Materials for Self-Consolidating Concrete

SCC and conventional concrete differ only in some minor proportioning modifications and a couple of admixtures. But in many cases, the contractor won't even know what's in the mix, but rather will specify the performance required in terms of workability (slump flow), passing ability, stability, and strength. Lafarge even has a name, Agilia, for its proprietary SCC mix.

Lafarge
This huge wall with heavily congested reinforcing was poured in a single lift with SCC. Lafarge North America

There are three types of SCC mixes:

  • high powder content and a high-range water reducing admixture (HRWRA)
  • low powder content, HRWRA, and a viscosity-modifying admixture
  • moderate powder content, HRWRA, and a moderate dose of VMA

The powder referred to here is all the fine materials, including cement, fly ash, ground granulated blast furnace slag, silica fume, and ground limestone filler. This thickens the mix, keeping the coarse aggregate stable in the matrix and also contributes to slump flow. Here are some things to understand about the materials in an SCC mix:

  • The fraction of coarse aggregate in the mix will be much lower (about 30%) and the size will be smaller (about ½ inch top size)
  • The amount of portland cement will typically be low—less than 300 pounds/cubic yard, or less than half of the total cementitious materials
  • Other cementitious materials will be high—fly ash in the range of 20 to 40% of total cementitious materials; slag in that range also, and often some silica fume
  • The powder, which includes the cementitious materials, may be supplemented with ground limestone.
  • The water-cementitious materials ratio is often quite low—as low as 0.27 in some mixes, although 0.4 is more typical.
  • Viscosity-modifying admixture (VMA) increases the viscosity to keep the mix stable. Without a lot of powder in the mix, VMA is used to thicken the mix and prevent segregation. "Every SCC mix we do in New York we use a VMA because the producers don't have a lot of clout with the quarry," said Euclid Chemical's Bill Phelan. "The quarries ship what they want but the ready-mix producers need to have a mix that works every day and the VMA keeps it consistent." In other parts of the country, though, well-graded, high-powder mixes tend to be the norm.
  • High-range water reducers for SCC mixes have recently been mostly those based on polycarboxylates, which have an incredible ability to increase slump at low dosages. Polycarboxylates, though, have created problems with foaming and adding unwanted air to the mix, so a de-foaming agent is added. This has not always been successful, although the admixture manufacturers are getting much better. There have also been some problems with sudden slump loss. Caroline Talbot, Euclid Chemical, recommends that for longer haul distances, the producer should consider using a naphthalene-based HRWRA instead of polycarboxylate.

Keep in mind that SCC mixtures are a little complicated, requiring just the right balance between all the materials, water, and admixtures. You can read an excellent description by Jeff Girard of how the properties of SCC are balanced between yield stress and viscosity by clicking here. Find more information on concrete mix design. Always experiment with a mix like this first before you start a job. "On any important concrete project, you need to do a test placement," said Phelan. "Don't be practicing on someone else's job!"

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