Sources: Oklahoma State University, Stillwater; CP staff
The American Association of State Highway and Transportation Officials’ new standard, “Characterization of the Air-Void System of Freshly Mixed Concrete by the Sequential Pressure Method,” validates a device measuring void spacing and air content in about 10 minutes. The Super Air Meter (SAM) is a modified version of the vessel used in ASTM C231, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method, and was developed at Oklahoma State University.
Void spacing has been shown to be a better indicator of freeze-thaw durability than total air content, SAM developers note, but challenging to pinpoint—especially in fresh concrete. The ability to accurately measure actual air-void spacing enables a lowering of air content requirements in many mixtures; in turn, practitioners can realize greater strength development in hardened slabs and structures and avoid quality control challenges rooted in high air content. Conversely, the SAM has also revealed how certain admixture combinations and construction practices can require higher air contents than typically specified to obtain the desired void spacing. Either way, the new meter allows adjustments to be made before the concrete hardens to ensure freeze-thaw durability.
The SAM’s primary modification from the ASTM C231 vessel is that two sequential pressure steps are applied to the concrete, at 14.5, 30, and 45 psi, to obtain a deformation figure. The difference between the first and second pressure steps yields the SAM number, which increases along with the average spacing between air voids. Excess spacing alerts users of the concrete’s susceptibility to freeze-thaw deterioration. A SAM number of 0.20 has been shown to correctly determine over 90 percent of the time whether the spacing between bubbles meets ACI Committee 201 Durability of Concrete guidelines. Agencies in 20 states and Canada are gauging air-void spacing with the SAM; staff in Michigan and Oklahoma departments have begun calling for measurements with the equipment in project specs.
Overseeing the SAM validation is Oklahoma State University’s Tyler Ley, Ph.D., P.E., Associate Professor of Civil and Environmental Engineering and Williams Foundation Professor; 405/744-5257; www.tylerley.com. More information can be found at www.superairmeter.com.