Concrete Formation Factor: Experimental and Modeling Methods

AUTHORS: Tanesi, J.; Montanari, L.; Carino, N.; and Cooper, M.

ABSTRACT: AASHTO R 101 includes formation factor (FF) as one of the main properties related to concrete mixtures’ fluid and ionic transport properties for corrosion and freeze–thaw service life modeling. However, FF determination requires knowledge of the concrete pore solution’s electrical resistivity, which is complex to measure directly. Hence, an alternative, indirect method, described in ASTM C1876 and AASHTO T 402, has been proposed to estimate pore solution electrical resistivity. The method involves immersing concrete specimens in an alkali-concentrated conditioning solution of known resistivity for a minimum of 7 days. At the end of the conditioning period, the standards assume equilibrium between the pore solution and the conditioning solution. Concerns within the industry regarding the ASTM and AASHTO approaches have included (1) how realistic the assumed equilibrium is between the pore solution and conditioning solution, (2) how the assumed equilibrium can affect the FF estimation, (3) the optimal time needed for equilibrium, and (4) handling and disposing of the caustic conditioning solution. This study examines various methods for assessing the FF of concrete mixtures by combining different conditioning techniques with pore solution modeling. Results show that sealed curing resistivity combined with pore solution modeling decreased the average error on the FF estimation by up to 65% compared with results of the current ASTM and AASHTO methods.