OPEN BOTTOM FIBER REINFORCED PRECAST CONCRETE ARCH UNIT

Background of the Invention

[0001] This invention relates to open bottom precast concrete arch units of the type constructed to be buried in the ground or soil and as generally disclosed, for example in U.S. Patents No. 3,482,406, No. 4,558,969, No. 5,281 ,053, No.6,161 ,342, No.6,205,717, No.6,406,220, No.6,408,581 and No. 6,640,505. Such precast concrete arch units are produced in various sizes and spans, for example, by Bebotech Corporation in Middletown, Ohio who offers part elliptical and part circular shapes and as a single or one-piece casting or as a twin or two-piece casting with spans from 12 feet to 84 feet. In such precast concrete arch units, it is common to have a wall thickness of 8 to 14 inches and to have embedded in the concrete walls steel reinforcing bars or rods which may be in the form of a mesh. The steel reinforcing bars or rods provide the concrete arch unit with sufficient strength to support the soil above the arch unit and any load which may be applied to the arch unit, for example, by the wheels of a vehicle on a road passing over the arch unit.

[0002] It is desirable to minimize the weight of a precast concrete arch unit without sacrificing strength for supporting a load in order to reduce the construction cost of the unit and the costs for handling, transporting and installing the unit at a construction site. Thus it is desirable to reduce the volume of concrete and the volume of reinforcing steel within a precast concrete arch unit in addition to reducing the labor required to precast the unit. Furthermore, when a one-piece open bottom precast concrete arch unit is used, it is desirable for the unit to be transported while resting on one end and be nested with other similar arch units for efficiently transporting a plurality of the arch units on a semi-trailer bed. While an arch unit may be precasted in two or more sections to facilitate transportation, substantial additional time and labor is required to assemble and connect the sections at the site where the arch units are installed.

Summary of the Invention

[0003] The present invention is directed to an improved one-piece open bottom precast concrete arch unit which is designed for underground use and to be buried in compacted soil. The arch unit of the invention provides all of the desirable features mentioned above and is effective to utilize the surrounding soil

mass as a resistance support for the arch unit and thereby minimize the production cost of the arch unit by substantially reducing the volume and weight of concrete and the weight of reinforcement within the arch unit. The arch unit of the invention eliminates the use of steel reinforcing bars or rods and thus eliminates the labor required to position steel reinforcement bars or rods between the forms which are used to precast the arch unit. [0004] In accordance with one embodiment of the invention, a one-piece arch unit is precast with opposite arcuate side wall portions integrally connected by an arcuate top wall portion. The side wall portions have a radius of curvature substantially greater than the radius of curvature of the top wall section, and the radius of curvature of the top wall portion is substantially less than the vertical height or rise of the arch unit and also substantially less than the span of the arch unit between the bottoms of the side wall portions. The precast concrete arch unit of the invention is reinforced with a predetermined volume of reinforcing fibers such as steel fibers which provide the arch unit with substantial ductility and flexure strength in order to utilize the resistance forces produced by the compacted soil mass surrounding the arch unit. Utilizing the compacted soil to resist bulging of the side wall portions of the arch unit enables a substantial load to be supported by the top wall portion.

[0005] The interaction between the arch unit of the invention with the surrounding soil in combination with the fiber reinforcement not only provides for eliminating steel reinforcement bars and rods, but also provides for substantially reducing the wall thickness of the precast concrete arch unit. The reduction in the wall thickness of the arch unit further increases the ductility of the unit and facilitates the transfer of loads and stresses within the arch unit. Additional economies are achieved by varying the wall thickness of the side wall portions to proportion the strength provided by the arch unit to the applied loads and stresses for the varying conditions around the perimeter of the arch unit. Tapering of the side wall portions also reduces the volume of concrete required to cast the arch unit, thereby further reducing the weight and production cost of the arch unit. In a modification of the invention, one or more elongated bands of longitudinally extending reinforcing fibers are connected or bonded to peripheral or arcuate sections of the top and side wall portions. [0006] Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

Brief Description of the Drawings

[0007] FIG. 1 is a perspective view of a plurality of underground open bottom precast arch units constructed in accordance with the invention and shown supported by parallel spaced concrete footers;

[0008] FIG. 2 is an end view of the arch unit of FIG. 1 and shown buried in compacted soil;

[0009] FIG. 3 is an end view of only the arch unit shown in FIG. 2 and illustrating the radii of curvature for the side and top wall portions of the arch unit along with the span, rise and wall thicknesses of the unit;

[0010] FIG.4 is a diagrammatic view of the arch unit shown in FIG.2 and illustrating, in exaggerated form, the deflection of the side and top wall portions in response to an applied downward load; and

[0011] FIG. 5 is a perspective view of a precast concrete arch unit constructed in accordance with the invention and having elongated bands of longitudinally extending reinforcing fibers bonded to the outer surface of the arch unit.

Description of the Preferred Embodiments

[0012] FIG. 1 illustrates a plurality or series of longitudinally aligned one- piece open bottom fiber reinforced precast concrete arch units 10 constructed in accordance with the invention and supported by parallel spaced elongated concrete footers 12 which may be cast in place at the construction site or be part of a poured concrete floor. The arch units 10 are buried in compacted soil 15 (FIG. 2) and define a passage 16 through which pedestrians, vehicles or water may pass. The arch units may also be provided with end walls to define a chamber in which water may be temporarily stored, for example, run-off water received from a large parking lot.

[0013] Referring to FIGS. 1 & 3, each of the precast concrete arch units

10 has a length L, for example, eight feet, and includes a pair of curved or arcuate opposite legs or side wall portions 18 integrally connected by a curved or arcuate top wall portion 22, and the arcuate length of each portion 18 and 22 may be generally the same, as shown by the lines 23 and 24. The arch unit 10 has a height or rise R from the bottom surfaces of the side wall portions 18 to the top inner surface of the top wall portion 22, and the arch unit has a span S between the bottom inner surfaces of the side wall portions 18.

[0014] Each of the side wall portions 18 has an inner radius of curvature

R1 , and the top wall portion 22 has an inner radius of curvature R2. Preferably, the radius R1 is greater than twice the radius R2, and the rise R is less than the radius R1. The radius R2 is also greater than 20 percent of the span S and less than 45 percent of the span. Thus the radius R2 is substantially smaller than the rise R, and the radius R1 may be greater than the span S, as shown in FIG. 3. When desired, the arcuate length of each side wall portion 18 may be shortened to reduce the rise R by relocating or inserting end walls within the arcuate forms used to precast the arch unit 10.

[0015] Each of the precast concrete arch units 10 is poured with a predetermined volume of reinforcing fibers F (FIG. 1) which are preferably steel with each fiber having a length preferably within a range of 0.25 inch to 3 inches and a length at least fifty times the cross-sectional thickness of the fiber. The density of the fibers as they are thoroughly mixed into the concrete before pouring the precast concrete arch unit 10 is preferably within a range of 0.25 and 2.0 percent by volume. One source of steel fibers which has provided satisfactory results are produced by Polytorx, LLC in Ann Arbor, Michigan and sold under the trademark HELIX. The twisted steel fibers of this Company are disclosed in U.S. Patent No. 6,060,163. However, other fibers also provide satisfactory results, such as the steel fibers produced by Bekaert Corporation and sold under the trademark DRAMIX. Synthetic or non-metal fibers may also be used, for example, fibers sold by Nycon, Inc. under the trademark NYCON. [0016] Each of the arch units 10 does not have any embedded steel reinforcing bars or rods, but has a relatively thin wall thickness to provide the fiber reinforced side wall portions 18 and top wall portion 22 with ductility and flexure strength. As shown in FIG. 3, the side wall portions 18 may also be tapered along its arcuate length from the top wall portion 22 which has a uniform thickness TT. The thickness TT of the top wall portion 22 may thus be greater than the thickness TB at the bottom or base of the side wall portions 18. Preferably, the thickness TB is within the range of 1.5 percent to 3.5 percent of the span S and within a range of 0.5 to 0.9 of the top wall thickness TT. For example, in an arch unit 10 having a span S of 16 feet, the top wall thickness TT may be within the range of 5 inches to 7.5 inches while the bottom wall thickness TB may be in the range of 2.5 inches to 4 inches.

[0017] Referring to FIG.4, an open bottom precast concrete arch unit 10 buried within the soil 15 is diagrammatically illustrated by a single line. When a downward load W is applied to the soil above the arch unit 10, for example, by

the wheels of a heavy vehicle, the top wall portion 22 tends to deflect downwardly, causing the side wall portions 18 to deflect or bulge outwardly, as greatly exaggerated in FIG. 4. However, the resistance forces produced by the compacted soil 15, as indicated by the arrows 30 substantially eliminates or minimizes the bulging of the side wall portions 18 with the result that the deflection of the top wall portion 22 is substantially eliminated or minimized. As a result, the possibility of cracking the fiber reinforced precast concrete arch unit 10 in response to a load W is substantially reduced.

[0018] Referring to FIG. 5, the wall thickness of the arch unit 10 or of any reinforced concrete arch unit may be minimized and/or the arch unit may be designed to support a heavier load W by adhesively bonding to the outer peripheral or arcuate surface of the arch unit one or more elongated bands 35 of longitudinally extending continuous fibers such as non-metal fibers or carbon fibers. A suitable epoxy adhesive may be used to form a positive bond between the bands 35 and the outer surface of the arch unit 10, and the bands need to extend peripherally over at least part of each of the side wall portions and part of the top wall portion in order to overlap the areas of the arch unit which receive the highest bending stress. However, each band may be more conveniently and quickly attached by having one continuous band 35 which also extends over the top wall portion, as shown in FIG. 5, instead of attaching shorter sections of the bands. Preferably, two peripherally extending thin bands 35 are attached adjacent opposite ends of the arch unit, as shown in FIG. 5, but may be spaced somewhat inwardly from the ends of the unit to provide for the attachment of strips of joint cover material when two arch units abut each other, as shown in FIG. 1. Each of the bands 35 may also have a second overlapping band 38 which may be shorter and applied only in each area of maximum bending stress. [0019] As apparent from the drawings and the above description, an open bottom fiber reinforced precast concrete arch unit constructed in accordance with the invention provides desirable features and advantages. For example, by eliminating conventional reinforcing bars and rods and using reinforcing fibers within an arch unit and substantially reducing the wall thickness of the arch unit, the arch unit is provided with ductility and flexure strength so that it may utilize the resistance forces exerted by the compacted surrounding soil to enable the arch unit to support a substantial load applied to the soil above the unit. In addition, the replacement of steel reinforcing bars or rods with the reinforcing fibers F and the substantial reduction in the wall thickness of the arch unit results in a substantial reduction in the volume of concrete used to form the arch unit

and a corresponding substantial reduction in the weight of the arch unit. These reductions provide for a significant reduction in the cost of producing the precast arch unit as well as the cost of handling, transporting and installing the arch unit at the construction site. Moreover, by avoiding the use of steel reinforcing bars or rods, the labor required for positioning the reinforcing bars or rods between the form surfaces for the arch unit, is eliminated.

[0020] Additional economies are achieved by varying the thickness of the side wall portions 18 of the arch unit 10 to proportion the strength provided by the arch unit according to the applied load and stresses around the perimeter of the arch unit. Bending stresses caused by concentrated loads to the top of the arch unit are transformed into thrusts that can be accommodated very efficiently in the arch unit. This transfer of loads and stresses is dramatically enhanced by increasing the ductility of the arch structure. Since the bending stresses in the arch unit diminish towards the base or bottom of the arch unit, the thickness of the side wall portions may be reduced towards the bottoms of the side wall portions. As another advantage, the discontinuity of the steel fibers F prevents corrosion of the reinforcing fibers, and this eliminates the need for additional concrete to protect the reinforcement as is required to protect continuous steel reinforcing bars and rods. Also the fibers add toughness to the precast arch unit, and the toughness resists damage to the unit during handling, shipping and installing. It is also apparent that a plurality of the arch units 10 shown in FIGS. 2 and 3 may be nested during storage and shipping to provide additional economies. It is also apparent that a plurality of laterally adjacent rows of the arch units as shown in FIG. 1 may be used for an underground water storage facility.

[0021] While the form of arch unit and its method of construction and use herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of arch unit and method, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims. What is claimed is: