Design methods for buried pipe are fairly well established, but durability, historically, has not been given proper consideration.
Excerpt from June 2016 issue of Concrete Pipe Info by the American Concrete Pipe Association
Determining a project design life and the durability, or service life, of a pipe are considerations as significant as its hydraulic and structural functions. The definition of a durable pipe contains three variables that must be evaluated; required performance, pipe properties, and service conditions. This Concrete Pipe Information reviews the significance of various physical and chemical factors which may be aggressive to concrete pipe; reviews the significance of pertinent service factors and concrete pipe properties, and durability design and performance of concrete pipe.
Durability, or service life, of a pipe material is as equally important as its ability to perform intended structural and hydraulic functions. The capability of the pipe to continue to perform satisfactorily for an economically acceptable period is a fundamental engineering consideration. Unfortunately, predictions of durability cannot be made with the same degree of precision as can structural and hydraulic performance, consequently, durability is not accorded adequate consideration. Durability is concerned with life expectancy, or the endurance characteristics of a material or structure. Much research has been directed to the durability of some pipe materials, but the varying nature of climate, soils and geology, Precast Concrete Pipe Durability fluid impurities, construction materials, and the construction process itself have prevented the development of a systematic and practical theory for predicting performance.
The problem has been compounded by the assumed requirement that pipe must last almost indefinitely. The U.S. Bureau of Reclamation1 defines a durable pipe as one that will withstand, to a satisfactory degree, the effects of service conditions to which it will be subjected. This definition contains three variables that must be evaluated; the pipe, the satisfactory degree of performance, and service conditions.
At the present time, there is no known material completely inert to chemical action and immune to physical deterioration. Concrete, under what might be considered normal exposure conditions, has a very long life.
Concrete pipelines have a history of excellent durability, and it is unlikely this record will change. Pipelines are beneath the ground where temperatures have very little variation, where atmospheric exposure is either not present or is greatly reduced, and where the materials in close proximity to the pipe may be non-aggressive. Laboratory test results, and damage records for cast-in-place concrete pavements and structures that have been exposed to atmospheric conditions, should not be related to buried precast concrete pipe unless it is determined that comparable conditions exist. Improper application of data could lead to over-design and excessive cost.
Aggressive Factors And Significance
The specific physical and chemical factors which can be aggressive to concrete pipe and which collectively account for practically all perceived durability concerns that could be encountered in traditional applications of the product include freeze-thaw and weathering, abrasion, acids, sulfates and chlorides. Conditions severe enough to result in actual durability problems for concrete pipe are, however, quite rare.
Freeze-thaw damage is caused by water penetrating into the concrete and freezing, which generates expansion stresses and disrupts the concrete if it does not have sufficient strength to resist the expansion stresses. Severity of exposure is usually described by the frequency of freeze-thaw cycles. Atmospheric exposure usually accompanies freeze-thaw 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 density and quality of the concrete. If the pipe is not buried, weathering could be American Concrete Pipe Association • email@example.com • www.concretepipe.org serious enough in some areas to warrant sealing the surface with a barrier coating. The high strength, low water-cement ratio concrete of precast concrete pipe inherently has excellent resistance to freeze-thaw forces.
Effluent velocity, by itself, does not create problems for concrete pipe within the ranges normally encountered. Below velocities of 40 feet per second, the severity of velocity-abrasion effects depends upon the characteristics of the bed load and the frequency of flows capable of moving the bed load through the pipe at abrasive velocities. Above velocities of 40 feet per second, cavitation effects can occur unless the wetted surface is smooth. Bed loads are usually more of an engineering flow problem than a question of pipe abrasion, particularly in a sanitary or storm sewer system, and can be controlled by proper design. Increasing the compressive strength of the concrete, and the specific hardness of the aggregates, increases abrasion resistance.
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