While adding superplasticizers at the ready-mix plant is preferred, long haul times may require adding them at the jobsite.
Credit: Cemstone While adding superplasticizers at the ready-mix plant is preferred, long haul times may require adding them at the jobsite.

{QUESTION} As a ready-mix producer, we can add superplasticizer at the plant for better control over the concrete. An engineer specifies the superplasticizer to be added onsite after the initial slump of the concrete is verified. A project to which we are supplying concrete is limited for space to test trucks prior to discharge and the contractor says it is slowing down the project. What are the deciding factors in determining where the superplasticizer is added?

{ANSWER} Let’s start by taking a look at the slump test and figuring out what it tells us. We begin by filling a specified cone in a specified manner (three lifts rodded 25 times each lift). Then we pull the cone vertically and let the concrete free-fall to a resting location. We call slump the measurement from the original height to the displaced center at the final resting place.

So it is measuring workability, finishability, placability, rheology, water-to-cement ratio, or how the concrete reacts to gravity and the lunar cycle. The slump may have been a pretty good indication of water-to-cement ratio although we now understand that many things can change the slump other than water content.

Simple changes such as sand content or the shape of the coarse aggregate can change the slump. Changing supplementary cementitious content or sources can adjust the slump. And, of course, today’s admixtures can make a 0.22 water-to-cement ratio look like a 0.90 water-to-cement ratio. Slump is no longer a means of design for the strength of a mixture.

The slump test measures consistency from batch to batch. We really do not have a true measure of its strength gain until seven to 28 days after we place concrete. We measure properties for consistency, slump, air content, and unit weight against historical data or trial batches to indicate the performance of concrete.

Often I give presentations concerning concrete production and testing and talk about unit weight being one of the best indicators of concrete performance and batching consistency. If the unit weight changes, something has changed in the mix.

The trouble is there are very few places where the testing agency conducts unit weight routinely. Therefore, we turn to slump testing. If a batch ticket indicates the proper material going into the batch each time and the slump is in a range of +/- 1 inch, the concrete consistency may be adequate. If the slump ranges several inches between batches, I would question the quality control of the batching system.

Therefore, if the slump test is measuring consistency in the batch, where and when the superplasticizer is added does not matter as long as it is consistent. Consider the following specification: The concrete shall arrive onsite with a maximum 3-inch slump. After superplasticizer is added the maximum slump shall be 8 inches.

In this case, four batches arrive, all at 2.5 inches. Then the superplasticizer is added and they all measure 7.5 inches. The fifth truck arrives at a 6-inch slump. It is rejected due to excessive slump, as it is obvious something happened in the batch to increase slump.

Now, consider adding superplasticizer at the ready-mix plant. The first four trucks would show up onsite at 7.5-inch slump; the fifth at 11.5 inches. This indicates the same issue in the batch and the truck would be rejected. Same outcome, but the admixture is already in the truck.

I prefer adding water reducers, including superplasticizers, at the plant. This doesn’t limit the ability to measure and control slump. Water reducers do not only increase slump but affect the cement hydration. Added benefits can be seen in the concrete mix by adding these chemicals earlier rather than later. There are exceptions. Long haul time may require adding admixtures at the jobsite.

Contractors should work with producers in each situation. Many mixes require engineering. If it’s designed for 7-inch slump, this should be the target. It’s up to producers to control the mix.

Contributed by Alf Gardiner, principal engineer with Braun Intertec. E-mail agardiner@braunintertec. For more, visit www.braunintertec.com.