# Vow..what a concrete !

In our journey into the mystery of a construction , we have so far covered concrete in the non-numerical aspects. Right ? Now let us also understand the mathematics involved .
This shall need a dedicated reading of this post.
Remember :
To supervise the manpower at site certainly requires skills but if you can do quantity estimation, bills verification etc and also control manpower , your compensation shall rise in steep curve.
So, please don’t ever skip the calculations part while learning the construction.
Volumetric batching of the concrete.

There are two distinct methods of proportioning the ingredients for concrete :
1. By the measuring box ( Hindi : Farma ):
2.By the buckets ( Hindi :  Ghamela ,Tagari  & Marathi :Pati,)

Both methods are in actual use at the construction sites where volumetric batching is followed.
Remember :
1.The RMC plants , however, do the batching of concrete based on the design mix and hence weigh batching is practiced.
2.Certain sites also practice the design mix at site and use weigh batching .

The discussion on S.nos 1 & 2 is excluded from the present post.

We are talking about  only those construction sites which manufacture the concrete at the site
( In short : Nominal mix , Mixed at site & cast in situ concrete ) .

1. By the measuring box ( Hindi : Farma ):
the dimensions of A Standard measuring box : 12″ X 12″ x 15 “
You must be thinking :Is there is any significance of keeping this size of the measuring box ?
Right ?
So  first let us see the importance of this  size.

The cement bag has a standard  weight of 50 Kg.
Cement has a bulk density of :1429 Kg/CuM.
Hence the  volume of 1 bag of cement shall be
= 50/1429 =0.03499 CuM.
Since 1 CuM = 1000 Litres
therefore, the volume of 1 bag of cement shall be 0.03499 CuM.X 1000 Litres = 34.99 Litre.
For practical purposes, we can consider this volume to be 35 Litre.

How do we use the measuring box for batching different grades of concrete ?
1.Let us say we want to mix a concrete of grade M20  .
We know that the Nominal mix for M20 grade  is : 1 :1.5 :3
since we are doing volumetric mix, this means that :
If we take
1 part of cement by volume,
we shall have to take 1.5 parts of sand measured in the same units and
3 parts of aggregate again measured in the same units .

So now , the same proportion 1 :1.5 :3 can also be written as :
1x 35 Litres : 1.5 x  35 Litres :3 x 35  Litres
or this can be further converted into :
1 bag of cement : 1.5 measuring boxes of sand : 3 measuring boxes of coarse aggregate.

Assuming that we have to maintain a water cement ration of 0.5 ,
we shall need 0.5 x 50 kg = 25 Kg or 25 Litres of water for one batch of concrete.

Now the steps are quite clear . Right ?

We start rotating the drum , pour about 10  Litres of water into the drum ,
Place measuring boxes of coarse aggregate in the mixer drum ,
Followed by 1.5 measuring boxes of sand ,
Add remaining water , and then finally place 1 bag of cement in the drum .

One batch of concrete is ready to pour after  rotating drum for about 1 to 3 minutes.
This concrete can now be transported to the actual location either through trolleys or hoist and trolleys.

There is no variation in the proportioning of ingredients and hence concrete produced has a consistent quality.

Local changes are not possible.
Like if we see that the aggregates are not properly blended, little sand is usually added to the mix.
This extra quantity of sand cannot be measured by the standard box.

2.By the buckets ( Hindi :  Ghamela ,Tagari  & Marathi :Pati,):

This method is followed at many sites.

First step in using the buckets is to know exactly how many buckets are filled up in one bag of cement ?
This check is essential since  buckets are produced in various sizes ( 12″ ,15″ and 18 ” diameter ) .

Suppose the buckets which we are using on the site are such that
exactly 4 buckets are filled up with  one cement bag.

Now , the same proportion of 1 :1.5 :3 can be written as :

4 buckets of cement : 1.5 x 4  buckets of sand : 3 x 4 buckets  of aggregate

or ,
1 bag of cement : 6  buckets of sand : 12 buckets  of aggregate
Now see the mixing sequence :

We start rotating the drum , pour about 10  Litres of water into the drum ,
Place 12 buckets of coarse aggregate in the mixer drum ,
Followed by  buckets of sand ,
Add remaining water , and then finally place 1 bag of cement in the drum .

It is possible to add an extra bucket of sand in this method for achieving a dense concrete.
Shifting the sand & aggregate in buckets is easier due to light weight .
Lifting & dropping material from buckets to the drum is convenient.