WO2010059169A1

WO2010059169A1 - Conductive concrete for heating and elelctrical safety

Info

Publication number: WO2010059169A1
Application number: PCT/US2008/084564
Authority: WO
Inventors: Christopher Y. Tuan; Lim Nguyen; Bing Chen
Original assignee: Board Of Regents Of University Of Nebraska
Priority date: 2008-11-24
Filing date: 2008-11-24
Publication date: 2010-05-27

Abstract

A conductive concrete is provided that contains various amounts of conductive materials. The conductive concrete is made by mixing cement, aggregate, water, and conductive materials. The conductive materials may include both metal fibers and metal particles. Particles made of carbon and graphite may be used to make the mixture easier to blend and pump. Conductive aggregates such as iron ore, copper ore, and taconite may also be used. Due to the electrical resistance in the conductive concrete mixture, heat is generated when connected to a power source. A microwave heating system which efficiently heats the conductive concrete by utilizing the radio frequency absorption properties of taconite is also disclosed. Several applications of the conductive concrete are disclosed including a conductive concrete slab, a radiant heating system, an indoor radiant heating system, and a high voltage system.

Description

CONDUCTIVE CONCRETE FOR HEATING AND ELECTRICAL SAFETY

BACKGROUND OF THE INVENTION

Conventional concrete is not electrically conductive. The electrical resistivity of normal weight concrete ranges between 6.54 and 11 kΩ m. A hydrating concrete consists of pore solution and solids, including aggregates, hydrates and unhydrated cement. The electric resistivity of the pore solution in cement paste is about 0.25 - 0.35 Ω m. Most common aggregates (e.g., limestone) used in concrete, with electrical resistivity ranges between 3 x 10² and 1.5 x 10³ Ω m, are non-conductive.

SUMMARY OF THE INVENTION In a first aspect, a conductive concrete slab comprising a conductive concrete mixture, comprising cement, a conductive aggregate, water, and conductive materials, wherein said conductive materials are selected from a group comprising carbon and graphite particles, and at least two electrodes embedded in said conductive concrete mixture at spaced locations is provided. In a second aspect, a radiant heating system comprising a plurality of concrete slabs in spaced relation, each concrete slab comprising a first layer of conventional concrete, a second layer made of an electrically conductive material situated atop said first layer; the second layer comprising a cementitious composite admixed with conductive materials, electrodes in contact with the second layer, a third layer comprising a thin coating of non- conductive material situated atop the second layer, and a voltage source that applies a voltage differential across the electrodes, thereby causing an electric current to flow through the second layer is provided.

In a third aspect, an indoor radiant heating system comprising a plurality of prefabricated conductive concrete panels affixed in spaced relation to an existing structure, wherein each conductive concrete panel comprises a cementitious composite admixed with conductive materials and at least two electrodes in contact with the panel, a voltage source that applies a voltage differential across the electrodes, thereby causing an electric current to flow through the conductive concrete panel, and an electrical control system that controls the voltage source according to a desired temperature setting is provided. In a fourth aspect, a high voltage system, comprising at least two conductive elements with a high voltage differential between the two conductive elements, at least one structural element proximate to at lease one of the at least two conductive elements, and a conductive concrete slab in electrical contact with the at least one conductive structural element and electrical ground is provided.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings which form a part of this specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views: FIG. 1 is a top plan view of the heated bridge deck system of the present invention;

FIG. 2 is a more detailed top plan view of a single row of concrete slabs spanning the width of a bridge deck;

FIG. 3 is a top perspective view of a section of a heated bridge deck system incorporating an electrical power source to heat the surface layer of the pavement;

FIG. 4 is a top perspective view of a section of a heated bridge deck system incorporating a microwave/radio frequency energy source to heat the surface layer of the pavement;

FIG. 5 is a side perspective view of a section an embodiment of the present invention utilized in an indoor radiant heating system;

FIG. 6 is a top perspective view of an embodiment of the present invention being utilized in an indoor radiant heating system;

FIG. 7 is a top perspective view of a section of an alternative embodiment of the present invention being utilized in an indoor radiant heating system; and FIG. 8 is a perspective view of an embodiment of the present invention being used to ground random currents generated on a high speed rail system.

DETAILED DESCRIPTION OF THE INVENTION CONDUCTIVE CONCRETE MIXTURE

Conductive concrete may be defined as a cement-based admixture, which contains a certain amount of electrically conductive components to attain a stable electrical conductivity to enable conduction of electricity through concrete. A conductive concrete mixture has been developed which contains various amounts of conductive materials. Due to the electrical resistance in the conductive concrete mixture, heat is generated when connected to a power source. The conductive concrete of the present invention is made by mixing cement, aggregate, water, and conductive materials. Type I or Type III Cement may be used and may comprise 12-16% of the total volume of conductive concrete. (Unless otherwise indicated, all percentages are based upon the total volume of conductive concrete.) Cement may more specifically comprise 14-16% by volume. The aggregate may contain some type of electrically conducting materials such as iron ore, copper ore, or taconite. Since the electrical conductivity of copper is about 6 times that of iron, copper-rich aggregates may be used if increased conductivity is desired. Using conductive aggregates will reduce the volume of steel particles and fibers required to maintain stable electrical conductivity. Alternatively, a chemical admixture may be added to aggregate to enhance electrical conductivity. Again, the objective of using a chemical admixture is to reduce the volume of steel particles and fibers required to maintain stable electrical conductivity. The aggregate used may be comprised of 10-25% fine aggregate and 10-25% coarse aggregate. Fine aggregate and coarse aggregate comprise 13-18% and 17-20% by volume respectively. Fine aggregate typically includes sand and gravel. For instance, Nebraska 47B may be used. The ratio by weight of water to cement should be between 0.3 and 0.4. The conductive materials may include both metal fibers and metal particles.

Particles made of carbon and graphite may be used to make the mixture easier to blend and pump. Low-carbon steel fibers having aspect ratios between 18 and 53 may be used. The fibers should be rectangular in shape with a deformed or corrugated surface to insure a bond with the concrete. Suitable fibers can be obtained from both Fibercon International and Novocon. Carbon and graphite products are commercially available and have good quality control. U.S. Patent No. 6,825,444 discloses a similar conductive concrete composition and is incorporated herein by reference.

The volume fractions of steel fibers and the carbon and graphite particles in the concrete mix may be optimized to provide the required conductivity and adequate mechanical strength. A range for achieving optimal mechanical strength and uniform, stable heating is a concrete mixture containing between 15 and 30% by volume carbon and graphite particles and between 1 and 3 % by volume steel fibers. More particularly, carbon and graphite particles and steel fibers comprise 20-25% and 1-2% by volume respectively. Mixtures in these ranges will provide good conductivity, high mechanical strength and a smooth finished surface. Mixtures with less than these amounts of fibers and particles may not efficiently conduct an electrical current and therefore may not efficiently heat the surface. Mixtures with more than these amounts of fibers and particles may create an unacceptably rough surface. The workability and surface finishability of mixtures in these ranges are similar to those of conventional concrete. Test results indicate that this mixture yields a compressive strength between 30 and 45 MPa (4500 to 6500 psi) and an electrical conductivity between 300 to 500 Ω -cm at 25⁰F.

Although not necessary, several optional components may be added to those discussed above in fabricating the conductive concrete mixture. Such materials include Class C Fly Ash, Silica Fume, and Superplasticizer (water reducer, High Range Water Reducer (HRWR)). One example conductive concrete composition may include: 1.5% steel fiber, 20% carbon and graphite particles, 15% cement, 2.5% Fly Ash, 1% Silica Fume, 18% fine aggregate, 20% coarse aggregate, 8% Superplasticizer, and water at a water/cement ratio between 0.3 and 0.4. If water reducer is used as the Superplasticizer, 4 oz./100 lbs. of cementitious materials are used. If HRWR is used, 16 oz./100 lbs. of cementitious materials are used. The Superplasticizer has no bearing on the conductivity, but improves the durability and workability of the conductive concrete mixture.

Several applications of the conductive concrete mixture disclosed above are discussed below.

BRIDGE DECK

U.S. Patent No. 6,825,444 discloses a heated bridge deck that uses electrodes embedded within conductive concrete and connected to a power source to remove snow and ice accumulation. The system disclosed includes molding a cement-based mixture containing conductive materials into pre-formed slabs and then placing them atop the paved surface of a bridge deck. Steel shavings are used in the cement-based mixture to improve conductivity. However, the use of steel shavings may cause concerns regarding a potential risk of eye injury. Additionally, there are potential issues with production and quality control, and the heating systems disclosed have potential energy efficiency issues. The FIGS. 1- 4 illustrate an exemplary embodiment of the present invention.

Particularly, they illustrate the use of the invention on a bridge deck. This embodiment is an improvement upon a heated bridge deck system disclosed in U.S. Patent No. 6,825,444, the disclosure of which is incorporated herein by reference.

Referring to the drawings in greater detail, and initially to FIGS. 1 and 2, a bridge deck designated generally by the numeral 20 is shown. Bridge deck 20 is comprised of a plurality of pre-formed concrete slabs 22 situated in horizontal spaced relation to one another as shown. Each horizontal row of pre-formed concrete slabs 22 spans the width of the bridge deck 20. A plurality of horizontal rows situated in spaced relation to one another span the entire length of the bridge deck 20. The concrete slabs 22 are formed of conductive concrete and have a pair of electrodes 24, 26 embedded therein as will be more fully described below. The details of the electrodes themselves are more fully disclosed in column 15, lines 1-58 of the 6,825,444 patent. The electrodes 24, 26 within each concrete slab 22 may be spaced four to six feet apart. Wire connectors 28 and 30 are secured at one end to the electrodes 24 and 26, respectively, by means well known in the art, such as a soldered, crimped, welded, or bolted connection. The opposite ends of the wire connectors 28, 30 extend outside of the concrete slabs 22 and are operably connected to a power source (not shown) by means well known in the art. Many suitable power sources are disclosed in columns 13-14, lines 55-67 and 1-41 of the 6,825,444 patent. The wire connectors 28, 30 are connected to the power source such that a first electrode 24 embedded in one concrete slab is situated next to a second electrode in the adjacent concrete slab. It is to be understood that the concrete layer of the bridge deck system of the present invention may be cast-in-place rather than comprised of pre-formed concrete slabs. In fact, a cast-in-place system may be more cost-effective for existing bridge decks.

As shown in FIG. 3, one embodiment of this system is comprised of a first layer 32, a second layer 34, a third layer 40, and a thermal insulating layer 36. The first layer 32 is the bridge deck and is formed of conventional concrete. A plurality of reinforcing bars 33 are embedded within the first layer 32 to increase strength as is well known in the art. This first layer 32 may often be about 152.4-203.2 mm or 6-8 inches thick.

The thermal insulating layer 36 is formed between the first layer 32 and the second layer 34 and is preferably about 12.7 mm or 0.5 inch thick. The thermal insulating layer 36 insulates the bottom face of the second layer 34 to prevent heat loss by conduction. The insulating layer 36 disclosed herein consists of a mixture of 50-99% mortar and 1-50% sawdust by volume. The insulating layer may consist of a mixture of 50% mortar and 50% sawdust by volume. This mixture provides efficient insulation and high enough mechanical strength to withstand the stresses due to automotive traffic. In addition, the costs associated with this insulating layer are quite low. Other insulating layers, such a polymer concrete (a concrete mixture containing a defined amount of polymer particles) also may be used. While the insulating layer 36 adds to the efficiency of the heating system of the present invention, it is not a necessary component for its construction. In fact, in many instances it may be desirable to eliminate thermal insulating layer 36.

The second layer 34 is formed of conductive concrete and may be about 50.8- 101.6 mm or 2-4 inches thick. A pair of electrodes 24, 26 is embedded in the conductive concrete layer 34 near the horizontal edge of the concrete slab. A power source 38 for applying electrical current to the electrodes is secured to the electrodes by wire connectors as described above.

The third layer 40 is a thin, non-conductive layer used as an overlay. In one embodiment, this non-conductive layer may comprise an epoxy coating of approximately 1/8 inch in thickness. In yet another embodiment, this non-conductive layer may comprise of regular concrete layer of approximately ¹A inch to ¥i inch thickness. The exposed surface 41 of the non-conductive layer constitutes the surface of the bridge deck.

In operation, an electric current passes through the conductive concrete thereby generating heat in the concrete due to its natural electrical resistance. This embodiment is preferred for use as an overlay atop an existing paved surface, although it is also well suited for use in the construction of new bridge decks, roadways, exit ramps, airport runways, street intersections, sidewalks, driveways, and other paved surfaces.

A further alternative embodiment is shown in FIG. 4. The bridge deck heating system illustrated comprises a first layer 42, a second layer 44, and a third layer 45. The first layer 42 is the bridge deck and is formed of conventional concrete. The second layer 44 is formed of conductive concrete, which constitutes the surface of the bridge deck. The third layer 45 is an ice/snow layer formed on the bridge deck surface 47. The relative thicknesses of the first two layers are as described above. A power source 46 for applying radio frequency (RF)/microwave energy to the conductive concrete layer 44 is attached to the conductive concrete by means well known in the art. While it is not necessary for the functionality of the system, a thermal insulating layer as disclosed herein may be disposed between the first 42 and second 44 layers and if so disposed, may increase the heating efficiency of this system. In operation, the conductive concrete and any ice thereon acts as a lossy resonator and resonates any RF/microwave energy applied to it. The application of an optimal RF/microwave frequency across this layer will cause the conductive concrete and the ice accumulation thereon to become excited. This excitation will generate heat (similar to the operation of a microwave oven) within the concrete and the ice, thus causing the ice to melt and the concrete to maintain a temperature high enough to resist ice formation. Taconite should be used for coarse aggregate instead of limestone. The full disclosure of this microwave heating system is given below.

MICROWAVE HEATING SYSTEM Unlike the electric heating systems disclosed in U.S. Patent No. 6,825,444, which rely on the efficiency of heat transfer to a conductive concrete surface in an adverse winter environment to melt frozen precipitation, microwave power directly heats the snow and ice formation in a manner similar to the heating process of a microwave oven. The surface of the conductive concrete, as disclosed above, along with the frozen precipitation will form a lossy microwave resonator. The microwave energy will be readily absorbed by the water molecules in the ice and snow, enabling direct heating for deicing applications. This system is potentially more energy efficient than using AC electric power. The radio frequency (RF) absorption coefficient of taconite is about 80 times that of ice. Likewise, the RF absorption coefficient of water at 1.5⁰C is about 20 times that of taconite. Therefore, if taconite is used to replace the limestone in the conductive concrete, the RF energy would be predominantly absorbed by the conductive concrete layer in an ice-covered taconite concrete surface, and a melting-ice-covered surface would readily absorb most of this energy for a direct RF heating of the ice water. The RF field in an ice-covered taconite concrete surface would propagate primarily in the taconite layer due to its larger dielectric constant. However, on a melting-ice-covered taconite concrete surface the RF field would propagate predominantly in the water layer since the dielectric constant of water is larger than that of taconite. These qualitative characteristics are significant because they lend support to a practical approach to using microwave deicing of taconite concrete surfaces. In the preferred embodiment, microwaves in the frequency range of approximately 300 MHz to 90 GHz are used. INDOOR RADIANT HEATING SYSTEM

Indoor radiant heating systems are commonly used in kitchens, bathrooms, and living rooms. Hydronic heating systems, pipes circulating heated fluids, are extensively used in the indoor radiant heating market. These systems often suffer from maintenance and reliability issues; due to the fact maintenance is nearly impossible in the event of a leak in a pipe. This embodiment of the present invention may be used to create indoor radiant heating systems free of the aforementioned maintenance and reliability issues. This embodiment is comprised of a plurality of prefabricated conductive concrete panels, similar to the conductive concrete slabs of the bridge deck system described above. FIG. 5 shows an illustration of a potential indoor radiant heating system embodied in a bathroom, for example. The indoor radiant heating system 50 is comprised of a plurality of pre-formed conductive concrete panels 52, 54. In this illustration, the plurality of panels comprises of a plurality of floor panels 52 and a plurality of wall panels 54 in spaced relation. In the preferred embodiment, the plurality of panels are placed together much like an engineered wood flooring systems, which are well known in the art. The thickness of a given panel will depend on its intended use. For instance, panels for use on the floor may be thicker than panels for use on the wall. The appropriate thickness range of a given panel may be between .5 to 1.5 inches thick. The preferable thicknesses of floor and wall panels are given respectively as ¹A" to W and ¹A" to 1". Panels may vary in size, shape and color. For instance, floor panels may be manufactured in squares measuring 4"x4" to 24"x24". The wall panels may be manufactured in squares measuring from 3"x3" to 6"x6". Other shapes, such as octagons and hexagons, may also be used. The plurality of panels may be affixed to existing structures using conventional means. For instance, the plurality of wall panels may be affixed to the existing wall 56 using prefabricated panels or similar means. Similarly, the plurality of floor panels may be affixed to the existing floor 58 using tongue- and-groove or similar means. Note that the indoor radiant heating system shown in FIG. 5 need not comprise of both wall and floor panels as illustrated. Rather, the system may comprise of only one of these components. Additionally, the system is not limited to wall and floor panels. Additionally, the concrete panels may come in many colors and textures and may have design patterns on the surface.

FIG. 6 illustrates a potential embodiment of a conductive concrete panel of the indoor radiant heating system as described above. The panel is comprised of a layer of conductive concrete 62 with electrodes 64, 66 embedded in a configuration as to reduce the voltage differential across the electrodes to be below 24 volts. A voltage differential is a concept well known in the art and can be defined as the potential difference between two points. Such electrode configurations are well known in the art. Ensuring that the voltage in the conductive concrete does not exceed 24 volts effectively heats the plurality of conductive concrete panels and ensures that the system is safe for human touch.

FIG. 7 shows an alternative embodiment of a conductive concrete panel of the indoor radiant heating system, which is similar to FIG. 6 expect that it further comprises of a thin non-conductive layer 82 affixed to the layer of conductive concrete 62. This non- conductive layer may range from 1/32 to 1/16 inches in thickness. Exemplary materials for constructing this layer include ceramic spray, polymer coating, and paint.

A control system similar to that disclosed in column 14, lines 44-67 of the 6,825,444 patent may be applied to control the operation of the indoor radiant heating system. In operation, a user may calibrate the control system by entering a desired temperature setting. The control system may then keep the plurality of conductive concrete panels at the desired temperature setting by controlling the voltage applied to the electrodes according to the feedback acquired by temperature sensors, such as infrared (IR) sensors. Humidity sensors may or may not be included. Various power sources such as those disclosed in the 6,825,444 patent may be used. HIGH-SPEED RAIL FOUNDATION FOR CORROSION PREVENTION

High-speed rail systems are common forms of transportation in certain parts of the world such as Europe and Japan. Most of these high speed rail systems use overhead electrification in the form of high- voltage cables to power the trains. These cables often carry voltages in excess of 1,500 volts. These high voltages increase the chance of random electrical currents being generated. Needless to say, these currents pose potential safety risks. Additionally, there is a concern that metallic parts in the rail system exposed to these high voltages will corrode at increased rates creating additional safety concerns. This embodiment of the present invention may be used to ground random currents generated by high voltage systems such as those used by high speed trains. FIG. 8 shows a high speed train 80 on a high speed rail system 82 being powered by a high voltage cable system 86. A conductive concrete slab 84 is placed beneath the high speed rail system as to be in electrical contact with the conductive structural elements of the high speed rail system. In operation, any random electric currents passing through any conductive structural elements of the rail system will be grounded by the conductive concrete slab. The conductive concrete slab may be constructed of the conductive concrete mixture as disclosed above. Those skilled in the art would appreciate that the exact configuration, size, and shape of the conductive concrete slab will vary according to the layout of the high speed rail system. Additionally, the present invention should not limited to use on high speed rail systems only, but rather it could be used in any high-voltage system that has conductive elements.

From the foregoing, it will be seen that this invention is one well-adapted to attain all the ends and objects hereinabove set forth together with other advantages which are obvious and which are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMSWhat is claimed is:

1. A conductive concrete slab comprising: a conductive concrete mixture, comprising cement, a conductive aggregate, water, and conductive materials, wherein said conductive materials are selected from a group comprising carbon and graphite particles; and at least two electrodes embedded in said conductive concrete mixture at spaced locations.

2. The conductive concrete slap of claim 1, wherein the conductive aggregate contains iron ore.

3. The conductive concrete slap of claim 1, wherein the conductive aggregate contains copper ore.

4. The conductive concrete slap of claim 1, wherein the conductive aggregate contains taconite.

5. The concrete slab of claim 1, wherein the slab is pre-cast into conductive concrete panels.

6. A radiant heating system comprising: a plurality of concrete slabs in spaced relation, each concrete slab comprising a first layer of conventional concrete; a second layer made of an electrically conductive material situated atop said first layer, the second layer comprising a cementitious composite admixed with conductive materials; electrodes in contact with the second layer; a third layer comprising a thin coating of non-conductive material situated atop the second layer; and a voltage source that applies a voltage differential across the electrodes, thereby causing an electric current to flow through the second layer.

7. The radiant heating system of claim 6, wherein the cementitious composite is admixed with carbon and graphite particles.

8. The radiant heating system of claim 6, wherein the thin coating of the third layer comprises an epoxy coating of approximately 1/8 inch in thickness.

9. The radiant heating system of claim 6, wherein the thin coating of the third layer comprises a concrete layer ranging from approximately ¹A inch to Vi inch in thickness.

10. An indoor radiant heating system comprising: a plurality of prefabricated conductive concrete panels affixed in spaced relation to an existing structure, wherein each conductive concrete panel comprises a cementitious composite admixed with conductive materials and at least two electrodes in contact with the panel; a voltage source that applies a voltage differential across the electrodes, thereby causing an electric current to flow through the conductive concrete panel; and an electrical control system that controls the voltage source according to a desired temperature setting.

11. The indoor radiant heating system of claim 10, wherein the electric current applied to the electrodes does not exceed approximately 24 volts.

12. The indoor radiant heating system of claim 10, wherein the plurality of prefabricated conductive concrete panels are between approximately 0.5 to 1.5 inches thick.

13. The indoor radiant heating system of claim 10 further comprising a second layer comprising a thin coating of non-conductive material situated atop said conductive concrete tile.

14. The indoor radiant heating system of claim 13, wherein the second layer comprising a thin coating of non-conductive material is constructed from a group consisting of ceramic spray, polymer coating, and paint.

15. The indoor radiant heating system of claim 13, wherein the second layer comprising a thin coating of non-conductive material is between approximately 1/32 and 1/16 inches thick.

16. The system of claim 10, wherein the electrical control system uses at least one infrared sensor to detect the temperature of the plurality of conductive concrete panels.

17. A high voltage system, comprising: at least two conductive elements with a high voltage differential between the two conductive elements; at least one structural element proximate to at lease one of the at least two conductive elements; and a conductive concrete slab in electrical contact with the at least one conductive structural element and electrical ground.

18. The high voltage system of claim 17, wherein the conductive concrete slab comprises a cementitious composite admixed with carbon and graphite particles.

19. The high voltage system of claim 17, wherein the conductive concrete slab comprises a conductive aggregate that contains at least one of the group consisting of iron ore, copper ore, and taconite.

20. The high voltage system of claim 17, wherein the at least two conductive elements comprise of conductive elements in a high speed rail system.

21. A microwave resonator comprising: a layer of conductive concrete having at least one surface upon which precipitation may accumulate; and at least one microwave energy source operable to apply microwave energy to the at least one surface of the layer of conductive concrete; wherein the layer of conductive concrete comprises; cement; a conductive aggregate containing taconite; water; and conductive materials.

22. The microwave resonator of claim 21, wherein the conductive materials are selected from a group comprising carbon and graphite particles.

23. The microware resonator of claim 21, wherein the conductive materials are selected from a group comprising metal fibers and metal particles.

24. The microwave resonator of claim 21, wherein the microwave energy source is operable to apply microwave energy having a frequency in a range of approximately 300 HMz to 90 GHz.