Q.: What do ACI and ASTM standards say about adding water to ready mixed concrete at the jobsite? Is it permitted? If so, when and how should it be added?
A.: ASTM's standard specification for ready mixed concrete (ASTM C 94-90) permits one addition of water under some circumstances. Section 11.7 says:
When a truck mixer or agitator is approved for mixing or delivery of concrete, no water from the truck water system or elsewhere shall be added after the initial introduction of mixing water for the batch except when on arrival at the jobsite the slump of the concrete is less than that specified. Such additional water to bring the slump within required limits shall be injected into the mixer under such pressure and direction of flow that the requirements for uniformity in Annex Al are met. The drum or blades shall be turned an additional 30 revolutions or more if necessary, at mixing speed, until the uniformity of concrete is within these limits. Water shall not be added to the batch at any later time.
Dick Gaynor of the National Ready Mixed Concrete Association says the words "on arrival" are important. If slump is low, the water addition should be made when the truck arrives at the jobsite--not after it has waited for some time. The water may be added at this time in one or more increments as the specified slump is approached. Once a slump within specification limits is reached, no further water additions are permitted. ACI's specifications for structural concrete for buildings (ACI 30189) also address this issue. Section 7.5.2 says:
When concrete arrives at the project with slump below that suitable for placing, as indicated by the specifications, water may be added only if neither the maximum permissible water-cement ratio nor the maximum slump is exceeded. The water shall be incorporated by additional mixing equal to at least half of the total mixing required. An addition of water above that permitted by the limitation on water-cement ratio shall be accompanied by a quantity of cement sufficient to maintain the proper water-cement ratio. Such addition shall be acceptable to the architect/engineer or his representative.
The June 1991 problem clinic answered a question about adding water to concrete at the jobsite. We quoted from ACI 301-89, which says water can be added only if the maximum permissible water-cement ratio isn't exceeded. Dick Elstner wrote from Hawaii to ask how an inspector can know whether the maximum permissible water-cement ratio is exceeded. Because of free moisture on aggregate surfaces, it's difficult if not impossible to know precisely how much mixing water is in the concrete. The batch man corrects for aggregate moisture but then he or the truck driver typically final-adjusts the mix by adding water until the design slump is reached. So how is the inspector to know how much mixing water is present (and thus know the water-cement ratio) of the delivered concrete?
The only way the inspector will know is if all water additions are recorded and the inspector has a record of those additions. Surface moisture contributed by the aggregate can be calculated if the aggregate surface moisture content is measured. This amount of water can be added to the amount batched at the plant, the amount of wash water left in the truck from the previous batch, and the amount of water added by the batch man or the driver.
An accurate estimate of mixing water content requires an accurate determination of aggregate moisture content. It also requires accurate measurement of other water added to the batch. The system is admittedly less than perfect. But until it's possible to quickly measure water-cement ratio of fresh concrete at the jobsite, there aren't many other alternatives when a maximum permissible water-cement ratio is specified.