Abstract:
A concrete slab construction is provided for use in both horizontal and vertical structures. The slab construction includes two layers. The first layer of concrete is a base layer comprised of traditional concrete materials. In addition, the concrete slab construction includes a second exterior layer of concrete which contains photocatalyst particles. Preferred photocatalytic materials include titanium dioxide in the anatase form. The concrete slab construction provides for reduction of green house gases and reduction in organic stains.

Description:
[0001]    The following patent application claims priority to U.S. Provisional Patent Application No. 61/685,323 filed Mar. 16, 2012 and U.S. Provisional Patent Application No. 61/739,519 filed on Dec. 19, 2012. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to concrete constructions, and more particularly to methods for producing concrete constructions. 
         [0003]    Concrete is a well known building material for commercial and residential applications. Concrete is durable, has good weight resistance, and provides excellent cost economy. Recently, concrete has become used more and more for flooring applications, but also for other structural applications which are highly visible to the public. Because of its widespread use, it is desirable to provide different colors and surface textures for the concrete. Well known techniques for improving aesthetics include adding color to the concrete. Other aesthetic techniques include modifying the surface texture of the concrete by imploring various finishes. 
         [0004]    Still an additional attempt to improve the aesthetics of concrete is to provide an aggregate finish. A first method of preparing an aggregate finish produces “integrally mixed aggregate concrete”. The integrally mixed aggregate concrete is prepared by integrating aggregates throughout the concrete slab. Thereafter, surface cement and fines are washed or removed from the concrete while the concrete surface is still in a plastic state so that the aggregate (such as stone, gravel, shell or glass) is left exposed on the surface of the concrete. Alternatively, a topical surface retardant is applied so as to allow the top layer of the concrete to be removed. The normal size of the aggregate is typically less than 1.5 inch in mean diameter. 
         [0005]    More recently, a surface seeded exposed aggregate method has been introduced. In this particular method, subsequent to pouring of the concrete, an aggregate is broadcast (also referred to as “seeded”) over the top surface of the concrete. Thereafter, the aggregate is troweled into the concrete so as to form a planar concrete upper surface. With the curing of the concrete, the previously broadcast aggregate is affixed in place but otherwise exposed. Preferably, the aggregate is nominally about 0.5 inch in diameter or less mean size such that the aggregate can be worked into the top surface and adequately affixed in place. 
         [0006]    Several patents have been filed to surface seeded exposed aggregate concrete. U.S. Pat. No. 4,748,788 describes a method of creating a surface seeded aggregate concrete wherein aggregate is broadcast into a concrete surface. A surface retardant vapor barrier is applied and a surface retardant is sprayed upon and washed from the surface. U.S. Pat. Nos. 7,322,772 and 7,607,859 describe methods for producing a surface seeded aggregate concrete which is intended to simulate quarried stone. Allegedly, the methods described in these patents provide improved aesthetics. Still additional U.S. Pat. No. 7,614,820 describes a method for producing a non-slick surface seeded aggregate concrete. Thus, numerous attempts have been made to create surface seeded aggregate concrete with a variety of characteristics including improved aesthetics. 
         [0007]    White cement is commonly used with white aggregates to produce a white concrete for greater decorative appeal. To achieve the white color, white cement typically includes chromium, manganese, iron, copper, vanadium, nickel and/or titanium. Titanium dioxide (“TiO 2 ”) has been introduced into cement for use in “architectural concretes” such as paving tiles. For example, U.S. Pat. No. 6,409,821, which is incorporated in its entirety herein, describes cement compositions containing photocatalysts such as titanium dioxide. The resulting concrete is alleged to have increased brilliance and color quantity as well as the ability to react with polluting materials. 
         [0008]    Unfortunately, the cement with photocatalyst properties is not suitable for conventional structural concrete projects such as walkways, walls, water features etc. Moreover, the cement compositions described in U.S. Pat. No. 6,409,821 produce only a substantially white appearance, whereas colored or non-homogenous appearances would be desirable. 
         [0009]    Thus, there is a significant need for an improved concrete composition which provides improved aesthetic capabilities while also providing photocatalytic properties for the removal of pollutants. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention addresses the aforementioned disadvantages by providing an improved layered concrete slab construction, and by providing an improved method for preparing the layered concrete slab construction. The term “slab” is to be interpreted broadly to include traditional horizontal concrete structures including driveways, streets, walkways and the like. However, the term “slab” is also intended to be interpreted to include partially vertical or vertical concrete surfaces such as walls, signs, water features, etc. 
         [0011]    In the most simple terms, the concrete slab construction includes a base layer of traditional concrete, preferably utilizing a traditional cement. Thereafter, a second layer of concrete is applied to the first layer of concrete. This second layer of concrete contains additional photocatalytic particles. In the presence of light, oxygen and water, the photocatalytic particles are capable of oxidizing polluting substances which come in contact with the exposed concrete surface. In addition to breaking down harmful greenhouse gases, the photocatalytic particles will also provide dramatic organic stain reduction. Specifically, the photocatalytic particles in the concrete cause organic stains to break down much more quickly thereby providing a self cleaning concrete. Suitable photocatalysts include titanium dioxide (TiO 2 ) or one of its precursors, tungsten oxide (WO 3 ), calcium titanate or strontium titanate (SrTiO 3 ). The preferred photocatalyst according to the present invention is titanium dioxide, and more preferably titanium dioxide in the form of anatase. Preferably, the titanium dioxide has an anatase structure of at least 5%, preferably at least 25%, and even more preferably at least 50%, and still even more preferably at least 70%. 
         [0012]    The method of producing the concrete slab construction of the present invention includes the initial step of preparing a subgrade or formwork, also commonly referred to as a framework, for receipt of poured concrete. Fill sand may be added to prepare the subgrade. Alternatively, wood, steel or other materials may be assembled to create an envelope for receipt of a concrete mixture. 
         [0013]    In a first preferred embodiment, a first mixture of traditional concrete is poured upon the subgrade or within the formwork. Though changing geographically due to supply and price, traditional concrete materials are known to those skilled in the art. However, preferably the concrete mixture includes Portland cement. After the first concrete layer has been poured, a second layer of concrete is poured upon the first layer of concrete to create an exposed surface. The exposed surface may be the top surface of a roadway or walkway, or the exterior of vertical constructions such as a wall or sign. Preferably, the second layer of concrete is poured upon the first layer of concrete while the first layer has not fully cured, which typically takes place within twenty-eight (28) days. Even more preferably, the second layer is poured while the first layer of concrete is still within a plastic (wet) state which preferably occurs within 24 hours of the first layer being poured. 
         [0014]    Importantly, the second mixture of concrete contains the additional photocatalyst particles such that the second layer of concrete contains more photocatalyst particles than the first layer of concrete. Once the second layer of concrete has cured, a slab of concrete is created wherein the first layer of concrete forms an interior layer of concrete adjacent to the subgrade, but the second layer of concrete forms an exterior layer having an exposed surface having increased amount of a photocatalyst particles. Again, preferably, the photocatalyst is titanium dioxide having at least 5% of the anatase structure, and even more preferably 25% of the anatase structure, and even more preferably 50% of the anatase structure. The most preferred photocatalyst is titanium dioxide having at least 70% of the anatase structure. 
         [0015]    A second method for creating the concrete slab construction of the invention includes pouring only a single mixture of concrete. This concrete mixture is a traditional mixture of concrete, preferably including Portland cement and conventional aggregates. Once the concrete has been poured, but while it is still in a plastic state, the photocatalytic particles, such as titanium dioxide in its anatase form, are broadcast on the concrete&#39;s exposed surface. Preferably, 1.5-16 ounces of titanium dioxide having at least 5% of the anatase structure are applied per square foot of concrete. The photocatalyst may be broadcast in a powdered form. Alternatively, the photocatalyst may be blended into a liquid solution, such as water or alcohol, and sprayed upon the exposed concrete surface. 
         [0016]    Once the photocatalyst has been broadcast to a desired coverage, the concrete&#39;s surface is floated, screed and/or troweled to fully work the photocatalyst into the concrete slab to ensure consistent and full penetration of the photocatalyst into the concrete. For this embodiment, the photocatalyst is only worked a very small distance into the concrete&#39;s slab structure, typically one-thirty-second ( 1/32)-one-half (½) inch of penetration. However, penetration of the photocatalyst into the concrete creates an exterior layer of concrete having a greater amount of a photocatalyst than the interior of the concrete. 
         [0017]    The concrete slab is allowed to cure providing a multi-layered concrete construction having underlying strength and wearability, but an exposed layer having increased aesthetic and photocatalytic properties providing reduction in greenhouse gases and enhanced organic stain removal. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is schematic flow chart of a first embodiment of the method for producing cement compositions containing photocatalyst particles of the present invention; 
           [0019]      FIG. 2  is a schematic flow chart of a second embodiment of the method for producing cement compositions containing photocatalyst particles of the present invention; 
           [0020]      FIG. 3  is a schematic flow chart of a third embodiment of the method for producing cement compositions containing photocatalyst particles of the present invention; 
           [0021]      FIG. 4  is a cross-sectional view of the various layers of the first embodiment of a cement composition containing photocatalyst particles; 
           [0022]      FIG. 5  is a cross-sectional view of the third embodiment of a cement composition containing photocatalyst particles; and 
           [0023]      FIG. 6  is a schematic flow chart of the fourth embodiment of the method for producing cement compositions containing photocatalyst particles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention addresses the aforementioned disadvantages by providing an improved layered concrete slab, and by providing an improved method for preparing a layered concrete slab. In the most simple terms, the concrete construction includes a base layer of traditional concrete utilizing a traditional cement, and a second upper layer of concrete containing photocatalyst particles upon the first layer of concrete. Preferably, the base layer of traditional concrete is much thicker than the upper layer of concrete containing photocatalyst particles. 
         [0025]    With reference to  FIGS. 1 and 4 , a first method of producing the two layer concrete construction of the present invention comprises the initial step  10  of preparing subgrade or formwork for concrete placement. It is envisioned that the present invention will most often require the preparation of the subgrade for creating substantially horizontal surfaces such as for walkways, driveways and the like. However, the present invention has application for producing partially vertical or substantially vertical concrete surfaces such as walls, signs, seatwalls, water features, etc. Where a vertical concrete construction is required, a traditional formwork utilizing wood, steel or other materials is assembled to create an envelope for receipt of a concrete mixture. The term formwork, also commonly referred to as a framework, is intended to be interpreted broadly to also include steel pans in the like. A typical subgrade  2  (shown in  FIG. 4 ) is prepared to the desired elevation and grade. Preferably, the subgrade is compacted to a desired compaction, such as 90%. Thereafter, the subgrade is preferably, though not necessarily covered with a layer of base course, illustrated as step  20  in  FIG. 1 . 
         [0026]    As reflected in step  40  shown in  FIG. 1 , the present invention includes the preparation and pour of a first layer  4  of concrete consisting of a standard concrete mixture. Conventional concrete mixtures can be selected, prepared and poured by those skilled in the art. A preferred concrete mixture includes Portland cement. Furthermore, preferably the concrete consists of at least 5.5 standard sacks of cement content per cubic yard. Preferably, the first layer of concrete is three (3) inches-twenty-four (24) inches thick. 
         [0027]    It is also preferred, though not necessary, that reinforcement members  5  be introduced into concrete layer. Reinforcement members  5  may include wire mesh, rebar, integral fiber mesh or the like so as to increase the resulting strength of the concrete slab. Color additives and decorative aggregates are considered unnecessary for the first layer  4  of concrete, and thus preferably, no color additives or decorative aggregates are introduced into the first concrete mixture. Also preferably, the first concrete mixture does not contain any calcium chlorides. As illustrated by the dashed lines in  FIG. 1 , the addition of the fill sand  20  and the introduction of the reinforcing members into the concrete layer are considered optional steps in the process of creating the two layered concrete construction of the present invention. 
         [0028]    After the first concrete layer has been poured, preferably, the concrete upper surface is floated, screed and/or troweled (step  50  in  FIG. 1 ) to create a substantially homogenous concrete surface having a substantially uniform finish. Preferably, the surface of the concrete is floated utilizing steel, aluminum, wood, fiberglass, or magnesium concrete bull float tools. Where the concrete slab has a larger square footage, the use of hand floats may be abandoned and instead more efficient screeds or motorized troweling finishing machines may be utilized. 
         [0029]    As illustrated in step  70  of  FIG. 1 , a second layer  8  of concrete is poured upon the first layer of concrete. Though not necessary, it is preferred that the second layer  8  of concrete be poured while the first layer  4  of concrete is not fully cured. Even more preferably the second layer  8  is poured while the first layer is within a plastic (wet) state which preferably occurs within 24 hours of it being poured. If a second layer of concrete is not poured within 24 hours, the first layer of concrete should be texturized (step  60 ) utilizing texturized trowels or rakes so as to create a roughened surface to provide better adhesion of the second layer  8  of concrete. Preferably, the second layer of poured concrete has a thickness thinner than the underlying base layer, and has a thickness one-sixteenth ( 1/16) inch to four (4) inches thick, though one-sixteenth ( 1/16) inch thickness would be unusual and four (4) inches thickness would be considered excessive and not preferred. 
         [0030]    This second layer of concrete may comprise a traditional concrete mixture. However, such concrete mixture also includes an additional photocatalyst. Any type of photocatalyst may be introduced into the concrete as long as it is capable of oxidizing in the presence of light, oxygen and water polluting substances which come in contact with the concrete. Suitable photocatalysts include titanium dioxide (TiO 2 ) or one of its precursors, tungsten oxide (WO 3 ), calcium titanate or strontium titanate (SrTiO 3 ). The preferred photocatalyst according to the present invention is titanium dioxide, and more preferably titanium dioxide in the form of anatase. Preferably, the titanium dioxide has an anatase structure of at least 5%, preferably at least 25%, and even more preferably at least 50%, and still even more preferably at least 70%. A preferred titanium dioxide is sold under the trademark P-25® by Degussa which is a mixture of 70:30 of anatase titanium dioxide: rutile titanium dioxide. An alternative acceptable titanium dioxide is Rockwood Pigments® Titanium Dioxide which is 100% titanium dioxide in a powdered form. 
         [0031]    Preferably traditional cement, such as Portland cement, are utilized in combination with the photocatalyst to create the second layer of concrete. Alternatively, white cements employing a photocatalyst may be utilized. Only a small quantity of the photocatalyst must be added to the concrete mixture. Preferably, the concrete mixture possesses at least 0.01% to 10% by weight of the photocatalyst. Even more preferably, the concrete mixture possesses 0.1%-1% by weight of the photocatalyst. 
         [0032]    Color pigments may be introduced into the second layer of concrete to provide color embellishment. Furthermore, as described in greater detail below and as illustrated in  FIG. 2 , decorative aggregates may be mixed into the second concrete layer  8  prior to it being poured to provide an integral aggregate concrete having photocatalytic properties. 
         [0033]    Once the second layer of concrete has been poured, its upper surface is floated, screeded and/or troweled to provide a substantially homogenous concrete surface having a uniform finish, as reflected in step  60  of  FIG. 1 . Once troweled, it is preferred that the concrete surface  7  (see  FIG. 4 ) be checked for ADA compliance. Sealants may be applied in step  130 , or a concrete surface may be simply allowed to dry to completion. Preferably, traditional surface sealants are not employed and instead penetrating sealants are applied to the photocatalytic concrete surface. Penetrating sealants are desirable instead of surface sealants so as to not unwantedly impede the photocatalytic properties of the concrete. 
         [0034]    Alternatively, if an aggregate has not been integrally mixed with the second layer of concrete, an aggregate  6  may be broadcast upon the second layer of concrete&#39;s upper surface (step  90  of  FIG. 1 ). Various aggregates may be utilized such as coarse sand, glass chips, organic materials such as seashells, metals or composite materials. It is preferred that the aggregate have a mean diameter of 0.5 inch or less. After being seeded with an aggregate, the upper surface is again re-troweled (step  100 ) to fully seed and work the aggregate  6  into the concrete slab to form a substantially planar concrete surface. As an alternative to traditional troweling, float tools may be utilized to float the aggregate into the concrete&#39;s upper surface. 
         [0035]    As illustrated in step  120  of  FIG. 1 , once the aggregate has been introduced into the concrete surface and the concrete surface has been troweled, screed or floated, the aggregate  6  is exposed. Exposure of the aggregate may be accomplished utilizing any of numerous methods known to those skilled in the art such as utilization of either surface retardants or chemical exposure methods. When utilizing a concrete surface retardant, a preferred surface retardant such as Grace Construction&#39;s Top-Cast® is applied to the second layer of the concrete&#39;s upper surface. Once the retardant is dried, which typically requires one (1) to two (2) hours, the retardant is removed utilizing water such as a pressure washer. As the surface retardant is removed, the retardant removes the upper layer of the concrete slab to expose the seeded concrete upper surface. As an alternative to utilizing a concrete surface retardant to expose the aggregate  6 , mechanical exposure may be accomplished utilizing brushes, sponges, rotary flooring machines with abrasive pads, handheld power tools with abrasive pads, or media blasting such as sand blasting. Mechanical exposure may include application of an acid solution to remove the upper portion of the concrete to expose the seeded aggregate. Advantageously, mechanical exposure methods can be utilized both the same day of concrete placement or in subsequent days after the concrete has hardened. 
         [0036]    After exposure of the aggregates within the second layer of the concrete&#39;s upper surface, the surface contaminants are removed by washing the concrete surface  7  utilizing a pressure washer, hose, scrub brushes, or cleaning machines. Preferably, the second layer of the concrete&#39;s upper surface is allowed to dry overnight and optionally a sealant is applied, as reflected in step  130  of  FIG. 1 . Various concrete sealants can be selected and utilized as can be determined by those skilled in the art so as to protect the photocatalytic concrete surface from the elements. Preferred sealants are penetrating sealants. Water based sealants can also be utilized in environmentally sensitive areas. It is preferred that the sealant be applied utilizing sprayers in accordance with the manufacturer&#39;s recommendations. Typically, a sealant is applied utilizing an airless high volume low pressure (HVLP) sprayer or roller or the like. 
         [0037]    Various modifications may be made within the scope of this invention. For example,  FIG. 1  illustrates embodiments where no aggregate is introduced into the concrete&#39;s surface or an aggregate is surface seeded upon the concrete&#39;s upper surface. However, in still an additional embodiment the aggregate is mixed integrally with the concrete and photocatalyst, which as illustrated in  FIG. 2  is preferably titanium dioxide. Again, the second layer of concrete including the titanium dioxide may also include color pigments. For this embodiment, the same initial steps are performed including preparing a subgrade or formwork in step  210 . Base course may be added in step  220 . Thereafter, reinforcement members  5  such as wire mesh, rebar, fiber mesh or the like may be introduced into or upon the subgrade and fill sand, as illustrated in step  230 . Again, a standard concrete mixture is poured upon the subgrade in step  240 . Once the first layer of concrete has been poured, the first concrete&#39;s upper surface is floated, screed or troweled to a desired level plane or grade. 
         [0038]    A second concrete mixture is prepared. The concrete mixture includes a photocatalyst, preferably titanium dioxide in the anatase form. In addition, the second layer of concrete, preferably one-sixteenth ( 1/16) inch to four (4) inches thick, includes a decorative aggregate integrally mixed with the concrete and titanium dioxide. Though this method requires substantially more aggregate than a surface seeded construction, the additional cost of mixing the aggregate throughout the entire concrete upper layer may be minimal when the upper concrete layer is not particularly thick. Still with reference to  FIG. 2 , once the integral aggregate mixture with a photocatalyst has been mixed, a mixture is poured in step  270  so as to create a concrete slab of desired thickness. The second layer of concrete is floated, screed or troweled to create a uniform and homogenous surface in step  280 . 
         [0039]    Once in a uniform state, in step  290  the aggregate is exposed by utilizing concrete surface retardants or abrasive mechanical apparatus. The upper surface of the concrete&#39;s second layer is then washed in step  300  to remove surface contaminants and the upper surface may be sealed by applying a penetrating sealant in step  310 . 
         [0040]    In still an additional vertical embodiment illustrated in  FIGS. 3 and 5 , the second layer of concrete is applied directly to a structural fill layer to provide a wall or other vertical surface such as a sign. For this embodiment, the concrete slab is prepared in similar manner to the methods described in  FIGS. 1 and 2 . A subgrade and/or formwork is prepared for the introduction of concrete (step  310 ). In addition, a structural fill layer is constructed for providing a base structure for application of the photocatalytic cement (step  320 ). The structural fill layer may be constructed of cement or structural foam. Preferably, the fill layer is a structural foam. If the structural fill layer is constructed of a dry concrete as part of a two part pour system, preferably its outer surface has been texturized by a texture trowel or rake or the like to provide adhesion of a second layer of concrete. Alternatively, a thin application, one-sixteenth ( 1/16) inch thick or greater, of photocatalytic cement can be applied as a paste with trowels, floats, or pneumatic applicators to the vertical structure while still in a plastic state. 
         [0041]    The photocatalytic cement is prepared as described above. The photocatalytic cement may include colorizing agents and/or decorative aggregates. The photocatalytic concrete is then poured within the formwork so as to cover the structural fill layer (step  330 ). The upper surface of the photocatalytic concrete is then floated, screed or troweled to provide a uniform smooth surface (step  370 ). Further, all vertical formwork members are removed so as to expose the vertical surfaces or partially vertical surface, of the photocatalytic concrete layer. 
         [0042]    Where the photocatalytic concrete has an integral aggregate  6 , the aggregate can then be exposed utilizing chemical retardants or mechanical exposure methods. If an aggregate is not integrally mixed with the photocatalytic concrete, an aggregate  6  may be broadcast onto the concrete surface as long as the concrete surface is still in a plastic state. After being broadcast upon the photocatalytic concrete, the aggregate is troweled into the concrete&#39;s upper surface and the aggregate is exposed utilizing surface retardants or mechanical exposure methods. Concrete penetrating sealants may be applied. 
         [0043]    In still an additional embodiment of the present invention,  FIG. 6  illustrates a method for producing a concrete slab construction containing photocatalyst particles wherein the particles have been broadcast on a traditional concrete mixture after it has been poured, but while it is still in a plastic state. Preferably a subgrade or formwork is prepared for accepting the concrete (step  410 ). Then, the subgrade or formwork may produce horizontal or vertical concrete surfaces such as walkways, driveways and the like as well as walls, signs, seatwalls and water features. Fill sand and reinforcing members may be employed in the formwork to provide increased strength (steps  420  and  430 ). As illustrated in step  440 , the embodiment of the present invention includes the pouring of the standard concrete mixture upon into the subgrade or formwork. As explained above, it is preferred that the concrete mixture include Portland cement of at least 5.5 standard sacks of cement content per cubic yard. Preferably, the concrete layer is poured to three (3)-twenty four (24) inches thick. 
         [0044]    After the concrete layer  440  has been poured, the concrete&#39;s exterior surfaces are floated, screed or troweled in step  450  to create a substantially uniform finish. After the concrete&#39;s surfaces have been floated or screed to a desired uniform and consolidated condition, but while the concrete surface is still in a plastic state, photocatalysts including titanium dioxide (TiO 2 ) or one of its precursors tungsten oxide, calcium titanate or strontium titanate is broadcast upon the still plastic concrete surfaces. In step  460  the photocatalyst may be broadcast utilizing a variety of broadcasting methods including hand broadcasting, mechanical broadcasting or pneumatically distributing the photocatalyst to a desired concentration per square footage. The preferred photocatalyst is titanium dioxide, and more preferably titanium dioxide in the form of anatase. Any additional titanium dioxide in the anatase form broadcast upon the plastic concrete surface is believed to provide photocatalytic effect. The photocatalyst may be broadcast while it is in a dry powdered form. Alternatively, the photocatalyst may be mixed into a liquid solution and broadcast by spraying the liquid upon the concrete&#39;s exposed surface. In still alternative embodiments, Rockwood Pigments® Titanium Dioxide, which is 100% titanium dioxide in a powdered form, is broadcast upon the concrete surface. Preferably, one-half (½)-sixteen (16) ounces of Rockwood Pigments® Titanium Dioxide is applied per square foot of concrete. Even more preferably, one-half (½)-eight (8) ounces of Rockwood Pigments® Titanium Dioxide is applied per square foot of concrete. 
         [0045]    Once the photocatalyst has been broadcast to a desired coverage, the concrete&#39;s surface is again floated, screed and/or troweled (step  470 ) to fully work the photocatalyst into the top one-thirty-second ( 1/32)-one-half (½) inch of the concrete slab to ensure consistent and full penetration of the photocatalyst into the concrete. Thereafter, the contaminants are removed by washing the concrete surface utilizing a pressure washer, a hose, scrub brushes, or cleaning machines. (See step  480 ). The concrete&#39;s upper surface is allowed to dry overnight and optionally a sealant is applied as illustrated in step  490 . 
         [0046]    Advantageously, the present invention of a multi-layer concrete construction provides the underlying strength and wearability of the underlying conventional concrete structure as well as an aesthetic and photocatalytic properties of the upper concrete layer. Preferably, the upper photocatalytic layer has a minimum thickness of (¼) inch, and more preferably has a thickness of (⅜) inch minimum and (4) inches maximum. Preferably, the underlying base layer of conventional concrete has a thickness greater than the photocatalytic upper layer of concrete, and has a thickness in the range of three (3)-twenty four (24) inches. 
         [0047]    While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention.