You have not mentioned the grade of concrete normally concrete weighs about 2400 kg/cum.*
So since you have ordered 6 cum the total weight of the concrete should be about 2400 x 6 = 14400 kg
Now when you weight the TM ( transit mixer ) it will show self-weight of TM + fuel inside the TM ( generally full tank is about 200 lit approx ) + water in the tank on TM ( 500 lit approx ) + any other material which is currently on the TM.
( please note that in case you intend to use large quantities then its better to weight all the TM's of that particular RMC suppliers by sending our reliable person which could be either storekeeper along with engineer once this is done you are aware of the approx weight of the TM as per their registration numbers, there is theft no matter how much you monitor it but if its a big project then ideally if we have our own weighing scale installed at the site then cases of cheating are controlled otherwise you can't do anything about it some amount of cheating is bound to happen.)
So considering a normal running TM of about 6 cum capacity it generally weights say about 10500 to 11000 kgs.
So when you first weigh the vehicle with concrete so ideally, the total weight should be 11000 kgs + 14400 kg =25400 kgs
After the concrete is discharged then again the vehicle should be weighed so now the calculation would be as follows:
Loaded weight of TM = 25400 kgs
Unloaded weight of TM = 11000 kgs
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Net weight = 14400 kgs
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So 14400 / 2400 = 6 cu.m
* Please note that depending on the grade of concrete being used generally the weight of concrete varies from 2400 kg/cum to 2450 kg/cum so derive your constants by maintaining the data for concrete received at site based on the RMC vendors.
One more way you can find if the correct amount of concrete was loaded in the TM was by seeing the BATCH MIX REPORT which comes along with the TM where you will see the material loaded in kgs and make an assumption.
Cheating is a way of life in India so you have to live with it, hence you have to be vigilant. Most of the customers don't weigh the TM and don't take precautions so that theft is minimized if you take care then such kind of cheating can definitely be arrested.
And the golden rule is always weighed each and every TM so the vendor knows that this party is vigilant and hence will not resort to cheating.

