Adding water to concrete on the truck weakens the final product permanently

Every gallon of water added to a yard of concrete reduces compressive strength by approximately 200 PSI. Per ACI 318, specified water-cement ratios exist for a reason. Asking the driver to add water because the mix is hard to work with trades short-term convenience for long-term failure. If the mix is too stiff, use a superplasticizer admixture (mid-range water reducer) that improves workability without increasing the water-cement ratio.