introduCtion External Load Crushing Strength Test (Three-Edge Bearing Test) A common method used to determine structural strength is to conduct an external load crushing strength (or threeedge bearing) test, which uses a machine designed to apply a crushing force in a true vertical plane, parallel to one diameter or the vertical centerline, and extending the full length of the wall of the specimen. The force is applied using the three-edge bearing method (pictured left), in which the test specimen is supported on two parallel longitudinal strips and the load applied through a top bearing beam. The load is applied at a steady rate along the pipe length until the formation of a 0.01-inch wide crack, which is measured using a specialized gauge. A 0.01-inch crack is considered formed when the point of the gage, without forcing, will penetrate 1/16 inch at close intervals through a one foot length of crack. If both the 0.01-inch crack and the ultimate (failure) load are required, the specified rate of loading need not be maintained after reaching the 0.01-inch crack load. The ultimate load is reached when the pipe will support no greater load. Since testing pipe to ultimate strength destroys the pipe section, destructive test procedures are costly, especially for larger size pipe and results in increased cost to the purchaser. Ultimate strength data has questionable usefulness since no correlation has been developed for the buried condition. Accordingly most agencies do not require ultimate strength testing. Some agencies do require testing to loads 10 to 15 percent greater than the specified 0.01inch crack load and, if no such crack occurs, the pipe are accepted. Still other agencies accept pipe on the basis of inspection of reinforcement placement, production quality, and material tests. ConCrete pipe stormwater treatment teChnologies box seCtions manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 21