Building a house: Here is the real cement-sand ratio that baffles many

Building a house is something for which a family takes utmost caution and makes the most strenuous preparation. The reason is it's the ultimate dream for many and involves one's savings of a lifetime. However, despite assiduous research, study and preparedness a majority of people might have something or the other to worry at the end of the construction. It could be a leakage, dampness or wear and tear. Most of the time the mixing of cement, sand and aggregate is doubted.

The fact is only a few have the knowledge of the correct ratio of cement, sand and aggregates. People often hold a vague idea of the proportion. The truth is even a slight change or mismatch in the ratio may produce a big difference in the quality of the concrete or mortar, which may in turn reflect on the strength of the building. Workers or contractors may throw the precision of the ratio to the winds for the sake of convenience or profit. Thus, knowing an engineer's guide to the ratio of mixing cement, sand and aggregates, which is fundamental in building construction, will not only keep doubts at bay but ensure the strength of the building.

The concrete
The general ratio of cement to sand to aggregate is 1:1.5:3. (M-20 Mix Designs as per IS-10262-2009). The tools and containers that are used at the site might not always conform to standard measurements. So the best practice would be to make a box of the size of 1 foot high, 1 foot wide and 1.1/4 long. Then fill the box with sand or cement using the 'chatti' or the container that's in use at the site. If it's cement, you can make the ratio as two boxes full of cement: three boxes full of sand and six boxes full of aggregates. If using the box every time for mixing is not easy, one can note the number of times the container is required to fill the box and complete the ratio for the rest of the materials accordingly. Meanwhile, for constructing bigger, multi-storey buildings in metros, the ratio of cement to sand to aggregate is fixed at 1:1:2 (M-25 Mix Designs as per IS-10262-2009)

The mortar
Usually, engineers recommend the cement-to-sand ratio for mortar for plastering as 1:3 for the ceiling as it needs to have the strongest mix to resist gravity. Meanwhile, the cement-to-sand ratio recommended for mortar mix on external walls is 1:4. The ratio of the same advised for internal walls is 1:5. The plastering for internal and external walls should have a minimum thickness of 12 mm. However, the thickness of the plastering of the ceiling should not exceed too far as it stands the risk of falling off.

Compaction
Using a vibrator properly is essential for making the compaction strong, and powerful. It helps avoid the harmful process of honeycombing (formation of vacuum spaces) in the concrete, which makes it prone to untimely wear-out. The more the vibrator is used the more efficient the concrete mix becomes during compaction.

The water factor
The quality of water is a major factor in determining the strength and efficiency of the mix. In some places, the water may be saline or may possess iron content. The presence of iron in the water will result in a chemical reaction in the mix, which will eventually render the concrete or the mortar weak and loose. Hence, it's always better to use potable water for mixing.

Tapered screeding
While constructing the roof terrace of a building an additional coating of mortar is laid to fill the gaps and as a protection and to avoid cracks and leaks. This process is called screeding. When this is done by keeping a thickness of 3 inches at one side of the layer and tapering down to 1 inch at the other side, it helps avoid water-logging on the roof and thereby reduces the chances of dampness in the building.

It is always better to add water in such a manner that the mix remains at a moderately workable level. The more the water is used the weaker the mix becomes, and on the other hand, using less water would make it less workable. Chemicals are also in use to make the mix workable. However, using too much of chemicals is not advisable either.
Information courtesy: Erin Jake Wilson, Sibin Babu Kuriakose; Staunch Consulting (Structural Engineering), Kochi