This article first appeared in the Precast Today 2023 Q2 edition.
By Terry Harris
Self-consolidating concrete – commonly referred to as SCC – has been available throughout North America for more than 20 years.
SCC is a highly fluid mixture designed to flow and consolidate under its own weight. The mix is cohesive and capable of filling spaces without bleeding. By now, most people in the precast concrete industry are aware of SCC’s benefits versus standard slump concrete.
But for those still contemplating whether to make the move to SCC, there are measures to consider.
Equipment, materials and operations
The first step is a plant evaluation to determine what, if anything, is needed for transition. NPCA and its members can be a resource here alongside chemical admixture suppliers.
Mixers and mix designs
Paste loss from a leaking mixer can cause significant issues, so mixers need to be leak-free to produce SCC as SCC’s viscosity can be significantly lower than slump concrete.
Mixes also may require a higher powder content than traditional concrete, which results in a lower water-to-cement ratio. So, mixers must be capable of handling these types of mixtures.
SCC typically is more sensitive to changes in water content, so regular moisture checks on aggregates are required to ensure adequate control.
As a facility transitions to SCC, the frequency of testing plastic properties initially increases to ensure that the mix is performing consistently and to evaluate the mix robustness.
Leaking buckets, tuckers, hoppers, chutes and other forms of conveyance can cause serious issues for SCC production.
Early on for SCC pioneers, one of the biggest issues was leaking forms. Even if everything else is perfect, SCC placed into leaking forms leads to ugly, hardened concrete with paste bleed at the corners, resulting in severe honeycombing.
In most cases, the materials a plant currently uses to make concrete will work for producing SCC. A typical exception is coarse aggregate.
Where a plant may be using a 1-inch or 3/4-inch aggregate, often a ½-inch or 3/8-inch coarse aggregate works better.
Fine aggregates certainly play a significant role in SCC production success, and, in general, the sand a plant already uses typically works.
Work alongside an admixture supplier on a paper evaluation for aggregate properties. Current and 30-day average gradations that include specific gravity, absorption and shape along with any deleterious materials such as mica or clay allow an experienced SCC practitioner to provide a reasonable evaluation.
While cement fineness plays a role in SCC stability, few cements are incompatible with SCC.
Additional powders such as slag, fly ash, silica fume, natural pozzolans, metakaolin and ground glass pozzolans all have various impacts on SCC properties and should be tested at early stages of incorporation.
Chemical admixtures play a vital role in successful SCC production, and admixture suppliers are the best resource to determine what is appropriate.
A polycarboxylate-based HRWR is required to produce a robust SCC mixture. A viscosity or rheology modifier also may be required with some aggregate combinations or if lower powder contents are desired.
What do I need?
Facilities starting down the road with SCC won’t know everything at the start. Like most transitions, experience is the best educator.
- What slump flow do I need?
- How much slump flow life do I need?
- What about air content?
- How about strength?
These are standard questions for any mix, and some may not have an obvious starting point based on a plant’s history with SCC. Suppliers and fellow NPCA members can offer guidance on these questions based on what pieces are being produced.
The slump flow question is important because proportioning for a 22-inch slump flow results in a different mix than proportioning for a 30-inch slump flow. The key is to proportion the mix for what is needed.
The mix cost can increase significantly when comparing a 30-inch flow to a 22-inch flow, and the difficulty of producing a consistent mix likely increases for high slump flows.
Evaluating how long the mix remains in the placeable range is important and will be different for each plant. It may even vary within a plant for different types of production.
Once a targeted slump flow is identified, ACI 237 offers information for powder contents required to produce a robust SCC mixture.
Strength and water-to-cement ratio
The next step is to make sure that strength and water-to-cement requirements are met using the powder content. The water required to produce an SCC mixture may be different than the amount used to produce slump concrete, but for mix development, start with the same water content.
Once the powder and water contents are determined, the target air content is determined.
A proportioning tool that shows workability, mix coarseness and a graphical representation of the combined aggregate gradation then determines aggregate contents is helpful. If such a tool is not available, use a simple spreadsheet.
There are a variety of available tools that allow a proportioner to see the paste content, mortar content and other key mix attributes before conducting the first trial batch.
Start with three or four different mixtures, then run them all in a laboratory mixer to see how they perform.
“See” is a key word, because looking at the mix while first in the mixer and then in the wheelbarrow after sitting are necessary to properly evaluate the mix and all of the plastic properties. Don’t overlook mix viscosity as mixtures with a lower T20 tend give a better formed finish.
Initial floor testing
Once lab trials achieve satisfactory results, the next step is to run a trial batch through the plant mixer.
The key here: Do not go straight from the lab mixer to production. The mixing action is significantly different in the plant mixer than in the lab, admixture efficiency may be different, and the time to have a stable mix may differ in the plant mixer versus the lab.
Run trial batches of the size expected during production, starting with the same material sequencing as already established. If it is possible to safely do so, visually check the mix for thoroughness and stability to determine the amount of mix time needed.
There is no magic mix time nor is there a perfect material sequence. There are only standard recommendations.
NO NEED TO RUSH
If the first production trial batch meets all of the requirements for slump flow, slump flow life, air, unit weight, stability, it is time to cast some product. If the mix is unsatisfactory, go back to the lab mixer and make adjustments. If it only needs some minor changes, then make the tweaks and run the trial.
If major changes are required, the best practice is to go back to the beginning and put together three or four mixtures to evaluate. But remember, fellow NPCA members are here to help. Do not hesitate to ask for help in troubleshooting. All of us had to start somewhere, and all of us needed help.
Once the mix is right, it is time to evaluate delivery and placing processes. Many publications, including ACI 237, offer guidance on placing SCC.
Be aware that the appearance of bug holes, sand streaking and other surface blemishes will be affected by the process.
Take videos to evaluate each production process step – from batching, discharge, delivery and then placement. This will show where in the production process that things went awry.
And remember: No matter how good an SCC mix is, excessive form oil can cause bug holes and possible discoloration. While placing the concrete observe:
- How far the concrete is flowing from the point of placement.
- Whether there is any blocking or segregation.
- The discharge speed.
- How high each lift is.
- Foaming or other signs of segregation.
It is important to check forms 10 to 15 minutes after placement to make sure the mix remains stable. With SCC, the proof is truly in the pudding – or in this case, in the hardened concrete. Properly proportioned, mixed and placed SCC into clean, lightly oiled non-leaky forms provide a better-formed finish than slump concrete.
And as with every step along the way, keep testing and try things out. Along with all of the standard tests, good spot checks for an SCC mixture include slump flow, J-ring, column segregation, rapid assessment of static segregation and VSI. It also is important to determine how long the slump flow life is for a mixture as this may change as materials and ambient conditions change.
Terry Harris is an ACI Fellow and the technical services director at Chryso and GCP.
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