Developing An Alternative Hardened Concrete Air Void Characterization Method

Villarreal, Ruben
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This study presents the development and analysis of an alternative characterization method for the hardened air void system in concrete. This study uses ASTM C457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete” as a baseline characterization method and attempts to thoroughly reduce the long sample preparation and characterization time required to complete the test. An investigation on the sample preparation requirements was first completed, in which new optimized cutting and polishing procedures were developed, along with improved contrast enhanced procedures, to accommodate the alternative characterization method. The new characterization method also eliminates the reliance on user-judgement and reduces human error. The new characterization method uses a Keyence digital optical microscope to analyze the processed hardened concrete sample, and automatically counts and measures the air void system. The microscope then provides an excel report on the analyzed system, which can then be processed to produce key hardened air void system information such as the maximum and minimum air void size, dispersion, circularity, air void percentage (amount of air) and further information such as the spacing factor and specific surface can be calculated from this data. This entire process was developed by producing four concrete mixtures with different air entraining dosages. The air entraining dosages were added at various ranges sufficient to provide a low (3-4%), moderate (5-7%), and high (> 8%) fresh air contents in the produced mixtures. A non-air entrained mixture was also included as a reference in the study. The mixtures were then tested for slump (ASTM C143), air content (ASTM C138), unit weight (ASTM C138), Super Air Meter number, and 7 and 14-day compressive strength (ASTM C39). Additional hardened samples were created from each mixture such that they can be cut, polished, and prepared for hardened air void analysis. A procedural analysis was completed to determine the minimum processing steps and time necessary to produce a surface that can be analyzed by both ASTM C457 and the alternative characterization method. The new alternative characterization method showed that testing laboratories would save time, money, and better consistent results by reducing human decision-making. The use of the Keyence VHX-7000 microscope, along with sample cutting, polishing, and color enhancing procedures discussed in this investigation, was found to give operator flexibility of running multiple analyses thru the use of the software in the Keyence microscope. Single run analysis with data stored within the file allows the operator to adjust boundaries, change the air void parameters such as circularity, max and min diameters, and re-run the test in minutes instead of hours.
Air entrained, Air void, Hardened concrete air void characterization
Villarreal, R. (2021). <i>Developing an alternative hardened concrete air void characterization method</i> (Unpublished thesis). Texas State University, San Marcos, Texas.