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3.2 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 tapes of slabs to serve as load-bearing element while providing the finished floor and ceiling surface. Similarly, reinforces concrete walls can provide architecturally attractive surfaces in addition to having the ability to resist gravity, wind, or seismic loads. Finally, the choice of size or shape is governed by the designer and not by the availability of standard manufactured members.
3.3 Fire Resistance
The structure in a building must withstand the effects of a fire and remind 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 buildings must be fireproofed to attain similar fire ratings.
3.4 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.
3.5 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.
3.6 Availability of Materials
Sand, gravel, cement, 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 frequently 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:
3.6.1 Low Tensile Strength ;As stated earlier, the tension strength of concrete is much low than its compressive strength (about1/10), and hence concrete is subject to cracking. In structural uses this is overcome by using reinforcement to carry tensile forces and limit crack width to within acceptable values. Unless care is taken in design and construction, however, these cracks may be unsightly or may allow penetration of water.
3.6.2 Forms 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 the construction of forms, the removal of these forms, and propping or shoring the new concrete to support its weight until its strength is adequate. Each of these steps involves labor and materials which are not necessary with other forms of construction.
3.6.3Relatively 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 structural requires a large volume and a greater weight of material than does a comparable steel structure. As a result long-span structural are often built from steel.
4. Time-dependent volume changes
Both concrete and steel undergo approximately the same amount of thermal expansion and contraction. Because these is less mass of steel to be heated or cooled, and because steel is better conductor than concrete, a steel structural 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.