Method of forming the primary core of a prestressed concrete pipe

Abstract

The present invention relates to a method of forming the primary core of a concrete pipe. A steel cylinder is disposed vertically on the base of the pipe forming machine. A conventional rotary packerhead having a diameter less than the inner diameter of the steel cylinder is lowered concentrically to the bottom thereof. Concrete is then introduced within the steel cylinder through the upper end thereof. The packerhead is rotated to force the concrete into the annular gap between the packerhead and the steel cylinder. As the primary core is formed, the packerhead is raised and rotated while concrete is introduced within the steel cylinder, thereby forming a primary core consisting of a steel cylinder having an inner lining of concrete. In an alternative embodiment, the steel cylinder is completely embedded within the concrete.

Description

FIG. 4 is a sectional view illustrating a concrete pressure pipe having a primary core consisting of a metal sleeve having an inner concrete lining and made in accordance with the method shown in FIGS. 1 and 2; and

FIG. 5 is a sectional view illustrating a concrete pressure pipe having a primary core consisting of a metal sleeve with inner and outer concrete linings and which may be made in accordance with the method shown in FIG. 3. If desired, a joint ring (not shown) can be welded to the bottom end of the steel cylinder 10 to effect a secure connection between pipe sections. The joint ring is conventional in the art and consists of an annular sleeve having an increasing diameter portion, the smaller end of which is welded to the steel cylinder 10. A rubber gasket (not shown) can be utilized to prevent fluid leakage from the joint.

A conventional packerhead of the type generally illustrated and described in U.S. Pat. No. 3,276,091, indicated generally at 14, is then lowered concentrically within the cylinder 10 to the base 12. The packerhead 14 includes a rotating cylindrically shaped base member 16, a plurality of radially spaced rollers rotatably secured to the upper side of the base member 16, and a circular plate 20 disposed above the plurality of rollers 18. The circular plate 20 has a plurality of apertures formed therein for allowing one or more upstanding concrete slingers 22, each attached to a respective one of the rollers 18, to extend therethrough above the circular plate 20. The entire packerhead 14 is mounted on a rotatable shaft 24 extending throughout the length of the steel cylinder 10.

After the packerhead 14 is lowered to the bottom of the steel cylinder 10, concrete 26 is introduced into the upper end of the steel cylinder 10. The concrete 26 falls under the influence of gravity onto the circular plate 20. As the packerhead 14 is rotated, some of the concrete 26 is effectively forced by the slingers 22 into the annular gap between the outer wall of the packerhead 14 and the inner wall of the steel cylinder 10. As is known, the shaft 24 rotates the base member 16 rotates in one direction while the rollers 18 are caused to rotate in an opposite direction to suitably radially compress the concrete 26 against the inside wall of the steel cylinder 10 to form a lining thereon. As the concrete begins to cure, the packerhead 14 is raised upwardly throughout the steel cylinder 10 while continuing to be rotated. In this manner, the concrete lining is applied to the entire length of the steel cylinder 10. The entire lining process typically occurs in less than five minutes. Once the lining is applied to the steel cylinder 10 and has sufficiently cured, the lined primary core can be removed and another steel cylinder 10 to be lined can be positioned in the apparatus. The lined primary core may then be allowed to naturally cure or be placed in a kiln to effect an accelerated final curing.

Referring now to FIG. 3, there is illustrated an apparatus for forming the primary core of a concrete pipe wherein the steel cylinder 10 is completely embedded within the concrete 26. An outer mold 28 is disposed coaxially about the steel cylinder 10 to create an annular gap therebetween. The concrete 26 is then introduced within the steel cylinder 10 and between the steel cylinder 10 and the outer mold 28. Vibrating means (not shown) can be provided to vibrate the outer mold 28 in an known manner to compact the concrete 26 between the steel cylinder 10 and the outer mold 28.

Typically, the standard length of the steel cylinder 10 is twenty feet. The inner diameter of the primary core generally ranges from twenty-four to sixty inches. It has been found desirable to increase the total wall thickness of the primary core, including the steel cylinder 10, in proportion to the inner diameter of the core. Typically, the total wall thickness of the primary core is approximately one-sixteenth of the inner diameter of the core. Thus, a primary core having an inner diameter of twenty-four inches would have a total wall thickness of one and one-half inches while a primary core having an inner diameter of forty-eight inches would have a total wall thickness of three inches.

It has been found desirable to utilize a "zero slump" concrete in the manufacture of either type of primary core. Zero slump concrete is drier than concrete used in the past in the manufacture of prestressed concrete pipe, requiring less cement and, thereby, creating a resultant obvious economic advantage. Also, the curing time to achieve a specified compressive strength required for prestressing is dramatically less with the utilization of zero slump concrete as opposed to high slump concrete. Furthermore, the utilization of zero slump concrete eliminates a recognized imperfection in prestressed concrete cylinder pipe known as slump cracking. Slump cracking refers to the wide spread occurrence of small fractures following circumferential prestressing of the primary core manufactured by the horizontal centrifugal method with high slump concrete.

A typical mixture of zero slump concrete consists of approximately 72 parts cement, 146 parts aggregate, and 208 parts sand, together with one part water by weight. For example, to form one cubic yard of zero slump concrete, the constituents would include 1875 pounds of sand, 1312 pounds of aggregate, 650 pounds of cement, and approximately nine gallons of water, depending upon the moisture content of the aggregate. Suitable results have been achieved by utilizing five-eighths inch diameter crushed stone aggregate and Portland Type Two cement.

After the primary core has been formed in either of the above-discussed methods, the core is prestressed by wrapping it with high tensile strength wire 30 (as shown in FIGS. 4 and 5) in a helical form at a predetermined spacing and tension for substantially the full length of the core. A cement mortar coating 32 (as shown in FIGS. 4 and 5) is applied to the primary core after it has been wrapped under tension with the high tensile wire. Typically, the mortar used consists of one part cement and not more than three parts of fine aggregate. The mortar is applied against the exterior surface of the primary core to produce a dense coating around the prestressed wires and cylinder. Concurrently with the mortar coating, a cement slurry is applied to the core. According to the conditions of the laying of the pipe in the ground and the chemical composition of the ground, the slurry may include epoxy resins which increase the mechanical and chemical resistance of the coating. The outer coating is cured in a manner similar to that described above. If necessary, a layer of bituminous material can be applied to the exterior of the cured coating to further protect the pipe from particularly aggressive chemicals in the ground.

The combination of a vertical pour of zero slump concrete with radial compaction of the concrete against the steel cylinder has several distinct advantages. First, the method of the present invention eliminates the void which commonly occurs between the steel cylinder and the concrete as the pipe structure cures in a horizontal disposition. Such a void is generally believed to be caused by the effect of gravity. The most dramatic manifestation of the phenomenon is the complete failure of the concrete to bind to the steel cylinder after curing. When this occurs, the concrete must be cleaned from the cylinder so as to salvage the cylinder for recycling.

Second, utilizing the method of the present invention produces a primary core which is homogeneous throughout its structure, thereby contributing to the strength of the finished product. With the conventional horizontal centrifugal method, there is a tendency for the coarse aggregate to move outwardly toward the surrounding steel cylinder, while the finer components of the aggregate are caused to be disposed inwardly towards the center or inner surface of the primary core. Such centrifugal action thereby causes a non-homogoneous mixture detrimentally affecting the overall strength and integerity of the primary core. Also, the finer components of the aggregate disposed adjacent the inner surface of the primary core by the previously known methods are predisposed to flaking and cracking. Furthermore, the inner surface of such a primary core must be sandblasted if the application of an epoxy or bituminous coating is desired. Such sandblasting is not required with primary cores formed in accordance with the present invention.

In accordance with the provisions of the patent statutes, the principle and mode of operation of the present invention have been explained and illustrated in its preferred embodiment. However, it must be understood that the present invention can be practiced otherwise and as specifically explained and illustrated without departing from its spirit or scope.

Claims

The method according to claim 4 including the step of forming one cubic yard of the zero slump concrete supplied in step (b) of a mixture of 1875 pounds of sand, 1312 pounds of five-eighths inch diameter crushed stone, 650 pounds of Portland Type Two cement, and approximately nine gallons of water.

1. A method of forming an adherent concrete lining on the inside surface of a cylindrical metal pipe and an adherent concrete casing about the outside surface of the metal sleeve the primary core of a prestressed concrete pipe comprising the steps of:

(a) positioning a cylindrical metal sleeve having a substantially fluid impervious side wall and a predetermined height in substantially vertical and coaxial relationship within a cylindrical mold having substantially the same height as the metal sleeve such that the outer surface of the metal sleeve is spaced from the inner surface of the mold;

(b) supplying concrete to the space between the outer surface of the metal sleeve and the inner surface of the mold while vibrating the mold to compress the concrete therebetween to form an adherent concrete casing about the outer surface of the metal sleeve;

(c) simultaneously with step (b), supplying a zero slump concrete to the interior of the metal sleeve;

(d) simultaneously with step (b), radially compressing the concrete in the metal sleeve against the inner surface thereof to form a concrete lining along the length of the inner surface of the metal sleeve; and

(e) removing the metal sleeve having the inner concrete lining and the outer concrete casing from the mold. 2. The method according to claim 1 including the step of forming the zero slump concrete supplied in step (c) of a mixture of approximately 72 parts cement, 146 parts aggregate, 208 parts sand, and one part water by weight.

The method according to claim 1 and further including the steps of:

(f) prestressing the primary core produced in step (e) by wrapping the core with a wire in helical form at a predetermined spacing and tension for substantially the full length of the core;

(g) subsequent to step (f), applying a protective coating to the exterior surface of the primary core to cover the prestressed wire.

. A method of producing a prestressed concrete pipe comprising the steps of:

(a) positioning a cylindrical metal sleeve having a substantially fluid impervious side wall in a substantially vertical orientation on a base of a rotary packerhead assembly;

(b) lowering the packerhead of the packerhead assembly within the metal sleeve to the base of the packerhead assembly;

(c) supplying a zero slump concrete to the interior of the metal sleeve;

(d) simultaneously rotating and raising the packerhead to radially compress the concrete over the inside surface of the metal sleeve to form an adherent concrete lining in the sleeve;

(e) curing the concrete lining on the inside surface of the metal sleeve to form a primary core consisting of the metal sleeve having the inner concrete lining thereon;

(f) prestressing the primary core of step (e) by wrapping the core with a wire in helical form at a predetermined spacing and tension for substantially the full length of the core;

(g) subsequent to step (f), applying a protective coating to the exterior surface of the primary core to cover the prestressed wire. 5. The method according to claim 4 including the step of forming the zero slump concrete supplied in step (c) of a mixture of approximately 72 parts cement, 146 parts aggregate, 208 parts sand, and one part water by

weight. 6. The method according to claim 4 and further including, prior to step (f), the steps of:

(h) positioning a cylindrical mold in a substantially vertical position about the metal sleeve such that the outer surface of the metal sleeve is spaced from the inner surface of the mold to form a uniform gap therebetween;

(i) supplying concrete to the space between the outer surface of the metal sleeve and the inner surface of the mold to form an adherent concrete casing about the outer surface of the metal sleeve; and

(j) removing the metal sleeve having the inner concrete lining and the outer concrete casing from the mold to form the primary core consisting of the metal sleeve having the inner concrete lining and the outer concrete casing thereon. 7. The method according to claim 6 wherein step (i) further includes the step of vibrating the mold to form the concrete casing about the outer surface of the metal sleeve. 8. The method according to claim 6 wherein step (i) is performed simultaneously with step (d). 9. The method according to claim 1 including the step of forming one cubic yard of the zero slump concrete supplied in step (c) of a mixture of 1875 pounds of sand, 1312 pounds of five-eighths inch diameter crushed stone, 650 pounds of Portland Type Two cement, and approximately nine gallons of water. PAR