Q. I recently saw a batch of self-consolidating concrete (SCC) for the first time. It is so much more fluid than standard concrete that some standard field tests are not applicable. So far it seems like most of the tests being developed for SCC are lab tests rather than field tests. Are there field tests for accepting a truckload of SCC?
A. Two tests for fresh SCC are available as ASTM standard test methods, but only one serves much use in the field. A test being developed at the University of Illi-nois looks promising for field use.
First, though, there are two factors to consider. The first is how it flows, including its ability to navigate changes of direction and get around obstructions such as rebar. The second is mix stability, or how well all the ingredients remain well-mixed and homogeneous. Mix stability is a concern both as the SCC flows into place and as it hardens.
ASTM C1611-05 “Standard Test Method for Slump Flow of Self-Consolidating Concrete” provides a measure of the sample's rheology, or how it flows. To perform the test, a standard slump cone, either upright or inverted, is filled, then rapidly removed. Rather than measuring the vertical drop of the concrete, which with SCC is almost the entire height of the cone, the diameter of the resulting sample is measured. Referred to as the spread, or slump flow, this measurement can help verify that the mix has the proper amount of whatever admixtures are being used to make it fluid.
In the field, the measured slump flow is compared to a target range determined during the mix design stage. For example, rather than looking for a 3-inch slump, the target for SCC might be a 28-inch slump flow. Depending on how the measured slump flow compares to the target range, the batch can be dosed with admixtures to meet the design. The particular dosages are based on tables developed during the mix design process. While this is not, strictly speaking, an acceptance test, is a valuable field indicator of needed adjustments.
The other standardized test currently available is ASTM C1621-06 “Standard Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring.” The J-ring test measures the effect of blockage, such as rebar, placed in the path of the SCC as it flows away from the slump cone and is more for lab use than field use.
The ASTM sub-committee on self-consolidating concrete is now finalizing a column segregation test that provides a measure of mix stability, but this is a lab test for use in qualifying mix designs rather than for field acceptance. The J-box, U-box and L-box are test methods being used to evaluate SCC's ability to fill forms, but none have yet been standardized.
At the sub-committee's meeting in Toronto in June, Leslie J. Struble, a professor at the University of Illinois, presented an overview of a new segregation probe that may provide a rapid and reliable field indication of SCC mix stability.
The probe, which is similar to a spindle for holding papers, is placed on the top surface of a concrete cylinder 2 minutes after it has been cast. It is allowed to settle for 1 minute, when its penetration depth is read. A stability rating is assigned based on that reading. If it settles less than 4 mm, it is rated “0.” Settlement of 4 to 7 mm is rated “1,” from 7 to 25 mm is rated “2,” and greater than 25 mm penetration is rated “3.” Ratings of 0 and 1 are considered stable, while ratings of 2 and 3 are unstable.
The probe weighs 18 grams and consists of a 2.38 mm diameter steel wire bent into a 100 mm circle and a 150 mm rod, which is at right angles to the plane of the circle. The segregation probe penetrates the cement paste at the top of the cylinder and settles atop the aggregates it encounters below the surface.
Correlation has been good between measurements using the probe, which can be done in a very few minutes, and results of the column segregation test, which takes about half an hour. Because even small changes in aggregate moisture content or admixture dosages can significantly affect the stability of SCC, this probe may prove useful in the field.