Concrete pipe classifications are determined by the strength of the pipe after a 28-day compression test. Many factors effect the strength including the design, materials used and the actual manufacturing process. For this reason, pipes are rated based on a compressive strength, from 4,000 to 8,000 psi (pounds per square inch).
The rating standards were created by the American Society for Testing and Materials (ASTM) in publication C76 for reinforced pipe and C14 for nonreinforced pipe. For pipes installed underneath traveled roadways, the American Association of State Highway and Transportation Officials (AASHTO) publication HS20 rating standard applies.
Reinforced Concrete Pipe
Reinforced concrete pipe used for sewer, storm water and waste conveyance purposes is graded into classes 1 through 5. Pipe is strength-tested with a design loading (D-load) standard. D-load tests the pipe under stress, including situations where the pipe is installed without bedding or lateral support. Class 3 pipe must support a minimum of 1,350 lbs. per foot (lb/ft). For class 4, the minimum is 2,000 lb/ft, and class 5 minimum is 3,000 lb/ft. The joints in all classes are also tested, and must hold up to 13 psi.
Nonreinforced Concrete Pipe
Nonreinforced concrete pipe used for sewer, storm drain or culverts applications are graded into classes 1 through 3. The testing is similar, with results expressed in pounds per linear foot (lb/Lft). Since the concrete pipe has no metal reinforcing, the testing focuses on maintaining strength over long runs of pipe.
Pipe Under Roadways
When piping is to be laid under a traveled roadway, the requirements for strength shift to a load based system. Instead of concentrating on measuring pipe strength over a run, the measurement is a minimum 16,000 lbs. axle load in a 10-by-20 footprint. Detailed information for calculating loads of this type is available in the American Concrete Pipe Association article in the resources section.