Buried pipes are grouped into two categories: rigid and flexible. A pipe that can deflect more than 2% in diameter without cracking is considered a flexible pipe; one that cannot deflect to this degree is deemed rigid. By this criterion, HDPE, PVC, and CS pipes are all classified as flexible and RC pipe as rigid. Rigid pipe is designed to resist external load by its inherent strength, whereas flexible pipe relies on the capacity of the surrounding soil to carry the load and provide stability. The review of the literature indicates that:

• All types of pipe can perform well, but the conditions for satisfactory long-term performance vary. Furthermore, the performance criteria are different for the different types of pipe: the severity of cracking is the main performance criterion for rigid concrete pipe, whereas the degree of deflection is the main performance criterion for flexible pipe.
• Installation and workmanship, which can affect short-term performance, are more critical for flexible pipe than for rigid pipe.
• Corrosion and abrasion are the prime causes of deterioration of CS pipe; protective coatings add only marginal value to pipe performance.
• PVC and HDPE pipe offer high resistance to chemical and biological attack in most corrosive environments. They also have superior abrasion resistance. However, creep, brittleness in low temperature, flammability and UV degradation are material characteristics common to plastic pipe materials.
• Concrete pipe is susceptible to chemical and biological attack, but has superior impact and fire resistance. It is also less likely to be damaged if adjacent pipes need to be excavated.

To determine the current state of practice, questionnaires were sent to 80 municipalities and 10 provincial departments of transportation (DOTs) across Canada. The survey included questions about inventory, design practices, installation procedures and performance of pipe materials. Thirty-eight municipalities and 6 DOTs completed and returned the questionnaire. The responses to the survey can be summarized as follows:

• Most existing storm and sanitary sewers are made of concrete, whereas 60 to 95% of existing culverts are constructed of CS pipe.
• A municipality or DOT, rather than a contractor or consultant, usually makes the choice of pipe product.
• Different bedding standards are used in different geographic regions.
• The age of concrete pipe ranges from newly installed to more than 75 years, while that of PVC pipe is less than 25 years.
• PVC pipe provides the greatest installation ease for storm and sanitary sewer applications because of its light weight and length. CS pipe has similar characteristics and is therefore the easiest type of pipe to install in culverts.
• Life-cycle cost has become an increasingly popular determinant in the choice of rehabilitation techniques. Excavation/replacement is the most common technique used in repairing pipes; it is followed by pipe lining.
• Traffic loading, burial depth and hydraulic flow are the most relevant design parameters.
• For further information, please contact Dr. Balvant Rajani at (613) 993-3810, fax (613) 954-5984, or e-mail balvant.rajani@nrc-cnrc.gc.ca