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.
No tolerances have been defined for this test, although one study cited in C 1621 found that two operators testing an SCC mix with average passing ability of 0.81 inches could achieve results with a standard deviation of 0.23 inches.
At the Revel, Atlantic County Concrete & Materials' project manager, Bob Todd, notes, “The spec on slump flow reads 25 inches, plus or minus 2 inches. With the J-Ring, we usually lose 2 inches, so the specified spread is 23, plus or minus 2 inches.”
But if we look at the tolerance on slump flow, it becomes obvious that this specification, and the J-Ring test itself, are meaningless. By applying the ±2-inch tolerances, the slump flow could be 25 inches with the J-Ring and 23 inches without, resulting in a negative 2-inch passing ability. “The tolerance in slump flow measurement (indicated in ASTM C 1611) is too high to make the difference in slump flow measurement using the J-Ring a meaningful indicator of passing ability,” says Koehler.
The stability test referenced in ASTM C 1611 is the Visual Stability Index (VSI), a completely subjective test where a number from 0 to 3 is assigned to the concrete used in the slump flow test. The technician assigns the concrete a value of 0 for no segregation to 3 for obvious segregation with a stack of aggregate in the middle of the slump flow test pad and paste (cement, water, and fines) spreading away from it. If there is water at the leading edge of the slump flow patty, then there is some segregation.
A more objective test for segregation has been developed and standardized as ASTM C 1610, Static Segregation of Self-Consolidating Concrete Using Column Technique. But this test is impractical for field use.
Several field tests for segregation have been suggested, with the best being the segregation probe developed by David Lange at the University of Illinois. In this test, a thin wire ring is placed on top of the fresh concrete and allowed to sink for two minutes. The depth of penetration can be directly correlated to segregation.Flowability and stability
Flowability and stability are the two workability features that define SCC. There are traditionally two ways to achieve these properties: admixtures or mix design. Both approaches require using a high-range water reducer (super-plasticizer), typically based on polycarboxylate ethers.
But the super only provides flowability, not the stability that is essential to a good SCC. Without stability, the product is a sloppy, segregated, unusable mix.
Achieve stability either by adding fines to the mix, in the form of fly ash, cement, and fillers (what the new ACI 237 report, Self Consolidating Concrete, calls powder), or by using a viscosity-modifying admixture (VMA), or both. The greater the slump flow desired, the more powder that is typically needed.
A typical SCC mix without using a VMA has a maximum sized aggregate of 1 inch and a water-cement ratio of 0.5 or less. The biggest difference from a typical concrete mix is adding fines as a replacement for coarse aggregate, as much as 8% of the total mix. This powder is typically ground limestone, silica fume, and fly ash.