In a prestressed concrete pressure pipe it is known to have the tensioned wire wrapping around a concrete pipe core terminate short of the bars outside joint-sealing surface at the end of the pipe core, embedded pretensioned wires extending axially of the core, and a tensioned ring either within or outside of the unwrapped or bare end of the core. In the present invention end portions of the pretensioned wires are bent outwardly at a hoop towards the unwrapped surface of the core and the hoop is pretensioned by the wires. The stress in the wires is thereby utilized to impart radially inwardly directed forces emanating from the bent-out portions of the wires and from the hoop to restrain flaring and cracking of the unwrapped end of the core.
The embodiments of the invention in which an
exclusive property or privilege is claimed are defined in
the following claims
1. A prestressed concrete pressure pipe comprising a moulded concrete pipe core with a spigot having an annular surface for engaging joint sealing means, a tensioned wire wrapping subjecting the core to circumferential compression, said wrapping having an end anchored short of said joint sealing means surface, said core containing a plurality of pretensioned wires spaced apart circumferentially of said core and extending substantially the length of the core, a pretensioned hoop contained in the concrete of said core, said hoop substantially concentric to the axis of said core and located axially inward from the end face of the core at the spigot end of the core, said pretensioned wires passing through said hoop in engagement with the inside of said hoop said pretensioned wires having bends at the points of engagement with said hoop from which portions of the wires continuing straight from said bends towards said end face of the core are inclined from their points of engagement with said hoop radially outwardly relative to the axis of said core whereby said inclined wire portions provide stress systems reacting radially inwardly onto the adjacent concrete of the core and together with said stressed hoop exert restraint against flaring of the spigot end of the core which otherwise could be caused by said tensioned wire wrapping.
2. A prestressed concrete pressure pipe according to claim 1, wherein said spigot includes a flange with said joint sealing means surface at the outside thereof.
3. A prestressed concrete pressure pipe according to claim 1 including anchoring means attached to the ends of said pretensioned wires.
4. A prestressed concrete pressure pipe according to claim 1, wherein said hoop is located from said end face a distance substantially equal to the spacing of the spigot end of said wire wrapping from said end face.
This invention relates to prestressed concrete pressure pipe in which pretensioned wires extend longitudinally of the wall of a monolithic concrete pipe core, and the core is circumferentially compressed by a highly tensioned wire wrapping. The core commonly has at least one end finished as a spigot adapted for entering a complementary outer member for forming a joint therewith.
It would be desirable to have the spigot end of the tensioned wire wrapping terminate immediately adjacent the end face of the pipe core if it were not of importance that a finite length of the outer surface of the core be left bare in order to seat a gasket against the concrete of the core for sealing a joint with an outer joining pipe member, such as a bell of another pipe, or a coupling sleeve. The end of the tensioned wire wrapping at the spigot is therefore anchored short of the surface which is provided for seating the gasket. Because of this, residual stresses caused by the tensioned wire wrapping remain in the concrete of the unwrapped spigot portion of the pipe core.
It is not unusual that the residual stresses resulting from the tensioned wire wrapping are high enough to overcome the strength of the core beyond the wrapping irrespective of that portion of the strength which is derived from the axial compressive effect of axially extending prestressed wires in the wall of the core. A consequence of residual stresses is that the spigot end of the core flares outwardly and ring cracks can develop at the inside of the core in the vicinity of the spigot end of the tensioned wire wrapping. Various efforts have been made to eliminate flexural cracking of the nature mentioned as disclosed, for example, in the U.S. Pat. Nos. 3,034,537 and 3,183,011.
One proposed mode for suppressing flaring and cracking invokes the use of a tensioned auxiliary wire binding about the spigot between the end face of the spigot and its gasket-receiving surface, but this entails supplemental winding and fastening procedures and requires that the main tensioned wire wrapping around the core be anchored more distant from the end face of the core than should be necessary.
Another mode for inducing circumferential compression in the spigot end of a concrete core is to form a ring at the ends of the longitudinally prestressing wires by interlinking loops at their ends with loops of other wires to which force was applied to tension the prestressing wires before the core was moulded. That mode is not sufficiently trustworthy because of the uncertainty of the amount of tension that can be imposed in the formed ring and, in any case, the construction would have limited application to the manufacture of pressure pipe for which prestressing wires of relatively light weight would be appropriate.
In accordance with the present invention end portions of longitudinally prestressing wires or rods which enter the spigot of a pipe core engage a hoop and are angularly inclined outwardly from the hoop to their ends which are closer to the end face and the outer surface of the spigot than the hoop. The longitudinal reinforcing for the core thus sets up a pattern of resisting forces in the concrete of the core at its spigot end which includes (a) axial compression, (b) radial bending stress at the end with the tension side at the external surface of the core and the compression side at the internal side, and (c) circumferential compression of the core near its end due to strain in the tensioned hoop. The strength of the spigot end of the core is thereby increased in proportion to the stresses resulting from the three force systems.
The resisting forces developed in the core varies with the stress in the prestressing wires and the angle which the deflected portion of each wire makes with the axially extending portion of the wire.
The highest radial thrust component derivable from deflected portions of the prestressing wires is obtainable with the deflected portions disposed perpendicularly to the axis of the core. In practice the angle of deflection should be within a range of about 15° to 90° and preferably as wide as the class, size and wall thickness of a pipe will permit.
According to one aspect of the present invention there is provided a moulded concrete pipe core with a spigot having an annular surface for engaging joint sealing means, a tensioned wire wrapping subjecting the core to circumferential compression, said wrapping having an end anchored short of said joint sealing means surface, said core containing a plurality of pretensioned wires spaced apart circumferentially of said core and extending substantially the length of the core, a hoop contained in the concrete of said core, said hoop substantially concentric to the axis of said core and located axially inward from the end face of the core at the spigot end of the core, said pretensioned wires engaging the inside of said hoop and having portions thereof deflected radially outwardly from their points of engagement with said hoop and towards said end face at the spigot end of the core.
Other aspects of the invention will be particularly pointed out in the claims appended hereto. The invention itself as to its objects and advantages, and the manner in which it may be carried out, will be better understood by referring to the following description and the accompanying drawing forming a part thereof.
In the drawing, FIG. 1 illustrates a pipe employing the invention with parts cut away for showing interior portions thereof;
FIG. 2 is a sectional view of an end portion of a mould on line 2--2 of FIG. 4;
FIG. 3 is a section of an end portion of a mould on line 3--3 of FIG. 4;
FIG. 4 is an end view of a segment of a mould end ring;
FIG. 5 is a profile of the spigot of a different pipe core; and
FIG. 6 is a profile of the spigot of still another form of a pipe core.
The concrete pressure pipe illustrated in FIG. 1 comprises a moulded concrete pipe core 10 which is reinforced and longitudinaly compressed by a plurality of prestressed wires 11 embedded in and bonded to the concrete of the wall of the core at equal intervals circumferentially around the core. The wires are of high tensile strength steel and the ends of each wire extend close to the respective end faces 12 and 13 of the core. As shown in the drawing, nuts 14 and 15 are attached to threads impressed into each wire, but other supplementary anchoring means may be used.
The left end of the pipe is fashioned as a spigot of sufficient length for entering into a bell of another pipe or other complementary member for making a joint therewith. There may be a spigot at each end of a pipe, or a spigot at one end and a bell at the other end which, in the pipe shown in FIG. 1, is a steel bell ring 16.
The spigot includes a flange 17 having an annular recess 18 for receiving a joint sealing means, such as a rubber gasket, and providing an annular surface 19 on the concrete core onto which the gasket may be seated.
The pretensioned wires 11 are bonded to the concrete of the core. The wires engage the inside of a pretensioned hoop 20 contained in the core. At the points of engagement of the wires with the hoop the end portions of the wires are directed angularly outwardly from their longitudinally disposed portions. The hoop is constructed from one or several rings of a high tensile strength steel wire, the ends of which are spliced to complete the hoop.
The hoop 20 is subjected to tangential tension when the wires 11 are tensioned in a mould before the mould is filled and the concrete hardens to form the core. When the core is formed and removed from the mould the inwardly directed force of the hoop is resisted by the concrete of the core.
A tensioned wire wrapping 21 extending between the fixed anchorages 22 and 23 circumferentially compresses the core and the bell ring of the pipe. The wrapping is applied in a manner known in the art by helically winding a tensioned high tensile strength steel wire around the core and the bell ring after the core has been thoroughly cured.
The spigot end of the wire wrapping at the anchorage 22 is necessarily located a short distance from the end face 12 of the core in order to leave bare an outside surface of the core for seating the sealing means which subsequently will be used to seal a joint with another pipe. A mortar or other protective coating 24 covers the full length of the tensioned wire wrapping 21. The edge 25 of the coating forms a side wall at one side of the gasket-bearing surface 19.
The pipe core is moulded in a rotatable cylindrical mould of which two different sections through an end ring 26 are shown in FIGS. 2 and 3. Another end ring (not shown) is mounted to the other end of a cylindrical shell 27 which forms the outside central wall of the mould. As thus constituted the mould is adapted for moulding in accordance with the roller suspension machine moulding technique, but the core can be formed in a centrifugal machine or other suitable mould.
Each wire 11 is engaged by a detachable gripper 28 threaded on the wire and having a head 29 which bears on the end ring 26. The far end of each wire is also engaged by a similar gripper which is supported by the end ring at that end of the mould.
The hoop 20 is placed in the mould to the outside of the wires 11 before the ends of the wires are gripped. It is held spaced from the inside radial surface 30 of the ring 26 by a number of tie-wires 31, one of which is shown in FIG. 3. An end of each of the wires 31 is twisted around and lashed securely to the hoop, and the other end of the wire is gripped by a removable anchoring pin 32 which is supported on the mould ring 26.
Each individual prestressing wire 11 is initially stressed under a measurable tension prerequisite for meeting the specifications for a particular pipe design. The number of longitudinal wires used in a core is dependent upon variable factors such as their size, the size and wall thickness of the core, the beam and crushing loading of a known expected earth cover and the maximum internal water pressure expected for a particular job. For a 48-inch diameter core designed to contain 281/4 inch prestressing wires about seven equally distributed tie-wires 31 suffice for retaining the hoop in position when the prestressing wires are subjected to tension in the mould, but more or less would serve equally well.
Upon initially tensioning the wires 11 by applying traction to their grippers at one end of the mould, a radial outward component of the applied tension is exerted on the engaged hoop 20 and the hoop is thereby subjected to a high tangential tension which subsequently exerts circumferential compression on the concrete of the core when the core is stripped of the mould.
After the mould has been filled with concrete and the concrete is sufficiently cured, all of the grippers are removed. Upon removal of the grippers 28 the radial outward restraint of the mould on the bent-out portions of the wires is transferred from the mould onto the concrete of the core. Thus the bent-out portions of the wires exert radially inward forces on the spigot end of the core which are effective to repress circumferential expansion or flaring of the spigot end of the core when the tensioned wire wrapping 21 is applied.
In FIG. 5 there is shown the profile of the spigot end of another concrete pipe core 33 having a relatively wide flange 34 and an annular gasket-receiving recess 35 with side walls constituted of the material of the core. The terminal coil 36 of the tensioned wire wrapping 37 is located adjacent the flange and the pretensioned hoop 38 is located in or near the diametrical plane of the terminal coil.
Tension was applied to the prestressed hoop 38 and the prestressed wires 39 in the manner previously described. In this modification the bond of the concrete to the wires is relied on for maintaining the wires and their deflected portions 40 stressed in tension. Should the pipe design and pressure requirements warrant, the ends of the deflected portions may be provided with anchoring means, either inside or outside of the core.
FIG. 6 shows an application of the invention in a concrete pipe core having a straight-walled spigot 41 and an 0-ring gasket 42 resting in a groove formed in the core. In this type of pipe the terminal coil 43 of the tensioned wire wrapping 44 is more removed from the end face 45 because a longer exposed surface of the core is needed to accommodate a socket of a joining pipe. The hoop 46 is located within the wall of the pipe as close to the end face as practicable. The presence of the pretensioned hoop and of the deflected portions 47 of the pretensioned reinforcing wires 49 and their reaction on the concrete inhibits the cracking of the concrete at the inside of the spigot.
The invention results in strengthening the spigot and effecting economy in the manufacture of any type of prestressed concrete pressure pipe having a circumferentially compressed concrete core with an exposed or bare surface at the spigot and compressed longitudinally by pretensioned wires or rods.
The invention may also be practiced if the various wire elements in the concrete pipe were replaced with equivalent high strength materials such as strands of glass filaments or fibers.