Measurement of air void system in lightweight concrete by X-ray computed tomography

An industrial lightweight concrete developed for pipe insulation is characterized using X-ray computed tomography to determine the size distribution of air voids within the paste fraction. Strategies to address practical difficulties are presented, particularly discerning air voids in paste from air voids within aggregate, and combining air void size distribution data collected at different resolutions. To address the first issue, a step-wise approach was employed to reduce the problem to a series of easier-to-solve two-phase grayscale threshold problems. The first step isolates a significant number of light weight aggregate particles on 2D slices using an active contour line approach and an effective threshold identified to discern between air void and solid voxels. The threshold is applied globally to the 3D data set, and the air void voxels removed. Next, a significant number of rectangular regions of interest from 2D slices are used to identify a second threshold between the solid portions of the lightweight aggregate and the paste. Finally, a 3D shape based criterion is used to isolate air voids in paste. Given the trade-off between resolution and sample size, it is necessary to conduct multiple scans using different resolutions and sample diameters in order to cover the full range of the air void size distribution. A power law curve is used to unite distributions collected at different resolutions.