Q. We are working with two water sources. One is treated soft water. The other is hard water from a well with a hardness of about 450.

Using the same mix, we have to add 6 to 7 liters more of hard water per cubic meter of concrete to obtain the same workability we get with the lesser amount of soft water. What is the best way to deal with this?

A. Richard Gaynor replied to this question by explaining the effects of batching and mixing techniques.

If the hard water is drinkable and meets the chemical requirements of ASTM C94 Table 2, and the concrete made with hard and soft water have the same air content, there is a problem in measuring the water, batching the water, or obtaining uniform mixing.

It's possible that if an air entraining admixture is batched with the water, the hard and soft water could react differently with the admixture and affect the concrete air content. Gaynor says he sees this sometimes when an air entraining admixture is batched into wash water, especially if the cement content is relatively low. The solution is to batch the admixture on the sand, not into the water. In lean mixes, extra air reduces the mixing water required to produce the same slump.

If an air entraining admixture is not being used, the cement could be entraining some air, which works to reduce the mixing water content. The other problem could be that the timing of water addition in the batching sequence is different and you are not getting uniformly mixed concrete.

For instance, if the well water is your usual source and the soft water is from an alternate source and is being added after all solids have been batched into the discharge end of the mixer, the batch will not be mixed well. In that case, the first concrete discharged will have a high slump and the last discharged will be dry.

This Q&A is from Aggregate Research Industries