US3234860A - Weakened concrete joint and method of forming same - Google Patents

Description

G. S. LACY Feb. 15, 1966 WEAKENED CONCRETE JOINT AND METHOD OF FORMING SAME Filed June 4, 1962 INVENTOR. 60/900 6. 4146' 1 United States Patent O WEAKENED CON CRETE JOlNT AND METHUD F FORMING SAME Gordon S. Lacy, Escondido, Califi, assignor to Clardon Investment Co., Escondido, Calif, a partnership of California Filed June 4, 1962, Ser. No. 199,694 11 Claims. (Cl. 94-18) This invention relates to poured concrete slabs and walls and more particularly to the provision of a weakened joint of a new and improved character at predetermined spaced intervals and so designed that fracture of the slab due to shrinkage or loading occurs substantially in the plane of the weakened joint and along irregular interlocking surfaces.

It is well known that large area slabs and walls of poured concrete are subject to shrinkage during curing of the cement, as well as to expansion and contraction due to temperature changes. For these reasons it is highly desirable that large expanses of concrete be provided with suitable provision at spaced intervals to accommodate relative movement between adjacent sections of the concerete to compensate or allow for shrinkage, temperature changes and the like variable factors. Various expedients have been resorted to heretofore in dealing with these problems. For example in the construction of concrete pavements and roadways and the like, it has been customary to pour individual slabs in end to end relation but separated from one another by an expansion joint of various types. One well known and widely used type of expansion joint comprises a laminated strip of fibrous strips impregnated with tar and extending for the full depth and width of the slab. This and related prior expansion joints are costly and time-consuming to install and leave the adjacent slabs totally disconnected from one another for all practical purposes. In consequence the application of a load to a corner area of a slab when the underlying ground or support is wet or soft often results in the application of severe tensile stresses on the corner of the slab causing it to break off.

To obviate this serious shortcoming it has been proposed to pour the concrete in continuous slabs and to cut a deep slot transversely of the slabs during a particular stage of the curing operation at spaced intervals using power driven diamond-tooth saws. This practice is quite effective in providing a weakened joint but is extremely time-consuming and costly. It is also found that the slots left by the saw cutting technique tend to become filled with foreign material or water. This material becomes thoroughly packed in the slots seriously interfering with the expansion of the concrete under high temperature conditions and the freezing of the water can introduce severe destructive forces.

By the present invention there is provided a new and improved weakened joint for concrete slabs and wall structures of all types. This joint features a narrow slot preferably filled by a continuous and permanent strip of inexpensive resilient material as for example a soft and resilient plastic of either a homogeneous or spongy character and of a suitable cross-sectional shape. One edge of this slot opens through one surface of the concrete, the slots customarily being generally parallel and appropriately spaced from one another in accordance with conventional practice. The depth of the slot is somewhat less than one-half the thickness of the slab structure and sufficient to assure occurrence of a fracture in a transverse area including the slotted area rather than in a portion of the slab to either side of the slot.

The cross-sectional shape of the material used to cast 3,234,860 Patented Feb. 15, 1966 the slot may take various forms, such as a simple narrow rectangle, or an inverted T with the stem or major dimension of the slot extending generally normal to and opening through one surface of the slab. A slot forming strip of inverted T-shape has the advantage that the failure crevice may occure along a wider zone in that it may be located in a vertical plane substantially anywhere between the opposite ends of the T-head. Hence it will be recognized that the failure crevice may occur in a generally zig-zag fashion crosswise of the slab thereby further increasing the strength of the interlock between the juxtaposed irregular faces of the crevice. This provides added assurance that adjacent slabs will share imposed loads equitably.

Irrespective of the cross-sectional shape of the slot forming strip, it may be desirable to provide the web of the material with suspension extension intended to extend beyond the finished surface of the slab to be formed and having a weakened line opposite the surface of the finished slab facilitating severing of the protruding portion of the strip following setting of the concrete. Alternatively and if desired, the strip of slot forming material may be withdrawn from the slab after the concrete has taken a partial set, although in a preferred mode of use, the material is left in the slot permanently for the purpose of excluding liquid and foreign material and accommodating by its resiliency changes in the width of the slot due to shrinkage and temperature changes.

The slot forming material or strip may be supported in various ways during pouring and setting of the concrete. If the slabs are not excessively wide, the strips may be supported on rigid members extending crosswise of the slabs and suitably supported at their ends. In other situations where the slabs are too Wide to permit of this expedient, stakes may be set at intervals crosswise of the slab and used to support rigid members which in turn support the slot-forming strips at intervals there along, after the concrete has taken a partial set, the rigid members together with the stakes are withdrawn and the openings left by the stakes are then filled with concrete mix flush with the exterior surface of the slab.

Accordingly it is a primary object of the present invention to provide an improved inexpensive and highly effective weakened joint for concrete as well as a method of forming a weakened joint as an incident to the pouring of the concrete mix.

Another object of the invention is the provision of weakened joints extending at spaced intervals crosswise of concrete slabs, walls and the like, and so constructed that one face portion of the slab as initially cast comprises continuous concrete whereas the other face portion is traversed by a deep narrow slot either left vacant or filled with resilient material capable of excluding foreign matter.

Another object of the invention is the provision of a weakened joint for concrete slabs extending crosswise of the slab and extending partially through the slab there- 'by forming a weakened cross-sectional area of concrete thereby to assure failure of the slab in this weakened zone and along an irregular surface effective to form a high strength interlock between the adjacent faces of the failure crevice.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which preferred embodiments of the invention are illustrated.

FIGURE 1 is a fragmentary perspective view of a short length of a concrete slab or wall showing a weakened joint incorporating the characteristic principles of this invention and in the process of being formed;

FIGURE 2 is an enlarged fragmentary transverse view through the weakened joint of FIGURE 1 and indicating a likely shape of the failure crevice likely to form in the slab;

FIGURE 3 is a fragmentary perspective view of an alternate mode of forming the weakened joint and particularly useful in slabs of wider widths;

FIGURE 4 is a transverse cross-sectional view of the joint shown in FIGURE 3;

FIGURE 5 is a cross-sectional view of a third embodiment of the weakened joint according to the present invention; and

FIGURE 6 is a fragmentary sectional view on an enlarged scale through the plastic strip shown in FIGURES 1 to 4.

Referring more particularly to FIGURES 1 and 2, there is shown by way of example a slab of concrete 10 poured along the ground and suitable for use as a drive-way and having a weakened concrete joint designated generally 11 extending crosswise thereof. This weakened joint includes a narrow but deep slot 12 extending across the entire width of the slab. The depth of slot 12 may vary over a considerable range but excellent results are achieved using a depth less than half but at least one fourth the thickness of the slab. This slot is formed during pouring of the concrete by the lower portion of a fiat strip 13 of suitable resilient material, secured as by nails 14 to a rigid supporting member 15. Strip 13 may be formed from various resilient materials having a homogeneous or cellular structure immune to attack by pavement chemicals and by moisture. Such materials include various plastics and such cellular materials as those formed from polystyrene, urethane and others. If made of a spongy plastic it is desirable that the cells be of the closed type to exclude moisture and other fluids.

Desirably a deep groove 16 of either a continuous or intermittent character is formed in one or both surfaces of member 13 and this is positioned to lie flush with the finished surface of the slab to facilitate severing of the portion above the groove and serving as means for suspending the strip proper from rigid member 15. As has been pointed out above, strip 13 is secured to the lower edge of mounting member 15 with groove 16 lying in the plane of the lower edge of member 15 as is clearly illustrated in FIGURE 2.

Before pouring the concrete mix, member 15 is suitably supported at its ends by stakes or by forms for the concrete, not shown, but understood as arranged along either lateral edge of slab 10. The slot forming material 13 then lies suspended in a vertical plane so that, upon pouring of the concrete mix, the lower portion of the strip will project downwardly into the mix and form a slot 12 of the desired depth.

After the poured concrete has taken a partial set or even after it has taken a complete set, member 15 together with the portion of the slot forming material 13 above severance groove 16 is torn or cut away flush with the top surface 18 of the slab. Or, if desired, all of the slot forming material may be withdrawn from slot 12 soon after the concrete has been poured and after it has taken a sufficient set as not to slump and fill the vacated slot. It will be understood that this interval between weakened joints 11 may vary over a considerable range depending on operating conditions the range of temperature changes during different periods of the year and other factors.

Under operating stresses or due to shrinkage or temperature changes posing severe strain on the completed slab, owing to the presence of slot 12, the failure of crevice 20 is certain to occur between the lower edge of slot 12 and the bottom surface of slab 10. Such failure invariably occurs along a highly irregular path, both ver- 'tically and horizontally, thereby providing a multiplicity of interlocks with the result that the juxtaposed faces of the crevice remain positively interlocked with one are other. It follows that the passage of a vehicle or other load across surface 18 of the slab is ineffective to depress the slab on one side of crevice 24 more than on the opposite side. Accordingly it will be understood that each of the adjacent sections of the slab will be constrained to share the load substantially equitably and that the load is transmitted crosswise of crevice 20 by the interlocking faces thereof. Owing to the fact that slot 12 is filled with the resilient material 13, dirt, liquid, and other foreign matter cannot enter the slot.

Referring now to FIGURES 3 and 4, there is shown an alternate mode of supporting the slot filling material 13. It will be observed that the rigid member 15 described in connection with FIGURES l and 2 is replaced by an angle iron 15' suitably supported at intervals therealong by an appropriate number of stakes 25 only one of which is here illustrated. Spaced at intervals along the downwardly projecting flange of member 15 and suitably fixed thereto are brackets 26 having resilient spring fingers 27 spaced apart suitably to receive and firmly grip the slot forming strip 13 astride its upper lateral edge. If desired one of the fingers 27 may be provided with an adjustable thumb nut or screw 28 Operable when rotated to adjust one of the fingers toward the other to provide a firmer grip on the strip 13. In the arrangement just described, slot forming strip 13' need be no wider than the depth of slots 12 thereby avoiding the need for wasting any of the strip material or for providing either the severance groove 16 or a suspension portion. customarily stakes 25 supporting rigid member 15' are driven to a proper depth with the lower ends of the strip supporting members 26 lying flush with the con templated finished surface of slab 10. Thereafter strip 13 is pressed upwardly between the gripping fingers 27 until the upper edge of the strips contacts the lower ends of support brackets 26. Thereafter the concrete mix is poured in the usual manner. After the concrete has taken a partial set, the opposite ends of members 15' are elevated along with stakes 25. Since the frictional grip of the concrete with strip 13' is stronger than the gripping action of fingers 27 therewith, strip 13' remains firmly in place in slots 12 as the stakes and members 15 are withdrawn. Thereafter the holes left by stakes 25 are filled with mix.

Referring to FIGURE 5 an alternative design of the slot forming material 13" is shown in its finished installed condition. Strip 13" is of inverted T-shape in cross-section and may be formed of the same material used in forming strips 13, 13' discussed above in connec tion with FIGURES l to 4. The length of the head portion 29 of member 13", as here shown, corresponds generally with the length of stem portion 30. However, these relative dimensions will be understood as non-critical and subject to wide variation in practice. The illustrated cross-sectional shape has the advantage of greatly increasing the strength and handling capabilities of the strip during the installation and pouring operation. Additionally the length of the head portion extending generally parallel to the upper and lower surfaces of the slab 10 tends to increase the positive interlocking character of the crevice 20' customarily forming in the weakened section of the slab between the lower surface of member 13 and the bottom of the slab. This is due in major part to the likelihood that crevice 20' can develop in any portion of the concrete underlying head 29 of the strip 13" as: well as in a much larger zig-zag path lengthwise of the. strip.

Although the weakened joint of this invention has beendescribed in connection with slabs poured on the ground. it will be understood that it is equally applicable to con-- crete slabs disposed in various planes and used in poured concrete constructions of all kinds including vertical,

horizontal and inclined Walls. If for any reason it is desirable that the slot forming material be eliminated after the concrete takes a set and particularly in applications where the described weakened joint is employed in wall structures, this may be done by either withdrawing the strips before the concrete sets or by applying a suitable solvent to the strip material. The solvent is applied and-left in contact with the material until softened sulficiently to permit the dissolved material to be flushed away using strong jets of water or the like. If polystyrene foam is used for the strip material, a solvent of the ketone class provides very satisfactory results economically. Acetone is particularly satisfactory and readily converts the polystyrene to a gas.

While the particular weakened concrete joint and method of forming same herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.

I claim:

1. An article of manufacture comprising a strip of homogeneous plastic material of uniform cross section comprising a major portion and a minor portion weakly connected together lengthwise thereof, said weak connection between said major and minor portions being sufficiently strong for use in suspending said major portion from said minor portion While concrete mix is being placed about said major portion to embed the latter in concrete mix to a depth such that said weak joint lies in a plane generally flush with the surface of said concrete mix and while said mix is taking a set, said weak connection being thereafter easily severable to disconnect the minor portion from the major portion of said strip leaving the latter embedded in and bonded to said concrete and exposed only along the portion thereof lying in a plane substantially flush with the finished surface of the concrete.

2. An article of manufacture as defined in claim 1 characterized in that said strip of plastic material includes at least one web portion generally normal to the exposed edge surface of said strip when embedded in concrete and having other web portions projecting laterally from said first mentioned Web portion.

3. An article of manufacture as defined in claim 1 characterized in that said major portion has voids therein out of communication with the lateral side wall areas of said strip and sealed against the entry of moisture and components of concrete mix while the latter is taking a set.

4. An article of manufacture comprising a continuous strip of semi-rigid material for embedment crosswise of one face of a monolithic slab of concrete to separate said one face into adjacent surface areas, said article being semi-rigid resilient homogeneous plastic material and having the same cross-sectional area from end to end thereof, and said article including a main body portion and a relatively small section extending along and Weakly joined to said main body portion of said strip and adapted for use in suspending the main body portion of the strip generally horizontally with said weak joint lying generally flush with said one face of a concrete slab wtule the concrete mix is being placed about the suspended strip and taking a set, and said relatively small section adapted to be easily separable generally flush With said one face of the slab after the mix has taken a set without disturbing the bond between said concrete and embedded sur faces of said strip.

5. That method of laying a continuous monolithic strip of concrete along a supporting surface having long narrow weakened zones of preselected intervals which method comprises driving stakes into the ground in lines extending across the strip to be concreted, supporting a rigid member on said stakes with its lower edge no lower than contemplated for the top surface of the concrete strip, suspending a noncompressed nonabsorbent strip of unitary homogeneous resilient plastic material from said rigid member and extending from edge to edge of said strip of concrete, pouring concrete mix in a continuous monolithic strip beneath and about said strip between the opposite ends thereof but leaving the top surface exposed, removing said stakes and said rigid member after the concrete has taken a partial set, and filling the stake holes with concrete mix flush with the upper surface of the concrete.

6. That method defined in claim 5 characterized in that said strip of resilient material includes a weakened connection of reduced cross-section extending lengthwise thereof, suspending said strip above the subgrade so that said weakened connection lies substantially in the plane of the upper surface of the strip of concrete as the latter is poured about the lower edge of said resilient strip, and removing the portion of said resilient strip along said weakened connection after the concrete mix has taken at least a partial set leaving the remaining lower portion of said resilient strip embedded in the concrete.

7. That method defined in claim 5 characterized in the use of plastic foam for said strip of resilient material, and further characterized in the step of removing said strip material after the concrete mix has taken a set by applying a solvent to the exposed surfaces of said strip effective to dissolve said plastic foam material.

8. That method defined in claim 5 characterized in the step of applying a liquid solvent to the exposed edge of said strip of resilient material after said concrete mix has taken at least a partial set whereby said strip is dissolved leaving a channel extending crosswise of said continuous strip of concrete.

9. That method of constructing concrete pavement having resilient separator members embedded therein with the upper edge thereof generally flush with the surface of the finished pavement, said method comprising suspending elongated resilient unitary separator members of homogeneous thermoplastic material having a plurality of flanges lying generally at right angles to one another lengthwise of said separators across the area to be paved, said separators being suspended from above with an edge of the uppermost flange generally flush with the intended outer face of the pavement being laid and with all remaining portions of all of said flanges positioned spaced substantially above the pavement subgrade for embedment and direct contact with concrete to either side and below said separator members, filling the area to be paved with concrete mix, and leveling the mix to lie flush with the uppermost edge of the upper flange of said separator members thereby leaving substantially all remaining portions of said flanges in direct contact with said concrete mix from end to end thereof.

10. That method defined in claim 9 characterized in that said separator members are generally T-shaped in cross-section and in that said members are so supported prior to the pouring of said concrete mix that one of said flanges lies in a generally vertical plane and that another flange projects laterally therefrom in a generally horizontal plane spaced substantially above the pavement subgrade, said vertically and horizontally disposed flanges cooperating with one another in anchoring the separator members rigidly within the concrete pavement and in sealing the finished pavement against entry of moisture along the interface between the concrete and the surfaces of said separator members.

11. That method of constructing monolithic concrete pavement having embedded therein resilient separator members extending only partially through the thickness of said pavement, said method comprising providing elongated separator strips of resilient homogeneous nonmetallic material having a major portion adapted to be embedded within and bonded to said pavement and a relatively small portion thereof weakly joined to said major portion and lying outside the surface of said pavement for use in supporting said major portion while concrete mix is being placed thereabout and While said concrete mix is taking a set, utilizing said small portion of said strip to hold the strip as a whole suspended above the pavement subgrade, placing concrete mix over the suhgrade to a level flush with the weak joint lengthwise of said separator strip, allowing said poured mix to take a set, and thereafter removing the exposed portion of said strip along said weakly joined area close to the surface of the finished concrete pavement.

References Cited by the Examiner UNITED STATES PATENTS 1,508,545 9/1924 Foster 94-51 1,516,100 11/1924 Hubbard 94-51 1,562,257 11/1925 Rodgers 94-51 Galassi 94-17 Heltzel 94-51 Robinson 50-70 Older 94-17 Thomas et al 94-18 X Fischer 94-18.2 Van London 94-18 X Kelley 94-17 Bartholow et al. 94-18 Carnes 94-17 X Maude 94-18 Worson 94-18 Middlestadt 94-18 Crone 94-18 JACOB L. NACKENOFF, Primary Examiner.

CHARLES E. OCONNELL, Examiner.

N. C. BYERS, Assistant Examiner.

Claims (1)

1. AN ARTICLE OF MANUFACTURE COMPRISING A STRIP OF HOMOGENEOUS PLASTIC MATERIAL OF UNIFORM CROSS SECTION COMPRISING A MAJOR PORTION AND A MINOR PORTION WEAKLY CONNECTED TOGETHER LENGTHWISE THEREOF SAID WEAK CONNECTION BETWEEN SAID MAJOR AND MINOR PORTIONS BEING SUFFICIENTLY STRONG FOR USE IN SUSPENDING SAID MAJOR PORTION FROM SAID MINOR PORTION WHILE CONTRETE MIX IS BEING PLACED ABOUT SAID MAJOR PORTION TO EMBED THE LATTER IN CONCRETE MIX TO A DEPTH SUCH THAT SAID WEAK JOINT LIES IN A PLANE GENERALLY FLUSH WITH THE SURFACE OF SAID CONCRETE MIX AND WHILE SAID MIX IS TAKING A SET, SAID WEAK CONNECTION BEING THEREAFTER EASILY SEVERABLE TO DISCONNECT THE MINOR PORTION FROM THE MAJOR PORTION OF SAID STRIP LEAVING THE LATTER EMBEDDED IN AND BONDED TO SAID CONCRETE AND EXPOSED ONLY ALONG THE PORTION THEREOF LYING IN A PLANE SUBSTANTIALLY FLUSH WITH THE FINISHED SURFACE OF THE CONCRETE.

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