Updated: a day ago
I double checked all the measurements! Why in the world the delivered concrete is not enough? Next time I´ll send to the weigh check the mixer trucks. I know that 1 cubic meter of concrete weights around 2350 kg/m³ so it will be easy to verify if the manufacturer is cheating on quantity.
But, is it really so?
What is the real weight of 1 cubic meter of concrete? The answer is: it depends!
Concrete is a conglomerate made of several ingredients: cement, water, fillers, sands, gravels, admixtures. Each and every ingredient has its own density (weight for a given volume unit). This simple consideration clearly clarifies that concrete cannot be defined with one only weight, even if approximate.
Now given that cement density is quite steady, that water density is 1 kg/l and the quantity of admixture used in 1 cubic meter of concrete is almost irrelevant on the final weight, what really matters is the density of the aggregates. But we have to remember that, no matter what, some air will always remain trapped inside the mass of concrete thus occupying volume although having no weight (please read the post "Concrete compaction"). This factor too will contribute to the final weight of 1 cubic meter of concrete.
So, is it possible to check for possible cheating? Of course, it is. And it´s even fairly easy; you just need some pieces of equipment:
A box with a given volume (you can even “do it yourself” weighing the tare of the container and then filling it up with water and weighing it again, the difference is the volume…please be accurate with the water level) a scale calibrated at 1 gr, a steel rod and a calculator.
This is the simple procedure:
1 Weigh the container of known volume (tare)
2 Fill it with concrete in several layers compacting accurately each and every one with the steel rod.
3 Weigh again (gross weight).
4 Get the net weight and divide by the volume of the container.
Finally you know, with a high degree of certainty, what is the real weight of 1 cubic meter of the delivered concrete, and you can even check every batch!
Now you have two possibilities to figure out if the quantity of concrete delivered is exactly the quantity you have purchased.
You can ask the manufacturer to declare the exact weight of 1 cubic meter of the concrete delivered and simply compare it with the result of the above-mentioned measurement, or
Randomly send to weigh mixer trucks while still full loaded and after the complete download. Now that you know the real weight of 1 cubic meter of that specific concrete, it will be totally easy to figure out what´s going on.
One last recommendation: remember that the weight of the concrete is different for different mix and that the weight of the concrete can vary during a casting, in case you didn´t ask for a “fixed” mix, if the type of the aggregates changes (in that case the manufacturer must provide with the same performances with exception of the weight, if not explicitly requested).