In many cases I have observed that masons prescribe even reinforcement steel in DPC, as if it were a beam! Thus it?s not only waste of concrete and steel but also sometimes because of additional requirement of concrete, the compromise on quality reducing cement quantity as if they are helping you!
Where as you lose on two counts! First you have to pay for additional concrete and in some cases unnecessary steel and then because of compromised quality damp-proofing doesn?t happen and your walls remain always damp throughout the year, if your building falls in high water table area or at least in rainy seasons! Thus my tip is to ask mason to do leveling using brick/stone (whatever type of foundation is the case) and then to allow only 2.5 cm thick rich concrete between plinth and wall!
Note: In the above I have intentionally mentioned fine aggregates and course aggregates instead of sand and stone chips. Where sand is not available, fine aggregates can be crusher dust and other small size stone products too. Similarly even brickbats are used as course aggregates where stone is not available. Thus to make the reference in general I have used the true engineering terms as fine and course aggregates).
Now let?s discuss regarding cover to reinforcement steel in RCC. I am not going to discuss engineering clarification. I would just explain the importance and delicacy of the cover.
Concrete is very strong in taking compressive loads where as very poor in taking tensile loads. Thus if structures like beams, slabs, columns (subject to tensile load) if constructed only in PCC (plain cement concrete), it starts to crack and collapse whenever it is subjected to tensile loads! That?s why reinforcement in forms of MS bars (Mild Steel bars) is made composite to concrete referring it as Reinforced Cement Concrete (RCC). Steel is poor in taking compressive load but superbly strong in resisting tensile loads!
Everybody might have come across with these RCC structures where MS bars are placed. One can note easily that these MS bars are always placed at the edges (bottom, top or sides depending upon the structures. It?s never been placed at middle. The reason is to have more LA (Lever-Arm). The distance between the centres of the MS bar?s cross-section to opposite edge is called lever arm.
More the lever-arm more is the resting tensile strength. However there is a limit of placing the steels close to the edge on two important aspects. First, to take the tensile loads coming on the concrete structure or RCC member, MS bars need to be adequately covered by the concrete. It?s because the transfer of load from concrete to MS bars occurred through the contact between concrete and steel that is the perimeter of the MS bars. If such covers are not available then the concrete would start cracking when the load is being transferred leading to failure of the structure.
Thus normal cover is specified as 1.5 times the diameter of MS bars used or the larger size of the course aggregate plus 5 mm whichever is higher. For example if you are using 8 mm MS bars, then cover must be of 12 mm from diameter consideration. If your course aggregate size is 20 mm then cover has to be 25mm. Thus 25 mm being higher than 12 mm, thus cover of 25 mm is to be used.
Another purpose of the cover is to ensure no external moisture to reach the MS bars which can inflict rusting of the steel making structure weak and failing! In normal atmosphere, the above said covers are sufficient. However, in severe climates (such as coastal areas, heavy raining areas etc), more cover is provided!
Now I think everyone may have understood how cover is a delicate balance aspect. If you increase more cover, mean you reduce lever arm thus reducing the strength of RCC! If you reduce cover, then although lever arm is increased, but it cannot sustain due to lack of adequate cover. That means cover is a very precise item. To ensure this precise cover, pre-cast cover blocks are always prepared in advance with the same grade of concrete in the structure. The pre-cast cover blocks are made accurately with specified size and ensure the proper cover as well as proper lever arm!
But what your mason does! He uses fresh concrete cakes as cover while concreting. Sometimes he places a stone chips or some other thing. They do the reinforcement binding in a faster way, and then raise it through concrete cakes arbitrarily.
What happens that time! Everything but proper cover is provided. Either he reduces cover at some places, making that part vulnerable to cracks in future or in some places provide additional cover reducing the lever-arm and reducing the tension resistance, which again makes it vulnerable to future cracks and collapse!
Thus next time, insist the mason to use pre-cast cover blocks. To have visual idea, please visit any nearby construction site supervised by engineers and see how pre-cast covers are being used! Thus don?t make your dream house to be poor in quality, strength and durability aspect! After all you build your dream home for once in your life!
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