National Cooperative Highway Research Program (NCHRP)
Peter Taylorptaylor@iastate.edu email >
Director, CP Tech Center
About the research
This report summarizes the research conducted on characteristics of the air-void system for securing long term freeze-thaw (F-T) durability of highway concrete. The work focused on three main areas:
- Investigate the hardened air-void system of in situ concrete in pavements that exhibited distress that seemed to be related to air void deficiencies
- Study the air-void systems of a wide range of concrete mixtures that could represent the mixtures employed for rigid pavement construction
- Investigate the accelerated test method for frost durability of laboratory mixtures based on a variety of test methods/variants
One observation is that finding a distressed concrete section where lack of a proper hardened air-void system was the sole factor for distress was a challenge. In most cases, air related problems were only one of the many contributors to the observed distress. Frost damage, in most cases, was manifested in joint damage and D-cracking, while the concrete exhibited acceptable performance at mid-panel.
Most of the tested cores, which were selected to represent different geographic locations, exhibited acceptable performance with a hardened air-void system with minimum air content of 4.2%, maximum spacing factor of 0.011 in., and minimum specific surface of 475 in.-1.
Correlations were established between the parameters of the air-void system of the laboratory mixtures in fresh and hardened states. These correlations were further compared to the field observations and formed the basis for proposing criteria for minimum air requirements in concrete pavements. Results indicated that a minimum fresh air content of 5% and a super air meter (SAM) number of 0.30 can be adequate for acceptable frost durability.
Producing high clustering rates proved to be a challenge in the laboratory, even though retempering and mixing in high temperatures were considered in the test matrix. This can be regarded as a positive sign, indicating improved chemistry of the chemical admixtures that can yield more robust air-void systems.
The standard AASTHO T 161 Procedure “A” freeze-thaw tank was successfully modified to computer control to perform the conventional F-T cycling, and also the modified test. Major changes included in the modified test included a drying period prior to testing, introduction of freezing fluid to one-dimensional capillary suction, a 12-hr F-T cycle to better simulate field conditions, and the ability to evaluate performance across a range of deicer or anti-icer chemicals.
The range of test variants were evaluated across concrete mixtures with differing air-system qualities. The initially proposed 56-cycle testing duration was insufficient to observe F-T deterioration in all but the worst performing samples. Consequently, samples were tested to 140 cycles, which provided sufficient exposure to observe deterioration. Air void parameters correlated with F-T performance for six of the seven mixtures evaluated using both the AASHTO T 161 Procedure “A” (300 cycles) and CDF-A:FT1 test (140 cycles). The CDF-A:FT1 test provided differentiation for the best performing samples, where AASHTO T 161 did not. The combination of computer control, more realistic exposure conditions, F-T cycle duration, and flexibility to include deicers does suggest it would be appropriate to consider adopting the new standard.
Recommendations for future research
Based on the observations made through different steps of this research, the following knowledge gaps were identified that are recommended for future research:
- It was observed in this study that the majority of distress in pavements was related to joints and aggregates. Whether or not the interaction with the air-void system can affect the rate of deterioration remains a question. Further investigation in this area can add significant value to the current knowledge. Accelerated testing of joint performance with and without deicing salts can also elucidate the role of the air-void system in long-term durability.
- The flatbed scanner measurements present agreement with the modified ASTM C457 measurements with the fixed-focus optical microscope for chords longer than 30 microns. Deviations increase when data from all chords are considered in comparisons. This could be further investigated in the future using a variety of air-entraining admixtures that yield a wide range of air voids in concrete.
- The SAM number measurements appear to be sensitive to the chemistry of the cementitious system. The extent and mechanisms behind such variations need to be further investigated.