PROPERTIES OF CONCRETE

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1) Properties of concrete

i) Strength

STRENTH OF CONCRETE
  • Concrete must be strong enough to withstand all kinds of imposed load without injury with required factor of safety.
  • If the concrete mix has been designed on the basis of maximum permissible water-cement ratio, keeping in view the requirements of durability, it will attain its required strength if properly placed in position and cured at proper temperature.

ii) Durability

DURABILITY OF CONCRETE

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 by capillary actions.
  • Expansion and contraction resulting from temperature changes and alternate drying and wetting.

Hence, concrete can be made durable by using quality of materials, by reducing the extent of voids, by suitable grading and proportionating the materials, by using adequate quantity of cement and lower water cement ratio.

iii) Workability

WORKABILITY
  • It is the most important property of concrete. In simple form, a concrete is said to be workable if it is easily mixed, handled, transported, placed in position and compacted. A workable concrete must be fluid enough so that it can be compacted with minimum labor.
  • A workable concrete does not result in bleeding and 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 segregates. Workability of concrete is affected by following:
  1. By reducing the proportion of coarse aggregate : finer will be the aggregate greater will be the workability.
  2. Shape of aggregates: rounded makes a concrete mix more workable than the angular one.
  3. By increasing the quantity of water to such limits that the water cement ratio is maintained.
  4. By increasing the quantity cement.
  5. By the process of mixing.

The workability of concrete can be measured by the following tests

  • Slump test
  • Compaction factor test
  • Vee- Bee test
Slump test
  • Concrete slump test is to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during the progress of the work.
  • Concrete slump test is carried out from batch to batch to check the uniform quality of concrete during construction.

Procedure for concrete slump test

SLUMP TEST
Slump test results
  • Following is the shape of the concrete slump that can be observed
SLUMP TEST RESULTS
  • True Slump – True slump is the only slump that can be measured in the test. The measurement is taken between the top of the cone and the top of the concrete after the cone has been removed as shown in figure above.
  • Zero Slump – Zero slump is the indication of very low water-cement ratio, which results in dry mixes. These type of concrete is generally used for road construction.
  • Collapsed Slump – This is an indication that the, water-cement ratio is too high i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.
  • Shear Slump – The shear slump indicates that the result is incomplete, and concrete to be retested.

1) Qualities of good concrete

  • Strength: The concrete should be able to withstand the stresses that it is subjected to. It is quite strong in compression but weak in tension.
    •DURABILITY: It should be durable enough to resist the effect of weathering agents.
  • Density: the concrete should be well compacted so that there are no voids or hollows left. It should weigh 3000 kg/cum
  • Water tightness: when used for construction of water retaining structures such as dams, elevated tanks and water reservoirs then this property of concrete becomes very important. Otherwise, the moisture inside the RCC would corrode steel and leakage would start resulting in the ultimate failure of the structure.
  • Workability: It should be easily workable.
  • Resistance to wear and tear: when used in floors and in the construction of roads the concrete should be able to withstand abrasive forces.

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