Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails, plant evaluations and comments on blog posts or magazine articles posted to Precast.org.
Chuck asks: How is SCC (self-consolidating concrete) defined by specifiers? Particularly, are viscosity modifying admixtures necessary to be included?
NPCA technical experts answer:
With regard to what defines SCC, a good source is ACI’s web page of standards, codes and articles about SCC, found here: Concrete.org/topicsinconcrete.aspx.
The definition of SCC is taken directly from Section 1.1 of PRC-237, formerly ACI 237R, and reads: “Self-consolidating concrete (SCC) is highly flowable, non-segregating concrete that can spread into place, fill the formwork and encapsulate the reinforcement without any mechanical consolidation. In general, SCC is concrete made with conventional concrete materials and, in some cases, with a viscosity-modifying admixture (VMA). SCC also has been described as self-compacting concrete, self-placing concrete and self-leveling concrete, which all are subsets of SCC.”
Note that while VMAs are mentioned, they alone do not define what constitutes SCC. Rather, it is merely the physical properties or behavior of the plastic concrete that is used to define SCC.
Granted, that definition does not make it explicitly clear what constitutes “highly flowable” concrete. We must delve into the ACI and ASTM specs a little further for that answer.
Starting with ASTM C143, the venerable slump test, the user is warned in Section 4.3, Note 2 that, “Concretes having slumps greater than about 9 inches (230 mm) may not be adequately cohesive for this test to have significance.”
Since ASTM C1611 on slump flow measurement conversely does not define a range of slump flow diameters, we must refer to ACI PRC-237
Section 8.2.2, where we learn that, “A common range of slump flow for SCC is 18 to 30 in. (450 to 760 mm).”
This is intuitively correct since, if one were to take the volume of the slump cone and convert that same volume to a dome shape 3 inches tall, the smallest the base diameter could possibly be is 17 inches. And we intuitively know that the resulting pile of concrete, be it SCC or otherwise, would not be perfectly dome-shaped but would be even flatter and wider at the base.
Karen asks: Are there any changes to the latest edition of the Quality Control Manual for Precast Concrete Plants that are currently under review regarding the definition of SCC?
NPCA technical experts answer:
Regarding the latest edition of the NPCA’s Quality Control Manual for Precast Concrete Plants, the QA/QC Committee made some changes to the 16th Edition of the QC Manual for 2023 to address SCC.
A PDF document available at Precast.org/certification/quality-control-manual outlines the changes. Additions have been made to the Commentary of Sections, and 3.1.1.1, 3.1.1.2, 5.3.1.1 and 5.3.1.2 have been added the new edition, which was finalized in February.