RES2020-09: Enhancing freeze-thaw resistance of Tennessee concrete mixes through improved air void testing

Researchers have been studying the intrinsic mechanism governing the freeze-thaw damage for decades and consistently found the generation of internal stress involved in the process, which is the result of: a) hydraulic pressure due to ice formation, with a 9% expansion in volume; b) osmotic pressure generated in the pore system by the movement of liquid water towards pores containing ice to restore thermodynamic equilibrium; and c) the pressure induced by the growth of crystals in pores and their interaction with pore walls (Wardeh et al. 2011; De Rojas et al. 2011; Vegas et al. 2009). It has long been recognized that the air-void system in concrete plays a critical role in the resistance of concrete to freezing and thawing cycles by providing additional space to reduce internal pressure caused by frozen water. The air-void system of concrete has been characterized by parameters including total air content, air-void spacing factor, specific surface (Powers and Willis 1949; Manns 1970; Zhang et al. 2010). Therefore, many laboratory test methods have been developed to determine these parameters of concrete mixes. The most widely used is still the pressure air test ASTM C231/C231M-10, first published in 1949, to quantifying the total air volume of fresh concrete mixes. However, the total air content cannot reflect the size and distribution of air voids in concrete, which have shown to be more effective in characterizing the freeze-thaw resistance of concrete. The air-void spacing factor and specific surface can be measured in hardened concrete in accordance with ASTM C457/C457M-12 and used to evaluate the size and distribution of air voids in hardened concrete. However, these two parameters demonstrate a high variability in data and the testing is lab-intensive and time-consuming. Another disadvantage of ASTM C457 is that, by the time air void system is found inadequate, the structure has already been built and little can be done (Tanesi et al 2016). Consequently, it is crucial to determine the air void system quality in real-time while concrete is still fresh and measures can be taken to ensure good quality air void system or, the concrete can be rejected. For the purpose of improving the laboratory testing, Dansk Beton Teknik developed the air void analyzer (AVA), which uses the Stoke’s law to measure bubble size distribution by timing the bubbles as they rise through a column of glycerol and water (Bly and Ventorini 2013). However, the AVA equipment requires a series of precautions when implemented and is very sensitive and relatively expensive (AASHTO 2013). Because of these difficulties, AVA has not been widely adopted by state DOTs (Distlehorst and Kurgan 2007; Bly and Ventorini 2013). Recently, a team led by Dr. Tyler Ley at Oklahoma State University developed a device called the super air meter (SAM) (Ley and Tabb 2013; Ley et al. 2017). SAM is a modified ASTM C231 Type B pressure meter gage and a restraint cage for safety purposes (Figure 1), which is capable of assessing air-void parameters including total air content, spacing factor and specific surface of fresh concrete. The major advantages of SAM is that it employs an inexpensive piece of equipment like pressure meter that most technicians are familiar with and it is easily performed on the job site providing results in real-time. Meanwhile, a parameter called SAM number has been proposed and correlated well to the spacing factor and freeze-thaw resistance of concrete. SAM has been standardized under AASHTO TP 118, Provisional Standard Method of Test for Characterization of the Air-Void System of Freshly Mixed Concrete by the Sequential Pressure Method. It has the potential to serve as a QC/QA tool for freeze-thaw resistance of concrete by state highway agencies (SHAs). The Tennessee Department of Transportation (TDOT) has no experience with the SAM meter and SAM number. TDOT is also concerned with the consistency of SAM number measurements and its suitability as a QC/QA tool. There is a need to examine the applicability of SAM meter and SAM number to Tennessee concrete mixes and to determine the threshold of SAM number for concrete with adequate freeze-thaw resistance in Tennessee.