US3520968A - Method of manufacturing self-stressed concrete pipe - Google Patents

Description

July 21, 1970 M. s. KRESTON METHOD OF MANUFACTURING SELF-STRESSED CONCRETE PlPE Filed Aug. 31, 1967 mm: S

I NVEN TOR.

United States Patent 3,520,968 METHOD OF MANUFACTURING SELF-STRESSED CONCRETE PIPE Max S. Kreston, Retllands, Calif., assignor t0 Stressed Pipe Research, Ltd., Littleton, Col0., a limited partnership of Texas Filed Aug. 31, 1967, Ser. N0. 664,701 Int. Cl. B28b 9/04 US. Cl. 264-228 3 Claims ABSTRACT OF THE DISCLOSURE A method of manufacturing concrete pipe comprising expansible concrete and internally, constraining reinforcing in which the pipe is initially cast in a conventional manner within a form; then, prior to the major growth period of the concrete, the form is removed and a longitudinal constraint is applied externally of the concrete to supplement the longitudinal, internal constraint afforded by the reinforcing; then, after the growth cycle is substantially completed, the external constraint is removed.

RELATED INVENTIONS This invention is related to the copending application of Edward K. Rice and William C. Cureton, entitled: Self- Stressed Concrete Pipe and Method of Manufacture, Ser. No. 453,705, filed May 6, 1965.

BACKGROUND OF THE INVENTION This invention utilizes an expansive concrete. Expansive concrete contains ingredients which cause the concrete to expand or grow during a critical period ranging from a few hours to several days. The amount of expansion may be merely enough to compensate for the shrinkage that normally occurs, or may be substantially greater; in fact, the expansion may be so excessive that unless the concrete is constrained during a critical period of its growth cycle, the concrete will disintegrate.

Examples of cement formulations which produce expansive concrete of the type suitable for use with the present method are found in Pats. 3,155,526; 3,251,701; and 3,303,037 issued to Alexander Klein. It has been found, for example, by tests conducted by T. Y. Lin and A. Klein and reported in the A.C.I. Journal, Proceedings vol. 60, No. 9, September 1963, pages 1187-1218; that concrete bodies of simple shape can be formed with highly expansive concrete if the concrete is cast about reinforcing steel, and the concrete is properly bonded to the steel before appreciable expansion takes place. The resulting concrete body is held in compression by the reinforcing and the reinforcing is placed under tension.

This type of product has been referred to as chemically stressed or self-stressed concrete, as distinguished from mechanically pretensioned or posttensioned concrete. It is well known that concrete has extremely high strength in compression, but no appreciable strength in tension. This has brought about the use of pretensioned and posttensioned structures which are so designed that under the intended loads, the concrete is held under compression by highly stressed steel tendons.

Previous use of posttensioned and pretensioned concrete pipe has established the advantages of such pipe or other pipe materials to carry or contain fluids under pressure; however, the cost thereof has been excessive. In theory, a self-stressed concrete pipe should have the advantages of conventionally stressed concrete pipe, yet previous attempts to produce a commercially acceptable self-stressed concrete pipe have not been successful due to the difficulties encountered in obtaining proper conice straint throughout the concrete pipe, especially at the ends thereof.

SUMMARY OF INVENTION The present invention is directed to a method of manufacturing self-stressed concrete pipe and included in the objects of the invention are:

First, to provide a method of manufacturing selfstressed concrete pipe whereby the concrete, including the concrete at the ends of the pipe is placed under compression and held so by the internal reinforcing so that the resulting pipe does not delaminate, disintegrate, or develop cracks even though subject to internal pressures of a magnitude tending to neutralize the compression loads within the concrete.

Second, to provide a method of manufacturing selfstressed concrete pipe which can be employed in combination with, or as a supplement to, most of the conventional processes now used in the manufacture of concrete pipe of all sizes; for example, but not limited to centrifugal casting, vertical casting using inner and outer forms, and packer head or tamp machines, or any repetitive process wherein concrete is cast in forms and the forms are removed and reused.

Third, to provide a method of manufacturing selfstressed concrete pipe, whereby axial expansion of the pipe is controlled not only by the permanent internal reinforcing within the walls of the pipe, but also by temporary external restraint, which may be, but is not limited to, threaded steel rods with locking nuts.

Fourth, to provide a method of manufacturing selfstressed concrete pipe which by selection of the amount and arrangement of internal reinforcing and temporary external restraint, the strength of the pipe may be varied to meet a wide range of combined internal pressure loads, earth loads and superimposed live loads.

Fifth, to provide a method of manufacturing selfstressed concrete pipe which adds only a minimal cost to the method of making conventional pipe yet materially increases the strength of the concrete pipe so that the self-stressed concrete pipe may be used in many installations where previously only steel pipe could be used.

DESCRIPTION OF FIGURES FIG. 1 is a fragmentary sectional view of a centrifugal casting mold with a completed pipe therein.

FIG. 2 is a similar fragmentary sectional view showing the concrete pipe after removal from the mold and provided with end and longitudinal constraints to control tensioning of the reinforcing during the expansion cycle of the concrete.

FIG. 3 is a partial end, partial sectional view of the concrete pipe in the condition shown in FIG. 2.

SPECIFICATION The method of manufacturing self-stressed reinforced concrete pipe may utilize any of the conventional apparatus employed to mold or form the pipe. For purposes of illustration, a centrifugal casting mold is shown. This conventional type of mold includes an outer form 1, comprising two or more sections which are joined by bolts 2, extending through flanges 3 at the confronting margins of the form sections. If the pipe to be cast is of the bell and spigot type, the form is enlarged at one end as indicated by 4 to cast the bell end of the pipe. If the pipe is of the double spigot type, then both ends of the mold are identical.

In order to contain the concrete, the outer form is provided with a spigot forming band 5 and a spigot ring 6. In conventional practice, the band 5 and ring 6 may be integral, but in the exercise of the present invention, it is desirable that they be separated. Suitably cemented or otherwise held in place within the band 5, is a band 7 which forms the seal ring groove.

A bell end ring 8 is fitted within the enlarged or bell end 4 of the outer form. The bell end ring may be conventional and includes an outwardly directed flange 9 joined to a bell forming band 10, which in turn is joined to an internal flange 11, offset from the flange 9.

As in conventional practice, prior to casting the pipe, a reinforcing cage is placed in the form. The reinforcing cage includes longitudinal reinforcing members 12, and circumferentially wrapped reinforcing 13.

In the casting of self-stressed concrete pipe, expansive concrete is substituted for conventional concrete. The term expansive concrete, as herein used, refers to concrete containing a portland-type cement to which has been added ingredients which cause the cement and the concrete containing the cement to expand in an amount sufficient to cause reinforcing contained in the concrete to be placed under tension. That is, the reinforcing prevents the concrete from expanding freely with the result that after the expansion cycle, the reinforcing is under tension and the concrete is under compression.

The expansion cycle actually starts upon mixing the concrete; however, the effective exansion begins as the concrete undergoes initial set and begins to bond to the reinforcing during a period beginning a few hours after the concrete has been mixed and continues for about twenty-four to seventy-two hours thereafter, or longer by selection of the various formulas as described in the aforementioned Klein patents. It is possible by controlling the percentages of the various ingredients to predetermine the duration of the expansion cycle and the amount of expansion within fairly close limits and also to control other characteristics of the concrete mixture. For example, it is possible to effect early bonding of the concrete to the reinforcing steel so that the steel may act as a constraint early in the expansion cycle.

Expansive concrete of the type suitable in the practice of the present invention, has been fully set forth in the hereinbefore mentioned Klein patents.

In the practice of the present invention, the concrete pipe, designated 14,- is removed from the form 1 after it has set sufficiently to be self-supporting, but before any significant expansion has occurred. The band is removed, but the ring 6 is retained as well as the bell end ring 8. The pipe thus removed includes a spigot end 15, having a gasket recess 16 and a bell end 17, having a bell cavity 18. However, the pipe may be of the double spigot type.

Before or at the beginning of the expansion of growth cycle, end restraining rings 19 are placed over the ring 6 and the internal flange 11 of the bell end ring 8, as shown in FIG. 2. The end restraining rings are joined by tie rods 20 which are secured by nuts 21. The rings 19 may be annular or may be solid plates, also these rings may be integral with the ring 6 and the bell end ring 8. The tie rods may be initially adjusted so that initially they are free of tension or may be tensioned to some selected amount by a torque wrench.

The number and diameter of the tie rods may vary depending upon the size of the pipe and also may vary depending upon the internal pressure for which the pipe is designed.

In some instances, the end restraining rings and tie rods may be mounted before the pipe is removed from the outer form 1. It is essential, however, that the pipe be removed from the outer form before the form acts as a significant constraint against radial exansion of the concrete for the reason that it is intended to place the circumferential reinforcing 13 under tension. That is, the the circumferential reinforcing is preferably the sole radial constraining means. This does not preclude the use of an outer form as a supplementary constraint providing that the form is capable of expanding in response to the forces generated within the concrete to permit proper tensioning of the circumferential reinforcing 13.

Usually, however, ample constraint may be obtained from the reinforcing 13.

Longitudinal constraint of the concrete pipe poses a more difficult problem. In the case of the circumferential reinforcing, the bond between the wire and the concrete is subject to loads which is transverse to the wire, whereas the bond between the longitudinal reinforcing and the con crete is longitudinally of the wire, that is, the bond is subjected to shearing forces and is of course, much less resistant to shearing forces than to forces applied perpendicular to the reinforcing. As a consequence, the longitudinal reinforcing, even though wrapped with circumferential reinforcing, is not adequate to hold the concrete so that the bond is destroyed or at least damaged so that control over the expansion of the concrete at the ends of the pipe is lost or impaired and the ends of the pipe are weakened.

By supplementing the longitudinal constraining effect of the longitudinal reinforcing 12 with the constraint afforded by the tie rods 20 and further by distributing the load over the ends of the concrete pipe by means of the rings 19, the spigot end ring 6, and the bell end ring 8 during the growth or expansion cycle, the proper bond is maintained and the optimum longitudinal compressive stress is setup in the concrete.

As indicated previously, the present method may be added to or supplement various methods of molding concrete pipe. If a vertical method is used, employing inside as well as outside forms, at least the outside form need be removed. The inside form may remain intact, as the concrete pipe expands away from the inside form. If the inside form remains intact, the tie bolts or tension bolts 20 may be placed in a ring outside the pipe rather than within the pipe. If a packer head method of forming pipe is used, no inside form is required so that control of the compression stresses in the concrete and tension stresses in the reinforcing may proceed in the same manner as that described in connection with the centrifugal casting of the concrete pipe.

While particular embodiments of this invention have been shown and described, it is not intended to limit the same to the details of the constructions set forth, but instead, the invention embraces such changes, modifications and equivalents of the various parts and their relationships as come within the purview of the appended claims.

I claim:

1. A method of manufacturing self-stressed reinforced concrete pipe utilizing a concrete, which, after being cast and set to self-sustaining condition, undergoes a chemical expansion cycle and, early in the expansion cycle, bonds to reinforcing contained therein; then, during further expansion, places the reinforcing under tension and the concrete under compression, said method characterized by:

(a) introducing said expansive concrete into a mold which contains a reinforcing cage having circumferential and longitudinally extending members, and

fully embedding the cage in the concrete;

(b) placing restraining end elements in external abutment with the ends of said cast pipe at the beginning of said effective expansion cycle;

(c) connecting said end elements by tension members, externally of said concrete, to restrain longitudinal expansion;

(d) removing the concrete body from the mold early in the expansion cycle and after it has set to selfsustaining condition;

(e) restraining the concrete solely by the internal reinforcing, and elements and tension members during the remainder of the effective expansion cycle;

6 (f) and removing said end elements and tension mem- (d) and removing said end constraints and tension elebers at the end of said effective expansion cycle. ments at the end of said effective expansion cycle. 2. A method, as defined in claim 1, wherein: (a) said tension members are placed within the bore References Cited 3 y i p p f hf d t 5 UNITED STATES PATENTS n e manu ac ure 0 rel orce concre e plpe, wherein the pipe is initially formed by a conventional gfi method in which a reinforcing cage is fully embedded in 2474660 6/1949 i i 264 42 the codngrete, the method of self-stressing the pipe charac- 27O9845 6/1955 ig: 56 X terize y:

(a) employing a concrete, which, after being cast and 10 329084O 12/1966 Mlddendorf 264228 X set to self-sustaining condition, undergoes a chemi- FOREIGN PATENTS cal expansion cycle and, early in the expansion cycle, 559,213 6/1958 canada bonds to reinforcing contained therein; then, during further expansion, places the reinforcing under ten- 15 OTHER REFERENCES sion and the concrete under compression; Chemical Prestressing of Concrete Elements Using (b) supplementing the longitudinal restraint produced Expanding Cements, Lin, T. Y. and Klein, A., Journal of by said reinforcing cage by placing end constraints the American Concrete Institute, September 1963, pp. at and in external abutment with the extremities of 1200-1203. the pipe and connecting the end constraints by exposed tension elements extending between the ends ROBERT F. WHITE, Primary Examiner of Sald P e; R. R. KUCIA, Assistant Examiner (c) exposing the formed concrete pipe to expansion during the major portion of its expansion cycle ex- S. C X.R.

cept as restrained by said reinforcing cage, end con- 264 229 231 straints and tension elements; i i i

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