Verifi Concrete Quality Blog

Is slump an indicator of concrete water content?

by Verifi

No, slump is not an indicator of concrete water content.

Simply put…nearly every factor about concrete affects slump, including material properties, material quantity, age, and temperature.  Chemical admixtures can be used to adjust the slump to whatever the contractor desires without changing the water content.

However…

A change in slump from one batch to the next is an indication that something has changed.

Water is one of the most likely factors to vary during production.  That’s because some sources of water, such as aggregate moisture content and water remaining in the drum, are notoriously difficult to measure precisely.

Additionally, variations in water are likely to have a much larger effect on slump, compared to variations in other materials.

Therefore, adjusting water content to reach a constant slump for a given mix after batching is one way to reduce, but obviously not eliminate, variation in water content.   This is commonly done in a central mixer based on amperage and can be done in the truck automatically by Verifi.

Differences in transit time and temperature mean that adjusting to a constant slump for a given mix at the jobsite may not result in reduced variation in water content.  Instead, adjusting to a constant slump with water after batching and with admixture at the jobsite can reduce variation in water content.

So, what does this mean?

1)      Engineers should not specify maximum slump as a means to control water in a mix design.  A concrete producer can design a mix with nearly any water content to achieve a target slump.

2)      Engineers should also not use slump at the jobsite to control water in a mix design.   For example, a concrete mix with a long haul time on a hot day may exceed the maximum water content but have less than the target slump.

3)      Producers can adjust water to a constant slump for a given mix design after batching as a way to reduce variation in water.  When used as part of an overall quality control program, this can be an effective way to compensate for difficult to control water sources such as aggregate moisture and residual wash water in the drum.

slump test

Verifi helps concrete producers control water content and quickly identify and address variations during the batching and delivery process, resulting in consistent quality concrete.  Learn more at verificoncrete.com.