JP4971233B2 - Manhole repair method - Google Patents

Description

  The present invention relates to a manhole repair method for lining and repairing an inner wall surface of a manhole.

  The manhole is generally made of concrete, and for a long time, the inner wall is corroded by hydrogen sulfide generated in underground pipes, particularly sewer pipes, and sulfur oxides in rainwater. In addition, since many manholes are buried in public roads, cracks may occur on the inner wall surface due to vehicle vibration. If corrosion of the inner wall of the manhole progresses or cracks occur in the inner wall, groundwater enters the underground pipe, and in the case of a sewage pipe, the amount of sewage increases due to the invaded groundwater, increasing the number of sewage treatment man-hours. There is also a possibility. If this state is left unattended, the manhole may collapse, so the inner wall surface of the manhole must be repaired at an appropriate time.

  As a method for repairing the inner wall surface of a manhole, for example, as described in Patent Documents 1 to 3, a plastic manhole repair material is arranged in the manhole so as to cover the inner wall surface of the manhole with a gap. In addition, it is known that a gap formed between the inner wall surface and the manhole repair material is filled with a curable material and cured to form a repair lining layer on the inner wall surface. The outer surface of the manhole repair material used here is to prevent the repair material from being too close to or in contact with the inner wall surface of the manhole, and to ensure that a gap with a predetermined dimension is formed. Are formed.

  Using such a repair method, even when the inner wall surface is severely corroded or groundwater leaks out from the inner wall surface, the repair lining layer has a sufficient repair effect and the inner surface is smooth. Can be formed in a short time.

  By the way, when using a resin material such as an epoxy resin as a curable material filled in the gap between the inner wall of the manhole and the manhole repair material, the manhole repair made of plastic due to the excellent adhesion of the resin material The material is firmly bonded to the inner wall surface of the manhole via a cured resin material. Therefore, a firm repair lining layer is formed by the manhole repair material and the resin material. However, the resin material is expensive, and if the resin material is used as the curable material, the repair cost becomes high. If we try to lower the repair cost, we will use cement-based grout material as the curable material, but cement-based grout material is not highly adhesive, so the grout where the plastic manhole repair material is cured There is a risk that the material will be easily peeled off from the material, and the repair effect may be reduced early.

  In order to use the grout material as a curable material or a filler and to firmly adhere the manhole repair material to the grout material, for example, as described in Patent Documents 4 to 6, on the outer surface side of the manhole repair material, It is conceivable to form protrusions (which may be protrusions or protrusions for forming gaps) embedded in the grout material with a retaining effect. However, the process of forming a manhole repair material having a protrusion having such a retaining function is complicated, and therefore, repairing a manhole using such a repair material results in an increase in cost. End up.

Japanese Patent Publication No. 7-23618 Japanese Patent No. 2641713 Japanese Patent No. 2687096 Japanese Patent Publication No.7-115365 JP-A-8-158389 JP 2001-248177 A

  SUMMARY OF THE INVENTION An object of the present invention is to provide a manhole repair method that can be implemented at a low cost and can form a strong repair lining layer with a manhole repair material and a grout material.

  The manhole repair method of the present invention for achieving this object is a manhole repair method for lining and repairing the inner wall surface of a manhole, and is laminated on a mold for forming a manhole repair material for lining. In the first step of preparing a molding material for repair material in which a liquid curable resin is impregnated with a reinforcing base material, at a stage where the curable resin is in a semi-cured state, or in a state where the curable resin is uncured The second step of pressing and attaching an anchor member made of the same material as the molding material for repair material to the outer surface of the molding material for repair material, and curing the uncured curable resin, The manhole repair material is arranged in the manhole so as to cover the manhole inner wall with a gap in the third step of constructing or preparing the manhole repair material having the anchor member fixed to the outer surface by removing from the mold. Step 5 and a fifth step of forming a repair lining layer in which a grout material is injected and cured in a gap between the manhole inner wall surface and the manhole repair material, and the anchor member is embedded in the grout material in a retaining state. Are provided. The repair lining layer is formed of a manhole repair material and a hardened grout material. Due to the retaining effect of the anchor member, the manhole repair material and the grout material are firmly bonded or fixed, and a strong repair lining layer is formed on the inner wall surface of the manhole. Moreover, the anchor structure of the manhole repair material used here uses the adhesiveness of the uncured curable resin of the molding material for repair material, and without using a special adhesive in the molding process of the manhole repair material, Since it is basically formed by simply pressing and fixing the anchor member to the repair material molding material, the manhole repair material can be configured easily and inexpensively. In addition, since the anchor member (pre-molded molded body) is the same material as the molding material for repair material, it has excellent adhesion to the molding material for repair material and is firmly fixed to the manhole repair material. Become. Here, the same material as the molding material for repair material means that the anchor member is formed of the same resin material as the molding material for repair material, or the same resin material as the molding material for repair material and It means that it is formed from a reinforced base material. The grout material is a backing material mainly composed of cement, and resin or the like is mixed as necessary. The manhole repair material is formed, for example, in a shape that can be arranged along the inner wall surface of the manhole or covers the inner wall surface of the manhole, and can be formed, for example, as a cylindrical body or an annular body. it can.

  When forming the gap forming protrusion on the outer surface of the molding material for repair material, a plate piece as an anchor member is attached to the gap forming protrusion, and the plate piece is around the gap forming protrusion. The anchor structure can be configured to extend to a position, for example, a lower peripheral position. When the grout material enters between the portion of the plate piece extending beyond the gap forming protrusion and the peripheral portion of the gap forming protrusion and hardens, the plate piece has a retaining effect.

  Moreover, an anchor member can be fixed to the outer surface of a manhole repair material, and an anchor structure can be comprised so that the through-hole extended along the outer surface of a manhole repair material may be formed. When the grout material enters into the through hole and hardens, the anchor member has a retaining effect.

  In addition, on the entire outer surface of the manhole repair material, an adhesive layer made of an uncured room temperature curable resin material is applied to form an adhesive layer, and this adhesive layer (uncured) is adhered in such a way as to sprinkle granular materials, By curing the adhesive layer, a granular material layer may be formed on the outer surface of the manhole repair material, and the adhesive strength between the grout material and the manhole repair material may be increased.

  The manhole repair method of the present invention is suitable for forming a high-strength repair lining layer while being easily implemented.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a perspective view of a manhole repair material used in the manhole repair method according to the present invention.

  The manhole repair materials 1 and 3 which are plastic cylindrical bodies and have a thickness of about 1.3 mm are stacked so as to cover the straight wall portion 7 of the inner wall surface 5 of the manhole (see FIG. 3). An oblique frustoconical portion having a cylindrical portion 9 (more specifically, a truncated cone portion having a trapezoidal cross section in which the angle formed by one side and the lower base is a right angle) 11 and the lower end A plastic manhole repair material 15 formed integrally from the cylindrical portion 13 and having a thickness of about 1.3 mm is stacked on the manhole repair material 3 to cover the inclined wall portion 17 of the manhole inner wall surface 5. (See FIG. 3). Each of the manhole repair materials 1, 3, 15 is cut at one circumferential direction over the entire length (full length in the longitudinal direction), and both end portions 19 a, 19 b, 21 a, 21 b, 23 a of the respective cut portions 19, 21, 23 are obtained. 23b is in an overlapped state (the figure shows the state pressed against the manhole inner wall surface 5). Since the outer end portions 19b, 21b, and 23b of the cut portions 19, 21, and 23 are formed so as to bulge outward over a circumferential width of about 100 mm, the inner diameter of the outer end portions 19b, 21b, and 23b is the inner end. It is formed to have a large diameter so as to accommodate the portions 19a, 21a, and 23a. Therefore, there is no step due to overlapping inside the cut portions 19, 21, and 23. The overlap width is usually 80 mm to 100 mm, but if the manhole inner wall surface 5 is severely corroded, the manhole repair materials 1, 3, 15 are further enlarged in diameter, so the overlap width may be about 30 mm. . In this case, a narrow concave portion extending in the vertical direction is formed inside the cut portions 19, 21, and 23.

  Since the upper end portions 25 and 27 of the manhole repair materials 1 and 3 are configured to expand outward (the height of the upper end portions 25 and 27 configured to expand outward is approximately 100 mm), The inner diameters of the upper end portions 25 and 27 are formed to have a large diameter so that the lower end portion 29 of the manhole repair material 3 and the cylindrical portion 13 (lower end portion) of the manhole repair material 15 are accommodated. There is no step due to the overlap inside. The upper end portions 25 and 27 of the manhole repair materials 1 and 3 are also formed with outward bulges 31 and 33 for accommodating the outer end portions 21b and 23b of the manhole repair materials 3 and 15, respectively. Note that the lower end of the manhole repair material 1 has a profile that follows the bottom outer edge of the manhole, and has a recessed tube opening 37 that extends along the upper side of the sewage pipe 35 (see FIG. 3). Has been processed.

  On the outer peripheral surfaces of the manhole repair materials 1, 3, and 15, there are provided a plurality of gap-forming ribs 39 and 41, which are concave portions on the inner side by swelling outward (manhole repair material 15 The rib 41 is not formed in the above. The protruding amount (height) to the outside of the rib 39 is about 10 mm, and the protruding amount to the outside of the rib 41 is about 3 mm, but on the outer end portions 19b, 21b, and 23b of the cut portions 19, 21, and 23 The protruding amount of the rib 39 provided on the outside is preferably about 7 mm. It is effective that the gap between the manhole inner wall surface 5 and the manhole repair materials 1, 3, 15 is 5 mm or more and 20 m or less. This is because if the gap is less than 5 mm, it is difficult to fill the grout material, and air retention may occur or the grout material may not reach the lower end. Conversely, if it exceeds 20 mm, the repaired manhole This is because the inside may become too narrow. In addition, when importance is attached to the smoothness of the inner surface, a finishing process is performed in which the inner recesses of the ribs 39 and 41 and the recesses generated inside the cut portions 19, 21, and 23 are filled with a curable resin or the like. In the figure, 43, 45, and 47 are hanging holes.

  The gap forming ribs 39 and 41 are each formed in a truncated pyramid shape, and an anchor member is fixed to a part of the gap forming ribs 39 and 41. The anchor member is formed as a plate piece 49, and one end side of the plate piece 49 is pressed and fixed to the outer surface of the gap forming ribs 39 and 41 by a fixing piece 51, and the other end side is a gap forming rib. The gap forming ribs 39, 41 extend to the lower peripheral position beyond the 39, 41. Therefore, between the other end side (lower end side) of the plate piece 49 and the outer surface of the manhole repair material 1, 3, 15, a gap recess (a gap or space in which the grout material enters and hardens) 53 (see FIG. 7). Will be formed.

  FIG. 2 is a diagram for explaining a process for forming the manhole repair material 1.

  To form the manhole repair material 1, first, a mold having a cylindrical outer surface (the outer surface of the mold is formed so as to correspond to the manhole inner wall surface 5 to be repaired, and a protrusion for forming the rib 39, etc. For example, glass fiber reinforced vinyl ester resin (molding material for repair material) 57 is laminated by a hand lay-up method (FIG. 2a). Next, when the vinyl ester resin is in an uncured state (specifically, when it is in a semi-cured state), the gap forming ribs 39 and 41 of the glass fiber reinforced vinyl ester resin 57 (the gap forming ribs 39 and 41). As shown in FIG. 2B, the plate piece 49 is pressed and pasted on the portion to be molded as (FIG. 2b). Here, in order to increase the fixing strength of the plate piece 49, the fixing piece 51 is attached to the outer surface of the glass fiber reinforced vinyl ester resin 57 so as to press the plate piece 49. The plate piece 49 is made of the same material as the manhole repair material 1 and is made of glass fiber reinforced vinyl ester resin. The fixing piece 51 is also made of the same material as the manhole repair material 1 and is made of glass fiber reinforced vinyl ester resin. However, the vinyl ester resin is uncured and has flexibility when applied. Then, when the vinyl ester resin is cured and the repair material molding material 57 and the fixing piece 51 are molded, die cutting is performed, and one circumferential direction of the obtained cylindrical body is cut in the longitudinal direction, and An FRP manhole repair material 1 having an anchor structure integrally is formed by processing the lower end into a predetermined profile (FIG. 1). The manhole repair materials 3 and 15 are also configured in the same manner as the manhole repair material 1, but in the case of the manhole repair material 15, a mold (an outer surface of the mold) having an oblique frustoconical outer surface having a cylindrical outer surface at the upper and lower ends. Is formed so as to correspond to the inner wall surface 5 of the manhole to be repaired, and has projections for forming ribs 39 and the like. The cut points 19, 21, and 23 may be formed at the time of molding.

  As a molding material for the manhole repair materials 1, 3, and 15, in addition to vinyl ester, thermoplastics such as unsaturated polyester, polyethylene, polypropylene, vinyl chloride, and polyamide, and thermosetting plastics such as acrylic and epoxy are used. You can also. Whichever material is used, the manhole repair materials 1, 3, and 15 need to have such a strength that the flexural modulus is at least 31.6 MPa (the flexural modulus is 31.6 MPa or more). (In this specification, the flexural modulus is measured in accordance with JIS K 6911 (1995)).

  3 and 4 are diagrams for explaining the arrangement process of the manhole repair material 1 in the manhole, FIG. 3 is a diagram showing the inside of the manhole before the manhole repair material 1 is inserted, and FIG. 4 is the manhole repair material 1. It is a figure which shows the state which was put in the manhole, pressed against the manhole inner wall surface 5, and has been arrange | positioned in the manhole.

  The inner wall surface 5 of the manhole is cleaned by spraying high-pressure water, but when the dirt is severe, cleaning with a rotating brush is also performed. Then, after step 58 is cut, the manhole repair material 1 is reduced in diameter so as to be in an overlapped state as shown by an imaginary line in FIG. Put in. The insertion work is performed by connecting a hanging string 61 to the hanging hole 43 of the manhole repair material 1 and lowering the manhole repair material 1 (FIG. 3). The manhole repair material 1 placed in the manhole is released from the overlapped state, and is placed on the bottom along the outer edge of the bottom so that the pipe opening 37 coincides with the sewage pipe 35.

  After the overlapping state of the manhole repair material 1 is released, the diameter expanding tool 63 is put into the manhole repair material 1 and a diameter expansion process is performed. The diameter expanding tool 63 is configured such that, for example, the diameter of a spring steel ring body cut at one place can be changed using a rack-pinion mechanism (not shown). Accordingly, the manhole repair material 1 is expanded in diameter by pressing the inner surface until the ribs 39 and 41 of the manhole repair material 1 are pressed against the straight wall portion 7 of the manhole inner wall surface 5 (see FIG. 4).

  Next, the cut portion 19 of the manhole repair material 1 is sealed with a two-component curable silicone resin 65, and between the pipe port portion 37 and the sewage pipe port 35 of the manhole repair material 1 and the lower end of the manhole repair material 1. The space between the bottom outer edges is sealed with an underwater curing epoxy putty 67. And a sufficient water-stop structure that can withstand a large hydraulic pressure under the manhole repair material 1 by injecting a small amount of water-curable curable resin (not shown) into the outer gap of the manhole repair material 1 and curing it. Configure.

  FIG. 5 is a view showing a state in which the other manhole repair materials 3 and 15 are placed in the manhole, pressed against the manhole inner wall surface 5 and placed in the manhole.

  The manhole repair materials 3 and 15 have a flexural modulus of about 169 MPa (the manhole repair material 1 also has a flexural modulus of about 169 MPa). It is hung by the hanging string 61 (refer FIG. 3) connected to 45 and 47 (refer FIG. 1), and is put in a manhole. The manhole repair material 3 inserted into the manhole is released from the overlapped state so that the lower end portion 29 is accommodated in the upper end portion 25 of the manhole repair material 1, and the plate piece 49 of the lower end portion 29 is the manhole repair material 1. Are stacked so as to be caught on the outer side of the upper end portion 25. Subsequently, the diameter expanding tool 63 is put into the manhole repair material 3, and the ribs 39 and 41 (in some cases, the fixing pieces 51 on the ribs 39 and 41) are placed in the manhole inner wall surface 5 in the same manner as the manhole repair material 1. The diameter of the manhole repair material 3 is increased until it is pressed against the straight wall portion 7, and the cut portion 21 (see FIG. 1) and the stacked portion 69 are sealed with a two-component curable silicone resin 71. Next, the manhole repair material 15 is put in the manhole, the plate piece 49 of the cylindrical portion 13 is caught on the outside of the upper end portion 27 of the manhole repair material 3, and the cylindrical portion 13 is accommodated in the upper end portion 27. The diameter of the manhole repair material 15 is expanded by the diameter expanding tool 63 until it is pressed against the upper end of the inclined wall portion 17 and the straight wall portion 7 of the manhole inner wall surface 5 (which may be the fixing piece 51 on the rib 39). The diameter expanding tool 63 used for expanding the diameter of the manhole repair material 15 is formed so that the outer surface thereof corresponds to the oblique truncated cone portion of the manhole repair material 15. And the cutting location 23 and the stacking location 73 are sealed with the two-liquid curable silicone resin 75, and the installation or arrangement of the manhole repair materials 1, 3, 15 is completed. In stacking, the work is performed so that the cut portion 21 coincides with the bulge 31 (see FIG. 1) and the cut portion 23 coincides with the bulge 33 (see FIG. 1).

  FIG. 6 is a view showing a state in which the grout material is injected and filled, and the repair work is completed.

After the completion of the installation of the manhole repair materials 1, 3, 15, the grout material (flowable grout material) 77 is placed over the entire gap 79 (see FIG. 5) between the manhole repair materials 1, 3, 15 and the manhole inner wall surface 5. Then, it is filled and cured to form a repair lining layer.

  When the grout material 77 is cured, the diameter expanding tool 63 is removed, and a step (not shown) is newly attached. Then, the appearance is finished and the repair work is completed. As shown in FIG. 7, the grout material 77 also enters and cures in the gap recess 53 between the lower end side of the plate piece 49 and the outer surface of the manhole repair material 1, 3, 15, and the manhole repair material 1, 3, 15 constitutes an effective repair lining layer. That is, the grout material 77 enters the back side 53 on the lower end side of the plate piece 49.

  FIG. 8 is a perspective view showing another manhole repair material, FIG. 9 is a view for explaining a manufacturing process of another manhole repair material, and FIG. 10 is a view showing another anchor member.

  The other manhole repair material 81 does not include the ribs 39 and 41, the plate piece 49, and the fixing piece 51, and is provided with another anchor member instead, but the manhole repair material 1 is provided for other configurations. Therefore, the same parts are denoted by the same reference numerals and description thereof is omitted.

  Another manhole repair material 81 includes an anchor member 85 having a semicircular cross-section (half-divided cylindrical shape) having a groove 83 extending over the entire length of the semicircular cross-section, which is fixed to the outer surface. A through-hole 87 extending in the vertical direction along the outer surface of the manhole repair material 81 is formed between the groove 83 and the outer surface of the manhole repair material 81. In order to form such a manhole repair material 81, first, a glass fiber reinforced, for example, is applied to a mold 89 having a cylindrical outer surface (the outer surface of the mold is formed to correspond to the manhole inner wall surface 5 to be repaired). A vinyl ester resin (molding material for repair material) 91 is laminated by a hand lay-up method (FIG. 9a). Next, in a state where the vinyl ester resin is in an uncured state (specifically, when the vinyl ester resin is in a semi-cured state), the anchor member 85 is pressed and pasted to an appropriate position on the outer surface (FIG. 9b). The anchor member 85 is made of the same material as the manhole repair material 81 and is made of glass fiber reinforced vinyl ester resin. Moreover, the anchor member 85 has a height of 10 mm and a height of 7 mm as required. Then, after the vinyl ester resin is cured and the repair material molding 91 is molded, die cutting is performed, one circumferential direction of the obtained cylindrical body is cut in the longitudinal direction, and the lower end is set to a predetermined profile. To form an FRP manhole repair material 81 integrally having an anchor portion 85 (FIG. 8). When another manhole repair material 81 is used, the manhole repair materials 3 and 15 also do not include the ribs 39 and 41, the plate piece 49, and the fixing piece 51, and another anchor member 85 is provided instead. Will be changed as follows. And the manhole repair material which changed the manhole repair materials 3 and 15 is manufactured similarly to another manhole repair material 81. FIG. As shown in FIG. 10, as the anchor member, the anchor member 95 (FIG. 10 a) having a triangular cross section having a groove 93 extending over the entire length of the triangular section, or the rectangular cross section including a groove 97 extending over the entire length of the rectangular section. The anchor member 99 (FIG. 10b) can also be used. The anchor members 95 and 99 are made of the same material as the anchor member 85, and have grooves 93 and 97, thereby forming a through hole extending in the vertical direction along the outer surface of the manhole repair material, like the anchor member 85. It will be. The anchor members 85, 95, and 99 have a retaining effect when the grout material enters the through hole and hardens. The anchor members 85, 95, and 99 function as gap forming protrusions as with the ribs 39 and 41.

  FIG. 11 is a perspective view showing still another manhole repair material.

  Furthermore, another manhole repair material 101 has a granular material layer (here, No. 3 (average particle size 1.2 to 0.6 mm) or No. 4 (average particle size 0.6 to 0) on the outer surface of another manhole repair material 81. .3 mm) silica sand layer 103 using silica sand) is fixed to the manhole repair material 81, and the description thereof is omitted. As shown in FIG. 12, another manhole repair material 101 is obtained by obtaining an FRP manhole repair material 81 integrally having an anchor member 85 (see FIG. 8), and then expanding the manhole repair material 81 into a flat plate shape. Then, an epoxy resin room temperature curable adhesive 105 is applied to the entire outer surface with a roller or a brush to form an adhesive layer 107 (FIG. 12a). Subsequently, while the adhesive layer 107 is uncured, the No. 3 or No. 4 silica sand 109 is entirely spread on the adhesive layer 107 to form the silica sand layer 103 (FIG. 12b). Then, the adhesive layer 107 is cured and the silica sand layer 103 is fixed to the adhesive layer 107, whereby the manhole repair material 101 of FIG. 11 is formed. When another manhole repair material 101 is used, the manhole repair materials 3 and 15 are also changed in the same manner as the manhole repair material 101. And the manhole repair material which changed the manhole repair materials 3 and 15 is manufactured by the method similar to another manhole repair material 101 further.

  As described above, the manhole repair method of the present invention can be widely applied to repair of the inner wall surface of a manhole.

It is a perspective view of the manhole repair material used with the manhole repair method which concerns on this invention. It is a figure explaining the formation process of a manhole repair material. It is a figure for demonstrating the arrangement | positioning process of the manhole repair material in a manhole, and is a figure which shows the manhole before putting a manhole repair material. It is a figure for demonstrating the arrangement | positioning process of the manhole repair material in a manhole, and is a figure which shows the state arrange | positioned in the manhole by putting a manhole repair material in a manhole, pressing against the inner wall surface of a manhole. It is a figure which shows the state which put the other manhole repair material in the manhole, pressed against the manhole inner wall surface, and has arrange | positioned in the manhole. It is a figure which shows the state which inject | poured and filled grout material and completed repair work. It is a figure which shows the lining layer for repair. It is a perspective view which shows another manhole repair material. It is a figure for demonstrating the manufacturing process of another manhole repair material. It is a figure which shows another anchor member. It is a perspective view which shows another manhole repair material. It is a figure for demonstrating the manufacturing process of another manhole repair material.

Explanation of symbols

1, 3, 15 Manhole repair material 5 Manhole inner wall 49 Plate piece 77 Grout material 79 Clearance 85, 95, 99 Anchor member

Claims (3)

A manhole repair method for lining and repairing the inner wall of a manhole,
A first step of preparing a molding material for repair material in which a reinforcing base material is impregnated with a liquid curable resin, which is laminated on a mold for molding a manhole repair material for lining;
A second step of pressing and fixing an anchor member made of the same material as the repair material molding material to the outer surface of the repair material molding material when the curable resin is in a semi-cured state;
A third step of curing the uncured curable resin, removing the molding material for repair material from the mold, and constituting a manhole repair material in which the anchor member is fixed to an outer surface;
A fourth step of disposing the manhole repair material in the manhole so as to cover the manhole inner wall surface with a gap;
A fifth step of forming a repair lining layer in which a grout material is injected and cured in the gap between the manhole inner wall surface and the manhole repair material, and the anchor member is embedded in the grout material in a retaining state. When,
A manhole repair method characterized by comprising: In the molding material for repair material prepared in the first step, a gap forming protrusion is formed on the outer surface, and the anchor member is attached to the gap forming protrusion, and this 2. The manhole repair method according to claim 1, wherein the manhole repair method is a plate piece extending from a gap forming protrusion to a position where a gap recess is formed between the outer surface of the manhole repair material. The manhole repair method according to claim 1, wherein the anchor member fixed to the outer surface of the manhole repair material forms a through hole extending along the outer surface.

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