Patent Publication Number: US-6711862-B1

Title: Dry-cast hollowcore concrete sandwich panels

Description:
BACKGROUND OF THE INVENTION 
     Concrete sandwich panels are well known in the art, and generally comprise spaced apart layers of concrete with an insulation layer sandwiched between the concrete layers. Connectors extend through the insulation layer and into the concrete layers to tie the concrete layers together when the concrete cures. 
     Concrete sandwich panel connectors normally are supplied with deformations or anchorage zones to provide notches, bosses, or other irregularities in the connector. Such connectors are usually installed in highly plastic concrete, which can flow into or around the deformations in the connectors, such that, upon hardening of the concrete, the connector and concrete are locked together. The consolidation of the concrete flowing into and around the irregularities in the anchorage zones of the connectors creates a mechanical interlock between the connector and the concrete. 
     In contrast, when sandwich panel connectors are installed in stiff or dry concrete, such as dry-cast concrete, the concrete is not capable of flowing into and around the irregular surfaces on the anchorage zones. Rather, the connectors create a hole in the concrete that remains after installation of the connectors. The connectors therefore are not anchored to the concrete, and can be easily pulled out with little or no load. 
     Extrusion is a common method used to produce lightweight, economical pre-cast concrete floor and wall panels. The extruded concrete normally includes longitudinal voids, or cores, such that the panels are commonly called “hollow-core panels.” Machines are used to slip form concrete with zero or low-slump into such hollowcore panels. Zero or low-slump material generally is defined as material having 0-1 inch of slump using standardized ASPM slump testing. This concrete, while including water or moisture, is very dry, and therefore will not flow around the sandwich panel anchorage zones. This concrete is commonly called “dry-cast.” 
     For this type of hollowcore panels, it is common to form sandwich panels using steel or stainless steel clips that must be anchored by hooking one end of the clips around a steel pre-stressing strand which is placed in the hollowcore layer during slip forming. In order to access the strand, the cured hollowcore concrete is excavated, and the connectors hooked around the exposed strand. The resulting hole in the hollowcore panel is then patched around the installed connector. This work is highly labor intensive and fails to provide a reliable anchorage of the connector in the concrete. The hooks of such steel clips can be straightened with a relatively small force, compared to the tensile capacity of the wire itself. Therefore, the pullout capacity of such anchorage clips is small. Also, the repair to the excavated concrete may leave voids around the wire clips. Since the wire clips are not embedded in the concrete, the clips are free to slide down the steel reinforcing strands in the hollowcore panel. This creates serious problems during handling and installation of the sandwich panels, with the face layer shifting more than an inch as the panel is moved to a vertical position. Furthermore, the excavation process can lead to zones within the panel wherein the reinforcing steel is not encased in the concrete. Because concrete creates a protective environment that slows the corrosion process for embedded steel, and because condensation is a common occurrence in sandwich panels, there is a serious probability that the reinforcing steel within the hollowcore panels will corrode and fail as a result of the installation of the hooked sandwich panel connectors or clips. 
     The installation of anchors or connectors in cured concrete using two-part epoxy adhesives is known in the art. This installation process requires that holes be drilled into the hardened concrete, which is highly labor intensive and time consuming. 
     Accordingly, a primary objective of the present invention is the provision of an improved dry-cast concrete hollowcore sandwich panel. 
     A further provision of the present invention is the provision of an improved hollowcore sandwich panel having connectors consolidated in the concrete layers. 
     A further objective of the present invention is the provision of a connection system that can be installed in dry or low-slump concrete. 
     Another objective of the present invention is the provision of a process for installing connectors in hollowcore sandwich panels. 
     A further objective of the present invention is the provision of a connection system, and a process for installing the connection system, that is positively anchored in the concrete layers of a sandwich panel, and does not allow large shear displacement of one layer of concrete relative to the other. 
     Another objective of the present invention is a concrete sandwich panel, and a method of producing the panel, without voids around the reinforcing steel strands contained in the panel. 
     A further objective of the present invention is the provision of hollowcore sandwich panels having a connection system with low thermal conductivity. 
     Still another objective of the present invention is the provision of hollowcore sandwich panels that the insulation system provides a uniform, verifiable spacing for the connectors. 
     Another objective of the present invention is the provision of a hollowcore sandwich panel having an improved concrete connection system. 
     A further objective of the present invention is the provision of a method for installing a connection system into a hollowcore sandwich panel utilizing minimum labor costs. 
     Another objective of the present invention is the provision of a hollowcore concrete sandwich panel that is economical to manufacture, and durable and efficient in use. 
     BRIEF SUMMARY OF THE INVENTION 
     The concrete sandwich panels of the present invention include a first hollowcore concrete layer and a spaced apart second concrete layer. Insulation is sandwiched between the concrete layers. Preferably, the hollowcore layers are constructed by slip forming zero or low-slump material, so as to have a plurality of voids and concrete webs. The hollowcore layer includes pre-stressing strands in some of the webs. The insulation layer includes a plurality of preformed holes. Holes are formed in the hollowcore layer before the concrete hardens and in alignment with the insulation holes. Adhesive, preferably a two-part epoxy or acrylic, is injected or otherwise supplied into the holes in the hollowcore layer. The adhesive provides a strong bond between the connector and the hollowcore layer. Connectors having low thermal conductivity are inserted through the insulation holes and into the holes in the hollowcore layer. A second concrete face layer is formed on top of the insulation, with the opposite ends of the connectors extending into the face layer, which consolidates around an anchoring surface on the upper end of the connectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an end elevation view of a dry-cast concrete hollowcore panel according to the present invention. 
     FIG. 2 is an enlarged elevation view taken along lines  2 — 2  of FIG.  1 . 
     FIG. 2A is a view similar to FIG. 2 showing an alternative embedment of the connector. 
     FIG. 3 is a schematic view illustrating the construction process for the panel of the present invention. 
     FIG. 4 is a perspective view of one type of tool that can be used to form the connector holes in the dry-cast concrete layer. 
     FIG. 5 is a perspective view of another tool that can be used to form the holes in the dry-cast concrete layer and inject adhesive therein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The concrete sandwich panel of the present invention is generally designated in the drawings by the reference numeral  10 . The panel includes a first concrete layer  12 , a second concrete layer  14 , and an insulation layer  16  sandwiched between the concrete layers  12 ,  14 . The plurality of connectors  18  extend through the insulation layer  16  and into the concrete layers  12 ,  14  to tie the concrete layers together after the concrete has hardened. 
     Preferably, the first concrete layer  12  is a hollowcore layer extruded by a slip-forming machine. The hollowcore layer  12  has a plurality of voids  20  extending longitudinally, with interconnecting webs  22  of concrete. In the enlarged view of FIG. 2, the webs are identified as  22 A,  22 B, and  22 C. The concrete layer  12  is preferably formed with a low-slump material commonly used in “dry-cast” processes. For purposes of this application, low-slump material includes zero slump material. 
     Preferably, the first concrete layer  12  is constructed by a slip-form machine using the low-slump material, which is very dry. The voids  20  are formed during the slip-forming extrusion process. A plurality of pre-stressing steel strands  24  are also placed in the first layer  12  during the extrusion process. The strands  24  run longitudinally and are positioned in some of the webs  22 , as seen in the drawings. 
     The insulation layer  16  has pre-formed holes  26 . A tool is used to push through the holes  26  and into the dry-cast concrete of the first layer  12  so as to form holes  28  therein. Thus, the holes  28  in the first concrete layer  12  are aligned with the holes  26  in the insulation layer  16 . 
     A connector  18  is adapted to extend through each of the holes  26  and into the holes  28 , as best seen in the enlarged drawing of FIG.  2 . More particularly, the connector  18  has a lower end  32  residing within the hole  28 , a central ribbed portion  34  residing within the hole  26  of the insulation layer  16 , and an upper end  36 . The lower end  32  and upper end  36  of the connector  18  has a tapered profile, or is otherwise irregularly shaped, so as to provide an anchoring surface  38 . The lower end  32  of the connector  18  is anchored in the first concrete layer  12  using an adhesive  40  which fills the hole  28 . The adhesive  40  may comprise any cementitious or plastic materials that can be injected into the concrete layer  12  or the hole  28 , set and harden, bond with wet concrete, and are chemically compatible with concrete. Preferably, the adhesive  40  is a two-part epoxy or acrylic which hardens to lock the connector  18  in the first concrete layer  12 . The upper end  36  is embedded in the second concrete layer  14 , which is more plastic and therefore consolidates around the anchoring surface  38  of the upper end  36  of the connector  18 . The connectors each have an enlarged flange  41  which limits the penetration of the connector  18  by engagement with the upper surface of the insulation layer  16 . 
     As an alternative to the connector shown in FIG. 2, the flange  41  and/or ribs  34  may be eliminated to provide a smooth central portion in a connector  18 A, as shown in FIG.  2 A. The depth of the embedment of the connector  18 A is limited to the depth of the hole  28  in the concrete layer  12 . The diameter of the preformed hole  28  can be minimized to reduce the opportunity for misalignment of the connection  18 A. 
     FIGS. 4 and 5 show two tools for forming the holes  28  in the first concrete layer  12 . FIG. 4 shows a simple probe  42  having a lower end  44 , a handle  46 , and a flange  48  between the lower end  44  and the handle  46 . The lower end  44  of the probe  42  is adapted to extend through the hole  26  in the insulation layer  16  and displace a portion of the concrete in the first layer  12 . The flange  48  limits the penetration of the probe  42  by engaging the upper surface of the insulation layer  16 . After penetration of the probe  42  into the first concrete layer  12 , the probe  42  is removed, thereby leaving the hole  28  in the concrete layer  12 . 
     FIG. 5 shows an alternative tool, including a shielded hollow probe  50 , which is adapted to displace the concrete in the first layer  12 , similar to the probe  42 , and automatically apply the adhesive  40  in the hole  28 . The probe  50  is connected by conduits  52 ,  54  to an epoxy container  56  and a catalyst container  58 . Flow of epoxy and catalyst from the containers  56 ,  58  is controlled by a trigger  60 . The probe  50  also includes known adjustment means for adjusting the mixture of epoxy and catalyst before it is ejected from the probe  50 . 
     In constructing the panel  10  of the present invention, the first concrete layer  12  is extruded by the slip-form machine, with the pre-stressing strands  24  laid in the webs  22  during the extrusion process. The insulation layer  16  with the predrilled holes  26  is then placed on top of the uncured concrete layer  12 . One of the probes  42 ,  50 , or any other suitable tool, is then used to form the holes  28  in the first concrete layer  12 . Adhesive  40  is supplied into the holes  28 , either simultaneously with the formation thereof, or immediately before the connectors  18  are inserted into the holes  26 ,  28 . As seen in FIG. 3, preferably, each connector  18  is forced downwardly through the insulation layer  16  and into the first concrete layer  12 , and then turned or twisted approximately 90° (as depicted by the arrows in the right hand portion of FIG. 3) so as to facilitate consolidation of the adhesive around the anchoring surface  38  of the connector  18 . The upper or second concrete layer  14  is then poured onto the insulation layer  16 , so as to embed the upper ends  36  of the connectors  18  therein. Since the second concrete layer  14  is relatively plastic, or is vibrated to consolidate it around anchorage end  36 , the concrete will consolidate around the anchoring surface  38  on the upper ends  36  of the connectors  18 . Upon hardening of the concrete layers  12 ,  14 , the connectors  18  will tie the concrete layers together to,form a composite panel having very little shear displacement between the concrete layers  12 ,  14 . Also, the connectors  18  are preferably made of material having a high R-value, so as to have low thermal conductivity. 
     The invention has been shown and described above with the preferred embodiments, and it is understood that many modifications, substitutions, and additions may be made which are within the intended spirit and scope of the invention. From the foregoing, it can be seen that the present invention accomplishes at least all of its stated objectives.