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Due diligence

Due diligence

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By Mike McGough, PE

Used successfully for more than a century in storm sewer and culvert applications, corrugatedsteel pipe (CSP) has undergone a renaissance of sorts over the last 50 years, resulting in better materials with a longer service life in challenging environments. Here's an overview of ways to choose the appropriate CSP products using an estimated material service life for various environmental ranges. When estimating the average invert service life, engineers and designers should also refer to the National Corrugated Steel Pipe Association's (NCSPA) CSP Selection and Service Life Guide or the Durability chapter of the Corrugated Steel Pipe Design Manual.

Environmental Considerations

When determining which CSP product is bestsuited for an installation, first determine the site's water-side environmental conditions for pH and resistivity. Normal environmental conditions have a pH range of 5.8 to 8 with a resistivity of R > 2,000 ohm-cm (ohms centimeter); mildly corrosive conditions have a pH range of 5 to 5.8 with a resistivity 1,500 . R . 2,000 ohm-cm; and corrosive conditions have a pH < 5 and a resistivity of R < 1,500 ohm-cm.

The presence of soluble salts must be considered as well. Soft water has a significant affect on galvanized coatings but does not affect the longevity of aluminized type 2 or polymer-coated CSP. Natural water contains calcium carbonate (Ca-CO3) in varying concentrations. Hard water has an excess of this dissolved salt, which is deposited on the pipe wall in the form of scale, serving to protect the underlying coating and extend the pipe's service life. When the calcium carbonate concentration in the effluent drops to less than 50 parts per million (ppm), the water is considered soft. At this concentration level there is no excess CaCO3 to deposit on the galvanized pipe, which prevents buildup of the protective scale layer.

There is a correlation between water softness and resistivity. A high resistivity level indicates a lower calcium carbonate concentration. Therefore, if a site has a resistivity = 10,000 ohm-cm, this could indicate that soft water is present. The design engineer should then consider using aluminum- coated type 2 or polymer-coated corrugated steel pipe.

The Federal Highway Administration (FHWA) established a classification system of abrasion levels based on flow velocity and bed load.

A study conducted by Corrpro Companies in 2002 found that soil-side durability often is not the limiting factor in designing CSP systems. Results indicated that 80% of plain galvanized installations have a soil-side service life in excess of 100 years and that nearly all have a soil-side service life in excess of 75 years.

The study also found that soil moisture contents below 17.5% did not exhibit any accelerated corrosion. Under most circumstances, corrosion rates are directly related to soil moisture content. However, for galvanized steel pipe, the soil moisture content primarily affects the activity of any chloride ions present and the chloride's acceleration of the corrosion. When the soil moisture content was below 17.5%, the chloride ion concentration did not significantly affect the corrosion rate of the zinc coating.

All corrugated steel pipes have a metallic coating for corrosion protection. When the coating selected does not provide the required service life or is outside the appropriate environmental conditions, an alternate coating system can be selected. Often the required service life can also be achieved by increasing the sheet thickness; this alternative should be weighed against the cost of supplemental coatings.

Service Life

Zinc-coated (galvanized) steel (AASHTO M36, ASTM A929) is produced with a coating weight of 610 grams per square meter (g/m2; 2 ounces/square foot) of surface (total both sides) to provide zinc coating thickness of 43 micrometers (?m; 0.0017 inch) on each surface. Zinc's excellent corrosion resistance is achieved in two ways: through the galvanizing process and through cathodic protection. Galvanizing provides a barrier coating of zinc over the steel, while in cathodic protection the zinc acts as an anode, sacrificing to oxidation before the steel. High corrosion rates in strongly acidic and strongly alkaline solutions can be attributed to the absence of film on the metal surface; stable films are present on the surface when the corrosion rates are low.

The service life of zinc-coated galvanized steel is determined using the American Iron and Steel Institute (AISI) Chart for Estimating Average Invert life for Galvanized CSP (see chart). The service life of CSP can be reasonably predicted based on the environmental conditions discussed above. This chart is based on 16-gauge galvanized CSP with a 610 g/m2 coating and can be applied to other thicknesses with the appropriate factor.

The other options are aluminumcoated type 2, polymer-coated, and pipe with other coating materials. Polymer coating (AASHTO M245, ASTM A742) is a laminate film that is applied over protective metallic coatings and is typically galvanized. The 10/10 grade (10 mils thickness, each side) is the product used most often. Polymer coating provides excellent adhesion to the base steel and extended corrosion- and abrasion resistance. The service life recommendations are based on extensive lab and field tests. Although polymer-coated CSP has not been in use quite as long as the galvanized or aluminized coatings, polymer CSP installations still go back nearly 50 years.

Polymer-coated CSP has been found to perform with essentially no deterioration, despite installation in some extreme environments. NCSPA and the Dow Chemical Co. have conducted laboratory tests to provide additional data on the service life of polymer-coated CSP. Sites contained environmental conditions with resistivity as low as 100 ohm-cm and pH as low as 2.1.

Testing indicates that polymer-coated CSP can withstand abrasion Level 3 conditions, and since polymer is a barrier coating, it exhibits uniformly predictable durability performance within defined environmental ranges. These criteria and the defined ranges for polymer-coated pipe are shown in the chart.

Aluminum-coated type 2 (AASHTO M274, ASTM A929) is a pure aluminum coating (no more than 0.35% silicon). The aluminum-coated type 2 is the most widely used supplemental coating for CSP. Aluminum is a reactive metal, but it develops a passive aluminum oxide coating or film that protects it from corrosion in many environments. This film is quite stable in neutral and many acid solutions, but is attacked by alkalies with pH greater than 9.

From a corrosion standpoint, aluminum coating has an advantage over galvanized coating in lower pH and in softwater conditions due to the formation of the oxide film. The coatings are essentially equal under abrasion and in waters where the zinc oxide film forms rapidly. Over the past 50 years, aluminum-coated type 2 CSP has been installed at thousands of sites ? many of which continue to be monitored for performance.

Additional service life can be gained by increasing the thickness of the base steel in accordance with the factors shown in the Chart for Estimating Average Invert Service Life, or with the use of additional coating systems. These coatings offer advantages over metallic coatings in the form of increased abrasion resistance, wider environmental ranges, and longer service life. Inherent in these coatings is less variability in performance, which is why specific add on service life values are recommended under various abrasion levels.

? McGough (info@ncspa.org) is thechief engineer for the National CorrugatedSteel Pipe Association.