Aug 17, 2004

22nd-Century Pipe

Installing concrete could lead to long-term savings

Economic growth has taken on new meaning because of GASB 34, which radically changes how state and local governments must report their finances. To include all existing major infrastructure assets, governments must perform condition assessments every three years. The National Cooperative Highway Research Program’s project 19-04 states, “How state DOTs respond to GASB 34 may have a significant impact on statewide costs of public borrowing, the long-term costs of infrastructure programs, and the proportion of agency funds devoted to construction versus preservation.”

Putting in the time

The economic benefits of applying an asset management approach to public infrastructure reinforces the choice of concrete pipe for sustainable drainage systems. A drainage system built today with low-maintenance reinforced concrete pipe (RCP) would last until 2100. It is truly a product that meets the needs of the present generation, without compromising the needs of future generations.
The three-edge bearing test, which is the traditional method for measuring concrete pipe strength, induces the most severe loading to which any pipe will be subjected. Load-carrying capacity when installed is increased by at least two-fold because of active soil pressure. Furthermore, concrete pipe’s structural strength can be adjusted by varying the wall thickness, concrete strength or the amount and shape of the reinforcing steel. Because structural integrity is derived primarily from the pipe, moderate changes to the soil envelope over time will not compromise the structural integrity of the pipe system. Most of the pipe-soil structure is delivered to the jobsite in the concrete pipe, minimizing problems associated with the installation process.
One of the critical, but often-overlooked, factors in project design is material service life. Data have firmly established that concrete pipe leads with a service life of 100 years and more. Wall thickness alone contributes substantially to the durability of concrete pipe, so scratches, gouges and abrasion are not factors in reducing service life. In addition, concrete is not flammable and the structural integrity of concrete pipe is not susceptible to temperature variations.
Concrete pipe is produced in 8-ft lengths, which may be an advantage on installations that require a trench box and less open excavation during installation. Moving the trench box after a pipeline section has been bedded and backfilled may result in disturbing the side fill. There is less risk of loss of side support with a concrete pipe installation than with other products.

Ready for installation

Concrete pipe joints can be designed to provide silt or water tightness with the ability to accommodate deflected joints, longitudinal, shear or vertical movement. Various joints and gasket designs are available for most installation conditions including culverts and storm and sanitary sewers. With more joints, line and grade is maintained and checked more frequently, pipe lengths can fit and be positioned in standard trench boxes easily, and longitudinal stresses in pipe walls are relieved when pipelines encounter non-uniform bedding foundations.
Even though concrete pipe is heavy, installation speed is more dependent upon rate of excavation than pipe placement. Weight is certainly an advantage when flotation is a concern and when compacting the sides of the pipe or under the haunches.
The concrete pipe industry promotes its product as having a roughness coefficient of 0.012, which is historically and widely accepted by hydraulics engineers. The 20% design factor in the coefficient considers the differences between laboratory and installed conditions. Laboratory results are usually obtained by using clean water and straight, new pipe sections without bends, manholes, debris or other obstructions.

The right material

RCP is produced using aggregates, cement, reinforcing steel and water. There are variations within these basic materials, but the engineering properties of these materials are well-known. These inert materials are commonly found in the natural environment and are recyclable. Culverts are often used in temporary applications to facilitate drainage during construction stages. While designers try to minimize cost, the salvage value of the pipe is sometimes overlooked. When pipe material is specified, it is essential to consider what happens to the pipe at the end of a project’s design life. Concrete pipe can be recycled and sometimes re-used.
Concrete is the most common building material in the world, and its attributes are readily accepted in the call for sustainable development. The concrete pipe industry has been contributing to sustainable development for decades. It encourages specifiers and contractors to choose a lasting product that will lighten the tax burden of future generations and not compromise future needs.

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