ASTM C 1611 also includes nonmandatory tests for viscosity and stability. The viscosity test is the T50 test, which is the time it takes for the concrete to spread to a diameter of 50 cm. This test also is called the T20 test,—the equivalent 20 inches of spread.
Specifications for T50 range from 2 to 10 seconds, depending on the intended use of the concrete, with a value between 2 and 5 considered a low viscosity concrete. A tolerance on this time should be agreed upon in advance since no established tolerances currently exist. Eric Koehler, with admixture manufacturer W.R. Grace, feels that T50 (or T20) may be a good quality-control tool for the concrete producer. “Instead of a maximum and minimum T20, minimizing the variation in T20 for a given mix would be advisable. Any changes in mixture proportions—especially water content—would be reflected in T20, even at a constant slump flow.”
Passing ability has been tested with several different apparatuses, including the L-Box, the U-Box, and the J-Ring. Today, the J-Ring Test has become the method of choice, especially in the field, and is standardized under ASTM C 1621, “Passing Ability of Self-Consolidating Concrete by J-Ring.” Here the slump flow is determined both with the J-Ring (a 12-inch-diameter rebar cage with 16 bars at 5/8-inch diameter) and without. The passing ability is defined as the difference. C 1621 states that “a difference less than 1 inch indicates good passing ability and a difference greater than 2 inches indicates poor passing ability.” No tolerances have been defined for this test, although one study cited in C 1621 found that two operators testing a mix with average passing ability of 0.81 inches achieved results with a standard deviation of 0.23 inches.
At the Revel, Bob Todd with concrete producer Atlantic County Concrete & Materials notes that “The spec on slump flow reads 25 inches ±2 inches. With the J-Ring, we usually are losing 2 inches, so the specified spread is 23 ±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. He prefers measuring the height of the concrete inside and outside the J-Ring, but that method has yet to be accepted by ASTM.
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 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 2 minutes. The depth of penetration can be directly correlated to segregation.
Robustness in SCC
Flowability and stability are the two workability features that define self-consolidating concrete. There are traditionally two ways to achieve these properties: admixtures or mix design. Both of these approaches require the use of one admixture: a high-range water reducer (superplasticizer), typically one based on polycarboxylate ethers.
But the superplastizer only provides the flowability not the stability that is essential to a good SCC. Without stability, all you have is a sloppy, segregated, unusable mix. Stability is achieved either by adding fines to the mix—in the form of fly ash, cement, and fillers (the new ACI 237 report, Self Consolidating Concrete, calls it powder), or by using a viscosity-modifying admixture (VMA), or both. The greater the slump flow desired, the more powder is 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 the addition of 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.
Another term that is gaining importance when referring to SCC is “robustness.” This is a mix’s ability to maintain its properties in the face of minor changes, such as water content (or aggregate moisture content), admixture dosage, cement type, aggregate gradation, or mixing and handling. SCC mixes tend to be less robust than ordinary concrete because the mix is more complex and minor variations can lead to serious problems on the jobsite both in terms of flow and segregation.
For many SCC mixes, the solution to this lack of robustness is to use high percentages of powder or high dosages of viscosity-modifying admixtures, or both. But the problem then is that with all the added fines and admixture, the cost of the materials begins to outpace the cost reduction from labor savings on the jobsite. Ask any contractor if they are willing to pay a higher price for a less predictable concrete mix in order to slightly reduce their labor costs, and they will show you the door in a hurry.
A robust and cost-effective SCC mix, though, is one a contractor will be glad to use. One way of providing that is with the iCrete system (see “Achieving Robust SCC Economically” in our online Editors’ Picks). Atlantic County Concrete is supplying SCC to the Revel project and has switched completely to this technology. “It’s more consistent and we are able to do it with a better blend of materials,” says Todd. “We also have been able to reduce our cost.”
Self-consolidating concrete is one of the greatest advancements in concrete technology in the past 20 years, but only if a robust mix can be provided in the field at a reasonable price. By using new technology and reasonable tolerances, producers have proven that they can accomplish this—now they just have to convince contractors.
