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{Q:} I am the QC manager for a ready-mix producer. We have our first large self-consolidating concrete (SCC) project soon. The engineer required test mixes to confirm slump, flow, spread, and compressive strength performance. We passed with flying colors. But the engineer is still nervous about the placement. What else should we do to ensure that the job goes smoothly?

{A:} There are two important phases in the prep work for a new SCC project. The first is careful development of a conservative mix design and a thorough material quality control program. You must ensure that you are producing robust SCC mixes—meaning that the mix will retain its properties even with minor variations in temperature, water content, and ingredients.

On any given day, almost any producer can batch acceptable SCC. The trick is to maintain this level of performance over an entire project. If you apply meticulous quality control, then most of your work is done.

The second phase is educating the project participants. This group can include contractors, engineers, and your own production and QC staff. All must be aware of the special procedures involved with adjusting SCC workability and how to deal with mixes that need adjustment. If you have written procedures in place for quality assurance and acceptance testing, take some time to modify a set of guidelines that specifically refer to SCC. If you do not have written policies, now is the time to start. Finally, it is time for hands-on practice.

Diane Hughes from Hughes Concrete says there are usually only two opportunities to make allowed adjustments to a mix. “It is important that the production crew is up to speed on the proper way to handle SCC,” says Hughes. To confirm SCC robustness in production conditions, use test batches. She recommends at least three pre-pour test batches. “Batch at least three cubic yards into each truck, and simulate the delivery, testing, and expected hold times on the job as closely as possible,” Hughes suggests.

Variations in moisture content of the sand and aggregate are the key source of difficulty when producing SCC. Hughes urges producers to identify a range of moisture they can foresee on the job. “A good starting point is ± 5% of total water, typically around 1.5 gallon/yard,” she says.

Batch the first test mix with your target water content to create a performance benchmark. Then, batch the second mix with an extra 1.5 gallon/yard of water, and the third with 1.5 gallon/yard of water held back.

Use these test batches to demonstrate the standard SCC test methods (slump flow, J-ring, and Visual Stability Index) to the crew. And if possible, have the contractor place it into forms to show flow performance. Production and QC personnel, and drivers should all participate.

This effort provides a visual example of how acceptable (and unacceptable) SCC mixes behave during batching, delivery, and testing. With this knowledge, team members can communicate to QC that something in the mix has changed or become questionable. Try to develop a good relationship with the contractor and the testing laboratory staff by inviting their key personnel to witness all or part of the process.

Jobsite

The second opportunity to make mix adjustments is at the jobsite. Review the project specifications to ensure that they include special provisions for SCC. Redline any provision that prohibits adding admixtures at the jobsite.