This is the addition of water to ready mixed concrete in a truck mixer after arrival at the location of the concrete placement. Such tempering of concrete may be done with a portion of the design mixing water which was held back during the initial mixing (referred to as trim water), or with water in excess of the design mixing water, at the request of the purchaser. The design mixing water is the quantity of water set by the mixture proportions for required performance of the concrete.
Water is added to concrete at the jobsite to increase its slump. When concrete arrives at the jobsite at a slump that is lower than that allowed by design or specification and/or is of such consistency so as to adversely affect the placeability of the concrete, water can be added to the concrete to bring the slump up to an acceptable or specified level. This can be done when the truck arrives on the jobsite provided the specified slump and/or water-cementitious materials ratio (w/cm) is not exceeded. Such an addition of water is in accordance with ASTM C94 – Specification for Ready Mixed Concrete.
The ready mixed concrete supplier establishes the proportions of materials for concrete mixtures according to industry standards to provide the intended performance. Addition of water in excess of the design mixing water will affect concrete properties, such as reducing strength (Figure 1), and increasing its susceptibility to cracking. If the purchaser requests additional water, in excess of the design mixing water, the purchaser assumes responsibility for the resulting concrete quality. The alternative of using a water reducing admixture or superplacticizer to increase concrete slump should be considered. Increasing the slump of concrete using admixtures usually will not alter concrete properties provided the mixture does not segregate. Consistent use of admixtures at the jobsite can reduce batch to batch variability. This option should be decided at a pre-pour conference as qualified personnel may need to be available at the jobsite.
To ensure that the design mixing water or specified w/cm is not exceeded, it is good practice for the concrete supplier to indicate on the delivery ticket the amount of trim water held back when concrete was batched. This sets the limit of the jobsite water addition.
When project specifications prohibit the jobsite addition of water, the concrete supplier should be notified so that the design mixing water can be added at the plant and provisions made to adjust the slump of concrete at the jobsite, if necessary, with the use of admixtures.
Some truck mixers are equipped with automated devices that monitor slump of concrete and add water to maintain a target slump. This occurs while the concrete is being transported to the jobsite. The device should be able to record the amount of water added and to terminate the addition based on set limits. ASTM C94 recognizes the use of these systems.
