Concrete care

Jagvir Goyal

Cement concretes of different mixes and cement sand mortars of different proportions are frequently used during the construction of foundations and raising of super structure of a house. The choice of right mixes for concretes and right proportions for cement sand mortars not only helps in achieving economy by bringing savings in costly item of cement but also help in better engineering as unnecessarily rich concretes and mortars do not get used. Let’s have a look at the desirable mixes of concrete and proportions of cement sand mortars:

Cement concrete items: Construction of a house involves using cement concrete for raising the structure of the house. These include lean concrete in the foundations, damp-proof course or plinth level beams, RCC lintels with or without projections and sun shades over doors and windows, RCC columns & beams, RCC slab, lean concrete in sub base of floors and concrete mix for corner ‘gola’ over the roof. All these items can have different mix of cement, sand and coarse aggregate commonly known as Bajri.

Cement Mortar items: Before fancy items like flooring, painting, lights, chinaware, CP fittings etc come into picture, items of work that contribute towards structural safety and strength of the house are to be completed. These include items that use cement sand mortar as a bonding or finishing material. These include brickwork in foundations, brickwork in super structure, plastering work on the walls and ceiling, plastering work on the external surfaces, deep pointing work if any on external surfaces, bearing plaster below beam ends, flush pointing over terrace tiles and brickwork in half brick walls. All these items can have different proportions of cement and sand in the mortar mix.

Concrete mixes: In the construction of a house, different concrete mixes come into play. In the lean concrete in foundations, a cement, sand and coarse aggregate mix of 1:5:10 or 1:4:8 is used. In the damp-proof course, a concrete mix of 1:2:4 is used. If plinth beams are provided, M20 concrete is used. In lintels, projections, sunshades over doors and windows, M20 concrete mix is used. In case of RCC columns also, mostly M20 or sometimes M25 concrete mix is used. In the RCC slab and integrated RCC beams, mostly M20 concrete is used. In the sub base of floors, a concrete mix of 1:8:16 is generally used. In the roof level gola, laid to avoid leakage of water from the joint of roof slab and parapet, concrete mix of 1:2:4 is generally used.

Designed concrete: Concrete can be produced in different proportions of cement, sand and coarse aggregate. These different proportions have different strengths. There are many factors that affect the strength of concrete but proportion of cement in it is a major factor. Differently proportioned concrete is denoted by different grades of concrete. The grades of concrete are M20, M25, M30, M40 and so on. The figures denote the compressive strength of that grade of concrete in Newton per Square mm. So M20 concrete has a strength of 20 Newton per square mm which is roughly equal to 200 kg per sq. cm. Similarly M25 concrete has a strength of 25 Newton per square mm. In houses, generally these two grades of concrete are used in RCC work. For M20 or M25 grades of concrete, design of concrete mix has to be evolved by taking samples of cement, sand and coarse aggregate. Normally, a common man building his house doesn’t go for getting the design of concrete mix for RCC work. He adopts the proportion of cement, sand and coarse aggregate on the basis of guidance by his architect or engineer, who mostly suggest a proportion of the three materials based on their experience in that area. It should be noted that a properly designed concrete results not only in better strength of concrete but saving in consumption of cement is also achieved. Thus, designed concrete proves economical and stronger.

Cement consumption for concrete grades: There is a common schedule of rates used in public works department and other construction departments for carrying out civil construction works. This common schedule of rates, known as CSR, suggests certain proportions of materials in different grades of concrete for guidance whenever concrete mix design has not been got done. For M20 grade concrete, mostly used in houses for RCC work, 6.8 bags of cement are suggested per cubic meter quantity of concrete. The sand quantity is 0.43 cubic meters and coarse aggregate is 0.85 cubic meters of concrete. In other words, for every one bag of cement, 2.23 cubic feet (cft) of sand and 4.4 cubic feet (cft) of coarse aggregate (bajri) are suggested. Thus, a house builder may ensure during concreting work that the labour should add 2.23 cft of sand and 4.4 cft of bajri. Similarly, for M25 concrete, sometimes used in houses, 7.30 bags of cement are suggested per cubic meter of concrete while the quantity of sand is 0.43 cum and of coarse aggregate is 0.85 cum. In other words, for every one bag of cement, only 2.0 cft of sand and 4.1 cft of bajri should be used for laying M25 concrete. Similarly, proportions for other grades of concrete can also be worked out.

As far as lean concretes like 1:4:8 are concerned, these mean one part of cement, 4 parts of sand and 8 parts of bajri. One cement bag has 50 kg weight or 1.2 cft volume of cement. So for 1:4:8 concrete, for every one bag of cement, 4.8 cft of sand and 9.6 cft of bajri shall be used. Similarly proportions of cement, sand and bajri for other lean concretes like 1:5:10 or 1:8:16 can be worked out.

Cement consumption in mortar: For 1:6 cement sand mortar, the volume of sand to be added is six times the volume of cement. Now, the volume of one bag of cement is 1.2 cubic feet (cft). Therefore, the volume of sand shall be 7.2 cft per bag of cement. Similarly, proportions of sand for every one bag of cement can be worked out for other proportions of cement sand mortar.

Mortar Proportions

Different mortar proportions are used in different items of work in a house. In brickwork in foundations, generally, a cement sand mortar of 1:6 is used. Sometimes, the engineer likes to make it richer and uses 1:5 proportion. In the superstructure brickwork, again, a 1:6 proportion cement sand mortar is used. In the wall plastering work also 1:6 cement sand mortar is commonly used. In case of plastering the underside of ceiling, the cement sand mortar is richer and a proportion of 1:3 or 1:4 is used for better strength and grip. In the bearing plaster done on the top of walls before laying of RCC slab over them, a rich proportion of 1:2 is used. In case, instead of plastering, deep pointing is got done on the external surfaces of brickwork walls, a rich proportion of 1:2 is used in cement sand mortar used in pointing work. Pointing is done on external surfaces of walls if the house owner likes the brick face work to be visible as a part of the elevation and doesn’t want to cover it with plaster. Sometimes, some walls raised in the house are only 4.5 inch thick. These are called ‘pardi’ walls or half brick walls. These act just as partitions and don't act as load-bearing walls. A cement sand proportion of 1:4 is used in raising these walls. Door jambs should be preferred to be plastered in rich mortar of 1:3 proportion.

Ensure right mixes and proportions during work

All the exercise of evolving concrete mix designs go waste when the proportioning of materials at site is left to the labour contractor or labour itself who are habitual of adding 2 baskets of sand and 4 baskets of bajri to the mixer for every one bag of cement. At the end of work, the house builder is left to brood over more or less than desired consumption of cement. Once a mix design has been evolved or adopted, it must be ensured that right quantity of sand and coarse aggregate is added to the mixer for every one bag of cement. To ensure this, exact number of baskets may be worked out by measuring the exact volume that a basket adds or by using steel boxes of measured volume instead of baskets. At the same time, eye should be kept on the quantity of water added to the mixer for every bag of cement. The quantity of water should be as per designed mix. Increase in quantity of water increases the water cement ratio which directly affects the strength of concrete.