Concrete mixer trucks typically have a water tank mounted on the truck. This water should never be added to the concrete batch unless approved by the engineer. Occasionally, the use of an absorptive aggregate, such as recycled aggregate, will cause a drop in the slump of the mix as water is withdrawn from the mortar into the aggregate. Even then, the water should be measured carefully using a certified gauge to ensure that the added water does not cause the concrete to exceed the water content of the approved mix design. Water should never be added to “re-temper” concrete that has begun to set. This can occur in hot weather or when delivery times are extended. It is also more pronounced when higher cement contents are used. The water tank should be full when the truck leaves the plant and full when it reaches the project.