Abstract:
A new process for creating a decorative surface on a cast concrete tile. A mold is prepared by coating with mold release. An aggregate of water, coloring dye, sand, Portland cement, and preferably filler material such as pea gravel is pre-mixed. Baking soda is mixed with a significant volume of water to create a high-viscosity paste. The paste preferably has a high solid to liquid ratio so that it can be crumbled into baking soda clumps of various sizes. The dampened baking soda clumps are sprinkled randomly onto the bottom surface of the mold (which will bear against what becomes the top surface of the cast tile). The pre-mixed aggregate is then added to the mold. Once the aggregate is cured, the cast concrete tile is removed. The baking soda clumps create complex voids in the tile&#39;s upper surface, producing a surface texture similar to limestone.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of patent application Ser. No. 11/265,839. The prior application listed the same inventors. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention relates to the field of building materials. More specifically, the invention comprises a method for producing a simulated limestone finish on the surface of cast concrete tiles. 
         [0006]    2. Description of the Related Art 
         [0007]    Concrete has been used to cast functional and decorative building materials for many years. It may be used, as an example, to pour a monolithic floor slab. A finished surface can be created on such a slab, so that no further flooring material is needed. 
         [0008]    Concrete is long-lasting and relatively inexpensive. One drawback, however, is its perceived lack of visual appeal. While some recent innovations in decorative surfaces have improved the appeal of concrete, it does not rival natural stone. Stone pavers or tiles display a natural variation in texture, luster, and color which many people find appealing. Stone pavers also feature cavities of varying depths with complex surface textures. For these reasons, most people prefer the look of natural stone. However, the price of stone—which can be ten times more expensive than concrete—often drives the consumer toward concrete. It would therefore be advantageous to provide a cast concrete product which mimics the desired surface look of natural stone. 
       BRIEF SUMMARY OF THE PRESENT INVENTION 
       [0009]    The present invention comprises a new process for creating a decorative surface on a cast concrete tile. A mold is prepared by coating with mold release. An aggregate of water, coloring dye, sand, Portland cement, and pea gravel is pre-mixed. Baking soda is mixed with a significant volume of water to create a high-viscosity paste. The paste preferably has a high solid to liquid ratio so that it can be crumbled into baking soda clumps of various sizes. The dampened baking soda clumps are sprinkled randomly onto the bottom surface of the mold (which will bear against what becomes the top surface of the cast tile). The pre-mixed aggregate is then added to the mold. The aggregate is then screed and compressed in the mold to minimize voids. A plastic cover is next added to reduce the moisture loss rate and increase the curing time. 
         [0010]    The wet aggregate comes into contact with the baking soda clumps. The baking soda reacts with the water in the aggregate to form sodium hydroxide and carbonic acid. A portion of the carbonic acid then tends to break down into water and carbon dioxide gas. The carbon dioxide gas produces voids and channels around the baking soda clumps as the aggregate cures. 
         [0011]    Once the aggregate is cured, the mold is separated into its component pieces and the cast concrete tile is removed. The residual baking soda is preferably removed. The upper surface of the cast tile will have been etched by the dampened baking soda, producing a variation in color and texture. The size of the baking soda clumps will also produce significant cavities in the surface. The production of the carbon dioxide gas provides a complex texture to the surface of these cavities. The ultimate effect is similar to natural stone. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view, showing a mold used to create a cast tile. 
           [0013]      FIG. 2  is a perspective view, showing the mold in an assembled state. 
           [0014]      FIG. 3  is a perspective view, showing the addition of the dampened baking soda. 
           [0015]      FIG. 4  is a perspective view, showing the filling of the mold. 
           [0016]      FIG. 5  is a perspective view, showing the concrete curing in the mold. 
           [0017]      FIG. 6  is a detail view, showing the surface finish of a cast tile. 
           [0018]      FIG. 7  is a perspective view, showing the process of adding water to the baking soda to form a high viscosity paste. 
           [0019]      FIG. 8  is an elevation view, showing the baking soda clumps on the bottom surface of the mold. 
           [0020]      FIG. 9  is a sectional elevation view, showing the reaction between the baking soda clump and the surrounding wet concrete. 
           [0021]      FIG. 10  is a sectional elevation view, showing the surface of the concrete after it has cured. 
           [0000]    
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   REFERENCE NUMERALS IN THE DRAWINGS 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   10 
                   mold 
                   12 
                   base 
                 
                 
                   14 
                   half frame 
                   16 
                   half frame 
                 
                 
                   18 
                   upper tab 
                   20 
                   lower tab 
                 
                 
                   22 
                   upper tab 
                   24 
                   lower tab 
                 
                 
                   26 
                   hole 
                   28 
                   pin 
                 
                 
                   30 
                   pin hole 
                   32 
                   mold cavity 
                 
                 
                   34 
                   baking soda 
                   36 
                   aggregate 
                 
                 
                   38 
                   shovel 
                   40 
                   filled mold 
                 
                 
                   42 
                   completed tile 
                   44 
                   void 
                 
                 
                   46 
                   color variation 
                   48 
                   back side 
                 
                 
                   50 
                   mixing container 
                   52 
                   water 
                 
                 
                   54 
                   mixer 
                   56 
                   water infused baking soda 
                 
                 
                   58 
                   soda clump 
                   60 
                   wet concrete 
                 
                 
                   62 
                   carbon dioxide bubble 
                   64 
                   large cavity 
                 
                 
                   66 
                   bubble cavity 
                   68 
                   bubble channel 
                 
                 
                   70 
                   dry concrete 
                   72 
                   textured surface 
                 
                 
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  shows the components of the mold used to create a cast tile. The mold is created by joining half frame  14  and half frame  16  to base  12 . Half frame  14  and half frame  16  are both “L” shaped pieces that form a square when joined at their ends. Half frame  14  has upper tab  18  that mates with lower tab  24  of half frame  16  when the two half frames are joined to form a square. Half frame  14  also has lower tab  20  that mates with upper tab  22  of half frame  16  when the square-framed mold is formed. Corresponding holes  26  of upper tab  18  and lower tab  24  and corresponding holes  26  of upper tab  22  and lower tab  20  align when the two frames are joined and the upper and lower tabs are mated. 
         [0023]    Base  12  has two pin holes  30  which are adapted to receive pins  28  when the frame is placed on the base. As described above, corresponding holes  26  of upper tab  18  and lower tab  24  and corresponding holes  26  of upper tab  22  and lower tab  20  are aligned when half frame  14  and half frame  16  are joined to form a square. Pins  28  are then inserted through holes  26  and pin holes  30  in base  12 . 
         [0024]    The assembled mold is illustrated in  FIG. 2 . Half frame  14  and half frame  16  form a square that is connected together and to base  12  by pins  28  as described above. Mold cavity  32  results from the attachment of the two half frames to the base. 
         [0025]      FIG. 3  shows how the mold is prepared to produce concrete tiles with a stone-like appearance. The surfaces of the mold are first prepared with mold release to prevent bondage of the concrete to the mold. Those skilled in the art are familiar with this part of the process and the various products that can be used to prevent bondage. Baking soda  34 , sodium bicarbonate, is next applied to the base  12 . The baking soda must be prepared in a controlled fashion. 
         [0026]      FIG. 7  shows one method of preparing the baking soda. The baking soda is placed in mixing container  50 . Water  52  is added while mixer rotates within the mixing container to mix the water into the baking soda. The mixing continues until the water is completely infused through the baking soda. Sufficient water should be added to produce a high-viscosity paste which can then be mechanically broken into clumps of a desired size. The baking soda to water ratio is significant. The table presented below describes the performance of various mixtures of baking soda and water, with the ratios being stated in terms of volume. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE ONE 
               
               
                   
               
               
                 Baking Soda 
                   
               
               
                 to Water Ratio 
                 Result 
               
               
                   
               
             
             
               
                 5.00 to 1 
                 Clumps are 0.5 mm to 2.0 mm 
               
               
                 4.75 to 1 
                 Clumps are 1.0 mm to 10.0 mm (avg. 5.0 mm) 
               
               
                 4.50 to 1 
                 Clumps are 2.0 mm to 13.0 mm (avg. 6.0 mm) 
               
               
                 4.25 to 1 
                 Clumps are 4.0 mm to 24.0 mm (avg. 12.0 mm) 
               
               
                 3.75 to 1 
                 Clumps are 25.0 mm+ 
               
               
                 2.50 to 1 
                 Forms a slurry that will not crumble 
               
               
                   
               
             
          
         
       
     
         [0027]    The particle size of the unwetted baking soda is quite small—well under 0.2 mm. The creation of the high viscosity paste allows the baking soda to be aggregated into much larger clumps. The clumps are created by mechanically shearing the water infused baking soda to break it into clumps. The shearing may be performed manually, or by using a mechanical shearing device. One skilled in the production process may create a suitable collection of clumps by kneading the paste using his or her hands. The sheared clumps will simply fall out of the hands where they may be collected. 
         [0028]    The size of the clumps used is significant. The reader may need an initial understanding of how the clumps are used to appreciate the significance of the clump size. A brief explanation of use will thus be provided at this time, with a more detailed explanation to follow. 
         [0029]    The water infused baking soda clumps are spread onto the lower surface of the concrete mold. Wet aggregate is then placed in the mold and left to cure. The baking soda clumps perform three basic functions: (1) They create large cavities in what will become the upper surface of the decorative tile; (2) The baking soda forms bubbles which displace some concrete and create a deeply textured surface in the large cavities; and (3) A small portion of the baking soda dissolves in water and flows away from the clumps over what will become the upper surface—thereby providing a conventional etching effect. 
         [0030]    The goal is to mimic natural stone. Thus, baking soda clumps below about 2.0 mm in size are not very useful because they will not create a cavity in the finished product that is large enough for an observer to see and appreciate. Adding more water to the baking soda paste tends to permit the creation of large clumps. However, adding more water also creates a clump which tends to slump and flatten when the concrete is added to the mold. Experimentation has shown that a usable range of baking soda to water ratio (state in terms of volume) is between about 5 to 1 and about 4 to 1. A more preferable range is between about 4.75 to 1 and about 4.25 to 1. The most preferred range is around 4.50 to 1. This ratio produces a good clump size and a nice variation in clump size. The clumps produced are fairly rigid and able to hold their shape when the wet concrete is added—thereby creating a large and fairly deep void in the finished surface. 
         [0031]    The exact baking soda to water ratio will depend somewhat upon the ambient temperature and humidity (as well as how long the baking soda has been exposed to ambient humidity). Thus, in humid conditions the ideal volumetric ratio could be 4.60 to 1, while in dry conditions it might drop to 4.40 to 1. Some adjustment may be needed to maintain the desired distribution of baking soda clump sizes—which is the ultimate objective. 
         [0032]    It is known in the art to spread fine baking soda powder over wet concrete to etch the surface. A typical particle size distribution of baking soda is 0.001 to 0.004 mm in diameter. These small particles simply dissolve in the water and create the etching effects via reacting with the water, calcium compounds, and silicon compounds in the concrete aggregate. They are too small to create voids or other visually discernible surface features. The baking soda clumps used in the present inventive process must be of a substantial size in order to create the desired voids and other visual effects. As explained previously, this means that most of the clumps need to be 2.0 mm or larger. 
         [0033]    Those skilled in the art will realize that the water infused baking soda can be mixed and crumbled using a wide variety of techniques. However this is done, the baking soda clumps thus produced are randomly spread across the surface of the base of the mold.  FIG. 8  shows a variety of soda clumps  58  resting on base  12 . Some of the clumps are fairly small (2 mm) while others are fairly large (10 mm) with a broad distribution of intermediate sizes. 
         [0034]    Concrete aggregate must then be made to produce the tiles. The aggregate can be any combination of concrete sand, gravel, cement, coloring agent and water. Those skilled in the art know that the precise formula can be varied to produce concrete with different appearances and properties. The aggregate is formed by mixing the aforementioned ingredients in a mixer. Additionally, the use of an ochre coloring agent is especially effective in creating stone-like coloration. A heterogenous coloration of the mixture can be enhanced by mixing the ingredients for three to five minutes, i.e., the coloring dye is unevenly distributed. 
         [0035]    The mold is then filled as shown in  FIG. 4 . The mold cavity is filled with aggregate  36  using shovel  38  or a variety of other known techniques. The mold cavity is filled completely with special care given to insure that the corners of the mold are filled and that the aggregate surface is even and level with the mold frame. A screed bar can be raked back and forth across the top of the mold frame to prevent the formation of ridges, lumps, or raised corners. 
         [0036]    The aggregate is then allowed to cure as shown in  FIG. 5 . Filled mold  40  is allowed to sit long enough for the aggregate to dry. Those skilled in the art know that the drying time required is dependent upon the aggregate&#39;s recipe and environmental conditions like temperature and humidity. The tiles can be covered with plastic wrap during the curing process to help the tiles hold in moisture. 
         [0037]      FIG. 9  shows a sectional elevation view through the soda clumps as the concrete aggregate is curing. Wet concrete  60  surrounds an envelopes soda clumps  58 . The baking soda (sodium bicarbonate) mildly reacts with the water in the surrounding concrete (as well as with the water within the clump) as expressed in the following reaction: 
         [0000]      NaHCO 3 +H 2 O→NaOH+H 2 CO 3    
         [0038]    Some of the carbonic acid then breaks down as expressed in the following reaction: 
         [0000]      NaHCO 3 +H 2 O→H 2 O+CO 2    
         [0039]    The reaction thus gives off a small quantity of carbon dioxide gas in the vicinity of the surface of the soda clumps. This is a mild reaction and not to be confused with the relatively violent reaction created when baking soda is mixed with an acidic substance such as citric acid or vinegar. The concrete aggregate is fairly alkaline and the gas formation rate is limited. A moderate quantity of carbon dioxide bubbles  62  form in the water surrounding the soda clumps. Some bubbles are small while others grow larger. Some bubbles aggregate and form channels in the wet concrete  60  (as seen in the right hand soda clump  58  shown in  FIG. 9 ). 
         [0040]    Those skilled in the art will know that the alkalinity of the concrete aggregate can be adjusted by adjusting the ratio of Portland cement to the other materials, as well as by adding modifiers such as weak acids. Adjusting the alkalinity will alter the carbon dioxide gas formation rate around the baking soda clumps. This will alter the amount of surface texture added by the gas bubbles to the voids created by the soda clumps. As explained in the present inventive method, the size of the baking soda clumps can be adjusted by varying the water content of the high-viscosity baking soda paste. Varying the clump size will vary the overall size of the voids in the finished surface produced by the clumps. Thus, one practicing the inventive process has the ability to vary the size of the voids and the surface texture of the voids. This permits many different types of stone to be accurately simulated. 
         [0041]    A small portion of the baking soda tends to dissolve in the surrounding water and spread as a film across base  12 . This portion acts like the prior art technique of spreading dry powdered baking soda across a concrete mold. It mildly etches the surface and produces pleasing color variations. 
         [0042]    Those skilled in the art will know that humidity and temperature control can be added to the curing process to produce a more evenly cured product. Once cured, the tile is released from the mold by removing the pins and tapping the frame with a hammer. 
         [0043]    The resulting tile that is produced by this process is shown in  FIG. 6 . Completed tile  42  serves as an illustration of some of the features and added benefits of this process. First, voids  44  are created where the aggregate cures around the space occupied by the baking soda and gases produced by the reaction of baking soda with the aggregate. This gives the surface of the tile a porous texture that is a similar to the surface of tiles made from natural stone. Additionally, color variation  46  is produced. The line illustrated in  FIG. 6  represents the boundary between subtly different hues. This boundary may actually appear blurry or mottled. Color variation is also influenced by the reaction of the baking soda and aggregate. This variation in color is often desirable as it mimics the coloration of natural stone. 
         [0044]      FIG. 10  shows the textured surface in much more detail. The reader should note that the resulting texture of  FIG. 10  corresponds to the soda clumps illustrated in  FIG. 9 . The soda clumps produce large and richly textured voids. Textured surface  72  includes large cavities  64  created by the baking soda clumps. Many smaller bubble cavities  66  lie along the boundary of each large cavity—producing a texture reminiscent of coral. Bubble channels  68  extending deep into the dried concrete are also formed at various locations. The reader should appreciate that the depiction in  FIG. 10  is two dimensional. The effect is in reality three dimensional with a great deal of pleasing complexity. The result is very similar (visually) to the appearance of natural stone. 
         [0045]    Different coloring agents can be used to mimic many variety of natural stone colors. 548 Ochre color, an effective coloring agent for producing a natural stone look, is commercially available from the New Riverside Ochre Company located in Cartersville, Ga. Other coloring agents can be used to imitate other naturally occurring stone colorations including dolphin grey, champagne, and rice white. Multiple coloring agents can even be used in the same batch to produce tiles with “swirls” of different colors. Furthermore, the degree of color variation can be controlled by adjusting the mixing time of the aggregate. A longer mixing time will result in a more homogenous coloration, and a shorter mixing time will result in greater color variation across the tile&#39;s surface. 
         [0046]    The process can be automated as well. One example of an automated process utilizes multi-cavity automated machinery to produce the tiles with limited human assistance. An automated mixer can be used to prepare the aggregate, and a controller can be used to coordinate mixing and pouring time intervals. An automated mixer and shearer can also be used to produce the water infused baking soda and to distribute the baking soda clumps into the molds. Using a conveyer belt or other means of locomotion, mold trays can be fed through various stations. First, the mold trays can be run through a station that sprays mold release. Second, the mold trays can be run through a station that randomly distributes baking soda across a two-dimensional field. The mold trays can then be sent to an injection site to be filled with aggregate. Finally, the trays can be circulated through an autoclave or other drying means to cure the concrete. A controller, like a programmable logic controller, can be used to coordinate the entire process. 
         [0047]    Although the preceding descriptions contain significant detail they should not be viewed as limiting the invention but rather as providing examples of the preferred embodiments of the invention. As one example, many types and shapes of molds can be used to produce the concrete tiles. Accordingly, the scope of the invention should be determined by the following claims, rather than the examples given.