The addition of water to fresh concrete in truck mixer upon arrival at the location of concrete is allowed only if some design mixing water is held back during initial mixing stage. Addition of water in excess of design mixing water would definitely cause an increase of water cement ratio leading to a reduction of concrete strength.
The purpose of adding water to fresh concrete is to increase the workability to facilitate placement of concrete. The use of water-reducing agent or superplasticizer could help resolve the problem but extra attention has to be paid on the segregation issue .
In case there is some buffer in the amount of water in fresh concrete so that addition of water would not result in exceeding the designed water cement ratio, water could be added based on the following rule of thumb:
5 liters of water per cubic meter of concrete gives about 25mm increase in slump.
