Patent Publication Number: US-6711866-B2

Title: Thin prestressed concrete panel and apparatus for making the same

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/684,874, filed Oct. 6, 2000, which is incorporated herein by reference. 
    
    
     THE FIELD OF INVENTION 
     The present invention relates to the construction and manufacture of thin prestressed concrete panels useful for architectural cladding of buildings and other purposes. 
     BACKGROUND OF THE INVENTION 
     Exterior cladding of a building is subjected to attacks from climatic conditions such as freeze-thaw cycles, moisture intrusion, ultra-violet rays, wind and seismic loading and sometimes vibration from traffic and other sources, amongst other things. 
     Precast concrete cladding systems have been used extensively on commercial buildings because of their durability and architectural appeal. However, precast concrete cladding, as used heretofore, is provided typically in heavy elements and its use has been limited to reinforced concrete or steel frame structures due to the load that it imposes on a building. Consequently, a building designed to carry the lateral and gravity loads imposed by the heavy concrete skin system is costly. Moreover, existing concrete panel systems are susceptible to permanent deformation from perpendicularly applied loads that create surface cracks in the tension face of the panel. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, concrete panels are prepared by casting panels of about 1.5 inch thickness or less, which contain prestressed tendons. The tendons are oppositely positioned between the mid-plane of the panel and each of the opposite faces and may be spaced either equidistantly or at different distances from the adjacent face, in spaced grids. The positioning of opposing tendons between the mid-plane and the opposite faces increases panel resilience and will effect return of a panel to its original shape after being flexurally deformed by loads imposed normal to its faces, such return being effected even if a crack has developed in the tension face of the panel. 
     OBJECTS OF THE INVENTION 
     It is, accordingly, an object of this invention to provide a prestressed concrete panel that is thin, light, durable and resilient. 
     Another object of the invention is to provide a prestressed concrete panel that may be field cut and easily installed on a structure. 
     Still another object of the invention is to provide an improved apparatus for casting prestressed concrete panels at a reduced cost. 
     The foregoing and other objects, features and advantages of the present invention are described further in the following detailed description, which proceeds with reference to the accompanying. 
    
    
     DRAWINGS 
     FIG. 1 is a plan view of the tendon layout in a panel made in accordance with the invention. 
     FIG. 2 is a fragmentary perspective view of a panel made in accordance with the invention. 
     FIG. 3 is a sectional view taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a sectional view taken along line  4 — 4  of FIG.  1 . 
     FIG. 5 is a plan view of a molding apparatus for forming a panel in accordance with the invention showing the position of the reinforcing tendons prior to the addition of concrete to the apparatus. 
     FIG. 6 is an enlarged, fragmentary sectional view of the apparatus taken along line  6 — 6  of FIG.  5 . 
     FIG. 7 is a fragmentary sectional view taken along line  7 — 7  of FIG. 6 showing the arrangement for positioning a pair of the tendons which extend lengthwise of the mold. 
     FIG. 8 is a view similar to FIG. 7 showing the arrangement for positioning a pair of tendons extending transversely of the mold. 
     FIG. 9 is a fragmentary view of a panel having a ribbed construction to provide a higher strength to weight ratio. 
     FIG. 10 is a fragmentary sectional view taken along line  10 — 10  of FIG. 9 showing tendon placement. 
     FIG. 11 is a fragmentary sectional view showing one arrangement for securing panels of the invention to a building surface. 
     FIG. 12 is a fragmentary sectional view of a different mounting arrangement. 
     FIG. 13 is a fragmentary sectional view of still another mounting arrangement. 
     FIG. 14 is a fragmentary sectional view of an arrangement for mounting a panel utilizing an imbedded plug receiving a threaded bolt. 
     FIG. 15 is a fragmentary sectional view of an arrangement for securing thin panels to a building surface. 
     FIG. 16 is a perspective view of a portion of a panel with another form of connector. 
     FIG. 17 is an enlarged view taken generally along the line  17 — 17  in FIG.  16 . 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIGS. 1-4, there is therein illustrated a preferred embodiment of the invention comprising a thin prestressed, reinforced concrete panel  10 , which may be, for example, approximately 50 inches in length, 25 inches in width and have a ⅝ inch thickness. This size is only illustrative since the panel may be made in a wide variety of sizes. A thin panel as used herein refers to a panel with a maximum thickness of approximately 1.5 inches. 
     The illustrated panel  10  is formed with an exposed face  12  and an opposite back face  14  each of which faces are flat and parallel to one another. Alternatively, the exposed face  12  may be textured rather than flat to achieve a desired architectural appearance on the panel. Panel  10  is shown as formed with a pair of opposite end faces, or edges,  16 ,  18 , and a pair of opposite side faces, or edges,  20 ,  22 . In the illustrated embodiment the side and end faces are beveled such that the back face  14  is of larger dimensions in length and breadth than the exposed face  12 . Such beveled faces are advantageous in that the exposed face  12  of the panel is less likely to chip during handling than is a panel having exposed and back faces that are perpendicular to the side and end faces. It therefore should be appreciated that the likelihood of chipping occurring is reduced as the angle between the exposed face  12  and an end or side face is increased. The beveled faces also facilitate removal of a cast panel from the mold in which it is formed. 
     Extending through the panel  10  between the end faces  16 ,  18  is a set of prestressed, parallel tendons, which may comprise a plurality of longitudinally extending pairs  30  of stainless steel wire ropes. Similarly, a second set of tendons is provided extending between the side faces  20 ,  22  which also may comprise a plurality of pairs  32  of pre-tensioned stainless steel wire ropes. 
     Referring more particularly to FIG. 4, each of the tendon pairs  30  comprises a first wire rope  34  which is spaced a distance d 1  (as measured to its center line) from the exposed face  12  and a second wire rope  36  which is positioned the identical distance d 1  from the back face  14 . The distance d 1  is preferably approximately equal to two times the diameter of the wire ropes, which in the case of a ⅝ inch thick panel, are preferably {fraction (1/16)} inch diameter, 7×7 strand stainless steel wire rope. In thicker panels the wire ropes, or tendons, may have a diameter up to approximately ⅛ inch. Such rope configuration facilitates formation of a secure bond between the concrete and the rope, and the positioning of the ropes no less than two diameters from the adjacent face will assure that no bond failure will result in the event of extreme loading upon the panel. The ropes  34 ,  36  of each pair  30  are spaced apart laterally, i.e., relative to the side faces  20 ,  22 . In a ⅝ inch panel a lateral spacing of about ½ inch is preferred. In thicker panels the lateral spacing between such ropes, or tendons, may be 1 inch or greater. 
     As best shown in FIG. 3, pairs  32  of wire ropes extend laterally with respect to the panel between the side faces  20 ,  22 . Each pair  32  comprises a first rope  40  and a second rope  42 . The first rope  40  of each pair  32  is positioned from the exposed face  12  by a distance d 2  which is greater than the distance d 1  by a distance equal to the diameter of the ropes so that the ropes  40  extend in a straight line from side face  20  to side face  22  and tangentially contact the ropes  34 . Similarly, the second ropes  42  are positioned from the back face  14  of the panel by the distance d 2  such that the ropes  42  tangentially contact the ropes  36 . 
     Prestressing of the wire ropes during the casting process should be limited so that the wire ropes do not excessively relax and lose their prestress over time. In the case of a ⅝ inch thick panel using 7×7, {fraction (1/16)} inch diameter stainless steel wire ropes, a satisfactory panel is obtained by prestressing the wire ropes to 315 lbs., which is approximately 70 percent of their breaking strength. This will result in a prestress in the panel of about 250 psi both longitudinally and crosswise of the panel. 
     The positioning of the prestressed tendons equidistantly from and on opposite sides of the mid-plane M of the panels, rather than in the mid-plane, greatly increases the panel&#39;s loading capacity and its resilience. A panel constructed as described with a minimum prestress of 200 psi on the tendons, should return to its original flat shape after being flexurally deformed even to the extent that a crack forms in the tension face of the panel. The tendons on the opposite sides of the mid-plane M of the panel may be spaced equidistantly therefrom so as to avoid an eccentric load which could cause warping of the panel. In the forming of the panel the surface textures and finishes should be accounted for in the positioning of the tendons. 
     Referring to FIG. 16, another embodiment of a thin prestressed reinforced concrete panel  10 A is illustrated. This panel may have a thickness T of about one inch between its opposed faces  12  and  14 . Mid-plane M is indicated intermediate faces  12 ,  14 . As mentioned previously thin panels according to the invention may have thicknesses up to about 1.5 inches. 
     In this embodiment tendon pairs  30  again are denoted having a first wire rope  34  and a second wire rope  36 . Tendon pairs  32  include a first wire rope  40  and a second wire rope  42 . 
     In this embodiment although the wire ropes, or tendons, in each pair are disposed on opposite sides of mid-plane M, between the mid-plane and their associated face of the panel, their spacing relative to their associated face of the panel, and relative to the mid-plane, are not equidistant. 
     By way of example, and for a specific application which may require compensating prestressing forces in the panel, the distance d 7  between tendons, or ropes,  34  and face  12  of the one inch thick panel may be about 0.4844 inch. Distance d 8  between wire rope, or tendon,  36  from its associated panel surface  14  may be on the order of 0.3281 inch. The distance d 9  between wire rope, or tendon,  42  and its adjacent surface  12  of the panel may be on the order of 0.3906 inch, and the distance d 10  between wire rope, or tendon,  40  and its associated face  14  of the panel may be on the order of 0.2344 inch. From this it will be seen that tendons  34 ,  36  in one set and  40 ,  42  in a second set are positioned at different, unequal, distances from the mid-plane M. 
     Depending on concrete mix characteristics and methods of pouring and vibrating the concrete, the concrete density may not be uniform throughout the thickness of the panel and the panel may bow or warp under prestress. To compensate for the variable density of the concrete and ensure that the panel does not bow or warp, the resultant force of the tendons should be slightly offset from the mid-plane of the panel. The amount of offset from the mid-plane can be determined by trial and error or calculated with standard engineering principles for a homogeneous material once a prototype panel has been constructed and warping is measured. 
     Explaining further, in pouring a panel such as that indicated at  10 A face  14  may be at the bottom of the mold (i.e., adjacent plate  60  of the mold as described below) and face  12  may be directed upwardly. In such process the panel may have a greater density near face  14 , than near face  12 . 
     To compensate for such varying density the tendons, as in FIG. 16, are offset toward face  14 , rather than being equidistant from the mid-plane. Typically, the centerline between a pair of tendons may be offset from the mid-plane toward face  14  by a distance of a much as 10 percent of the total thickness of the panel. 
     Also, the tendons of each pair, while it is desirable they be close to one another laterally, should not be positioned in the same plane normal to the surfaces of the panel but should be offset therefrom to avoid creating a weak plane in the concrete. In addition, minimizing spacing between pairs of tendons increases the resilience and strength of the panel. Thus, the pairs of tendons should be spaced close enough together such that a reinforcing grid is created to disperse point loads and reinforce corners and edges of the panel. A maximum spacing of eight times the panel thickness between each pair of tendons is preferred. 
     For panels which are exposed to moist atmospheres, it is desirable that the tendons be non-corrosive. In place of stainless steel wire rope, carbon fiber tendons or glass fiber tendons or others could be used. In any event, a tendon must have a surface suitable for forming a firm bond between the tendon and the concrete. The tendon material should also be strong enough to limit relaxation over time so as not to lose the prestress applied thereto. High strength stainless steel of approximately 200 ksi has proved to be satisfactory. 
     The concrete mix utilized should be one that will have durability under the climatic conditions to which the panel will be exposed such as freeze/thaw cycles, and should be resistant to shrinkage so that prestress will not be lost and the panel&#39;s architectural appearance will be maintained. To optimize the properties of the concrete, the aggregate size preferably should not exceed one-third the panel thickness and the concrete mix should have a low water-cement ratio. A mixture of aggregates can be used to provide the desired architectural look. 
     Depending on the coarseness of the concrete mix, it may be difficult to obtain a flat back face  14  on a panel. In such a case, a layer of sand and cement backing mix, which preferably should be between {fraction (1/16)} and ⅛ inch in thickness, may be applied to the casting form to provide a back face and achieve a flat surface. Tables 1 and 2 below detail a suitable concrete mix and a backing mix which I have found to form suitable panels. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Concrete Mix 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Percent of 
                   
               
               
                   
                   
                 Total 
                 Specific 
               
               
                 Materials 
                 Brand 
                 by Weight 
                 Gravity 
               
               
                   
               
               
                 White cement 
                 Riverside 
                 18.0% 
                 3.15  
               
               
                 Silica Fume 
                 Master Builders 
                  0.0% 
                 2.2  
               
               
                   
                 Rheomac SF100 
               
               
                 Total Cementitious 
               
               
                 Material 
               
               
                 Fine silica Sand #70 
                   
                 10.0% 
                 2.273 
               
               
                 Silica sand #30 
                   
                 10.0% 
                 2.346 
               
               
                 Silica sand #8 
                   
                 10.0% 
                 2.353 
               
               
                 {fraction (3/16)}” Black basalt 
                   
                 14.0% 
                 2.700 
               
               
                 No. 2 Crushed Granite 
                   
                 30.6% 
                 2.514 
               
               
                 (¼ inch) 
               
               
                 Water 
                   
                  7.2% 
                 1    
               
               
                 Color 
                 Davis 860—black 
                 0.09% 
               
               
                 High range super 
                 Master Builders 
                 3.080 
                 OZ/100 lbs 
               
               
                 plastizier 
                 Rheobuild 3000 FC 
                   
                 cement 
               
               
                 Entrained Air 
                 Master Builders MB 
                 1.030 
                 OZ/100 lbs 
               
               
                   
                 AE 90 
                   
                 cement 
               
               
                 Water/Cement ratio 
                 Including Silica Fume 
                 40.0% 
               
               
                 Water/Cement ratio 
                 W/O Silica Fume 
                 40.0% 
               
               
                 Total Wgt 
                   
                 100.0%  
               
            
           
           
               
               
            
               
                 Mix unit wgt 
                 144.4 lb/cuft 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Backing Mix (when required for leveling of back face) 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Percent of 
                   
               
               
                   
                   
                 Total 
                 Specific 
               
               
                 Materials 
                 Brand 
                 by Weight 
                 Gravity 
               
               
                   
               
               
                 White cement 
                   
                 24.5% 
                 3.15  
               
               
                 Silica Fume 
                   
                  0.0% 
                 2.2  
               
               
                 Total Cement 
                   
                 24.5% 
               
               
                 Fine silica sand #70 
                   
                 21.0% 
                 2.273 
               
               
                 Silica sand #30 
                   
                 21.0% 
                 2.346 
               
               
                 Silica sand #8 
                   
                 21.5% 
                 2.353 
               
               
                 Water 
                   
                 12.0% 
                 1.000 
               
               
                 High range super 
                 Master Builders 
                 3.4 
                 OZ/100 lbs 
               
               
                 plastizier 
                 Rheobuild 3000 FC 
                   
                 cement 
               
               
                 Entrained Air 
                 Master Builders 
                   
                 OZ/100 lbs 
               
               
                   
                 MB AB 90 
                   
                 cement 
               
               
                 Water/Cement ratio 
                 w/o fume 
                 49.0% 
               
               
                 Total Wgt 
                   
                 100.0%  
               
               
                   
               
            
           
         
       
     
     Referring now to FIGS. 5 and 6, therein is shown an example of a suitable molding apparatus for forming a panel containing prestressed wire rope in accordance with the invention. The illustrated apparatus comprises a frame  50 , comprising opposite longitudinal side members  52 ,  54  and end members  56 ,  58 . A preferred embodiment of the molding apparatus is provided with a frame dimensioned to cast a panel measuring approximately 50 inches in length, 25 inches in width and ⅝ inch in thickness. Alternatively, larger panels, which may be, for example, of 15 feet or more in length, 6 feet or more in width, 1.5 inches thick may be cast and field cut into smaller usable panels. The frame may be reinforced with suitable bracing (not shown) to maintain the rigidity of the frame members  52 - 58  as tension is applied to the wire ropes. 
     Suitably supported on the frame  50  is the bottom plate  60  of a mold. The upper surface  66  of the plate  60  may be flat and smooth or may be textured so as to form a desired texture on the cast panel. A continuous bulkhead  62  comprising opposite side portions  68 ,  68 ′ and opposite end portions  70 ,  70 ′ is mounted to the sides and ends of the plate  60  and will define the side faces and end faces, respectively, of the panel cast therein. 
     As best shown in FIG. 6, the top edge  72  of the bulkhead defines a plane parallel to the upper surface  66  of the plate  60  and is spaced therefrom by a distance equal to the desired thickness of the panel  10 . In the embodiment shown, the bulkhead  62  is formed so that the casting surface  64  thereof slopes upwardly from the upper surface  66  of the plate  60  at an included angle of about 105 degrees. The inclined casting surface  64  is desirable in that it forms the beveled side and end faces of the panel and facilitates the removal of the panel from the mold once the concrete has cured. It should be appreciated, however, that mold walls of a different inclination or walls that are perpendicular to the bottom plate could be utilized in the present invention. In any case, a mold release material is preferably applied to the upper plate surface  66  and the bulkhead casting surface  64  to assist in removing the panel once the concrete has cured. 
     As best shown in FIG. 7, the bulkhead end portions  70 ,  70 ′ are each provided with a plurality of pairs of slots  74 ,  76  through which the ropes  34  and  36 , respectively, can be threaded prior to pouring the concrete into the mold. The bottoms of the slots  74  are each spaced from the plane of the bottom plate upper surface  66  by a distance d 3 , which is equal to one and a half times the diameter of the ropes. The bottoms of the slots  76  are each spaced from the plane defined by the top edge  72  by a distance d 4 , which is equal to two and a half times the diameter of the ropes. This spacing will position the ropes supported thereby equidistantly from the adjacent face of the cast panel and equidistantly from the mid-plane M of the cast panel, see FIGS. 3 and 4. 
     Referring to FIG. 8, slots  78 ,  79  are similarly formed in the side portions  68  to properly position the wire ropes  40 ,  42 , respectively, that extend laterally with respect to the mold. The bottoms of slots  78  are each spaced from the plane of the upper surface  66  by a distance d 5 , which is equal to two and one half times the diameter of the ropes, and the bottoms of slots  79  are each spaced from the plane defined by the top edge  72  by a distance d 6 , which is three and a half times the diameter of the ropes. Thus, the depths of the slots  78 ,  79  are such that the wires  40  will be positioned above the wires  34  in tangential engagement therewith, and the wires  42  will be positioned beneath the wires  36  in tangential engagement therewith, as best shown in FIG.  6 . 
     The above-noted dimensioning and positioning of the slots for receiving and holding the wire ropes during the panel forming process would be modified as needed to properly position the wire ropes, or tendons, for different panels, such as described above in regard to FIG.  16 . 
     Tensioning means are provided for applying tension to the wire ropes during the casting and hardening of the panel. Referring more particularly to FIGS. 5,  6  and  7 , the illustrated tensioning devices are each arranged to apply tension to a set of three pairs of wire ropes. Since the tensioning devices are substantially identical, only the devices for a single set of wire ropes will be described in detail. Suitably mounted to the frame element  58 , as by welding, is a dead head  80  into which are threaded three pairs of bolts  82 ,  84 ;  86 ,  88 ; and  90 ,  92 , which define posts around which a wire rope is reeved as more particularly described hereinafter. A bushing  100  (indicated in dotted lines in FIG. 6) is disposed on each of the posts  84 ,  86 ,  88  and  90  to facilitate movement of the wire rope around the posts with minimal friction. 
     Attached, as by welding, to the opposite frame element  56  is a pair of brackets  94  and  96  in which is journaled a shaft  98 . Secured to the shaft  98  are three posts  95 ,  97 , and  99 , around which a tension element is reeved. Each post  95 ,  97 , and  99  has a bushing  101  to minimize the sliding friction of the wire rope as it is tensioned. Secured to one end of the shaft  98  is a stressing drum  102 , which is adapted to be releasably engaged by a pair of ratchets  104 ,  106 , pivotally mounted to the bracket  96 . The opposite end of the shaft  98  is formed with a hex head  108  adapted to be engaged by a torque wrench (not shown) for effecting rotation of the shaft when tension is to be applied to the wire rope engaged thereon. 
     Referring more particularly to FIG. 5, in the embodiment shown it is convenient first to position in place the wire ropes forming the laterally extending pairs  32  and thereafter the wire ropes forming the longitudinally extending pairs  30 . Thus, one end of a rope is secured to a post  82 , of a side mounted dead head by tightening a nut  109  on the bottom end of the post  82  so that the rope end is securely held between the upper surface of the dead head  80  and a washer  111  disposed on the upper end portion of the post. The rope is then laid in a notch  78  of the adjacent bulkhead side portion  68  and transversely of the mold and into the opposing notch  78  in the opposite side portion  68 ′, thus forming the first course  40  of one of the pairs  32  of tendons. The rope is carried around the post  95  and thence laced back across the mold positioning it in the notches  79 ,  79 ′ adjacent the notches  78 ,  78 ′ in which the first course  40  was positioned, thereby forming the course  42 . It is then passed around the posts  84  and  86  as shown in FIG. 5, and back again to the opposite side of the mold, positioning the rope in the notch  78  adjacent the post  86 , and the similar notch positioned opposite thereto adjacent post  97 . The rope is then passed around post  97  and back across the mold positioning it in the notches  79 , around the posts  88  and  90 ; thence back across the mold and around the post  99 , and finally back to post  92  to which it is secured in a conventional manner. In similar fashion three additional wire ropes are laid laterally of the mold, between the other deadheads  80  and stressing drums  102  along the mold sides. 
     Thereafter the wire ropes extending lengthwise of the mold can be laid in place so as to provide the pairs  30  of ropes  34 , and  36 . Deadheads and stressing drums, as described above, are mounted to the frame and side members, as may be seen in FIG.  5 . Two ropes in the illustrated embodiment are reeved around the posts in the deadheads and stressing drums, but in this instance the portion of a rope forming the course  34  of a set is passed under the previously stretched ropes  40  and a rope forming a course  36  is passed over the ropes  42 , as best seen in FIGS. 3 and 4. As previously mentioned, the rope courses  34 ,  36  extend through slots  74 ,  76 , and  74 ′,  76 ′, formed in the end portions  70 ,  70 ′, respectively, of the bulkhead  62 , the slots being dimensioned so that when the ropes are stressed and taut the longitudinal and lateral runs of the rope are tangential to one another in their straight and stressed condition. 
     When all of the ropes are in place, and the proper tension applied thereto, a concrete mix of desired composition may be poured into the mold. If desired, a texturing composition or element may be applied upon the upper surface  66  of the bottom plate  60  prior to mounting the ropes in place. The concrete is preferably poured into the mold from a vibrating hopper (not shown) that is moved across the mold evenly to distribute the concrete to the desired level. The mold may be mechanically vibrated to further ensure even distribution of the concrete in the mold and to effect release of entrapped air. Preferably, the top surface of the concrete is leveled with a vibrating screed (not shown) which can be drawn across the edges  72  of the bulkheads  62 . If necessary or desired, a sand and cement backing mix can be applied to the top surface to fill any voids and assist in creating a flat surface. Since a panel tends to warp if moisture is allowed to escape from one of the surfaces of the panel and not the other, the upper surface of the panel in the mold is preferably covered with a wet mat during the initial curing of the concrete. 
     Once the initial set of the concrete has been accomplished, which will occur in approximately two hours with the mix described in Table 1 above, the mold and the concrete panel therein are preferably steam cured at 120-140° Fahrenheit for about 18 hours until the panel has developed sufficient strength (approximately 3,000 psi) to anchor the cables therein to hold their prestress. It should be appreciated that the actual time required for setting and curing of any particular panel will vary depending on panel thickness and concrete mix. When the panel has developed sufficient strength, the tension on the ropes is released by cutting the exposed tension elements extending through the bulkhead  62  with wire cutters or a similar device. The panel is then removed from the mold which may be facilitated by introducing compressed air between the casting surface and the panel. As previously mentioned, the inclined mold casting surface  64  facilitates the removal of the cast panel from the mold. 
     If desired, the panel may be allowed to continue to cure in a moist environment for an appropriate time, usually about five days, after being removed from the mold. Additional curing is desirable in that it increases the panel&#39;s resistance to shrinkage and its ability to maintain prestress. After curing of the panel is completed, an appropriate finish can be formed thereon by sandblasting or otherwise, and a sealer may be applied to the panel surfaces. 
     Referring now to FIGS. 9 and 10, there is illustrated a panel  110  constructed with a ribbed configuration with tension elements extending therethrough. The ribbed construction is advantageous in that it provides a higher strength to weight ratio than a flat faced panel. As will be apparent to those skilled in the art, the panel  110  will be cast in a mold having a waffle iron type of configuration, so that the cast panel when inverted will appear as shown in FIG.  9 . The panel  110  has tendons extending through longitudinal ribs  112  and lateral ribs  114 . Again, the tendons are preferably stainless steel wire rope, but can be of any other suitable material giving consideration to the environment in which the panel will be utilized. Extending through each of the longitudinal ribs  112  is a first wire rope  134  and a second wire rope  136  that is in this case, positioned vertically beneath the rope  134 . The wire ropes  134 ,  136  are preferably positioned substantially equidistantly from the centroidal plane C of the panel, i.e. a plane through the centeroid of the panel and parallel to the flat face  12 . It is desired that the forces exerted by the stressed tendons be centered in such plane and field experience with a particular panel configuration may require that particular tendons in a panel be relocated closer to or further from the centeroidal plane to achieve such force centering and avoid warping of a panel. Extending through each of the lateral ribs  114  is a wire rope  140  and a wire rope  142  positioned vertically therebeneath. Wire ropes  140 ,  142  are positioned so that each rope  140  is beneath and tangential to the uppermost wire ropes  134 , and each wire rope  142  is immediately above and tangential to the lowermost wire ropes  136 . The spacing of wire ropes in each pair at equal distances from the centroidal plane C of the panel ensures the panel does not bow or warp and effects return of the panel to its original shape after being deformed by perpendicularly applied loads. 
     As discussed previously, various characteristics of the panel may warrant offsetting of the tendons toward one face of the panel, such as they are not equidistant from the mid-plane. This occurs with a panel such as that described in relation to FIGS. 9 and 10 also. 
     Referring now to FIG. 11, there is therein shown an arrangement for securing a pair of panels made in accordance with the invention to the surface  150  of a building wall  152 . A vapor barrier  154  may be placed against the surface  150  and the panels secured in position by means of clips  156 ,  158  held by screws  160 , or other suitable fastener to the wall  150  of the building  152 . The clips  156 ,  158  are each formed with legs  161 ,  162 , respectively, which are adapted to be received in kerfs  164  formed by a suitable masonry saw in the end walls of the panels. A backing rod  168  may be positioned between the legs  161 ,  162  to provide a surface on which a sealant  170  can be applied for sealing the adjacent ends of the panels from the elements. Spacers  172  may be positioned between the surface  150  and the panels to allow for air circulation behind the panels. 
     FIG. 12 shows still another clip arrangement that could be utilized to secure a panel  10  in which a kerf  200  is formed in an end wall thereof to receive a rib  202  provided on a clip  204 , and FIG. 13 illustrates still another arrangement in which a panel  10  can be seated within a channel  206  formed in a clip  208  suitably secured to a building wall  152 . FIG. 14 illustrates a still further fastening arrangement utilizing a clip  210  and an imbedded plug  212  for receiving a securing bolt  213 . 
     FIG. 15 discloses an arrangement particularly useful for securing an adjacent pair of thin panels  214 ,  216 , i.e. panels less than three-fourths inch in thickness, to a building surface  220 . With such arrangements a conventional vapor barrier  224  is suitably secured to the building surface  220  and thereafter spacers such as hat channels  226  secured in place. Precast panels are then mounted by means of two part anchor clips  230  comprising a first part  232  having a base leg  234  which is secured to the building surface by a screw  236  or the like, an outstanding leg  238  and a flange  240  adapted to seat in a step  244  formed in an edge of the panel  214 . The other part of the clips comprises a base leg  246  secured to the base leg  234  and the building by a screw  248 , an outstanding leg  250  and a flange  251  which fits in a step  244  formed in the adjacent edge of the panel  216 . A sealant  252  may be applied over the flanges  240 ,  251  and the opening between the clip parts and panel edges to provide a weather tight seal. 
     FIGS. 16 and 17 illustrate another connector arrangement, indicated generally at  270 , for connecting a panel to adjacent support structure. Connector  270  includes a formed sheet metal clip  272  having a substantially planar central portion  274 , a return bend portion  276  adjacent one side thereof, and a reverse bend portion  278  adjacent an opposite side thereof. A circular opening  280  extends through main portion  274 . Member  272  is adapted to be connected to an adjacent pair of wire ropes, or tendons, as indicated generally at  34 ,  36  in FIGS. 16 and 17. 
     The connector  270  also includes a screw plug  284 . The screw plug has external threads thereon permitting it to be screwed into opening  280 . The screw plug also is internally threaded for receiving a screw connector to secure the panel to an adjacent support structure. 
     In the process of forming a panel the clip  272  is connected to wire ropes, or tendons, such as those indicated at  34 ,  36 , and screw plug  284  is screwed into opening  280 . The screw plug is screwed into member  272  to a position in which its inner end engages wire rope  36  to clamp the connector securely to rope  36 . The panel concrete then is cast about the connector. The connector thus is embedded in the panel and is adapted to receive a screw connector. Other fastening arrangements will be obvious to those skilled in the art. 
     In addition to using panels made in accordance with the invention as wall panels, the panels could be utilized as floor covering, counter tops, lightweight traffic surfaces on structures and other surfacing environments. 
     Having illustrated and described the preferred embodiments of my invention, it should be apparent to those skilled in the art that the invention permits of numerous modifications and changes in arrangement and detail. I claim all such modifications and changes as come within this scope and purview of the appended claims.