US3415914A - Method of casting multi-duct concrete conduits - Google Patents

Description

DeC- 10, 1968 1 D. WALKER ETAL 3,415,914

METHOD OF CASTING MULTI-DUCT CONCRETE CONDUITS Filed March 9, 1965 5 Sheets-Sheet 1 4 50i M i7 i '/gy Dec. 10, 1968 1 D. WALKER x-:TAL 3,415,914

METHOD OF CASTING MULTI-DUCT CONCRETE CONDUITS Filed March E, 1965 5 Sheets-Sheet 2 At/ gf Dec 10, 1968 l.. D. WALKER ETAL 3,415,914'

METHOD OF CASTING MULTI-DUCT CONCRETE GODUITS Filed March 9, 1965 5 Sheets-Sheet 3 Dec- 10, 1968 L. D. WALKER ETAL 3,415,914

METHOD OF CASTING MULTI-DUCT CONCRETE CONDUITS Filed March 9, 1965 5 sheetsheet 4.

Dec. 10, 1968 L. D. WALKER ETAL 3,415,914

METHOD OF CASTING MULTIDUCT CONCRETE GONDUITS 5 Sheets-Sheet 5 Filed March 9, 1965 www NPN

3,415,914 METHOD F CASTING MULTI-DUCT CONCRETE CONDUITS Larry D. Walker, Riverside, Calif., and Clarence H. Jorgensen, Jr., Stamford, Conn., assignors to American Cement Corporation, Los Angeles, Calif., a corporation of Delaware Filed Mar. 9, 1965, Ser. No. 438,322 7 Claims. (Cl. 264-86) ABSTRACT 0F THE DISCLOSURE A method of casting a concrete conduit in which a slurry of concrete is pumped into a mold chamber around an elongate core extending through the chamber. After suiiicient uncombined water is expressed from the slurry to form a partially dried self-supporting casting, the dimensions of the mold chamber are increased transversely of the core, but not along the length of the core, by an amount which is less than that productive of separation of the boundaries of the chamber from the partially dried casting, thereby permitting the core to be withdrawn from the chamber without damage to the casting.

This invention relates to multi-duct concrete conduits and, more particularly, to an improved method, and to apparatus for carrying out the improved method, for forming such conduits.

Multi-duct concrete conduits are used to encase underground telephone lines, for example. It is desirable that the duct passages formed in each length of the conduit Ibe of uniform diameter. It is also desired that the concrete be of substantially uniform density throughout the conduit section.

Presently known conduits frequently show non-uniform density and/ or diameter along the length of the ducts within the conduit. Also, the methods which have been used in the past to fabricate such products have led to a high percentage of rejects either `because of cracking of the product as it is removed from a casting mold, or because the density variation exceeds the applicable tolerance limits, or because of a greater than tolerable variation in duct diameter. A high reject rate leads to a high cost of the finished product.

To handle the concrete mix satisfactorily, the concrete mix must be wet, i.e., in the form of a slurry. The slurry is injected into a mold cavity which has one or more duct-defining cores therein. The slurry must be dried in the cavity to dene a self-supporting preform. In prior art concrete conduit casting apparatus, the mechanisms provided for drying the slurry often were such as to cause defects in the product.

This invention provides an improved method for casting multi-duct concrete conduits. The method may be performed rapidly and, in contrast to 4many of the prior art methods, its practice leads to economical use of the concrete mix. Additionally, this invention provides a simple mechanism for drying the wet sluny to provide a self-supporting conduit preform, but which does not lead to the casting of defective products as was common in prior art procedures.

Generally speaking, this invention provides a method of casting a concrete conduit `and includes the step of forming an enclosed mold chamber which has an elongated core extending through the chamber. The mold chamber is filled with a wet slurry of concrete and sufficient uncombined Water is expressed from the slurry to form a partially dried, self-supporting casting in the chamber. Preferably the water is expressed solely by the application of pressure to the slurry in a manner which does United States Patent O Patented Dec. 10, 1968 not require a reduction in the volume of the mold charnber. Additionally, the method includes the step of increasing the dimensions of the mold chamber a predetermined `amount transversely of the core, but not lengthwise of the core. The core is then withdrawn from the chamber and the partially dried casting is thereafter removed from the mold chamber.

The above mentioned and other features of the present invention are more fully set forth in the following detailed description of a preferred embodiment of the invention. This description is presented in conjunction with the accompanying drawings wherein:

FIG. l is a perspective view of a section of multi-duct concrete conduit provided by this invention;

FIG. 2 is a front elevation view, with parts broken away, of conduit casting apparatus according to the present invention;

FIG. 3 is an enlarged fragmentary elevation view, with parts broken away, of the portion of the casting apparatus not shown in the central part of FIG. 2;

FIG. 4 is an enlarged cross-sectional elevation view of a portion of the lower end of the mold cavity provided by the apparatus shown in FIG. 2;

FIG. 5 is a fragmentary top plan view of the front of the casting apparatus shown in FIG. 2;

FIG. `6 is a cross-sectional elevation view taken along line 6-6 of FIG. 5;

FIG. 7 is an elevation view, with parts broken away, of a core shown in FIG. 2;

FIG. 8 is an end view of the core shown in FIG. 7; and

FIG. 9 is a semi-schematic view of the.` mold cavity and the mechanism for preparing and injecting a wet concrete slurry into the cavity.

A multi-duct concrete conduit unit 15 is shown in FIG. 1. The conduit unit defines a plurality of circular ducts 16 `which extend from end 17 to end 18 of the unit. Preferably, there are six ducts 16A-16F `which are arranged in two parallel rows of three ducts each, the ducts in each row being aligned with the corresponding ducts in the other row. Along the major portion of the length of conduit unit 15 from end 18, the unit has an external surface 19 which is convoluted so that the wall thicknesses of the ducts are substantially constant adjacent the exterior of the unit. Adjacent end 17, however, the exterior of the unit is configured to define a portion 20 of a coupling by which each unit is connected to an adjacent unit.

It is to be understood that the method and apparatus described herein are not restricted to the casting of any specific conduit unit configuration. Accordingly, conduit unit 15 has been shown merely for illustrative purposes. Conduit units like that shown, however, are fabricated in accord with specications issued by telephone companies, for example. Such conduits are used as casings through which underground telephone cables are run. In the usual case the specifications require that the diameters of ducts 16A-16F not vary appreciably along the length of the conduit unit, and that the density of the concrete be substantially uniform throughout the finished product. As indicated above, a conduit unit fabricated in accord with the procedures of the prior art often are of non-uniform density and manifest significant variations in the diameters of the ducts. As will be seen, however, practice of this invention provides a product which cornplies with these criteria.

FIG. 2 shows a conduit casting machine 25 adapted for the production of cast concrete conduits of the type shown in FIG. l. The machine includes a foundation `comprised of a horizontal bed plate 26 supported above a supporting surface 27, such as a concrete floor, by a plurality of rigid columns 28. A top plate 29 is supported in spaced apart relation to the bottom plate by left and right sides beams 30 and 31, respectively, and by a rear beam 32 which is similar to the side beams. The side and rear beams are rigid and are spaced apart'from each other transversely of the machine as shown in FIG. 2.

The casting machine also includes a plurality of mold sections, namely, a top mold section (not shown), a bottom mold section 33 (see FIG. 4) which is provided in the form of a sintered metal plate, and left, right, forward and rear mold sections 34, 35, 36, 37, respectively. The upper and lower mold sections are carried on the under and upper sides, respectively, of a pair of parallel, horizontally disposed, spaced apart closure plates 38 and 39, respectively. The upper closure plate 38 is movably connected to the underside of top plate 29 by a toggle link mechanism 40 which is operated by a hydraulic ram (not shown for the sake of clarity of illustration) connected to a lever arm 41 extending laterally from one of the links of mechanism 40. Mechanism 40 is not described in detail at this point since it is similar both in structure and in operation to the link mechanisms which mount the side and back mold sections to the associated beams, and which will be described in detail below. A similar toggle link mechanism 42 operated by ram 43 movably mounts lower closure plate 39 to bottom plate 26.

Side - mold sections 34 and 35 are mounted to the opposing surfaces of strongbacks 45 and 46, respectively. Similarly, forward and rear mold sections 36 and 37 are mounted to the opposing faces of forward and rear strongbacks 47 and 48 (see FIGS. 2 and 9). The front strongback is carried by a front or door beam 49.

Each of the vertically disposed strongbacks 45-48 are movably mounted to the adjacent vertically disposed beam by a toggle link mechanism 52 and by a pair of guide pistons 53. As shown in FIG. 2 with respect to the toggle link mechanism which mounts strongback 45 to beam 30, each toggle link mechanism includes an upper pair 54 and a lower pair 55 of toggle link assemblies, the assembliel in each pair being horizontally spaced apart from each other. The toggle link assemblies are substantially identical and comprise two pivotally connected links 56 and 57. Each link 56 is pivotally connected to the inner face of the respective beam, eg., the face of the beam 30 which opens to strongback 45. Each link 57 is pivotally connected to a vertically extending stiffener bar 58 which is a part of the adjacent strongback. Preferably each strongback includes a pair of horizontally spaced apart stiffener bars.) The upper and lower link assemblies of each link mechanism are interconnected by a pair of cross links 59, the opposite ends of which are pivotally connected to pivot pins 60 which pivotally interconnect links 56, 57 in each toggle link assembly. A hydraulic ram 61 is included in each toggle link mechanism 52 and has one end pivotally connected to the beam, eg., beam 30, and the other end pivotally connected to one of pivot pins 60 between the link assemblies of assembly pairs 54 or 55.

Upon operation of ram 61, strongback 45, for example, is caused to translate toward and away from beam 30; that is, the strongback is moved in a manner such that it does not rotate but is moved entirely laterally relative to the beam. Guide pistons 53 are mounted to the strongback and are engaged in journal sleeves 62 secured to the beam to support the strongback and to assure that movement of the strongback relative to the beam is purely translatory.

As indicated above, the toggle link mechanisms 40 and 42 connected to the upper and lower closure plates, respectively, are similar to toggle link mechanisms 52. Guide pins (not shown) are slidably connected between the closure plates and the adjacent foundation structure and serve the same function as do guide pistons 53.

Assuming that door beam 49 is disposed in the position shown in FIG. 5, operation of toggle link mechanisms 52 moves the mold sectionsl'into and out of relation in which the mold sections define a vertically elongated, substantially rectangular molding cavity 65 (see FIG. 9). In such a relation the strongbacks fit together so that the volume enclosed between the strongbacks and between the closure plates is essentially water tight. The contour of the mold cavity is defined by the mold sections.

As shown most clearly in FIGS. 3, 4, and 9, each mold section is substantially similar in construction. Accordingly, only mold section 37 will be described in detail. The differences in conguration of the various mold sections is readily apparent from an examination of FIG. 9 in conjunction with FIG. l. The upper portion of mold section 37 includes an open lattice of horizontal and vertical members 66 and 67, respectively, which preferably are conventional steel fiat bars notched and interfitted with one another in a conventional manner. Horizontal members 66 are configured to define the con-.Pollutions appearing in exterior surface 19 of the conduit unit 15 shown in FIG. 1. The lattice formed by members 66 and 67 Iis mounted in a frame 68 (see FIG. 4) which is bolted to the inner face of strongback 48. A perforated metal plate 69 is laid over the surface of the lattice opposite from the strongback and follows the contour defined by members 66. In a presently preferred embodiment of this invention, the holes in the perforated plate are approximately l/gg in diameter. A wire mesh screen 70 is laid over the perforated plate and, when the mold sections are in mold delining relat-ion to one another, defines a portion of the surface of the mold cavity. In a presently preferred embodiment of the invention, screen 70 is a 200 mesh screen.

As indicated above, the upper and lower mold sections are defined by sintered metal plates such as sintered metal plate 33. Each closure plate 38, 39 is recessed, as at 71 (see FIG. 4), and the sintered plate is `disposed in this recess. An annular clamp plate 72 is disposed over the sintered plate and is secured to the respective closure plate by screws 73 which pass through the sintered metal plate.

In order that coupling portion 20 of the conduit unit shown in FIG. l may be cast into the unit, the lower end of mold cavity 65 is defined by suitably contoured sintered metal blocks 74 and 75 associated with the side and front and with the rear mold sections 34, 35 and 36, 37, respectively.

Blocks 74 and 75 are secured to the corresponding strongback so as to abut the lower ends of the mold section lattice frames 68 described above. Strictly speaking, however, each of the vertical mold Section5 is comprised of one block 74 or 75 and the associated screened lattice.

As shown in FIG. 2, a plurality of duct dening cores A-80F are carried in the upper portion of casting apparatus 25. The cores correspond in num-ber and in geometrical arrangement one to another to the number and arrangement of ducts 16A-MF in conduit unit 15. The cores are .movable through a single opening (not illustrated) in top plate 29 and through a corresponding plurality of openings (not illustrated) in upper closure plate 38 into and out of mold cavity 65. In order that the cores may be moved into and out of the cavity, each core has its upper end connected to a corresponding one of a plurality of hydraulic rams SIA-SIF. The rams are mounted to a ram carrier plate 82 which is supported above top plate 29 by a plurality of columns 83.

Cores SOA-SOP are substantially identical and, therefore, only core 80A is illustrated and described in detail. As shown in FIGS. 7 and 8, core 80A is a rigid elongated cylindrical member. The core has a conduit unit duct defining portion S6 which is of constant diameter and which extends lalong the major portion of the length of the core. The diameter of core por-tion 86 corresponds to the diameter of the duct which the core is provided to define in conduit unit 15. Accordingly, core portion 86 has a length at least as long as the length of the conduit unit to be fabricated in apparatus 25. Preferably core portion 86 is coated with a material, such as tetrafluoroethylene, which has a low coefficient of friction and which facilitates removal of the core from the cast preform. The core has a lower end 87 adjacent which the core defines a cylindrical reduced diameter portion 88 configured to mate with a corresponding aperture (not shown) formed through lower closure plate 39. An O- ring 89 is disposed in a circumferential recess around reduced diameter portion 88 so that, when the core is engaged with closure plate 39, a water-tight seal relative to mold cavity 65 is maintained. Similarly, a sleeve 90 is provided around the core adjacent the upper end of core portion 86. The sleeve is configured to mate with the corresponding aperture defined in closure plate 38 through which the core is inserted into cavity 65. An O- ring 91 is disposed around the sleeve to maintain a water-tight seal with 4the upper closure plate when the core is disposed in the mold cavity.

As shown in FIG. 7, core 80A is hollow at least from adjacent the lower end of the core portion 86 to lower end 87 by virtue of a bore 93 formed in the lower portion of the core. The bore is a portion of means which are provided for draining from mold cavity 65 water which is expressed from concrete slurry in the mold cavity during that stage of manufacture of a conduit unit in which the slurry is dried to define a self-supporting conduit unit molding or preform. Additional portions of this drainage system are defined by a pair of slots 94 which are formed in the exterior of core portion 86. The slots preferably extend along the elongate extent of the core and are disposed approximately 90 apart from each other around the circumference of the core. The lower ends of the slots communicate with bore 93 through lateral passageways 95. A screen 96, preferably of the same mesh as screen 70 described above, is disposed across the opening of each of slots 94 to the exterior of core portion 86. The screens preferably are flush with the exterior surfaces of the core. A screen backup bar 97 extends longitudinally of each slot and engages the rear surface of the screen to prevent the screen from being lbuckled inwardly into the slot by the pressures which are applied to the screen during the fabrication of a conduit unit.

Additional portions of the water drainage system referred to above are illustrated in FIG. 4. Horizontal members 66 of the lattices of the mold sections, e.g., section 35, and lattice frames 68 are drilled out or notched along the interfaces between the lattices and strongbacks. The rear surfaces of blocks 74 and 75 are vertically grooved, as at 100, in line with the lattice and frame notches. The lower ends -of grooves 100 open to a peripheral recess 101 formed in the lower end of each of the strongbacks which, when the strongbacks are in mold cavity defining relation, is aligned with a recess 102 formed in the upper surface of lower closure plate 39 peripherally of sintered metal plate 33. A duct 103 communicates with recess 102 and is adapted to be connected `to a suitable` hose or the like for carrying away water entering the duct from recess 102. As water is expressed from concrete slurry present in cavity 65, the water passes through screens 70 and perforated plates 69 into the interior of the lattices of mold sections 34-37. The water then passes downwardly through grooves 100 to duct 103. Walter expressed from the portion of the slurry adjacent sintered metal 'blocks 74 and 75 passes through the blocks within which sintered metal plate 33 is disposed, a plurality of drain notches 104 are formed in the portion of the lower closure plate between Irecess 71 and recess 102 in order that water not be retained in recess 71. Accordingly, water which percolates through sintered metal plate 33 from the slurry passes through notches 104 to recess 102 and to duct 103.

As will be seen from the following description of the operation of casting apparatus 25, one of the mold sections which cooperate to define mold cavity 65 must be movable away lfrom the adjacent mold sections with which it cooperates. Accordingly, door beam 49 is hinged relative to the bed and top plates. As shown in FIGS. 2, 5, and 6, the door beam is mounted at its upper and lower ends to the corresponding ones of a pair of vertically spaced substantially identical door arch members 110. To simplify the illustration of this invention, only upper door arch member is shown; it is to be understood that a descripition of the upper door arch member and its mounting mechanism will sufiice as a descripition of both members. The front strongback is mounted to door beam 49 by -a toggle link mechanism 52 and by guide pistons 53 in accord with the above description.

Referring to FIG. 6, a door hinge shaft 111 is keyed in a vertically bored boss 112 formed at the right end of each door arch member. The opposite ends of the shaft are rotatably mounted by suitable bearings in bed plate 26 and in top plate 29. The top plate defines an annularly bored boss 113 within which the door hinge shaft bearings are disposed. A sleeve 114 is keyed, as at 115, to the upper end of shaft 111 and is adapted to be connected to a rotary shaft (not shown) of a door operating mechanism 116. In a presently preferred embodiment of this invention, mechanism 116 is an Ex-Cell-O Rotac Model RN-lOS-IV hydraulic torque motor.

The end of each arch member 110 opposite from hinge shaft 111 defines a boss 117 through which is formed a vertical bore 118. The top plate and the bed plate each define a boss 119 which is so positioned as to be aligned with the adjacent boss 117 when the door is in its closed position. Each boss 119 is vertically bored as at 120. A door locking pin 121 is mounted in bore 120 by suitable journals for vertical movement into and out of engagement with a suitable journal 122 carried in the adjacent end of door arch member bore 118. A double acting hydraulic ram assembly 124 is mounted to each boss 119 and has a piston 125 connected to a respective one of lock pins 121. Extension of piston 125 moves the lock pin into engagement with the door arch member when the arch members are in the door-closed position to maintain the door beam in the position shown in FIG. 5 against the forces which are applied thereto during the casting of a conduit unit.

As shown schematically in FIG. 9, casting apparatus 25 includes a cement slurry mixer and a slurry pump 131. The pump is disposed to receive the slurry discharged from the mixer and to discharge this slurry to slurry injection nozzles 132 mounted inside mold sections 34 and 35. The pump is connected to the slurry injection nozzles by suitable ducts 133 which, at least adjacent the side strongbacks, preferably are flexible to accommodate the movement of the 131 preferably is a positive displacement pump of the piston type and develops discharge pressures of `from 125-225 lbs. per square inch.

Preferably the concrete mix which is: used in fabricating conduit unit 15 is composed of water, cement, silica, and asbestos. A suitable formulation consists of 50% cement, 34% silica, and 16% asbestos, although the asbestos content may be varied from l0-16% of the weight of the dry constituents of the mix. The dry constituents are mixed thoroughly and then are wet mixed with approximately 23 gallons of water for each 115 lbs. of dry mix. The wet mixing process is carried out as fast as possible (approximately 10-25 seconds) in the high speed mixer. Wet mixing process is critical and is continued only long enough to assure that the dry constituents are disbursed through the water. If the wet mix ing process is continued too long, the absorption of water by the asbestos is excessive and too thick a slurry results. If the slurry is too thick, hydraulic pressure cannot be applied properly to inject the slurry into mold cavity 65. Moreover, the slurry is of a nature such that it must be injected into the molding cavity substantially immediately upon completion of the wet mixing process. If this is not done, the slurry tends to set and to clog the injection nozzles.

Preferably there are four slurry injecting nozzles 132 disposed in vertically spaced apart relation in the central portion of each side mold section 34, 35. it is within the scope of this invention, however, that any number and pattern of injecting nozzles may be used.

The fabrication of a conduit unit is commenced by moving the strongbacks and the mold sections carried thereby into the mold defining relation shown in FIG. 9. The upper and lower closure plates are also moved into position and engage the adjacent ends of the strongbacks to seal the mold cavity. Core rams 81 are then operated to move the cores into position in the mold cavity. During this operation the cores pass through the respective apertures in upper closure plate 38 and O-rings 89 and 91 are engaged with the lower and upper closure plates, respectively. Accordingly, a water-tight mold cavity is provided.

Preferably the cores are oriented in the mold cavity so that screened slots 94 open to those portions of the cavity which correspond to the walls of the ducts 16A-16F located adjacent the central portion of the conduit unit.

Immediately upon completion of the wet mixing of the concrete mix, the slurry is injected as fast as possible into mold cavity 65. The rapid pumping of the slurry into the cavity prevents clogging of the injection nozzles and slurry ducting. The pumping operation continues until a major portion `of the water contained in the slurry in the cavity is expressed from the cavity through the water drainage devices described above. That is, the water is expressed from the slurry solely by means of the pressure developed by the pump provided for injecting the slurry into the cavity. In the presently preferred apparatus, it has been determined that the water expression process is complete when the period between pump strokes is approximately 10 seconds. At this point approximately 20 of the 23 gallons of water present in the slurry have been expressed from the chamber so that the partially dried mix in the molding cavity defines a self-supporting conduit molding or preform which weighs about 140 lbs.

Cores SGA-SGF are then withdrawn from the mold cavity. The slurry has a tendency to expand as the solid constituents absorb water. Because of the expansion of the slurry, the pressures developed within the mold cavity are not completely dissipated when pump 131 is shut down at the end of the water expression process. The pressure exerted upon the cores by the expanded slurry may prevent removal of the cores unless unreasonably great forces are applied to pull the cores from the mold cavity. In order that the cores may be withdrawn by rams of reasonable size, the volume of the mold cavity is increased in a direction laterally of the elongate extents of the cores but not parallel to the cores. In other words, strongbacks -48 are moved a predetermined distance laterally from the cores toward the corresponding vertical beams. The movement of the strongbacks is critical. Preferably they are moved only about /gg-/lg, If the degree of mold cavity expansion is too great, the preform tends to break as the cores are withdrawn from the cavity since the preform then does not have sufficient lateral support by the walls of the cavity. If the degree of cavity expansion is too small, then the pressure exerted on the cores by the expanded but partially dried slurry remains too great and the cores cannot be withdrawn from the cavity.

The upper and lower closure plates are moved only through a distance sufficient to free the strongbacks from clamped relation between the closure plates. It is necessary to move the closure plates only a few thousandths of an inch in order to accomplish this. Accordingly, it may be said that the carrier plates are not moved vertically of the cores as the volume of the mold cavity is increased. if, prior to the removal of the cores from the preform, the closure plates are moved apart a distance greater than that required to unclamp the strongbacks, the preform may be deformed as the cores are moved.

The mold cores are then withdrawn from the mold cavity according to a predetermined sequence. First the core (core A) located at one end of one of the two rows of cores is withdrawn. Then the core (core 801:) located at the other end of the other row is withdrawn. Next the core (core 80C) located at the other end of the one row is withdrawn, after which the core (core 80D) located at the one end of the other row is withdrawn. The remaining core of the one row (core 80B) is then withdrawn. Lastly, the remaining core (core 80E) of the other row is withdrawn. Alternatively, cores 80A and 801:, cores 80C and 80D, and cores 80B and 80E may be withdrawn as pairs.

The upper and lower closure plates 38, 39 are then removed from engagement with the preform by operation of toggle link mechanisms 46 and 42, respectively. The toggle link mechanism which interconnects rear strongback 48 with rear beam 32 is then operated to remove the rear mold section from engagement with the preform. Similarly', the front mold section is moved away from mold defining engagement with side mold sections 34 and 35 and the front of the casting apparatus is opened by operation of door operating mechanism 116. The side mold sections, however, are left in the positions to which they were moved just prior to withdrawal of the cores from the cavity. The side mold sections thus continue to engage the preform and support the preform above lower closure plate 39. A suitable pallet or the like is then inserted below the preform to receive the preform as strongbacks 45 and 46 are moved apart.

The casting apparatus is then prepared for the next casting operation by fiushing slurry ducts 133 with water under high pressure. This is necessary since a portion of the slurry used to cast the conduit unit just removed from the apparatus remains in the ducts, The asbestos present in this residual slurry will have absorbed so much water that the slurry should not be used in casting the next conduit unit. Preferably the casting operation is carried out in approximately 5 minutes and, therefore, the slurry present in the ducts will not have had an opportunity to set sufficiently to prevent fiushing of ducts 133.

In order that most economical use of the concrete mix may be obtained, it is preferred that the length of ducting between slurry injection nozzles 132 and the discharge from pump 131 be kept to a minimum.

The operation of casting apparatus 25 preferably is automatically controlled. An interlocking mechanism is provided in this control system to prevent opening of the front of the apparatus before the pressure applied to the slurry in the cavity has been released. A suitable mechanism (not shown) responsive to the period between strokes of the piston of pump 131 is provided. This mechanism shuts down the pump when the period between pump strokes reaches a duration which corresponds, by empirical determination, to sufficient expression of Water from the cavity. Shutting down of the pump places rams 43 of the upper and lower closure plate toggle link mechanisms in condition for operation so that the closure plates may be moved sufficiently to unclamp the strongbacks. As closure plate toggle link mechanisms 40 and 42 are operated to retract the closure plates following withdrawal of the cores from the preform, a cam mounted to the end of each of lever arms 41 operates a limit switch 141. Operation of the limit switches 141 allows operation of rams 61 and 124 and of door opening mechanism 116 in accord with the above described sequence.

In the apparatus described above, the only mechanism which is provided for expressing water from the wet slurry injected into the mold cavity is pump 131. Water is expressed from the slurry solely by means of pressure applied to the slurry by the same mechanism which is used to inject the slurry into the mold cavity. The mold chamber is not constricted to remove this moisture from the concrete disposed in the cavity. This is contrasted with prior art casting apparatus in which the expression of water often was obtained by a reduction in the volume of the molding chamber. For example, the apparatus described in Patent 2,650,409 issued Sept. 1, 1953 to Carbon C. Dubbs, for example, discloses the use of an expansible rubber sleeve which defines the exterior surface of a cylindrical core and which is expanded to decrease the volume of the molding cavity of the casting apparatus. Operation of such a sleeve frequently caused variations in density in the material molded in the molding cavity. Also the operation of such a sleeve resulted in manufacture of a conduit in which the diameter of each duct in the conduit was greater centrally of the conduit than at the ends of the conduit. The apparatus of the present invention, however, does not suffer from these disadvantages. The concrete in the preform provided by this invention is of uniform density throughout the preform since the slurry is dried to a self-supporting state solely by means of pressure applied substantially uniformly to the slurry. Moreover, cores SUA-SGF have a fixed constant diameter, thereby assuring that the diameter of the ducts formed in the preform are of constant diameter.

After a preform fabricated in accord with the above described procedures has been removed from the casting apparatus, it is autoclaved to drive off the remaining uncombined moisture present in the preform. The finished product weighs about 115 lbs.

The apparatus described above has the feature that it enables the manufacture of dimensionally uniform cast concrete conduits. This dimensional accuracy is provided by the above described method of concrete casting which features the step of expanding the volume of the molding cavity a predetermined amount laterally of the cores prior to the removal of the cores from the mold cavity.

The novel fabrication step of expanding the volume of the mold cavity prior to removal of the cores from the preform permits the use of relative small capacity rams for operating the cores. Such rams have a relatively small diameter and thus may be positioned so that the axes along which their pistons move are close together. This means that the spacing between ducts 16 in conduit unit is dictated only by the thickness of concrete required to provide a structurally adequate device, rather than by the diameter of the rams provided for pulling the cores from the conduit preform. If rams of high capacity were required to pull the rams, the spacing between the conduit ducts very well might necessarily exceed the spacing required in a structurally adequate conduit. Accordingly, no more concrete than is required because of structural considerations need be used in the fabrication of conduits according to this invention. Economical use of concrete means the provision of a lightweight product. l

While the invention has been described above in conjunction with specific apparatus and in conjunction with a specific conduit unit configuration, this has been merely by way of example and is not to be considered as limiting the scope of this invention.

What is claimed is:

1. The method of casting a concrete conduit including the steps of:

(a) forming an enclosed porous mold chamber having an elongated circumferentially nexpansible core extending therethrough,

(b) pressure filling the mold chamber with a wet slurry of concrete,

(c) expressing suficient uncombined water from the slurry in the mold chamber to form a partially dried self-supporting casting by continuously exerting said filling pressure,

(d) slightly increasing the dimensions of the mold chamber transversely of the core, but not lengthwise of the core, by an amount which is less than that productive of separation of the boundaries of the chamber from the partially dried casting,

(e) withdrawing the core from the mold chamber, and

(f) removing the partially dried casting from the mold chamber.

2. The method of casting a concrete conduit including the steps of:

(a) forming an enclosed porous mold chamber having an elongate circumferentially core extending therethrough,

(b) injecting a wet slurry of concrete into the mold chamber under pressure,

(c) applying further pressure on the slurry in the chamber to express sufficient uncombined water therefrom to form a partially dried self-supporting casting,

(d) slightly increasing the dimensions of the mold chamber in all directions transversely of the `core but not lengthwise of the core by an amount which is less than that productive of separation of the boundaries of the chamber from the partially dried casting,

(e) withdrawing the core from the mold chamber, and

(f) removing the partially dried casting from the mold chamber.

3. The method of forming a cast concrete conduit including, in sequence, the steps of:

( a) forming an enclosed porous mold chamber having a plurality of substantially cylindrical circumferentially nexpansible spaced cores extending therethrough,

(b) pumping a wet slurry of concrete into the mold chamber under pressure to fill the mold cham-ber,

(c) applying further pressure to the slurry in the chamber from the pump to express sufficient uncombined water from the slurry to form a partially dried self-supporting conduit preform,

(d) slightly increasing the dimensions of the mold chamber in all directions circumferentially of the cores but not lengthwise of the cores by an amount which is less than that productive of separation of the transverse boundaries of the chamber from the partially dried casting so that the casting is supported in the chamber by engagement with said boundaries,

(e) withdrawing the cores from the mold chamber according to a predetermined sequence,

(f) removing the preform from the mold chamber,

and

(g) heating the preform to drive off any uncombined water remaining therein to harden the preform to a finished product.

4. A method according to claim 3 wherein six cores extend through the mold chamber, the cores being arranged relative to one another in two rows of three cores each with the cores in one row being aligned adjacent the corresponding cores in the other row, wherein the steps of withdrawing the cores from the mold chamber include the steps of, in sequence:

(a) withdrawing the core located at one end of one row,

(b) withdrawing the core located at the other end of the other row,

(c) withdrawing the -core located at the other end of the one row,

(d) withdrawing the core located at: the one end of the other row,

(e) withdrawing the remaining core from the one row, and

(f) withdrawing the remaining core from the other row.

5. A method according to clai-m 3 wherein six cores extend through the mold chamber, the cores being arranged relative to one another in two rows of three cores each with the cores in one row being aligned adjacent the corresponding cores in the other row, wherein the step of withdrawing the cores from the mold chamber includes the steps of, in sequence,

(a) simultaneously withdrawing the core located in one end of one row and the core located at the other end of the other row,

(b) simultaneously withdrawing the core located at the other end of the one row and the core located at the other end ofthe other row, and

(c) simultaneously withdrawing the remaining cores.

6. The method of forming a cast concrete conduit including the steps of:

(a) forming an enclosed porous mold chamber having an elongate circumferentially inexpansible core extending therethrough,

(b) injecting a wet slurry of concrete into the mold chamber under pressure by means of a pump,

(c) expressing sufficient uncombined water from the slurry in the mold chamber to form a partially dried self-supporting conduit preform solely by applying further pressure from the pump to the slurry,

(d) slightly increasing the dimensions of the mold chamber a predetermined amount transversely of the core but not lengthwise of the core by an amount which is less than that productive of separation of the boundaries of the chamber from the partially dried casting,

(e) withdrawing the core removing the partially dried casting chamber.

7. The method of forming a cast concrete conduit including the steps of:

(a) forming an enclosed porous mold chamber having a plurality of elongated circumferentially inexpansible cores extending therethrough,

(b) mixing a wet slurry of concrete,

from the mold chamber, and from the mold (c) substantially immediately injecting the slurry into the mold chamber under pressure by means of a pump to ll the mold chamber,

(d) expressing sufficient uncombined water from the slurry in the mold chamber to form a partially dried self-supporting conduit preform solely by applying further pressure from the pump to the slurry,

(e) slightly increasing the dimensions of the mold chamber a predetermined amount in all directions `circumferentially of the cores but not lengthwise of the cores by an amount which is less than that productive of separation of the transverse boundaries of the chamber from the partially dried casting so that the casting is supported in the chamber by engagement with said boundaries,

(f) withdrawing the cores from the mold chamber according to a predetermined sequence,

(g) removing the preform from the mold chamber,

and

(h) heating water remaining finished product.

the preform to drive off any uncombined therein to harden the preform to a References Cited UNITED STATES PATENTS 1,864,365 6/1932 Montgomery 264-86 2,731,699 1/1956 Dubbs 264-87 2,983,021 5/1961 Millard 264-87 2,052,818 9/1936 Freyssinet 264-86 722,464 3 /1903 Stevens 264-87 2,964,822 12/ 1960 Tompkins 264-86 2,650,412 9/1953 Dubbs 264-87 2,731,699 1/1956 Dubbs 264-87 2,983,021 5/1961 Millard 264-87 ROBERT F. WHITE, Primary Examiner. K. I. HOVET, Assistant Examiner.

U.S. Cl. X.R.

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