CONCRETE PIPE Freeze-Thaw and Weathering Freeze-thaw damage is caused by water penetrating into concrete and freezing, which generates stresses and disrupts the concrete if it does not have sufficient strength to resist these stresses. Severity of exposure is usually described by the frequency of freeze-thaw cycles. Atmospheric exposure usually accompanies freezethaw action, which complicates the situation. Thus, instead of a pure freeze-thaw situation, thermal stresses and evaporative surfaces with concentration effects and crystallization of various soluble salts in the pore structure could combine to provide an accelerated weathering effect. Normally, concrete pipe is not exposed to this combined set of conditions. When it has been, however, its performance has been excellent, primarily due to the high quality of the concrete. In some circumstances, weathering exposure could be serious enough to require sealing the surface with a protective coating. Such circumstances are not common, but can occur. Velocity and Abrasion Although velocity related abrasion is a subject that has been discussed extensively, very few generally accepted conclusions have been reached. There have been very few field observations and these have provided only limited data. Invert erosion has been observed in some culverts, but it has often been a composite phenomena in which velocity related abrasion was augmented by corrosion from aggressive waters. Velocity, by itself, does not create problems for concrete pipe within the ranges normally encountered. At velocities of 120 m (40 feet) per second, or greater, cavitation effects can be serious unless the surface is smooth and internal offsets at joints are closely controlled. Within the range of velocities up to 120 m (40 feet) per second, the severity of velocity related abrasion effects depends upon the characteristics of the bed load. A bed load is the quantity of solids being moved through the pipe by the flow velocity. Bed loads may be intermittent or continuous and vary in particle size, hardness and specific gravity. They are usually more of an engineering flow problem than a question of pipe abrasion, particularly in a sanitary or storm sewer system, and can normally be controlled by proper design. Particle size and density are usually smallest in sanitary sewers, larger in combined and storm sewers, and largest in culverts. Specific hardness of the particles is subject to wide variation and is quite significant, because if the particles are not as hard as the concrete, the particles will be abraded rather than the pipe. Increasing the compressive strength of the concrete along with an increase in the specific hardness of the aggregate used, increases abrasion resistance. PAGE 54 LAFARGE PIPE eMANUAL