US1781699A - Reenforced concrete pipe - Google Patents

Description

Nov. 18, 1930. w. c. ARMLEY 1,781,599

REENFORCED CONCRETE PIPE Filed May 7, 1927 wuemtoz UNITED STATES PATENT-.- OFFICE WALTER C. PAIRMLEY, OF UPPER MON'ICLAIR, NEW JERSEY nnn'uroncnn CONCRETE PIPE Application -fi1ed May 7, 1927. Serial No. 189,613.

This invention relates to a method and means for reenforcing concrete structures, such as conduit pipes, etc., and has for its primary object to provide reenforcing means around the pipes which will materially lessen the tendency of longitudinal cracking of the concrete of which the pipes are constructed.

A further object of the invention is to arrange the reenforcing means in such a manner that the tensile stresses caused by the application of a load upon jthepipes will be sustained by the reenforcing means thereby preventing stressing of the concrete pipes beyond their proper limits.

Another object of the present invention is to permit a predetermined amount of initial compression to be placed within the concrete structure of the pipes particularly at those points which will later be placed under tensile stresses due to the application of a load so that the concrete structure will not be under tension at all, or at least to only a very small degree.

A still further object of the invention is to providereenforcing means about the concrete pipes, which will be substantially free in movement with respect to the concrete by which it is supported so that stresses created in the reenforcing means by the application of a load to the pipe sections will be distributed substantially equally throughout the reenforcing means.

Another object of the inventionis to provide adjustable reenforcing means for the concrete pipe sections so that by proper ad-' justment the concrete will be placed under ini- 'tial compression and the reenforcing means =under initial tension so that the compression in the concrete acts oppositely to the effect produced by internal pressure or by tension moments from external loads and results in greatly strengthening the concrete pipe sections.

With the object above indicated and other of a section of concrete pipe showing the arrangement of the reenforcing means;

Fig. 2 is a transverse cross sectional view taken on line 2 2 of Fig. 1, and showing the fidjusting means for the reenforcing memers;

Fig. 3 is a transverse cross sectional View of a modified arrangement; and

Fig. 4 is a transverse cross sectional view taken on the line 4-4 of Fig. 3.

The general method of manufacture of concrete pipes has been to embed in the molded concrete forming the walls of the pipes a skeleton of reenforcing steel, of mesh or rod type. When the concrete hardens the reenforcement becomes firmly embedded in the concrete. The fundamental principles underlying reenforced' concrete in general are two: 1st, the nearly equal expansion and contraction of the two materials caused by changes of temperature which permits reasonable team work between the two and, 2nd, the fact that there is a great adhesion between the concrete and the reenforcement, thus permitting the reenforcing steel to exert its superior strength most efliciently in reenforcing the concrete at those points where the latter is subjected to the greatest tensile stresses. i

In reenforced concrete pipe work certain, methods as now practiced are objectionable and it is the purpose of this invention to minimize or wholly avoid these objections. 1

place with practically no internal stress being set up within the mass of the concrete. The resulting pipe will therefore, when subjected to bending stresses of loading, be able to withstand these stresses up to the normal limit of the tensile strength of the concrete.

The modulus of the concrete in tension therefore, usually runs two or three hundred pounds per sq. in. and it is this transverse sult that the steel becomes subjected to a can tain amount of initial compression and the concrete to a corresponding initial tension.

This adverse effect of the reeinforcement frequently causes longitudinal cracking of pipes, the tension in the concrete being in excess of its tensile strength. Whether the pipe actually cracks or not before it is subjected tointernal or external loading is more or less immeterial since the first effect of the steel reenforcement is to produce an initial tension in the concrete and cause it to crack at an earlier stage than it would otherwise have done. Furthermore, the cracks when they do occur under loads will be wider than they would be if the reenforcement had not been-in a state of initial compression, and the steel under loading stresses will not actually be stressed up to the limits which are calculated to be its working limits.

Another gain which the invention accomplishes is that it permits a certain amount of initial compression to be put into the concrete particularly at those points where subsequently the concrete will be under tensile stresses. This initial compression will be such amount that ultimately the concrete will be relatively in tension to only a small degree or not at all. It furthermore makes it possible to produce a structure in which the concrete can be stressed up to its full compressive stren h with very little or no tension upon t e tension side of the section,

I or, in fact with the entire section of the pipe actually still in compression when the latter is withstanding a considerable bending moment or stress from external loads or internal pressure. It also makes it possible to stress the reenforcement up to any desired degree and the higher the limits the less will be the tendency for the concrete to crack on the tension side of the pipe section.

There can be no question that any process,

manipulation or design which lessens the tendency of concrete to crack is an improvement of the first importance over-present practice in reenforced concrete construction, especially since at present it is not expected that the tension in-the reenforcement shall exceed about three to five thousand pounds per square inch before the concrete actually cracks. Under hydrostatic pressuremuch of I coming carrie crete may be due to leakage through minute cracks that escape detection of the eye. In fact, it has been known for many years that concrete may be ru tured and water under pressure may pass t rough althou h the extension is only one third or less 0 that nec essary to produce visible cracking. Herein lies a most fundamental weakness of present practice in reenforced concrete particularly when applied to structures sustainin hydrostatic pressures. The methods out ined in this invention are ca able of entirely over these defects in practice as generally. out at the present time.

When a hoop is placed about a pipe 10 as indicated b 11 in. Fig. 1, either around the outside of t e pipe or embedded more or less deeply in the pipe, the hoop when brought into mitial tension by some means. such as a saddle 12 through which the ends of the hoop extend and which are held tight by nuts 13, or by othermeans, it subjects the con crete of the entire pipe to compression, the intensity of which is a maximum at the inner surface of the pipe and decreases according to a certain law to a minimum at the outer surface of the pipe. This compression in the concrete acts opposite to the effect produced either by internal water pressure or by tension moments from external loads. The pipe therefore is greatly strengthened by the laws sure of the hoop.

ing a circular structure such as a pipe, a bar rel, etc? is not new, but what I do claim is that by the performance of certain things in connection therewith I do produce an entirely new and novel eflfect.

Suppose, for instance, that a hoop with initial tension encircles the pipe 10 as shown at 11 in Figs. 1 and 2. If the hoop is embedded in the concrete in the ordinary manner, or in a groove 14 about the exterior of the pipe and the groove filled with cement mortar 15the hoop becomes cement bound in the concrete. It thus ceases to act in the manner of an ordinary barrel hoop and becomes reenforcement in the pipe and acts according to the laws of reenforced concrete.

Suppose now that the pipe, lying in a horizontal position, is subjected to a vertical load at the top. The load produces tension effects upon the inner surface of the pipe at the top and bottom and on the outside at the horizontal points. It therefore tends to increase theinitial tension of the hoop at the springing lines and to decrease it at the top and bottom. At the top and bottom a point will be reached as the load is increased where the compression on the outside of the pipe will exactly neutralize the initial tension in the hoop. With any further addition of load the hoop, at the top and bottom will cease entirely to act in the manner of a barrel hoop but will begin to sustain compression at an intensity of the same number of times the unit compression in the concrete, that the modulus of the steel is reater than the modulus of the concrete. t the same time the original compression in the intradosal portions of the ring at the top and bottom caused by the original tension in the hoop will be neutralized by the tension induced by the load. The tension in the concrete therefore will be less than it would have been if there had been no hooping, by the amount of the compression in the concrete which was caused by the hoop tension.

At the springing lines the conditions will be exactly reversed. That is, the tension in the extrados of the ring will be added to the hoop tensionand the compression in the intrados of the section will be added to that .caused by the initial hoop tension. The effect therefore of the hoop is entirely different from what is experienced in the case of an ordinary barrel hoop or similar cases of hooping in wood stave pipes, tanks, etc.

Again, consider the hoop as shown at 11" in Fig. 3. If this hoop is given initial tension and then cement bound in the mortar 15 of the concrete, its action against the effects of external loads will be quite different from that described in the former case. That is to say: while at the springing line the effect will be as heretofore described, at the top and bottom the loading compression on the outside of the pipe will be added to the initial compression in the pipe caused by the initial tension in the hoop, and on the inside of the pipe section the original tension in the hoop will be increased by the tension increment induced by the load. With the hoop in this position it will ,act in the same direction throughout as the loading tensions in the pipe, and these tensions in the concrete will induce an additional tension in the hoop throughout all thoseportions of the pipe section where tension is produced by load.

In order to give the hoop initial tension it is necessary that the concrete of the pipe shall have sufiicient hardness and compressive strength to withstand the reaction of the hoop. The means whereby this initial tension is induced is similar to that shown in Fig. 2. The ends of the hoop 11 extend through suitable openings in a saddle 12 and are drawn tightly about the pipe by nuts 13, such as is commonly used on wood-stave pipes, tanks, etc. or by any other means such as electric welding after the rods have been pulled to a predetermined tension. Suitable grooves 14 are cast in the pipe and it then becomes possible to apply the hoops within the grooves after the pipes have attained a certain amount of curing and hardness and the hoops given an arbitrary amount of initial tension desired. These grooves can be 'molded in the exterior surface of the pipe and be of variable depths, according to the requirements.

There are also conditions when it is not desirable to have the hooping cement bound but to have it embedded in grooves and fully covered with cement for protection against the elements, or to have it cast directly in the concrete mass. Suppose a hoop embedded in a groove shown at 14 or 14: but with the hoop protected with some suitable material such as asphalt, grease, paper or other means, so that it will remain free to slip within the body of the concrete when fully embedded.

Its, effect in combination with loads will be} quitedifierent from that of the hoop when cement bound, as discussed above.

If the original hoop 11 is normally circular in position and is then subjected to the stresses caused by a slight flattening of the pipe under-load, the hoop will cease to be circular inform but will be changed into substantially an ellipse. With the hoop free to slip within the mass of the concrete its total length will be altered by a very slight amount only, if the deflection of the pipe is within the usual limits. The result is that the tension in the hoop will remain unchanged or will be only very slightly increased with the application of the load. Furthermore, the intensity of the tension will be practically constant .throughout the entire length of the hoop. The effect therefore at the crown will be to add to the compression in the concrete at the extrados of the pipe and add to the compression of the pipe at the springing lines, by an amount due to the initial tension in the rod. In case the tension in the intrados at the top and bottom due to the load is exactly equal to the compression induced in the same regions by the initial tension in the hoop there will be no tension in the concrete and the combined effect will be the same as when a load is applied normally to a section of an arch at the limit of the middle third. With any further addition of load the tension will remain practically constant in the hoop instead of passing from a zero tension into compression as in the case above. Any further resistance to the load must thenceforward be sustained entirely by the tensile strength of the concrete in the intrados orby other reinforcement introduced for that purpose.

Consider now the case of the hoop shown 'at 11 in Fig. 4 and with the condition that the hoop is free to slide within the grooves of the concrete. It was noted above that when a load is applied to the pipe and the hoop is cement bound a variable tension incrementis added to that already existing in the hoop, the intensity of this added tension being a maximum at the top and bottom and near a maximum at the sides, and decreasing in either direction from these points to zero tensions in the intermediate regions. On

the other hand, when the hoop is free to slide within the grooves of, the concrete pipe, with the application of load as described, there will be an equalization of the tension increments due to the load so that the actual tension in the hoop will be nearly constant throughout its entire length. There is also this further difference which distinguishes the behavior of this hoop from the one in cirtical to begin with. With any ange. in shape of the 1pipe due to application of a load, the elliptica shaped hoop will become more flattened. It therefore will not remain of nearly constant length as in the case where it was circular in form, but with each added increment of deflection an appreciable and increasingly large increment will be'added to the length of the hoop.

- From the above discussion it becomes aparent that each of the positions for reenorcement and manner of embedment de-- scribed has its uses and particular suitability to add this .reenforcement, for it is plain that certain portions of the ring otherwise would not be sufliciently well protected by the presence of the reenforcement. This secondar reen orcement may be of any kind desire rods or mesh or other form. Just how it would act in one tratefits use in com ination with hooping.

Take the case where the hoop'is normally circular, as shown at 11 in Fig. 2. As stated above the hoop tension brings all the fibers of the pipe into a certain amount of compression, the intensity being greatest at the inside and gradually decreasing according to a certain curved law to a lesser amount at the outside. It was shown that when-the tension efiect of a load at the top: of the pipe becomes equal and opposite to the induced compression effect of the hoop there is neither compression nor tension in the inner portion of the-section. Now any further increase in loading would bring tension increments to bear upon these inner portions of the concrete. If no reinforcement were provided to counteract these tension increments this part of the section of the pipe from this point on would act in efiectlike a plain non-reinforced concrete ipe, and the ultimate strength would epend upon the tensile cular form, and that is thatthis hoo is elliparticular case will illus- Y strength of the concrete at this point. If, on theother hand, some reinforcement is provided for this portion, the final tension increments of the load will be taken up partially or wholly by this added reinforcement and they will not be thrown entirel into the unprotected concrete. A further ustification for this procedure may be cited in the fact which can be shown that the larger the amount of reenforcement there is in the section, it this steel is embedded in the concrete in the usual way, the greater will be the tendency for the concrete to crack from shrinka e. With a relatively smaller amount of reen rcement at this point the tendency to initial cracking of the pipe will be considerably reduced.

From the positions of the reenforcement as shown at 11' and 16, it is evident that a style of reenforcement shown in Fig. 4 would not be'appropriate if the pi e were turned or laid so that the regions 0 maximum tensions did not come' where the hooping is placed, but providing the pipe is so placed, the reenforcement when'in the position shown in Fig. 4 will be more'efiective than it would be if it were placed in the normally circular form as shown in Fig. 2. On the other hand, the pipe shown in Fig. 2 has the advantage of being suitable for laying in any position irrespective to the directions of the principal forces. The question as to which or what form of reenforcement described should be used will depend upon the special conditions to be met.'-

The same effects as described can be duplicated if instead of placing the hoopin in grooves of uniform or varying depthsfli the pipes are first cast of less than full thickness. The hooping then can be added with the intention of having it either cement bound or not cement bound as the case may be, after which an additional thickness can'be cast about the reenforcement, embedding the same and bringing the total thickness to the required amount. The provision which has been made for laying the hoops in grooves and then filling. them with mortar is there fore one of expediency and not necessity.

Figs. 2 and 4 show hoops embedded in groves of various depths molded in the body ofthe pipe. If it is desired to have the hoops embedded in protective coverings so as to permit their slipping within the body of the concrete, the molding of grooves in the exterior of the pipe, or other method described above, is not necessary. If the rods be properly covered with suitable rotective material, as heavy greases, etc. wit the ends lapped and tied and laced so that the lie immediately inside of the outer wall 0 the pipe and inside of the outer shell of the pipe mold, the concrete can then be inserted within the mold in the ordinary way and the pipe finished in the usual manner. When theniold is removed from the pipe and the concrete has been given time to acquire the required amount of hardening the threaded ends of the rods can be pulled out and the saddle 12 or other form of coupling, applied. The nuts 13 can now be turned tightly against the saddle and the hoop 11 pulled up to any desired amount of unit tension. The hoop being coated, as described, permits it to slip through the concrete and the entire circumference of the hoop brought into uniform tension. This action would not be possible if the hoops were embedded in the ordinary manner and allowed to become cement bound in the concrete.

While I have illustrated the preferred structure embodying the invention 1t is to be understood that other methods of producing the same result may be resorted to without departing from the spirit of the invention as defined in the appended claims.

I-Iaving'thus described my invention what I claim is:

1. A concrete pipe section having a plu rality of grooves extending circumferential ly of said pipe, reenforcing means disposed in said grooves under tension and adapted to create an initial compression in said pipe, means for sealing said reenforcing means within said grooves, and means for preventing adhesion between said reenforcing means and said sealing means.

2. A concrete pipe provided with grooves,

tension rods insaid grooves adapted to create an initial compression in said pipe, cementitious material in said grooves and encircling said rods, and means for preventing adhesion between said cementitiousmaterial and said rods.

3. In a concrete pipe section, reenforcing means under tension, and means for preventing adhesion between the reenforcing means and said concrete pipe section.

4. In concrete conduits and the like, reenforcement placed around the conduit in such ,a manner as to create initial compression in the. latter, and means permitting relative movement between said reenforcement and said conduit to equalize the strain throughout the length of the reenforcement ivvhgn the conduit is subjected to external 5. In a concrete pipe section, a plurality of reenforcement members encircling said section, means whereby initial tension is produced in said reenforcement members, and means applied to said members to facilitate slippage thereof relative to the concrete whereby tensile stresses from external loads will be .equally distributed throughout the length of the reinforcement.

6. In a concrete conduit section, a. reenforcing member of dissimilar shape, means whereby an initial tension may be produced 'in the reenforcing member after the material of the concrete has partially hardened, and

other means provided to prevent said memj ber from becoming cement bound in the material of the conduit.

7. In a concrete pipe section, circumferentially extending grooves, reenforcing bands therein, and embedded in a yielding plastic material.

8. The method of making concrete pipe sections, which consists in molding the pipe section with reenforcing means embedded therein so as to permit relative movement between said reenforcing means and the material of the wall of the pipe section, and subsequently tensioning said reenforcing means whereby said material is subjected to an initial compression uniformly distributed circumferentially of the pipe section.

9. The method of making reenforced concrete pipe sections which comprises, treating reenforcement members to prevent the adhesion of concrete to said members, molding the pipe section with said member embedded in the concrete, and subsequently placing said members under tension whereby the concrete is subjected to uniformly distributed initial compression.

In testimony whereof, I hereunto aflix my signature.

' WALTER G. PARMLEY.

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