US2833186A - Reinforced pavements - Google Patents

Description

May 6, 1958 c. DOBELL REINFORCED PAVEMENTS original Filed March 11, 195s REINFORCED lPAVEMENTS Curzon Dobell, Nassau, Bahamas, assignor to The Preload Company, Inc., New York, N. Y., a corporation of Delaware i Continuation of application Serial No. 341,770, March 11, 1953. This application November 19, 1956,Serial No. 622,896

3 Claims. (Cl. y94-4--Z2) This invention relates to the construction of concrete surfaces resting on a sub-base, such as pavements or road, ways, and more particularly. to theY construction of concrete roadways that are held under continuouslongitndinal compression in such a way that cracking and buckling are avoided and expansion joints become unnecessary.

At present, in particular in designing runways forV heavy bombers, engineers `are faced with the problem of using either Iabnormally thick vand expensive slabs of reinforced concrete to spread the load over the sub-base suiciently so that little or no deformation will occur, or trying to nd something such as asphaltic materials which will deform under the wheel loads and return to normal levels after the load has rolled on. To date, pref stressed concrete is the only medium'which offers both an economical slab thickness which can sustain the loadings, and a material which has elasticity enough to deform under the wheels and return to normal when the load has passed.

The common diiculties encountered in concrete roads a time with the prestressed reinforcing wires or rods running lengthwise and crosswise of the section, ordinary cracking may be prevented. However, expansion joints is another object to create a pavement which would be l a continuous slab having a uniform concrete section in compressive stress and a uniform steel section in tensile stress, both over a substantially unlimited length, with the elastic shortening caused by the compressing of the slab absorbed in increments by the use of temporary construction joints. vIt is yet another object to avoid and eliminate costly expansion joints -between sections of pavement. And it is yet a further object to provide an easily laid roadway notV requiring elaborate preparation of the,

reinforcing elements. It has been found that this can be done by the method and arrangement set forth in this application and that as a result cracking-and buckling.

may be avoided.

lBrietly, this invention consists in prestressing a concrete road 'by diagonally placed elements so that the ends of the prestressing elements can be tensioned from the edges of the slab and simultaneously providing a means and method to take up the longitudinal distance that the concrete of the road loses by reason of its being placed under longitudinal compression, by providing temporary lconstruction joints that are later-filled, allowed to set and compressed.

The result is a continuous concrete pavement vunderv y sucha mannerthat there is no tendency of the roadf to buckle or crack even when thelongitudinal compression a is increased by increases in temperature that are restrained from causing corresponding increases in length.

It is preferred,.in applying the principles of this inven-A tion, -to use `prestressing material of a quality such that relatively high tension may be applied to it so that after the concrete has undergone any plastic flow of which it is capableand the material of the prestressing element has taken on any permanent elongation that it will take on as a'result of the forces bearing upon it, there will still be a high degree of tension in the. prestressing elements and a high degree of compression in the concrete.

An understanding'of both the details and the principles of this invention may best be had by an immediate ref- --ierence to the drawingswhich illustrate the preferred mode are still necessary between the sections to compensate t for changes in length of the sections due to changes in temperature and ordinarily these must be made quite wide in 4order both to provide for access to the ends of the longitudinal wires or rods so that they may be tight-Y tions as a whole under compression, and at the joint bein-accordance with the principles of this invention asv4 tween two adjacent sections serious buckling eouldgoccur. In the use of longitudinal stressing the length of the section is determined by the loss of prestress between is increased requiring elaborate precautions yto accommodate the increased movement.

"'pavement constructed in accordance with this invention; f

of application of the principles of this invention and to the following detailed description of lthis mode of application of this invention. l

In the drawings, Figure l is a plan view of a concrete while Figure 2 shows thel same in more detail;

Figure 3 is a cross section of Figure 2 taken along line 3 3; and

The object of the present invention is to provide a syse* tem of stressing pavements in which the slab is stressed f` continuously alongl its length and when possible continuously along its breadth without any interruptions in the stressing reinforcing for expansion joints or 'the like. It

Figure 5 is a detail .view of the end anchorage for the reinforcing rods.

Figure. 6 is a diagram showing the slab compression under various conditions of compression.

Pavements or slabs on any subbase maybe constructed extending reinforcing elements 12 and 13, which are coated to prevent bonding with theconcrete or placed in preformed holes. Balanced stresses are providedby inclining one series of elements 12 oppositely from Vthe` other series 13. It is necessary that the ends of the ele- '-ments be arranged in a method to avoid interference with the tensioning device. A recess 14 is provided at each end ofeach of these elements so as to providel access to it for tensioning the element and a nut 15 or other appropriate means for holding the element intenysion isalso provided. The detail in Figure 5 shows the atented May 6, 1958 Figure 4 is a cross section of Figure 2 taken along line I end anchorage which is a threaded barl over which is placed a bearing plate 25 followed by a washer 26 and iinally the nut 15.` A suitable `bearing plate -for a l 4inch bar would be steel plate 11/2 inches thick and 5 inches by 6 inches` in area. Any othertype of, end anchorage adaptedito secure the elements will -be adequate. The concrete may be prevented from bonding to the elements in any of the known ways. For example, the elements may be `covered with asphalt before the concrete l is molded around them or they may be placed in preformed holes in the concrete. e

As can be seen from Figure 2, a concrete roadway is cast in a series of spaced sections 10 and 11 but the reinforcing elements extend across thefspace between the sectionsiso that some of the reinforcing elements lie partly in one section and partly in another.

After the concrete has set, the elements 12 and 13 are tensioned to place the concrete under both lateral and longitudinal compression. As a result of their compression it is natural that the length and width of each concrete section will diminish to a certain extent by elastic contraction.` The length of a A500 ft. road section, for example, will decrease some three or four inches.

This `invention contemplates that in order that this Vde-` crease in length takes place from the ends of the section toward the center and that it be compensated,appropri ate instrument for forcing the ends apart, such as a hydraulic jack shownV schematically at 20 is provided so that it will lit between the abutting ends of the adjacent sections and force their ends apart. A pump 21 is connected to the jack, and the jack, which consists of an elongated pressure chamber that isflexible so that it can expand, is put under pressure and presses against the abutting ends of the adjacent sections so as to force them apart as tension is placed on the reinforcing material, The abutting ends of the adjacent sections may be made somewhat thicker to form a haunch 23 as shown in Figure 3 so as to properly accommodate this hydraulic jack.

After the tension has been placed on` the reinforcing elements and the concrete placed under compression by the combined action of this tension and the pressure of the `hydraulic jack 20, the space above the hydraulic jack 20V is filled with concrete or other suitable material 22 as shown in Figures 3 and 4. The pressure may be released in the hydraulic jack 20 and the jack withdrawn after the material filling the joint has hardened sutiiciently. The compression in the concrete then is transferred against the material filling between the sections and the whole section is placed under substantially equal compression.

In my scheme the jacks and tensioned reinforcing serve entirely different purposes. The` concrete is placed under the required compression by the post-tensioning of the reinforcing. By post-tensioning is meant the stressing v of the elements after the concrete has set. However, it would be impossible to prestress a pavement for any great length -by post-tensioning the reinforcing alone without providing for some periodic adjustment of the elastic strain in the concrete due` to prestressing. This adjust- `mentwould amount to some 3.5 ft. per mile at a 300 out interrupting the continuity of the rods passing through p. s; i. stress. This movement is resisted by friction between the pavement and the sub-base.

To absorb the elastic strain in the concrete due to prestressing, temporary construction joints are provided at intervals of such frequency (between 200 and 500 ft. depending on subsoil friction values), so as to reduce the frictionloss between joints to a reasonable minimum. Jacks are placed between the vertical surfaces of the concrete at these joints. The prestressing is preferably placed diagonally in the slab, probably in the form of high strength bars, at any desired angle to the axis of the pavement which provides the desired ratio of longitudi. nal and transverse stress. These bars are coated so that they will act as sliprods to avoid the. cost of subsequent grouting, or maybe placed inspreforrnedholes.Y

As the tensioning of the reinforcing progresses along the pavement the jacks are placed in the construction joints and are actuated to increase the gap in the joint, as measured by micrometers, by the amount of the elastic shortening of the concrete in the length of this joint. If the joint as cast was l inch wide and the elastic shortening in a 500 ft. slab 0.35 inch, the jack would be used to enlarge the joint from 1 inch to 1.35 inches as the prestressing progressed across the joint.

The jacks would then be used to temporarily increase the elastic shortening (such as to increase the joint to 1.7 inches), `which would increase the compression in the concrete at the joint for a short period, and thus reduce the frictional loss at the mid-point between the two joints. The gap would then be relaxed back to the 1.35 inches, at which gap spacing the joint would be grouted with mortar. After the mortar has retained sufficient strength'the jack would be removed, thus transferring the `full force of the pre-tensioned steel across the mortared joint, and the jacks would be reused at a subsequent joint. By the use of temporary over-stressmg as described, 'the mid-point between the joints would vreceive an increased compression, which would be at least partially retained afterthe jack was removed, resultmg m a more uniform distribution of compression.

This is shown in the diagrammatic sketch of Figure 4 wherein the concrete roadbed r has a length l. If the Vcompression which is normally expected to be maintained is applied tol the roadway, the maximum compresslon will be realized at the ends of the slab while there will be substantially no compression at the mid-point of the slab. This is shown in line a with the maximum 'compression d at the slab end. lf double the compression is applied, the compression over the slab will be exemplitied by linefb, showing the increment y due to the extra compression, and further showing the compression now existing at the mid-point of the slab. Reducing the gap or the compression back to the normally expected standard results in line c, wherein a more evenly smoothed olutbcompression results across the entire length of the s a It has .been found preferable, in this construction, to use steel wire or rods or other suitable materialshaving a tensile strength of at least 150,000 pounds per square inch, and to tension wires or rods initially to a tension of atleast 100,000 Vpounds per square inch. It is preferable,but not entirely necessary, to use concrete for filling the joints,of,the same type as that use in the slab in order-to 4have a uniform material throughout.

Adiscussion of the h'aunch 23 is in order. By providing a thickened portion of the concrete slab there is provided an adequate bearing surface for the jack 20 withthe gap between the slabs, and without interrupting the continuity ofthe concrete poured in the gap to Vtill it after the jack has forced the sections apart. When this concrete has set and the jack tension is removed, the

vconcrete iniwthe gap is placed under the same compression as the rest of the pavement, `and by the use ofthe haunch, the concrete in the gap is the same thickness as 1nthe rest of the pavement.v An alternative to the v haunch isthe use of a series ofnotches in the ends of the f slabs at the joints vto retain 'a jack set on top of the slab,

and, having the concrete under a relatively uniform compression and the prestressing elements under a relatively uniform tension, is in a condition where it is quite stable and there is little or no tendency to buckle or crack even under relatively wide changes in temperature.

Any tendency to buckle is resisted by the prestressing elements. As these are continuous along the enti-re length of the roadway, there are no weak points or interruption of the section at which buckling may occur.

Many modifications and improvements that will be apparent to those skilled in the art are considered to be within the scope of the appended claims.

This application is a continuation of my application Serial No. 341,770, tiled March ll, 1953, now abandoned,

I claim:

l. The method of constructing a long concrete slab resting on a sub-base which comprises forming a series of slab sections between which temporary transverse joints are provided at intervals, incorporating in the sections and across the joints high strength reinforcing elcments in unbonded engagement with the sections, permitting the sections to set until they have acquiredconsiderable strength, tensioning said reinforcing elements, increasing the width of the temporary joints by forcing them apart with an expanding means to an amount corresponding to the `amount of elastic shortening which would occur in the sections if unrestrained by -friction with the sub-base, lling the extender joints with a material capable of attaining sucient strength to resist substantial compression and removing the expanding means after the joint material has attained such strength to transfer compression from the sections to the joint material.

2. The method of constructing a long concrete slab resting on a sub-base that comprises forming `a series of sections between which temporary transverse joints are provided at intervals, incorporating in the sections and across the joints high strength reinforcing elements extending diagonally across the joints and through the side edges of the sections, said reinforcing elements being in unbonded engagement with the section, permitting the sections to set until they have acquired considerable strength, separately tensioning said reinforcing elements to provide a predetermined compression in the sections causing an elastic shortening of the sections, increasing the width of the temporary joints by an amount equal to the elastic shortening of the sections at the joints while tensioning is in progress by forcing them apart with an expanding means, filling the joints while in the enlarged position with material capable of achieving a set of suiiicient strength to resist substantial compressive forces, and relaxing the expanding means after the joint material has set so as to transfer the compression from the sections to the joint material.

3. A method as defined in claim 2, with the addition of forcing the sections apart a-t a pressure in excess of that required to create the ultimately desired compression in the roadway, holding them at the increased pressure a sufficient time to permit the compression to be distributed to the center of the sections, partially releasing the compression before lling the space between the sections With concrete.

No references cited.

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