CONCRETE PIPE Autogenous Healing Small cracks in concrete, under damp or wet conditions, will in time be sealed by the process of autogenous healing. This natural phenomenon has been studied extensively and reported in the technical literature for over 50 years. Two examples of the many reported in the literature will serve to illustrate autogenous healing. A piece of 30-inch unreinforced concrete pipe, when tested at Port Angeles, Washington on May 17, 1928, cracked under a three-edge test load of 2430 pounds per linear foot but did not fall apart and was laid aside. On April 8, 1931, the pipe was placed in the same position as when tested in 1928 and withstood a load of 2570 pounds per linear foot by the three-edge method. Both tests were made by the Engineer of Tests, Washington State Highway Department. A 61-inch extra strength reinforced concrete pipe was installed under a rock fill on U.S. 410 in the State of Washington near the summit of the Cascade Mountains in 1929. Some of the pipe sections, which were provided with elliptical reinforcement, were incorrectly installed. Cracks developed in the crown and invert of the pipe which, when inspected seven years later in August of 1936, were found to have been completely sealed as the result of autogenous healing. A generally accepted explanation of autogenous healing is that the sealing results from bonding material consisting of calcium carbonate and calcium hydroxide crystals. There is greater strength when the crack is healed under water. It is hypothesized this results from the higher concentration of carbon dioxide than found in air at 95 percent relative humidity, and in the greater ease of migration of the soluble calcium hydroxide in the paste if appreciable water is present. Strength of the bond is considered directly related to bridging of the crack and the percentage of the crack that is filled with the calcium carbonate crystals. The autogenous healing process is particularly applicable to concrete pipe, since moisture is present in considerable amount in practically all installations. PAGE 64 LAFARGE PIPE eMANUAL