On the occasion of Concrete Day, Nagesh Puttaswamy looks into the basic mixture so vital in construction

As everybody knows, the journey of building a home is tedious. With the construction industry still in the unorganised sector, there are no fixed regulations to control the way it works, except for some guidelines as building bye-laws. Amongst the owners, contractors or small-time builders not many are aware of the other regulation or guidelines given by the Bureau of Indian Standards (BIS) regarding the quality of the materials used in construction or the guidelines to be followed in the construction practice.

This article is a small effort to educate people in knowing the components that go into the concrete mixture.

Cement

Cement is one of the two basic construction materials which is manufactured in controlled conditions, the other being steel. Cement is manufactured by burning limestone, shale and few other ingredients at a temperature of 1200 to 1400 degrees Centigrade. This form lumps called clinker which is then grinded to powder form along with gypsum. There are two types of cement for which Bureau of Indian Standards (BIS) has drawn up specifications that can be used in normal construction work. The BIS guidelines ensure that the cements sold satisfy a minimum specified quality parameters.

There is a perception that blended cement is not good for construction, but this is not true. In fact it is much better for plastering and masonry work. The blended ones are made by mixing industrial bye-products such as fly-ash or Ground Granulated Blast Furnace Slag (GGBS). The mortar and concrete made of these cements have better durability and chemical resistance. In fact many underground structures are built with slag cement or the concrete is mixed with GGBS.

Important facts

* The colour of cement is not an indicator of strength or any other of its quality parameters. Its colour comes from the raw materials used in the manufacture.

* Technically cement is something like glue or gum when it is mixed with water (cement-water reaction produces a gel called CSH gel). Addition of more water will dilute the gum and it affects the quality of the gum. Similarly the addition of more water to cement would dilute the CSH gel and make it weak.

* Cement requires a specific amount of water to complete the chemical reaction; any excess water will not participate in the chemical reaction but remains in the mass as water.

* Use of less cement in any work will produce less gel, hence less binding, and addition of more cement may not help much.

* Use of blended cement will give you better results as the blending materials like fly ash or GGBS are slow in reacting, hence they would be releasing the CSH gel slowly over a period of time. It has been observed that if this period is as long as 90 days or even a year, it would strengthen the concrete or mortar better. It is also called secondary strengthening.

* The chemical reaction of cement is irreversible, hence it is advised to take proper guidance from your engineers or from the technical help given by the cement manufacturer.

Concrete

Concrete has been used as construction material for over 2000 years, since the time of the Roman civilisation, for construction of buildings, roads and other monuments. The cement in those times was the ash that came out of the volcanoes (called the pozzulonic ash) and lime was used as binding material.

Concrete is a mixture of cement, water, sand and broken stones (jelly) in a definite proportion which is a workable mass initially and hardens over a period of time. The concrete is normally recognised by its strength which is the compressive stress required to make a 150mm x 150mm x 150mm cube crack or fail in a machine called Concrete Testing Machine. It is designated as M10, M20, M25 and so on. M20 concrete means this concrete can withstand a pressure of 20 Mega Pascals (relates to 200 kg force over an area of 1 cm x 1 cm).

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.

* 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 should be clean river sand or manufactured sand. Silt content (clay particles) in sand is very harmful for the concrete or mortar made. The manufactured sand will not have this problem; however, quarry dust is not manufactured sand. The permissible silt content in sand is five to six per cent. The filter sand supplied in big cities is not recommended.

* Course aggregates or crushed stones or jelly as it is popularly known provides the mass and strength to the concrete mass. We should have a mix of all sizes of stone pieces below the specified size, for example, 20mm aggregates should be a mix of 20, 10, 6 mm aggregates, so that small pieces sit in between big pieces and give concrete a good mass.

* 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. An ideal water-cement ratio for houses and other normal work with M20 concrete should never exceed 0.50 (i.e. 25 litres of water for one bag of cement). Restricted water quantity is also difficult to handle. Admixtures, available as plasticisers, can be used only with advice taken from the technical team of the cement manufacturer or the admixture manufacturer.

* For procedures like curing, vibrating or compacting of concrete, one has to ensure that there are no leakages in the form work or centring. Modern cement reacts faster than old cement. 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.

(The author is structural engineer, and is a member of the Indian Concrete Institute, Karnataka-Bangalore centre)

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