Controversia aluminio-concreto

The aluminum and concrete controversy
s a conduit, aluminum offers a number of important advantages. It has a high resistance to atmospheric corrosion, even under conditions of high humidity or pollution with chemical dust or fumes; it is safe electrically, and it offers a substantial saving (up to 50 percent) in man-power because it is light in weight, easy to handle and easy to bend. Thepicture, unfortunately, is not all rosy. A number of cases have been reported in which excessive corrosion of aluminum conduits has resulted in cracking and spalling of the concrete in which it was embedded. In cases of corrosion where the concrete cover was too thick to result in concrete failure, the conduit has collapsed inwardly. One such project which received considerable publicity was theWashington, D.C. stadium. Extensive spalling and cracking developed after the stadium was opened in 1961. Investigation revealed that the deterioration was caused by expansion of corrosion products surrounding aluminum conduit embedded in the concrete. This type of problem has been reported under such varying climatic conditions as those experienced in New York, California, Pennsylvania, Illinois,Virginia, Kansas, Tennessee, Florida, and Ontario. The consequences of these isolated experiences have been the banning of aluminum conduit in concrete by such groups as the Army Corps of Engineers, District of Columbia Building Department, and the Public Buildings Service. Agitation by the International Association of Electrical Inspectors has brought recognition of the problem in the 1965 NationalElectrical Code. Suitable corrosion protection approved for the condition will now be required for any metal raceway buried in concrete or in the ground. Considerable research is being conducted in this area since the causes of aluminum corrosion in concrete are not conclusively established. So far, two schools of thought seem to have evolved. One school believes that, while it is true that aluminumin concrete corrodes only under a special environment, it is difficult to assure the prevention of the factors that make up this environment under actual field conditions. The other school believes that the generally satisfactory performance of aluminum conduit in concrete demonstrates the practicability of its continued use, especially if preventive measures are taken when conditions encouragingcorrosion are likely to be present. Aluminum is highly resistant to the corrosive effects of

A

Typical aluminum conduit specimens (left) before test, (middle) after 28 days in concrete free of chloride, and (right) after 28 days in concrete containing 4 percent calcium chloride and steel coupled to the aluminum.

many environments because it develops a thin, invisible oxide coating. Underneutral or nearly neutral conditions (pH 4.5 to 8.5), this coating is further strengthened by the formation of additional hydrated alumina. While the alkaline solutions in concrete may dissolve the existing oxide coating, they tend to form new films which are highly resistant barriers against further attack. Extensive corrosion occurs only when a galvanic action is established by the interactionof dissimilar metals in concrete containing chloride ions and water. In the case of aluminum conduit, the aluminum acts as the anode, the steel as the cathode, and the concrete as the electrolyte. Instead of the normal protective coating, this galvanic action results in the formation of chemically complex aluminum corrosion products having considerably increased volume. The disrupting forces arethe result of this increased volume of corrosion products. Aluminum industry researchers and some other investigators believe that serious aluminum corrosion problems occur only when a galvanic reaction is set up; in other words, when chloride salts are present in appreciable quantities in the concrete along with aluminum and steel. The Aluminum Association and its members have investigated...