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66 Summary and Recommendations for Research Conclusions This report summarizes the research conducted on characteristics of air-void systems for securing long-term F-T durability of highway concrete. The work focused on three main areas: ⢠Investigation of the hardened air-void systems of in situ concrete in pavements that exhibited distress that seemed to be related to air-void deficiencies, ⢠Study of the air-void systems of a wide range of concrete mixtures that could represent the mixtures employed for rigid pavement construction, and ⢠Investigation of accelerated test methods 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 is the sole factor for distress is a challenge. In most cases, air-related problems were only one of the many contributors to the observed distress. Further, frost damage was manifested in joint damage and D-cracking, although the concrete exhibited acceptable perfor- mance 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.â1. Correlations were established between the parameters of the air-void system of the labora- tory mixtures in fresh and hardened states. These correlations were further compared with 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 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 at high temperatures were conducted. This finding indicates that changes in the chemistry of the chemical admixtures implemented by the manufacturers appear to yield more robust air-void systems. The research findings also indicate modifications to the standard AASHTO T 161 Proce- dure âAâ, including modification of the test procedure as well as the F-T tank-to-computer control to perform the conventional F-T cycling. Major modifications to the test procedure included a drying period prior to testing, introduction of freezing fluid to one-dimensional capillary suction, a 12-hour F-T cycle to better simulate field conditions, and the ability to evaluate performance across a range of deicer or anti-icer chemicals. A 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 C H A P T E R 5
Summary and Recommendations for Research 67 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 âAâ (300 cycles) and CDF-A:FT1 test (140 cycles). The CDF-A:FT1 test reflected slightly better the effect of F-T cycles on concrete samples in water that were expected to exhibit good F-T resistance, but was similar to T 161 in identifying poorly perform- ing mixtures. The combination of computer control, more realistic exposure conditions, F-T cycle duration, and flexibility to include deicers are features proposed for incorporation in the test procedure. Recommendations for Future Research Based on the findings of this research, the following knowledge gaps were identified and recommended for future research: ⢠Distress in pavements was mostly related to joints and aggregates. Whether or not the inter- action with the air-void system can affect the rate of deterioration is unknown. Further investigation in this area is needed. 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 agree 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. Further investigation using a variety of air-entraining admixtures that yield a wide range of air voids in concrete is needed. ⢠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.