Assessment of Flatbed Scanner Method for Quality Assurance Testing of Air Content and Spacing Factor in
Concrete
by
Sona Nezami
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Department of Civil Engineering University of Toronto
© Copyright by Sona Nezami 2013
ii
Assessment of Flatbed Scanner Method for Quality Assurance
Testing of Air Content and Spacing Factor in Concrete
Sona Nezami
Master of Applied Science
Department of Civil Engineering University of Toronto
2013
Abstract
The flatbed scanner method for air void analysis of concrete is investigated through a
comparison study with the standard ASTM C457 manual and Rapid Air 457 test methods. Air
void parameters including air content and spacing factor are determined by image analysis
of a large population of scanned samples through contrast enhancement and threshold
determination procedures. It is shown that flatbed scanner method is giving comparable
results to manual and Rapid Air 457 methods. Furthermore, a comparison of the air void
chord length distributions obtained from the two methods of flatbed scanner and Rapid Air
457 has been implemented in this research. The effect of having different settings in the
scanning process of scanner method is also investigated. Moreover, a threshold study has
been performed that showed the flatbed scanner method can be employed in combination
with manual and Rapid Air 457 methods as a time and cost saving strategy.
iii
Acknowledgments
It is with my immense gratitude that I acknowledge the support and help of my Supervisor
Professor Karl Peterson whose supervision, guidance, and encouragement from the
preliminary to the concluding part of my research enabled me to develop a clear
understanding of the subject. I would like to thank the other member of my thesis
committee, Professor Daman Panesar for her guidance and continued interest in the
subject. I would like to extend my thanks to Hannah Schell, Jana Konecny, Jixing Jiang from
the Ministry of Transportation of Ontario, Gerald Anzalone from Michigan Technological
University and Kenneth Totty from Grace Construction Products company who provided the
basis for my research. I would also thank my best friend Hadi Malekghasemi who has greatly
helped and encouraged me in every step of my research. Most importantly, I wish to thank
my parents without whom I would never reach this stage of my life. They bore me, raised
me, supported me, taught me, and loved me and to them I dedicate this thesis. Finally, I
express my genuine appreciation towards the Almighty.
iv
Table of Contents
Abstract .................................................................................................................................................. ii
Acknowledgments ................................................................................................................................. iii
List of Tables ........................................................................................................................................... v
List of Figures ......................................................................................................................................... vi
Chapter 1 ................................................................................................................................................ 1
Introduction ............................................................................................................................................ 1
Chapter 2 ................................................................................................................................................ 5
Flatbed Scanner versus Manual Method ............................................................................................... 5
2.1 Sample Preparation .................................................................................................................. 6
2.2 Scanning Process ...................................................................................................................... 8
2.3 Image Analysis ........................................................................................................................ 12
2.3.1 Theoretical ....................................................................................................................... 12
2.3.2 Threshold Determination ................................................................................................ 14
2.4 Data Analysis .......................................................................................................................... 22
2.5 Threshold Study ...................................................................................................................... 28
2.6 Scanner Settings ..................................................................................................................... 32
2.6.1 Automatic Brightness Adjustment .................................................................................. 32
2.6.2 Image Sharpening Setting ................................................................................................ 34
2.7 Point Count ............................................................................................................................. 36
Chapter 3 .............................................................................................................................................. 39
Flatbed Scanner versus Rapid Air 457 Method ................................................................................ 39
3.1 Sample Preparation ................................................................................................................ 40
3.2 Image Collection and Analysis ................................................................................................ 43
3.3 Air Content and Spacing Factor Results ................................................................................. 45
3.4 Chord Length Distribution ...................................................................................................... 47
3.5 Threshold Study ...................................................................................................................... 49
Chapter 4 .............................................................................................................................................. 52
Conclusion ........................................................................................................................................ 52
References ............................................................................................................................................ 55
v
List of Tables
Table 2.1: Statistical Analysis Results. .................................................................................................. 26
Table 2.2: Threshold Values for All and Individual Labs. ...................................................................... 27
Table 2.3: Statistical Data of Different Sample Populations (AC). ........................................................ 29
Table 2.4: Statistical Data of Different Sample Populations (VF). ........................................................ 30
Table 3.1: Statistical Analysis Results. .................................................................................................. 47
Table 3.2: Statistical Data of Different Sample Populations. ............................................................... 50
Table C.1: Point Count Detailed Results. .............................................................................................. 69
Table D.1: Flatbed Scanner versus Manual Method Database. ........................................................... 72
Table E.1: Flatbed Scanner versus Rapid Air 457 Database ................................................................. 91
vi
List of Figures
Figure 2.1: Grinding Apparatus. ............................................................................................................. 7
Figure 2.2: Automated Lapping Machine (Top View). ............................................................................ 7
Figure 2.3: Adhesive-Backed 600 Grit Silicon Carbide Paper. ................................................................ 8
Figure 2.4: Final Polish Machinery. ........................................................................................................ 8
Figure 2.5: Samples after a) Placing Stickers and b) Blackening and Powdering (Scale Bar Represents
20 mm). .................................................................................................................................................. 9
Figure 2.6: Screen Shot of Adobe Photoshop Script Showing User Interface and Scanned Slab Image.
.............................................................................................................................................................. 11
Figure 2.7: Histograms and Probability Density Functions of Optimum Threshold Levels Determined
for Void Frequency for a) Normal Distribution with μ = 52 and b) General Extreme Value Distribution
with μ = 39. ........................................................................................................................................... 18
Figure 2.8: Histogram and Probability Density Functions of Optimum Threshold Levels Determined
for Air Content Fit with a) a Normal Distribution with μ = 71, and Fit with b) a General Extreme Value
Distribution with μ = 47. ....................................................................................................................... 19
Figure 2.9: Influence of Threshold Selection Method on Air Content Measurements, with Assumed
Paste Content of 30%: a) Dual Threshold (VF52, AC71) Normal Distribution Method, b) Single
Threshold (62) Normal Distribution Method, c) Dual Threshold (VF39, AC47) Extreme Value
Distribution Method, d) Single Threshold (43) Extreme Value Distribution Method. ......................... 20
Figure 2.10: Influence of Threshold Selection Method on Spacing Factor Measurements, with
Assumed Paste Content of 30%: a) Dual Threshold (VF52, AC71) Normal Distribution Method, b)
Single Threshold (62) Normal Distribution Method, c) Dual Threshold (VF39, AC47) Extreme Value
Distribution Method, d) Single Threshold (43) Extreme Value Distribution Method. ......................... 21
Figure 2.11: Manual versus Flatbed Scanner Air Content (All Labs). ................................................... 23
Figure 2.12: Manual versus Flatbed Scanner Spacing Factor (All Labs). .............................................. 24
Figure 2.13: Manual versus Flatbed Scanner Air Content (Lab 3). ....................................................... 24
Figure 2.14: Manual versus Flatbed Scanner Spacing Factor (Lab 3). .................................................. 25
Figure 2.15: Manual versus Flatbed Scanner Air Content (Lab 3 Excluded). ....................................... 25
Figure 2.16: Manual versus Flatbed Scanner Spacing Factor (Lab 3 Excluded). .................................. 26
Figure 2.17: False Negative Zone for Spacing Factor (Lab 3 and All Labs). .......................................... 27
Figure 2.18: Air Content Threshold of 72 (Proposed Threshold) versus 71 (True Threshold). ............ 30
Figure 2.19: Void Frequency Threshold of 55 (Proposed Threshold) versus 52 (True Threshold). ...... 31
Figure 2.20: Air Content Threshold of 78 (Proposed Threshold) versus 71 (True Threshold). ............ 31
Figure 2.21: Void Frequency Threshold of 57 (Proposed Threshold) versus 52 (True Threshold). ...... 32
Figure 2.22: Without Automatic Adjustment versus Automatically Adjusted Histogram (Air Content).
.............................................................................................................................................................. 33
Figure 2.23: Without Automatic Adjustment versus Automatically Adjusted Histogram (Spacing
Factor). ................................................................................................................................................. 33
Figure 2.24: Sharpened versus Normal Scanner Setting (Air Content). ............................................... 35
Figure 2.25: Sharpened versus Normal Scanner Setting (Spacing Factor). .......................................... 35
vii
Figure 2.26: Spacing Factor Comparison of Point Counting and 30% Assumed Paste Contents. ........ 37
Figure 2.27: Spacing Factor Comparison of Manual and Flatbed Scanner 30% Assumed Paste
Contents. .............................................................................................................................................. 38
Figure 2.28: Spacing Factor Comparison of Manual and Flatbed Scanner 30% Assumed Paste
Contents. .............................................................................................................................................. 38
Figure 3.1: Polishing Apparatus. ........................................................................................................... 41
Figure 3.2: Surface Lapping with Water and Silicon Carbide Grit Suspension. .................................... 41
Figure 3.3: Polished Slabs (Left) and Close-up (Right). ......................................................................... 42
Figure 3.4: Slab after Blackening and White Powder (Left) and after Threshold Application (Right). . 43
Figure 3.5: Wiping Away Excess Powder (Left) and Performing Analysis (Right). ................................ 43
Figure 3.6: Air Content Comparison of Flatbed Scanner and Rapid Air 457. ....................................... 46
Figure 3.7: Spacing Factor Comparison of Flatbed Scanner and Rapid Air 457. .................................. 46
Figure 3.8: Chord Length Distributions from Rapid Air 457, and Flatbed Scanner at Thresholds of a)
115 and b) 140. ..................................................................................................................................... 48
Figure 3.9: Comparison of Flatbed Scanner Spacing Factor Results Using Void Frequency Threshold
Levels of 115 and 120. .......................................................................................................................... 51
Figure A.1: Manual versus Flatbed Scanner Air Content (All Labs). ..................................................... 57
Figure A.2: Manual versus Flatbed Scanner Spacing Factor (All Labs). ................................................ 57
Figure A.3: Manual versus Flatbed Scanner Air Content (No Lab ID). .................................................. 58
Figure A.4: Manual versus Flatbed Scanner Spacing Factor (No Lab ID). ............................................. 58
Figure A.5: Manual versus Flatbed Scanner Air Content (Lab 1). ......................................................... 59
Figure A.6: Manual versus Flatbed Scanner Spacing Factor (Lab 1). .................................................... 59
Figure A.7: Manual versus Flatbed Scanner Air Content (Lab 2). ......................................................... 60
Figure A.8: Manual versus Flatbed Scanner Spacing Factor (Lab 2). .................................................... 60
Figure A.9: Manual versus Flatbed Scanner Air Content (Lab 3). ......................................................... 61
Figure A.10: Manual versus Flatbed Scanner Spacing Factor (Lab 3). .................................................. 61
Figure A.11: Manual versus Flatbed Scanner Air Content (Lab 4). ....................................................... 62
Figure A.12: Manual versus Flatbed Scanner Spacing Factor (Lab 4). .................................................. 62
Figure A.13: Manual versus Flatbed Scanner Air Content (Lab 5). ....................................................... 63
Figure A.14: Manual versus Flatbed Scanner Spacing Factor (Lab 5). .................................................. 63
Figure A.15: Manual versus Flatbed Scanner Air Content (Lab 6). ....................................................... 64
Figure A.16: Manual versus Flatbed Scanner Spacing Factor (Lab 6). .................................................. 64
Figure A.17: Manual versus Flatbed Scanner Air Content (Lab 7). ....................................................... 65
Figure A.18: Manual versus Flatbed Scanner Spacing Factor (Lab 7). .................................................. 65
Figure A.19: Manual versus Flatbed Scanner Air Content (Lab 8). ....................................................... 66
Figure A.20: Manual versus Flatbed Scanner Spacing Factor (Lab 8). .................................................. 66
Figure B.1: Optimum Threshold Value Distributions for a) Air Content and b) Void Frequency. ........ 67
viii
List of Appendices
Appendix A: .......................................................................................................................................... 57 Comparison Plots of Manual and Flatbed Scanner for Different Labs ............................................. 57
Appendix B............................................................................................................................................ 67 Normal Distributions for Threshold Determination of Rapid Air 457 versus Flatbed Scanner ........ 67
Appendix C ............................................................................................................................................ 68 Point Count Method Detailed Results .............................................................................................. 68
Appendix D ........................................................................................................................................... 71 Flatbed Scanner versus Manual Method Database ......................................................................... 71
Appendix E ............................................................................................................................................ 90
1
Chapter 1
Introduction
Air void parameters of air entrained concrete are closely linked to the freeze and thaw
deterioration of concrete and this phenomenon has for a long time lead the researchers
and the industry to evaluate these parameters for quality control of concrete infrastructure.
In Ontario, Air Void System (AVS) analysis is performed on all new Ministry of
Transportation concrete construction, and the parameters of air content and spacing actor
are used to define the quality of air entrainment [1]. Basically, a minimum volumetric
content of air should be provided and the bubbles must be close enough to one another to
protect the paste from freeze and thaw damage [2]. Air void spacing factor equations have
been introduced by Powers, Philleo, Attiogbe, and Pleau and Pigeon, and Snyder [3].
However, the spacing factor equation developed by Powers is the basis for American
Society for Testing and Materials (ASTM) C457 Standard Test Method for Microscopical
Determination of Parameters of the Air-void in Hardened Concrete [3]. It is described as a
parameter related to the maximum distance in the cement paste from the periphery of an
air void [3,4].
ASTM C457 is frequently used for evaluation of air entrainment in concrete. However,
microscopical examination of polished plane surface of concrete based on this standard is a
tedious and operator dependent procedure. Therefore, in recent years the importance of
achieving automated methods which are less operator dependent and are capable of
2
providing air void parameters of hardened concrete within shortest amount of time has
lead researchers to evaluate different ideas to automate the air void analysis of hardened
concrete.
Most of the new methods use digital image processing and contrast enhancement methods
to analyze air void system of concrete [5,6,7,11]. Rapid Air 457 and flatbed scanner methods
are two methods capable of providing air void parameter results comparable to the
standard manual test method according to a few small scale studies [7,8]. Both of these
methods are much less tedious compared to the manual test method [6,8,9,10,11,12] and
therefore, have gained popularity. For the Rapid Air 457, the operator sets a threshold level
for each analysis to distinguish air voids. The total traverse length, the length traversed in
air, the length traversed in paste, and the number of air voids intersected are provided by
this method. The air content, specific surface and spacing factor are then calculated.
Jakobsen et al. [6] describe techniques for Rapid Air 457 automatic analysis of concrete
samples as well as data from a round robin study in which samples were circulated to 7
different laboratories for automatic air void analysis. Also, for comparative study samples
were analyzed manually using linear traverse and point counting methods according to
ASTM C 457 [6]. The flatbed scanner method uses an ordinary flatbed scanner to scan the
prepared samples. Image analysis is then implemented to give air void parameters such as
air content and spacing factor. This method is very cost effective and convenient in
comparison with both the manual and Rapid Air 457 methods of analysis since it is
compromised of a computer and an inexpensive scanner and also the whole process of
scanning and analysis takes around just a few minutes depending on the brand of scanner.
3
Assessment of the amount and size distribution of entrapped air in concrete using a high
resolution flatbed scanner has also been implemented recently [13] in which image of the
curved surface with no distortion is provided by rolling the core surface on the scanner
using a traverse trolley. High resolution industrial computed tomography (CT) x-ray scan
method is another emerging technology to measure air void parameters in hardened
concrete [14, 15]. Yun et al. [14] have evaluated the ability of x-ray tomography to
quantitatively analyze the distribution of paste-void spacing in concrete. As a non-
destructive technique, CT scan can produce resolutions on the order of 10 microns or less.
However, other than cost and safety issues, small diameter samples are required to attain a
satisfactory resolution in CT scan.
Among the old and new air void system analysis methods, the flatbed scanner has been
under research and examination since scanners have become inexpensive and widely
available. In this study the flatbed scanner method of air void analysis is compared with
manual examination and Rapid Air 457 methods of analysis using 324 and 105 samples for
these comparisons respectively. Similar studies have been performed, but never with such
large population of samples [7,11,17]. A threshold optimization routine described by
Peterson et al. [8, 19] is employed along with scripts written in Adobe Photoshop to analyze
the scanned images and report the required air void parameters. For chapter 2 of this
dissertation, a set of 324 samples with air content and spacing factor parameter results
from manual examination are analyzed by the flatbed scanner method and the results are
used for comparison between the two methods. Another set of 105 samples with Rapid Air
457 parameter results are analyzed by the flatbed scanner method and the results are
4
compared and reported in chapter 3. In both of these chapters a more detailed threshold
study for image analysis of the samples has been provided.
.
5
Chapter 2
Flatbed Scanner versus Manual Method
The Ministry of Transportation of Ontario (MTO) requires air void analysis of concrete to be
conducted by approved laboratories for quality assurance as specified in the OPSS 1350
Material Specification for Concrete Materials and Production. In this regard air content of
over 3 percent (%) by volume and spacing factor below 0.23 mm are the accepted limits of
these parameters and the contractor receives a penalty by failing to achieve these limits. On
the other hand, for spacing factors between 0.1 and 0.15 mm and air contents in the range
of 4.5 to 6.5 volume %, the contractor receives bonuses [1]. Manual air void analysis is
performed according to MTO Test Method LS-431 [18] through microscopical examination
of concrete cores. However, as mentioned in chapter 1, this analysis is very time consuming
(up to several hours per sample) and operator dependent resulting in variation of the
results. Flatbed scanner as an automated method has the potential to reduce variation of
the results, and also solves time and cost issues involved with the manual method of
concrete core examination. Prior to this research, comparison between the results of this
method with the manual method has not yet been implemented in a large scale. In this
chapter the results of 324 samples analyzed by both the flatbed scanner and manual
methods are compared and discussed. Two important air void parameters, namely the air
content and spacing factor found by both methods are compared statistically. A detailed
6
study for determination of threshold using different portions of samples has also been
implemented in section 2.5.
Also, specific populations of samples were scanned with different scanner settings in order
to capture their impact on the results of air content and spacing factor (section 2.6).
Moreover, a point counting procedure is employed to estimate paste content percentage
for a population of 77 samples (section 2.7).
2.1 Sample Preparation
In order to eliminate variations between test results from a single concrete core due to
different sample preparation procedures, all of the sample surfaces coming from different
laboratories were ground, and re-polished for this study. The laboratory-prepared surfaces
were removed (about 1 mm) by applying hand pressure on a rotating water-cooled 80 grit
diamond 200 mm diameter platen shown in Figure 2.1. Then the sample surfaces were
lapped flat to within 2 μm from edge to edge for a period of 12 minutes using water and
loose 600 grit silicon carbide on the automated lapping machine shown in Figure 2.2.
7
Figure 2.1: Grinding Apparatus.
Figure 2.2: Automated Lapping Machine (Top View). Due to possible erosion of edges that define the perimeter of the entrained air-voids
exposed at the surface during polishing, a dilute solution of nitrocellulose in acetone (5:1
acetone to commercial clear fingernail polish) was applied on the polished surfaces [16]. As
a final step, surfaces were polished on a rotating lap covered with a sheet of adhesive-
backed 600 grit silicon carbide paper shown in Figures 2.3 and 2.4.
8
Figure 2.3: Adhesive-Backed 600 Grit Silicon Carbide Paper.
Figure 2.4: Final Polish Machinery. It should be considered that due to the limitations of machinery used in this study (i.e. 300
mm diameter platen of the automated lapping machine) sample sizes could not exceed 100
mm × 75 mm. Therefore, the cores were cut into two approximately equal segments
representing the top and bottom of the core. Also, the surfaces were cleaned with air-water
pressure between grinding, lapping and polishing steps.
2.2 Scanning Process
In this stage the prepared samples were scanned and analyzed to get the required
parameters such as air content and spacing factor. Before scanning, stickers were placed on
9
the corners of the polished surfaces (Figure 2.5a) to prevent the polished surface from
scratching the scanner surface.
a)
b)
Figure 2.5: Samples after a) Placing Stickers and b) Blackening and Powdering (Scale Bar Represents 20 mm).
10
An EPSON Perfection V500 photo scanner was used to collect all of the images in this study.
As a first step 24 bit color images were collected at a resolution of 125 dpm (3,175 dpi) for
further studies involving point counting which is covered in section 2.7 of this study.
A second scan is performed after the following contrast enhancement (Figure 2.5b) steps:
The polished surface of samples were darkened by drawing approximately overlapping
parallel lines with a wide tipped black opaque pigment marker (SAKURA Color Products
Corporation, Japan). Darkening was implemented in two coats, changing the orientation
90° between coats.
Then, the dry darkened surface was covered with 2 μm median size white powder
(NYCO Minerals Inc. NYAD 1250 wollastonite) and the powder was pushed into the
sample surface filling nearly all of the surface voids. A razor blade was then used to
scrape away excess powder. Also, surface was wiped off with an oily fingertip leaving a
shiny black and white surface.
A fine tipped black marker (Sharpie brand) was used to darken voids in aggregates with
the help of a stereo microscope.
After preparing the sample surfaces, 8-bit grayscale images at the resolution of 125 dpm
(3,175 dpi) were collected with all automatic image enhancement scanner software options
deactivated. A flat steel plate having black and white vinyl electrical tape was placed on top
of each sample to act as a reference defining the upper and lower brightness limits in the
analysis (Figure 2.6). In the Adobe Photoshop script, the first step is to identify the location
of the black and white reference. A histogram stretch is performed by the script on the
entire image to assign a value of zero for any portion of the sample darker than the black
11
reference and a value of 255 for any portion of the sample brighter than the white
reference. Therefore, the threshold level used to distinguish between air and non-air may
remain consistent from scan to scan.
Figure 2.6: Screen Shot of Adobe Photoshop Script Showing User Interface and Scanned Slab Image.
12
2.3 Image Analysis
As mentioned earlier, in this study the air content and spacing factor are the two
parameters required by the MTO, and these parameters were used for comparison of
flatbed scanner results with manual examination results. Image analysis for the scanned
images was implemented in Adobe Photoshop software using scripts. In many image
analysis schemes, including the flatbed scanner method, a simple threshold level must be
set in order to identify features of interest. However, a description of the threshold
determination routine used in this study first requires a full explanation of air void
parameters and formulas involved with it. Therefore, in this section these parameters and
formulas is summarized and then threshold determination will be described in details.
2.3.1 Theoretical
Air void parameters defined in ASTM C457 are as follows:
Average Chord Length ( ): the average length of the chords formed by the intersection of
the voids by the line of traverse; the unit is a length.
Paste-air Ratio (p/A): the ratio of the volume of hardened cement paste to the volume of
the air voids in the concrete.
Paste Content (p): the proportion of the total volume of the concrete that is hardened
cement paste expressed as percentage by volume.
Spacing Factor ( ): a parameter related to the maximum distance in the cement paste from
the periphery of an air void, the unit is length.
13
Specific Surface (α): the surface area of the air voids divided by their volume, expressed in
compatible units so that the unit of specific surface is a reciprocal length.
Void Frequency (n): voids per unit length of traverse; the number of air voids intercepted by
a traverse line divided by the length of that line.
Below are formulas from ASTM C457 which are used in this study to calculate air content,
void frequency and spacing factor:
Air Content (A), in %:
Void Frequency (n):
Average Chord Length ( ):
or
Specific Surface (α): α
Paste Content (p), in %:
Paste-Air Ratio (p/A):
Spacing Factor ( ) =
where:
= total length of traverse.
= traverse length through air.
= traverse length through paste.
14
2.3.2 Threshold Determination
In this study, for determination of a threshold level, manual AVS test results of MTO-
approved laboratories are used to “train” the automated scanner method [19]. In other
words, for any individual sample a threshold level exists that can best approximate the
“true” value of the air void parameter. In this study, results from MTO-approved
laboratories are taken as the true values. The threshold for the scanned image is adjusted
until the measured air void parameter by flatbed scanner analysis best matches the results
of MTO manual examination. Therefore, by analyzing a significant population of scanned
images from samples with manual AVS test results, a threshold optimization routine has
been developed and is applied to find a single threshold level that is appropriate for all of
the samples. Since air content (AC) and void frequency (VF) are the only physical
parameters measured by the flatbed scanner analysis, thresholding is implemented based
on these two values.
In the flatbed scanner method of analysis a series of lines are extracted with a total line
length on the order of 4 m. The air content parameter is determined by the number of air-
pixels in the line divided by the total number of pixels in the line. The void frequency
parameter is measured by the number of air-void intercepts with the line divided by the
total line length. From these two parameters and the paste content from mix design
information, the spacing factor can be computed. Paste content can also be determined by
manual point counts performed on the color images. This is explained in details in section
2.7.
15
Air content and spacing factor are the only two parameters with specified limits in
OPSS1350. These two values were also the only parameters provided for this study by the
MTO for more than half of the samples. Therefore, an assumed paste content of 30% was
substituted into the spacing factor formula for these samples to determine the void
frequency. The assumed value of 30% for paste content is based on typical MTO concrete
mixtures, and has been used in previous air entrainment studies [7]. For the rest of the
samples, values for specific surface were also included, along with air content and spacing
factor. Having specific surface known for these samples, the void frequency was calculated
directly using the specific surface and air content values without the need to assume a value
for paste content.
The calculation procedures are illustrated below:
If specific surface ( ) is known, void frequency ( ) can be found using cord length:
Paste content not included
If specific surface is unknown void frequency ( ) can be found using spacing factor and
assuming paste content of 30%:
Spacing Factor ( ) =
If
=
where
, and
Then
16
If
Specific surface can be found from =
Then void frequency will be found from the following:
and
Once the manual results for void frequency were calculated for each sample the ideal
threshold levels were determined using a script that iterates through different thresholds to
best approximate the manually determined values for air content and void frequency. To
perform the iterations for all of the 324 samples in this study the script required only a few
hours to find appropriate thresholds for air content and void frequency. In order to help
determine a consistent value of threshold suitable for the two parameters, the entire
population of optimum threshold levels were fit using both normal and extreme value
distributions, and the parameter μ used as the fixed threshold level (Figures 2.7 and 2.8).
Following are the equations of the two distributions:
Normal Distribution
Generalized Extreme Value Distribution
Two different approaches of thresholding were applied: a single threshold approach, and a
dual threshold approach. These two approaches were explored along with the use of
normal and extreme value distributions to identify the best case for threshold
determination. In single thresholding, the average of the void frequency and air content
thresholds was used, while, in the case of dual thresholding, separate air content and void
17
frequency thresholds were used individually. Having two different distributions, four cases
can be investigated as shown in Figures 2.7 and 2.8. Note that threshold levels must consist
of whole numbers from 0 to 255 (all threshold units in this paper were rounded to the
nearest whole number).
To find the best thresholding method, air content and spacing factor parameters were
assessed according to two types of errors, Type I and Type II. According to OPSS 1350 and
MTO, air content above 3% and spacing factor smaller than 0.23 mm are acceptable values
for these parameters. A Type II error (false-negative) represents a situation where flatbed
scanner reports a spacing factor of smaller than 0.23 mm while the true value (manual data)
is above 0.23. In the case of air content, a Type II error (false-negative) represents a
situation where the flatbed scanner recognizes more than 3% air content while the true
value is below 3%. Type II false-negative errors must be avoided, especially from the
owner’s perspective. On the other hand, a Type I error (false-positive) occurs when flatbed
scanner finds spacing factor larger than 0.23 mm, while the true value is below 0.23mm. For
air content, a Type I error (false-positive) occurs when the flatbed scanner reports a value
below 3% air content while the true value is above 3%. It should be noted that a Type I error
is not as crucial for the owner as it is for the contractor. Ideally, none of the data points
should fall within the false-positive and false–negative zones. These areas as well as the
number of data points that fall in each of them are shown in Figures 2.9 and 2.10. Based on
the above considerations, the use of a normal distribution, and dual thresholding came to
be the best choice as it minimizes both Type I and II errors.
18
a)
b)
Figure 2.7: Histograms and Probability Density Functions of Optimum Threshold Levels
Determined for Void Frequency for a) Normal Distribution with μ = 52 and b) General
Extreme Value Distribution with μ = 39.
19
a)
b)
Figure 2.8: Histogram and Probability Density Functions of Optimum Threshold Levels
Determined for Air Content Fit with a) a Normal Distribution with μ = 71, and Fit with b) a General Extreme Value Distribution with μ = 47.
20
a) b) c) d)
Figure 2.9: Influence of Threshold Selection Method on Air Content Measurements, with Assumed Paste Content of 30%: a) Dual Threshold (VF52, AC71) Normal Distribution Method, b) Single Threshold (62) Normal Distribution Method, c) Dual Threshold (VF39, AC47) Extreme Value
Distribution Method, d) Single Threshold (43) Extreme Value Distribution Method.
21
a) b) c) d) Figure 2.10: Influence of Threshold Selection Method on Spacing Factor Measurements, with Assumed Paste Content of 30%: a) Dual Threshold (VF52, AC71) Normal Distribution Method, b) Single Threshold (62) Normal Distribution Method, c) Dual Threshold (VF39, AC47) Extreme Value
Distribution Method, d) Single Threshold (43) Extreme Value Distribution Method.
22
2.4 Data Analysis
In the previous section the normal distribution dual threshold approach was chosen as the best
case, in which a threshold value of 52 was used for the void frequency and a threshold of 71
was used for the air content. Using these two thresholds the scanned images were analyzed by
the Photoshop scripts and the results of this analysis (flatbed scanner) were compared with
those of manual method. Results of all 324 samples coming from at least 8 different MTO
approved labs are plotted for both air content and spacing factor (Figures 2.11 and 2.12). In all
of the plots in this section, other than data points, a trend line of linear regression analysis and
the x=y line (called the line of equity in this study) are illustrated. The best case for our study
occurs when data are as much as possible on and around the line of equity which means that
flatbed scanner is giving the same air content or spacing factor as the manual method of AVS
analysis.
It can be seen that the results are comparable and flatbed scanner appears to output air
content and spacing factor values near to those of manual method (Figure 2.11 and 2.12).
As explained in section 2.3.2, for a smaller population of 121 samples coming from Lab 3 in this
study, specific surface parameters were known. For these samples, the void frequency was
directly calculated with no assumption for the paste content in the thresholding stage. To
explore the effect of assuming a paste content value in determination of void frequency
threshold, two sets of plots are given in this study for air content and spacing factor. Figures
2.13 and 2.14 are comparison plots of Lab 3 results for air content and spacing factor, while,
Figures 2.15 and 2.16 present all of the results excluding Lab 3.
23
A statistical analysis of the three sets of plots is reported in Table 2.1. In this table, average
deviation from zero difference between manual and flatbed scanner values are calculated as
shown below; and reported along with regression analysis of the results.
=
According to the regression analysis (Table 2.1) Lab 3 data has given a slightly better R2 value.
Also, deviation for this particular lab is to some extent less as illustrated in Table 2.1. The
performance can also be assessed through the number of false negative occurrences for
spacing factor. As shown in Figure 2.17, 14 data points fall in the false negative zone for all 324
samples (4.3%), however, in the case of lab 3 only 1 of 121 data points (0.8%) falls in this zone.
Figure 2.11: Manual versus Flatbed Scanner Air Content (All Labs).
24
Figure 2.12: Manual versus Flatbed Scanner Spacing Factor (All Labs).
Figure 2.13: Manual versus Flatbed Scanner Air Content (Lab 3).
25
Figure 2.14: Manual versus Flatbed Scanner Spacing Factor (Lab 3).
Figure 2.15: Manual versus Flatbed Scanner Air Content (Lab 3 Excluded).
26
Figure 2.16: Manual versus Flatbed Scanner Spacing Factor (Lab 3 Excluded).
Table 2.1: Statistical Analysis Results.
Samples Air Content Spacing Factor
--- Deviation
(vol%) R2
Deviation (mm)
R2
All Labs 1.634 0.44 0.049 0.36
Lab 3 1.539 0.5 0.036 0.56
Lab 3 Excluded
1.688 0.4 0.055 0.33
27
Figure 2.17: False Negative Zone for Spacing Factor (Lab 3 and All Labs).
A database containing all of the data and analysis results is provided in Appendix D (Table D-1).
Also, a detailed comparison between the results of flatbed scanner and manual method for all
other labs has been implemented and the plots are presented in Appendix A. Figures of air
content and spacing factor showing all of the lab results with different colors is presented in
Appendix A as well. Table 2.2 shows threshold values determined for individual labs.
Table 2.2: Threshold Values for All and Individual Labs.
Lab ID Optimum Air
content Threshold
Optimum void frequency Threshold
No Lab ID 42 60
Lab 1 79 69
Lab 2 44 61
Lab 3 48 78
Lab 4 66 61
Lab 5 39 27
Lab 6 94 74
Lab 7 53 84
Lab 8 41 52
Average Value 52 71
28
2.5 Threshold Study
Threshold values mentioned and used above were found using all 324 samples. In other words,
all 324 manual results were used to train the flatbed scanner analysis. However, to explore
whether a smaller population of samples can give the same threshold values, average air
content (AC) and void frequency (VF) thresholds are found for different population of samples.
The results are reported in Tables 2.3 and 2.4.
From the statistical results in tables 2.3 and 2.4, it can be concluded that half of the samples
can be used to ideally predict the required thresholds for analysis of all samples with average
deviations as small as 1.25 for air content (AC) and 3.07 for void frequency (VF). Also, choosing
one third and one fifth of the total 324 samples to find the threshold gives very close answers
to those of all samples. Selecting one tenth of samples however reports acceptable threshold
values in the case of AC only. In the case of VF threshold, choosing one tenth of samples results
in a high variance among the five populations corresponding to a lower reliability. In order to
verify the accuracy of the threshold coming from half of the samples, AC and VF deviated
threshold values (or 71+1 72 and 52+3.07 55) are used and the resulting air content and
spacing factor values are compared with those analyzed with the true threshold AC and VF
values of 71 and 52 (Figures 2.18 and 2.19). As it is obvious in these figures, choosing only half
of the samples for the purpose of thresholding and applying this threshold to all other images is
ideal for flatbed scanner analysis. Also, using threshold values of one third and one fifth of total
samples gave a threshold value of 78 (71+7 78) for AC and using this threshold gives suited
results when used for all samples analysis (Figure 2.20). Use of the threshold from one tenth of
samples for AC was not encouraging due to high variance reported in Table 2.3. In the case of
29
VF using threshold values coming from one third, one fifth, and also one tenth of samples (or
52+4 57) has given a fitted results when used for all samples analysis (Figure 2.21). This
statistical study considering that the samples used for this study are from different concrete
structures shows that employing a combination of manual and flatbed scanner methods where
only a portion of the samples need to be analyzed manually is valid for threshold determination
of flatbed scanner method. For a large number of samples similar to this study, a set of 160
samples seems to be a very good representative of all samples for threshold determination.
Table 2.3: Statistical Data of Different Sample Populations (AC).
Number of Samples out of 324 Samples
Trial # Average AC Threshold
Value (True Value = 71)
Variance and Average Deviation from True
Value
162 (half of total population)
1 71
Variance=1.5 Deviation=1
2 72
3 71
4 70
5 72
108 (one third of total population)
1 64
Variance=38 Deviation=6
2 68
3 65
4 77
5 63
66 (one fifth of total population)
1 65
Variance=52 Deviation=7
2 79
3 69
4 59
5 74
32 (one tenth of total population)
1 51
Variance=152 Deviation=12
2 67
3 75
4 79
5 87
30
Table 2.4: Statistical Data of Different Sample Populations (VF).
Number of Samples out of 324 Samples
Trial # Average VF Threshold
Value (True Value = 52)
Variance and Average Deviation from True
Value
162 (half of total population)
1 49
Variance= 9.4 Deviation=3.07
2 51
3 52
4 55
5 57
108 (one third of total population)
1 46
Variance= 11 Deviation=3.3
2 49
3 50
4 52
5 54
66 (one fifth of total population)
1 48
Variance=13 Deviation=3.6
2 50
3 51
4 52
5 59
32 (one tenth of total population)
1 51
Variance= 22 Deviation= 5
2 52
3 54
4 54
5 62
Figure 2.18: Air Content Threshold of 72 (Proposed Threshold) versus 71 (True Threshold).
31
Figure 2.19: Void Frequency Threshold of 55 (Proposed Threshold) versus 52 (True Threshold).
Figure 2.20: Air Content Threshold of 78 (Proposed Threshold) versus 71 (True Threshold).
32
Figure 2.21: Void Frequency Threshold of 57 (Proposed Threshold) versus 52 (True Threshold).
2.6 Scanner Settings
In this section, possible effects of scanner settings on the image and therefore on the results of
flatbed scanner method are investigated. Settings such as automatic brightness adjustment and
auto sharpening were chosen to be further studied in this section.
2.6.1 Automatic Brightness Adjustment
In most scanner driver software, there is the possibility for the automatic adjustment of the
brightness of the scanned image. In an 8-bit grayscale image, 0 represents the darkest and 255
the brightest pixels in an image. The EPSON scanner used in this study performs an automatic
adjustment based on the brightness histogram for each scanned image. In other words, it
adjusts the locations of the bright and dark end points, and the midpoint. A subset of the 324
33
samples was scanned using the automatic adjustment, and repeated without the automatic
adjustment, and the results are presented in Figures 2.22 and 2.23.
Figure 2.22: Without Automatic Adjustment versus Automatically Adjusted Histogram (Air
Content).
Figure 2.23: Without Automatic Adjustment versus Automatically Adjusted Histogram (Spacing
Factor).
34
As shown in Figures 2.22 and 2.23, most of the data points are on or near the line of equity
indicating that automatic brightness adjustment does not have a significant effect on the
results for both air content and spacing factor. The reference black and white vinyl electrical
tape present at the top of each scanned image (section 2.2 and Figure 2.6) is what the Adobe
Photoshop script uses as end points. Therefore, even if the end points and mid points vary from
scan to scan, as they do when the automatic brightness is used, the effect is mitigated by the
utilization of the black and white reference included in each scan.
2.6.2 Image Sharpening Setting
Another common setting in almost every scanner is automatic sharpening which is used to
make the images sharper and less blurry. An important consideration is that some of the details
in the image might be changed with automatic sharpening. Therefore, in flatbed scanner
method it is of crucial importance to have the sharpening and other auto-adjustment tools
turned off while scanning. In order to investigate how these tools can affect the results of
flatbed scanner method, a small population of the samples were scanned having auto-
sharpening turned on and the results are compared with those of normal setting (auto-
sharpening off) and presented for both air content and spacing factor in Figures 2.24 and 2.25.
As it can be seen in these figures, sharpening the images has a considerable negative effect on
the results, in the case of spacing factor, but not as pronounced in the case of air content. This
is due to the fact that sharpening enhances the contrast between brighter and darker regions.
This results in inclusion of small air bubbles that are not literally present. Therefore, Adobe
Photoshop script reports a higher void frequency, which leads to a lower value for spacing
35
factor. However, air content is not considerably affected because these added fake air voids are
very small, and therefore do not change the overall air content very much.
Figure 2.24: Sharpened versus Normal Scanner Setting (Air Content).
Figure 2.25: Sharpened versus Normal Scanner Setting (Spacing Factor).
36
2.7 Point Count
As mentioned in section 2.3 a paste content value of 30% was assumed for the purpose of
image analysis in this study. In an effort to explore whether paste content can be better
approximated, manual point counting was implemented on the 24 bit color images collected
from the samples prior to the black and white treatment. To carry out point counting, images
are divided into 500 frames with a cross-hair at the centre in Adobe Photoshop using a script.
Then, for each frame, an operator answers “yes” or “no” as to whether or not the cross-hair is
on an aggregate. With this manual point counting aggregate fractions are determined as
follows:
where
P = Paste content (vol. %) where paste is defined as the total concrete volume minus the aggregate and air volumes. A = Air content (vol. %) from automated AVS test result. Agg = Aggregate content (vol. %) from manual point count.
A population of 77 samples coming from Lab 3 was selected for point counting in this research.
Spacing factor values calculated with individual paste content values coming from point
counting were compared with those of calculated using the 30% paste content assumption
(Figure 2.26). Furthermore, manual spacing factor values are plotted with flatbed scanner
spacing factors calculated based on individual point count results (Figure 2.27) and with flatbed
scanner spacing factor values resulted from 30% assumed paste content (Figure 2.28). As
illustrated in Figure 2.26 the flatbed scanner reports slightly higher spacing factor values using
point counted paste content results. Deviation from manual results in both cases of point
37
counting paste content results and 30% paste content assumption are 0.047 and 0.036 mm
respectively. It should be considered that although this point counting method can assist in
predicting the paste content values, it is operator dependent and in this study only one
operator has performed the point counts. A table containing individual point counting results is
provided in Appendix C (Table C-1).
Figure 2.26: Spacing Factor Comparison of Point Counting and 30% Assumed Paste Contents.
38
Figure 2.27: Spacing Factor Comparison of Manual and Flatbed Scanner 30% Assumed Paste Contents.
Figure 2.28: Spacing Factor Comparison of Manual and Flatbed Scanner 30% Assumed Paste Contents.
39
Chapter 3
Flatbed Scanner versus Rapid Air 457 Method
Rapid Air 457 method of analysis is being used by many companies and universities around the
world and as previously mentioned it is capable of providing air void parameter results
comparable to the ASTM C457 standard [7, 10]. In this chapter this automated method is
compared with the flatbed scanner method of air void analysis. Similar studies have been
performed comparing the Rapid Air 457 and flatbed scanner methods, but never with such a
large population of samples as in this study [7,11,17].
In order to investigate the correlation between flatbed scanner results with those of Rapid Air
457, air content and spacing factor results of 105 samples analyzed by both the Rapid Air 457
and flatbed scanner methods are found and compared. Other than air content and spacing
factor normalized void frequency chord length distribution graphs from both methods are
plotted and studied. Also, different populations of samples are studied to find a specific number
of samples which would be adequate to find an acceptable threshold for image analysis of all
samples.
This chapter includes materials that are also presented in a paper by the author that is accepted
for publication in the proceedings of the 35th annual meeting of the international cement
microscopy association (ICMA).
40
3.1 Sample Preparation
For both automated methods, the sample surfaces need to be flat and smooth, and clear of any
rough texture. Polishing was performed on a Struers Abripol machine, using a steel screen
embedded with 250 µm diamond. The screen attaches to a magnetic rotating platen. The
specimen is then attached to a sample holder via double sided tape. A yellow crayon is used to
draw a crisscross grid across the surface to be polished. The sample holder is then attached to
the polisher head and the process begins. Depending on the condition of the screen and
concrete, the yellow crayon is removed in as little as 35 seconds or up to 2 minutes. The 250
µm screen is then changed for the 68 µm screen and the same process is repeated for 1 to 2
minutes. Note that water is used as the lubricant for this process (Figure 3.1).The specimen
surface is then rinsed with water, patted dry and then held under an air hose to remove water
from voids until completely dry. At this point, the surface appears very smooth with a light
sheen. A 12-inch cast iron lapping platen with concentric circles cut into the surface is then
placed onto the Abripol machine along with attaching the specimen holder. The machine runs
for two minutes while it is wetted with a splash containing a solution of 800 grit silicon carbide
(12 µm) and water (10/90 grit/water ratio) (Figure 3.2). This process is then repeated with a
1000 grit silicon carbide (7 µm). The specimen is then rinsed, dried with air hose and placed into
a 100 oven for 2 hours. Figure 3.3 shows examples of the polished surfaces.
41
Figure 3.1: Polishing Apparatus.
Figure 3.2: Surface Lapping with Water and Silicon Carbide Grit Suspension.
For contrast enhancement of the samples, they are blackened with an inked stamp pad and left
for 30 minutes to dry followed by gently removal of the excess ink with a soft, lint free cloth.
Then, a non-abrasive white powder (barium sulfate, BaSO4) of small enough particle size (3-7
µm) is applied to fill all the voids on the blackened polished surface. A small vibrating table is
used to ease the filling voids with the powder. The excess powder is removed using the palm of
the hand. Note that a very small amount of mineral oil can be rubbed onto the palm to enhance
the removal process.
42
Figure 3.3: Polished Slabs (Left) and Close-up (Right).
Finally, the aggregates are blackened with a marker to exclude aggregate voids (Figure 3.4), and
the sample placed on the Rapid Air 457 stage for analysis (Figure 3.5). A camera and a
motorized stage are used to collect digital images of the prepared surface. The pixel dimensions
in the images are 1.1 by 1.1 µm, which is suitable for the detection of air void intercepts with
diameters on the order of 10 µm. The operator sets a threshold level for each analysis to detect
the air voids. Traverse lines are applied to the images. The total traverse length, the length
traversed in air, the length traversed in paste, the number of air voids intersected, and the
chord length distribution are provided by this method. The air content, specific surface, and
spacing factor are then calculated, using a pre-determined value for paste content [12,20].
43
Figure 3.4: Slab after Blackening and White Powder (Left) and after Threshold Application (Right).
Figure 3.5: Wiping Away Excess Powder (Left) and Performing Analysis (Right).
3.2 Image Collection and Analysis
After analysis by the Rapid Air 457, small stickers were placed at the four corners of each
sample, and the samples were scanned in 8-bit grayscale at a resolution of 125 dpm (3,175 dpi)
44
with the EPSON scanner similar to chapter 2. Then, the same image analysis in Adobe
Photoshop as is explained in chapter 2 was applied to find AVS parameters.
Air content and void frequency are the only physical parameters directly measured by both the
Rapid Air 457 and the flatbed scanner methods. The spacing factor is calculated using these
parameters, along with a pre-determined value for the paste content. In this study, the paste
content was computed from the mix design. For the Rapid Air 457, a single threshold level is
selected by the operator. For the flatbed scanner routine employed here, two thresholds are
used: one to determine air content, and another to determine void frequency as explained in
chapter 2 as well. This approach is used to minimize errors that may occur due to the
competing nature of the air content and void frequency parameters. To efficiently detect small
voids, the threshold level tends to be set lower, which at the same time will inflate the values
for air content. By performing the thresholding separately for each parameter, the errors are
minimized [19]. In order to find a set of appropriate thresholds, a subset of samples for which
the air void parameters are already known are analyzed iteratively by the script. The threshold
levels that yield the closest match to the air content and spacing factor values reported by the
Rapid Air 457 for each individual sample are recorded. Then, a normal distribution is used to
find the average value for each threshold (Figure B-1 in Appendix B). These values are in turn
set as constant for all of the analyses. Using the entire population of 105 samples, threshold
values of 132 and 115 are found for air content and void frequency respectively. Again, it
should be noted that due to the discrete nature of the 8-bit grayscale images, threshold values
are restricted to integer values.
45
3.3 Air Content and Spacing Factor Results
The results of the two automated methods are plotted in Figures 3.6 and 3.7. In these plots
both the trend line of the linear regression analysis and the x=y line (the line of equity) are
illustrated. As shown in Figures 3.6 and 3.7, there exists a reasonably good correlation between
the results of both methods. Average deviation from difference of zero (deviation from equity
line) between the results is 0.64 vol % in the case of air content and 0.06 mm for spacing factor
(Table 3.1). Regression analysis of the results yields an R2 value of 0.88 for both the air content
and spacing factor (Table 3.1). Although the correlation is reasonably good, the flatbed scanner
appears to consistently overestimate the spacing factor at values above 0.3 mm relative to the
Rapid Air 457 results. Furthermore, there are a few notable outliers present in the spacing
factor data. It should be noted that since spacing factor values higher than 0.3 mm are well
beyond the MTO limit of 0.23 mm, the slight overestimation by the flatbed scanner is not as
crucial; that is, samples that have exceeded the acceptable limit are still readily identified. A
database containing all of the data and analysis results is provided in Appendix E (Table E-1).
46
Figure 3.6: Air Content Comparison of Flatbed Scanner and Rapid Air 457.
Figure 3.7: Spacing Factor Comparison of Flatbed Scanner and Rapid Air 457.
47
Table 3.1: Statistical Analysis Results.
Air Content Spacing Factor
Deviation (%) R2 Deviation (mm) R2
0.64 0.88 0.06 0.88
3.4 Chord Length Distribution
A comparison of chord length distributions of the two methods was also performed. The
comparison is complicated by the fact that two different thresholds are used by the flatbed
scanner method, which would yield two different chord length distributions, one resulting from
the void frequency threshold level of 115, and another resulting from the air content threshold
level of 132. In order to compare the two methods, the chord length distribution resulting from
the void frequency threshold is used to include the smallest air voids detected by the scanner.
Figure 3.8a illustrates distributions of the two methods for a single sample with air content and
spacing factor values within the “good” range (air content = 4.74 vol. %, spacing factor = 0.14
mm). It can be seen that for chord lengths above 0.07 mm, the flatbed scanner reports slightly
more air void intercepts than the rapid Air 457, and below 0.07 mm, the flatbed scanner
reports slightly less air void intercepts than the Rapid Air 457. This same pattern is also seen for
the rest of samples. To also explore the influence of choice of threshold level for the flatbed
scanner method, Figure 3.8b shows the chord length distribution obtained using a threshold
level of 140. The difference between the flatbed scanner chord length distributions at
thresholds of 115 and 140 is negligible.
48
Figure 3.8: Chord Length Distributions from Rapid Air 457, and Flatbed Scanner at Thresholds of
a) 115 and b) 140.
49
In an associated comparative investigation undertaken at W.R. Grace, a selection of these same
‘reference’ samples, which had been previously tested in the Grace Laboratory in Cambridge,
were also tested using the University of Toronto Rapid Air 457 and on a new second Rapid Air
457 machine at the Grace Laboratory. The results of chord length distribution and associated
data obtained for the second Grace Rapid Air 457 equated reasonably well with the data shown
in Figure 3.8 for the Toronto Rapid Air 457 machine for the selected samples. However, it was
also noted that the frequency of the extremely fine chord lengths as indicated by the peaks at
0.03mm was noticeably higher on both the Toronto machine and new Grace machine, than had
been previously recorded on the older Grace machine. This effect was most pronounced for a
coarser mean sized air void reference sample. Close examination of the camera and associated
threshold imaging on the new Rapid Air 457 machine indicated that additional ‘noise’ was now
being included in the subsequent air void analysis, this being primarily recorded as an increase
in the frequency of the very fine chord lengths, such as shown in Figures 3.8. Fine tuning of the
image capture settings was subsequently able to remove this effect giving the new machine
comparable results to the older Grace system, without significantly affecting the other air void
parameters.
3.5 Threshold Study
To explore whether a smaller population of samples can give the same threshold values,
average void frequency thresholds are found for different populations of samples. The results
are as follows in Table 3.2.
50
Table 3.2: Statistical Data of Different Sample Populations.
Number of Samples out of 105 Samples
Trial #
Average Void Frequency
Threshold Value (True Value =115)
Variance and Deviation from
True Value
52 (half of total population)
1 106
Variance=18.96
Deviation=4.5
2 113
3 115
4 116
5 119
35 (one third of total population)
1 108
Variance=28.96
Deviation=5.4
2 112
3 116
4 120
5 123
21 (one fifth of total population)
1 109
Variance=45.04
Deviation=8.6
2 118
3 121
4 126
5 128
The statistical data from Table 3.2 shows that choosing only half of the samples to explore a
threshold for all samples gives a very close answer to that of coming from all samples, with a
deviation as small as 4.5. On the other hand, choosing one third and one fifth of total
population to find the threshold, also gives small amount of deviation. However, it must be
noted that in comparison with the population containing half of the samples, third and fifth
portions introduce a higher variance. In order to verify the accuracy of the threshold coming
from half of the samples, the most deviated value (or 115+4.5 120) is used as a threshold and
the spacing factors are compared with those analyzed with the true threshold value of 115
(Figure 3.9). As shown in Figure 3.9, choosing only half of the samples for the purpose of
thresholding and applying this threshold to all other images appear sufficient for flatbed
scanner analysis. This statistical study shows that only a portion of the Rapid Air 457 samples
need to be used for determination of appropriate threshold levels for the flatbed scanner.
51
Figure 3.9: Comparison of Flatbed Scanner Spacing Factor Results Using Void Frequency
Threshold Levels of 115 and 120.
.
52
Chapter 4
Conclusion
The flatbed scanner as an automated method for air void analysis is investigated in this study.
Contrast enhancement and image analysis has been used to measure air content and spacing
factor based on threshold optimization of prepolished scanned concrete samples. The
performance of this method has been studied by comparing the results of air content and
spacing factor with those of manual ASTM C457 method for a large population of 324 samples
received from MTO. It is shown that the results of both methods are in an acceptable
agreement with 1.6 volume percent of average air content and 0.049 mm of spacing factor
deviation. However, for a smaller population of the samples where more manual information
(i.e. void frequency) was known, the deviation came to be less (1.54 volume percent deviation
for air content and 0.036 mm deviation for spacing factor). Different populations of samples
were also studied to find a specific number of training samples which would be adequate to
find an acceptable threshold for image analysis of all samples. Neither the Rapid Air 457 nor the
flatbed scanner approaches provide a value for paste content. An assumed paste content of
30% was used in this study which affects the accuracy of the results. Therefore, a point
counting method was explored on scanned images from a subset of samples and the results of
spacing factor were compared with those of 30% assumed paste content and a deviation
difference of 0.01 mm was captured between the two results. The most important drawback of
flatbed scanner method is considered to be its dependency on the determination of threshold
values based on manual test results. To investigate this issue, a threshold study is also
53
performed and showed that smaller population of samples can be used to report a threshold
representing that of coming from all samples. Therefore, to examine a large population of
samples (e.g. above 300 samples) a smaller population of those samples (i.e. 160 samples) need
to be examined by the manual method to report the required threshold values for flatbed
scanner method analysis. This approach can lead to an invaluable air void analysis considering
flatbed scanner method’s time efficiency and less operator dependency.
Furthermore, a comparison of flatbed scanner and Rapid Air 457 methods of air void analysis is
performed in this study. In addition to air content and spacing factor values, chord length
distribution from both of methods are plotted for this comparison study. In the case of both air
content and void frequency flatbed scanner reported very similar values to those of Rapid Air
457 (0.64 volume percent and 0.06 mm deviation). In this case the threshold study also showed
that a smaller population of samples can be a very good representative of all samples for
threshold determination. Also, comparison of the normalized chord length distributions
showed that for chord lengths above 0.07 mm, the flatbed scanner reports slightly more air
void intercepts than the Rapid Air 457, and below 0.07 mm, the flatbed scanner reports slightly
less air void intercepts than the Rapid Air 457. One possible explanation for the discrepancy
could be that the higher resolution of the Rapid Air 457 images (1.1 x 1.1 µm pixels) allows for
the detection of air voids too small for detection buy the flatbed scanner method (8 x 8 µm
pixels).
Finally, it is concluded that the flatbed scanner as a time and cost saving method is fairly
qualified to output results that are in agreement with the manual and Rapid Air 457 methods
54
and it can be effectively used in combination with any of them. Future work could expand on
the effect of sample preparation on the results of flatbed scanner method.
55
References
[1] Ontario Provincial Standard Specification, “Material Specification for Concrete – Materials and Production”, OPPS.PROV 1350, (2010). [2] Sutter, L. L., “Evaluation of Methods for Characterizing Air-Void Systems in Wisconsin Paving Concrete”, Wisconsin Highway Research Program, WHRP 07-05, (2007). [3] Snyder, K. A., “A Numerical Test of Air-Void Spacing Equations”, National Institute of Standards and Technology Building Materials Division, Gaithersburg, (1998). [4] ASTM Standard C457: Standard Test Method for Microscopical Determination of Parameters of the Air-void in Hardened Concrete, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, USA, (2012). [5] Zhang, Z., Ansari, F., and Vitillo, N., “Automated determination of entrained air-void parameters in hardened concrete”, ACI Materials Journal, 102 (1), (2005). [6] Jakobsen, U. H., Pade , C., Thaulow , N., Brown, D., Sahu, S., Magnusson, O., De Buck, S., and De Schutter, G., “Automated air void analysis of hardened concrete — a Round Robin study”, Cement and Concrete Research, 36, 1444–1452, (2006). [7] Ramezanianpour, A. M., Hooton, R. D., and Dean, S., “Evaluation of Two Automated Methods for Air-Void Analysis of Hardened Concrete”, Journal of ASTM International, V. 7 (2), (2009). [8] Peterson, K., Carlson, J., Sutter, L., and Van Dam, T., “Methods for threshold optimization for images collected from contrast enhanced concrete surfaces for air-void system characterization”, Materials Characterization, 60, 710-715, (2009). [9] Mateusz, R., Olek, J., Zhang, Q., and Peterson, K., “Evaluation of the critical air-void system parameters for freeze-thaw resistant ternary concrete using the manual point-count and the flatbed scanner methods”, Journal of ASTM International, v 7, n 4, April (2010). [10] Peterson, K. W., Swartz, R. A., Sutter, L. L., and Van Dam, T. J., “Air Void Analysis of Hardened Concrete with a Flatbed Scanner”, 24th ICMA proceedings. San Diego, CA, April 8–11; (2002). [11] Carlson, J.; Sutter, L.; Van Dam, T.; and Peterson, K., “Comparison of Flatbed Scanner and Rapid Air 457 System for Determining Air Void System Parameters of Hardened Concrete”, Transportation Research Record (Concrete Materials), n 1979, p 54-59, (2006).
56
[12] Zalocha, D., and Kasperkiewicz, J., “Estimation of the Structure of Air Entrained Concrete Using a Flatbed Scanner”, Cement and Concrete Research, 35, 2041 – 2046, (2005).
[13] True, G., Searle, D., Sear, L., and Khatib, J., “Voidage assessment of concrete using digital image processing”, Magazine of Concrete Research, 62, 857–868, (2010). [14] Yun, T.S., Kim, K.Y., Choo, J., and Kang, D. H., “Quantifying the distribution of paste-void spacing of hardened cement paste using X-ray computed tomography”, Material Characterization, 73, 137–143, (2012). [15] Kim, K.Y., Yun, T.S., Choo , J., Kang, D.H., and Shin, H.S., “Determination of air-void parameters of hardened cement-based materials using X-ray computed tomography”, Construction and Building Materials, 37, 93–101, (2012). [16] Roberts, L. R., Scali, M. J., “Factors Affecting Image Analysis for Measurement of Air Content in Hardened Concrete,” Proceedings of the 6th International Conference on Cement Microscopy, Albuquerque, NM, USA, March 26-29, pp. 402-419, (1984). [17] Jana, D., “A Round Robin Test on Measurements of Air Void paramewters in Hardened Concrete by Various Automated Image Analyses and ASTM C457 Methods,” Proceedings of the 29th International Conference on Cement Microscopy, (2007). [18] Ministry of Transportation, Ontario Test Method LS-431, “Method of Test for MicroscopicaL DETERMINATION OF AIR VOID SYSTEM PARAMETERS IN HARDENED Concrete, for Referee Testing”, Laboratory Testing Manual, Rev. No 26., (2001). [19] Peterson, K., Sutter, L., Radlinski, M., “The Practical Application of a Flatbed Scanner for Air-Void Characterization of Hardened Concrete” Journal of ASTM International, v. 6, n. 9, (2009). [20] Elsen J., “Automated Air-Void Analysis on Hardened Concrete – Results of a European Intercomparison Testing Program,” Cement and Concrete Research, vol. 31, pp. 1027-1031, (2001).
57
Appendix A:
Comparison Plots of Manual and Flatbed Scanner for Different Labs
Figure A.1: Manual versus Flatbed Scanner Air Content (All Labs).
Figure A.2: Manual versus Flatbed Scanner Spacing Factor (All Labs).
58
Figure A.3: Manual versus Flatbed Scanner Air Content (No Lab ID).
Figure A.4: Manual versus Flatbed Scanner Spacing Factor (No Lab ID).
59
Figure A.5: Manual versus Flatbed Scanner Air Content (Lab 1).
Figure A.6: Manual versus Flatbed Scanner Spacing Factor (Lab 1).
60
Figure A.7: Manual versus Flatbed Scanner Air Content (Lab 2).
Figure A.8: Manual versus Flatbed Scanner Spacing Factor (Lab 2).
61
Figure A.9: Manual versus Flatbed Scanner Air Content (Lab 3).
Figure A.10: Manual versus Flatbed Scanner Spacing Factor (Lab 3).
62
Figure A.11: Manual versus Flatbed Scanner Air Content (Lab 4).
Figure A.12: Manual versus Flatbed Scanner Spacing Factor (Lab 4).
63
Figure A.13: Manual versus Flatbed Scanner Air Content (Lab 5).
Figure A.14: Manual versus Flatbed Scanner Spacing Factor (Lab 5).
64
Figure A.15: Manual versus Flatbed Scanner Air Content (Lab 6).
Figure A.16: Manual versus Flatbed Scanner Spacing Factor (Lab 6).
65
Figure A.17: Manual versus Flatbed Scanner Air Content (Lab 7).
Figure A.18: Manual versus Flatbed Scanner Spacing Factor (Lab 7).
66
Figure A.19: Manual versus Flatbed Scanner Air Content (Lab 8).
Figure A.20: Manual versus Flatbed Scanner Spacing Factor (Lab 8).
67
Appendix B
Normal Distributions for Threshold Determination of Rapid Air 457 versus Flatbed
Scanner
Figure B.1: Optimum Threshold Value Distributions for a) Air Content and b) Void Frequency.
68
Appendix C
Point Count Method Detailed Results
69
Table C.1: Point Count Detailed Results.
70
Continued...
71
Appendix D
Flatbed Scanner versus Manual Method Database
72
Table D.1: Flatbed Scanner versus Manual Method Database.
*
73
Continued…
74
Continued…
75
Continued…
76
Continued…
77
Continued…
78
Continued…
79
Continued…
80
Continued…
81
Continued…
82
Continued…
83
Continued…
84
Continued…
85
Continued…
86
Continued…
87
Continued…
88
Continued…
89
Continued…
* In the aggregate type column, Igneous Meta stands for Igneous Metamorphic.
90
Appendix E
Flatbed Scanner versus Rapid Air 457 Database
91
Table E.1: Flatbed Scanner versus Rapid Air 457 Database.
92
Continued...
93
Continued...