place to conform with the secondary reverse 
crown 38 by clamping them to the temperature controlled table top, 
although they may also be straight. Then they are welded in place. 
Referring to FIG. 4, the next step in the manufacturing process results in 
the unfinished panel shown in FIG. 4. To achieve the stage of manufacture 
shown in FIG. 4, the unfinished panel shown in FIG. 3 is stretched by 
placing it on a stretcher table and activating a plurality of clamps, such 
as six, that clamp with compression force on both sides of the face sheet 
12 and also pull against the end rails 20. For example, one compressive 
clamp may be placed substantially in each of the four corners of the face 
sheet 12. The clamps are tightened and then the incomplete panel shown in 
FIG. 3 is subjected to pulling stresses, or in other words is put into 
tension by pulling in a direction parallel to the y-axis with a force of 
25,000 to 45,000 lbs. (11,400 to 2,050 kg). This force is distributed in 
directions and planes other than the pulling direction in a complex 
pattern, but the result of the pulling is to stretch the side rails 18 and 
the face sheet 12 about 1/16 inch (0.158 cm), although well within the 
linear range of the stress-strain curve and well short of the elastic 
limit or yield point of the metals. Conventional hydraulic expansion 
devices are used to put the incomplete panel of FIG. 3 under tension. 
While the incomplete panel of FIG. 3 is under this tension, brace means are 
inserted between the cross-ribs 22 to fill the spaces between the 
cross-ribs and thereby prevent the face sheet 12 from returning to its 
original dimensions even after the hydraulically generated external 
tension forces are removed. In the illustrated example, nine reinforcing 
members 24 are set in place and welded to the adjacent and perpendicular 
members. The reinforcing members are disposed in one or more straight 
lines parallel to the y-axis and intermediate of the two long sides or 
other boundary edge of a face sheet. It is important that there is no gap 
between the reinforcing members 24 and the adjacent elements to which they 
are welded when the welding is completed, so the reinforcing members 24 
are coped to fit closely into their intended spaces. As illustrated in 
FIG. 4, the coped reinforcing members are shaped to conform snugly with 
the contour of the adjacent adjoining members when the incomplete panel of 
FIG. 3 is in the stretched condition and any possible remaining gap is 
welded closed. 
Still referring to FIG. 4 in particular, the coped reinforcing members 24 
are disposed in a line along the longitudinal centerline, or y-axis of the 
face sheet 12 and are in contact with the face sheet 12 as well as the 
adjoining members that project outwardly from the rear side 14 of the face 
sheet 12. Adjacent to each end rail 20, however, there are two coped 
reinforcing members 24 spaced apart to divide the width of the face sheet 
12 into thirds. Although it is possible and workable to include only a 
single coped reinforcing member 24 in these two end channels 42 that are 
formed toward the short edges of the panel 12, it has been found that the 
additional reinforcement provided by having a second coped reinforcing 
member 24 at each end of the finished prestressed lightweight panel 10 
provides a more rigid face sheet 12 at the points where the greatest 
hydrostatic pressure will be developed, namely, at the bottom of the form 
(either end being capable of serving as the bottom of the form). When the 
weld beads and all elements have solidified, the external tension forces 
are removed. 
Strain gauge measurements reveal that the tension remaining in the face 
sheet 12 after all clamps and tensioners have been removed from the panel 
of FIG. 4 is at least about 2,000 lbs. (900 kg) in a direction parallel to 
the y-axis. It is believed that the coped reinforcing members are now in 
compression and lock in the stress and strain of the face sheet 12 that 
was caused by the tension forces applied in the preceding step. It is 
apparent that only a portion of the total force remains locked into the 
panel 10 using the process described to the point. Additional potential 
tension in the face sheet can be realized as actual tension in subsequent 
steps that add the spine reinforcement member 26, as disclosed below. 
At this point, the panel as shown in FIG. 4 still maintains a shape similar 
to the shape of the face sheet shown in FIG. 1, that is, both the crown 30 
and the secondary reverse crown 36 remain in the panel, although they are 
not as pronounced as before the tension forces were applied because those 
forces tend to flatten the sheet and to remove at least a portion of the 
crown 30. That is, the face side 16 of the face sheet is still slightly 
bent toward the face side along the narrow ends of the panel as 
illustrated in FIG. 5 where the curvature is exaggerated for purposes of 
illustration. The panel thus formed is complete and is suitable for many 
applications. 
Referring now to FIG. 6, there is shown an additional step that is 
performed in the manufacture of an alternative embodiment of a prestressed 
lightweight panel 10 intended for use in heavy load applications. In this 
embodiment, an anti-bowing means, such as the spine reinforcement member 
26, is attached to the rear side 16 of the face sheet 12 to prevent the 
panel, and especially the face sheet 12, from bowing, i.e., becoming more 
concave when subjected to heavy loads. 
A spine reinforcing member 26 consisting of a length of aluminum bar stock 
is disposed along the y-axis and connected to a stop 46 by the rod 48 and 
the fastener 50. The rod 48 is passed through an aperture in the 
right-hand end rail 20. The left-hand end rail 20 also includes an 
aperture through which a rod 48 is passed, the other end of the rod being 
attached to the spine reinforcing member by the fastener 50. A stop and 
hydraulic expander 52 is attached to the end of the left-hand rod 48. At 
this point, the spine reinforcing member 26 lies loosely along the coped 
reinforcing members 24 and the cross-ribs 22. A pair of clamps, one 
located on each side rail 18 where they are intersected by the x-axis, 
clamps the face sheet 12 firmly to the table top to insure the panel will 
bend in the desired direction only. The stop and hydraulic expander 52 is 
activated creating tension forces in the spine reinforcing member 26 
parallel to the y-axis, until the face side 16 of the face sheet 12 flexes 
or bows and becomes substantially flat in side elevation. That is, the 
curvature illustrated in exaggerated form in FIG. 5 is removed. When the 
face sheet 12 is substantially flat no additional force is applied through 
the stop and hydraulic expander 52, but the tension then in the spine 
reinforcing member 26 is maintained. If desired, the face sheet 12 may be 
bent a little beyond a flat state so that it has a longitudinal convexity 
of about 1/8 inch (0.318 cm), which will be removed by the tension in the 
face sheet 12, which slightly stretches the welded spine reinforcement 
member 26, so that the finished panel 10 is substantially flat. Installing 
the spine reinforcement member 26 increases the tension in the face sheet 
12 by about 2,000 lbs. (900 kg), for a total locked in tension of about 
4,000 lbs. (1,800 kg). Thus the embodiment of the prestressed lightweight 
panel 10 having the spine reinforcement member 26 is stiffer and less 
subject to deflection than the embodiment that does not include it. Then 
the spine reinforcing member 26 is welded to the coped reinforcing members 
24 and the weld beads are allowed to cool. It is important that the spine 
reinforcing member 26 be welded at each weldment sequentially from one end 
to the other. 
Then the stop 48 and the stop and hydraulic expander 52 are removed, along 
with the rods 48 and the fasteners 50. If desired, the apertured tab ends 
of the spine reinforcing member 26 that were attached to the pulling 
apparatus can be cut off to provide a superior finished appearance. The 
final completed prestressed lightweight panel 10 is shown in FIG. 7. 
It is believed that the primary forces now acting within the prestressed 
lightweight panel 10 include tension forces and stress and strain within 
the face sheet 12 primarily oriented in directions parallel to the y-axis; 
some tension forces parallel to the length of the side rails 18, 
particularly in those portions of the side rails 18 closest to the rear 
side 14 of the face sheet 12; compression forces in the side rails 18, 
particularly in the portions farthest from the face sheet 12, which are 
trying to return to their original straight shape; compression forces in 
the coped reinforcing members 24, which were inserted and fixed into place 
when the face sheet 12 was stretched and which apparently lock in much of 
the remaining tension forces found in the face of the finished prestressed 
lightweight panel 10. Finally, in the preferred embodiment that includes 
the spine reinforcement member 26, tension forces in the spine 
reinforcement member directed substantially parallel to and along the 
y-axis remain in the member 26 after manufacture. It has been found that 
the resulting finished prestressed lightweight panel 10 is dimensionally 
stable, indicating that these counter-acting forces are in equilibrium. 
In use, when the poured concrete or other external vector exerts a force 
against the face side of the face sheet, that force must compress the side 
rails 18, which are already in compression, and the spine reinforcement 
member 26, which is in tension, before the face sheet 12 itself can be 
deflected. 
In an alternative embodiment that may use even lighter weight materials for 
some members, the panel may include one or more stiffening members 28, as 
shown in FIG. 7. The stiffening member 28 is preferably an aluminum pipe 
pushed through apertures drilled in the side walls of the cross-ribs 22, 
and welded at each joint, that is, where the stiffening member 28 
penetrates a coped reinforcing member 24. The stiffening member 28 is 
welded to the face sheet 12 while the panel is stretched longitudinally, 
that is, at the stage of the process shown in FIG. 4. Upon release of 
tension in the panel, the member 28 is placed in compression and resists 
the tendency of the panel to rebound. One or more stiffening members 28 
may be used in conjunction with or in lieu of the coped reinforcement 
members 24. Naturally, other means of further stiffening the resulting 
structure can be developed. It would be simple enough, for example, to 
employ more than one spine reinforcing member 26. 
In an alternative embodiment, a flat face sheet 12 is pulled in tension 
along the y-axis and a pair of side rails 18 comprising a brace means are 
placed along the edges of sheet 12 that are parallel to the y-axis. While 
the face sheet is being pulled, the side rails are welded to the face 
sheet. Additional longitudinal members may also be welded to the face 
sheet while it is under tension. The tension is put into the face sheet 12 
by pulling on it in directions parallel to the y-axis with a force of 
25,000-45,000 lbs. (11,400-2,050 kg). Each side rail is welded to the face 
sheet in a number of places and the welds must be laid down sequentially 
from one end of each side rial 18 to the other to lock the tension forces 
into the face sheet 12. When the welds have solidified and cooled, the 
external tension forces can be released from the face sheet 12. Then the 
face sheet 12, which has been stretched, but not beyond its elastic limit, 
tends to rebound because for the sheet 12 to shorten, the side rails 18 
would also have to become shorter. Thus the tension forces in the sheet 12 
place the side rails 18 in compression and the two opposing forces cancel 
each other and permit no significant dimensional change in any member. 
Naturally additional members, such as cross-ribs, reinforcement members, 
spine reinforcement members, or stiffening members could also be added to 
the prestressed lightweight panel of this alternative embodiment. 
As a further aid to preventing or significantly reducing deformation of the 
face sheet 12 when under load, such as the hydrostatic loading of poured 
concrete and the additional loading that results when the concrete expands 
as it sets, compression resistant structural members of particularly 
lightweight and light resistance to compression may be positioned between 
the face sheet 12 and structural members backing the face sheet 12, such 
as the cross-ribs 22 and the reinforcing members 24. In the illustrated 
example, the cross-ribs 22 are of cross sectional hat shape and form a 
void against the face sheet. The area of the face sheet 12 within the ribs 
22 and between the ribs 22 is subject to the localized bulging, called 
pillowing. 
To combat this pillowing, the void is filled with a compression resistant 
material, such as a high strength plastic foam, aluminum composite 
honeycomb, or as shown in the illustrated example, a resin impregnated 
kraft paper based honeycomb shape 60. Alternatively, the void may be 
filled by an injectable mix of expanding chemicals to cure into a 
compression resistant foam to resist the pillowing phenomena under the 
ribs 22. 
While it is known in the prior art to employ a weak foam as a void filling 
to seal against entrance of extraneous concrete when used in a concrete 
form, such void fillings do not provide a resistance against deformation. 
In operation, the prestressed lightweight panel 10, when designed for use 
as a concrete form, is subjected to loading of 1,000 to 1,200 lbs. per 
square foot (420-506 kg/m.sup.2) at the bottom of an 8 foot (2.4 m) panel, 
loadings that are extremely typical and common in pouring foundations for 
houses. In such circumstances the prestressed lightweight form panel 10 
shows much less deflection and maintains its degree of flatness better 
than unstressed panels. In addition, the panel 10 described herein weighs 
73 lbs. (33 kg), approximately 20 lbs. (14 kg) lighter than the average 
form panel in the concrete industry. 
Additionally, use of the above described process of manufacture enables 
production of a substantially flat faced form panel of 0.080" aluminum 
skin. Heretofore, such an unpretensioned thickness of skin, under 
conventional production methods could not be made sufficiently flat for 
commercial acceptance. For example, the disclosed method of manufacture 
permits construction of a concrete form panel with 0.080" face sheet 
thickness having a deviation from absolute flatness of 41% less than the 
deviation from absolute flatness of a unpretensioned form panel of 0.080" 
face sheet thickness. Not only does the disclosed method of manufacture 
enable practical production of such a thin sheet form panel, but the form 
panel is surprisingly stronger and more resistant to deflection under load 
than an unpretensioned panel. Engineering calculations reveal that an 
unpretensioned form panel of 0.080" face sheet thickness has approximately 
two times the deflection under a 1900 p.s.f. hydrostatic load than a form 
panel having the same thickness of face sheet and under a 4,000 
pretension. 
In the other measurements using a pretensioned concrete form panel of 0.094 
face sheet thickness and a honeycomb void filling shape resistant to 
compression, it was determined that the subject structure had only 27% of 
the lateral deflection and 49.4% of the longitudinal deflection of an 
unpretensioned form panel without honeycomb. Accordingly, the pretensioned 
structure with compression resistant void filling represented a 
significant improvement in strength. 
It is to be understood that while certain forms of this invention have been 
illustrated and described, it is not limited thereto, except and insofar 
as such limitations are included in the following claims.