Developing An Alternative Hardened Concrete Air Void Characterization Method
Date
2021-12
Authors
Villarreal, Ruben
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Abstract
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.
Description
Keywords
Air entrained, Air void, Hardened concrete air void characterization
Citation
Villarreal, R. (2021). <i>Developing an alternative hardened concrete air void characterization method</i> (Unpublished thesis). Texas State University, San Marcos, Texas.