|Excerpt from:||Corrugated Plastic Pipeline
Published by the Corrugated Plastic Pipe Association
Volume No. 3
The study conducted for the CPPA by L.H. Gabriel, Ph.D., P.E., California State University, Civil Engineering Department does much to draw a focus on the attributes, vulnerabilities and properties of pipes in corrosive situations, Gabriel states however, that drawing generalizations should be limited to the degree that laboratory conditions reasonably meet real world field conditions. It is extremely difficult to match laboratory conditions with field conditions in the organic soils of the Mid-Atlantic coastal plain and Southeastern USA. Organic soils are defined as those soils affected by the natural decay of sub-surface vegetation. This in turn creates extremes of chemical variety whereby the measured pH can vary with the seasons and the intenseness of rainfall.
The run-off from dry washes, ephemeral streams and rills that require storm culverts is often quite different from that of nearby rivers that are easily measured, and commonly composed of a water-sediment mix from many diverse sources that allow pH readings to level out. Hard data on the MEAN pH of a specific site's soil-water component is difficult and research expensive-intensive to come by; hard data on the EXTREMITY of a site's pH that may influence the rate of corrosion would be even more difficult and expensive to obtain. Consequently, HISTORICAL EXPERIENCE with corrosion in a given area receives more weight than pH alone in the selection of a non-corrosive pipe material.
Based on the HISTORICAL method of identifying corrosive soils, states from New York to Florida have set aside counties where extra efforts must be made to deal with corrosion in pipe systems. In this respect, the NC-DOT has outlined twenty-four counties in Eastern North Carolina where uncoated metal pipes are not allowed for use on NC-DOT stormwater control projects because of the experienced corrosion on uncoated pipes in these counties:
Paragraph E on page 685 of the 1990 North Carolina DOT Specifications for Roads and Structures identifies 24 counties where uncoated aluminum alloy or galvanized steel pipes may not be used.
"Pipe meeting the applicable requirement of Subarticle 1032:3 (A) for aluminized pipe may be substituted for bituminous coated galvanized steel culvert pipe as provided in paragraphs 1 through 5 below, except that paragraphs 1, 2 and 3 shall not apply in the counties of Beaufort, Bertie, Bladen, Brunswick, Camden, Carteret, Chowan, Columbus, Craven, Currituck, Dare, Gates, Hertford, Hyde, Jones, Martin, New Hanover, Onslow, Pamlico, Pasquotank, Pender, Perquimans, Tyrell and Washington."
Until this date, metal pipes with bituminous coatings have been required in these counties of predictable pH extremes and known corrosiveness. One such county is the coastal county of Brunswick, where 29 of 36 soil and water features identified by the Soil Conservation Service (SCS) county soil survey are cited as highly corrosive to either uncoated steel or concrete.
|Flooding||High Water Table||Subsidence||Risk of Corrosion|
|Map symbol & soil name||Hydro-
|Months||Depth (ft)||Kind||Months||Initial||Total||Uncoated Steel||Concrete|
|Lu- Lumbee||D||Rare||-||-||0.0-1.5||Apparent||Nov - Apr||-||-||High||High|
|C||None||-||-||0.5-1.5||Apparent||Nov - Apr||-||-||High||High|
|C||None||-||-||1.5-3.5||Apparent||Jun - Dec||-||-||Moderate||High|
|D||Frequent||Brief||Nov - Apr||0.5-1.5||Apparent||Dec - Mar||-||-||High||High|
Sample of table excerpted from pages 117 and 118 of the Brunswick County, NC Soil Survey which lists the full 36 soil types and their corrosive levels.
Another county is the inland county of Bladen where 55 of 61 soil and waste features identified by the County SCS Soil Survey are cited as being highly corrosive to either uncoated steel or concrete. The NC-DOT specs listed above also note that uncoated aluminum alloy pipes are not to be used in either county as well.
|Flooding||High Water Table||Risk of Corrosion|
|Map symbol & soil name||Hydro- logic group||Freq- uency||Dur-
|Months||Depth (ft)||Kind||Months||Uncoated Steel||Concrete|
|Na - Nahunte||C||None||-||-||1.0-2.5||Apparent||Dec - May||High||High|
|NoA, NoB - Norfolk||B||None||-||-||4.0-6.0||Apparent||Jan - Mar||Moderate||High|
|NuB - Norfolk Urban land||B||None||-||-||4.0-6.0||Apparent||Jan - Mar||Moderate||High|
|Oc - Ocilla||C||None||-||-||1.0-2.5||Apparent||Dec - Apr||High||Moderate|
Sample of table excerpted from pages 174 to 177 of the Bladen County, NC Soil Survey which lists the full 61 soil types and their corrosive levels.
The imperviousness of bituminous coated pipes to corrosion, however, is always suspect on every construction job. This is because normal construction handling often bends or dents pipes, cracking their protective coating. In addition, sunlight frequently dries out the protective tar-like coating if stored, and not installed soon after manufacture. Temperatures in excess of 85ºF that are nearly a daily occurrence in the Southeastern USA from May to September, also melt the coating which causes it to run, and thin out in spots. This allows the gritty abrasiveness of sediment that usually flows through these unpredictable pH EXTREME prone soils to more easily strip the remaining thin bituminous protective layer allowing corrosion to begin.
"The fear of corrosion brought on by such normal, routine construction events often causes project managers to forego stocking bituminous coated pipe on the site in reserve for installation. Managers opt instead for delivery on the day of installation to cut down on extra handling events to prevent pipe damage that may lead to corrosion later", says Eugene Carter, a local pipe distributor from Shallote, NC. Following this construction organizational practice, the project manager knows he is opening himself up to the possibility of unexpected rain delays and equipment breakdowns, which often accompany the daily scenario of these pipes being installed; too many unpredictable components to organize and schedule results in lost time with more construction loan interest.
HDPE corrugated plastic pipes, while more expensive in some sizes than pipes manufactured from competing materials, not only allow a construction manager to have more choice in timing his pipe installation to suit each particular job, but also offer a cheaper "installed-in-place" pipe due to installation labor savings. "This is particularly true in the larger sizes where more labor is needed to man handle the installation," adds Carter.
It would seem that the recent advent of ASTM F405 / AASHTO M252 and 294 chemically inert, corrosive free HDPE (High Density Polyethylene) corrugated pipes with tough abrasion resistance in diameters of 12" and larger, as described by Dr. L.H. Gabriel, P.E. of California State University in his study of ABRASION RESISTANCE for the CPPA, will offer a much need option to bituminous coated pipes for construction managers installing culvert stormwater control systems in corrosive soils.
Houston Crumpler, Jr. is Secretary-Treasurer of Crumpler Plastic Pipe Inc., Roseboro, NC and a member of CPPA research committee.
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