|Developed at Oklahoma State University, the Super Air Meter helps users better understand freeze-thaw durability potential before their concrete is placed. Technicians reach a SAM number through two cycles involving three levels of chamber pressure.|
A modified version of the ASTM C231, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method vessel, the Super Air Meter (SAM) measures air-void spacing and air content of plastic concrete in about 10 minutes. 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 the fresh concrete.
The ability to accurately measure actual air-void spacing in the fresh concrete 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 avoid 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 standard ASTM C231 device 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. That benchmark reflects investigation of 300-plus lab and field mixtures measured at Oklahoma State University and the Federal Highway Administration Turner-Fairbank Highway Research Center in Virginia. As part of an ongoing Transportation Pooled Fund Program study, 10 state departments of transportation initially deployed the SAM in field concrete work, including Illinois DOT (staff shown here). Test ease and accuracy have expanded the equipment’s user base: 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.
Durability indicators from SAM measurements are also being compared to hardened specimens’ performance reflected in ASTM C666, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. An AASHTO Provisional Standard referencing the equipment has been approved in early-2015 balloting. 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.