Handling & Installation Comparisons

Manufacturers of high density polyethylene pipe (HDPE) claim that the longer lengths, lighter weights and fewer joints of their products yield substantial cost savings. This claim, however, is difficult to prove using actual field data. In some cases, the very attributes of concrete pipe that competitors cite as disadvantages prove beneficial to the installation and performance system.

Lengths

Concrete pipe is produced in shorter lengths than most flexible products. Reasons fro this include the mass of the product, shipping requirements and manufacturing methods. The lengths of concrete may be an advantage on installations that require a trench box. The length of concrete pipe allows for a shorter trench box and less open excavation during installation. Moving the trench box after having bedded and backfilled the pipe, may result in disturbing the side fill. This could result in the loss of side support so critical to an HDPE installation. ASTM D 2321 "Standard Practice for Installation of Thermoplastic pipe for Sewers and Other Gravity-Flow Applications" states that movable supports should not be used below the top of plastic pipe unless approved methods are used for maintaining the integrity of embedment material.

Weights

Concrete pipe is heavier per foot (or meter) than flexible but, in most common sizes, even flexible pipe is beyond OSHA limits for routine manual lifting. Machinery is required to handle both products. In addition, installation speed is more dependent upon rate of excavation than pipe placement.

The heavier weight of concrete also is an advantage when flotation is a concern. It is well documented that the comparative lighter weight of flexible pipe makes it subject to lateral movement when compacting on the sides of the pipe. HDPE also tends to lift vertically off the required grade as material under the haunches is compacted. Select material is important to a successful installation of flexible pipe. HDPE installation procedures recommend that the trench width be established by the designer and ASTM D2321 requires all material in the haunch area be placed by hand.

Joints

The increased number of joints is a perceived shortcoming of concrete pipe while in fact it may be an advantage. Line and grade is maintained and checked frequently. Various joints and gasket designs are available for most installation conditions from culverts, to storm sewers, to sanitary sewers.

Concrete pipe can also be designed for jacking, micro-tunneling and low-head pressure applications. Concrete pipe is available in several different classes and shapes for each diameter. This gives the designer considerable versatility. By varying the class of pipe, the type of bedding or the installation type, a myriad of designs are available. Cost Analysis of Pipe Envelope (CAPE) is a design aid to help evaluate the material costs for projects.

Other Factors

With concrete pipe, the majority of the pipe-soil structure is delivered to the job site in the pipe. In some instances, up to 100% of the pipe-soil structure is provided to the job site, thereby minimizing potential problems and negative impact of contractor error in the installation process. HDPE pipe with its low stiffness comprises 5% or less of the soil-pipe system at delivery. This places most of the burden of performance in the field on the installation method, type of backfill material and adequacy of field inspection. Poor site conditions, such as weak native soils and groundwater, further aggravate the problem. A wider trench may be required for HDPE pipe than for concrete to provide adequate stiffness to support the pipe.