Examples of deteriorating concrete are commonly known to all of us. Often the trouble comes from using a poor quality of concrete, but frequently good concrete has been unintentionally subjected to conditions that lead to disintegration. When this happens, the owner is understandably upset and the contractor may find himself blamed for a failure which might have been avoided if both parties had been better informed about the natural limitations of concrete. Acids combine with calcium compounds in hydrated cement to form soluble substances which are easily eroded, thus producing concrete disintegration. Many acids are used directly in industrial processes; in other industries, acids develop as a result of bacterial growth in waste or spilled material. If their presence is anticipated, a bituminous or polysulfide coating can be applied to the concrete. Bases are chemical compounds such as ammonia water and caustic soda which yield hydroxyl ions in water solutions; they neutralize acids into salts and have pH values above 7.0. When portland cement concrete is made with non-alkali-reactive aggregates, it is highly resistant to strong solutions of most bases. Calcium, ammonium, barium and strontium hydroxides are normally harmless, but sodium hydroxide may cause damage. Salts are chemical compounds usually formed by reaction between acids and bases. Some of the salts best known for their damaging effects on concrete are the sulfates. Soluble sulfates occur naturally in many parts of the world and are potentially harmful to concrete. Resistance to naturally occurring sulfates can best be achieved by use of a cement with a limited amount of tricalcium aluminate. Trace amounts of sugar in freshly mixed concrete are sufficient to retard the setting, but dry sugar has no effect on hardened concrete. Sugar solutions and sugar-containing food products may cause deterioration. Air drying for at least 28 days following curing helps concrete resist attack from sugar solutions.