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The use of de-icing salts on roadways, used to reduce the freezing point of water, is probably one of the primary causes of premature failure of reinforced or prestressed concrete bridge decks, roadways, and parking garages. The use of epoxy-coated reinforcing bars and the application of cathodic protection has mitigated this problem to some extent. Also FRP rebars are known to be less susceptible to chlorides. Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to the effects of deicers.
Another important source of chloride ions is from sea water. Sea water contains by weight approximately 3.5 wt% salts. These salts include sodium chloride, magnesium sulfate, calcium sulfate, and bicarbonates. In water these salts dissociate in free ions (Na+, Mg2+, Cl-, SO42-, HCO3-) and migrate with the water into the capillaries of the concrete. Chloride ions are particularly aggressive for the corrosion of the carbon steel reinforcement bars and make up about 50% of these ions.
In the 1960's and 1970's it was also relatively common for Magnesite, a chloride rich carbonate mineral, to be used as a floor-topping material. This was done principally as a levelling and sound attenuating layer. However it is now known that when these materials came into contact with moisture it produced a weak solution of hydrochloric acid due to the presence of chlorides in the magnesite. Over a period of time (typically decades) the solution caused corrosion of the embedded steel rebars. This was most commonly found in wet areas or areas repeatedly exposed to moisture.
5. Reinforced Concrete Structures
Concrete and reinforced concrete are used as building materials in every country. In many, including the United States and Canada, reinforced concrete is a dominant structural material in engineered construction. The universal nature of reinforced concrete construction stems from the wide availability of reinforcing bars and the constituents of concrete, gravel, sand, and cement, the relatively simple skills required in concrete consturction, and the economy of reinforced concrete compared to other forms of construction. Concrete and reinforced concrete are used in bridges, buildings of all sorts, underground structures, water tanks, television towers, offshore oil exploration and prooduction structures, dams, and even in ships.
Mechanics of Reinforced Concrete Concrete is strong in compression but weak in tension. As a result, cracks develop whenever loads, or restrained shrinkage or temperature changes, give rise to tensile stresses in excess of the tensile strength of the concrete. In the plain concrete beam, the moments due to applied loads are resisted by an internal tension-compression couple involving tension in the concrete. Such a beam fails very suddenly and competely when the first crack forms. In a reinforced concrete beam, steel bars are embedded in the concrete in such a way that the tension forces needed for moment equilibrium after the concrete cracks can be developed in the bars.
Factors Affecting Choice of Concrete For a Structure The choice of whether a structure should be built of concrete, steel, masonry, or timber depengs on the availability of materials and on a number of value decisions.
Economy Frequently, the foremost consideration is the overrall cost of the structure. This is, of course, a function of the costs of the materials and the labor necessary to erect them. Frequently, howere, the overall cost is affected as much or more by the overall construction time since the contractor and owner must allocate money to carry out the construction and will not receiever a return on this investment until the building is ready for occupancy. As a result, financial savings due to rapid constructiong may more than offest increased material costs. Any measures the designer can take to standardize the design and forming will generally pay off in reduced overall costs.
In many cases the long-term economy of the structure may be more important than the first cost. As a result, maintenance and durability are important considerations.