Patent Publication Number: US-2005141961-A1

Title: Method and apparatus for forming longitudinal joints in concrete

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates generally to a method and apparatus for forming longitudinal joints in concrete as it is being slip-formed and, more specifically, to a method for using a knife beneath a slip-forming machine, such as for slip-forming a section of concrete roadway, that forms a longitudinal joint in the roadway that does not require sawing, cleaning, sealing or maintenance.  
      2. Background of the Prior Art  
      It has become very common to use slip-forming techniques in the forming of concrete roadways. A machine, such as that described in U.S. Pat. No. 4,073,592, is used to form one or more lanes of a concrete roadway by slip-forming. Wet or plastic concrete is added to the machine and it is formed into the desired roadway configuration by a plurality of forming surfaces. The concrete is applied by the machine to a prepared road bed, typically including reinforcing steel to strengthen the formed roadway. In a multiple lane roadway, reinforcing steel is used to span the lanes in a transverse direction to tie the lanes together. The reinforcing steel is effective at preventing lateral separation of the lanes but allows some degree of flexure along the joint between the lanes.  
      Cured concrete expands and contracts with changes in temperature. The expansion and contraction can lead to the formation of cracks which weaken and lead to breaking of the concrete. The larger the unitary structure of the concrete, the more likely it will crack due to expansion and contraction. Accordingly, one method of preventing the formation of cracks, at least in areas where cracks are not desired, it to form the concrete into reduced size sections that will be more resistant to cracking. In forming a roadway, if more than one lane is formed at a time, the width of the multiple lanes is typically too great to be left as a unitary structure. The approach to preventing undesired cracking has been to form a longitudinal joint down the formed roadway to divide the roadway into multiple lanes. The joint provides a place for concrete expansion, contraction or flexural movements to occur in a straight line and controlled manner.  
      The conventional method of constructing a longitudinal joint in a roadway paving project comprises four distinct operations. After the plastic concrete has been placed and molded, it is allowed to harden just sufficiently to carry the load of a concrete saw and associated equipment. The first step is sawing a longitudinal joint into the hardened concrete. The joint is typically between about one-eighth and three-eighths inch wide and to a depth of ⅓ of the thickness of the pavement being formed. Next, the joint is blown clean using a high capacity air compressor. The third operation, usually done twenty-four to forty-eight hours later, is installing a foam backer rode or rope into the joint down to about one inch below the pavement surface to create a base. The final step is to fill the joint reservoir above the backer rod with a sealant such as a hot, liquid asphaltic rubber sealant. This process requires the use of a saw, blades that can cut concrete and the aggregate used in the concrete, a lubricant, air or water cleaning of the groove, a drying period, insertion of the backer rod, placement of the sealant, and periodic maintenance to repair the sealant. Accordingly, this practice is expensive, noisy, dirty, and time-consuming.  
      A need exists for a method of forming longitudinal joints in a roadway that does not require post-forming processing, is inexpensive, noiseless, clean, fast, and without requiring periodic maintenance.  
     SUMMARY OF THE INVENTION  
      The invention consists of a method and apparatus for forming longitudinal joints in concrete as it is being slip-formed. A knife member having a depending knife is supported in a floating relationship on the surface of the roadway being formed. The knife extends below the surface of the concrete. As the slip-forming machine travels forwardly, the knife member travels longitudinally across the surface of the formed, uncured concrete. The knife forms a groove in the concrete. Preferably, the knife member also includes a horizontal float that assists in smoothing the surface of the disturbed concrete. Also preferably, the knife member and associated knife are vibrated as the knife forms the groove to assist in the operation of the knife and smoothing of the concrete surface by the horizontal float of the knife member.  
      The knife member is positioned under the slip-forming machine and aligned to form the joint, most generally coinciding with the centerline of the roadway being formed. The knife member can be mounted either forwardly or rearwardly on the machine, provided that the knife extends into the plastic concrete. The knife pushes aside each piece of aggregate encountered in its path, creating a longitudinally extended, vertical strip of concrete paste. This strip is a plane of weakness and a straight line crack or joint later occurs in the hardened concrete along the weakened plane, thereby forming an induced crack or joint. The joint appears as a longitudinal hairline crack. The joint is held tightly closed by deformed steel reinforcing tie bars positioned transversely at mid depth in the concrete at periodic spacing along the joint, as in the conventional roadway design. Sealing of this tight joint is not usually needed.  
      An object of the invention is to provide a method for forming a longitudinal joint in concrete that is performed in a single pass as the concrete is being formed.  
      Another object of the invention it to provide apparatus for forming a longitudinal joint in concrete that readily mounts on existing concrete forming machinery and is operable concurrently with such machinery.  
      A further object of this invention is to provide a method and apparatus for forming longitudinal joints in concrete that is inexpensive, timely, easy, efficient, and which reduces or eliminates maintenance of the joint.  
      These and other objects of the invention will be understood by those of skill in the art upon a review of this specification, the associated drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of a longitudinal joint formed in the conventional manner.  
       FIG. 2  is a perspective view of a knife member of the present invention, with trowel and vibrator attached.  
       FIG. 3  is rear perspective of the knife member of  FIG. 2  mounted on the underside of a concrete slip-forming machine.  
       FIG. 4  is rear perspective of the knife member of  FIG. 2  mounted on the underside of a concrete slip-forming machine and including an optional rubber bushing mount.  
       FIG. 5  is a schematic diagram of a possible arrangement of the trowel and process of forming a longitudinal joint according to the present invention.  
       FIG. 6  is a plan view showing a hairline crack formed along the longitudinal joint.  
       FIG. 7  is a cross-sectional view of a sample of concrete removed from the area of a longitudinal joint formed according to the present invention showing the aggregate moved to either side of the joint and the concrete paste strip along which the crack or joint forms. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
      In the conventional process for forming a longitudinal joint in concrete, the concrete is formed and allowed to harden sufficiently that it will support the weight of a concrete saw, its operator and associated apparatus. The saw is used to cut a longitudinal groove in the hardened concrete that is typically between one-eighth and three-eighths inch wide, as illustrated in  FIG. 1 . It is important that this operation be completed during a window of time in the hardening process of the concrete that may last less than about 12 hours; failure to create the sawn joint within the window is likely to result in uncontrolled cracking of the roadway which requires substantial time and expense to repair. Cutting the concrete requires the use of an expensive concrete saw, frequent replacement of blades, a lubricant, and of course an operator. Not uncommonly, the operation must be carried out at night in order to complete it during the optimal window and so lights may also be required. A large amount of silicaceous dust is formed (up to 4000 lbs. per mile) and the saw is very noisy in operation. The operator must wear a mask and ear protection.  
      The sawn groove is cleaned out using a high capacity air compressor and small tractor, once again requiring the expense of two additional machines, two operators, and resulting in the production of a large amount of dust. Backer rod is inserted into the cleaned groove about an inch below the surface of the concrete. Hot liquid asphaltic rubber is used to fill the void in the crack above the backer rod. It is common to require removal and replacement of the sealant at approximately seven-year intervals.  
      There is illustrated in  FIG. 2 , generally at  10 , a knife member of the present invention. The knife member  10  includes a vertically disposed knife  12  which extends downwardly from a horizontally disposed float  14 . The forward end section  16  of the knife  12  extends forwardly of the float  14  and includes a sloped forward edge. The knife  12  and float  14  are secured to a mounting frame  18  used to mount the trowel  10  on a concrete slip-forming machine (not shown). A vibrator  20  is mounted on the knife member  10  and is used as described below to vibrate the knife member  10  during operation.  
      The knife member  10  is illustrated in  FIGS. 3 and 4  mounted on the underside of a concrete slip-forming machine  22 . A rearward end portion  24  of the knife  12  extends upwardly to form a mounting member for the knife member  10 . An optional rubber bushing  26  is attached to the upper end of the rearward end portion  24  and is received about a horizontal mounting pin  28  of the slip-forming machine  22  ( FIG. 4 ). The rubber bushing  26  assists in mechanically isolating vibrations in the knife member  10  from the slip-forming machine  22  while permitting the slip-forming machine  22  to draw the knife member  10  through the plastic concrete formed by the machine  22 . The knife member  10  is also mounted to the machine  22  at a forward position by the use of bolts or the like (not shown) extending through the openings  28  and  30  in the knife  12  ( FIG. 2 ) and through corresponding opening in the slip-forming machine  22  (not shown).  
      It is well known in the art that the aggregate used in the concrete is usually much harder than the Portland cement that fills the space between the aggregate. This is one of the reasons why it is difficult to saw concrete; the saw must cut through not only the cement, but also individual pieces of aggregate that are present along the path of the saw. Additionally, when cracks form in concrete, they almost always travel through the cement and around the aggregate. This results in a crack that meanders not only at the surface of the concrete but through the depth of the concrete as well. Indeed, one of the reasons that the longitudinal joints are formed is to define a controlled path for the crack and avoid cracks in undesired areas of the concrete.  
      An optional vibrator  20  may be used to vibrate the knife member  10  during operation. Vibration assists in reducing the resistance of the knife  12  as it is drawn through the plastic concrete and will assist in forming a smoother joint. Moreover, in the process of the present invention, the vibration of the knife  12  assists in the important function of pushing aside aggregate that are encountered by the knife  12  as it passes through the concrete. If the knife  12  is not vibrated and a large or irregular aggregate particle is encountered, the knife  12  may push the aggregate particle ahead of the knife  12 , possibly creating a void in the concrete and abnormally disturbing the surface of the concrete. Forming a slope in the forward or leading edge of the knife  12  and preparing a sharpened or ground leading edge assists in deflecting aggregate downwardly and under the knife  12  and the vibrator  20  both assists in this process and in deflecting the aggregate to either side of the knife  12 . Another advantage of vibrating the knife member  10  is to assist in consolidating the concrete around the knife  12  and under the float  16 . A principal, unanticipated advantage of the deflection of the aggregate below and to either side of the knife  12  is the formation of a vertical strip of concrete paste or cement that is free of aggregate. This strip is a weakened zone or area of the concrete. Any crack induced in the area of the strip will follow the weakened strip and be constrained along the strip. Accordingly, the concrete paste strip performs the function of the sawn groove of the conventional technique in providing a controlled, constrained path for crack formation.  
      In a preferred embodiment, the vibrator  20  is a hydraulically actuated vibrator of the type already in common use on concrete slip-forming machines. Accordingly, operators of concrete slip-forming machines have existing inventories of suitable vibrators, including experience with operating and repairing the vibrators, and the machines themselves have supplies of pressurized hydraulic fluid, control systems, and supply lines needed to operate the vibrator  20 . While too high of a frequency of the vibrator tends to disrupt the concrete, creating voids, the vibrator  20  may be operated at a wide range of frequencies depending on the conditions. Most commonly, vibrations in the range of from about 20 cycles per second and about 50 cycles per second will produce satisfactory results.  
      A typical roadway forming operation using the method and apparatus of the present invention is illustrated schematically in  FIG. 5 . A belt placer  32  places uncured or plastic concrete in the general location of the finished roadway. Following the belt placer  32  is the slip-forming machine or paver  22 . The knife member  10  of the present invention is, in this embodiment, mounted at the rearward or trailing end of the paver  22 . Behind the paver  22 , manually operated bull floats  34  are used to further smooth the surface of the formed concrete roadway. Thereafter, tining to roughen the surface of the roadway is done and the roadway is allowed to cure.  
      In a preferred embodiment, the knife  12  is of a depth to create a strip of concrete paste that is approximately one-fifth to one-half, and more preferably about one-third, of the thickness of the concrete roadway being formed. The thickness of the knife  12  must not be so large that the gap in the concrete does not close with the concrete paste. The knife preferably is between about one-sixteenth and about one-quarter inch thick and, more preferably, about one-eighth inch thick. The length of the knife  12  is believed to have an effect at preventing possible “rebonding” or “aggregate interlock” which may disrupt the aggregate-free strip of cement paste from forming behind the knife  12 . That is, if the knife  12  is not sufficiently long, it is possible that the aggregate will not have been displaced fully to the side of the knife  12  by the time it has passed and so may drift back in to the area or zone of the concrete paste strip. In the preferred embodiment, the knife  12  is between about 20 and about 45 inches long and the slope of the leading end portion is between about 20 degrees and about 60 degrees relative to horizontal.  
      The strip of concrete paste that is formed by the knife  12  is illustrated in  FIG. 7  generally at  36 , which shows a cross section of a sample core taken from a formed roadway. The strip  36  is evident as the smooth, upper portion of the core sample in the photograph, the smooth surface indicating that the cross-section was taken through the aggregate-free zone of cement paste created when the aggregate particles were deflected to the side of the knife. In contrast, the presence of the aggregate  38  is clearly seen in the region of the core below the strip created by the knife.  
      Within a short amount of time after formation of the roadway and the longitudinal joint, a hairline crack, shown at  40  in  FIG. 6 , forms along the joint. The hairline crack is usually so tight that no sealing is needed. Moreover, the hairline crack is in a line very close to parallel to the roadway.  
      While the foregoing description of a preferred embodiment has focused on concrete containing aggregate, the method and apparatus will also work in forming a longitudinal joint in fiber-reinforced concrete. In such concrete, it may be desired to decrease the angle of attack of the leading edge of the knife  12  to assist in preventing the knife  12  from carrying the fibers.  
      Although the invention has been described with respect to a preferred embodiment thereof, it is to be also understood that it is not to be so limited since changes and modifications can be made therein which are within the full intended scope of this invention as defined by the appended claims.