Abstract:
A paving brick with a means for reducing mechanical stress on at least one edge of a top surface, particularly when used in a paving system for roads, streets, sidewalks and like when heavy vehicle load applies a force on the top surface. In one embodiment the means for reducing mechanical stress is a rounded edge. In another embodiment the means for reducing mechanical stress is an angle sidewall having an angle theta between 75 and 85 degrees.

Description:
FIELD 
       [0001]    The present invention relates generally to paving bricks and specifically to road surfaces comprising paving bricks with improved wall and edge designs for increased durability when exposed to vehicle traffic and other environmental stresses. 
       BACKGROUND OF THE INVENTION 
       [0002]    The use of artificial paving bricks with various geometric shapes and used as paving elements to form sidewalks, court yards, roads, streets, and the like is well known. Such paving bricks and the surfaces they form are expected to carry vehicle traffic, wheelchairs, bicycles and the like, and to endure mechanical forces caused by freezing, heating, and earth movements which can impart mechanical stresses on the paving bricks and cause long term degradation or sudden cracking and spalling at stress concentration points, particularly at the top surface edges. 
         [0003]    In U.S. Pat. No. 3,969,851 the inventor describes several artificial paving stones with interlocking protrusions at the side surfaces to resist displacement when loads are applied. While effective at keeping the paving stones in place, the sharp edges and corners are known to create weak points for spalling and cracking, and are not durable under heavy vehicular loads. 
         [0004]    In U.S. Pat. No. 5,707,698 the inventors describe a water permeable paving system with channels formed along the sides of the paving bricks which, when the bricks are placed together, create channels to allow water to pass through. While an effective means of increasing the water permeability of the paving system, these channels create stress concentration points that can lead to cracks and spalling when the bricks are subjected to environmental stresses and vehicle traffic. 
         [0005]    In U.S. Pat. No. 4,761,095 the inventors describe an artificial paving stone with irregular sidewalls designed to engage with neighboring stones and anchor them against lateral shifting and displacement during vehicle motion. The inventors further describe a chamfered top edge which, while easy to manufacture, creates sharp corners that is known to cause cracks and spalling under heavy vehicle loads owing to the sharp angles created at the chamfer edges. 
         [0006]    In addition to these problems of the paving bricks of the prior art, there is an increasing desire in the marketplace for paving bricks which are slightly porous and therefore permeable to moisture, and the porosity of such paving bricks further exposes them to damage at any sharp corners or edges owing to their methods of construction and, in some cases lack of solid reinforcement. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide a paving brick of the kind described above with novel features designed to improve the load bearing capacity and durability of a surface made with such bricks. In one embodiment, paving bricks are provided with rounded edges in order to limit contact between adjacent bricks when laid adjacent to each other. The rounded edges begin approximately two thirds of the distance from the bottom of the paving brick, and in some embodiments have a radius of curvature from 1 mm to 4 mm, such as 3 mm, and such as 2 mm. 
         [0008]    Another embodiment of the invention includes at least one sidewall which is angle inward such that the top surface of the paving brick is smaller in at least one of a width and length dimension than a bottom surface of a paving brick. The inward angle is such that the angle between the at least one sidewall and the bottom surface is less than 85 degrees and greater than 75 degrees, such as 80 degrees, and is defined as angle theta. 
         [0009]    Another embodiment of the present invention is to provide a water permeable paving brick designed in such a way that the load bearing capacity and durability of a roadway or other surface made with such bricks is clearly improved. When porous artificial paving bricks are employed there is a risk of breakage and the top edges and corners, particularly for porous paving bricks having a compressive strength between 40 MPa and 70 MPa and a tensile strength of between 0.1 MPa and 0.2 MPa. In this range of compressive and tensile strengths a paving system made of bricks known in the prior art have been found by the inventors to be susceptible to cracking and spalling at the top edges when heavy vehicle traffic is applied. 
         [0010]    A further mechanical strength property of paving stones is known as the modulus of rupture, or flexural strength, which is defined as the maximum stress the material can withstand under a three-point bend test. This is test is defined in ASTM standard C67-02 entitled, “Standard Test Method for Sampling and Testing Brick and Structural Clay Tile”, Section 6.0—Modulus of Rupture, and is a measure of the brittleness of a material under bending load. The inventors have found that porous stones of the kind used for permeable paving, having a dry modulus of rupture between 10 MPa and 20 MPa are particularly suited for the novel features of the present invention. 
         [0011]    The compressive strength of paving stones may be tested according to ASTM standards C192 and C39, and the tensile strength of paving stones may be tested according to ASTM standard C190. 
         [0012]    A more complete appreciation of the present invention and its scope can be obtained from the following detailed description of the invention, drawings, and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a top down view of a prior art paving system. 
           [0014]      FIG. 2  is a perspective view of a novel brick design for paving. 
           [0015]      FIG. 3  shows two adjacent paving bricks with a sloping edge design. 
           [0016]      FIG. 4  shows two adjacent paving bricks with a rounded upper edge portions. 
           [0017]      FIG. 5  is a perspective view of two adjacent paving bricks with rounded upper edge portions. 
           [0018]      FIG. 6  is a top view of a paving system formed with bricks of the prior art adjacent to a paving system formed with bricks of the present invention. 
           [0019]      FIG. 7  is a perspective view of a paving system formed with bricks of the prior art adjacent to a paving system formed with bricks of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Reference will now be made in detail to specific embodiments of the invention. Examples of the specific embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to such specific embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. 
         [0021]    As set forth below, a paving system is described comprising paving bricks with novel designs that limit mechanical stress build-up and chipping of the corners and edges of the paving bricks when heavy loads are applied to the paving system. In some embodiments, the paving bricks are porous to allow water to pass through at a rate of more than 1,000 grains per 24 hour per square meter, such as 1,500 grains per 24 hour per square meter, such as 3,000 grains per 24 hour per square meter, and such as 4,000 grains per 24 hour per square meter. 
         [0022]      FIG. 1  is a top down view of a prior art paving system. Edge region  102  and corner region  101  depict the cracking and spalling that occurs on the sharp or chamfered edges of the prior art. 
         [0023]      FIG. 2  is perspective view of one embodiment of the present invention. In this embodiment the width dimension of the bottom surface  203  is greater than the width dimension of the top surface  204  such that angle  207  is less than about 85 degrees and greater than 75 degrees, such as 80 degrees, and shall be defined as angle theta in the accompanying claims. The angle theta introduced therein allows adjacent paving bricks, as shown in  FIG. 3 , to touch at their bottom edges  305  and lock in place, helping to resist movement under vehicle load while providing gap  304  between adjacent top surfaces to allow for expansion and contraction during heating and cooling cycles, and drastically reduce the stress applied to the top edge and corner surfaces of adjacent paving bricks under load. In a typical paving installation the gap between adjacent pacing bricks is filled with a porous or non-porous grout (not shown), as commonly known in the art. 
         [0024]      FIG. 4  is an end view of one embodiment of the present invention showing two adjacent paving bricks with rounded top edges  407 . In one embodiment, the ratio of the height  402  of the paving brick to the height  401  of the unrounded edge is between 1.5 and 3. In other words, the rounded portion  407  should begin no more than two thirds of the total height  402  from the bottom surface  408  of the paving brick in order to allow sufficient contact between adjacent bricks to create friction and lock the brick in place. By introducing a rounded edge  407  the stress concentration at the edge  407  is reduced, and gap  406  is created to allow for expansion and contraction during heating and cooling, and to allow for movement of the bricks during heavy vehicular loads. In some embodiments, gap  406  may be filled with a porous or non-porous grout compound (not shown) as commonly known in the art. 
         [0025]      FIG. 5  is a perspective view of the adjacent bricks as shown in  FIG. 4 . In this example gap  505  created by rounded edge  507  provides spacing for the expansion and contraction during heating and cooling, and allows for movement of the bricks during heavy vehicular loads. 
         [0026]      FIG. 6  is a top view of a paving system of the prior art with sharp edges constructed adjacent to one embodiment of the paving bricks of the present invention. Due to the dramatically reduced stress concentration from the rounded edges of  407 , the distance between adjacent bricks  601  is greatly reduced without adversely affecting the durability of the paving system. In contrast, the distance  602  between paving bricks of the prior art is required to be much larger, in some cases two to three times larger, in order to minimize cracking and spalling at the top edges to an acceptable level. This required extra spacing adds cost due to the additional filler material (not shown) that must mixed and poured at the installation site. In some instances the additional spacing requiring reduces the aesthetic value of the paving system, and in other instances limits the types of vehicle travel that are acceptable on the paved surface. 
         [0027]      FIG. 7  is a perspective view of a paving system of the prior art adjacent to the paving bricks and system of the present invention. In this experimental example, several vehicles weighing in the range of 2,000 to 4,000 pounds travelled over each surface, and the corners of the prior art bricks exhibited cracked corners and edges  701  due to the stress concentration at the sharp edges, whereas the bricks of the instant invention having rounded edges were observed to remain intact. In a separate experiment, not shown, the inventors have found a similar result for bricks having trapezoidal shapes wherein the angle formed between the side wall and bottom surface was less than about 85 degrees and greater than 75 degrees, for example 80 degrees as depicted in  FIGS. 2 and 3 .