Patent Grant
9228349
This application claims the priority benefit of Korean patent application serial no. 10-2012-0101782, filed on Sep. 13, 2012. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
1. Field of the Invention
The present invention relates to a method of enhancing fire resistance of high-strength concrete by mixing a spalling reducer (fiber cocktail) that is a hybrid of a polypropylene fiber and a steel fiber into the concrete and performing proof stress reinforcement (transverse confinement) on main steel bars using a wire rope and spacers.
2. Discussion of Related Art
As the height of buildings increases, the strength of concrete increases. Due to the recent technological development of high-strength concrete, there is increased need of a spalling prevention method as a plan for ensuring fire safety performance of a high-strength concrete column applied to a super high-rise building when a big fire breaks out, as well as a plan for improving load bearing capacity.
The high-strength concrete has characteristics of lower permeability and higher density than ordinary concrete. When exposed to high temperature, the high-strength concrete undergoes spalling due to thermal stress caused by a temperature difference between the interior and exterior of the high-strength concrete and a sharp increase in pore pressure of the interior of the high-strength concrete. For this reason, to ensure structural fire safety of super high-rise buildings to which the high-strength concrete is applied, a composite technique for preventing spalling and simultaneously improving load bearing capacity is required.
In general, to prevent spalling, a sharp rise in temperature should be suppressed, and a moisture content of the concrete should be equal to and less than a range of 5 to 6%. Further, a ratio of water to cement should be maintained at a adequate level. However, these requirements are merely minimum requirements for preventing spalling. To prevent spalling of the high-strength concrete applied to high-rise buildings, additional countermeasures based on a mechanism by which spalling takes place are studied in addition to such basic requirements.
Spalling of concrete advances in a primary spalling process in which the concrete is exposed to high temperature and is accompanied by an exfoliation phenomenon caused within 20 minutes by surface expansion, and secondary and tertiary spalling processes caused by an increase in vapor pressure of pores. Due to a loss of dynamic behavior capability of main bars in the primary spalling process, and a loss of a cross-section in the secondary and tertiary spalling processes, the building may collapse.
Known spalling prevention methods based on a spalling mechanism include a method of applying fireproof paint, a method of using polypropylene and metal lath, a method of applying a fireproof board, and a method of using a mixture of polypropylene and steel fibers.
First, the method of applying fireproof paint has an advantage in that construction is easy while a surface temperature can be controlled, but a disadvantage in that proof stress can be sharply reduced due to separation of a foamable material when a fire breaks out.
Next, the method of using polypropylene and metal lath has an advantage in that scattering caused by spalling can be reduced, but a disadvantage in that securing of concrete fluidity (coagulation of polypropylene that is organic fiber) and quality control are difficult.
Here, polypropylene (a kind of organic fiber, and called PP fiber) is melted when internal temperature of concrete continues to increase during a fire, thereby forming a microstructure serving as a passage through which vapor can be discharged. The metal lath is a kind of metal mesh that is installed for traverse confinement of steel bars arranged in the concrete.
Next, the method of applying a fireproof board has an advantage in that heated temperature can be controlled, but a disadvantage in that an effective space is reduced due to an increase in cross-section of a member.
The method of using a mixture of polypropylene and steel fibers has an advantage in that compressive strength is increased due to an increase in tensile strength due to the steel fiber, but a disadvantage in that securing of fluidity and quality control are also difficult.
In addition, the higher the strength of high-strength concrete, the smaller the size of a pore contributing to discharge of vapor. There is a disadvantage in that there is a limit in preventing spalling based on a method of mixing in the organic fiber.
Further, the existing method of applying fireproof paint mentioned above emphasizes prevention of spalling, but both the prevention of spalling and securing of load bearing capacity are required because actual columns are load-bearing members.
Particularly, the fiber (e.g. polypropylene fiber) mixing method has an advantage in that spalling can be prevented by facilitating discharge of vapor through pores secured by a low melting point in the event of a fire, but a problem in that the load bearing capacity can be significantly reduced by pores generated when the fiber melts in a fire.
Therefore, when the fiber mixing construction method is applied as a method of controlling spalling of a high-strength concrete column, a method capable of securing the load bearing capacity should always be applied. Otherwise, fire resistance of the high-strength concrete column applied to super high-rise buildings cannot be secured.
Here, the conventional metal lath is used as a means for confining steel bars. However, there is a limit in securing the load bearing capacity of the high-strength concrete column using only the metal lath such as the steel mesh. Further, there are problems in that constructability is low and there is no alternative but to be limited to concrete pouring in the long run.
a shows an example of a conventional column structure reinforced by wire ropes 20.
That is, main steel bars 10 are arranged inside a column structure in a vertical direction. In place of conventional shear steel bars (transverse reinforcing bars or stirrups), wire ropes 20 wind the main steel bars 10, and opposite ends thereof are connected by fasteners 30. Spacers 40 are supported inside the main steel bars 10.
However, the wire ropes 20 are vertically spaced apart from each other around the main steel bars one by one. Consequently, it can be found that one shear steel bar is reinforced by one wire rope 20. In this main steel bar reinforcement, the wire rope arranging method for securing fire resistance is not separately disclosed.
b shows an example of conventional construction of a spiral shear reinforcement (formed of a reinforcing fiber and a resin) 50. It can be found that this spiral shear reinforcement is continuously disposed along main steel bars in a spiral pattern in a vertical direction, but the wire rope arranging method for securing fire resistance is not separately disclosed.
However, when the fiber mixing method (e.g. polypropylene fiber) mainly used as the existing spalling prevention method is applied to the high-strength concrete column to which the load is applied, this may serve as a major cause of abrupt brittle fracture due to generated pores. Accordingly, the present invention is directed to a method of enhancing fire resistance of high-strength concrete by mixing a spalling reducer (fiber cocktail) into the concrete in order to control spalling and simultaneously performing shear reinforcement of main steel bars using shear stiffeners based on a wire rope and spacers.
To achieve the object, first, the present invention replaces conventional stirrups with a wire rope based on a test. The wire rope is easily handled due to a thin piano wire form and is easily disposed in a spiral shape because it winds around main steel bars and gives tensile strength.
Here, when the stirrups are used for the main steel bars, this has a transverse reinforcement effect of the main steel bars, but the main steel bars are not reinforced between the stirrups. As such, the concrete between the stirrups leaves much room for brittle fracture (at a portion at which no stirrups are arranged) due to spalling.
That is, when a given portion of a reinforced concrete structure such as a column structure undergoes brittle fracture due to fire, there is a possibility of the entire column structure effectively collapsing. Thus, a very narrow spacing may be set between the stirrups. In this case, economic efficiency is considerably reduced, which is not advantageous.
Accordingly, the present invention uses the wire rope in place of the stirrups. The wire rope has a thin piano wire form, is easy to work with, and is very easy to wind around the main steel bars in a spiral shape.
While conventional art discloses use of such a wire rope to reinforce the main steel bars in a composite spiral shape, the wire rope is vertically arranged along the main steel bars and has no effect on the prevention of spalling. The wire rope is partly wound in a spiral shape and does not effectively reinforce the main steel bars in between the wire rope.
Typically, steel bars may be machined, but it is very difficult to arrange the steel bars on the main steel bars in a desired shape such as a spiral shape. As such, there is no alternative but to use a reinforced fiber rather than the steel bars. A spiral shear stiffener based on the reinforced fiber is merely spirally disposed at given intervals in a vertical direction, which may be regarded as an arrangement method that has no effect of preventing spalling.
For this reason, the present invention is designed to maximize the effect of preventing spalling in the event of a fire by continuously winding the wire rope around the main steel bars of a column structure which are vertical steel bars in a spiral shape in a vertical direction so as to range from 100 to 140 turns, and setting a vertically disposed spacing to about (⅔)L, where L is a spacing between conventional stirrups, to perform shear reinforcement of the main steel bars with the wire rope.
Further, although a polypropylene (PP) fiber and a steel fiber may be mixed into the concrete of a conventional column structure made of high-strength concrete, a problem with this mixture is fluidity of the poured concrete. To solve this problem of fluidity, the present invention is designed to add a superplasticizer of 6 kg/m3 to 14 kg/m3 in construction methods according to embodiments of the present invention.
Further, in the construction methods according to embodiments of the present invention, the polypropylene fiber of the high-strength concrete is designed to be mixed in at a concentration of 1.4 kg/m3 to 1.6 kg/m3.
Further, in the construction methods according to embodiments of the present invention, the steel fiber of the high-strength concrete is designed to be mixed in at a concentration of 35 kg/m3 to 45 kg/m3.
Further, in the construction methods according to embodiments of the present invention, the polypropylene fiber is designed to be mixed in such a manner that two polypropylene fibers having different diameters are mixed in at a mixing ratio of 7:3 to 5:5.
Further, in the construction methods according to embodiments of the present invention, the reinforced concrete structure is designed to have tie bars arranged in upper and lower sections thereof, each of which occupies (⅕)L of a net section L, and the wire rope is wound in a remaining intermediate section (⅗)L thereof.
According to the present invention, it is possible to effectively enhance the fire resistance of high-strength concrete by improving the fire resistance based on spalling reducer and improving the structural performance of the main steel bars of the concrete structure based on the wire rope and the spacers. Thus, in a high-strength concrete region, it is possible to provide concrete having a stable structure in the event of a fire.
The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:
a is a view showing how to reinforce main steel bars using wire ropes in the conventional art;
b is a view showing how to reinforce main steel bars using a spiral shear stiffener in the conventional art;
Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. While the present invention is shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention.
For the sake of clarity and concision, technical matters not central to the present invention will not be described. Throughout the appended drawings and the description, parts that appear in more than one drawing or are mentioned in more than place in the description are consistently denoted by the same respective reference numerals.
The present invention suggests a method of improving fire resistance of high-strength concrete by mixing a walling reducer (fiber cocktail) hybridizing a polypropylene fiber and a steel fiber as in
The polypropylene fiber (PP fiber) forms micro channels (pores) in the concrete due to a low melting point of 170° C., thereby helping effectively reduce vapor and pore pressures of the concrete to minimize the occurrence of spalling.
When the steel fiber is additionally mixed in, it reduces the occurrence of cracks in a high-strength concrete member after the polypropylene fiber is melted (due to a tensile strength of the steel fiber) and blocks heat penetration from the outside.
Particularly, when the fiber cocktail, i.e. the polypropylene fiber and the steel fiber, is used as a fireproofing mixture for the high-strength concrete vulnerable to spalling (because the high-strength concrete has a very high moisture content), it can be found that this helps to secure fire resistance.
Further, mixing ratios of the high-strength concrete are given in Table 1 below.
In Table 1, “unmixed” and “mixed” indicate a case in which the fiber cocktail is not used and a case in which the fiber cocktail is used; W/B indicates a percentage of a binder (cement) based on water; S/a indicates a percentage of a sand volume based on an aggregate volume; Air indicates a volume of air; W indicates water; C indicates cement; Slag indicate (blast furnace) slag; F/A indicates fly ash; S/F indicates silica fume; S indicates sand; G indicates gravel; Ad. indicates advanced air-entrained (AE) water reducing agent (superplasticizer); and PP and Steel indicate a PP fiber and a steel fiber (hybrid).
The high-strength concrete may indicate concrete whose compressive strength is equal to or greater than 40 MPa. The maximum compressive strength with which this high-strength concrete can be manufactured by a concrete mixer at a current technical level is 60 MPa. High-strength concrete having a maximum compressive strength of 100 MPa cannot be manufactured by the concrete mixer but can be manufactured in a laboratory. The constituent materials themselves are not different from one another, but each constituent material for securing desired compressive strength may be adequately changed.
In the present invention, an important factor associated with the high-strength concrete is a superplasticizer. The superplasticizer is added to the high-strength concrete in order to secure fluidity of the high-strength concrete, because the fluidity of the high-strength concrete is considerably reduced by the fiber cocktail (the hybrid of the PP fiber and the steel fiber) mixed into the high-strength concrete for fire resistance.
The problem is how much the fluidity should be secured in the high-strength concrete of 40 MPa or more in order to obtain an optimal mixture.
For this reason, in the present invention, the superplasticizer is designed to be mixed within a range of 6 kg/m3 to 14 kg/m3. The reason is that, as a result of testing the fluidity after ready-mixed concrete is made, the fluidity is not secured when the superplasticizer is mixed in at a concentration of 6 kg/m3 or less, and a maximum mixing amount for securing fluidity is 14 kg/m3. When the superplasticizer is mixed in at a concentration of 14 kg/m3 or more, the fluidity can be secured. However, since a manufacturing cost including a material cost is increased, the mixing range of the superplasticizer in the present invention is limited to the range of 6 kg/m3 to 14 kg/m3.
Here, a mixing amount of the PP fiber in the high-strength concrete may range from 1.4 kg/m3 to 1.6 kg/m3, and a mixing amount of the steel fiber may range from 35 kg/m3 to 45 kg/m3.
Further, the PP fiber may be used by mixing two fibers, and a mixing ratio used for the mixture is adequately adjusted within a range of 7:3 to 5:5. As an example, Fiber-1 (having a length of 10 mm, a diameter (denier) of 5, and a percentage of 30%) and Fiber-2 (having a length of 5 mm, a diameter (denier) of 3, and a percentage of 70%) are mixed within the range of the mixing ratio.
The reason is that, when the fibers have the same diameter, a phenomenon in which the fibers conglomerate when mixed is increased, and simultaneously the discharge of vapor is inefficient because the pores formed in the event of a fire are constant in size.
When only a fiber mixing method is applied to a high-strength concrete column, test results comparing the fire resistance in an unloaded state of the column (structure) with that in a loaded state are given as in Table 2 below.
The fire resistance test was performed on a 100 MPa high-strength concrete column to which the fiber mixing method is applied, and the loaded fire resistance test was performed according to a KS testing method based on KS F 2257-1. Brittle fracture of the column occurred due to a sharp pore increase caused by fiber mixture and buckling of middle steel bars caused by a load. The unloaded fire resistance test was performed according to an unloaded fire resistance testing method based on No. 2008-334 announced by The Ministry of Land, Infrastructure and Transport of Korea and showed that a fire resistance of 180 minutes could be secured in the case of the fiber mixing method.
Here, actual columns are structural members supporting a load. The column in the unloaded state cannot be applied to a building. As such, to serve as the column member in the loaded state, a fiber mixing method for preventing spalling in the event of a fire, a technique for preventing buckling of main steel bars, and a technique for improving proof stress of the column member should be applied at the same time.
Therefore, to secure a fire resistance of the loaded high-strength concrete column, a spalling prevention method based on the fiber mixture and a method of securing the load bearing capacity based on the transverse reinforcement of the main steel bars should be applied at the same time. Otherwise, the fire safety of the column applied to a high-rise building cannot be guaranteed.
Accordingly, in the present invention, the main steel bars of a reinforced concrete structure are shear-reinforced to prevent buckling of the main steel bars using support spacers along with a transverse reinforcement arrangement using wire ropes.
A means for preventing the spacing between the main steel bars from being changed by the wire ropes tightening around the main steel bars, keeping the spacing between the main steel bars constant, and preventing buckling of the main steel bars is a support spacer 200 of the present invention.
The support spacer 200a of
The support spacer 200a is made up of bearing rings 220 for increasing an area of contact with the main steel bars and a linear body 210. A turnbuckle 230 is installed so as to be able to adjust a length of the support spacer. Here, the turnbuckle 230 is installed at both sides of the body 210 so as to be able to adjust the length of the support spacer at both sides or at one side of the body 210, so as to be able to adjust the length of the support spacer at one side only.
The support spacer 200b of
In the present invention, to shear-reinforce the main steel bars of the reinforced concrete structure using the wire ropes as transverse reinforcement arranging units, the wire rope 110 as shown in
This wire rope 110 has a body 112 of a predetermined length. The body 112 is installed around the main steel bars of the reinforced concrete structure and is fixed to the main steel bars by eyes 116.
The wire rope 110 may perform the same as or better than a steel bar because it is lighter than an ordinary steel bar and has a much greater tensile strength than the steel bar, for instance, four times greater than the steel bar or two times greater than a composite material reinforcing bar. Further, the greatest advantage of wire rope 110 is that constructability is remarkably improved due to flexibility.
The eyes 116 of the wire rope 110 are formed in a closed loop shape by binding opposite ends thereof with clips 130 such as crimp sleeves. The eyes 116 may be fixedly bound by the clips 130 in consideration of load efficiency and convenience of machining the wire rope. That is, a method of forming each end of the wire rope 110 in a loop shape and crimping the end may be used.
First, the fiber cocktail made of the PP fiber and the steel fiber is mixed into the concrete.
Next, the support spacers 200a as described above are installed between the main steel bars at the corners of the reinforced concrete structure in a diagonal direction.
Finally, as shown in
Continuously, as shown in
Here, the wire rope is wound in the closed shape so as to be able to transversely reinforce the main steel bars in various shapes such as a quadrilateral shape, a rhombic shape, or an octagonal shape.
a and 7b are cross-sectional views showing a state in which a wire rope and support spacers are installed in accordance with a second embodiment of the present invention.
The second embodiment is the same as the first embodiment except that support spacers 200b having the fixtures bent in the hook shape are installed in place of the support spacers 200a.
As shown in
In another embodiment of the present invention, the main steel bars can be prevented from being widened and deformed using closed spacers in place of the support spacers as described above.
The closed spacer 200c of
The closed spacer 200d of
First, the fiber cocktail made of the PP fiber and the steel fiber is mixed into the concrete.
Next, the support spacers 200c as described above are installed at the inner circumference connecting the main steel bars of the reinforced concrete structure.
Finally, as shown in
The fifth embodiment is the same as the fourth embodiment except that the closed spacers 200d having the fixtures bent in the hook shape are installed at the outer circumference connecting the main steel bars of the reinforced concrete structure.
The aforementioned embodiments of the present invention may be used along with a conventional method of arranging tie bars. As an example,
Referring to
In the methods according to embodiments of the present invention described up to now, the wire rope, the spacers, and the distance adjustors are easily installed compared to conventional shear steel bars, so that work efficiency is excellent.
Further, since the proof stress of the main steel bars can be reinforced by providing the transverse reinforcement using the wire rope and preventing buckling and deformation of the main steel bars using the spacers, fire resistance and earthquake resistance of the column can be secured.
Thus, the fire resistance of high-strength concrete can be effectively improved by improving the fire resistance based on the fiber cocktail and improving the structural performance of the main steel bars of the concrete structure.
Although exemplary embodiments of the present invention have been described, the present invention is not limited to these embodiments and various modifications, additions and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.
[Fire Test of High-Strength Fireproof Concrete Column]
To develop a method of constructing high-strength fireproof concrete based on a fiber mixture and wire rope reinforcement, high-strength fireproof concretes having compressive strengths of 60 MPa and 100 MPa were mixed as in Table 3, and properties of constituent materials used for the mixtures are given in Table 4.
The high-strength fireproof concretes were produced from a batch plant of H company according to strength-specific mixing tables of the high-strength fireproof concretes, and flow rates and air amounts of uncured concretes were measured. Then, the concretes were cured and their compressive strengths were measured. As a result, it could be confirmed that a compressive strength of a design strength or more could be secured, as shown in Table 5.
To develop the method of constructing high-strength fireproof concrete based on a fiber mixture and wire rope reinforcement, an applied load and a load ratio were calculated with respect to column specimens having strengths of 60 MPa and 100 MPa, and the fire resistance test was performed on the column specimens having dimensions of 500×500×3000, as shown in Table 6.
An actual fire test was performed by realizing fixed end conditions using bolts and steel sections under heating conditions to simulate standard fire (ISO fire) conditions based on KS F 2257-1, and on two-end boundary conditions of the column which were the same as the actual structure.
The high-strength fireproof concrete column specimens based on the fiber mixture and the wire rope reinforcement were manufactured as in
(1) Temperature Estimation
As a result of performing the fire resistance test (temperature estimation) on the column specimens having strengths of 60 MPa and 100 MPa to develop the method of constructing high-strength fireproof concrete based on a fiber mixture and wire rope reinforcement, average and maximum temperatures of the steel bars according to application of the method were measured as shown in Table 7.
(2) Fire Resistance Estimation
As a result of performing the fire resistance test (shrinking estimation) on the column specimens having strengths of 60 MPa and 100 MPa to develop the method of constructing high-strength fireproof concrete based on a fiber mixture and wire rope reinforcement, shrinkages and shrinking rates of the specimens according to the application of the method were measured as shown in Table 7.
As a result of performing the fire resistance test, maximum spalling depths and weight losses of the column specimens showed that spalling was reduced more in the specimens to which the method was applied compared to the specimens to which the method was not applied, as shown in Table 9.
To develop the method of constructing high-strength fireproof concrete to which fiber mixture and wire rope reinforcement were applied, post-fire damage resulting from performing the fire resistance test under ISO fire conditions is shown in
(3) Fire Resistance Estimation of High-Strength Concrete Column Having Strength of 60 MPa
Shrinkages of the specimen resulting from performing the fire resistance test on the fireproof concrete column specimen having a strength of 60 MPa is shown in
(4) Fire Resistance Estimation of High-Strength Concrete Column Having Strength of 100 MPa
Shrinkage of the specimen resulting from performing the fire resistance test on the fireproof concrete column specimen having a strength of 100 MPa is shown in
As a result of analyzing the shrinkage of specimen S-4 to which the fireproof construction method was not applied and the shrinkage of specimen S-6 to which the method of mixing the wire rope and the fiber cocktail was applied, it was shown that S-4 underwent shrinkage that exceeded an allowable deformation and an allowable strain at 44 minutes, and that S-6 did not exceed the allowable deformation and the allowable strain up to 180 minutes. Thus, when the fiber cocktail was additionally mixed with the wire rope reinforcing method, which was the existing plan for improving the transverse confining force of the steel bars, the fireproof concrete column specimen could secure a fire resistance of 180 minutes, meaning it could be applied to all buildings of 14 stories or more.
In the present invention, the fire resistance test was performed on the column specimens which had strengths of 60 MPa and 100 MPa and were used to develop the method of constructing high-strength fireproof concrete based on a fiber mixture and wire rope reinforcement, under ISO fire conditions, and the fire resistance based on KS F 2257-1 was estimated. It was concluded that the results could be used as basic data for on-the-spot application of the high-strength fireproof concrete. Main research results were as follows.
(1) As a result of determining the fire resistance of the column member based on the KS F 2257-1 standard, a fire resistance of 146 minutes was obtained in the case of specimen S-1 which had a strength of 60 MPa and to which no construction method was applied. S-1 could be applied to buildings of 12 stories or less as a column that could support 552 tons.
(2) When the wire rope construction method in which the tie bars were replaced with the wire rope at the same volume ratio in the high-strength concrete column having a strength of 60 MPa was used as the steel bar transverse confinement construction method, the high-strength concrete column could secure a fire resistance of 180 minutes, meaning that it could be applied to buildings of 12 stories or more as a column capable of supporting 552 tons without separate fireproofing treatment.
(3) When the wire rope construction method in which the tie bars were replaced with the wire rope at the same volume ratio in the high-strength concrete column having a strength of 60 MPa was mixed with the fiber cocktail construction method for preventing spalling, the high-strength concrete column could secure a fire resistance of 180 minutes, meaning that it could be applied to buildings of 12 stories or more as a column capable of supporting 552 tons without separate fireproofing treatment.
(4) As a result of determining the fire resistance of the column member based on the KS F 2257-1 standard, it was shown that a fire resistance of 43 minutes was obtained in the case of specimen S-4 which had a strength of 100 MPa and to which no construction method was applied. S-4 could not be applied as a fireproof structure.
(5) When the wire rope construction method in which the tie bars were replaced with the wire rope at the same volume ratio in the high-strength concrete column having a strength of 60 MPa was used as the steel bar transverse confinement construction method, the high-strength concrete column could secure a fire resistance of 69 minutes. Thus, it could be applied to buildings of 4 stories or more as a column capable of supporting 552 tons without separate fireproofing treatment.
(6) When the wire rope construction method in which the tie bars were replaced with the wire rope at the same volume ratio in the high-strength concrete column having a strength of 60 MPa was mixed with the fiber cocktail construction method for preventing spalling, the high-strength concrete column could secure a fire resistance of 180 minutes. Thus, it could be applied to buildings of 12 stories or more as a column capable of supporting 737 tons without separate fireproofing treatment.
It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers all such modifications provided they fall within the scope of the appended claims and their equivalents.
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| 10-2012-0101782 | Sep 2012 | KR | national |
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| 20140068946 A1 | Mar 2014 | US |
