Patent Publication Number: US-2005118297-A1

Title: Molded concrete foundation element and method for its manufacture

Description:
FIELD OF THE INVENTION  
      The present invention is generally in the field of solid concrete elements. More particularly, the invention is concerned with centrifuge-cast foundation elements, their manufacture and an apparatus for manufacturing same.  
      Such elements are often referred to in the art as ‘armor units’ or ‘armor elements’.  
      The term “foundation element” used hereinafter in the specification and claims is used in its broadest aspect and denotes a variety of foundation/construction elements, e.g. as a bed for marine construction, wave breakers, dams, supportive walls, soil foundation and consolidation, etc.  
     BACKGROUND OF THE INVENTION  
      Use of foundation elements for various construction purposes is well known. At times, large rocks are used for such purposes. However, a disadvantage concerned with using rocks is the availability of such large rocks and the expenses involved with quarrying and transportation of the rocks to the work site.  
      Rather than using rocks, there is an ever-growing use of molded foundation elements which are relatively cheap and which may also be molded at or adjacent the work site. Even more so, in molding such elements, one may also control the mechanical properties of the elements, e.g. the compressive strength, weight, wear resistance, etc., by controlling different parameters such as type of concrete used, additives used (binders and aggregates), amount of liquid added, entrapped air, etc. Still, one may control the shape and the size of the elements to thereby impart them with various properties so as to meet requirements of a particular construction site.  
      Presently, foundation elements of the concerned type are molded in a harmonized manner, i.e. the distribution of aggregate material (typically gravel) through a section of the element is essentially equal.  
      U.S. Pat. Nos. 4,976,291 and 5,035,850 disclose a concrete-type composite pipe produced by rotating a drum mold while casting concrete there into to form a concrete layer of a uniform thickness using a centrifugal force exerted on the cast concrete, casting a corrosion protective layer on an inner surface of the formed concrete layer, scattering aggregates on the inner surface and accelerating the rotation to cause the aggregates or the like to form an intermediate layer between the concrete layer and the corrosion protective layer.  
      It is an object of the present invention to provide a concrete-molded foundation element formed with a durable crust, enveloping a core consisted of aggregated material indexed in an inverted segregation dispersion. It is a further object of the present invention to provide a method for production of such foundation elements and an apparatus for carrying out such a method.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a construction element formed with a durable concrete crust having an external surface with a relatively high compressive strength, said crust enveloping a core made of aggregate material indexed in inverted segregation dispersion.  
      According to the present invention there is provided a foundation element, symmetrical about a longitudinal axis thereof, said element formed with a crust made of at least normal-strength concrete, said crust enveloping a core made of aggregate material dispersed such that the size of the aggregate material increases from fine to large about a radial section of the element and where space between larger aggregate material is occupied by smaller sized aggregate material. The construction element is molded in a centrifugal process.  
      The arrangement is such that large aggregate material is disposed adjacent an outer surface of the crust and small aggregate material is disposed at the center of the element.  
      In accordance with a particular embodiment of the invention, the core material comprises waste material. Such waste material may consist of fly ash, polymeric waste material, radioactive contaminated material, etc. This is an environmentally friendly method for getting rid of such waste material.  
      In accordance with an other particular embodiment, the construction element is fitted with at least one hoisting-eye. By a preferred embodiment, the hoisting-eye is received within an indention or depression such that it does not project beyond a top surface of the foundation element.  
      The construction element in accordance with the present invention may have different cross-sections, e.g. cylindrical, triangular, square, hexagonal, octagonal, etc. According to a particular design, where the cross-section of the construction element is polygonal, the number of faces is at least five, so as to avoid significantly differing distance from the center of the core.  
      In accordance with a further aspect of the present invention, there is provided a method for manufacturing a foundation element symmetrical about a longitudinal axis thereof, said method comprising: 
          (i) obtaining a centrifugal mold formed with a bottom base and side walls extending upwards therefrom;     (ii) introducing at least normal-strength concrete into the mold;     (iii) rotating the mold so as to generate centrifugal force acting on the cement thus forming a peripheral crust;     (iv) decreasing rotation speed of the mold and introducing an additional amount of at least normal-strength concrete into the mold to form a bottom base crust, continuous with the peripheral crust;     (v) introducing into the mold aggregate core material comprising graded material indexed between large size and small sized substance material;     (vi) rotating the mold at high speed whereby the said crust envelopes the core such that the size of the aggregate material increases from fine to large about a radial section of the element;     (vii) stopping the mold and adding an additional amount of at least normal-strength concrete into the mold to form a top base crust, continuous with the peripheral crust; and        

      (viii) drying the element.  
      According to a modification of the invention, steps (ii) to (v) may be replaced by introducing a mixture comprising cement and a mixture of aggregate material comprising graded material indexed between large size and small sized substance material.  
      In accordance with still a modification, any time after step (v) liquids may be drained or suctioned from the mold which was drawn liquid may then be replaced by substitute material such as cement.  
      The invention is further concerned with an apparatus for molding a foundation element, comprising a mold mountable on a rotatable plate member; said mold comprising a base, peripheral side walls extending therefrom and a top cover attachable to the side walls; said top cover comprising an inlet opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For better understanding the invention and its different aspects, and to see how it may be carried out in practice, reference will now be made to the accompanying drawings, by way of example only, in which:  
       FIG. 1A  is a longitudinal section through a foundation element in accordance with the present invention;  
       FIG. 1B  is a horizontal section along lines II-II in  FIG. 1A ;  
       FIG. 2A  is a schematic representation of an apparatus for manufacturing a foundation element in accordance with the present invention;  
       FIG. 2B  is a longitudinal section of a mold used in the apparatus of  FIG. 2A ;  
       FIGS. 3A  to  3 E are schematic illustrations of the steps for manufacturing a foundation element in accordance with the present invention; and  
       FIG. 4  is a schematic representation illustrating manufacturing of foundation elements at a marine site. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Attention is first directed to  FIGS. 1A and 1B  of the drawings illustrating a foundation element in accordance with an embodiment of the present invention, generally designated  20 . The element  20  has a hexagonal cross-sectional shape and is essentially symmetrical about its longitudinal axis. However, it is appreciated that other symmetrical shapes are possible too, though in particular circular or such shapes in which the faces are disposed at significantly blunt angles, e.g., an octagonal cross-section, etc. The size of the element  20  may vary, depending on the intended use.  
      Element  20  has a crust  22  typically made of normal to high-strength concrete, having a compressive strength of at least 20 MPa. The crust  22  has an outer surface  24  obtaining its shape and pattern from the mold in which it is molded, as will be discussed hereinafter. The cement slurry/paste used for obtaining the concrete crust is made of a mixture of Portland, or other type of cement, with water and at times some additives, as known per se, imparting the concrete with desired parameters e.g. corrosive-resistance, etc. The ratio of cement to water determines the strength of the concrete, namely its compressive strength, and according to the intended use of the foundation elements one decides which concrete to use.  
      Dispersed inwardly of the crust layer  22  there is large aggregate material in the form of large stones  26  (crushed rock and large gravel), which during the centrifugal molding process are forcefully urged towards the surface  24  of the crust layer  22 . It is apparent that the space between larger aggregate material is occupied by smaller sized aggregate material.  
      Inward from the large aggregate material  26  and in the voids and gaps therebetween, there is dispersed medium size aggregate material  30 , e.g. small stones, gravel, etc., with a low amount of cement therebetween. Further inward there is fine aggregate material  40 , e.g. sand, filling the gaps between the small aggregate material and comprising a minimal percent of cement.  
      The innermost layer of the element  20  comprises, in the molding process, the excessive water. After drying what remains is fine aggregate material  40  e.g. various types of sand, and air voids  
      As can be further seen in  FIG. 1A , the foundation element  20  has a top base  46  with a downcast central portion at  48  which appears during the drying process of the element  20 . Further noted, the element  20  is fitted with two hoisting hooks  50 , inserted into the foundation element during the molding process, which do not extend beyond the upper edge  54  of the element. This ensures that in stacking like elements on top of one another the hosting hooks  50  do not deform.  
      Reverting now to the aggregate core material, it is apparent that it consists of material indexed in an inverted segregation dispersion such that large aggregate material  26  is disposed adjacent the outer surface  24  of the crust  22  with fine aggregate material  40  disposed at the center of the element  20 . This arrangement is obtained during the molding process of the core of the element  20 , which is carried out under centrifugal forces, whereby the heavier material is radially urged toward the outer surface  24  of the crust  22 .  
      The core material may comprise waste material in various forms, e.g. granulated or powdered material, fibers, compressed material, crushed material, etc. The waste material may be any environmentally hazardous material of which it is desired to dispose of, e.g. polymeric material, fly ash, radioactive-contaminated material, etc. It is thus advantageous that when a foundation element comprises waste components, e.g. radioactive contaminated material, the crust  22  be made of high-strength concrete, e.g. having a compressive strength of at least 50 MPa, so as to increase safety.  
      Further attention is now directed to  FIGS. 2A and 2B  illustrating a form of an apparatus useful in manufacturing a foundation element in accordance with the present invention and as disclosed hereinabove. The apparatus generally designated  70  is in the form of a towable platform  71  and is self-provided with a power unit  72 , typically a motor fitted with a control panel  74  and a revolution counter  76 . A mold  78  is fixable to a rotatable plate  79  mounted on the platform  71 . The rotatable plate  79  is fitted with a transmission  80  for imparting it with rotary motion about its longitudinal axis. The mold  78  is fitted with a cover  88  supported at neck  84  by a support arm  86  pivotably articulated at a pin  87  to the platform  71 . The arm  86  serves also for closing and opening and opening the cover  88 , possibly by the aid of a pneumatic or hydraulic piston  90 , for assisting in pivotally displacing arm  86  in the direction of arrow  92 .  
      As can further be noted in  FIG. 2B , mold  78  comprises a base plate  100  from which upwardly extend side walls  102  which give rise to form a mold having a hexagonal cross-section. Each of the walls  102  is pivotally secured to the base plate  100 . Alternatively, the sides walls  102  are separated from the base plate  100 .  
      The side walls  102  are fitted at their bottom ends with engagement pins  110  for securing into corresponding openings  112  formed in the rotatable plate  79  ( FIG. 2B ) to thereby transfer rotary motion from the rotating plate  79  to the mold  78 .  
      Cover  88  of the mold  78  comprises a plurality of downwardly extending projections  116  ( FIG. 2A ) for engagement with corresponding brackets  118  at the sides walls  102 , which cooperate together to secure the mold in particular during rotation thereof when significant centrifugal forces act on the walls  102 .  
      It is further noted that the top cover  88  is formed with two depressions  120 , each of which is formed with an opening for receiving a hoisting eye-hook  50  to be integrally molded with the foundation element  20  (see  FIGS. 2B, 3D  and  3 E).  
      The device of  FIG. 2B  differs from the device shown in  FIG. 2A  in that the cover  88  is integrally formed with a supporting neck portion  128  extending from a funnel-like opening  130 , through which cement and other ingredient material may be introduced into the mold. The neck portion  128  is embraced by a suitable bearing of the support arm  86 , coaxially above a corresponding support at the bottom side of the rotary plate  112 . Also noted in  FIG. 2B , at least some of the side walls  102  are provided with hoisting hooks  134  for lifting the mold by a suitable hook  136  depending from a crane, etc. ( FIG. 2B ).  
      It is to be appreciated by a versed person that the mold  78  may have different cross-sections and different sizes as previously discussed in connection with the foundation element. Furthermore, the inner surfaces  138  of the walls  102  may be textured to thereby impart the external surface of the foundation element  20  with a corresponding decorative texture. Even more so, the walls  102  of the mold  78  may have a non-parallel cross-section, e.g. a rhombus-like cross-section.  
      In order to understand the method of manufacturing a foundation element in accordance with the present invention, further attention is now directed to  FIGS. 3A-3E .  
      In a first step, a mold  78  is placed on rotatable plate  78  and is rotatably fixed thereto by means of engagement pins  110  projecting into corresponding openings  112  of the rotating plate  79  (see  FIG. 2B ). Then, the mold cover  88  is placed over the mold  78  and is secured in position by means of support arm  86 . In the embodiment of  FIG. 2B  the side walls are arrested by a peripheral groove  89  receiving the upper edge of the side walls and preventing them from yielding and radially displacing under centrifugal force. However, in the embodiment of  FIG. 2A , the side walls are arrested by the projections  116  of cover  88  projecting into the corresponding brackets  118  of the side walls  102 .  
      A cement slurry/paste is poured through funnel  130  into the mold  78 , while the mold  78  is rotating at a relatively high speed, thus imparting the slurry with centrifugal forces urging it against the inner walls  102  of the mold  78 . After some time, when the cement is partially solid and retains its position as in  FIG. 3A , forming a side wall crust  140 , the rotational speed of the mold  78  is slowed down. At this point the crust  140  remains stable and maintains its form.  
      After the rotation of the mold  78  is significantly slowed down, an additional amount of cement slurry is introduced through funnel  130  allowing it to fall to the bottom of the mold  78  and form a bottom crust  141  ( FIG. 3B ).  
      Then an aggregated core material is introduced through funnel  130 . The aggregate material comprises a mixture of large stones  142 , small stones, e.g. gravel  144  and fine aggregate material, e.g. various types of sand  146 . Upon insertion of the aggregate mixture, the speed of rotation of mold  78  is increased thereby urging the aggregate material to disperse in a so-called inverted segregation dispersion whereby the large stones  142  are forced to penetrate into the enveloping crust  140  formed in the previous step (see  FIG. 3A ) and such that the fine aggregate material  146  is disposed at the center of the mold  78 , as in  FIG. 3C . As an option, the aggregate material may be introduced into the mold  78  at an indexed order, i.e. first the large stones  142  and finally the fine aggregate material  146 .  
      The speed of rotation of the mold  78  and the duration of rotation depend on the consistency of the concrete as well as on the size and specific weight of the aggregate material. If desired, waste material may be introduced into the core material, e.g. waste polymeric material, fly ash, radioactive contaminated material, etc. of which it is desired to dispose of in an environmentally friendly manner. For such a purpose, it is desired that the crust layer  140  be of at least normal and preferably high-strength concrete, depending on the intended use of the foundation element  20  and on the ingredients of the core material. For example, for marine use and where radioactive-contaminate material is introduced as waste material, it would be preferable that the crust be of high-strength type and have a compressive strength of at least 50 NPa. Furthermore, it may be advantageous that in the first step, a corrosive-resistant layer be applied on the inner surface  138  of the mold  78  or together with the slurry forming the crust  140  to impart the foundation element  20  with corrosive resistance.  
      In accordance with a modification of the invention, liquid may be drained from the mold or suctioned by a suitable pump arrangement (not shown) to allow substituting the removed liquid, typically water, by a heavy substance such as, for example, a cement slurry or other reinforcing or adhering agent.  
      Turning now to  FIG. 3D , a pair of eye-hooks  154  is introduced into the mold through suitable openings formed in the cover  88  such that only a looped portion  156  projects from the opening in the cover  88  and do not exceed over an uppermost edge of the side walls  102 . Then, a concrete slurry is introduced through funnel  130  so as to form a top crust  160  of the foundation element  20 .  
      The process then ends by elevating the cover  88  (by means of arm  86 ) and hoisting away the mold  78  and allowing the molded element to consolidate and heal at a suitable drying site ( FIG. 3E ). Then, after a predetermined period of time the side walls  102  are removed and the foundation element  20  is ready for use as disclosed hereinabove.  
      As an alternative, rather then first molding the crust  22  and then adding the aggregate material, one may combine these steps by introducing into the mold  78  a mixture comprising the cement and the mixture of aggregate material comprising graded material indexed between large sized and small sized material, whereby the cement and the aggregate material will disperse during rotation of the mold  78  at high speed, under the influence of centrifugal force.  
      Turning now to  FIG. 4  there is illustrated schematically a marine site where a plurality of foundation elements  180  are to be laid. A foundation-element manufacturing site  182  is in the form of a floating barge or ship, where water supply is from the sea (with necessary water-treating means provided) as well as fine aggregate material (e.g. sand) which is sucked from the sea bed by means of a pipe  188 . The large aggregate material  26 , e.g. stones and gravel are stored aboard or may be transferred thereto. The foundation elements  180  manufactured on board the floating barge are then transferred by cranes  190  or floating barge  192  to the site at which the foundation elements  180  are to be laid.  
      Obviously, foundation elements of different shape and size are used as desired according to various engineering and other considerations.