Q.: We have had a request for our recommendations as to the type of concrete to use in a rock salt storage bin. Can you help us?
A.: If kept completely dry, rock salt (sodium chloride) has little effect on concrete other than abrasion. It is likely, however, that at some time during its service life, a rock salt storage bin would become moist from leakage, condensation, or storage of moist salt. If so, some sodium chloride solution would be absorbed by the concrete. Although there is some slight possibility of expansion cracking from this cause, the more serious possible effect is from magnesium chloride, often found in rock salt as an impurity. This may cause cracking, followed by corrosive attack on the reinforcing steel. The best way to prevent trouble is to use concrete of high strength (hence high abrasion resistance) and low absorption. These properties are enhanced by proportioning concrete to have as low a water-cement ratio as practicable. A hard rock aggregate of low absorptivity would be helpful. Ordinary Type I portland cement can be used. At least 2 inches of cover should be provided over the steel. It might also be well to use air-entrained concrete to prevent salt-scaling in case the concrete ever becomes frozen while wet.