Superior Corrosion Resistance
When defining the durability of a drainage product, both corrosion resistance and abrasion resistance must be considered. The protection afforded by aluminum’s tough, oxide surface film barrier permits aluminum to resist the detrimental effects of corrosion-abrasion cycles.
The superior corrosion resistance of our CAP is achieved by a special aluminum clad culvert material. The rugged core alloy 3004-H34 is itself rated as highly corrosion resistant. But aluminum culvert coil is also clad on both sides with alloy 7072, which is anodic to the core alloy—protecting it both physically and electro-chemically against corrosion.
The aluminum alloys in our CAP have a proven history of excellent corrosion resistance. This is primarily due to a thin, tenacious, inert oxide barrier that forms on the metal surface when exposed to air. This tough, tightly-adhering oxide barrier cannot be easily removed. If damaged or affected by an aggressive environment, this oxide barrier will reform. This is referred to as a “self-healing” effect. The oxide barrier appears on the pipe surface as a grayish-white coating that will build up over time.
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Service-life expectancy studies on installed aluminum drainage products have been conducted since the early 1960s by state and federal agencies, and many are ongoing. Most agencies are now predicting a minimum of 50 years service-life for 0.060” (16-gage) aluminum drainage products in soil/water environments with a pH range of 4.0 to 9.0 and a resistivity of 500 ohm-cm or greater. In addition, good performance may be expected in seawater environments of 36 ohm-cm when the pipe is backfilled with a clean, granular material.
As a matter of good design, there should be no dissimiliar metals in contact with or bonded to the pipe. Experience has shown that galvanized steel fasteners are compatible with aluminum. Other metal-to-metal systems must be insulated with non-conducting coatings or materials.
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Superior Abrasion Resistance
Aluminum’s superior abrasion resistance has been proven through years of exposure to wet/dry abrasion-corrosion cycles. In normally abrasive runoffs, aluminum will only peen with minimum metal loss.
The Aluminum Association presented a paper to the Transportation Research Board in January 1969 reporting on more than 1,000 culverts.* An updated report was presented in 1986.** Both reports included a method of predicting abrasion performance of aluminum corrugated drainage pipe, whereby peak energy curves were converted into a service-life chart.
If a proposed pipe is expected to be installed in a stream with high velocity (more than 15 fps) runoff and with heavy bed load (especially angular rocks with sharp corners), it is recommended that the Aluminum Association abrasion papers be reviewed.
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Vertical live and dead loads are partially transferred by Corrugated Aluminum Pipe to the surrounding soil envelope through earth arching, creating a composite aluminum/soil structure. This composite action provides an economical, structurally sound approach to pipe design. With loads distributed uniformly around the entire circumference of round pipe, the unit pressure at the top and bottom is significantly less than the unit pressure of a rigid pipe under identical loading conditions.
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Satisfactory site preparation, trench excavation, bedding, and backfill operations are essential to develop the strength of any flexible conduit. In order to obtain proper strength while preventing settlement, it is necessary that the soil envelope around the pipe be of good granular material, properly placed, and carefully compacted.
Pomona Pipe Products' engineers are qualified to help you design a proper foundation, adequate bedding, and backfill. (Reference: ASTMB 788)
If the adjacent embankment material is structurally adequate, the trench requires only a bottom clear width of the pipe’s span plus sufficient room for compaction equipment.
Bedding preparation is critical to both pipe performance and service life. The bed should be constructed to uniform line and grade to avoid distortions that may create undesirable stresses in the pipe and/or rapid deterioration of the roadway. The bed should be free of rock formations protruding stones, frozen lumps, roots, and other foreign matter that may cause unequal settlement.
It is recommended that the bedding be a stable, well graded, granular material. Placing the pipe on the bedding surface is generally accomplished by one of two methods to ensure satisfactory compaction in the haunch area. One method is shaping the bedding surface to conform to the lower section of the pipe. The other is carefully tamping a granular or select material in the haunch area to achieve a well-compacted condition.
Satisfactory backfill material, proper placement, and compaction are key factors in obtaining maximum strength and stability.
The backfill material should be free of rocks, frozen lumps, and foreign matter that could cause hard spots or decompose to create voids. Backfill material should be well graded, granular material that meets the requirements of AASHTO M145. Backfill should be placed symmetrically on each side of the pipe in six-inch to eight-inch loose lifts. Each lift is to be compacted to a minimum of 90 percent density per AASHTO T180.
A high percent of silt or fine sand in the native soils suggests the need for a well graded, granular backfill material to prevent soil migration, or a geotextile separator can be used.
During backfill, only small tracked vehicles (D-4 or smaller) should be near the pipe as fill progresses above the top and to finished grade. The engineer and contractor are cautioned that the minimum cover may need to be increased to handle temporary construction vehicle loads (larger than a D-4). Refer to Heavy Construction Loads below.
Salt Water Installation
In salt water installations, the bedding and backfill around the pipe must be clean granular material. If the backfill is subject to possible infiltration by the adjacent native soil, the clean granular backfill should be wrapped in a GEOTEXTILE.
For minimum-cover applications, Pomona Pipe Products recommends that a properly designed flexible or rigid pavement be provided to distribute live loads and maintain cover heights.
- Heavy Construction Loads
For temporary construction vehicle loads, an extra amount of compacted cover may be required over the top of the pipe. The height of cover shall meet the minimum requirements shown in the table below. The use of heavy construction equipment necessitates greater protection for the pipe than finished grade cover minimums for normal highway traffic.
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** "The Mechanisms of Abrasion of Aluminum Alloy Culvert Related Field, Experiences and a Method to Predict Culvert Performance"
** "Abrasion Resistance of Aluminum Culvert Based on Long-Term Field Performance"