Some concrete facts

Reinforced concrete forms a major component in any construction. But what are the quality parameters required to see the mixture hold the building strong? This column will bring some concrete solutions for quality buildings...



BUILDING STRENGTH: In non-engineered reinforced concrete constructions, the right mix ratio has to be followed. PHOTO: K. RAMESH BABU

BUILDING STRENGTH: In non-engineered reinforced concrete constructions, the right mix ratio has to be followed. PHOTO: K. RAMESH BABU  

In building construction reinforced concrete forms a major component. Invariably all the multi-storeyed flats we build are made with reinforced concrete frame. In reinforced concrete work, the most important requirement for good behaviour is good quality of concrete, which is not usually achieved in non-engineered construction. Here are some simple guidelines for making concrete of adequate strength and durability.

In non-engineered reinforced concrete constructions, the proportions of concrete mix are to be kept as 1:11/2:3 by volume of cement: sand: aggregates. Under no circumstance should water to cement ratio (w/c ratio) of more than 0.45 be adapted. In non-coastal areas, concrete strength should be 200 kg/sq cm, and in coastal areas like Chennai it should be a minimum of 300 kg/sq. cm. In coastal areas, the w/c ratio should be restricted to 0.40. The best way to make concrete mixture is by using whole bags of cement. For measuring sand and aggregate, a wooden/steel box with handles having a volume equal to one sack of cement will be most accurate as well as convenient to use.

Where mixing is done manually, without using a power-driven mixer, it should be done on an impervious platform, say, using iron sheets or cemented floor. For making a mix of 1:11/2:3, six boxes of aggregates should first be measured and flattened on the platform, and then three boxes of sand should be spread on the aggregate and finally two full sacks of cement opened on top. The material should first be mixed thoroughly in dry state so as to obtain uniform colour and then added with water.

The quantity of water should be enough to make a soft ball of the mixed concrete in hand. A little wetter mix is better for hand compaction and drier mix where vibrator is used for compaction. On any account water in excess of 0.45 w/c ratio should not be used. It is advisable to limit w/c ratio to 0.4. If necessary, suitable plasticisers (workability aids) can be used.

The quality of not only the concrete surface but also the strength of concrete depends on the quality of the formwork and its imperviousness to the leakage or oozing out of the water and cement through the joints.

Wooden or steel sheet formwork with well-formed surface and joints between planks or sheets should be used. Use of water resistant plywood for the skin of the formwork will give very good surface for the concrete.

While placing reinforcing bars, the following points must be taken care of as otherwise the structure will get into undefined weakness. Minimum clear cover to the reinforcement: 20 mm to the bars in slabs, 25 mm to bars in beams and 40 mm to the bars columns. In large columns, say 450 mm in thickness, the cover should be 40 mm. For achieving proper cover, mortar blocks of required size should be made. They should be properly installed between the bars and formwork. Tying bars with thin soft binding wire will ensure the proper placement of bar. Mortar bricks should be of good quality so that they do not introduce local weakness below the rebar.

The binding wire should be turned inward after binding so that they do not touch the erected formwork.

The concrete should normally be cast in one continuous operation so as to avoid discontinuity of more than one hour. Mixed concrete should not be allowed to stay on the platform for more than 45 minutes and must be placed in the forms and compacted continually. Hand compaction must be done by rodding through the freshly placed concrete. Simply levelling the surface with trowels will leave voids in the mass. It may be mentioned that lack of compaction results in large reduction in concrete strength, hence utmost attention should be given to this factor. When vibrators are used, formwork should be checked to ensure proper water tightness and to withstand vibration effects of wet concrete.

Concrete work requires water curing for a minimum of 14 days so as to gain strength, otherwise the concrete becomes brittle. Concrete slabs may be kept under water by ponding of water over it by making barriers around the edges. Columns should be kept covered with wet empty gunny bags. Keeping the side forms intact on the beam webs will prevent the evaporation of water from the concrete surface and help in curing. Covering any concrete surface with polythene sheets after wetting the surface will help retain the moisture for longer period of time. Curing should be continuous and not intermittent.

We saw various aspects of ensuring quality of concrete construction in this column. In the forthcoming columns we shall see the importance of choosing the appropriate materials such as cement, sand, aggregates and so on to ensure long service life for the structures that we build.

(The author is retired Dean (Dept. of Civil Engg.), Anna University, Chennai) Feedback to

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