A reasonable tolerance on SCC spread is plus or minus 2.5 inches.
A reasonable tolerance on SCC spread is plus or minus 2.5 inches.
Slump flow, a test that is often performed with an upside down cone, measures SCC's flowability.
Slump flow, a test that is often performed with an upside down cone, measures SCC's flowability.

For more information, also see Achieving Robust SCC Economically

Concrete that flows into rebar-congested forms under its own weight with no vibration and no segregation is the dream of every placing contractor. Today, most ready-mixed producers can provide that dream material —self-consolidating concrete (SCC).

In fact, the basic understanding of how to produce SCC has existed in North America for at least 10 years. Producers know that if they combine the proper amounts of coarse and fine aggregate with cementitious materials, fines or powder, water, and admixtures, the concrete will flow.

Design engineers also know that such a mix is easy to place, even with congested reinforcement and complex forms providing confidence in the structure's ultimate strength. Architects likewise realize that SCC mixes produce very high-quality concrete surfaces with a nearly polished appearance, with no bugholes or honeycombing.

Most importantly, the contractor favors SCC. The material is easy to pump, needs no consolidation, and eliminates the vibrators and the workers to handle the task. This also increases jobsite safety because workers need not be positioned atop the forms to consolidate the material.

“I could never keep a good vibrator operator,” a contractor once told me, “because the job was usually assigned to the lowest ranking laborer. As soon as I trained him, he would find a new job.”

With all these factors going for it, SCC would seem to be an obvious choice for any concrete going into formwork. So why is it still relatively rare in site-cast construction? One estimate puts the amount of SCC produced in the U.S. at only about 1% of total production, compared to as much as 10% in Europe.

One reason, besides the ingrained reluctance of contractors and designers to adopt anything new, is that simple standardized test methods for quality control did not exist for several years. But today, most engineers believe the slump flow test and the J-ring test have largely resolved this.

But even with these tests, one of the greatest barriers to acceptance has been the difficulty producers and inspecting engineers have had agreeing on the parameters for accepting SCC's flow consistency and robustness.

Testing and tolerances

There are three primary characteristics of good SCC:

  • High flowability, which allows the SCC to fill forms without consolidation.
  • Passing ability, which allows the concrete to flow through closely spaced reinforcement.
  • Resistance to segregation, or stability, which implies no separation of the paste and aggregate. Stability is needed both during transport and placement (dynamic stability) and after the concrete is placed but before it sets (static stability).
  • Flowability is measured using the slump flow test, now standardized as ASTM C 1611, “Slump Flow of Self-Consolidating Concrete.” This is basically a standard slump test, although the slump cone is often inverted to make it easier to fill, and rather than measure the vertical slump, since the concrete spreads out to a thin layer, the testing technician measures the diameter of the resulting concrete patty. Typical SCC mixes have slump flows between 19 and 30 inches.