FACTORS AFFECTING CHOICE OF CONCRETE FOR A STRUCTURE

The choice of whether a structure should be built of concrete, steel, masonry, or timber depends on the availability of materials and on a number of value decisions.

1. Economy

Frequently, the foremost consideration is the overall cost of the structure. This is, of course, a function of the costs of the materials and the labour and the time necessary to erect them. Concrete floor systems tend to be thinner than structural steel systems because the girders and beams or joists all fit within the same depth, or the floors are flat plates. This produces an overall reduction in the height of a building compared to a steel building, which leads to a

• Lower wind loads because there is less area exposed to wind
• And savings in cladding and mechanical and electrical risers

Frequently, however, the overall cost is affected as much or more by the overall construction time since the contractor and the owner must allocate money to carry out the construction and will not receive a return on their investment until the building is ready for occupancy. As a result, financial savings due to rapid construction may more than offset increased material and forming costs. The materials for reinforced concrete structures are widely available and can be produced as they are needed in the construction, as opposed to structural steel, which must be ordered and partially paid for in advance to schedule the job in a steel fabricating yard.

Any measures the designer can take to standardize the design and forming will generally pay off in reduced overall costs. For example, column sizes may be kept the same for several floors to save money in form costs, while changing the concrete strength or percentage of reinforcement to allow for changes in column loads.

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 types of slabs to serve as load bearing elements while providing the finished floor and ceiling surfaces. Similarly, reinforced 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. 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 fire proofing or other details. Structural steel or timber buildings must be fireproofed to attain similar fire ratings,

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 re seldom a problem.

5. Low maintenance

Concrete members inherently require less maintenance than do structural steel or timber members. This is particularly true if dense, air-entertained 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.

6. 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 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:

1. Low tensile strength

As stated earlier, 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 crack widths to within acceptable values. Unless care is taken in design and construction, however, these cracks may be unsightly or may allow penetration of water.

2. Forms & 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 the 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 labour and/or materials which are not necessary with other forms of construction.

3. 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.

4. 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 deflection or cracking. Furthermore, deflections will tend to increase with time, possibly doubling, due to creep of the under sustained loads.