JP2002246158A - Current-carrying heating sheath wire, structure for melting snow on road surface, and construction method for forming structure of melting snow on road surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a linear heating element having good current carrying efficiency, and provide a structure for melting snow on road surface, including the linear heating element, and a construction method for forming the structure for melting snow on the road surface. SOLUTION: A current-carrying heating sheath wire 1 includes a core wire 2 for current- carrying and heating, an insulation layer 3 of cross-linked polyethylene for coating the core wire 2 for current-carrying and heating, and a sheath material layer 4 of chloroethene containing an infrared element 5 for coating the insulating layer 3. The infrared element is preferably a mixture of 70-95% graphite infrared elements and 5-30% ceramic infrared elements. The structure for melting snow on a road surface is a structure, which a road surface layer 7 is laid on a base 6. The infrared element 5 is mixed in the road surface layer, and the current-carrying heating sheath wire 1 is laid in a boundary part of the base 6 and the road surface layer 7. The construction method for forming the structure for melting snow on a road surface fixes a current-carrying heating sheath wire temporarily fixing member 9, which includes an unevenness part 10 for temporarily fixing the current-carrying heating sheath wire 1 on a thermoplastic resin sheet with a predetermined space on the base, and strikes the road surface layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current-carrying heating sheath wire, a snow melting road surface structure, and a method for forming a snow melting road surface structure which are used on a road surface in a snowfall area to exhibit a snow melting effect. In the present invention, surface portions such as roadways, sidewalks, parking lots, garages, stairs, approaches, road shoulders, and airports are collectively referred to as road surfaces.

[0002]

2. Description of the Related Art Various measures have been taken to remove or melt snow on a road surface in a snowfall zone. As one of the measures, a method is known in which a current-carrying heating element is buried on a road surface and the heat is generated to melt snow. A linear heating element and a planar heating element are used as the energizing heating elements. In the sheet heating element, for example, as shown in JP-A-10-172725, a far-infrared radiator having a heat retaining function and a far-infrared ray having a high heat conducting function are provided in a resin sheet constituting an outer cover of the sheet heating element. A device including a radiator (hereinafter, also referred to as a far-infrared device in the present invention) is also used.

[0003]

However, although the planar heating element has a relatively good energization efficiency, it is the same as the presence of planar foreign matter in the road structure from the roadbed to the road surface layer.
There is an unavoidable disadvantage that a discontinuous surface portion is formed, and the layer constituting the road structure peels off due to the discontinuous surface portion, and the layer is easily damaged. The linear heating element has a small heating element area with respect to the road surface area, and it is desired to further improve the power supply efficiency. Attempts are being made to promote snow melting from the road structure. 2. Description of the Related Art In recent years, a far-infrared ray element has been actively mixed in each layer constituting a road structure, especially in a road surface layer, especially in the vicinity of the surface of the road surface layer. There are many types of far-infrared elements to be mixed, but those with more optimal performance are being sought. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a linear heating element having better power supply efficiency, and to provide a snow melting road surface structure including a linear heating element and a method for forming a snow melting road surface structure.

[0004]

In order to solve the above-mentioned problems, an electric heating sheath wire as a linear heating element according to the present invention comprises:
It comprises a core wire for energizing and heating, an insulating layer made of cross-linked polyethylene for covering the core wire for energizing and heating, and a sheath material layer made of vinyl chloride containing a far-infrared ray element for covering the insulating layer.
The far-infrared device is a graphite-based far-infrared device 70.
It is desirable to be a mixture of about 95% and a ceramic far-infrared element of 5 to 30%. The snowmelt road surface structure of the present invention is a structure in which a road surface layer is laid on a roadbed, wherein a far-infrared ray element is mixed in the road surface layer, and a boundary between the roadbed and the road surface layer or the road surface. The current-carrying sheath wire is laid below the surface layer. The method for forming a snowmelt road surface structure according to the present invention employs an energization method in which irregularities are formed on the thermoplastic resin sheet so that the energization heat generation sheath wires can be temporarily fixed at predetermined intervals in order to improve the fixation of the energization heat generation sheath wires. The heating sheath wire temporary fixing member is fixed to the roadbed, and the road surface layer is cast.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a linear heating element is used as an electric heating element instead of a planar heating element. FIG. 1 is an explanatory perspective view showing one embodiment of an energized heating sheath wire as an energizing heating element of the present invention, in which a part of each layer is sequentially peeled off. Since the current-carrying heating element buried on the road surface serves as a heat source for melting snow falling in a severe cold season, it is not always necessary to increase the temperature. Therefore, as the energizing and heating core wire 2 constituting the energizing and heating sheath wire 1 of the present invention, any resistance wire having a resistance value of about 0.04 to 3 Ω / m can be used. For example, various metal wires are used, and among them, nickel chromium, copper nickel alloy and the like are preferable. The conductive heating core wire 2 may be a single wire or a stranded wire. For example, a cable type in which six stranded wires are wound around a core wire may be used. By using a stranded wire, the electric heating and heating core wire has increased flexibility and is easy to handle when laid on a road surface. Considering that the wire is a buried wire, the core wire and the stranded wire can be the same resistance wire.

For example,. Copper nickel alloy (about nickel
50%) 0.32mm wire with core wire and 6 stranded wires
And a stranded wire (apparent outer diameter 1.0 mm)
And the conductor resistance value is 870Ω / km (20 ° C.),. Same material
0.50mm wire rod with 7 strands (1.5m outer diameter)
m), the conductor resistance value is 360 Ω / km.
. The alloy is a copper-nickel alloy (about 10% nickel)
Seven stranded wires of 0.5mm wire (appearance outer diameter 1.5mm)
Then, the conductor resistance value is 73Ω / km. Laying width 1
m, the pitch interval of the meandering wiring is 50 mm, and the meandering width is 950.
mm and a rated voltage of 200 V, for example,
Stranded wire, the design standard 130W / m ^TwoWith an area of 4.2
m^Two, Design standard 150w / m^TwoAt 3.9m^Two, Design criteria 1
70W / m ^Two, Design standard 200W / m^Two3.4m at^TwoTona
You.

[0007] The heating and heating core wire 2 is covered with an insulating layer 3. As the insulating layer 3, various synthetic resins which are excellent in electrical insulation, heat resistance, weather resistance and the like and satisfy hardness and the like can be used. In the present invention, a crosslinked polyethylene is used. The thickness of the insulating layer 3 is not particularly limited, but is 0.5 to 1.5 mm.
It is preferable that the thickness be, for example, 0.8 mm. Since the current-carrying sheath wire 1 is repeatedly subjected to stress from a passing vehicle or the like, it is desirable to have the sheath material layer 4 on the outer surface. As the sheath material, flexibility, high elasticity, heat resistance, weather resistance, adhesiveness with the inner insulating coating material, etc. are required, for example, fluorine resin, silicon rubber,
Examples thereof include a heat-resistant vinyl resin or a mixture thereof. In view of the fact that asphalt is often used for the road surface layer, and also in consideration of dispersibility when mixing a far-infrared ray element described later, vinyl chloride is used as the sheath material layer material in the present invention.

In order to improve the current-carrying snow-melting efficiency of the current-carrying sheath wire 1, a far-infrared element 5
Is mixed. The far-infrared element absorbs heat from the outside and emits far-infrared rays. The emitted far-infrared rays destroy clusters in water, so that snow and ice are efficiently melted. Various types of far-infrared devices are known, and any of them can be basically used. For example, SiO ₂ · Al ₂ O ₃ · Fe ₂ O ₃ · TiO ₂ ·
Examples include ceramic-based materials such as MnO.CaO.MgO, those obtained by blending metal germanium or germanium oxide with the above-mentioned ceramic-based materials, molding and sintering, and graphite-based materials obtained by heat-treating graphite at a high temperature. For example, Japanese Patent Publication No. 4-10197 discloses various far-infrared radiators.

Among ceramics, powders obtained by sintering and pulverization based on a composition of 65% alumina and 35% silica are particularly preferred. Among graphite-based materials, in particular, graphite for electrodes is used, and
Apply a high voltage of 1500 to 2000 V at 000 ° C.
It is preferable that the powder is treated for 5 to 1 hour to obtain a powder having a particle size of 150 μm or less. As graphite for electrodes, besides using virgin material, shavings generated during electrode molding,
Collected products of the electrode rod and the electrode plate can also be suitably used. It goes without saying that the virgin material and the shavings / recovered product can be mixed and used at an appropriate ratio.

[0010] Far-infrared ray element 5 mixed in sheath material layer 4
May be used alone or in combination. As the composite material, a combination can be appropriately selected, but a combination of the above-mentioned suitable graphite-based material and the above-mentioned suitable ceramic-based material is preferable, and the ratio thereof is 70 to 95% of the graphite-based far-infrared device. The ceramic far-infrared ray element is preferably 5 to 30%, and the former is 80 to 90%.
%, And the latter 10 to 20% are particularly preferred. The thickness of the sheath material layer 4 is preferably thicker from the viewpoint of characteristics. However, if the thickness exceeds 2.2 mm, the cost increases, and the bending efficiency decreases to lower the work efficiency. It is preferable to set it to 2 mm or less. For example, by setting the sheath thickness to 2.2 mm, the surface temperature of the sheath material surface is about 3 to 4 ° C. (about 1
2%) higher.

The heating characteristics of the current-carrying sheath wire of the present invention are shown in Table 1 as an example. In Table 1, the energized heat-generating sheath wire used was used in Example 1 described later, and was energized with a rating of 145 W, and used in Example 2 described below, and was energized with a rating of 170 W , Are shown. The temperature shown in Table 1 is the surface temperature of the energized heat-generating sheath wire when energized at an ambient temperature of 14 ° C. in an airless state in the atmosphere.

[0012]

[Table 1]

The energized heating sheath wire 1 is embedded in the road surface. FIG. 2 shows an embodiment of the snow melting road surface structure of the present invention,
It is shown as an explanatory view partially showing a cross section. The road surface basically includes a roadbed 6 and a road surface layer 7. In the present invention, a layer in which the energized heating sheath wire 1 is buried or whose lower surface is in contact with the energized heating sheath wire 1 is referred to as a road surface layer 7, and a layer below the road surface layer 7 is referred to as a roadbed 6. Therefore, depending on the construction mode, the roadbed 6 is a layer subjected to the heating asphalt stabilization treatment, a concrete layer is hit, a bridge or a viaduct is an iron plate structure, or a steel plate structure. When the concrete or the like is cast, it refers to, for example, the road surface layer 7 is composed of one layer of asphalt, one layer of concrete, two layers of different properties, and two layers of an asphalt layer and a concrete layer. In this case, the term refers to a case where the base material is composed of two layers of a base layer and a high-performance pavement layer provided with water permeability, a case where a tile is attached to the surface thereof, and the like.

The energized heating sheath wire 1 according to the present invention comprises a conventional
Like the energized heating wire in which the far-infrared ray element is not used, it is buried at the boundary between the roadbed 6 and the road surface layer 7 or buried under the road surface portion 7. Subbase 6 and road surface layer 7
Even when buried at the boundary of the road surface, since it is a linear heating element, a sufficient contact area between the roadbed 6 and the road surface layer 7 can be obtained.
It does not hinder the connection mode. The thickness of the road surface layer 7 above the buried current-carrying sheath wire 1 of the present invention is:
It is preferable that it is about 6 to 15 cm at the beginning of the laying.
The road surface layer 7 is provided so as not to affect the energized heating sheath wire 1 when the road surface layer 7 is gradually cut and repaired as the vehicle passes. More preferably, 7 to
10 cm.

In order to efficiently melt snow on the road surface, a far-infrared ray element 8 is mixed into the road surface layer 7.
By mixing the far-infrared element 8, the sunshine and the outside air temperature before snowfall can be advantageously used for melting snow, and the heat from the current-carrying heating element buried on the road surface can be effectively and efficiently used for melting snow. As the far-infrared ray element 8 to be mixed into the road surface layer 7, the far-infrared ray element 5 exemplified as one previously mixed into the sheath material layer 4 can be used. In view of economics, graphite-based far-infrared devices, particularly the above-mentioned preferred sintered and pulverized graphite-based particles, can also be suitably used here.

A far-infrared element 8 to be mixed with the road surface 7
Can substitute for sand as fine aggregate, and the amount is 5
Up to 30%. Preferably, 7-1
5%. In addition, at least in the upper layer portion of the roadbed 6,
A similar far-infrared element 8 can be mixed. By mixing the far-infrared ray element 8 into the roadbed 6, it is expected that a part of the heat that moves downward from the energized heating element will travel upward as far-infrared radiation.

The energized heat-generating sheath wire of the present invention is meandered and temporarily fixed on a roadbed to form a road surface layer. Appropriate means may be used for the meandering wiring and temporary fixing, but it is preferable to use a current-carrying sheath wire temporary fixing member 9 as shown in FIG. 4 as shown in FIG. Preferably, the electrically conductive heating sheath wire temporary fixing member 9 is formed in a concave portion 10 (FIG. 3 in FIG. 3) on a thermoplastic resin sheet of vinyl chloride, polystyrene, polypropylene, polyethylene, ABS, or the like, leaving a sufficient space for temporarily fixing the electrically conductive heating sheath wire 1. , Because it is shown in the opposite direction,
(Which looks like a convex part). The remaining space becomes the locking groove 11 of the energized heating sheath wire 1. Recess 1
The length of 0 is the heating and heating sheath wire 1 for meandering wiring.
(Refer to the cutout display section in FIG. 2). The width of the recess 10 is appropriate. If the depth of the recess 10 is about the thickness of the energized heating sheath wire 1, the energized heating sheath wire 1 can be wired at a position in contact with the roadbed 6, and by adjusting the depth of the recess 10, the surface of the road surface can be adjusted. The height from the roadbed 6 embedded in the layer 7 is adjusted.

When the strength is required, it is preferable to provide a flange portion 12 for the electrically-heated sheath wire temporary fixing member 9. The flange portion 12 and the locking groove 11 may be directly adjacent to each other, or may be provided separately from each other via the same plane as the wall of the concave portion 10 (the latter case is shown in FIG. 3). Has been). Concave portion 10 of electrically heated sheath wire temporary fixing member 9
By sandwiching the energized heating sheath wire 1 in the locking groove 11 formed therebetween, the wiring mode of the energized heating sheath wire 1 can be easily determined, and the energized heating sheath wire is temporarily fixed using a nail or the like at the bottom of the concave portion 10. By fixing the member 9 to the roadbed 6, temporary fixing of the meandering wiring of the energized heating sheath wire 1 can be performed extremely easily.

By using a thermoplastic resin sheet, in particular, a vinyl chloride sheet, as the electric heating sheath wire temporary fixing member 9, for example, when asphalt material is used for the road surface layer 7, heat is applied by heating for fixing the asphalt material. The plastic resin sheet softens and collapses in shape,
It does not behave as a foreign substance in the asphalt that would impair continuity. When the energized heating sheath wire 1 is laid in a meandering wiring, in order to adjust the parallelism, spacing, etc. of the energized heating sheath wire 1, together with the energized heating sheath wire temporary fixing member 9, a narrow tape of a thermoplastic resin sheet is used. As shown in FIG. 4, the energized heating sheath wire 1 can be used as a temporary fixing tape member 13 as shown in FIG. Electric heating sheath wire temporary fixing tape member 1
Like the electrically-heated sheath wire temporary fixing member 3, it is also preferable that the material 3 be melted by heating for fixing the asphalt material and mixed and dispersed with the asphalt.

As another embodiment of the temporary fixing member for the meandering wiring, an electric heating sheath wire temporary fixing member 14 as shown in FIG. 5 is preferably used. The energized heating sheath wire temporary fixing member 14 is formed by forming a concave groove portion 15 at an appropriate interval on a long flat plate portion 15. The electric heating sheath wire 1 is fitted into the concave groove. The temporary fixing of the meandering wiring of the current-carrying sheath wire 1 can be extremely easily performed by fixing the current-carrying sheath wire temporary fixing member 14 to the roadbed 6 using a nail or the like at an appropriate flat portion. This is the same as the case of the sheath wire temporary fixing member 9. By using a long flat plate-shaped current-carrying sheath wire temporary fixing member 14, FIG.
As shown in the figure, winding can be performed in a state in which the energized heat-generating sheath wires 1 are arranged in a meandering manner at intervals, so that the meandering wiring can be constructed in a place apart from the construction site such as a factory, and can be easily installed in the construction site. And laying at the construction site can be made more efficient.

[0021]

Examples and Comparative Examples Examples and comparative examples of the present invention are shown below, but these are not intended to limit the present invention. [Example 1] A stranded wire (apparent outer diameter 0.96 mm) of seven copper nickel alloy (about 50% nickel) wires having a diameter of 0.32 mm is prepared as a resistance wire for a current-carrying sheath wire. Polyethylene is prepared as a resin for the insulating layer.
As a sheath material layer material, a graphite-based far-infrared device powder (fixed carbon 9
9.00 or more, ash content 0.1% or less, true specific gravity 1.7 g / c
m ³ , particle size 150 μm or less (including 58.8 μm or less 58.8)
%) And 600 ° C. while applying a voltage of 1750V.
85% ceramics far-infrared device [65% alumina and 35% silica particles are sintered and pulverized and have a particle size of 5.27 μm or less (including a maximum distribution particle size of 1.69 μm). 23.4%))]] A composition containing 20% of the mixed far-infrared ray element powder mixed at a ratio of 15% was granulated by a twin-screw extruder.

An insulating layer having an apparent thickness of 0.8 mm (outer diameter of 2.5
mm), followed by electronic crosslinking. Further, the outermost layer of the crosslinked electric heating core wire was extruded and coated so as to have a composite far infrared ray element-containing vinyl chloride sheath material layer having a thickness of 2 mm (outer diameter 6.5 mm), thereby producing an electric heating sheath wire. . PVC sheet (0.50mm
Thickness), the center line spacing of the locking grooves formed between the concave portions is 50 mm,
A current-carrying sheath wire temporary fixing member was prepared by continuously forming a recess having a width of about 17 mm and a depth of about 15 mm in a row so as to have a locking groove width of about 6.4 mm.

On the roadbed stabilized with heated asphalt having a thickness of 80 mm, the two temporary fixing members for the current-carrying sheath wire are placed.
Using the set, the energized heating sheath wire manufactured in Example 1 was meandered to the width of the road lane, and was fixed to an appropriate recess of the energized heating sheath wire temporary fixing member with a nail. The energized heating sheath wire was disposed at a position of approximately 5 mm above the roadbed. 10% of the sand as fine aggregate mixed in the asphalt was replaced with the same graphite-based far-infrared ray element as used in Example 1, and a high performance was obtained from above the temporary fixing member of the current-carrying sheath wire fixed with the nail. Asphalt pavement. The thickness of the high-performance asphalt pavement was 90 mm on the roadbed. When examining the inside of the high-performance asphalt pavement, the energized heating sheath wire was properly stopped at the position where it was temporarily fixed, and the energized heating sheath wire temporary fixing member softened and collapsed and did not retain its original shape. No significant differences were found in the performance of the pavement compared to the other parts.

[Example 2] In Example 1, a 0.5 mm diameter wire (having an apparent outer diameter of a stranded wire of 1.5 mm) and a sheath material layer thickness of 2 mm were used as the resistance wire for the current-carrying sheath wire.
A groove having a center line spacing of 50 mm was formed in a vinyl chloride sheet (0.5 mm thick, 30 mm width) as a temporary fixing member of 2 mm (outer diameter of the current-carrying sheath wire) and a current-carrying sheath wire. Except for using the one shown in FIG. 5, a snow melting road surface structure provided with an energized heating sheath wire was formed in the same manner as in Example 1. The result of examining the inside of the high-performance asphalt pavement was the same as in Example 1.

Comparative Examples 1 to 4 The following were prepared as comparative examples. Comparative Example 1 = Thickness of sheath material layer without using far-infrared ray element
An example of laying out in the same manner as in the second embodiment using a current-carrying heating wire manufactured in the same manner as in the first embodiment except that the width is set to 2 mm. Comparative Example 2 = An example in which a road surface layer of 10 cm was laid with high-performance asphalt containing a far-infrared ray element using a planar heating element. Comparative Example 3 = An example in which a road surface layer of 7 cm (high-functional asphalt containing a far-infrared ray element is used for a surface layer of 4 cm) was laid using another company's current-carrying sheath wire. Comparative Example 4 = An example in which a 9 cm road surface layer was laid with high-performance asphalt without using a heating element. With respect to the snowmelt road surface structures obtained in Examples 1 and 2 and Comparative Examples 1 to 4, an electric current melting test was carried out under the conditions of an external temperature of 2 ° C. and an amount of snow of 2 cm / h. Table 2 shows the experimental results. In Table 2, “element” means “far-infrared ray element”.

[0026]

[Table 2]

From the test results shown in Table 2 and a series of test results whose description is omitted, the following can be said. 1. The snow melting effect of Examples 1 and 2 in which the far-infrared ray element was mixed into the energized heating sheath wire and the road surface layer was remarkable. 2. Even when the far-infrared ray element was simply mixed into the road surface layer, a large snow melting effect was observed. 3. The construction using the sheath wire temporary fixing member prevented displacement of the sheath wire and was easy to perform. 4. The design criteria for the snowmelt road surface structure using the current-carrying heat-generating sheath wire of the present invention are as follows: average snowfall per hour to 2 cm, minimum temperature average value -3 ° C, 130 W / m ² , 2.5 mm · -5
℃ with ^{150W / m 2, 170W / m} 2 in 3cm · -7 ℃,
200 W / m ² at 3 to cm · −10 ° C. can be realized.
By the way, according to the catalog, the design standard of the linear heating element sheath product of the other company used in Comparative Example 3 is 20 cm at ｃｍ2 cm · −6 ° C.
^{0W / m 2, 250W / m} 2, 3 in the 2.5cm · -10 ℃
It is 300 W / m ² at cm · −15 ° C.

[0028]

By using the current-carrying sheath wire of the present invention, the efficiency of the current-carrying heating element in snow melting on the road surface can be greatly improved, and power consumption can be reduced. Further, the snow melting road surface structure can be formed quickly and easily by using the energized heating sheath wire temporary fixing member.

[Brief description of the drawings]

FIG. 1 is an explanatory perspective view of one embodiment of an energized heating sheath wire of the present invention, in which a part of each layer is sequentially peeled off.

FIG. 2 is an explanatory view showing a snow melting road surface structure according to an embodiment of the present invention, partially including a cross section.

FIG. 3 is an explanatory view showing one example of a current-carrying sheath wire temporary fixing member used in the snow melting road surface structure forming method of the present invention in a direction opposite to a used state.

FIG. 4 is an explanatory diagram showing a state in which an energized heating sheath wire is temporarily fixed to an energized heating sheath wire temporary fixing member and set on a roadbed.

FIG. 5 is an explanatory view showing another embodiment of a temporary fixing member for an energized heating sheath wire of the present invention.

FIG. 6 is an explanatory view showing a construction mode in a case where the energized heating sheath wire temporary fixing member shown in FIG. 5 is used.

[Explanation of symbols]

 1: energized heating sheath wire 2: energized heating core wire 3: insulating layer 4: sheath material layer 5: far-infrared ray element 6: roadbed 7: road surface layer 8: far-infrared ray element 9: energized heating sheath wire temporary fixing member 10: Concave portion 11: Locking groove 12: Flange portion 13: Temporary fixing member for energized heating sheath wire 14: Temporary fixing member for energized heating sheath line 15: Flat plate portion 16: Depressed groove portion

Claims (4)

[Claims] An electric heating and heating core wire, a cross-linked polyethylene insulating layer covering the electric heating and heating core wire, and a vinyl chloride sheath material layer containing a far-infrared ray element covering the insulating layer. A current-carrying sheath wire. 2. The current-carrying sheath wire according to claim 1, wherein the far-infrared device is a mixture of 70 to 95% of a graphite-based far-infrared device and 5 to 30% of a ceramic-based far-infrared device. 3. A road surface structure in which a road surface layer is laid on a roadbed, wherein a far-infrared ray element is mixed in the road surface layer, and a boundary between the roadbed and the road surface layer or the road surface layer. A snow melting road surface structure, characterized in that the energized heat-generating sheath wire according to claim 1 or 2 is laid at a lower portion of the road. 4. An electric heating sheath wire temporary fixing member formed with irregularities capable of temporarily fixing an electric heating sheath wire at a predetermined interval on a thermoplastic resin sheet is fixed to a roadbed,
4. A method for forming a snow-melt road surface structure according to claim 3, wherein a road surface layer is formed thereon.