Entrained Air-Void Systems for Durable Highway Concrete (2021)

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63 Guidance on Protocols for Testing and Analysis Protocols for Testing and Analysis Air Content From the field-testing data, it was observed in several cases that pavement constructed using the same equipment and materials and exposed to the same environmental conditions can exhibit different levels of deterioration over time. Poor correlation was observed for the hardened air-void data for cores representing dis- tressed F-T pavements and durable sections. This indicates that the air-void system is not the only factor that influences frost durability of concrete pavements. In other words, although a minimum amount of air is necessary in hardened concrete, it is not the sole factor governing durability, and other contributing factors need to be addressed to deliver a frost-durable pave- ment. Aggregate damage and chemical attack in joints were observed to be the most dominant forms of distress in the investigated pavements. A modified analysis based on ASTM C457 was performed to determine the properties of the air-void system in the hardened state. Data obtained from durable pavements suggest the following requirements for hardened air-void systems when using the ASTM C457 method: • Hardened air content greater than 4.5%, • Spacing factor less than 0.011 in., and • Specific surface greater than 475 in.–1. The following requirements are recommended for flatbed scanner measurements: • Hardened air content greater than 4.5%, • Spacing factor less than 0.012 in., and • Specific surface greater than 450 in.–1. The following requirements are recommended for SAM measurements: • Fresh air content greater than 5.0%, and • SAM number of 0.30 and lower. Observations for Air-Void System Testing Using the SAM Based on the SAM evaluation: • SAM data collected in the laboratory appeared to correlate reasonably well with hardened air properties collected from the same batch. • Alkali content of the cement and chemical composition of SCMs will influence the SAM number and its correlation with the spacing factor in the hardened state. C H A P T E R 4

64 Entrained Air-Void Systems for Durable Highway Concrete Observation for Air-Void System Testing Using the Flatbed Scanner Data obtained indicated that flatbed scanner data correlated well with the hardened air properties obtained from the same batch. Observations and Recommendations Based on Clustering • Systems with LA cements that are not retempered appear to reduce the risk of clustering. The use of a stable AEA and lower-LOI fly ash is recommended. • According to the data obtained from concrete mixtures with the same w/cm ratio, reductions in compressive strength of up to 35% could occur due to clustering. Observation and Recommendation for Improvements in Testing for Freeze–Thaw Durability AASHTO T 161 “A” correlated well with concrete air system parameters and is recommended as the method for evaluating frost durability of concrete. Identification of Issues Affecting Implementation The following issues were found to affect use of the SAM: • Meters need to be calibrated on a regular basis (and also after any repairs) as recommended by the device manufacturer. • Operators must be well trained in operating and cleaning the equipment to reduce variability. • Pressing the lever while the pressure in the bottom chamber is higher than that of the top chamber can result in movement of water or paste or both into the top chamber, causing false readings. • Correlation between the SAM number and the spacing factor can depend on the cement chemistry, the quality of the incorporated SCM, and the stability of the AEA. The following issues can affect use of automated microscope equipment for air-void system analysis: • Selection of a proper threshold value for analysis using the fixed-focus optical microscope is a function of sample preparation and the amount of light provided during the scan. This number cannot be generalized from one laboratory to another, and possibly not even from one test setup to other systems available in the same laboratory. Refer to Appendix A for the recommended test procedure. • High quality and uniform polishing are necessary to ensure repeatability and reliability of the data. • Any problems with sample preparation, e.g., availability of residual paste or powder on the surface of specimens, can affect the data obtained. • Availability of sources of external light can affect the quality of the scan and the outputs. The following issues can affect flatbed scanner implementation: • High quality and uniform polishing are necessary to ensure repeatability and reliability of the data. • Any problems with sample preparation, e.g., availability of residual paste or powder on the surface of specimens, can affect the data obtained.

Guidance on Protocols for Testing and Analysis 65 • The selection of optimized thresholds is necessary to ensure reliable data. The following observations and recommendations will further improve CDF testing: • The microcomputer control presents a robust, flexible, and reliable method to replace mechanical or previously computerized F-T cabinets. The equipment is inexpensive, and newer Wi-Fi enabled versions allow remote monitoring and control. Once calibrated for the temperature sensor, the equipment can run flawlessly and, therefore, is recommended for either AASHTO T 161 “A” or any of the CDF variants evaluated herein.