DISADVANTAGES OF REINFORCED CONCRETE AS A STRUCTURAL MATERIAL
To use concrete
successfully, the designer must be completely familiar with its weak points as
well as its strong ones. Among its disadvantages are the following:
1)
Concrete has a very low tensile strength, requiring the
use of tensile reinforcing.
2)
Forms are required to hold the concrete in place
until it hardens sufficiently. In addition, false-work or shoring may be
necessary to keep the forms in place for roofs, walls, and similar structures
until the concrete members gain sufficient strength to support themselves.
Formwork is very expensive. In the United States its costs run from
one-third to two-thirds of the total cost of a reinforced concrete structure, with
average values of about 50%. It should be obvious that when efforts are made
to improve the economy of reinforced concrete structures the major emphasis is
on reducing formwork costs.
3)
The low strength per unit of weight of concrete leads
to heavy members. This becomes an increasingly important matter for long-span
structures where concrete's large dead weight has a great effect on bending
moments.
4)
Similarly, the low strength per unit of volume of
concrete means members will be relatively large, an important consideration for
tall buildings and long-span structures.
5)
The properties of concrete vary widely due to
variations in its proportioning and mixing. Furthermore, the placing and curing
of concrete is not as carefully controlled as is the production of other
materials such as structural steel and laminated wood.
6)
Two other characteristics that can cause problems are
concrete's shrinkage and creep.
