Adding Water to Concrete on the Jobsite
The addition of water to concrete on the jobsite is still one of the biggest problems facing the ready-mix concrete industry today. Concrete is one of the few building materials whose quality and performance characteristics can be imperiled after the product is transported from the manufacturing facility.
How to Address Issues with Water and Concrete Integrity
On many jobsites, the addition of water is at the discretion of the finishers, whose concerns for in-place concrete durability and strength are often swept aside in favor of ease and speed of placement. So, is it OK to add water to concrete on the jobsite? And, if so, are there any guidelines or specifications that might help the industry protect itself from product liability problems and unjust replacement costs?
Fortunately, ASTM C94, "Standard Specification for Ready-Mixed Concrete," addresses these problems in detail. Unless otherwise specified, water can be added, one time only, to bring the slump of that particular load within the range specified for the project. But the addition of water cannot increase the water-cement ratio above the maximum permitted by the mix design. This presupposes that the person responsible for adjusting the slump knows how much more water can be added to the truck before the proper water-cement ratio is exceeded. Many ready-mix concrete producers are solving this problem by printing the necessary information on the delivery tickets, easily done at plants using computerized batching.
If all the water allowed by the approved mix design has already been added and the slump is still below the requirement, the accepting authority has the option of adding more water at his own risk or rejecting the load.
It is impossible to bring concrete exactly to a specified slump on a consistent basis. ASTM C94 has provided some suggested tolerances that should be followed if the project specifications do not mention any.
If the specifications for slump are written as a target requirement, not a "maximum" or "not to exceed" requirement, the tolerances are as follows:
Tolerances for Nominal Slumps
Specified Target Tolerance
2 inches and less ±0.5 inch
Greater than 2 inches and less than or equal to 4 inches ±1.0 inch
More than 4 inches ±1.5 inches
When the project specifications for slump are written as a "maximum" or "not to exceed" requirement, the tolerances are as follows:
If 3 inches or less If more than 3 inches
Plus tolerance 0 0
Minus tolerance 1.5" 2.5"
One major problem encountered by concrete producers occurs when a mixer truck is delayed on the jobsite. Although it might have been within the specified range of slump when the truck arrived, hot temperatures can cause the concrete to lose enough workability that the specified slump can only be reached by exceeding the water-cement ratio. ASTM C94 covers this eventuality, too. It states that, "Concrete shall be available within the permissible range of slump for a period of 30 minutes starting either on arrival at the jobsite or after the initial slump adjustment, whichever is later." It further states, "If the user is unprepared for discharge of the concrete from the vehicle, the producer shall not be responsible for the limitation of minimum slump after 30 minutes have elapsed starting either on arrival of the vehicle at the prescribed destination or at the requested delivery time, whichever is later."
In other words, once the truck arrives at the jobsite, and assuming it isn't early, the driver is responsible for keeping the slump within the permissible range for about 30 minutes. If it takes longer than that to discharge the load, and more water is needed to maintain the slump, that water is the responsibility of the user.
For further protection of the concrete producer, ASTM C94 also imposes a limit on the amount of time the concrete can remain on the truck. The user must have discharged all the concrete from the truck within 90 minutes of batch time. This limit can be extended, at the discretion of the contractor, if the concrete is workable enough to permit placement without the further addition of water.