US3789759A

US3789759A - Surface relief of concrete and method therefor

Info

Publication number: US3789759A
Application number: US00285001A
Authority: US
Inventors: R Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-08-30
Filing date: 1972-08-30
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05

Abstract

Surface relief of concrete shapes is described wherein an explosive composition is embedded a shallow depth in the concrete shape and is thereafter detonated after the concrete has acquired a permanent set. The detonation removes the over burden of concrete and relieves the surface of the concrete in an aesthetically pleasing manner. The preferred explosive composition is a detonating cord which can be embedded in the concrete in a regular and orderly pattern and at a uniform depth, e.g., about 1/2 to 1 inch beneath the surface of the concrete shape when the concrete is poured into its containing form.

Description

waited States Patent [1 1 J ones SURFACE RELIEF OF CONCRETE AND METHOD THEREFOR [52] [1.5. CI 102/23, 125/1, 264/84 [51] Int. Cl. F42d 1/00 [58] Field 05 Search 102/22, 23, 27; 264/84 [56] References Cited UNITED STATES PATENTS 11/1964 Smith 102/27 R 3/1966 Andrew et al. 4/1973 Regan 264/84 FOREIGN PATENTS OR APPLICATIONS 12/1970 Germany 102/27 R Primary ExaminerVerlin R. Pendegrass Attorney, Agent, or Firm-Robert E. Strauss ABSTRACT Surface relief of concrete shapes is described wherein an explosive composition is embedded a shallow depth in the concrete shape and is thereafter detonated after the concrete has acquired a permanent set. The detonation removes the over burden of concrete and relieves the surface of the concrete in an aesthetically pleasing manner. The preferred explosive composition is a detonating cord which can be embedded in the concrete in a regular and orderly pattern and at a uniform depth, e.g., about l/2 to 1 inch beneath the surface of the concrete shape when the concrete is poured into its containing form.

11 Claims, 5 Drawing Figures PATENTEU FEB 74 FIGUQE 1 FIGURE FIGURE 5 SURFACE RELIEF OF CONCRETE AND METHOD THEREFOR DESCRIPTION OF THE INVENTION This invention relates to amethod for the preparation of concrete shapes having pleasing and aesthetic surface reliefs and to such concrete shapes.

Concrete is extremely versatile and inexpensive material of construction. Consequently, the material has found wide acceptance in the building industry for the fabrication of fences, vertical panels, walls, pillars, beams and the like. The highly plastic nature of the material which serves to its advantage by permitting it to be poured into an endless variety of shapes and forms is, aesthetically, an undesirable characteristic, since it commonly results in concrete being formed into shapes having smooth, unbroken and uninspiring surfaces.

Some attempts have been made to impart artistic relief patterns to the surfaces of concrete shapes using molds formed of plaster, rubber, gelatin, wood, fiberglass and the like. One approach which has been highly successful is that described in my prior U.S. Pat. Nos. 3,515,779 and 3,702,180 wherein a liner formed of polystyrene foam is placed in a concrete form and the intaglio pattern of the lines casts an artistic pattern on the surface of the concrete.

The use of preformed molds or mold liners unavoidably results in orderly and, often, repetitous artistic patterns on the surface of concrete shapes. in many instances, this is desirable and adds to the aesthetic impact of the surface. Frequently, however, it is desirable to have a highly random and irregular quality in the surface relief.

Again, some attempts have been made to provide irregular and random reliefs in concrete surfaces. Commonly used methods are sandblasting or bushhammering of the surfaces of cured concrete shapes.

Other methods are employed before the concrete is cured such as scrubbing or washing of the cement from the surface of the concrete.

All of these techniques require a considerable amount of hand labor and only achieve exposure of the aggregate of the concrete. Commonly, the depth of the surface relief achieved is only very slight and isuniform across the entire surface.

It is an object of this invention to provide a method to impart aesthetically pleasing textures to the surfaces of concrete shapes.

It is also an object of this invention to provide such a method requiring a modicum of hand labor.

It is an additional object of this invention to provide means or apparatus for the safe and protected practice of the aforesaid method.

Other and related objects will be apparent from the following description of the invention:

I have now found that the aforementioned objectives can be achieved by a method that comprises placing at a slight depth beneath the surface of a concrete shape .a water-resistant high explosive at a concentration of from 3 to about 400 grains per square foot of desired relieved surface area, providing igniting means with said explosive composition and, when the concrete has reached a permanent set, detonating said exposive composition by activating said igniting means whereby the surface of said concrete is blasted from said shape.

The preferred method employs a cord-type explosive composition such as Primacord which can simply be placed beneath the concrete surface, either before the concrete hardens or placed in grooves formed or cut in hardened concrete and then covered with motar or fresh concrete. The detonating cord can be placed in a random or orderly fashion, as desired.

The exposion tears the surface of concrete covering the explosive composition from the concrete shape. Generally, the explosive composition is placed at a depth from 174 to about 1 inch beneath the surface and this covering thickness of concrete is removed from the shape by the explosion in a varied manner; removal to a maximum depth occurs immediately adjacent the composition and. at a progressively'decreasing depth with increasing distance from the explosive composition.

The explosion also achieves an exposed surface which cannot be achieved by any other manner of treatment. Some of the aggregate of the concrete is shattered by the explosion so that the resultant surface does not merely expose the aggregate, but, instead, some of the aggregate is cleavedor severed and its interior is also exposed. This results in a very decorative and aesthetically pleasing appearance to the concrete surface.

The invention will now be described by reference to the FIGURES, of which:

FIG. 1 illustrates placement of the preferred cordlike explosive composition;

FIG. 2 illustrates the ignition system;

FIG. 3 illustrates a means and method of protecting the explosive while the concrete hardens;

FIG. 4 illustrates the surface of the concrete after the explosive composition has been detonated;

FIG. 5 illustrates avariation of the surface of concrete possible with the invention.

Referring now to FIG. 1, an outer containing form for a slab of concrete is shown at 10 comprising sidewalls l2 and 14 and

end walls

16 and 18. The form rests on a flat surface such as previously poured slab of concrete 20 and can be separated therefrom if desired by a film of plastic 22 as a parting membrane. The form is conventional in structure however, the walls are slightly higher than conventionally used, e.g., if a wall of 6 inches thickness is customarily used or required for the necessary structural strength, the height of the wall is increased about 0.5 to 1.5 inch, preferably 0.5 to 0.8 inch to provide the desired depth of concrete to confine the explosive composition. The added height can be achieved by selection of wiser boards or material used to frame the form or strips or blocks of material 24 can be added to the edge of the framing material as shown by

strips

13,15, 17 and 19.

Opposite sides of the form are provided with a plurality of grooves 26 which are about l/ l 6 to inch wise and a depth equal to the desired depth of the explosives composition in the concrete. Depending on the final appearance desired on the finished surface of the wall, the grooves can be placed at equal or orderly spaces or can be randomly placed along the wall. If desired, similar grooves could be placed in the upper edges of the other two walls to provide a checked or criss-crossed placement of the explosive composition. This, however, greatly increases the labor for placement of the explosive composition and is not preferred.

The concrete is poured into the form and tamped, screeded and trowelled in the conventional manner, however, the extensive towelling and smoothing of the conventional surfacing isnt necessary. Instead, the explosive composition is placed beneath the surface of the wet concrete.

While any of a variety of commercially available explosive composition, such as described hereinafter, can be used, the most preferred is a detonating cord wherein a high explosive composition such as pentaerythritol tetranitrate, PETN, or cyclotrimethylenetrinitramine, RDX, is contained within a flexible sheathing formed of metal; plastic or fabric. There are a variety of these detonating cords available such as Primacord, Primex and Bona cord. Examples of suitable explosive compositions are: mild detonating fuse, MDF, which contains a plasticized PTNE explosive; completely confined mild detonating fuse,.CCMDF, which has a metal cladding about the cord; and flexible linear shape charge, FLSC, which is a metal-clad cord with a triangular cross-section having one concave side to provide a shaped charge configuration to the cord. The amount of explosive in the cord can vary from about 1 to 400 grains per linear foot, preferably from 5 to 50 grains per linear foot and, most preferably, from to about 12 grains per linear foot and, most preferably, form 10 to about 12 grains per linear foot. Generally, 50 grains per foot is the maximum used to avoid any danger of weakening the structural integrity of the concrete panel while a maximum of 12 grains is preferred for economy of the method.

The detonating cord is placed in the concrete while the latter is still wet, i.e., while it isstill plastic. This can be performed by laying the cord between grooves on opposite sides of the form and on the surface of the concrete, preferably after the latter has been screeded and tamped and then pulling the free ends of the cord.

taut so that the cord is pulled the dsired depth below the concrete surface. If desired, the cord can be worked to and fro across the surface to insure that it cuts into and beneath the surface. The cord can then be looped about the one end of the form as shown at 28 and returned to the opposite side of the form through the next adjacent groove. Alternatively, the cord can be cut into a plurality oflengths slightly longer than the length or width of form 10 and the free ends can extend outside of the form at each end. After placement of all the cords, the latter can be rolled, floated or tamped to move them the desired beneath the surface.

The grooves 26 and, hence, cord 30, can be placed at intervals between about 2and 12 inches, preferably from 3 to about 6 inches. The depth of the grooves and, hence, of cord 30, can be from /1 to 2, preferably from 178 to 1 inch below the top of the form, i.e., below the top surface of concrete 32. The spacing between lengths of cord, depth of placement and density of explosive in the cord is interrelated. If it is desired that the entire surface of the panel be removed, i.e.. relieved, cords of 12 grains explosive per foot placed at a depth of about 54 inch should be placed apart no further than about 4 inches. Spacings of up to 6 to 8 inches can be used with cord of fifty grains explosive per foot placed at a depth of A to 3'4 inch and complete removal will still be attained.

When the cords are placed at greater center-tocenter spacings than indicated, there will be some irdistance regularly shaped bands of unrelieved texture across the surface of the concrete. Even this, however, can be artistically appealing and, for this reason, there is not invariable dictate on either the order or the frequency of spacing of the cords 30 across the surface of the concrete.

A simple method to embed the cords beneath the surface of concrete 32 is shown by roller 34. This roller has a plurality of longitudinal fins or blades 36 which have a width approximately equal to the desired depth of placement of the explosive cord. The blades are about equal to the width of the surface and the roller can extend slightly further to rest on the upper edges of

sides

12 and 14. The cord can be placed on the surface of the concrete, pulled taut between the grooves and then rolled beneath the surface bypassing rolIer 34 across the concrete surface in the illustrated manner.

FIG. 2 illustrates the manner in which the cords are secured to an ignition system. The free ends of the cords are knotted securely to a transverse cord 40, preferably by a clove hitch knot, and one end of the cord 40 is secured to an ignition device-such as a conventional blasting cap 42. The knots can then be covered by a layer of electricians friction tape. The blasting cap is, in turn, connected by lengths of metal wires 44 which extend to a safe area where they are connected to a source of electrical energy such as a handoperated magneto 46 which is commonly used to ignite blasting caps.

Adequate safety precautions should be taken to prevent premature ignition of the explosive composition by vandals or by accidents. Generally, the concrete should harden for a period of several days to several weeks before detonating the explosive; about 4 to 8 days are preferred. During this time, the concrete panel can be guarded or a suitable cover can be locked over the exposed ends of the detonating cord. FIG. 3 illustrates a suitable protective means. This is shown on form 10 by end brackets 48 which are simple angle brackets which are riveted or permanently secured to the sides 12 and -14 of form 10. A protective metal plate such as angle bar 50 is placed over grooves 26 and the free ends of cords 30 and is rigidly secured to brackets 48 by welding or locked thereto by a hasp and padlock means, not shown.

The concrete panel is relieved after suitablehardening by activating the ignition system. Preferably the panel is permitted to rest on the ground in a horizontal position and the explosion will throw loosened concrete and aggregate upwardly for a distance of about 3 to 15 feet. A suitable weighted cover, e.g., a mattress, tarpaulin, etc., can .be laid over the concrete surface to minimize the throwing of debris therefrom. When the concrete panel is uprighted, most of the debris will fall from its surface and the remainder can be washed or brushed off.

FIG. Typically, illustrates the texture 'of the relief that can be attained when the cords have sufficient explosive composition density and are placed closely enough together and deeply enough to attain complete removal of the overburdening concrete. Tpically, this could be 12 grain per foot cord placed inch below the concrete level and on 3 inch centers.

The surface 52 of panel 60 has aslightly corrugated appearance with grooves 54 separated by higher ridges 56. The grooves are smooth and sharply defined and are formed by the cord placement in the concrete. The explosion moves outwardly and upwardly from the cord, diminishing in effectiveness with increasing distance from the cord and forming the ridges 56.

The surface of the concrete is relieved and the aggregate 56 is exposed. Some of the aggregate will also be observed to have been shattered so that its interior surface is also exposed.

If desired, various aggregates can be used to enhance the surface appearance. Colored aggregates, polished aggregates such as beach pebbles, or aggregates of a selected size range can be used in the entire concrete mix. The concrete panel can also be prepared in two pours, the first being a conventional mix up to th level of grooves 26 and the second pour being of a concrete mix containing the aggregate of a selected color or size.

A residue from the sheathing of the detonating cord will be found in the grooves 54. When cords which have a thin metallic coating are used, e.g., cords having an aluminum, copper or lead coating, a thin, straight metallic line will be embedded in the groove. This also provides a very aesthetically pleasing appearance. Accordingly, use of metal-sheathed explosive compositions is a preferred embodiment of the invention.

FIG. 5 illustrates a section 70 ofa panel formed when the explosion does not remove the entire surface, either because the explosive density was too low or the cords were placed too far apart or were to shallow in the concrete to achieve complete removal. This, too, provides an interesting and irregular relief pattern. The grooves 62 are separated by flat-topped ridges 64 and inclined areas 66 of surface relief. Areas 66 are similar to those shown and described with regard to FIG. 4. The flattopped areas, however, extend the length of the panel, parallel to grooves 62 and have a varied or feathered edge along the side of area 66.

The preceding discussion has been directed to using various detonating cords as the explosive composition. These, of course, are preferred and generally comprise a high explosive material within a sheathing. The sheathing can be extruded as a tube from plastic or metal or can be a film of plastic or metal wrapped about a core of the highly explosive material. Altematively, the sheathing can be a fibrous material, woven of fibers of plastic, e.g., nylon, polypropylene, etc., or of cotton, wool and the like. The sheathing can comprise one or more layers.

The explosive ingredient of the explosive composition can be any high explosive material including any of the following known high explosives which are present in major amounts, e.g., at least about 50 percent and preferably from 15 to 100 percent:

TABLE 1 EXPLOSlVE INGREDIENTS A. Primary mercury fulminate lead oxide diazodinitrophenol lead styphnate nitromannite B. Secondary .trinitrotoluene trinitrophenylmethylnitramine cyclotrimethylenetrinitramine pentaerythritol tetranitrate ammonium picrate picric acid ammonium nitrate dinitrotoluene ethylenediaminedinitrate nitroglycerine nitrostarch Various non-explosive diluents and inerts can also be present in minor amounts, e.g., in amounts up to about 25 weight percent of the mixture. These can be:

aluminum waxes, petroleum, beeswax, carnauba, etc.

diphenylamine nitiates of metals such as calcium, potassium mononitrotoluene ferrosilicon wood pulp calk diphenylamine sulfur carbon Some low explosive ingredients can also be present, again in minor amounts up to about 25 percent, and these include: sodium or potassium nitrate, sulfur, charcoal and mixes thereof, e.g., black powder, nitrocotton, etc. i Y

While some of the explosive materials in Table l are normally liquid, they can, nevertheless, be used by compounding with a suitable absorbent solid such as calk, pulp, carbon, sulfur, etc., in the manner well known in the art. The powders can also be waterproofed by coating the powders or their inert carrier such as the sheathing of detonating cord with wax or other water-resistant fillers.

The preferred explosive ingredient is a secondary explosive which requires an initiating detonation. The primary explosives can also be used, however, these require a great deal of caution in handling and are,'therefore, not the preferred explosive ingredient. As with conventional practice, the primary .explosive ingredient can be used in the initiating fuse or detonator. Thus, mercury fulminate or lead ozide are commonly used in blasting caps or fuses to create a detonation of sufficient intensity to initiate detonation of the secondary explosive ingredient.

The explosive ingredient should be admixed in or with a carrier which is a material that'will impart a structural integrity to the composition. With th'e preferred detonating cord, the plastic, cloth or metal sheathing serves this purpose by surrounding the powdered explosive ingredient. Other compositions can, however, be used. An example of another composition would be the filling of a molded plastic material with a high explosive powder. Thus, the powder could be admixed with plastics either in a powdered or molten state. Suitable polymers are: homo-and co-polymers of vinyl chloride, ethylene, propylene, vinyl acetate, isobutylene, butadiene, styrene, acrylonitrite, acrylomide, etc., and the mixture could be extruded into sheets, fibers or the like. Plasticiziers and some inert fillers such as previously mentioned could also be present in minor amounts, e.g., less than about 25 percent. Suitable plasticizers are the conventionally used resin plasticizers such as the esters of alkanols or alkylene glycols with monoand polycarboxylic acids, e.g., esters of alkanols with dibasic acids such as succinic, adipic, phthalic, terephthalic, naphthadioic, etc., or esters of glycols such as ethylene, propylene, hexarnethylene glycol and mono-basic acids such as acetic, propionic, valeric, caprylic, decanoic, lauric, etc. An example of a commercially available material of such a composition is Detasheet which is a sheet of plasticized polyisobutylene that is loaded with sufficient PETN to be detonatable.

While the invention has been described with reference to various preferred embodiments, it is apparent that other modes of practice can be used. Thus, a simple method for embedding the explosive composition such as a detonating cord in the concrete can be to place the cord across the face of the form before the concrete is poured and attach at least one end of the cord to the form with a tension spring. This is shown in FIG. 3 where the endsof the cord 30 are attached to springs 49 which are hooked over the heads of nails 50. The concrete can be poured into the form and it will force the cords to the bottom of the form. The concrete is then vibrated in a conventional manner and the tension of springs 49 will pull the cords up until they lie in the plane between grooves 26.

Other embodiments can also be used such as the placement of the explosive composition adjacent the bottom surface rather than the top surface of form Ml. tee explosive composition can then be detonated while it rests on the ground and no protective cover need be placed over the concrete panel. This construction will also permit placing the crane or hoist connector on the top surface and permit the direct lifting of the panel into place without reversing it. Conventional connectors such as the two inch diameter helical springs which are welded or secured to the internal reinforcement of the panel can be used for attachment of the lift cable. If desired, the panel can be partially raised or tilted by the crane or hoist and the explosive composition can then be detonated. Also, if desired, the panel can be fully raised to a vertical position and the explosive composition then detonated, however, care should be taken to insure adequate bracing of the panel in its upright position.

The explosive composition can also be embedded in the concrete after it has hardened by cutting grooves in the concrete to the desired depth and a explosive compostion such as the detonating cord can be laid in the grooves. The cord or composition can than be covered with a layer of concrete or mortar which can be applied over only the grooves or across the entire face of the concrete panel. The concrete or mortar can contain quick curing additives such as the conventional aluminite or calcium chloride to hasten the curing of the concrete or mortar.

When the exposed face of the panel is to be blown off, i.e., when the panel is not resting on this face, it is desirable to cover the exposed face with a protective mat. A conventional mat such as a rug formed of long or continuous steel wire coils or a mat formed of rubber pads wired together can be used in lieu of the mattress pads previously mentioned. The rubber pads can be cut up sections of old tires.

Although no prior mention has been made of the reinforcement of the panels, conventional reinforcement .can be used since the explosive force is directed outwardly, no significant weakening of the panel will be experienced. I

From the preceding discussion it is apparent that the invention is not intended to be limited to the illustrated modes of practice. Instead, it is intended that the invention be defined by the means and their obvious equivalents and the steps and their obvious equivalents set forth in the following claims.

I claim:

11. A method for the preparation of a concrete surface having an irregularly relieved surface texture that comprises:

a. pouring concrete into a containing form;

b. embedding at a-depth of from A to 2 inches below the exposed surface of the concrete at intervals no closer than about 2 inches, a continuous explosive composition comprising a detonating strip formed of a water resistant high explosive material and a carrier therefor to provide a concentration of said explosive material of from 3 to about 400 grains per square foot of said surface;

c. securing igniting means in communication with said explosive composition;

d. permitting said concrete to harden to a permanent set above said explosive composition; and

' e. detonating said explosive composition by activating said igniting means whereby the surface of said concrete is blasted from above said explosive composition in an irregular pattern.

2. The method of claim 1 wherein said explosive composition is embedded in freshly poured concrete.

3. The method of claim 1 wherein said explosive composition is a detonating cord comprising a highexplosive material within a sheath.

4. The method of claim 2 wherein said explosive composition is a detonating cord comprising a high explosive material within a sheath.

5. The method of claim 4 wherein said detonating cord has a concentration of high explosive material from 5 to 50 grains per linear foot and is placed across said surface at intervals fr0m'2 to 12 inches and at a depth of from /2 to 1 inch.

6. A combination of a containing form having sidewalls and a bottom defining a container, concrete within the form and distributed across the concrete at a depth of from A to about 2 inches beneath an exposed surface thereof at intervals no closer than about two inches, a continuous explosive composition comprising a detonating strip of a water resistant high explosive material and carrier therefor at 21 concentration of said high explosive material of from 3 to about 400 grains persquare foot of said surface.

7. The combination of claim 6 wherein said explosive composition is a detonating cord comprising a high explosive material within a sheath.

8. The combination of claim 6 wherein said detonating cord has a concentration of high explosive material from 5 to about 50 grains per linear foot and is placed in said concrete at regular intervals at 2 to about 12 inch spacing.

9. A panel of concrete having embedded at a depth of from A to about 2 inches beneath at least one of its exposed surfaces and at intervals no closer than about two inches, a continuous explosive composition comprising a detonating strip of a water resistant high explosive material and carrier therefor, at a concentration of said high explosive material of from 3 to about 400 grains per square foot of said surface.

10. The panel of concrete of claim 9 wherein said ex plosive composition is a detonating cord having a concentration of high explosive material from 5 to about after the concrete has hardened and covered with a layer of fresh concrete which is permitted to harden before the step (e) of detonation of said explosive composition.

UNITED STATES PATENT OFFICE CETIFICATE 0F CUECTIN Patent No. 3 789 759 Dated Feb 5 1974 Robert L. Jones Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 57, delete "165" and insert -3/8 line 57, delete "wise" and insert wide Column 3,

line 17, delete "PTNE" and insert PETN line 55,

delete "178" and insert 1/2 Column 4, line 58,

"Typically," should read 4 line 62, delete "Tpically" and insert Typically". Column 5, line 14, delete "th" and insert the Signed and sealed this 17th day of September 1974.

(SEAL) Attest:

McCOY M GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents I My 1050 (10-69) USCOMM-DC 60376-P69 U.$ GOVERNMENT PRINTING OFFICE I I959 0-365'334,

line 17, delete "PTNE" and insert PETN line 55,

delete "178" and insert 1/2 Column 4, line 58,

(SEAL) Attest:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. lsysgl/w Dated 9 Robert L. Jones Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 17, delete "PTNE" and insert PETN line 55, delete "178" and insert l/Z Column 4, line 58,

(SEAL) Attest: v

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents I 0 (10- USCOMM-DC 6O376-P69 U.S. GOVERNMENT PRlNTING OFFICE: 1959 0"355'334,

Claims

1. A method for the preparation of a concrete surface having an irregularly relieved surface texture that comprises: a. pouring concrete into a containing form; b. embedding at a depth of from 1/4 to 2 inches below the exposed surface of the concrete at intervals no closer than about 2 inches, a continuous explosive composition comprising a detonating strip formed of a water resistant high explosive material and a carrier therefor to provide a concentration of said explosive material of from 3 to about 400 grains per square foot of said surface; c. securing igniting means in communication with said explosive composition; d. permitting said concrete to harden to a permanent set above said explosive composition; and e. detonating said explosive composition by activating said igniting means whereby the surface of said concrete is blasted from above said explosive composition in an irregular pattern.

3. The method of claim 1 wherein said explosive composition is a detonating cord comprising a high explosive material within a sheath.

5. The method of claim 4 wherein said detonating cord has a concentration of high explosive material from 5 to 50 grains per linear foot and is placed across said surface at intervals from 2 to 12 inches and at a depth of from 1/2 to 1 inch.

6. A combination of a containing form having sidewalls and a bottom defining a container, concrete within the form and distributed across the concrete at a depth of from 1/4 to about 2 inches beneath an exposed surface thereof at intervals no closer than about two inches, a continuous explosive composition comprising a detonating strip of a water resistant high explosive material and carrier therefor at a concentration of said high explosive material of from 3 to about 400 grains per square foot of said surface.

9. A panel of concrete having embedded at a depth of from 1/4 to about 2 inches beneath at least one of its exposed surfaces and at intervals no closer than about two inches, a continuous explosive composition comprising a detonating strip of a water resistant high explosive material and carrier therefor, at a concentration of said high explosive material of from 3 to about 400 grains per square foot of said surface.

10. The panel of concrete of claim 9 wherein said explosive composition is a detonating cord having a concentration of high explosive material from 5 to about 50 grains per foot and placed in said concrete at regular intervals of 2 to about 12 inch spacing.

11. The method of claim 1 wherein said concrete is provided with a plurality of cord-receiving grooves and said explosive composition is placed in said grooves after the concrete has hardened and covered with a layer of fresh concrete which is permitted to harden before the step (e) of detonation of said explosive composition.