There is no such relation of slump to yield of the concrete.
Concrete yields to 1 cum when all specified quantity required for 1 cum of the concrete is mixed.
Before going to yield test first understand how it yield.
During process of mix design, we use the specific gravity of all material to calculate the weights of each material as per absolute volume of each.
What is specific gravity, its indirectly the density of material excluding all voids and considering it as a complete solid without any cavities in it.
So we can also relate specific gravity as Kg per cum for doing calculations when deriving weights of each material with respect to its volume in a mix.
Base on the specific gravity of each material, its weights are calculated as per proportions to get the exact 1 cum of concrete when those materials are mixed in specified proportion.
How the yield or volume of concrete can get affect after mixing of concrete as per the design.
Volume of produce concrete can only change or differ in following situations;
Specific gravity of aggregates changed (aggregates are taken from other source) - when aggregates become lighter in weight or the specific gravity of material reduced - it will increase your volume of the concrete as we are taking same weight of that material whose corresponding volume is increased.
when material becomes heavy, its specific gravity get increased which will produce a small volume for same weight of material and can affect on volume of concrete.
In above cases difference will be less than 5% unless there is change in source of aggregates and aggregates are light weight or heavy weight comparing to old one.
The biggest possibility when there is variation in yield of concrete by more than 5%, either the calibration of RMC plant is gone or the RMC manufacturer is playing with concrete for making more profit.
What you can do when such issues occurs;
Get the copy of original mixes of concrete and check the specific gravity of materials.
Verify the current stock aggregates specific gravity from third party laboratory.
Do check for what moisture corrections are done.
Do cross check values of material from concrete batch report with original mix design.
Do a lab trial at RMC plant where all materials are measured on weight scale which should yield to volume.
If you find the lab trial yields to exact volume, then the problem might be in RMC plant, it may be malfunctioning or out of calibration.
If you find, the lab trials wont give the yield, its clear indication of, the properties of materials used for producing concrete are changed and need to revise the concrete mix design to adjust the change in specific gravity of materials.
To ensure RMC manufacturer is not doing any criminal thing with concrete, do add clause in your purchase order or work order.
Concrete will be accepted only after verification of yield at site, if concrete fails to give yield, it will be rejected and will not be payable.
