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Suitability of Material for Architectural and Structural Function A reinforced concrete system frequently allows the designer to combine the architectural and structural functions. Concrete has the advantage that it is placed in a plastic condition and is given the desired shape and texture by means of the forms and the finishing techniques. This allows such elements as flat plates or other types of slabs to serve as load-bearing elements while providing the finished floor and ceiling surfaces. Similary, reinfored concrete walls can provide architecturally attractive surfaces in addition to having the ability to resist gravity, wind, or seismic loads. Fnally, the choice of size or shape is governed by the designer and not by the availability of standard manufactured members.
Fire Resistance The structure in a building must withstand the effects of a fire and remain standing while the building is evacuated and the fire is extinguished. A concrete building inherently has a 1-to 3-hour fire rating without special fireproofing or other details. Structural steel or timber building must be fireproofed to attain similar fire ratings.
Rigidity The occupants of a building may be disturbed if their building oscillates in the wind or the floors vibrate as people walk by. Due to the greater stiffness and mass of a concrete structure, vibrations are seldom a problem.
Low Maintenance Concrete members inherently require less maintenance than do structural steel or timber members. This is particularly true if dense, air-entrained concrete has been used for surfaces exposed to the atmosphere, and if care has been taken in the design to provide adequate drainage off and away from the structure.
Availability of Materials Sand, gravel, cement, and concrete mixing facilities are very widely available, and reinforcing steel can be transported to most job sites more easily than can structural steel. As a result, reinforced concrete is frenquently used in remote areas.
On the other hand, there are a number of factors that may cause one to select a material other than reinforced concrete. These include:
Low Tensile Strength As stated eailier, the tensile strength of concrete is much lower than its compressive strength,and hence concrete is subject to cracking. In structural uses this is overcome by using reinforcement to carry tensile forces and limit crackwidths to within acceptable values. Unless care is taken in design and construction, however, these cracks may be unsightly or may allow penertration of water.
Form and Shoring The construction of a cast-in-place structure involves three steps not encountered in the construction of steel or timber structures. These are (a) the construction of the forms, (b) the removal of these forms, and (c) propping or shoring the new concrete to support its weight until its strength is adequate. Each of these steps involves labor and/or materials which are not necessary with other forms of construction.
Relatively Low Strength Per Unit of Weight or Volume The compressive strength of concrete is roughly 5% to 10% that of steel, while its unit density is roughly 30% that of steel. As a result, a concrete structure requires a larger volume and a greater weight of material than does a comparable steel structure. As a result, long-span structures are often built from steel.
Time-dependent volume changes. Both concrete and steel undergo approximately the same amount of thermal expansion and contraction. Because there is less mass of steel to be heated or cooled, and because steel is a better conductor than concrete, a steel structure is generally affected by temperature changes to a greater extent than is a concrete structure. On the other hand, concrete undergoes drying shrinkage, which, if restrained, may cause deflections or cracking. Furthermore, deflections will tend to increase with time, possibly doubling, due to creep of the concrete under sustained loads.