Strong construction happens only with the right ingredients – II

While we learnt about cement as an important component of concrete mixture, let us now understand concrete and an important part of construction work.

Concrete

Concrete is a mixture of cement, water, sand and broken stones in a definite proportion which is a workable mass initially and hardens over a period of time. The concrete is normally recognized by its strength which is the compressive stress required to make a 150mm x 150mm x 150mm cube crack.

The process of making and using cement concrete is covered by BIS guidelines. The making of concrete to get specific properties (like strength, for specific type of work etc.) is called concrete mix proportioning. The strength of concrete is influenced by the quality, strength and other properties of the ingredients in concrete like sand, jelly or crushed stones and cement.

Ingredients

Cement

Cement bags with ISI mark with a number below indicate what type of cement you are using. The quality reduces with time. However, if it is stored in well-protected rooms, it can be used up to 60 days, but cement beyond 90 days from manufacture is not recommended.

Sand

Sand should be clean river sand or manufactured sand. Silt content in sand is very harmful for the concrete or mortar. The filter sand supplied in big cities is also not recommended.

Stones

Course aggregates or crushed stones provide the mass and strength to the concrete mass. We should have a mix of all sizes of stone pieces below the specified size so that small pieces sit in between big pieces and give concrete a good mass.

Water

Water is the life of concrete and until cement comes in contact with it, the mortar many not achieve any binding property. However, excess water would dilute the binding property and affect its strength. This is referred to as water-cement ratio by engineers. A lower water-to-cement ratio gives better strength.

Admixtures, available as plasticizers, can be used only with advice taken from the technical team of the cement manufacturer or the admixture manufacturer.

Points to remember:

  • For procedures like curing, vibrating or compacting of concrete, one has to ensure that there are no leakages in the form work.
  • It is better to keep concrete moist all along – start spraying water on concrete from about three or five hours of concreting. This reduces the appearance of hairline cracks.
  • The curing should continue for about 28 days in concrete and 12 to 15 days for plaster and mortars.
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