The hardening of cement in concrete or mortar is not a true chemical reaction. The resulting crystallization of the amorphous cement is subject to alteration when in contact with many corrosive materials. These include soil acids, brine, ceiling surfacing, alkali, lactic acid, and cinder acids. Normal chemical action on the James River Bridge in Virginia to the concrete from salt spray, ice abrasion, tide and weather caused massive repairs only five years later. Protection against the natural aging influence of salt water aided by the physical action of tide and wind has resulted in the recognition that especially dense concretes are necessary and that moisture-tight surfacing is economically advisable. Insufficient air and moisture control to the steel reinforcing bars caused rusting which eventually bursted the precast concrete beams at a lumber drying kiln in Michigan. In this case, the concrete should have been waterproofed and the concrete covering the reinforcing should have been at least 2 inches thick. Excessive use of calcium chloride for snow removal, together with the leaching of tannic acid from the soil adjacent to new concrete curbs caused the complete disintegration of the curbs in a plant during the first winter. The footings were under the water line and after hardening the pits were once again flooded. During the reconstruction, the pits were kept clear of water for a month. Another potential problem is expanding shales and clays that cause large damage to structures built thereon. The best protection seems to be to build buildings on cored piles.