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Dear Reader ,
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.
For a new reader , it is suggested to please first read the following posts for general understanding of the subject and then return to this page .
1. A concrete proposal for you:
2.You mean you don’t know RMC ?
3.Confirmed concreting ?
4.Entangled in RCC ?
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.
A CEMENT BAG WEIGHS 50 KG AND HAS A VOLUME OF 35 LITRES.
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 ) .
THIS KIND OF CONCRETE WE ARE TALKING IN THIS ARTICLE. NOMINAL MIX, PROPORTIONED , MIXED & PLACED AT SITE
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1. By the measuring box ( Hindi : Farma ):
the dimensions of A Standard measuring box : 12″ X 12″ x 15 “
THE MEASURING BOX [ FARMA ] WHICH MEASURES 12″ X12″ X15 ” OR 1.25 CFT. OR 43.75 LITRES
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 3 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.
Advantages of this method :
There is no variation in the proportioning of ingredients and hence concrete produced has a consistent quality.
Disadvantage :
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.
USING THE BUCKETS FOR MEASURING SAND & AGRREGATES involves first measuring capacity. in this case : 1 cement bag = 4 buckets
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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
HEAD LOAD : Aggregates being placed in the drum through buckets
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 6 buckets of sand ,
Add remaining water , and then finally place 1 bag of cement in the drum .
Advantages of this method :
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.
Disadvantage :
There is lot of variation in the proportioning of ingredients since each bucket can be filled up at different levels
and hence concrete produced may not be consistent .
There is always a possibility that one or two buckets are missed or added extra since individual labour carries one bucket full of material at a time.
Keeping a track of material actually placed in the drum is difficult for the novice on the site.
CONCRETE MIXED AND PLACED AT SITE