Precast Solutions - March/April 2008 - (Page 13)

cover. Are there concerns about reactive/corrosive soil or groundwater conditions? Materials engineers often choose HDPE pipe for its inert chemical properties. Yet RCP stands up very well in these environments and also offers equally high material impermeability, especially when mix designs include fly ash, silica fume and other cementitious materials. Because of the variability of its native soils, current culvert design procedures and standard specifications, WIDOT limits the maximum diameter for plastic pipe to 36 inches. Allowable diameter for plastic pipe varies among the state agencies. Some DOTs allow use of 60-inch diameter, the maximum HDPE size manufactured at this time. In many cases, fill height tables determine the pipe material and class. Samples of pipe specifications may be found in the online version. Geotechnical engineers in Georgia evaluate the soil conditions (pH and resistivity) and prepare a modified table that is project-specific. The project-specific (or modified) allowable materials table is then used in state transportation construction plans for contractors to use when selecting suitable pipe material (see the online version for an example). installation and recommended fill materials to reach its manufacturer-specified service strength. Liability. Dr. Patricia Galloway, P.E. (see sidebar “Engineer’s Liability: The Difference between RCP and HDPE,” page 13) provides sobering information on the engineer’s liability in specifying materials for pipe installation. Some view the engineer’s liability to be ENGINEER’S LIABILITY: THE DIFFERENCE BETWEEN RCP AND HDPE BY DR. PATRICIA D. GALLOWAY, P.E. Many engineers do not understand or appreciate the differences between reinforced concrete pipe (RCP) and highdensity polyethylene (HDPE) pipe and the potential for liability when specifying each product. While RCP is a rigid structure that is designed, built and tested as a structure before it arrives at the construction site, the structure of HDPE pipe is actually built and tested in the field. Therefore, HDPE pipe is not an “approved equal” substitute for RCP. Corrugated HDPE pipe, rather than RCP, is typically specified by design engineers as a Cost. Engineers for private developers may consider HDPE pipe for drainage to comply with an owner’s desire to keep costs at a minimum, with little consideration for service life. State DOTs, as publicly funded agencies, also must weigh the material costs of available pipe materials. The general perception seems to be that the initial cost of RCP is higher in most applications for pipe under 60 inches in diameter than the initial cost for plastic pipe. The installed cost of plastic pipe (which may include engineered backfill, inspection and laser testing), however, may prove equal to or greater than RCP costs. result of assertions by HDPE pipe manufacturers regarding its lower cost and superior attributes. In some cases, HDPE may be a reasonable alternative to RCP, depending on the specific project requirements and design life. However, designers must be cognizant of all aspects and design responsibilities of using any pipe material before specifying it, because materials and their service lives may differ. Some ASTM/AASHTO specifications place significant responsibility on the engineer regarding installation essentials to ensure service performance. Applicable sections in ASTM D 2321-04 include requirements placed on the engineer with regard to installing thermoplastic pipe. Engineers also must recognize that HDPE pipe requires an engineered installation in which the engineer must be involved in the construction activities. The engineer must take responsibility to ensure that post-installation deflection testing has been performed and documented. Because the prime structure in an HDPE pipe system is the soil envelope and the pipe-soil interaction can account for up to 90 percent of an installation’s success, it is Strength. RCP arrives on site as a proven structure with a tested concrete compressive strength that is known to the engineer, usually having attained greater than 90 percent of its maximum strength. Plastic pipe arrives on site as a non-structure, at about 10 percent of the installed system’s required strength, and requires proper contractor critical that this determination (interaction between pipe, embedment material and native soil) be made by a geotechnical or soils engineer. Dr. Patricia Galloway, P.E., is CEO of the Nielsen-Wurster Group and past president of ASCE. This text is an excerpt reprinted with permission of Galloway from an article entitled “Read between the Pipes – Concrete pipe offers safety benefits when used in storm sewer systems,” copyright Scranton Gillette Communications Inc. MARCH/APRIL 2008 | PRECAST SOLUTIONS 13

Table of Contents Feed for the Digital Edition of Precast Solutions - March/April 2008

Precast Solutions - March/April 2008 Contents Smooth Move Riding the Waves Comparing RCP With Plastic Pipe Cost-Value Analysis of Storm Sewer Pipe Concrete Testing On a Grand Scale Building Confidence

Precast Solutions - March/April 2008

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