materials of concern also depends on the associated scale of the
primary observations relative to the grain or aggregate size of
the studied materials, especially when localized estimates are
not adequate for field level or macro scale characterizations.
Because the scan system employed in these experiments has a
fixed field of view (4,000 ×4,000 pixels for a slice), the resolution
of the image pixels depends on the sample size of the specimen. To
explore the effects of resolution on the property estimates, three
Ohio sandstone cores with diameters of 5, 12.6, and 24 mm,
corresponding to X-ray spatial resolutions of 1.5, 3.67, and
6.27 μm, respectively, were scanned to generate identically sized
images (4,000 ×4,000 pixels). Thresholds were selected to match
the measured porosity, and subsequently, the other properties were
obtained (a summary is provided in Table 3).
The wide range of tortuosity values computed from the random
walk suggests that coarser resolution (i.e., larger mean voxel size)
tends to be inadequate when detecting possible connected pores.
This effect was also reported in the study by Promentilla et al.
(2009), in which the results are based on images (matrices) of
different sizes representing the same specimen. The results of
specific surface area did not show a monotonic correlation with
the resolution, partially because the porosity was nearly identical
and did not reflect an effect on the pore connectivity. The
permeability estimates in this case were mostly affected by the
difference in tortuosity estimates and were understandably greater
for finer resolution, smaller sized voxel dimensions.
Seven commonly used porous building materials were examined by
using X-ray CT. The materials included three seemingly homo-
geneous natural stones (two sandstones and a limestone) and four
heterogeneous engineered materials (three concretes and a brick).
Scanned images of each building material were processed to recon-
struct their geomorphic structures and two approaches were exam-
ined for thresholding in the analysis of these samples. Random
walk simulations were performed on the reconstructed pore struc-
tures to compute properties that are relevant to transport phenom-
ena, such as tortuosity, specific surface, and permeability. The
computed porosities and permeabilities were compared to the
The calibration-based method for permeability analysis seemed
to provide better estimates than the histogram-based method,
especially when the effective porosity was used in Eq. (3).
Presently, the latter is more commonly used, but may not be appro-
priate for cementitious and heterogeneous building materials such
as concrete and brick. The reconstructed geomorphic structures of
these heterogeneous engineered materials varied greatly when
the thresholds for image analysis were selected based on the
Measured and computed permeabilities were generally within
one order of magnitude of each other when the thresholding
was based on measured effective porosity. The computed perme-
abilities compared better with the measured permeabilities when
the thresholding was based on the measured effective porosity
than the histogram, particularly for the heterogeneous, artificial
Finally, the relatively homogeneous and heterogeneous pore
structures of commonly used natural and engineered building
materials can be captured by X-ray tomography.
Support for this work was provided by Defense Threat Reduction
Agency, HDTRA1- 08-C-0021. We thank Ms. Lindsay Meador for
help in the laboratory.
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Table 3. Parameters Estimated for Ohio Sandstone of Different Sizes
Resolution (μm) 1.5 3.67 6.27
Sample diameter (mm) 5 12.5 24
Porosity (%) 11.5 11.2 11.8
Tortuosity 4.6 21.6 58.3
Specific surface area (m−1)4.5×1042.8×1044.0×104
Permeability (m2)21 ×10−14 8.1×10−14 1.8×10−14
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