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Concrete Has Long-Term Resistance to Seawater
We expect to be engaged in the design of some undersea structures. These will be large and we must do whatever is necessary to make them permanent. They will be located near shore and not subjected to tremendous pressures, but we are concerned about sulfate attack. We believe we ought not to rely on any kind of protective coating because of the danger that might occur if the coating were to become damaged, and the difficulty of repairing or renewing the coating. Are there any concretes that will survive, without protective coatings, attack by sulfate in seawater almost indefinitely?
Portland cement concrete is probably more widely used in marine environments than any other construction material. Examples of Roman concrete structures exposed to seawater for 2000 years on the shores of the Mediterranean are still intact. A jetty at Southampton, the first reinforced concrete marine structure in Great Britain, was built in 1899 as a reinforced concrete deck supported on concrete piles. An inspection in 1956 reported in the British magazine Concrete and Constructional Engineering showed the jetty to be in excellent condition with no signs of deterioration. In 1941 Homer Hadley, a consulting engineer, made a comprehensive survey of structures along the Pacific coast of both the United States and Canada and found no evidence of sulfate attack from seawater or of deterioration due to freezing and thawing. In the 1950's, Bryant Mather of the U.S. Army Corps of Engineers made an extensive review of the performance of concrete in coastal areas of the United States and naval installations in the Philippines and other Pacific islands. He concluded that "the service records indicate that good concrete can endure for a period of 50 years and more without excessive maintenance requirements. The records indicate the necessity of an adequate cover (preferably 3 inches or more) of dense, waterproof concrete over reinforcing steel." Most concrete structures in seawater exposures, if they have been designed and built by good practice, are in good condition. It is believed that when performance has been poor the cause has been poor concrete or failure to use good practices. Seawater is mildly aggressive, principally because of the soluble sulfate it contains. There must be cognizance of this during design and appropriate precautions taken. Generally, the precautions do not involve selecting or using unusual materials or procedures, nor do they cause any significant increase in cost of construction. Information necessary for producing concrete suitable for seawater construction is available, in materials standards published by the American Society for Testing and Materials (ASTM). Publications of the Portland Cement Association (PCA) and the American Concrete Institute (ACI) supply recommendations on how to use the materials. Field and laboratory tests indicate that for concrete exposed to seawater the tricalcium aluminate (C
3A) content of the cement should not exceed 8 percent in order to assure adequate sulfate resistance. ASTM designations C 150 and C 595 show that the following types of cement are required to meet this limitation: portland cement Types II, IIA, IV and V; portland blast-furnace slag cement Types IS(MS) and IS-A(MS); and portland-pozzolan cement Types IP(MS), IP-A(MS), P(MS) and PA(MS). If the specifier wishes, he may add a limitation of 8 percent C
3A to specifications for ASTM Types III and IIIA. Types IV and V have limitations of 7 and 5 percent C
3A respectively. In some parts of the country other types of portland cement may normally contain less than 8 percent C
3A. In addition to the proper selection of cement, other requirements for quality concrete such as low water-cement ratio (0.50 by weight, as recommended in ACI 211.1-77, Table 5.3.4b), adequate air entrainment, low slump, good consolidation, uniformity, adequate concrete cover over steel reinforcement (minimum 2 inches but preferably 3 inches) and sufficient moist curing are essential requirements for securing economical and durable concrete in a marine environment. Well-designed concrete can also be relatively impermeable. Early in 1977 the Civil Engineering Laboratory, Naval Construction Battalion Center, Port Hueneme, California, found that an uncoated 66-inch-diameter concrete sphere with a 4-inch-thick wall placed and left in the ocean at a depth of 2800 feet had allowed only slightly more than 9 gallons of water to enter in a period of five years and four months. The sphere also supported less growth of small sea life on its surface than either the anchor chains or a similar, coated concrete sphere. Properly designed and built, concrete structures for seawater exposure should serve, trouble-free, at least 50 and probably 100 years or more.