{Question} We have been running into issues with rapid slump loss at our precast plant. We have had difficulties tracking down the source of the problem and need more guidance. What do you suggest?

{Answer} Rapid slump loss can be caused by many things. Typically, it is caused by removing water of convenience or the ineffectiveness of the water reducer to maintain slump.

Concrete typically has more water per cubic yard than needed for hydration. This extra “water of convenience” allows the concrete to be placed, consolidated, and finished. Water is used up during hydration, accounting for typical slump loss, although rapid slump loss indicates water may be used up in other fashions.

Rapid slump loss can be as fast as 2 inches in 15 minutes and go from 5-inch slump to zero inches in 60 minutes. This causes strength to decrease, as water is added to the mix to compensate for slump loss. In precast operations where the concrete is transported in buckets or non-mixing vehicles, slump loss may cause the loss of an entire batch of concrete.

Aggregate is the first place to look. Aggregates at lower moisture than saturate surface dry (SSD) conditions when added to the mixer will absorb water during mixing and transporting. Absorption of water by aggregate typically requires a longer time than a quick mix cycle.

Adding water at this point may not be detrimental to the water/cement ratio, as it replaces water which is being taken out of the mix by the aggregate. The potential problem comes from retempering a mix (adding water and remixing a mix to restore workability so the mixture is placeable or usable). Retempering can dramatically increase air content or clumping of air around aggregate, causing strength loss. Make sure your aggregates are at or above SSD when introduced to the mixer.

Cement chemistry can influence slump loss. First look for false set. False set is not typically an issue in ready-mix concrete, as a truck mixes through false set and regaining slump is easy.

Cement producers put gypsum in cement mills with the clinker for grinding. Gypsum is a source of sulfate, which reacts very quickly with the tricalcium aluminate (C3A) so that the cement does not flash set. C3A, without the sulfates, reacts within minutes, and would not allow the concrete to be placed. Sulfates react with C3A to form other compounds which slows its hydration. Sulfate phase of the cement can be in several forms: full-hydrate, hemi-hydrate, or anhydrate. Large amounts of hemi-hydrate usually cause false set.

Measuring false set

Use ASTM C359, Standard Test Method for Early Stiffening of Hydraulic Cement, to measure false set, but you can perform a quick analysis in any mixer. Charge the mixer (concrete or mortar) and mix the concrete for a typical mix cycle, no more than three minutes. Stop the mixer, leaving the concrete inside and gage how easy it is to insert something, such as a stick, into the concrete.

Probe the concrete for 11 minutes. If false set occurs, you will feel dramatically more resistance from three minutes to 11 minutes. At this time, turn the mixer back on for 30 seconds and see if the workability returns. If it returns to the original consistency, it is likely a false set issue.

This cement chemistry must be fixed at the cement plant. The cement supplier may tell you this cannot be fixed. This may be true, as many cements can have other field issues if they run at lower amounts of hemi-hydrate.

Next, there may be flash set issues with the cementitious products. The same experiment as described above with a pause in the mixing cycle can test for flash set. If after turning the mixer on again for 30 seconds the concrete does not return to original workability without additional water, it is likely flash set. If there is insufficient sulfate going into solution fast enough, the C3A will hydrate quickly and cause flash set.

Many fly ash sources also contain high levels of C3A; combinations of cement, fly ash, and admixture may cause insufficient amounts of sulfate to be present in the mix. Removing or replacing one material often solves the problem. Providing additional sulfates is risky, as the concrete element you make will increase in size.

Finally, consider the admixture. If slump loss more than 30 minutes is greater than you can tolerate, then review your admixture. Admixtures have different pot lives, or slump retention times. New admixtures are being sold as slump retention admixtures. These will increase slump of a mixture and maintain that slump for a designed time.

Contributed by Alfred Gardiner, principal engineer with Braun Intertec. Email agardiner@braunintertec.com. For more, visit www.braunintertec.com.