Embodiments of the invention relate to an on-grade joint-stability system for on-grade concrete slabs. Embodiments of the system may include: a first on-grade concrete-slab portion; a second on-grade concrete-slab portion that is separated from the first on-grade concrete-slab portion by a joint; a first on-grade plate having a first portion and a second portion, the first portion of the first on-grade plate being positioned underneath, and connected to, the first concrete-slab portion, and the second portion of the first on-grade plate being positioned underneath the second concrete-slab portion; and a second on-grade plate having a first portion and a second portion, the first portion of the second on-grade plate being positioned underneath the first concrete-slab portion, and the second portion of the second on-grade plate being positioned underneath, and connected to, the second concrete-slab portion, such that height differentials across the joint are substantially prevented.

CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/707,353, which was filed Aug. 11, 2005, and which is incorporated herein by reference. This application also claims the benefit of U.S. patent application Ser. No. 10/489,380, now U.S. Pat. No. 7,481,031, which was filed Sep. 13, 2002, which is incorporated herein by reference, and U.S. Provisional Application No. 60/318,838, which was filed on Sep. 13, 2001, which is incorporated herein by reference. 


   BACKGROUND 
   U.S. Pat. No. 6,354,760, which is entitled System for Transferring Loads Between Cast-in-Place Slabs and issued Mar. 12, 2002, to Russell Boxall and Nigel Parkes, discloses a load plate for transferring loads between a first cast-in-place slab and a second cast-in-place slab separated by a joint. The load plate has at least one substantially tapered end adapted to protrude into and engage the first slab. The load plate is adapted to transfer between the first and second slabs a load directed substantially perpendicular to the intended upper surface of the first slab. 
   PCT application WO 03/023146 A1, which was published Mar. 20, 2003, is entitled Load Transfer Plate for in Situ Concrete Slabs, and for which Russell Boxall and Nigel Parkes are applicants and inventors, discloses a tapered load plate that transfers loads across a joint between adjacent concrete floor slabs. The tapered load plate accommodates differential shrinkage of cast-in-place concrete slabs. When adjacent slabs move away from each other, the narrow end of the tapered load plates moves out of the void that it created in the slab thus allowing the slabs to move relative to one another in a direction parallel to the joint. Tapered load plates may be assembled into a load-plate basket with the direction of the taper alternating from one tapered load plate to the next to account for off-center saw cuts. 
   BRIEF SUMMARY 
   This Brief Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Brief Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
   Embodiments of the invention relate to an on-grade joint-stability system for on-grade concrete slabs. Such a system may include: a first on-grade concrete-slab portion; a second on-grade concrete-slab portion that is separated from the first on-grade concrete-slab portion by a joint; a first on-grade plate having a first portion and a second portion, the first portion of the first on-grade plate being positioned underneath, and connected to, the first concrete-slab portion, and the second portion of the first on-grade plate being positioned underneath the second concrete-slab portion; and a second on-grade plate having a first portion and a second portion, the first portion of the second on-grade plate being positioned underneath the first concrete-slab portion, and the second portion of the second on-grade plate being positioned underneath, and connected to, the second concrete-slab portion, such that height differentials across the joint are substantially prevented. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  is a side view of an on-grade joint-stability system for on-grade concrete slabs in accordance with embodiments of the invention. 
       FIG. 2  is a top view of the system of  FIG. 1 . 
       FIGS. 3-7  are flow charts showing steps for stabilizing a joint between on-grade concrete-slab portions in accordance with embodiments of the invention. 
   


   DETAILED DESCRIPTION 
   In the following description of the various embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which are shown, by way of illustration, various embodiments of the invention. Other embodiments may be utilized and structural and functional modifications may be made without departing from the scope and spirit of the present invention. 
   Load plates of the type disclosed in the issued U.S. patent and the published international patent application discussed above are well suited to transferring loads between load-bearing concrete slabs that are at least approximately 6 inches deep. The phrase “load-bearing slabs” refers to floors designed to accommodate fork lifts and other relatively heavy loads. 
   For situations in which load transfer is not required, such as, sidewalks, malls, and in stores in which forklifts do not ride along the floor, shallow floor slabs, for instance, floor slabs that are less than approximately five inches deep, are typically used. 
   Although load transfer may not be needed, joints between shallow floor slabs should be stabilized to prevent adjacent slabs from developing height differentials relative to one another. Height differentials of this type are tripping hazards, which may undesirably cause people to trip, fall, get injured, and initiate related personal-injury litigation. 
   Slabs can curl due to differential shrinkage throughout the slabs depth. Different lengths curl more or less. In saw-cut joints, this curling of slabs occurs. Joint stability (i.e., preventing differential vertical movement between adjacent slabs) is desirable so that the slabs curl together. 
   If concrete floor slabs are shallow, for instance less than approximately five inches deep, concrete may not consolidate (i.e., fill in void spaces) as desired if conventional plate arrangements, such as those disclosed in the issued U.S. patent and the published international patent application discussed above, are used. Aggregate used in concrete is measured according to the smallest dimension of the particle. For example, a three-quarter inch aggregate may, in fact, be three-quarter inch in width, but substantially larger in length, e.g., 1.25 inches. Particles of such size below a conventional load plate located at the mid-depth of the slab may cause voids to occur below the plates when the slab thickness is less than approximately five inches. Conventional plate arrangements may be used, however, when the slab thickness is at least six inches, such as floors that are designed to handle use of forklifts. 
   Moreover, slabs having a specified height of four inches may actually be only 3.25″ deep in particular places due to tolerances in the level of the subgrade. Based on the considerations discussed above, using plates located halfway up the height of the slabs is associated with various shortcomings. 
   Embodiments of the invention are directed to on-grade plates for use with on-grade concrete slabs less than approximately five inches deep for the purpose of insuring joint stability rather than for traditional load-transfer functionality. “On-grade concrete slabs,” as used herein, refers to concrete slabs placed on a subgrade and/or a subbase. The subgrade is the natural in-place soil. The subbase is generally a compactible fill material that brings the surface to a desired grade. 
   In accordance with embodiments of the invention, trapezoidal plates may be situated on the subgrade or subbase. Plates having other shapes, including, but not limited to, a circle or a rectangle, may also be used. Plates may be triangular shaped. A pointed end may, however, present a safety hazard and may produce undesirable stress concentrations. Therefore, the pointed end may be omitted such that the plate takes on a generally trapezoidal shape. 
   The plates permit substantially full consolidation of the concrete slab for slab thicknesses down to approximately four inches deep. If such a plate is at grade with a 4″ slab, it produces a situation above the plates that is similar to an 8″ slab with plates embedded at a height of 4″. In this way, plates in accordance with embodiments of the invention avoid under-consolidation of concrete beneath the plate and spalling of concrete above the plate as may happen if the concrete cover above the plate is too thin. 
   The wide end of the trapezoidal plate may have either a stirrup or stud protruding into a concrete-slab portion to create a positive connection between the plate and the concrete-slab portion. The plates may be situated in an alternating fashion such that each successive plate is rotated 180 degrees relative to its neighboring plates. For instance, referring to  FIG. 2 , plate  106 - 1  has its wide end oriented to the left, plate  106 - 2  has its wide end oriented to the right, and plate  106 - 3  has its wide end oriented to the left. As is discussed in more detail below, alternating the orientation of the plates in such a way operates to prevent height differentials across joints between slab portions thereby preventing a trip hazard despite movement of the slabs due to slabs settling, shrinking, crowning, and the like. 
   On-grade plates oriented alternately work together to prevent height differentials between adjacent concrete slabs as follows. Referring to  FIGS. 1 and 2 , slab portions  100 - 1  and  100 - 2  are cast in place and divided via saw cut  102  and crack  104 . Plates  106 - 1  and  106 - 3  are positioned such that they will be positively connected, via their respective stirrups  108 - 1  and  108 - 3 , to slab portion  100 - 1 . Similarly, plate  106 - 2  is positioned such that it will be positively connected, via its stirrup  108 - 2 , to slab portion  100 - 2 . Although not shown in  FIG. 2 , additional on-ground plates  106  may be oriented in alternating directions (as is the case with plates  106 - 1 ,  106 - 2 , and  106 - 3 ) at a joint between slab portions. 
   If slab-portion  100 - 1  moves upward, then the plates  106 - 1 ,  106 - 3 , and any additional plates oriented the same way, underneath slab portion  100 - 1  will be lifted via the positive connection established by stirrups  108 - 1  and  108 - 3  between plates  106 - 1  and  106 - 3  and slab-portion  100 - 1 . Lifting of the plates in this way will result in the respective portions of the plates  106 - 1 ,  106 - 3 , and any additional plates oriented the same way, that are positioned underneath slab portion  100 - 2  to lift slab portion  100 - 2  thereby preventing a height differential across the saw cut  102 . 
   If slab-portion  100 - 1  moves downward, then the portion of plate  106 - 2 , and any additional plates oriented the same way, underneath slab portion  100 - 1  will be pushed down. This will cause slab portion  100 - 2  to be pulled down through the stirrup on plate  106 - 2  (and through the stirrups on other plates oriented in generally the same direction) thereby preventing a height differential across the saw cut  102 . 
   The principles discussed above with respect to preventing height differentials across saw cut  102  apply to upward and downward movement of slab-portion  100 - 2 . Namely, if slab-portion  100 - 2  moves upward, then the portion of plate  106 - 2 , and any additional plates oriented in generally the same direction, underneath slab portion  100 - 1  will lift slab portion  100 - 1  thereby preventing a height differential across the saw cut  102 . 
   If slab-portion  100 - 2  moves downward, then the portion of plates  106 - 1 ,  106 - 3 , and any additional plates oriented the same way, underneath slab portion  100 - 2  will be pushed down. This will cause slab portion  100 - 1  to be pulled down through the respective stirrups  108 - 1  and  108 - 3  on plates  106 - 1  and  106 - 3  (and through the stirrups of other plates oriented across saw cut  102  in generally the same direction as plates  106 - 1  and  106 - 3 ) thereby preventing a height differential across the saw cut  102 . 
   Instead of (or in addition to) a stirrup  108 , other means for positively connecting a plate  106  to a slab portion  100  may be used. For example, a headed stud that protrudes from the plate at a location relatively close to the saw cut may be used. 
   In accordance with embodiments of the invention, a blockout sheath with foam or fins inside of the blockout sheath may be used to create voids to the sides of the plates. Techniques of this type are well known in the art, are discussed in the issued U.S. patent mentioned above, and, therefore, do not need to be discussed herein in detail. 
   The plates may be made of steel or any other suitable material. To prevent corrosion, an epoxy coating may be applied to the plates and/or a vapor barrier may be used under the slabs. 
     FIGS. 3-7  are flow charts showing steps for stabilizing a joint between concrete on-grade slabs in accordance with embodiments of the invention. Referring to  FIG. 3 , a positive connection between a first portion of a first on-grade plate and a first portion of an on-grade concrete slab is established, wherein a second portion of the first on-grade plate is positioned underneath a second portion of the on-grade concrete slab that is separated by a joint from the first portion of the on-grade concrete slab, as shown at  300 . A positive connection between a second portion of a second on-grade plate and the second portion of the on-grade concrete slab is established, wherein the first portion of the second on-grade plate is positioned underneath the first portion of the on-grade concrete slab such that the first and second on-grade plates substantially prevent height differentials across the joint from occurring, as shown at  302 . 
   Referring to  FIG. 4 , if the first portion of the on-grade concrete slab is trying to move downward relative to the second portion of the on-grade concrete slab, the yes arrow will be followed, as shown at  402 . Then, the first portion of the on-grade concrete slab pushes the first end of the second on-grade plate downward thereby causing the second on-grade plate to pull the second portion of the on-grade concrete slab downward via the positive connection between the second portion of the second on-grade plate and the second portion of the on-grade concrete slab, as shown at  404 . 
   Referring to  FIG. 5 , if the first portion of the on-grade concrete slab is trying to move upward relative to the second portion of the on-grade concrete slab, the yes arrow will be followed, as shown at  502 . Then, the first portion of the on-grade concrete slab pulls the first end of the first on-grade plate upward, via the positive connection between the first portion of the first on-grade plate and the first portion the on-grade concrete slab thereby causing the second end of the first on-grade plate to push the second portion of the on-grade concrete slab upward, as shown at  504 . 
   Referring to  FIG. 6 , if the second portion of the on-grade concrete slab is trying to move downward relative to the first portion of the on-grade concrete slab, the yes arrow will be followed, as shown at  602 . Then, the second portion of the on-grade concrete slab pushes the second end of the first on-grade plate downward thereby causing the first on-grade plate to pull the first portion of the on-grade concrete slab downward via the positive connection between the first portion of the first on-grade plate and the first portion of the on-grade concrete slab, as shown at  604 . 
   Referring to  FIG. 7 , if the second portion of the on-grade concrete slab is trying to move upward relative to the first portion of the on-grade concrete slab, the yes arrow will be followed, as shown at  702 . Then, the second portion of the on-grade concrete slab pulls the second end of the second on-grade plate upward, via the positive connection between the second portion of the second on-grade plate and the second portion the on-grade concrete slab thereby causing the first end of the second on-grade plate to push the first portion of the on-grade concrete slab upward, as shown at  704 . 
   Although the subject matter has been described in language specific to structural features and/or methodological acts, the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.