Q. We have just equipped several of our mixers with an on-board slump monitoring system. As the truck travels to or waits at the jobsite, the system monitors the mix's slump. When the system's controller senses a significant stiffening of the mix, it will maintain the load's targeted slump by adding water. In most cases, the water quantity is relatively small. Before discharge, the driver inspects the load's final slump and uniformity.

How should we report the water addition?

A. According to ASTM C 94, Standard Specification for Ready-Mixed Concrete, the producer must only furnish the customer a delivery ticket for each batch that reports the name of the ready-mix producer and the plant from which the material was batched; ticket serial number; date; truck number; purchaser's name; job name and location; concrete class or designation; load-out time; and water added by the receiver.

Since the water added by the slump monitoring system occurs before delivery, it is not required to alert the purchaser of this addition.

But remember that the standard allows the customer to request additional information for certification purposes to be included on the ticket. One of these allowable requests could require reporting data from the slump monitoring system.

According to the standard, the customer may request the reporting information necessary to calculate the batch's total mixing water. The standard defines total mixing water as free water on the aggregates, batch water (including ice), washwater retained in the mixing drum, and water the truck driver adds from the truck's water tank.

To comply with the standard in this circumstance, the producer must include any water added by the slump monitoring system to the water quantity reported on the ticket.

To avoid purchaser concerns of unreported mix water additions by the slump control systems, some producers alert the concrete contractor that the new system is in use in his area. These producers send a letter describing how the system works to the purchaser before the project's start.