Patent Publication Number: US-2010129152-A1

Title: Method of covering an area with a layer of compressible material

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     Asphalt paving is widely used to for roadbeds, parking lots, and other surfaces where a relatively smooth, durable surface is needed. Typically, the contour of a foundation surface, such as a roadbed, is prepared with a motor grader or other machine. A mat of hot asphalt concrete, commonly and hereinafter referred to as asphalt, is laid over the foundation surface with an asphalt paving machine that slowly moves over the area. After the mat of hot asphalt is deposited on the roadbed, a roller or other compression machine compresses the asphalt and produces a relatively smooth finish on the top surface of the asphalt layer. Not only does compressing the asphalt mat finish the top surface of the asphalt layer, it also compresses the roadbed and forces the irregularities of the roadbed into the asphalt. Additionally, compressing the asphalt presses together the mineral aggregate particles in the asphalt mixture to enhance the structural integrity of the final asphalt layer. 
     Typically it is desired that the asphalt paving operation produce a predefined upper surface elevation contour over the paved area. The foundation on which the asphalt is to be laid may be a layer of crushed rock or other material, or in some cases an appropriately graded earth roadbed. Such surfaces typically have irregularities, and commonly it is desired that these irregularities not be reflected in the final top surface of the asphalt layer. In the past it has been common to deposit an asphalt mat having a uniform pre-compression thickness. The thickness of the mat is chosen so that when the mat is subsequently compressed, the high spots of the mat are at or near the desired elevations. The low portions of the compressed mat are then filled in with additional asphalt material and the compression step repeated. It will be appreciated, however, that making multiple passes with an asphalt paving machine and a compacting roller over portions of the area to be paved is time consuming and expensive. It is seen that there is a need for a method of depositing asphalt or other compressible material in which only a single deposit step, and a reduced number of compression steps are needed to achieve the desired elevation level for the upper surface of the compressed asphalt or compressible material layer. 
     SUMMARY OF THE INVENTION 
     This need is met by a method of paving an area with asphalt to a desired elevation using an asphalt paving machine that deposits a mat of asphalt of a controlled thickness on a foundation surface, and then using a compacting machine to compact the asphalt to the desired elevation. The method includes the steps of determining the elevation of the foundation surface over the area, and determining the difference between the elevation of the foundation surface and the desired elevation over the area. A mat of asphalt is deposited over the area. The mat has a thickness that varies over the area, with the mat being thicker in the areas having a lower foundation elevation and thinner in the areas having a higher foundation elevation, such that the mat is sufficiently thick to allow for compression of the mat over the area to the desired elevation. The mat of asphalt is compressed during one or more compression operations such that the upper surface of the mat has the desired elevation over the area. 
     The step of determining the elevation of the foundation surface over the area may comprise the steps of scanning the contour of the foundation surface over the area, and storing the results of the scanning operation. The step of scanning the contour of the foundation surface over the area may include the step of using a laser scanning device to determine the elevation of the foundation at a plurality of points spaced over the area. The step of scanning the contour of the foundation surface over the area may include the step of using a GNSS survey device to determine the elevation of the foundation at a plurality of points spaced over the area. The step of scanning the contour of the foundation surface over the area may include the step of scanning the foundation from a scanner mounted on the asphalt paving machine to determine the elevation of the foundation at a plurality of points spaced over the area ahead of the asphalt paving machine while the asphalt paving machine is depositing an asphalt mat of varying thickness. The step of scanning the contour of the foundation surface over the area may include the step of scanning the foundation from a scanner mounted on a vehicle that travels ahead of the asphalt paving machine to determine the elevation of the foundation surface at a plurality of points spaced over the area. 
     The step of depositing a mat of asphalt over the area may comprise the step of depositing a mat which has a thickness that is substantially 1.25 times the difference between the desired elevation and the elevation of the foundation surface at each of a plurality of points spaced over the area. The step of depositing a mat of asphalt over the area may comprise the step of depositing a mat which has a thickness that is substantially a constant K times the difference between the desired elevation and the elevation of the foundation surface at each of a plurality of points spaced over the area. The method may further comprise the steps of determining the elevation of the upper surface of the mat after compression, and adjusting the value of K to compensate for determined offset from the desired elevation of the upper surface over the area. The step of determining the elevation of the upper surface of the mat after compression may include the step of measuring the elevation of the upper surface of the mat after compression with a laser scanner directed to the surface behind the compacting machine. The step of adjusting the value of K to compensate for determined offset from the desired elevation of the upper surface over the area may comprise the step of computing an adjusted value for K as the ratio of the thickness of the uncompressed asphalt mat deposited on the foundation surface to the thickness of the compressed asphalt mat. The method may further comprise the step of determining the thickness of the compressed asphalt mat by subtracting the elevation of the foundation surface at a point in the area from the elevation of the upper surface of the mat after compression at the same point in the area. 
     A method of covering an area with a compressible material to a desired elevation using a machine that deposits a layer of the compressible material of controlled thickness on a foundation surface, and then using a compacting machine to compact the compressible material to the desired elevation, comprises the steps of determining the elevation of the foundation surface over the area, and determining the difference between the elevation of the foundation surface and the desired elevation over the area. A layer of compressible material is deposited over the area. The layer has an upper surface that varies in elevation over the area, with the layer upper surface being higher in the areas having a lower foundation elevation and lower in the areas having a higher foundation elevation. The layer is sufficiently thick to allow for compression of the layer of compressible material over the area to the desired elevation. The layer of compressible material is compressed during one or more compression operations such that the upper surface of the layer after compression has the desired elevation over the area. 
     The step of determining the elevation of the foundation surface over the area may comprise the step of scanning the contour of the foundation surface over the area, and storing the results of the scanning operation. The step scanning the contour of the foundation surface over the area may include the step of using a laser scanning device to determine the elevation of the foundation at a plurality of points spaced over the area. The step scanning the contour of the foundation surface over the area may include the step of using a GNSS survey device to determine the elevation of the foundation at a plurality of points spaced over the area. The step of scanning the contour of the foundation surface over the area may include the step of scanning the foundation from a scanner mounted on the machine that deposits a layer of the compressible material to determine the elevation of the foundation at a plurality of points spaced over the area ahead of the machine while the machine is depositing a compressible material layer of varying thickness. The step of scanning the contour of the foundation surface over the area may include the step of scanning the foundation from a scanner mounted on a vehicle that travels ahead of the machine that deposits the compressible material to determine the elevation of the foundation at a plurality of points spaced over the area. 
     The step of depositing a layer of compressible material over the area may comprise the step of depositing a layer which has a thickness that is substantially 1.25 times the difference between the desired elevation and the elevation of the foundation surface at each of a plurality of points spaced over the area. The step of depositing a layer of compressible material over the area may comprise the step of depositing a layer which has a thickness that is substantially a constant K times the difference between the desired elevation and the elevation of the foundation surface at each of a plurality of points spaced over the area. The method may further comprise the steps of determining the elevation of the upper surface of the layer after compression, and adjusting the value of K to compensate for any offset from the desired elevation of the upper surface over the area. The step of determining the elevation of the upper surface of the layer after compression may include the step of measuring the elevation of the upper surface of the layer after compression with a laser scanner directed to the surface behind the compacting machine. The step of adjusting the value of K to compensate for any offset from the desired elevation of the upper surface over the area may comprise the step of computing an adjusted value for K as the ratio of the thickness of the uncompressed compressible material layer to the thickness of the compressed compressible material layer. The method may further comprise the step of determining the thickness of the compressed compressible material layer by subtracting the elevation of the foundation surface at a point in the area from the elevation of the upper surface of the layer after compression at the same point in the area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagrammatic longitudinal sectional view of an area, such as a road bed, showing a foundation surface over which compressible material has been laid to a desired grade or elevation; 
         FIG. 1B  is a diagrammatic sectional view, similar to  FIG. 1A , showing the compaction and fill levels achieved in multiple paving steps; 
         FIG. 1C  is a diagrammatic sectional view, similar to  FIG. 1B , showing compaction levels; 
         FIG. 1D  is a diagrammatic sectional view, similar to  FIG. 1C , useful in explaining the method of the present invention; 
         FIG. 1E  is a diagrammatic sectional view, similar to  FIG. 1D , useful in explaining the manner in which the volume of the asphalt dispensed and applied is controlled; and 
         FIG. 2  is a diagrammatic view of an asphalt paving system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1A  illustrates a road  10  that has been paved with asphalt  12  to a desired grade  14  along its top surface  15 .  FIG. 1A  is taken along a vertical plane that extends longitudinally along the center of the road  10  and thus illustrates elevation variations that occur along the length of the road. The asphalt  12  has been paved over a foundation  16  defining a foundation surface  18 . The top surface  15  of the asphalt  12  is shown as essentially flat even though the surface  18  of the foundation  16  varies significantly. To achieve this, the thickness of the asphalt mat  12  varies significantly along the road  10 . Several points should be noted. First, although a short segment of the road, such as that shown in  FIG. 1A , may be horizontal, the inclination of the road will vary along its length. Second, although the description here defines control of the paving operation along the length of the road, a control across the width of the road is also utilized and described, below. 
       FIG. 1B  shows a road  10  in section, in a view similar to that of  FIG. 1A , and is useful in understanding the prior art paving process. Corresponding elements have been given the same reference numerals in all of the drawings.  FIG. 1B  illustrates the process of paving a road in which an asphalt mat or layer of uniform thickness  20  is deposited on the upper surface  18  of foundation  16 . After the asphalt mat  12  is deposited, it is compressed to a thickness  22 . It will be noted that a surface  14 ′ results from compression that is below the desired grade  15 . At this point, additional layers of asphalt are deposited on the surface  14 ′ in the lower elevation areas and compacted. At least one additional layer of asphalt is then applied over the entire area and compacted to bring the multiple layers of asphalt up to the desired elevation  15 . In  FIG. 1C , a paving process similar to that of  FIG. 1B  is shown. In the process of  FIG. 1B , the initial layer of asphalt applied to the foundation brings the pre-compacted elevation of the asphalt mat up to the desired grade. Once the mat is compacted, all portions of the surface  14 ′ are below the desired grade. In the  FIG. 1C  paving process, on the other hand, the asphalt mat  12  has a pre-compression thickness  20  that is sufficient to result in the highest portions of the compressed mat  12  achieving the desired elevation. In both cases, however, the compressed mat  12  has portions that have a below grade top surface, and that must be filled in with additional asphalt in additional steps. 
       FIG. 1D  illustrates a method by which the appropriate amount of asphalt can be deposited as a mat having a thickness  20  that varies over the surface of the roadbed. In order to pave an area with asphalt to a desired elevation using an asphalt paving machine that deposits a mat of asphalt of a controlled thickness on the foundation surface, and then use a compacting machine to compact the asphalt to the desired elevation, the existing elevation of the foundation surface level  18  of the foundation  16  must be determined over the area to be paved. The difference, ΔH G-F , between the elevation H F  of the foundation surface  18  and the desired elevation H G  over the area is determined. A mat  12  is deposited over the area. The mat  12  has a thickness  20  that varies over the area, with the mat being thicker in the areas having a lower foundation elevation and thinner in the areas having a higher foundation elevation. The mat thickness is selected such that it is sufficiently thick to allow for compression of the mat over the area to the desired elevation in one or more compression operations. It has been found that by depositing a mat with a thickness that is approximately 25% greater than would be the case with an incompressible mat, the mat can be compressed in a single compression operation, or in a reduced number of compression operations, such that the upper surface  14  of the mat is uniformly at the desired elevation  15  over the area. In other words, the method contemplates the deposition of an asphalt mat that has a thickness adjusted to be approximately 1.25 H G-F . 
     The step of determining the elevation of the foundation surface  18  over the area may comprise the steps of scanning the contour of the foundation surface  14  over the area, and storing the results of the scanning operation. The mapped surface contour can then be used later in developing the appropriate thickness for the mat of asphalt before the mat is applied. The step of scanning the contour of the foundation surface  18  over the area may include the step of using a laser scanning device  40  ( FIG. 2 ) to determine the elevation of the foundation at a plurality of points spaced over the area. This data may be stored in memory  41 . The step of scanning the contour of the foundation surface over the area may include the step of using a GNSS survey device, such as receiver  42 , to determine the elevation of the foundation at a plurality of points spaced over the area. It will be appreciated that the elevation of the paving machine  43  may be determined by use of the GNSS receiver with the elevation of the foundation surface with respect to the machine  43  determined with the scanner  40 . The GNSS receiver then permits the elevation of the foundation to be referenced to a coordinate system that matches the grade plans for the road. It will be further appreciated that the elevation of the foundation surface may be mapped by other known survey techniques. The step of scanning the contour of the foundation surface  18  over the area to be paved may include the step of scanning the foundation from a scanner  40  that is mounted on the asphalt paving machine  43  to determine the elevation of the foundation  18  at a plurality of points spaced over the area ahead of the asphalt paving machine, while the asphalt paving machine is depositing an asphalt mat of varying thickness. Alternatively, the scanner  40  may be mounted on a vehicle that travels ahead of the asphalt paving machine. 
       FIG. 1D  illustrates depositing a mat of asphalt  12  over the area which has a thickness  20  that varies over the area, with the mat being thicker in the areas having a lower foundation elevation with respect to desired grade elevation and thinner in the areas having a higher foundation elevation with respect to desired grade elevation. It has been determined that depositing a mat  12  which has a thickness  20  that is substantially 1.25 times the difference between the desired elevation  15  and the elevation of the foundation surface  16  at each of a plurality of points spaced over the area compensates for compression of the mat. It will be noted that at each of the points x i  the overfill is approximately 25% of the fill height to the desired grade. In other words, H FILL =1.25 ΔH G-F  for each value of x i . It will be appreciated, however, that the overfill of 25% is an approximation that may need to be altered depending on the particular makeup of the asphalt material being used, on its temperature, and on other factors. It may, therefore, be necessary to adjust the amount of overfill to compensate for such variations. It is contemplated that a mat which has a thickness that is substantially a constant K times the difference between the desired elevation and the elevation of the foundation surface at each of a plurality of points spaced over the area may be deposited. If the factors that determine the percentage of overfill should change during a paving operation, it may be desirable to change the constant K also during the paving operation. This may be accomplished by determining the elevation of the upper surface  14  of the mat  12  after compression, and adjusting the value of K to compensate for the offset from the desired elevation of the upper surface over the area. The step of determining the elevation of the upper surface  14  of the mat  12  after compression may include the step of measuring the elevation of the upper surface of the mat after compression with a laser scanner  44  directed to the surface of the mat behind the compacting machine. More specifically, the output from the laser scanner  14  and the output from a GNSS receiver  46  are used to determine the elevation of the surface  14  at specific points behind the compacting machine  48 . The step of adjusting the value of K to compensate for determined offset from the desired elevation of the upper surface over the area comprises the step of computing an adjusted value for K as the ratio of the thickness of the uncompressed asphalt mat deposited on the foundation surface to the thickness of the compressed asphalt mat. The thickness of the compressed asphalt mat is determined by subtracting the elevation of the foundation surface at a point in the area, as measured by sensor  40 , from the elevation of the upper surface of the mat after compression at the same point in the area. 
     It will be appreciated that although the paving methods are described above in the context of asphalt concrete paving, these methods may be used in conjunction with applying and compressing a layer of any type, and then compacting the compressible material. It will be further appreciated that although these methods are described in the context of changing the amount of overfill along the length of a roadway as the roadway is being paved to compensate for variations in the elevation of the foundation in the direction of the roadway, this same method may also be used to adjust the level of overfill across the width of the roadway to compensate for variations in the elevation of the foundation across the roadway. In order to make such an adjustment, of course, it is necessary that the asphalt paving machine have a screed arrangement that controls variations in the elevation of the deposited asphalt across the direction of machine travel. 
       FIG. 1E  illustrates the manner in which the volume of the asphalt material being dispensed along the roadbed or other area is determined. The dispensed differential volume of the asphalt at a differential point x i  along the roadway is: 
       Volume=(Width)( H   FINAL   −H   FOUNDATION )(Δ x   i )(1.25) 
     The paving machine therefore determines the volume of asphalt that is to be deposited on the foundation surface as the paving machine moves along foundation surface  18 . This volume of material is ejected from the paver  43  and the elevation of the asphalt behind the paver  43  is determined by a screed  47  following the paving machine  43 . The asphalt is deposited by a dispensing unit  48  in the disperser under control of control unit  50 . 
     It will be appreciated that the present system determines the true height of the base foundation of the roadbed that is to be filled in with asphalt, and that this elevation measurement is taken so that it is in the same coordinate system as the desired road model. It will be appreciated that the system of  FIG. 2  measures the actual level of compaction and then takes this into account in dispensing the asphalt so that the compaction process produces a compacted asphalt service at the desired level in a single operation without adding additional layers of asphalt and with a reduced number of compaction cycles. 
     Other aspects, objects, and advantages of the embodiments can be obtained from a study of the drawings, the disclosure, and the appended claims.