There is an engineering term called ‘Water-Cement (WC) Ratio’ which is very important for strength of concrete and mortar. For simple understanding this is a ratio of water in kg and cement in kg to be used for a particular grade of concrete or a particular type of mortar. It should be noted that lesser the WC ratio more the strength. The ratios are fixed considering different grade of concrete and workability.
In building construction people always use nominal mix (volumetric mix) which in fact obsolete now. It’s always advised to get a design mix. A design mix is the proportion of cement, Coarse aggregates (stone chips), fine aggregates (sand/crusher dust), water in weight (instead of volume).
This is easily obtainable in any laboratory. It’s generally cheaper and better strength is achieved considering the materials available locally. Nominal mix is always costlier as it requires more cement than requirement. The volumetric mix was an old idea when cements available in the country were much inferior quality. Nowadays design mix is common. Even though somebody uses volumetric mix, it too has a water-cement ration or simply how much water to be used in a batch or cum.
I think everybody is clear by this time that the ratio between weight of water and weight of cement plays a greater role in both concrete and mortar. It generally ranges from 0.60 to 0.37. Up to water-cement ration 0.45, the concrete/mortar is workable. However, if in any case we need more strength such as M25, M30 or even more, the water-cement ration is reduced up to 0.37.
In such cases for workability various admixtures are used which are not water yet gives provide workability (Workability can also be understood that it gives a fluidity to the concrete or mortar to which mason can work). After final setting time, the admixtures evaporate leaving the water-cement ratio in tact ensuring the intended strength.
From above two paragraphs I think one thing must be clear to everybody that water-cement ratio is very important and lesser the ratio more the strength thus vice versa. Now just remind how many times you have ignored the fact that masons have a general habit to mix additional water to increase the workability of concrete or mortar! It would have appeared to you that it’s normal process! How can the mason work on dry concrete/mortars?
But while adding water the mason in fact diluting the strength of mortar and concrete under your nose. Further if concrete or mortar became dry, then it may have crossed the initial setting time, thus making it useless! In many cases I have seen Masons keep mortars over-night for reuse! Those are simple useless things being utilized at your dream home, under your nose at your cost!
Remedy is simple! Get the mix design from a laboratory near or from any PWD engineer. Ensure right quantum of ingredients including water is used. Ask them to prepare just required mortar or Concrete to be consumed within an hour. No need to mix in the morning and to be used for entire days. It should be mix and consumed with in sixty minutes (or whatever initial setting time is mentioned for a particular brand and lot of cement!). Of course never and ever allow your mason to use extra water to add in already prepared mixes!
In next article I will discuss how you are fooled during the use of DPC and cover to reinforcement.
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