Slump and water addition rule of thumb:
It is not uncommon in the concrete industry for the contractor to add water to the load prior to or even
during the unloading process to increase the slump and improve the workability of the concrete. The rule
of thumb: One gallon of water will increase the slump of 1 yard of concrete by approximately 1 inch. This
should only be taken as a rule of thumb as there are various other conditions, like temperature and air
content, that will change the water required to increase concrete slump.
Another important point is that water should not be added after any significant quantity of concrete has
been discharged from the mixer because the quantity of concrete being adjusted is uncertain as is the
impact of the water addition on the concrete properties. ASTM C94/C94M permits the measurement of
slump and air content from a preliminary sample from the initial portion of the discharge so that
adjustments for slump and air can be made to a full load of concrete.