Advances in measuring parameters of the air-void system in hardened concrete

To improve the freezing and thawing resistance of concrete, air bubbles are entrained and distributed evenly and closely. Evaluation of the parameters of the air-void system in hardened concrete is detailed in ASTM C457 “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.” Microscopical methods are used to determine traverse lengths of air voids, aggregates, and pastes on the polished surface of concrete samples. Since this manual measurement is time-consuming, tedious, and dependent on skills of operators, automated methods are preferred. This study recommends improvements to both the ASTM C457 test protocol and the metrics that are used to characterize the air void system. A flatbed scanner is used to acquire a single high resolution image of the polished surface of a concrete sample. The concrete surface is polished and treated with phenolphthalein and orange powder to facilitate segmentation of air bubbles, paste and aggregate phases. The image is processed using ImageJ, Photoshop, and Matlab software. A three-phase image is generated with air, paste, and aggregate phases shown in white, gray, and black, respectively. Using the three-phase image, ASTM C457 parameters can be readily determined by computer. To validate the new approach, six groups of concrete samples were examined in a “blind study.” The concrete samples were donated by CTLGroup which had previously evaluated the air void system using the standard ASTM C457 test method. A good agreement between the two methods was shown except in the case of samples with lightweight aggregates. The mechanism of freezing and thawing damage is complicated and theories proposed by researchers cannot explain all the observations or establish a clear relationship between the spacing of air void, the freezing rate and the paste properties. In many respects, these theories complement each other. Air entrainment is effective and reliable to resist freezing and thawing for concrete designed for outside exposure in the cold climate. The structure of the air-void system is critical for the frost resistance of concrete and the spacing factor is one of the most significant parameters of the air-void system. A new approach to spacing factor is developed in this study using two-dimensional images to provide better information than linear traverse or point count methods. The average distance of pastes to the nearest air void, the percentage of protected pastes, the area fraction of air voids, and the surface area of air voids are two-dimensional parameters which can be used to evaluate the freezing and thawing performance considering the physical properties and the mechanism of freezing of concrete. A comparative study is conducted between the two-dimensional parameters and the one-dimensional parameters in ASTM C457 and the advantages of the two-dimensional parameters are presented