There are three main aspects to consider before embarking on building a house. Yes, engineers will make suggestions. Yes, there will be professionals to help you, but if you have the opportunity to obtain firsthand information about these basic yet significant information by spending a few minutes reading this blog then why not !
- Mix proportion/ ratio
When the concrete is produced by taking standard proportions of concrete ingredients, that’s mixed proportion. Concrete ingredients consist of cement, sand and aggregate stones along with water. From a layman perspective, mixing concrete is a cakewalk and one might wonder “what the big fuss about it is; Simply, mix the cement, sand and stones with water and get it over with! Isn’t that straightforward”?
No. it’s not as trivial as it is sounding because there is a specific RATIO in place. As engineering professionals, time and again we end up hearing people constantly asking us about which cement in particular to choose for their building. We firmly tell them that it is only relevant if the ratio is maintained while proportioning the concrete.
So, what is the right ratio?
The right ratio actually depends upon the grade of concrete. As far as M20 grade concrete is concerned, its 1:1.5:3. That means, for every 1 part of cement, 1.5 parts of sand and 3 parts of aggregate stones, in volume is required to prepare M20 grade concrete. Here we are talking about M20 grade because, in Nepal, mostly M20 grade concrete is used when it comes to residential buildings.
Thus, compromising in ratio is compromising with concrete strength. And, compromising with concrete strength would be to compromise with the entire construction. Therefore, you; who is building your ideal house must be mindful of this fundamental yet significant information about mix proportions.
- Cover block
The major purpose of cover block revolves around the prevention of corrosion of rebar. The cover block must be firmly embedded in concrete between formwork and rebar. What that does is it shields the rebar from environmental effects that may result in corrosion. However, it would be a mistake if someone use stone or brick as a cover block.
Therefore, the cover block raises the rebar 20 to 50 mm above the poured concrete and becomes a permanent and inherent aspect of it. As a result, it ensures that no rebar drifts and that the needed concrete covering is minimized during the pour.
Cover block varies according to the structure. That means different structures require different sizes of cover block. For example:
- Slab- 20 mm
- Beam- 25 mm
- Column/Pillar – 40mm
- Foundation- 50 mm
For a concrete to gain its optimum strength, three things stand out. Moisture, temperature and time. So, what we do to get these three crucial elements is we apply a curing process to it. It prevents possible shrinkage of concrete. Curing of concrete should commence as soon as the concrete has reached its first setting time (6 to 8 hours after pouring) and should continue for a fair amount of time according to the stipulated criteria.
In Nepal, we follow code of practice IS456:2000 to carry out curing and it says that we should apply the curing process for at least 7 days. Curing can be done in different ways. Let’s talk about the most efficient and cost effective method.
- Jute Bag/ Gunny bag method of curing – This is a typical curing process, especially for structural concrete. Basically the whole idea of it is to prevent the exposed surface of concrete from drying out. Therefore, we keep on watering it daily for at least 3 to 4 times a day. Coverings for vertical and sloping surfaces must be adequately secured. It is an extremely effective method for various structures that include isolated footing, pillar, staircase and beams.
- Ponding method of curing: It is considered as one of the best methods for curing. We apply a ponding method of curing when it comes to horizontal slab, raft footing, roads and airfield pavements. Here the first and foremost thing is to make a mixture maintaining 1: 10 Ratio of cement and sand and divide it in areas as a number of rectangles, shaping it like a pond. Then, what we do is we fill waters between the ponds, not letting it dry out.
Therefore, these are the three key pieces of information that one must be mindful of before building their house; that would be proper mix proportional understanding, usage of cover block and an application of curing process. Like we have said it earlier, yes there would be professionals to help you out but knowing these important things beforehand would not harm you at all. It allows you to supervise the construction in a rightful way.
- The ratio should be 1:1.5:3 while proportioning the concrete for M20 grade concrete which we usually use in residential houses in Nepal.
- Cover block should be made to prevent the rebar from corrosion. It should be made and must not be replaced with stone or a brick. The sizes of cover blocks vary with the kind of structures.
- Curing should be carried out to provide optimum strength to the concrete and to avoid the shrinkage and cracks in it. The most efficient and cost effective method would be the Jute/Gunny bag method of curing and ponding method of curing.