US 3601448 A
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United States Patent  lnventor Stanford C. Stone Deerfield, 111.
 Appl. No. 817,783
 Filed Apr. 21, 1969  Patented Aug. 24, 1971  Assignee Gas Development Corporation Chicago, Ill.
 METHOD FOR FRACTURING CONCRETE AND OTHER MATERIALS WITH MICROWAVE  References Cited UNITED STATES PATENTS 3,430,021 2/1969 Watson 219/1055 3,443,051 5/1969 Puschner 219/1055 Primary Examiner- Ernest R. Purser Attorney-Dominik, Knechtel & Godula ABSTRACT: A low-noise method and apparatus for fracturing concrete and other solid materials by coupling microwave energy into the material in a fashion such as to generate independent heat patterns using at least two or more microwave applicator horns that are spaced apart from each other. The power density of the microwave energy coupled into the solid material is established and maintained below a predetermined threshold level above which violent and explosive type reactions occur. The spaced heat patterns or zones produced cause the heated material to expand, so as to place high tensile forces or stresses on the unheated material between the heat patterns. These forces or stresses cause failure to occur between the heat patterns, and ultimately across the heat patterns themselves.
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METHOD FOR FRACTURING CONCRETE AND OTHER MATERIALS WITH MICROWAVE ENERGY apparatus for carrying out these methods. More particularly,
the invention relates to improved methods and apparatus for thermally inducing stresses in such types of solid materials, to cause failure thereof in compression or tension in a known and predictable manner.
The apparatus presently most commonly used for breaking solid materials such as those mentioned above is the pneumatic hammer. While extremely effective, the use of pneumatic hammers often causes public complaints when they are used in densely populated areas. The reason is that both the pneumatic hammer itself and the air compressor that supplies high pressure air to the hammer produce irritating noise of highintensity. This noise is particularly objectionable in, areas where hospitals and schools are located. This noise also excludes, or at least restricts, the scheduling of excavation operations at night, which, in many cases, such as pavement removal for repairing or laying underground cable and pipes, would reduce the downtime and, the inconvenience to the public.
Recently, considerable effort has. beenand is being made to develop a low-noise method for breaking solid material, particularly concrete pavementpElectric and gas utility companies are especially interested in thedevelopment of such a method because of the need to remove concrete pavement to lay new underground cables and pipes, or to repair existing cables and pipes.
One low-noise method recently developedinvolves the use of a so-called-plasma torch which actually melts theconcrete that is to be removed. Improved torch designs and fuel gas systems have been developed during'the investigation of the plasma torch as an effective low-noise device forconcrete pavement removal, however, its use has been substantially abandoned because of the problem of disposing of the molten concrete, which presents difficulties in making deeper cuts. Accordingly, industry is still seeking an effective, operable,
low-noise method and apparatus for breaking solid material.
,In U.S. Pat. No. 2,859,952, there is disclosed a method of mining using high-frequency magnetic energy, to fracture and comminute the oreas mined so thatthe latter'will not be in large unwieldy boulders, but in relatively small fragments. The fracture operation achieved results from producing localized expansion so as to subject adjacent areas of the rock contain ing the ore to tensile stresses beyond the fracture strain thereof.
The method and apparatus of this patent may be classified as a low-noise method of breaking solid material, however, its use is generally restricted to mining-type operations where the ore or material has a metallic content response to highfrequency magnetic radiation, and where his only necessary or desired to produce a large number of random, uncontrolled fractures in the solid material. The ore or material thereafter can be more easily broken up into small fragments. Furthermore, the extent or direction of the fractures is not known or predictable, hence the method cannot be used to, for example, generate a straight-crack along a desired course. The large amount of energy, approximately 25 kilowatts, andthe apparatus required to generate this amount of energy also make the disclosed method less than desirable.
in U.S. Pat. No. 3,2l9,280, there is disclosed anothermethod and apparatus for splitting nonmetallic brittle materials using ultrahigh-frequency oscillations. The apparatus of this patent likewise generates or produces exceptionally high internal tensions in the interior of the stone. These tensions, in turn, cause a large number of random cracks or fractures to occur so that the stonethereafter can be easily broken upinto small fragments.
More particularly, the apparatus of this last-mentioned patent comprises a radiating element in the form of an injector having two relatively perpendicularly polarized radiators each radiating substantially into the same local space. In operation,
the injector is introduced into a hole drilled into the stone for locally concentrating the ultrahigh-frequency oscillations in the interior of the stone at the area of the injector so that the stone cracks and thereafter can be broken up into smaller pieces.
The method of the present invention is a low-noise method, and involves coupling microwave energy into the solid materials such as concrete or rock to thermally induce stresses leading to failure in compression or tension, in a known and predictable manner. The apparatus for coupling the microwave energy into the solid material is adapted to generate independent heating patterns by, for example, using at least two or moremicrowave applicator horns that are spaced apart from each other. The applicator horn or horns for generating the heat patterns are positioned on or slightly above the solid material to be fractured. The spaced heat patterns or zones produced cause the heated material to grow" or expand, due to its coefiicient of heat expansion, so as to place high tensile forces or stresses on the unheated material between the heat patterns. These forces or stresses cause failure to occur between the heat patterns, and ultimately across the heat patterns themselves. In, this manner, a substantially straight crack between adjacent heatpatterns along a desired course can be produced. The crack that is generated is a hairline crack, whichmay extend completely through the solid material, depending upon its thickness, however, in each case, it is substantially weakened.
For example, the method and apparatus can be used to break up extremely hard and extremely thick slabs of concrete such as may be used fora support base for heavy equipment or the like. In the past, pneumatic hammers usually have been used for this job also. Normally, the fractures or cracks developed do not extend completely through these slabs, however, the slabs are weakened so that they thereafter can be more easily and more quickly broken up, with less wear or damage resulting to the equipment. Once cracked and weakened, the bit of the pneumatic hammer can be more easily driven into the slab to break it up.
This same technique of weakening the concrete so that it can be more easilybroken up with a pneumatic hammer also can be applied in mining, tunneling, and other similar applications. For example, in mining, the solid material to be mined can be initially weakened by cracking it through the application of microwave energy so that the mining drills and/or bits, pneumatic hammers and the like can break up the precracked material more easily. This factor also causes less wear or damage to the mining equipment. In tunneling, the same technique can be used.
Accordingly, it is an object of the present invention to provide improved methods employing microwave energy for breaking solid material which is electrically or dielectrically lossy in nature, and the apparatus for carrying out these methods.
Another object is to provide improved methods and apparatus of the above type, for thermally inducing stresses in such types'of solid material, to cause failure thereof in compression or tension in a known and predictable manner.
A still further object is to provide a completely self-contained, mobile microwave system which is capable of cracking solid material of the above described type.
Still another object is-to provide improved methods and apparatus of the above-described type which is capable of crackingthe solid material in substantially the same period of time generally required with presently available apparatus such as hydraulic and pneumatic hammers and the like.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the several steps and relation of one or more of such steps with respect to each of the others and the apparatus embodyingfeatures of construc- FIG. 2 is a front plan view of a multihom applicator, exemplary of one embodiment of the invention;
FIG. 3 is a side plan view of the multihom applicator of FIG.
FIG. 4 is a top plan, pictorial-type view of the microwave system of the multihorn applicator of FIGS. 2 and 3;
FIG. 5 is a side plan, pictorial-type view of the microwave system of the multihorn applicator of FIGS. 2 and 3;
FIGS. 6 and 7 are a front plan view and a side plan view of one of the applicator horns of the multihom applicator of FIGS. 2 and 3;
FIG. 8 is a graph showing the relative signal strength, in
decibels, of the microwave energy penetrating various thicknesses of concrete;
FIG. 9 is a view generally illustrating the heat patterns or zones generated by the multihom applicator of FIGS. 2 and 3;
FIG. 10 is a graph representative of the relationship between the spacing between the applicator horns and/or heat zones with respect to the thickness of concrete;
FIG. 11 is a perspective view of a multihom applicator exemplary of another embodiment of the invention;
FIGS. 12 and 13 are a partially sectionalized side plan view and a bottom plan view of a microwave cavity for asphalt removal; and 1 FIGS. 14 and 15 are a side plan view and a bottom plan view of an applicator horn which is adjustable to vary its energy distribution area.
Similar reference characters refer to similar parts throughout the several views of the drawings.
Referring now to the drawings, in FIGS. 2 and 3 there is illustrated a multihom applicator 10 which has four (as illustrated) applicator horns 11-14. These applicator horns 11-14 when linearly aligned are capable of generating or producing a controlled or predictable crack, along a desired course, with little or no attendant noise, in a manner described more fully below.
The multihorn applicator 10 is constructed of two identical modular frames 15 and 16 which are adapted to be affixed together to form an integrated unit. These modular frames 15 and 16 advantageously are fabricated from high-strength aluminum tubing welded together to provide a lightweight, sturdy support for the microwave components contained within them. As can be best seen in FIGS. 4 and 5, the microwave components contained with each of the modular frames 15 and 16 comprise a microwave power-generating device 20,
which in the illustrated embodiment, is magnetron 21 that is; capable of delivering an output of 5 kilowatts, at a frequency of approximately 2.450 MHz. The magnetrons 21 can be, for
example, a Type YJl 190 magnetron manufactured by Amperex, or another type microwave power generator.
The magnetron 21 is coupled to the wave guide system, by means of a right angle transition unit 22 and a wave guide transition unit 23 which is a unit for coupling together wave guides of different sizes. A bidirectional coupler 24 for monitoring forward and reflected microwave power for determining the VSWR of the microwave system is coupled between the wave guide transition unit 23 and an E-H tuner 25 which is used to match and balance discontinuities of the microwave system to reduce power loss. The E-I-I tuner 25 is coupled by means of a 90 H-plane bend 27 to a power dividing tee 28 which divides the microwave power into two equal branches, and the latter is coupled to a pair of flexible wave guides 32 by means of a pair of 90 I-I-plane bends 29 and 30. Aftixed to the opposite ends of the flexible wave guides 32 is a 90 E-plane bend 34, for providing a 90 direction change in the E plane. The applicator horns 11-14 are affixed respectively to one of the E-plane bends 34.
A microwave power supply 35 (FIG. 1) is coupled to the magnetrons 21 by means of coaxial cables 36 and 37 which preferably are heavy-duty high voltage cables. A liquid-type heat exchanger 38 also is provided, and the latter is coupled to the magnetrons 21 by means of the water hoses 39. As can be seen in FIG. 1, in the illustrated embodiment, both the microwave wave power supply 35 and the heat exchanger 38 are housed in a motorized van 40 which also is adapted to receive the multihom applicator 10, so that the system can be easily transported from one location to another. The van 40 also is advantageously provided with a special transmission having a power takeoff gear assembly 41 which is drivingly coupled with a 25 Kv.-a. AC generator 42 that generates the 3-phase, 450.volt, 60 cycle current for powering the power supply 35 of the microwave system. In this manner, a completely self-contained unit is provided so that no outside source of power is required and the multihorn applicator 10 is usable in any remote location.
The van 40 also includes an I-beam trolley-hoist system (not shown) to facilitate loading and unloading of the multihom applicator 10. The latter has eyehooks 43 affixed to each of the modular frames 15 and 16, for receiving a hook for lifting them. The modular frames each also have swivel casters 44 affixed to them, to permit them to be easily moved about on the concrete pavement.
The applicator horns 11-14, as can be best seen in FIGS. 6 and 7, are rectangular-shaped horns, and each of them preferably has a skirt-type shield 46 of metal screening affixed about the lower ends'thereof, for preventing stray radiation. These shields 46 also preferably are weighted about the peripheral edges thereof so that the edges seat on and conform to the surface of the concrete pavement. Vent holes 47 are provided in the applicator horns to permit the escape of moisture driven out of the concrete. A window or plug 48 of Teflon or other low-loss material which is transparent to microwavesis affixed within each of the applicator horns to prevent extraneous material such as moisture, concrete dust and chips from entering and damaging the wave guide system and/or the magnetrons. In operation, as more fully explained below, the horns horns 11-14 preferably are positioned on or just slightly above the surface of the concrete pavement. Accordingly, an adjustable assembly 49 is incorporated into each of the modular frames 15 and 16, for raising and lowering the applicator horns to position them. The adjustable assemblies 49 also are adapted to horizontally or laterally adjustably position the applicator horns 11-14 so that the lateral spacing between them can be varied, for reasons set forth more fully below.
The construction of the multihom applicator 10 resulted after numerous tests to determine the factors which contribute to the successful cracking of concrete pavement. While all of the factors involved are not fully known, it has been determined that successful cracking does depend on the microwave energy distribution pattern and the power density (watts per square inch) applied to the concrete, and the total energy coupled into the solid material by all of the applicator horns.
For example, the results of one of the many tests conducted indicates that the design or geometry of the applicator horns is not critical, provided the applicator horns are electrically tuned so that there is a reasonably close impedance match with the particular solid material to be fractured.
Still other tests on determining the behavior of microwaves inside concrete and the effect of the applicator horn geometry on the energy distribution pattern indicate that there is less likelihood of spalling and cratering occurring when microwave energy is applied to the solid material through two or more applicator horns than there is when only a single applicator horn is used and the same level of microwave energy horns as applied through any one of the multiple applicatorv horns.
Spalling and cratering is an explosive-type reaction, and
cracking may or may not take place. If cracking does result, it generally is of a random nature, propagating to the nearest discontinuity, flaw or boundary-grain in the material. This explosive-type reaction is generally the reaction which the method of the above-mentioned US. Pat. No. 3,219,280 depends upon to fracture and comminute the stone to produce relatively small fragments. With this type of operation, it is virtually impossible to generate or produce a predictable crack in concrete or other solid material.
With the multihorn applicator 10, the microwave energy is coupled into the concrete pavement in a manner such as to thermally induce stresses leading to failure in compression or tension, in a predictable manner. While the multihorn applicator 10.is illustrated and described as having four.applicator horns 11-14, it is to be realized that a single applicator horn capable of generating two or more independent heat patterns or any number of individual applicator horns greater than two can be used, as will be apparent from the description below.
More particularly, the applicator horns 11-14 are spaced apart from each other so as to generate independent-heat patterns or zones in the concrete. In order to produce a substantially straight crack along a desired course, the applicator horns are linearly aligned, and are spaced on or slightly above the concrete to be fractured. The microwave energy emitted by the applicator horns 11-14 is generally equally distributed, as can be seen in FIG. 8 which is a graph indicating the relative signal strength, in decibels, of the microwave energy which penetrates various thicknesses of concrete. In the illustrated example, the applicator horns 1 1-14 are spaced approximately 8 inches apart, and the plot A represents the relative signal strength of the microwave energy penetrating l.75 inches of concrete; plot B, 3.50 inches of concrete; plot C, 5.25 inches of concrete; and plot D, 7.00 inches of concrete.
The spaced heat patterns produced by the applicator horns cause the heated material to expand, due to its coefficient of heat expansion, so as to place high tensile forces or stresses on the unheated material between the heat patterns. These heat patterns generally appear as discolored areas on the concrete, as represented by the reference numerals 50 in FIG. 9. As soon as a crack develops, it is usually detectable by a damp or moist line which forms on the surface of the concrete, along the path of the crack. This damp or moist line results from water or moisture in the concrete migrating to the surface, upon being released when the bond between the concrete particles is broken.
In numerous tests, it is found that the heat patterns must reach a certain temperature, which temperature varies depending upon the composition of the concrete, before a fracture will be initiated. Also, these tests indicate that there is a certain threshold limit of power density (in watts per square inch) which can be applied to the surface of the concrete,
' above which violent or explosive reactions occur. These violent and explosive reactions, as indicated above, generate a crater or cause the concrete to spall. The fracture or cracking rate in lineal feet per minute increases in proportion to the total power applied to the concrete, however, as indicated above, the power density applied to the concrete preferably is established and maintained below the level at which the explosive reactions occur. In some cases, it is desirable to pulse or progressively increase the energy output of each of the applicator horns 11-14 or power density applied to the surface of the concrete, to facilitate cracking. There is a relationship of the spacing between the applicator horns 11-14 and the thickness of the concrete for optimum operation. The graph of FIG. 10 is representative of this relationship, and it can be seen that with concrete 8 inches in thickness, the applicator horns ll-14 can be spaced approximately 6 inches apart. With concrete 2 inches in thickness, the applicator horns 11-14 can be spaced approximately 12 inches apart. By spacing the applicator horns substantially as indicated in FIG. 10 and applying power in the manner described above, concrete can be fractured, in approximately 3-8 minutes, depending 'upon its thickness, hardness and composition.
As indicated above, the magnetrons 21 included in the modular frames 15 and 16 are each rated at 5 kilowatts or a total of 10 kw. power. Therefore, the maximum power output of each of the four applicator horns fed by the magnetrons is approximately 2.5 kilowatts. The fracture or cracking rate in lineal feet per minute increases in proportion to the total power applied to the concrete, however, as indicated above, there is an established energy density threshold level which should not be exceeded. Accordingly, it would be ad vantageous to have a greater number of applicator horns, each of which is capable of providing an energy density output of at least this established threshold level.
In FIG. 11, there is illustrated a multihorn applicator 52 constructed of four rather than two modular frames 53-56 which are adapted to be affixed to one another to form an integral or composite unit. The modular frames 53-56 are generally like the modular frames 15 and 16 above, however, each of them includes a magnetron 57 for supplying microwave energy to its associated applicator horn 68-71, respectively, at at least the established energy density threshold level. The principle advantages of the multihorn applicator 52 is its greater versatility in the sense that the spacing between the applicator horns is not restricted or limited, individual power control and replacement of parts, to mention but a few. Otherwise, its operation is generally like that described above, in the case of the multihorn applicator 10.
In many instances, the concrete to be fractured is covered with a layerof asphalt which must be removed before the concrete can be fractured. In FIGS. 12 and 13, there is illustrated a microwave cavity 60 for asphalt removal comprising a box 61 of metal having a pair of microwave mode mixers 62 therein, for applying uniform microwave radiation to an asphalt surface. The microwave mode mixers 62 each includes a number (6 as illustrated) of propeller blades 63 mounted on a shaft 64 which is rotatably driven by means of a motor 65. A baffle 66 is disposed between the two mode mixers, as can be best seen in FIG. 13.
In operation, the microwave cavity 60 is installed in the multihorn applicator 10, or 52, in place of two of the applicator horns. When the microwave energy is introduced into the microwave cavity, the rotating blades 63 of the mode mixers 62 deflect the microwaves in all directions. The additional deflections at the cavity walls further disperse the microwave energy inside the cavity, to uniformly radiate the asphalt beneath it. Once the asphalt paving is heated to removable state with microwaves, it can be easily removed with other mechanical means.
In addition to its application for fracturing concrete pavement, sidewalks and the like, the method and apparatus of the invention also can be used to break up extremely hard and extremely thick slabs of concrete such as used for support bases for heavy equipment and the like. These slabs can be initially cracked using a multihorn applicator of the type described above to weaken it, so that a pneumatic hammer or other similar device thereafter can be more easily used to break up the slab.
This same technique of weakening the concrete also can be applied in mining, tunneling and other similar operations. The rock, stone and the like to be mined can be initially weakened by cracking it using microwave energy in the manner described above, so that the mining drills and bits, pneumatic hammers and the like can break up the precracked material more easily and more quickly with less attendant wear or damage to the removal equipment. In tunneling, the same technique can be used.
When fracturing extremely hard and extremely thick slabs of concrete, and in mining, tunneling and other similar operations, it is believed that the mechanism of thermal fracture is dependent upon a thermally induced stress which will exceed the ultimate strength of the concrete or rock. The driving force for propagation of cracks is provided by the elastic ener gy stored in the concrete or rock at the instant of fracture. The mechanism of energy dissipation which tends to arrest propagating cracks is the effective surface energy required to producethe newly formed cracked surfaces.
Localized heating, such as that caused by the microwave energy coupled into the solid material by the individual applicator horns, produces temperature gradients which will create stresses in the material. Concrete and particularly rocks have low thermal conductivity and appreciable thermal expansion, hence if the temperatures are high enough and the gradients steep enough, then the induced stresses will overcome the strength of the material, thus causing fractures in compression or tension.
In some applications, it may be desirable to increase, or decrease, the size" of the heat patterns generated by the applicator horns, in order to apply more power to the concrete, but at the same energy density. Accordingly, it wouldbe advantageous to have applicator horns which have adjustable energy distributions. An arrangement of this nature also is ad-.
vantageous in that it eliminates the need for replacing the applicator horns with a microwave cavity, such as the microwave cavity '60, for applications where asphalt or other similar material is to be weakened" for removal. In the latter case, the energy distribution of the applicator horns is merely enlarged so as to encompass a larger area.
Adjustable energy distributions for the applicator horns can be provided electrically or mechanically. One manner in which this can be accomplished mechanically is illustrated in FIGS. 14 and 15, wherein an applicator horn 85 is shown having two slidably adjustable horn jaw halves 86 and 87 which are affixed together in overlapping relationship by means of pins 88 disposed within the slots 89. As can be seen, these horn jaw halves are adjustable with respect to one another in a fashion such that the dimensions of the horn cavity and the output aperture thereof can be enlarged or made smaller, to thereby vary the energy distribution thereof.
As indicated above, cratering and spalling preferably is avoided while producing a controlled crack or fracture, however, a void in the solid material, a delamination thereof or other defect in the solid material may cause either cratering or spalling to occur. The explosive-type reaction which results, in many cases, will blow" the applicator horn or horns off of the surface of the solid material being fractured. Accordingly, some means must be provided to protect against these forces from being transmitted to the magnetron or other microwave generating equipment, to protect the latter from damage.
In the case of the multihom applicator 10, the flexible wave guides 32 permit the applicator horns 11-14 to move vertically, and simultaneously absorb the forces which would be transmitted to the magnetrons if the wave guides were of a solid, rigid construction. In the case of the multihom applicator 52, it may be noted that the applicator horns 68-71 are slidably telescopically coupled to the wave guides 80. With this arrangement, the telescopic coupling permits the applicator horns to vertically raise and lower, and in this fashion, the explosive forces imposed on the applicator horns are absorbed.
It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are eff ciently attained and certain changes may be made in carrying out the above method and in the construction set forth. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
Now that the invention has been described, what is claimed as new and desired to be secured by Letters Patent is:
l. A low-noise method employing microwave energy for fracturing in a known and predictable manner solid material such as concrete, rock or other compounded or coalesced material which is electrically and/or dielectrically lossy in nature comprisingthe steps of: generating microwave energy; coupling said microwave energy into said SOIlCI material y passing it through a plurality of spaced-apart applicators; controlling and maintaining the power density of the microwave energy coupled into said solid material to below a predetermined threshold level above which violent and explosive type reactions occur, said threshold level being dependent upon the composition of said solid material and the number of applicators being used to couple said microwave energy into said solid material; heating said solid material with said microwave energy to create at least two spaced-apart heating patterns at a temperature such as to create stresses with the solid material at least between said heat patterns that exceed the strength of and thereby cause failure of said solid material; and thereafter terminating the application of said microwave energy.
2. The low-noise method of claim 1, wherein the spacing between said applicators and hence said heat patterns is established in accordance with the thickness and composition of the solid material to be fractured.
3. The method of claim 2, wherein said applicators and hence said heat patterns created within the solid material are linearly aligned, and the failure of the solid material is in the nature of a crack extending between and across said heat patterns in substantially a straight line.