Concrete producers often batch less than the maximum amount of water allowed by the mixture design, which allows them to compensate for plant and job conditions while also ensuring concrete quality. A common way of doing this is to “trim water” in concrete or “hold back” a certain amount of water during batching. This water is available to be added after batching.
For example, during the second half of November 2013, concrete producers using Verifi batched an average of 3.6 gal/yd3 less than the maximum water content. This water was then available for Verifi to add automatically. In the same period, Verifi added an average of 0.6 gal/yd3.
The design water content for a mixture should be for conditions requiring the highest water content—namely, a plausible combination of the maximum expected slump, haul time, temperature, etc. In other cases, the concrete is delivered with less than the design water.
Here are four reasons why producers trim water in concrete.
Slump loss after batching. To account for slump loss, producers need some water available to be added either at the site or during batching. If water is to be added at the site, it should be trimmed at batching.
Variations in water quantity. Aggregate moisture, residual water in the drum before batching, and water to wash down the drum after batching can be difficult to measure precisely, even when controlled within ASTM C94 tolerances. To avoid batching too far above the target slump, producers would rather trim water and add back after batching. In a wet batch plant, this water is often added before the load is discharged into the truck. In a dry batch plant it is added by the driver or by an automated system such as Verifi.
Variations in other materials. The properties of cementitious materials, aggregates, and other materials vary, which affects slump. To meet both the slump requirements of the contractor and the maximum water content of the mixture design, producers can batch below the target slump and water content, then add water to reach the target slump, without exceeding the maximum water content.
Contractor slump increases at site. The contractor often wants to increase the slump at the site, above the target slump, to ease placement. Technically, the contractor usually takes possession of the concrete at this point and is responsible for any consequences of water addition. However, some producers reserve water in the mixture to accommodate this added water without exceeding the maximum water in the mixture.
To learn more about how Verifi helps control concrete water content, visit verificoncrete.com.