Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 14: Plan Cement Concrete

Properties Of Concrete

The main properties of concrete are discussed under the following heads.

In addition to these a good concrete should be able to resist wear and corrosion and it should be water – tight, compact and economical.

1. Strength:

   The concrete must be strong enough to withstand without injury all the imposed stresses with the required factor of safety. When the concrete mix has been designed on the basis of maximum permissible water - cement ratio, keeping in view of the requirements of durability, it will develop the required strength if properly placed in position and cured. After placing, concrete should not be allowed to dry rapidly because moisture is very much essential for the develoment of its high strength. Curing temperautre also has a great effect on the strength development of concrete. To develop a given strength, longer time of moist curing is required at lower temperature than is necessary while curing is done at higher temperatures.

2. Durability:

   It is the property of concrete by virtue of which it is capable of resisting its disintegration and decay which may be caused due to

   • Use of unsound cement.
   • Use of less durable aggregate.
   • Entry of harmful gases and salts through the pores and voids present in the concrete.
   • Freezing and thawing of water sucked through the cracks or crevices by capilary action.
   • Expansion and contraction resulting from temperature changes and alternate drying and wetting.

   Thus the concrete can be made durable by using good quality of materials like cement and aggregate, by reducing the extent of voids by proportionating the materials and ensuring its through mixing, desired placing, compaction and curing.

3. Workability:

   Despite all its importance, workability is the most elusive property of concrete and is quite difficult to define and measure. In the simplest form concrete is said to be workable if it can be easily mixed, handled, transported, placed in position and compacted.
   Lubricating effect of the cement – paste which in turn is solely governed by the degree of dilution, effects the workability of a concrete mix. A workable concrete mix must be fluid enough so that it can be compacted with minimum labour. A workable concrete does not result in bleeding or segregation. Bleeding of concrete takes place when the excess of water in the mix comes up at the surface and segregation is caused when coarse aggregates have a tendency to separate from the fine aggregates. Workability of a concrete mix is affected by

   • Reducing the proportion of coarse aggregate.
   • The finer is the grading the greater will be the workability. Fine aggregate increases the workability.
   • Shape of the aggregates has a great effect on the workability of concrete.

   Rounded aggregate renders a concrete mix more workable than the angular one.

   • Increasing the quantity of water to such limits that the water cement ratio is maintained.
   • Increasing the quantity of cement.
   • The process of mixing.

   The requirement of workability varies according to the nature of job and the obstruction to the full flow of concrete caused by the spacing and nature of reinforcement.