COMPARISON OF AASHTO PLASTIC PCC AIR DETERMINATION TECHNIQUES

A field investigation of the four current American Association of State Highway and Transportation Officials (AASHTO) Standard Test Methods for determining air content of plastic portland cement concrete (PCC) was conducted. The four methods are: (1) Air Content of Freshly Mixed Concrete by the Pressure Method, AASHTO T 152-97; (2) Air Content of Freshly Mixed Concrete by the Volumetric Method, AASHTO T 196-96; (3) Mass per Cubic Meter (Cubic Foot), Yield, and Air Content (Gravimetric) of Concrete, AASHTO T 121-97; and (4) Air Content of Freshly Mixed Concrete by the Chace Indicator, AASHTO T 199-00. Duplicate air tests were conducted on 32 three-cubic-yard field batches of Tennessee Department of Transportation (TDOT) Class A PCC using all four currently available AASHTO plastic PCC air determination techniques. Methods were compared statistically using the range of the duplicate tests. Further, the research team analyzed method mechanics, applicability, logistics and costs. The research team recommends that the TDOT continue using the pressure method for normal weight aggregate concrete and the volumetric method for lightweight aggregate concrete. The two methods had similar precision and accuracy for TDOT Class A PCC. In the vast majority of cases, the true air content of TDOT Class A PCC appears to be below the observed value produced by the pressure method and above the observed value produced by the volumetric method. The gravimetric method and Chace Air Indicator were found to have several disadvantages when compared to current TDOT methods. The recommendations are based primarily on mechanics and applicability.