Determining the Air Void Parameters of Concrete Using Digital Image Analysis of Polarized Light Micrographs

The ASTM C457 test has long been a standard used to obtain the air void parameters of concrete materials. These air void parameters provide valuable information that has been linked to the performance of concrete under conditions such as freezing and thawing cycles. The standard test procedure involves linearly traversing a cut and polished section of a concrete specimen while a technician observes it under a microscope. Chord lengths of material constituents that the technician observes along the linear traverse are recorded and later used to calculate air void parameters statistically. This procedure is long and tedious, which makes it susceptible to human error due to operator fatigue.

This study proposes and implements a new test method for evaluating concrete air void parameters using an image analysis method. A polishing procedure along with a differential interference contrast microscope are used to obtain high contrast images of material constituents, which provide raw data for the image analysis method. Because of the high contrast that can be obtained, cement paste, air voids in the cement paste, and aggregate materials in the concrete can be distinguished from one another based on these images. An image analysis program has been written for this study which linearly traverses these images and records the chord lengths of material constituents in a similar way to the standard ASTM C457 test. The chord length data must be processed further, however, because features in the images can be truncated by the edge of the image. Correction calculations for this problem are implemented in the image analysis algorithm.

Two specimens which have been previously tested using the standard ASTM C457 method by the Virginia Transportation Research Council, (VTRC), are used in this study. The air void parameters obtained using the new test are compared directly with the results obtained by VTRC for the two specimens. Statistical comparisons indicate that the results of the new test are indeed significant, showing the potential it has for practical implementation. There are drawbacks to the test including a long polishing procedure, but this process can be automated. The new test appears to have excellent potential for practical application, but it should be emphasized that the test has only been implemented using materials in two concrete specimens. Further study on a variety of other concrete materials would be required for implementation in a standard procedure.