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.
Establishing tolerances on slump flow is critical to prevent rejection of batches. For example, the new Revel resort in Atlantic City, N.J., includes two 1900-room hotel towers and 650,000 square feet of casino and dining/retail/entertainment space. SCC is being used for all formed concrete, such as columns and walls.
The tolerance on the SCC mix is ±2 inches on a specified spread of 25 inches. This performance tolerance is the same regardless of the specified strength, which ranges from 6000 psi to 12,000 psi.
Other project specifications have different tolerances. The tolerance on a recent Australian project was 700 mm ± 20 mm (27.5 inches ± 0.8 inches). Such a tolerance is unreasonable.
Based on testing done for ASTM using several laboratories and technicians, spreads can vary up to 3 inches between testing technicians, even if they use the same test apparatus.
According to Colin Lobo, vice president of engineering for NRMCA, ASTM subcommittee C09.40 has balloted a tolerance to include in C 94, “Specification for Ready Mixed Concrete,” that would establish a tolerance of ±2.5 inches on slump flows greater than 22 inches (±1.5 inches on slump flows less than 22 inches). This seems a reasonable tolerance on such an imprecise test.
ASTM C 1611 also includes non-mandatory 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 is also sometimes called the T20 test, switching to the equivalent 20 inches of spread.
Specifications for T50 range from two 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.A quality control tool
Eric Koehler, a research and development engineer 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,” he says. “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.”
To perform this test, slump flow is determined both with the J-Ring (a 12-inch diameter rebar cage with 16 5/8-inch diameter bars) 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.”