Analysis of Microcrack Behavior in Mass Concrete

ANALYSIS OF MICROCRACK BEHAVIOR IN MASS CONCRETE

ENRIQUE J. VILLAVICENCIO CAMACHO

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN BUILDING CONSTRUCTION

UNIVERSITY OF FLORIDA

2006

Enrique J. Villavicencio Camacho

To my father: thank you for your hard work, sacrifice, and dedication. Your priority in life has been fulfilled: to provide all your children with an education. I admire you and love you always.

This work is also dedicated to my wife, Bárbara. Thank you for your patience, love, and support.

ACKNOWLEDGMENTS

Thank you to the thesis supervisory committee, Dr. Abdol Chini, Dr. Ajay Shanker, and

Dr. Robert Stroh, for their guidance and advice. Special thanks to Dr. Abdol Chini, director of the University of Florida M.E. Rinker, Sr. School of Building Construction, for presenting the challenge and opportunity of working on this project.

Sincere thanks are due to the staff of the Florida Department of Transportation Materials

Research Laboratory for providing the funding and facilities to conduct this project. I wish to thank Tanya Riedhammer for her assistance in the SEM imaging, and Omar Osmanzai of Allied

High Tech, Inc. for his assistance in the image analysis applications. Finally, sincere appreciation is due to Hassan Mozahab for his assistance and collaboration on this project.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF TABLES...... 7

LIST OF FIGURES ...... 8

ABSTRACT...... 10

CHAPTER

1 INTRODUCTION ...... 12

Background...... 12 Cracking...... 12 Objective...... 14 Scope of Work ...... 14 Literature Review ...... 14 Sample Selection and Specimen Preparation Technique ...... 14 Imaging...... 15 Data Analysis...... 16 Summary...... 16

2 LITERATURE REVIEW ...... 17

Introduction...... 17 Mass Concrete ...... 17 Studying Microcracks in Concrete ...... 18 Specimen preparation procedures...... 19 Image analysis procedures...... 19 Microcrack quantification procedures...... 19 Specimen Preparation Techniques...... 20 Impregnation Techniques ...... 21 Epoxy Impregnation Technique ...... 21 Wood’s Metal Method...... 23 Image Processing and Analysis ...... 23 Microcrack Quantification...... 25

3 SPECIMEN PREPARATION METHODOLOGY...... 28

Introduction...... 28 Concrete Sample Selection...... 28 Sample Preparation Technique...... 31 Lab Procedures for Sample Preparation ...... 32 Diamond Saw Cutting ...... 32

Sectioning Procedure...... 32 Epoxy Impregnation Procedure...... 33 Grinding and Polishing Procedures...... 36 Preparation for Imaging...... 39 Specimen Preparation Test ...... 40 Oven-Dried Specimens...... 41 Summary...... 41

4 IMAGE ANALYSIS METHODOLOGY ...... 43

Introduction...... 43 Image Acquisition...... 43 Image Analysis and Microcrack Quantification...... 46 Summary...... 51

5 RESULTS...... 52

Introduction...... 52 Test for Validity of Specimen Preparation Procedure...... 53 Statistical Analysis Method...... 53 Analysis of Effect of High Curing Temperatures on Microcracking ...... 56

6 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ...... 60

Summary...... 60 Conclusions...... 61 Recommendations...... 62

APPENDIX

A IMAGE ANALYSIS DATA RESULTS AND ANOVA STATISTICS...... 64

B GLOSSARY ...... 69

LIST OF REFERENCES...... 70

BIOGRAPHICAL SKETCH ...... 72

LIST OF TABLES

Table page

3-1 Concrete sample mixes and labels...... 30

3-2 Adiabatic temperature rise data for concrete with cement type B, 95°F placing temperature, and high temperature curing...... 31

3-3 Mixing schedule of epoxy kit per Structure Probe, Inc. product manual...... 35

3-4 Grinding and polishing procedure for manual preparation of specimens with MPrep3 and consumables by Allied High Tech Products, Inc...... 38

3-5 Concrete specimen preparation methods for microstructural analysis of concrete...... 42

4-1 Concrete specimen and images list...... 45

4-2 Length of noise, microcracks and voids at three different magnification factors (Soroushian, 2003)...... 48

5-1 Microcrack density values for oven-dried (OD) and regular specimens...... 54

5-2 Microcrack density values for high temperature (HT) and room temperature (RT) specimens...... 57

A-1 Results of one-way ANOVA for density values of oven-dried (OD) and standard specimens...... 64

A-2 Results of one-way ANOVA for density values for room temperature (RT) and high temperature (HT) sample specimens...... 65

A-3 Specimen image analysis database...... 66

LIST OF FIGURES

Figure page

2-1 Schematic diagrams showing (a) a basic SEM column, (b) electron beam interaction volume with a sample (Bunday et al., 2005)...... 24

3-1 Concrete samples. A) 95B00PRT and B) 95B00PHT...... 30

3-2 Diamond blade saw used for initial cutting operation at the FDOT Materials Research Lab in Gainesville, Florida...... 32

3-3 Apparatus and lubricant for sectioning procedure. A) Trim saw and B) Sectioning fluid lubricant #60-20110 by Allied High Tech Products, Inc...... 33

3-4 Section view of remnant specimen after red-dyed ethanol replacement...... 34

3-5 Ultra-low viscosity epoxy kit by SPI Industries...... 34

3-6 Failure to polymerize epoxy solution in final stage of replacement...... 35

3-7 Concrete specimens in mounting cups inside the oven...... 36

3-8 MPrep3 grinder and polisher at the University of Florida School of Building Construction Concrete and Soils Lab...... 37

3-9 Various consumables used for grinding and polishing procedure...... 38

3-10 Step 6 of the polishing procedure A) Specimen polished with a 0.05 m colloidal alumina suspension. B) Specimen dried after polishing...... 39

3-11 Extra fine point permanent marker by Pilot used for specimen grid markings...... 39

3-12 Polished specimen ready for imaging under SEM...... 40

4-1 SEM equipment at the University of Florida Advanced Material Characteristic Lab in Gainesville, FL, model Hitachi S-3000N...... 43

4-2 Specimen grid imaging system A) Finished specimen prior to imaging, B) Grid labeling system...... 45

4-3 Screen capture of the segmentation step in the image analysis program...... 47

4-4 Screen capture of the measurement condition filter using the FibreLength measuring tool...... 47

4-5 Screen capture of the final step in microcrack detection with manual selection of microcracks and other features...... 49

4-6 Output image from AxioVision software ...... 50

5-1 Average microcrack density comparison of oven-dried and non-dried specimens...... 53

5-2 Box plots of microcrack densities for oven-dried (OD RT & OD HT) and non-dried (RT & HT) sample specimens...... 55

5-3 Average microcrack density comparison of room temperature and high temperature sample specimens...... 58

5-4 Box plots of microcrack densities for room temperature (RT) and high temperature (HT) sample specimens...... 59

6-1 Microcrack formation output images. A) Oven-dried specimen, B) Regular specimen...... 62

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science in Building Construction

ANALYSIS OF MICROCRACK BEHAVIOR IN MASS CONCRETE

Enrique J. Villavicencio Camacho

December 2006

Chair: Abdol Chini Major Department: Building Construction

Mass concrete applications in Florida require controls to monitor temperature gradients

between the core and surface of mass concrete structures. Florida Department of Transportation

(FDOT) structural standards specify maximum allowable temperature differentials of 35°F.

FDOT specifications only require control of gradients and not maximum temperatures. The

objective of this project was to determine if high temperature levels during curing increase

microcracking in mass concrete.

Historically, FDOT mass concretes have experienced temperatures between 180°F and

200°F during curing. This study examined the potential of microcrack development at high

temperatures and provides recommendations towards the potential need to monitor maximum

curing temperatures during concrete pours.

Concrete samples were prepared with FDOT design mixes using cements from Florida suppliers with high heat of hydration characteristics. The concrete samples were maintained in adiabatic conditions during curing, and test specimens were prepared from these samples. Two types of specimens were evaluated in this study, a room temperature cured specimen and a high temperature cured specimen. The specimens were prepared for examination under scanning electron microscope. Identification of microcracks was achieved with an image analysis

threshold process that distinguished noise, voids, and microcrack features in the image field.

The image analysis software quantified the total length of microcracks in each image. Crack densities for each sample were calculated and a statistical comparison of crack densities from high temperature and room temperature specimens was performed. Results showed no significant difference in microcrack development in the specimens prepared from high temperature cured samples. A recommendation to modify FDOT mass concrete structure design standards could not be justified from the basis of microcrack formation.

CHAPTER 1 INTRODUCTION

Background

The American Concrete Institute (ACI) defines mass concrete as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking” (ACI 207.1R-

96, 2003). The Florida Department of Transportation (FDOT) Structures Design Guidelines specify that during mass concrete pours, contractors must ensure that temperature differentials between the core of the concrete structure and its surface do not exceed 35°F (FDOT, 2002).

Many of these structures are built for bridge foundations, bridge piers, abutments, and dams amongst others. When constructed, these concrete structures experience increased temperature levels during curing. Cement hydration is the exothermic chemical reaction that produces the temperature rise in the material. Improper monitoring of temperature during curing can lead to cracking, reduced service life, and loss of structural integrity.

Cracking

Mass concrete practices were largely developed from concrete dam construction where temperature-related cracking was first identified. Temperature-related cracking has also been experienced in other thick-section concrete structures, including mat foundations, pile caps,

bridge piers, thick walls, and tunnel linings. Cracks in any direction in any type of building

material regardless of their dimensions signify volume changes in the material. In the case that

the volume change is sufficient to produce cracks of appreciable width, the structural integrity of the material may be seriously affected. Random cracking from material-related causes can pass through a massive concrete element and the crack widths can vary from hairline to wide (ACI

224R-01, 2001). Hairline cracks inside a structure cannot be identified until they propagate later

in the life of the concrete. The long-term formation of a crack may result from slow propagation of networks of microscopic cracks (“microcracking”).

The spawn of microcracks due to elevated temperatures presents an area of concern for engineers and contractors building mass concrete structures. Microcracks can propagate to stress induced cracks or thermal stressed cracks after concrete hardening. Noticeable increases in microcrack density during early stages of curing, may affect the long term structural integrity of mass concrete structures. Furthermore, crack formation during curing will have adverse effects on concrete since it is most susceptible to environmental factors during curing than when it is fully hardened. Microcrack features adversely affect durability and can be associated with the initiation of water and contaminant seepage into the structure.

If found that an effect of curing mass concrete in extreme high temperature conditions is the formation of additional cracks due to high temperature rise, methods will need to be implemented to not only control temperature differential, but also total temperature rise. In order to test for the formation of microcracks under these conditions, a simulation where concrete is maintained at elevated curing temperatures without temperature gradients must be prepared.

As specified by ACI, the reduction in temperature of mass concrete between the surface and interior must be limited to the temperature differential dictated by the tensile strength of the concrete at that age (ACI 224.1R-01, 2001). FDOT requires thermal control plans to maintain temperature differentials of 35°F or less between the core and the surface. However, FDOT does not set a maximum temperature rise requirement for the concrete, only a differential. This presents the following problem: Does concrete reaching high temperatures during curing, say between 180°F to 200°F, experience crack formation due to their elevated temperature conditions?

The type of cracking that was studied in this project is microcracking. Several methods of analyzing concrete microstructures have been implemented in past research. The specimen preparation and imaging techniques used for this project are described in detail in Chapters 3 and

Objective

Research was needed to investigate if high temperatures in mass concrete result in crack formation. This study was an extension of the research project “Adiabatic Temperature Rise in

Mass Concrete” performed by Chini and Parham for FDOT (Chini et al., 2005) and the project

“Detection of Microcracks in Mass Concrete Cured at Elevated Temperatures” performed by

Shah (Shah, 2004) as masters thesis for the University of Florida School of Building

Construction.

The objective of this project was to identify if high temperature conditions during curing result in significant differences in microcrack formation. As part of the continuation of the aforementioned research projects it was also an objective to improve on the past efforts of specimen preparation and image analysis techniques.

Scope of Work

Literature Review

A comprehensive review on previously performed research studies on concrete specimen preparation for microscopic evaluation was undertaken. Additionally, prior research on image processing and analysis of microcrack observation and quantification was also investigated.

Sample Selection and Specimen Preparation Technique

In the preceding study conducted at the University of Florida, “Adiabatic Temperature

Rise of Mass Concrete in Florida” (Chini et al., 2005), concrete samples were prepared using different mix designs, placing temperatures, and curing temperatures. Some of the concrete

samples were cured in heating chambers that simulated the core temperature conditions of mass concrete. Meanwhile, a second set of samples prepared with the same mix designs were cured under room temperature conditions. A selection of these samples was used to test for microcrack formation. First, the specimen preparation technique used to prepare the concrete specimens for microscopic imaging was tested for validity. The first step in performing this study was to select two sets of samples from the mix design that experienced the highest curing temperature conditions and its room temperature cured counterpart. Smaller specimens are then cut from the concrete samples and later embedded in epoxy and prepared for microscopic analysis.

One of the challenges presented by this project was to establish a specimen preparation method that will yield the closest approximation to the natural conditions of mass concrete at its core, without the introduction of secondary microcracks and excessive experimental error. The concrete will be analyzed under the assumption that the conditions found in the sampled images will be representative of the in situ conditions of the concrete. Since the samples used for this study were prepared under laboratory controlled adiabatic conditions, factors of influence such as tri-axial stresses and thermal induced stresses, which are present in all applications of concrete during and after placement, will be ignored. This presents an experimental limitation for the modeling. However the adiabatic controlled conditions under which the samples were prepared provide the most viable alternative for our model and the objectives of this study.

Several specimen preparation techniques use in the materials microscopy industry were considered. Upon choosing the most appropriate technique for our study, the technique will be utilized for specimen preparation.

Imaging

The images generated from the microscope are analyzed with computer imaging techniques for quantification of microcracks. One of the challenges was to establish a uniform

process for quantifying cracks in the sampled images. Commercial computer applications for the

analysis of electronic images were utilized. Once the density of microcracks in the images was

quantified, an analysis of the density results reveals if significant changes in crack behavior occur under high temperature conditions.

Data Analysis

Microcrack concentrations for samples cured at room temperature and high temperature conditions were compared. An analysis of variance was implemented to determine if there were significant differences in the findings.

Summary

Considering that internal temperatures ranging from 180°F to 200°F have been recorded in mass concrete structures at FDOT projects, it is prudent to consider the possible adverse effects that high curing temperature conditions can have on these structures. Accordingly, the research conducted in this study was performed to evaluate if increased microcracking is observed under these conditions.

CHAPTER 2 LITERATURE REVIEW

Introduction

The literature review performed for this project investigates the current state of the art

regarding the following topics:

• Characteristics of cracking in mass concrete and effects of high curing temperatures

• Preceding research on observation of microcracks in concrete.

• Methodology of concrete specimen preparation for microscopic study.

• Image analysis processes for detection and quantification of microcracks in concrete.

Mass Concrete

In the design of mass concrete structures thermal action, durability, and economy are the

main factors that are taken into consideration. Because of the large masses involved, heat of

hydration of the concrete presents considerably more problems than in other structural concrete,

and some of the provisions made to limit the temperature rise have a vital effect on curing and other practice applications. In warmer climates, as is the case in the state of Florida, the concrete temperature is kept to a minimum during placement and hydration by the use of low-heat Type II and IV cements, fly ash or other pozzolan replacement, refrigeration, special curing techniques, and heat-dispersing steel forms. In general pour lift plans, placing sequence, and form removal are planned carefully to avoid thermal shocks which would result in cracking of the concrete surface, Hurd (1995).

The control of the thermal action exhibited by mass concrete is essential to restraint

cracking caused by temperature differentials and thermal stresses. When the tensile stresses due

to the differential temperatures (and its subsequent differential volume changes) exceed the

tensile stress capacity, concrete will crack. Per ACI 224.1R-4, cracking in mass concrete can

result from a greater temperature on the interior than on the exterior. These temperature

gradients can be caused by the following conditions: concrete heating up more in the center from the liberation of heat during cement hydration and/or more rapid cooling of the exterior relative to the interior. In either case, the root cause of thermal cracking is the temperature gradients.

Consequently, FDOT specifications require thermal gradients not to exceed a maximum differential of 35°F. However, curing temperatures in FDOT projects have been recorded between 180°F and 200°F, and no standards are required for maximum curing temperature. This condition raises concern over the initiation of microcracks at the time of curing resulting from elevated temperatures. Patel et al. (1995) reported no marked differences in the microstructural features present in concrete cured at 60°F, 108°F, and 115°F. The study also concluded that the presence of microcrack networks was evident within the matrix of all concretes, but that higher degree of microcraking was observed in sample cured at 185°F and sample with eighteen years in-service use. The author further explains that the reason for additional microcraking may be explained by the expansion of air in the poorly dispersed entrained air voids. This revelation does not provide sufficient evidence that microcracking is caused by the increase temperature condition but more by a high air void concentration.

Studying Microcracks in Concrete

While the macrostructure of concrete can be observed unaided, the microstructure must be observed from high resolution images generated from a microscope. Conventionally, microcracks are defined as cracks whose widths are about a few micrometers (<10µm),

Ammouche et al. (2001). Microcrack lengths can vary significantly.

One weakness of studying concrete microstructure is the lack of a standardized specimen preparation technique for microscopic observation. Although this may hinder the general

acceptance of this type of study, it represents the only reasonable procedure available for

evaluating the microstructural behavior of concrete.

The development of an automated procedure for the identification of features in cement

paste is also paramount to the success of the study of microcracking in concrete. Digitized

images are acquired and cracks can be identified based on local changes in image characteristics.

Darwin et al. (1995) reported that in early work, cracks were identified manually, which was

naturally subjective and susceptible operator fatigue which could further reduced objectivity.

The procedure for quantification and imaging presents another difficulty in the study of microcracks. The principal processes in the microscopic study of concrete will be discussed later in this chapter. The main difficulties with the analysis of microcrack behavior are summarized:

Specimen preparation procedures

Most specimen preparation techniques require pre-drying treatment that can alter the in situ

state of the material. Cement paste is a moisture sensitive material in which the slightest drying

can induce secondary microcracking. This presents a question of whether the microcracks

observed under microscope are found in the natural (in situ) state of the concrete, or whether

they are induced by the specimen preparation procedures.

Image analysis procedures

A systematic process for image analysis is required. He availability of scanning electron

microscopes and the relative ease of obtaining images of even very poor specimens has led to the

proliferation of images that are meaningless or even deceptive if nor correctly interpreted,

Detwiler et al. (2001).

Microcrack quantification procedures

As mentioned previously, test results may be biased by the subjectivity of the operator,

who often has to convert a complex image into a binary result. In addition to operator bias,

microcrack measurements can significantly vary with magnification factor from the image

acquisition process.

Specimen Preparation Techniques

Before reviewing the techniques developed to study microcracks in concrete, the characteristics of an appropriate technique need to be defined. Per Hornain et al. (1996), ideal specimen preparation techniques should not induce any cracks during the preparation of the sample. Therefore, procedures which involve prior drying of the specimens should not be used.

The ideal technique should be simple, economic, rapid, and should be able to detect very fine cracks, Hornain et al. (1996). Finally, the ideal method has to be coupled to an image analysis system in order to yield quantitative information.

Specimen preparation is fundamental to the microscopic analysis of concrete. As stated by

Soroushian et al. (2003),

Preparation of concrete samples so that the features of interest (microcracks and voids) develop a distinct contrast against the body of concrete in the selected microscopy technique is the prerequisite for application of modern automated image processing and analysis techniques to concrete microscopic images.

Poor preparation methods can lead to erroneous diagnoses of microstructural conditions. All of

the steps involved in the preparation of the specimens affect what is seen in the microscope, but

there is more than one right way to prepare concrete specimens for microscopic examination.

The specific procedures will depend on the purpose of the investigation, the equipment available,

the examination technique to be used, and the personal preference of the investigator, Detwiler et

al. (2001).

Standardized methods of preparing specimens for the purposes of this research were not

found. However, literature regarding the evaluation of current ASTM standards was available.

In Roy et al. (1993) an evaluation committee provided recommendations to standard

specifications, practice, and test methods as defined by ASTM and ACI. The authors analyzed

the method of impregnation using fluorescent and epoxy microscopy as established by ASTM

C856, Standard Practice for Petrographic Examination of Hardened Concrete. Their report

stated that there exist no reliable methods for standardizing a procedure for the analysis of

microcracks that is independent of operator bias during image analysis and specimen preparation. Furthermore the committee concluded that such a method for undertaking petrographic examination of concrete may not be technically mature enough to warrant inclusion in an ASTM standard practice.

Impregnation Techniques

Impregnation techniques are used to assess the microstructure of cement-based materials.

Such techniques can include fluorescent dye, red dye, or low viscosity epoxy as the impregnating agents. Although these techniques offer interesting possibilities for investigating concrete microstructure, they usually require drying of the specimens prior to impregnation. The most common techniques for drying are oven drying, vacuum drying, freeze drying, and solvent exchange. Oven drying temperatures above 95ºF should be avoided because of the destructive effect on the microstructure and its tendency to induce cracking, Detwiler et al., 2001.

Meanwhile solvent drying works utilizes dehydration and does not seriously affect the outward morphology of the specimen. A procedure that consists of epoxy-ethanol replacement was found and is described next.

Epoxy Impregnation Technique

In Struble et al. (1989), a process for sample preparation was developed which involved a three-step procedure to replace pore water from concrete. The process was later reported by

Stutzman et al. (1999) in greater detail. The technique involves a two stage counter diffusion process, where water present inside concrete voids is replaced with 200% ethanol and then

ethanol by an ultra-low viscosity epoxy solution. The steps towards achieving the replacement

process do not involve high pressure vacuum or extreme temperature exposure. Once voids and

cracks are replaced with epoxy, lapping and polishing procedures are undertaken. The purpose

of replacing voids and cracks with epoxy was to maintain integrity of microstructure and

strengthen material during the grinding and polishing steps. The grinding and polishing of the

surface to be visualized is an inevitable destructive process. Therefore, an epoxy binder is used

to impregnate the sample and provide additional structural support to the surface during grinding and polishing.

The advantages of the procedure developed by the authors include: not having to dry the

specimen with the application of heat or high vacuum pressure, and it provides an economically

viable alternative to specimen preparation. The only disadvantage found is the required heating

of epoxy during curing. The selection of the type of epoxy depends on the objective of the study.

In this case, the choice of epoxy was determined by the viscosity of the fluid. In addition, the

epoxy had to be resistant to oils, lubricants and other solvents used during specimen preparation.

The ultra-low viscosity kit utilized was manufactured by Structure Probe, Inc. and is currently

available in the market. Chapter 4 of this report describes in greater detail the procedure

developed by Struble et al., which was used as part of the specimen preparation for this study.

Other impregnation techniques have been utilized with variations of impregnating agent

characteristics, such as the use of fluorescent dye or red dye, but alternative methods with

comparable efficiency in cost and simplicity where not found. One method developed by

Nemati presented an interesting approach which deserves mention even though it is not suitable

for the timeframe of this project.

Wood’s Metal Method

In Nemati et al. (2001) researchers developed a stereological approach for studying the mechanical behavior of concrete. A process for looking at microstructure of concrete was

implemented, and then the acquired microscopic images were altered to project crack

characteristics. This project used Woods metal, which is a metal in liquid phase with a melting

point between 158ºF and 191ºF. The Woods metal was used to preserve the microstructure of

stress-induced microcracks in concrete. The impregnation took place during the curing of the

cylinders, which were later stressed to mechanically generated cracking in the concrete.

A detailed specimen preparation process for microscopic analysis was not described.

However, the need to impregnate the concrete specimens after sectioning was clearly not

required because the features of interest (microcracks and voids) were filled by the Woods metal

material during the curing stages. The images were analyzed using gray level histogram

thresholds and filtering. The equipment used to insert the Woods metal in the concrete was not

economically viable for this project. In order to conduct this procedure, the sample

impregnations have to be implemented during the curing of the concrete, and not after the

samples are fully hardened.

Image Processing and Analysis

The features and capabilities of imaging via specialized microscope equipment are

discussed. Recent trends in microscopic analysis of concrete involve the use of equipment such

as a scanning electron microscope (SEM) and image analysis software. An optical microscope

has inadequate depth of field and resolution to view the microstructure of concrete. Therefore

the use of scanning electron microscopy is preferred for image acquisition. The SEM requires no

special coating on the samples and the ability to view detailed features in the concrete

microstructure make it the most suitable option for image analysis.

SEMs are microscopes that map a target with a focused electron beam. Figure 2-1 presents a schematic diagram of a basic SEM. An electron gun generates electrons of various energies.

This electron “beam” is accelerated to several kilovolts of energy down an optical column, which is made “monochromatic” using an aperture, focused with multiple electrostatic objective lenses, and steered over the target using electric or magnetic fields (Bunday et al., 2005). The beam is then decelerated so that the energy is on the order of hundreds of volts. The final beam spot size

is approximately 10 nm.

The SEM used for this study is from the University Of Florida Department Of Civil

Engineering. The computerized equipment generates an image from the electron beam that is

TV-rastered over the specimen. The beam interacts with the specimen surface, generating many secondary electrons and backscattered electrons at the point of impact and within the interaction volume, as illustrated in Figure 2-1 (b). SEMs use secondary electrons because they have low energy and thus have a very shallow escape depth from the specimen, which is important for quality surface imaging (Bunday et al., 2005).

Figure 2-1. Schematic diagrams showing (a) a basic SEM column, (b) electron beam interaction volume with a sample (Bunday et al., 2005).

As the beam is scanned over a field of view (which is inversely related to magnification), a detector collects the secondary electrons from each pixel so that an intensity map is generated.

The magnification factors can range up to 500X. Investigations conducted by Soroushain (2003) were performed to observe the characteristics of microcrack identification at different magnification factors. The optimal magnification for our study will be based on previous research and the observed quality and image field size generated by SEM at different magnification factors.

Microcrack Quantification

In Ammouche et al. (2001), microcracks in were analyzed using imaging algorithms which quantified the microcrack patterns. The technique for quantification of cracks was developed from the resulting binary image treatment where cracks were “skeletonized” and later measured using software algorithms. The specimens were impregnated with red dye to highlight cracks and voids. The developed image treatment process made it possible to systematically analyze specimens with respect to mechanically damaged samples (Ammouche et al., 2001).

The algorithms developed by Ammouche provided a way of measuring individual crack patterns. However, the process did not provide a means for analyzing crack density in the sample concrete area. The authors developed the image analysis algorithms via a proprietary software Visilog 4 environment.

After acquiring an image, a method needs to be implemented to process the image and allow for a quantitative analysis of the specimen. For our study, a quantitative analysis requires the measurement of all microcracks within the image field and the calculation of the crack density in each image. The image processing should differentiate between voids and cracks, and boundary cracks around aggregates or between phases.

Soroushian et al. (2003) used segmentation of images from grey level to binary images by

thresholding approach for viewing microcracks in samples. The research compared manual

threshold operations with automated threshold operations which included factorization, entropy and moment. In manual thresholding, the low threshold level was set to zero and the best upper limit threshold was manually determined. The optimum high threshold level was found by comparing original gray image and the image after the application of manual threshold on a binary image.

When features of interest (microcracks and voids) were highlighted, the most distant contrast between microcracks, voids and concrete background was obtained. The authors reported that there were no significant difference between manual thresholding and automated thresholding operations. The author recommended that depending on magnification factor, the threshold length (measured in number of pixels) for distinguishing noise from microcracks and voids should be selected based on the visual observations of noise (error) impact on removal criteria.

For purposes of this study a semi-automated process will be used. It involves the use of

AxioVision software by Carl Zeiss. The software module AutoMeasure will be used to delineate microcracks in an image field and quantify the total length of microcracks in the image. There are 256 discrete intensity (gray) levels available on the 8 bytes graphics card used to scan the specimen images. The process first involves the identification of features as potential cracks based on local changes in gray levels. The rules used to determine the level are relatively lenient to insure that the vast majority of the cracks are identified. The next step involves the elimination of non-crack features via filtering by crack length and operator observation. In the final step the geometric properties of the microcracks are recorded. This data is used to

determine the density of cracks by dividing the total lengths of cracks by a constant image

surface area. The resulting density values will be statistically compared for all specimens prepared. The purpose of analyzing the resulting crack densities is to determine if additional microcrack formation is present in concrete samples cured under adiabatic high temperature conditions.

CHAPTER 3 SPECIMEN PREPARATION METHODOLOGY

Introduction

In this chapter, the materials and methods used to prepare the concrete sample specimens for microscopic analysis are presented. First, the concrete sample selection criteria are discussed, followed by the specimen preparation technique. The laboratory work for specimen preparation was carried out at the Concrete and Soils Lab of the University of Florida M.E.

Rinker, Sr. School of Building Construction.

This project has two objectives: first, verify the validity of previously applied concrete sample preparation techniques and second, analyze the specimens to determine if high curing temperatures result in additional microcrack formation. The specimen preparation and imaging research methodology is presented in two chapters. Chapter 4 will discuss the procedures for image acquisition via scanning electron microscopy, and the image analysis and quantification of microcracks.

Concrete Sample Selection

The first step was to select concrete samples whose characteristics would be favorable for microcrack formation at high curing temperatures. The specimens used in this project were obtained from the concrete samples prepared in the research study “Adiabatic Temperature Rise in Mass Concrete in Florida” conducted at the University of Florida for the Florida Department of Transportation (Chini et al., 2005). In the referenced study, concrete samples were prepared from mixes representative of FDOT approved mass concrete design mixes.

The cement material used in their research was outsourced from different Florida cement manufacturers. First, two cements were selected based on the chemical properties that would produce the highest heat of hydration characteristics. For purposes of their study, these were

identified as cements “A” and “B”. The source of cements A and B were Florida Rock

Industries in Newberry, FL and Florida Mining and Material (Cemex) in Brooksville, FL, respectively. The authors then prepared twenty different mixes using the aforementioned

AASHTO Type II cements.

The mixes were prepared by combining cements A and B with various proportions of pozzolanic materials (fly-ash and slag) as well as other common concrete mix constituents. In total, twenty mix designs and their respective samples were prepared. The objective of their research study was to develop and recommend a set of adiabatic temperature rise curves for typical mass concretes used in the state of Florida (Chini et al., 2005). The authors investigate the effects of the cement mixes properties on concrete compressive strength and adiabatic temperature rise during curing.

The concrete samples were given mix designations based on their material characteristics.

The same nomenclature was used for our research and consists of an eight character alpha numeric label based on the following parameters:

• First and second character: Placing temperature, 73°F (73) and 95°F (95)

• Third character: Cement type, (A) or (B)

• Fourth and fifth characters: Percentage of fly-ash or blast furnace slag material expressed in percentage (00, 20, 35, 50, 70)

• Sixth character: Pozzolanic material, Slag (S) or Fly-ash (F). When 0%, (P)

• Seventh and eight characters: Curing temperature, room temperature (RT) and high temperature (HT)

For example, concrete sample “95B00PHT” corresponds to a concrete sample placed at 95°F, mixed with cement type B and 0% pozzolanic material (or 100% cement), and cured under high

temperature conditions. Some of the concrete mixes with assorted pozzolanic content are shown in Table 3-1 for clarity.

Table 3-1. Concrete sample mixes and labels. Cementitious Placing Cement Curing Sample label Material Proportions Temp Type Condition High Temp. 95B00PHT 100% Cement 95°F B Room Temp. 95B00PRT 80% Cement and High Temp. 95B20FHT 95°F B 20% Fly Ash Room Temp. 95B20FRT 65% Cement and High Temp. 95B35FHT 95°F B 35% Fly Ash Room Temp. 95B35FRT 50% Cement and High Temp. 95B50SHT 95°F B 50% Slag Room Temp. 95B50SRT 30% Cement and High Temp. 95B70SHT 95°F B 70% Slag Room Temp. 95B70SRT

The concrete mix samples for high temperature and room temperature curing conditions

were cured in different molds. Samples cured at room temperature were cured in 4 in. by 8 in.

plastic cylinder molds. Meanwhile, the samples cured at high temperature conditions were

placed in thermal curing chambers. The thermal curing chambers monitored the curing

temperature to maintain an adiabatic condition. The chambers were connected with

thermocouples to a micro-controller that regulated temperature and documented the adiabatic

temperature rise during curing. Figure 3-1 shows the samples used for this study.

A B Figure 3-1. Concrete samples. A) 95B00PRT and B) 95B00PHT.

The data provided in Table 3-2 show the observed temperature rise and maximum temperature reached over the first 14 days of curing.

Table 3-2. Adiabatic temperature rise data for concrete with cement type B, 95°F placing temperature, and high temperature curing. 100% Cement 20% Fly Ash 35% Fly Ash 50% Slag 70% Slag Time 95B00PHT 95B20FHT 95B35FHT 95B50SHT 95B70SHT (day) Temp. Max. Temp. Max. Temp. Max. Temp. Max. Temp. Max. Rise Temp. Rise Temp. Rise Temp. Rise Temp. Rise Temp. 14 85.9°F 184.6°F 77.6°F 167.5°F 69.9°F 170.8°F 82.0°F 177.8°F 79.9°F 173.9°F

The temperature data in Table 3-2 indicate sample 95B00PHT had the highest temperature rise of 85.9°F, followed by 95B50SHT with a temperature rise of 82.0°F. The conditions inside the thermal chambers sustained adiabatic conditions as samples 95B00PHT and 95B50SHT, reached maximum curing temperatures of 184.6°F and 177.8°F, respectively.

The decision to select sample 95B00PHT for specimen preparation was made for clear reasons. Sample 95B00PHT constitutes the worst case scenario of all available samples for potential of microcracking due to high temperature conditions. The high temperature sample

95B00PHT and its room temperature counterpart, sample 95B00PRT, were cut to take sample specimens and prepared them for microscopic analysis.

Sample Preparation Technique

The method utilized for specimen preparation was similar to the epoxy impregnation process developed by Struble and Stutzman (Struble et al., 1989). Preparation of concrete specimens for microscopic analysis involves various processes, depending on the type of microscopy analysis to be conducted. ASTM C 856 gives good overviews of preparation methods for samples to be examined using optical microscopy (Detwiler, et al. 2001).

The process of concrete impregnation with epoxy developed by Struble et al. (1989) involves a counter diffusion method wherein the pore water of concrete is replaced with ethyl

alcohol, and then by a low viscosity epoxy. The method does not require pre-heating of the concrete specimen nor does it require applying vacuum pressure for impregnation of the epoxy.

What this method provided was a viable economic method with minimum destructive procedures which best suited the needs of this project. The only drawback from this preparation procedure was the heating of the specimen during the polymerization of the epoxy, which will be discussed later in this chapter.

Lab Procedures for Sample Preparation

Diamond Saw Cutting

The first step was to cut the concrete sample blocks. The concrete samples had been stored in moist rooms at the FDOT Materials Research Laboratory to avoid drying shrinkage cracking.

Samples were extracted by cutting 0.5 in. thick pieces with a water lubricated diamond blade saw, shown in Figure 3-2.

Figure 3-2. Diamond blade saw used for initial cutting operation at the FDOT Materials Research Lab in Gainesville, Florida.

Sectioning Procedure

The sectioning operation was performed with a diamond wafer saw; model Trim Saw by

Allied High Tech Products, Inc. shown in Figure 3-3. The sampled pieces were cut down to 1.5

in x 1.5 in. x 0.25 in. thick specimens. The cuts were performed manually at 480 RPM with a 6

in. diameter, 0.02 in. thick diamond metal bond blade. The blade was kept lubricated with a low

speed propylene-glycol based cutting fluid #60-20110 by Allied High Tech Products, Inc. (also

shown in Figure 3-3). The cutting fluid is designed exclusively for low speed cutting

applications (less than 500 RPM). This fluid reduces heat generation from friction while

removing cut debris from the specimen.

A B Figure 3-3. Apparatus and lubricant for sectioning procedure. A) Trim saw and B) Sectioning fluid lubricant #60-20110 by Allied High Tech Products, Inc.

Epoxy Impregnation Procedure

Once the specimens were cut down to the desired size, the process of epoxy impregnation was begun. The first step in this technique is to replace the water in the voids and pores of the

concrete with 200% proof ethanol made by Sigma-Aldrich, Inc. The specimens were placed in a

lidded jar and fully immersed in the ethanol. A control specimen was used to monitor the depth

of replacement of the pore water with the ethanol. The control was a remnant of the original sample after the sectioning procedure. The remnant was placed in a jar and immersed in a dyed

ethanol bath. The dye that was used was a red dye by PolyScience, Inc. The companion

specimen was cut every day to observe the depth of replacement, see Figure 3-4. Alcohol-pore water replacement was found to be approximately 0.0394 in/day. After five days the control specimen was cut and the red dye coloring demonstrated evidence of replacement to a noticeable depth, see Figure 3-4.

Figure 3-4. Section view of remnant specimen after red-dyed ethanol replacement.

Once the ethanol replacement was complete, the specimens were infiltrated with an ultra- low viscosity epoxy mix. The epoxy kit was from Structure Probe, Inc (SPI-CHEM). The kit components are shown in Figure 3-5.

Figure 3-5. Ultra-low viscosity epoxy kit by SPI Industries.

The epoxy replacement procedure was performed based on manufacturer recommendations and the specifications found in the product manual. The SPI-CHEM ultra-low viscosity epoxy kit consists of four chemicals which are mixed together to obtain the desired quantity of epoxy solution. Table 3-3 shows the proportions for preparing the epoxy.

Table 3-3. Mixing schedule of epoxy kit per Structure Probe, Inc. product manual. Chemical Mixing Schedule Vinylcyclohexene dioxide (VCD) 10 g n-Octenyl succinic anhydride (n-OSA) 20 g Butanediol Diglycidyl Ether (BDE) 0.3 g Dimethylaminoethanol (DMAE) 0.3 g

The protocol for the epoxy infiltration process was as follows:

• Stage 1: Fully submerge specimen in a 3:1 ethanol to epoxy solution for 12 hours,

• Stage 2: Fully submerge specimen in a 1:1 ethanol to epoxy solution for 12 hours,

• Stage 3: Fully submerge specimen in a 1:3 ethanol to epoxy solution for 12 hours,

• Final stage: Fully submerge specimen in a 100% epoxy solution for 12 hours.

One of the obstacles encountered during the epoxy replacement procedure was to keep the specimens at each stage isolated from the presence oxygen. The manufacturer’s recommendation was to perform the procedure in a dry nitrogen environment with a glove bag purged of air. However, this alternative was not feasible for our project given the resources available. Instead, careful attention was placed during the preparation of the epoxy and ethanol- epoxy solutions to avoid entrainment of air bubbles into the solutions.

Initial attempts at this procedure were unsuccessful because of failure to fully polymerize the 100% epoxy solution after its final replacement stage. Figure 3-6 shows specimens in mounting cups with the un-polymerized epoxy mix.

Figure 3-6. Failure to polymerize epoxy solution in final stage of replacement.

Based on manufacturer recommendations, in order to better control the polymerization of the epoxy solution, the specimens had to be heated at 140°F during each stage of replacement.

Afterwards, epoxy replacement was performed by heating the specimens in their mounting cups to 140°F, see Figure 3-7.

Figure 3-7. Concrete specimens in mounting cups inside the oven.

Grinding and Polishing Procedures

The grinding and polishing steps are performed to provide a level, reflective surface for

SEM imaging. The specimens have been embedded with an epoxy solution and are now ready for grinding and polishing. These are vital steps in the sample preparation process. Grinding is done primarily to expose a clear surface layer and to remove excessive epoxy coating on the surface of the specimen. Careful attention must be placed during the grinding procedure because excessive grinding can cause damage to the concrete specimen and result in secondary cracking.

All grinding and polishing processes were performed on the MPrep3 Grinder-Polisher Machine by Allied High Tech Products, Inc. shown in Figure 3-8.

Figure 3-8. MPrep3 grinder and polisher at the University of Florida School of Building Construction Concrete and Soils Lab.

The grinding and polishing procedure is a multi-step process that involves the use of

various materials and techniques. The step-by-step procedure is best described in Table 3-4.

This procedure was developed in conjunction with product application specialists from the

manufacturer of the consumables and equipment used on the project. The six step procedure was

performed manually.

In order to comply with industry standard practices, the procedure was developed and

specialists from the specimen preparation equipment manufacturer. The process combines the

use of abrasive paper for grinding, diamond paste polishers with polishing cloths and special lubricants from the same manufacturer. The specimens were prepared in a systematic manner in order to reduce deviations between the different operators that prepared the samples. By establishing a constant procedure the experimental and operator-induced errors are better controlled. Recommendations to improve these procedures are briefly described in Chapter 6 of this study.

Table 3-4. Grinding and polishing procedure for manual preparation of specimens with MPrep3 and consumables by Allied High Tech Products, Inc. Steps 1 2 3 4 5 6

Abrasive material size 180 grit 320 grit 600 grit 6 μm 1 μm 0.05 μm Silicon Silicon Silicon Poly- Poly- Colloidal Abrasive type Carbide Carbide Carbide crystalline crystalline Alumina Paper Paper Paper Diamond Diamond

Grinding Grinding Grinding Glycol Glycol Polishing Carrier Disc Disc Disc Suspension Suspension Suspension

White Polishing Cloth None None None Gold Label Chem-Pol Label

Coolant Water Water Green Lube Green Lube Green Lube None

Platen Speed / 250 RPM / 250 RPM / 250 RPM / 250 RPM / 250 RPM / 250 RPM / Direction Comp** Comp** Comp** Contra* Contra* Contra* Manual applied Approx. 8- Approx. 8- Approx. 8- Approx. Approx. Approx. 6- pressure 10 lbs 10 lbs 10 lbs 10-12 lbs 10-12 lbs 8 lbs

Duration 1:30 min 1:00 min 1:00 min 1:30 min 1:30 min 1:00 min *Contra: Platen and specimen rotate in opposite direction. **Comp: Specimen stays in fixed position.

Figure 3-9. Various consumables used for grinding and polishing procedure.

In Figure 3-9 some of the consumables used during this preparation stage are shown for illustration. After step 3, which involves grinding with 600 grit size paper, the surface becomes smooth and ready for polishing. Polishing is done to remove the surface damage resulting from the grinding of the specimen surface. Polishing also provides proper reflectivity of the specimen

surface, which is essential in obtaining a clear picture when scanning the images with the SEM

equipment.

The polishing operation involves the use of special polishing cloths with diamond suspension pastes decreasing in particle size from 6μm to 1μm, and a final polish with a 0.05μm

colloidal alumina suspension. The details of each polishing step are shown in Table 3-4. The

final polishing application with the colloidal alumina suspension is being shown in photo A of

Figure 3-10. After each polishing step was complete the specimens were cleaned with a micro

organic soap (#148-10000 by Allied High Tech) and dried with compressed air cans, see photo B

below.

A B Figure 3-10. Step 6 of the polishing procedure A) Specimen polished with a 0.05 m colloidal alumina suspension. B) Specimen dried after polishing.

Preparation for Imaging

Once the specimens are ready for viewing, they are marked with a 0.25in by 0.25in grid matrix. The grid was created by tracing with an extra-fine point marker by Pilot, shown in

Figure 3-11.

Figure 3-11. Extra fine point permanent marker by Pilot used for specimen grid markings.

The use of the Pilot marker is effective for the SEM image acquisition system since it provides a temporary traceable coordinate system for identifying the location of each image being scanned. For each grid cell a single image was scanned. The specimen images were scanned by a manufacturer certified SEM technician. The resultant specimen ready for imaging is shown Figure 3-12. As shown in this figure, X marks were placed on the grid cells which contained aggregate components which were of no interest to the project. For purposes of this project, microcracking was evaluated at cement paste areas and interfacial transition zones (ITZ).

The ITZ is defined as the boundary area between aggregates, fine and course, and the cement paste.

Figure 3-12. Polished specimen ready for imaging under SEM.

Specimen Preparation Test

One of the goals of this research was to test the specimen preparation technique for reliability. The procedure had to be tested in order to determine if significant amounts of secondary microcracks were produced during the specimen preparation process. In order to accomplish this, a control set of specimens was created along with the high temperature and room temperature specimens. The specimens were divided into oven-dried specimens and regular specimens for comparison.

Oven-Dried Specimens

The control specimens were identified as oven-dried (OD). Prior to epoxy impregnation a

specimen from the same sample source was dried at 250°F for 12 hours and later impregnated

with the same epoxy solution. No water-ethanol replacement was required since all pore water

in the specimens should have evaporated from heating them at a temperature over 212°F.

Control specimens were prepared with the previously described sectioning, grinding, and polishing process.

The purpose of conducting this procedure validity test was to check if the developed

processes were tampering the data and providing invalid results. The technique of inducing

drying shrinkage cracks via oven-heating would also shed light on whether the preparation

techniques produce damages that would significantly altered the in situ conditions of the

concrete specimens.

Summary

A total of 10 specimens were prepared using the equipment and techniques mentioned in

this chapter. In the following chapter the process of image acquisition and image analysis for

crack identification and quantification are explained.

The determination of what specimen preparation technique to use was based on several

factors that would influence the viability of this study. The procedure for specimen preparation

was chosen to answer the question of microcrack behavior as unequivocally and economically as

possible. The specimen preparation procedure selected was the epoxy impregnation technique as

developed by Struble and Stutzman (Struble et al., 1989). Table 3-5 summarizes the selection

criteria used to determine the specimen preparation techniques used for this study.

Specimens were prepared for image analysis with the goal of determining whether the high temperature curing conditions resulted in increased levels of microstructural cracking. The

Table 3-5. Concrete specimen preparation methods for microstructural analysis of concrete. ASTM Required Temp. Applied Cost Equipment Method Standard Microscopy Exposure Pressure Effective Availability Reference Equipment Epoxy Impregnation C 856 140°F Negligible Yes SEM Yes

Dye Impregnation None Room Negligible Yes Optical Yes Dye & Fluorescent 280psi C856 140°F No SEM Yes Epoxy Impregnation Nitrogen 280psi Wood's Metal None 200°F No SEM Yes Nitrogen ESEM Method None 37°F Water vapor No ESEM No (Humidity control) High Pressure Epoxy C 856 176°F High No SEM Yes Impregnation specimen preparation steps of grinding and polishing were conducted based on manufacturer’s recommendations. However, several problems were encountered during the replacement and mixing of the ultra-low viscosity epoxy. In Chapter 6, recommendations are given for alternative products, equipment and procedures that could improve the quality of test results.

CHAPTER 4 IMAGE ANALYSIS METHODOLOGY

Introduction

Image analysis tools were used to identify and quantify microcracks in the concrete

specimens. In the literature review, prior research studies in the field of image analysis for

microcrack quantification were presented. In this second chapter of the research methodology,

the processing and analysis of the images obtained from the specimens are explained. Imaging methods were used based on the precedent research studies and in conjunction with the tools available in industry image analysis software. The equipment, computer software tools, and mathematical procedures used to determine the quantity of microcracks are described.

Image Acquisition

The first step in the imaging phase is image acquisition via scanning electron microscope

(SEM). Image acquisition was performed at the Advanced Material Characteristic Lab at the

Department of Civil and Coastal Engineering, University of Florida, Gainesville. This facility

houses a Hitachi S-3000N, variable pressure Scanning Electron Microscope (SEM), which is shown in Figure 4-1.

Figure 4-1. SEM equipment at the University of Florida Advanced Material Characteristic Lab in Gainesville, FL, model Hitachi S-3000N.

The microscope equipment allows for the viewing of the concrete specimens without covering the surfaces with a conductive material. The SEM works with a focused beam of electrons that scans across the specimen and measures signals resulting from the electron beam interaction with the surface of the concrete specimen. Computerized equipment translates the information produced by the SEM into high resolution digital images. The three major types of signals generated by the SEM are secondary electrons, backscattered electrons, and x-rays.

Backscatter electrons are highly energized beams that can be used to distinguish between the particles in the concrete surface on the basis of variation in brightness and grayscale value. As reported by Stutzman, backscatter electron and x-ray imaging tools are useful in identifying cracks in the cement paste with grayscale alteration (Stutzman et al., 1999). In the research performed by Stutzman, images were generated from the SEM scanning procedure and manipulated to uncover features of interest such as microcracks and voids.

All SEM images for our study were generated with the following settings:

• Vacuum: 30 Pa • Accelerating voltage: 15.0 KV • Scale: 300μm • Pixel Type: 8-bit grey levels • Image Size: 1280 x 960 pixels (870μm x 650μm) • Magnification factor: 150X

The images were scanned at the same scale, pixel type, and magnification factor. After a few trial images, it was determined that a magnification factor of 150X was most suitable for microcrack analysis. The images generated at 150X portrayed microcracks of appreciable width and length with the largest field area. Previous work by Soroushian used several magnification levels ranging from 125X to 500X in image analysis (Soroushian et al., 2003). The selected magnification factor was also beneficial because it revealed a wide array of cement paste and

ITZ conditions which had to be included in the analysis.

After digital images are obtained from the SEM, they are analyzed for microcrack identification and quantification. As described in Chapter 3, each specimen was labeled with a grid system that was used to identify individual matrix cells and areas of interest. The grid system is illustrated in Figure 4-2 below.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 X 20 21 22 23 24 25 26 27 28 X 29 30 31 32 33 34 A B Figure 4-2. Specimen grid imaging system A) Finished specimen prior to imaging, B) Grid labeling system.

The ten specimens prepared for imaging are listed in Table 4-1. As shown, the total number of images generated was 328.

Table 4-1. Concrete specimen and images list. No. of Images Specimen Obtained 95B00PHT-1 30 95B00PHT-2 27 95B00PHT-3 30 95B00PRT-1 43 95B00PRT-2 35 95B00PRT-3 21 95B00PODHT-1 42 95B00PODHT-2 22 95B00PODRT-1 43 95B00PODRT-2 35 Total 328

Analyzing each image individually without the assistance of computer software tools would be very time consuming and subject to excessive user bias. Therefore, an imaging analysis process with automated tools was used to accomplish the analysis.

Image Analysis and Microcrack Quantification

The main challenge of the image analysis process was to find a system that would enable the analysis of numerous images in an automated, time effective, user-friendly manner. The system should automatically identify and measure microcracks in the image field. A market study was performed to find a software program with the following capabilities:

• Grayscale analysis of images. • Crack identification based on crack length parameters. • Filtering of noise and voids. • Quantification of total crack length in the image area. • Highlight of microcracks for illustrative purposes. • Output of data for statistical analysis.

The image analysis software selected for this study was AxioVision version 4.4.1 by Carl

Zeiss. Microcracks, voids, and noise could be distinguished visually from the SEM images.

However, an automated mapping of the cracks was done to find the total length of the microcracks per image. The AxioVision program enabled the measurement of each microcrack detected.

In order to identify microcracks in the image, segmentation techniques were used.

Segmentation is a process by which the images are partitioned into meaningful areas based only on the intensity of the pixels. The intensity of each pixel is defined by a grayscale value.

Grayscale analysis involves the use of threshold levels for the different grey level values present in the image. Images were scanned at 8-bit grey levels, which contain grayscale values between

0 and 256. The optimum threshold range was determined by comparing the original image with the feature highlighted image generated via the manual threshold operation. With the

AxioVision software the features which fell within the threshold range where highlighted in red.

The grayscale histogram threshold could be modified manually to find the range of grayscale

values that highlighted features of interest. As seen in Figure 4-3 many of the features that are delineated by the manual threshold procedure are not microcracks.

Manual threshold adjustment

Figure 4-3. Screen capture of the segmentation step in the image analysis program.

However, the microcracks were identified and filtered out by using pixel length criteria as

seen in Figure 4-4.

FibreLength filter

Figure 4-4. Screen capture of the measurement condition filter using the FibreLength measuring tool.

The software module AutoMeasure in the AxioVision program provides a useful tool in

measuring crack length. The highlighted features were measured with the FibreLength tool of

the AutoMeasure module. The FibreLength tool allows for the measurement of crack lines that

are irregular and curved. The FibreLength algorithm estimates the length of the fiber-shaped

outline along the microcrack or highlighted feature. The selections based on the FibreLength tool

are shown highlighted in green in Figure 4-4.

The pixel length criteria for filtering out noise and other unwanted highlighted features was

established from the data provided in previous research studies. The equivalent micron lengths

were calculated using the scaling feature in AxioVision, where 1 pixel is equal to 0.6818μm. A

microcrack length of 30 pixels, or 20μm, was determined as the bottom threshold for the

microcrack length selection criteria. Table 4-2, shown below, was taken from the research paper

published by Cement and Concrete Research Vol. 33, p. 1960 (Soroushian, 2003) and shows

similar values for mean void lengths at different magnification factors.

Table 4-2. Length of noise, microcracks and voids at three different magnification factors (Soroushian, 2003). 125X magnification 250X magnification 500X magnification Length (pixels) Micro- Micro- Micro- Noise Voids Noise Voids Noise Voids cracks cracks cracks Min 2 18.4 8 2.82 29 6.72 2 29 35.5

Max 106 478 612 82 756 906 165 948 745

Mean 15.5 97.7 107 16.8 130 205 24.4 198 271

A sensitivity analysis was performed to determine if the 20μm length filter would alter the

results of total crack length obtained. After attempting filters with minimum crack lengths of 10,

15 and 20μm it was found that the data reacted proportionally and did not deviate significantly between images. This is justified by the results of Soroushian because mean noise features were found between 15.5-24.4 pixels in length.

Further filtering of large voids or noise could be performed manually by deleting highlighted features. Per ASTM C856 section 9.1.6,

Heat used while impregnating concrete with thermoplastic wax or resin will cause cracking if the concrete is heated while it is wet, and will alter the optical properties of some compounds, such as ettringite. Artifacts may therefore be produced and compound identification made difficult. These artifacts may be mistaken as original features. Care must therefore be used in evaluating a particular feature and indexing it as original in the specimen, or produced during the removal of the specimen from the structure or during laboratory processing.

This was the case for some features that were identified during image analysis which were believed to be caused artificially. The most common example of this type of feature was water evaporation marks on the specimen surface. The characteristic circular patterns contained microcrack networks inside the circular areas, and distinguishable round cracks around the perimeter. These features were manually removed by the program operator once identified.

Figure 4-5 depicts this typical condition.

Typical water mark

Epoxy filled void

Figure 4-5. Screen capture of the final step in microcrack detection with manual selection of microcracks and other features.

Figure 4-5 shows the step in the program where individual features were manually selected for removal. Other large areas that were eliminated consisted of larger voids filled with the

epoxy resin. Figures 4-5 shows an example of such deleted features. An example of a final output image produced by the imaging software is shown in Figure 4-6. The microcracks highlighted in the image have length values that are listed in a separate output table. The microcrack length data for each image was gathered in spreadsheet format. Once image analysis was complete, a database of all crack measurements was created.

Figure 4-6. Output image from AxioVision software

The database allowed for the calculation of total microcrack length per image. Since the total crack length was obtained, Equation 4-1 could be used to compute the microcrack density values:

ρ = Σ Li / A Equation 4-1

Where:

ρ = Crack density [µm/µm2]

L = Microcrack length [µm]

A = Viewable image area [µm2], (870µm x 650µm = 525,000 µm2)

Final analysis of the crack density values per image and the mean values per specimen provided the data required to determine if microcracks formation varied significantly in room temperature and high temperature cured samples. Also, the results for the over-dried specimens will show if specimen preparation had an effect on microcracks formation. The presentation and analysis of these results is discussed in Chapter 5.

Summary

Despite all precautions taken during specimen preparation procedure there still exists the probability that captured images will contain different sources of error. Sources of error can be associated with factors such as leftover impregnating agent on the surface of the lapped specimen, uneven distribution of light under the microscope, improper adjustment of brightness and contrast of the microscope, damaged surfaces from the impregnation and specimen preparation procedure, visual identification of cracks based on manual thresholding, and operator bias.

The microcrack patterns in the concrete specimens were identified and quantified for analysis of microcrack behavior. The results were compiled in spreadsheet format by the

AxioVision program and analyzed utilizing Microsoft Excel statistical tool add-ins. The collected data is shown for reference in Appendix A, Table A-3. The analysis of results is presented in Chapter 5.

CHAPTER 5 RESULTS

Introduction

This chapter provides the results of the microcrack density values obtained from the analysis of images from the concrete specimens. The data collected with the imaging software gave total crack lengths per image for 328 images and 10 specimens. A summary of the crack length data is tabulated in Appendix A. The results from the first four specimens were used for analysis of the validity of the specimen preparation procedure. The image results came from samples 95B00PRT-1 and RT-2, 95B00PODRT-1 and ODRT-2, 95B00PHT-1 and HT-2, and

95B00PODHT-1 and ODHT-2, which represent a total of 277 images. The analysis was done by comparing the results of the microcrack densities for high temperature and room temperature cured specimens with their oven-dried counterparts. The pre-dried control specimens were

subject to induced dry shrinkage cracking and should exhibit larger density values.

A total of six specimens, three room temperature (RT), and three high temperature (HT),

were prepared and analyzed for comparison of microcrack density values. The six specimens

produced a total of 186 images for use in the analysis. The goal of this study was to investigate

if additional microcracking occurred due to high temperature curing conditions. The findings

presented in this study may have an impact on state of Florida mass concrete specifications.

Presently, total temperature rise during curing is not considered a detrimental factor to the

structural integrity of mass concrete. If the study can demonstrate that elevated temperatures

during curing lead to the formation of a significantly different density of microcracks, controls

for maximum temperature rise will need to be incorporated into FDOT project specifications. .

Test for Validity of Specimen Preparation Procedure

The results of microcrack densities for the oven-dried and regular specimens are shown in

Table 5-1. The results show that average crack densities for oven-dried specimens are larger

than the average densities in the specimens that were not dried. The average crack density for

oven-dried specimens is 2.92% while the non-dried show an average density of 1.49%, as seen in

Figure 5-1. The most important reason for conducting this test was to show that the images generated after specimen preparation were representative of the in situ condition of the concrete

samples. It became evident that a larger quantity of microcracking was observed for the oven-

dried specimens. This was expected since the concrete samples were induced with shrinkage

cracks via the oven-driying procedure. A 98% increase in microcracking was observed.

3.00% Oven-dried specimens 2.50% 2.92%

Non-dried 2.00% specimens

1.50%

1.48% 1.00%

0.50%

Figure 5-1. Average microcrack density comparison of oven-dried and non-dried specimens.

Statistical Analysis Method

An analysis of variance model (ANOVA) was used to evaluate the data shown in Table 5-

1. Statistical analysis was performed with the StatPro software plug-in, 2002 version by Palisade

Table 5-1. Microcrack density values for oven-dried (OD) and regular specimens. Image Microcrack Density Values [μm/μm2] (%) No. HT-1 HT-2 RT-1 RT-2 OD HT-1 OD HT-2 OD RT-1 OD RT-2 1 1.32% 4.15% 1.89% 1.10% 2.29% 2.34% 2.83% 3.37% 2 1.06% 2.69% 1.42% 0.55% 2.50% 5.05% 2.63% 3.01% 3 1.85% 1.66% 1.35% 0.48% 0.98% 3.73% 2.64% 3.84% 4 0.30% 2.34% 1.39% 0.54% 1.19% 2.43% 2.11% 5.13% 5 0.71% 2.66% 0.86% 0.63% 0.69% 2.80% 2.28% 3.71% 6 0.91% 3.18% 0.79% 0.52% 0.26% 4.16% 2.40% 4.10% 7 0.80% 2.75% 0.94% 1.23% 4.30% 4.26% 3.13% 5.96% 8 0.93% 4.08% 0.84% 0.81% 4.30% 6.36% 1.14% 5.92% 9 0.98% 2.93% 0.70% 0.94% 0.97% 7.17% 3.47% 4.74% 10 0.43% 2.60% 0.41% 1.50% 0.72% 5.01% 3.13% 5.18% 11 0.62% 2.05% 0.25% 1.50% 0.97% 9.12% 2.94% 3.61% 12 1.32% 2.35% 0.53% 1.03% 0.73% 2.34% 2.97% 1.08% 13 0.92% 2.46% 1.89% 1.10% 2.29% 5.00% 2.83% 3.37% 14 0.61% 1.97% 0.39% 1.03% 1.77% 2.38% 2.59% 6.51% 15 0.41% 2.15% 0.16% 1.82% 2.59% 4.41% 1.40% 3.46% 16 0.84% 1.64% 1.01% 0.75% 2.50% 2.86% 2.76% 5.34% 17 0.33% 2.31% 0.47% 1.38% 1.62% 6.51% 1.94% 5.25% 18 0.75% 1.22% 0.56% 1.02% 2.80% 3.32% 0.90% 2.60% 19 0.69% 3.23% 0.63% 0.95% 2.00% 3.29% 1.56% 2.56% 20 0.75% 2.36% 0.80% 1.69% 1.24% 3.98% 1.56% 2.35% 21 0.85% 1.47% 0.41% 0.96% 1.93% 3.08% 1.30% 3.97% 22 1.14% 2.23% 0.62% 3.20% 2.12% 2.88% 1.10% 2.48% 23 0.33% 2.80% 1.34% 2.34% 3.04% * 3.25% 4.38% 24 0.48% 4.72% 0.87% 2.14% 1.90% * 1.72% 3.98% 25 0.72% 5.33% 0.71% 1.79% 2.46% * 2.47% 3.89% 26 0.68% 2.46% 0.94% 3.15% 2.12% * 1.72% 2.58% 27 0.68% 2.95% 1.00% 2.63% 3.35% * 2.51% 1.70% 28 0.75% * 0.83% 4.54% 3.00% * 2.29% 1.98% 29 1.09% * 0.96% 3.32% 2.22% * 1.03% 1.48% 30 0.48% * 0.88% 2.69% 2.12% * 1.08% 1.53% 31 * * 0.67% 1.35% 2.33% * 1.63% 2.03% 32 * * 0.71% 1.79% 2.46% * 2.47% 3.89% 33 * * 1.10% 1.45% 1.93% * 1.00% 1.19% 34 * * 0.68% 0.96% 2.94% * 1.74% 1.04% 35 * * 1.35% 3.05% 1.86% * 0.83% 2.92% 36 * * 1.11% * 2.31% * 1.10% * 37 * * 0.66% * 1.45% * 1.64% * 38 * * 0.63% * 1.54% * 2.16% * 39 * * 0.46% * 1.35% * 1.81% * 40 * * 0.42% * 2.23% * 1.61% * 41 * * 0.65% * 1.95% * 0.86% * 42 * * 0.55% * 2.02% * 1.50% * 43 * * 0.79% * * * 1.86% * Ave. 0.79% 2.69% 0.83% 1.60% 2.03% 4.20% 2.00% 3.43% Set Average = 1.48% Set Average = 2.92%

Corp., for use in Microsoft Excel. The microcrack densities were tested for statistical inference

using a one-way ANOVA approach with unstacked variables at a 95% confidence level.

Confidence intervals for mean differences were also calculated using the Tukey confidence

interval correction method. Tukey's method considers all possible pairwise differences of means

at the same time and is considered a conservative approach for sample populations with unequal

sample sizes. Detailed ANOVA results tables are included in Appendix A for all statistical analysis procedures.

The statistical test for the analysis of microcrack density data of the oven dried and non- oven dried specimens was performed to determine if the observed data is consistent with the

ANOVA null hypothesis that all sample population means do not vary significantly. In Figure 5-

2, box plots of crack densities for oven-dried and non-dried sample specimens are shown.

OD RT-2 OD RT-1 RT-2 RT-1 OD HT-2 OD HT-1 HT-2 HT-1

0.00% 2.00% 4.00% 6.00% 8.00% Microcrack density [μm/μm2] (%)

Figure 5-2. Box plots of microcrack densities for oven-dried (OD RT & OD HT) and non-dried (RT & HT) sample specimens.

ANOVA analysis reinforced the conclusion that significant differences do exist between

the sample population means. It is expected that the density data for the oven-dried samples vary

considerably within the sample specimens since the effects of overheating will produce irregular

results in the microcracking behavior. This reasoning explains the larger variances within most

the oven-dried sample data shown in Figure 5-2. Meanwhile variances between the sample

means are also quite large for OD and non-dried samples. As shown in Table A-1 of Appendix

A, the mean densities between specimens RT-1, RT-2, HT-1, HT-2 and their oven-dried

counterparts, OD RT-1, OD RT-2, OD HT-1, and OD HT-2 are significantly different.

Analysis of Effect of High Curing Temperatures on Microcracking

The first part of the analysis showed that the procedure for specimen preparation with

epoxy impregnation, lapping, grinding and polishing, provided a valid representation of the in situ conditions of the concrete microstructure. A different test result would have otherwise

dismissed all other imaging analysis data in this study. Instead, it was found that the average

crack density was increased by over 50% due to the induced dry shrinkage cracking.

The room temperature and high temperature cured sample specimens prepared were

95B00PRT-1, RT-2 and RT-3, and 95B00PHT-1, HT-2 and HT-3, respectively. At first hand,

the average crack density results indicate that the average crack density for the room temperature

sample group is higher that the high temperature sample group density. This result is

contradictory with the expectations of the project, were high temperature conditions would

produce larger quantities of microcracks. This result provides no evidence of increased levels of microcrack behavior in high temperature conditions. Table 5-2 shows the data from the microcrack quantification and density calculations and Figure 5-3 illustrates the obtained mean microcrack density values for room temperature and high temperature sample specimen groups.

However, in order to discover how the sampled means are significantly different from another a second stage ANOVA analysis was performed. The second stage analysis is accomplished via confidence intervals. The data from the confidence interval analysis is presented in Table A-2 of

Appendix A.

Table 5-2. Microcrack density values for high temperature (HT) and room temperature (RT) specimens. Image Microcrack Density Values [μm/μm2] (%) No. HT-1 HT-2 HT-3 RT-1 RT-2 RT-3 1 1.32% 4.15% 0.85% 1.89% 1.10% 2.31% 2 1.06% 2.69% 1.26% 1.42% 0.55% 3.54% 3 1.85% 1.66% 0.99% 1.35% 0.48% 2.43% 4 0.30% 2.34% 1.29% 1.39% 0.54% 3.20% 5 0.71% 2.66% 0.24% 0.86% 0.63% 4.46% 6 0.91% 3.18% 0.37% 0.79% 0.52% 2.54% 7 0.80% 2.75% 0.96% 0.94% 1.23% 2.19% 8 0.93% 4.08% 0.23% 0.84% 0.81% 3.03% 9 0.98% 2.93% 0.37% 0.70% 0.94% 2.94% 10 0.43% 2.60% 0.44% 0.41% 1.50% 2.32% 11 0.62% 2.05% 0.42% 0.25% 1.50% 2.58% 12 1.32% 2.35% 0.85% 0.53% 1.03% 4.04% 13 0.92% 2.46% 0.31% 1.89% 1.10% 3.55% 14 0.61% 1.97% 0.31% 0.39% 1.03% 2.85% 15 0.41% 2.15% 0.26% 0.16% 1.82% 3.22% 16 0.84% 1.64% 0.46% 1.01% 0.75% 2.98% 17 0.33% 2.31% 0.37% 0.47% 1.38% 5.06% 18 0.75% 1.22% 0.19% 0.56% 1.02% 2.04% 19 0.69% 3.23% 0.37% 0.63% 0.95% 4.02% 20 0.75% 2.36% 0.29% 0.80% 1.69% 2.93% 21 0.85% 1.47% 0.18% 0.41% 0.96% 1.27% 22 1.14% 2.23% 0.20% 0.62% 3.20% * 23 0.33% 2.80% 0.15% 1.34% 2.34% * 24 0.48% 4.72% 0.29% 0.87% 2.14% * 25 0.72% 5.33% 0.23% 0.71% 1.79% * 26 0.68% 2.46% 0.40% 0.94% 3.15% * 27 0.68% 2.95% 0.29% 1.00% 2.63% * 28 0.75% * 0.34% 0.83% 4.54% * 29 1.09% * 0.53% 0.96% 3.32% * 30 0.48% * 0.37% 0.88% 2.69% * 31 * * * 0.67% 1.35% * 32 * * * 0.71% 1.79% * 33 * * * 1.10% 1.45% * 34 * * * 0.68% 0.96% * 35 * * * 1.35% 3.05% * 36 * * * 1.11% * * 37 * * * 0.66% * * 38 * * * 0.63% * * 39 0.46% 40 0.42% 41 0.65% 42 0.55% 43 0.79% Ave. 0.79% 2.69% 0.46% 0.83% 1.60% 3.02% Set Average = 1.32% Set Average = 1.82%

2.00%

1.80% RT sample specimens 1.60% ] (%) 2 1.40% m HT sample

μ 1.82% specimens m/ 1.20% μ 1.32% 1.00%

0.80%

0.60%

0.40% Microcrack density [ density Microcrack

0.20%

0.00%

Figure 5-3. Average microcrack density comparison of room temperature and high temperature sample specimens.

Table A-2 shows incongruous statistical information which follows the pattern found in the variation of mean values for the sample densities. The differences in density means between all pairs of specimens are both significant and non-significant for different pairings. No statistical evidence exists to corroborate the null hypothesis test that all means are equal. This disparity is better appreciated from the box plots shown in Figure 5-4. From Figure 5-4 it is evident that large variances exist between and within the sample densities. For specimens RT-2, RT-3 and

HT-2 large variances occur within their sample data. Meanwhile relatively smaller variances occur within the densities of samples RT-1, HT-1 and HT-3. The large variances in the data are most likely a result of experimental error in the sample preparation and image analysis processes.

One of the sources of error which may be influential in this particular example is operator bias during image analysis. In order to accomplish the automated quantification of microcracks three operators were required to perform the analysis of the 328 images. Nevertheless, the

RT-3

RT-2

RT-1

HT-3

HT-2

HT-1

0.00% 1.00% 2.00% 3.00% 4.00% 5.00% Microcrack Density [μm/μm2] (%)

Figure 5-4. Box plots of microcrack densities for room temperature (RT) and high temperature (HT) sample specimens. results have shown that no correlation exists between high temperature levels during curing and the density of microcracks in the concrete samples.

In Chapter 6 several recommendations will be given based on the findings of this study.

Due to time constraints the test and imaging procedures could not be repeated over several iterations for sensitivity analysis. An in-depth analysis of the possible sources of error should be performed to better control experimental error.

CHAPTER 6 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

Summary

The main objective of this project was to determine if a significant increase in microcrack formations are developed as a result of high temperature levels during curing of mass concrete structures. In addition, the method for preparing concrete specimens for microscopic study was tested for suitability. The following is a summary of the steps taken to complete the objectives of this project.

A literature review was conducted to identify industry practices for specimen preparation in microscopy analysis. Various research papers related to the detection and quantification of microcracks in concrete were evaluated. The precedent research provided the guidelines for determining the resources required to implement the specimen preparation and image analysis techniques. Previous image analysis procedures via SEM-generated images were studied to benchmark available techniques for automated quantification and detection of microcracks. The literature review showed that techniques for specimen preparation exist since the 1980s, some of which require equipment and resources that were out of the scope of this project. Other procedures would undermine the requirement to study the concrete in situ conditions. The techniques that involved the least destructive petrography procedures were used for the project.

The literature review revealed that no standardization of specimen preparation techniques existed. No assurance was found that a specific procedure could be performed without the byproduct of secondary cracking. The in situ study of concrete microstructure is an area of great debate. Evidently, physical limitations such as cutting and grinding the concrete during the preparation process produce surface deformations and add secondary cracking. In spite of this,

the specimen preparation process used for this study represents the most viable, cost effective

method available.

The first step in the experimental process was to determine if the specimen preparation technique selected was not inducing enough microcracking to significantly tamper the results.

Once the technique was verified, additional specimens were prepared to provide a larger sample

population for measuring microcrack generation during curing at elevated temperatures.

The specimens used for the study were extracted from a systematically selected batch of concrete mix samples. Two sets of specimens were paired for comparison. Each specimen cured at high temperature had a room temperature counterpart specimen. Similarly oven-dried control specimens were prepared to test the quality of the specimen preparation.

The images generated from each sample specimen were analyzed with an imaging analysis computer program. The software tools provided outputs of data with the total length of microcracks found in the image fields. The criterion used for the differentiation of cracks from voids and other concrete matrix features was established.

The scanned image fields via SEM at 150X magnification produced images sizes of

650μm x 870μm. The total image area analyzed in this project represents only 1% of the total surface area of the concrete specimens.

Conclusions

The following conclusions can be made after the completing the analysis of the results for this study:

• The procedure for specimen preparation was tested for quality and significant differences were observed in the microcrack density data. The oven-dried specimens displayed approximately 98% more microcracking than the regular specimens. Figure 6-1 illustrates microcrack quantification output images for both conditions.

A) B) Figure 6-1. Microcrack formation output images. A) Oven-dried specimen, B) Regular specimen.

• Research revealed no relationship between concrete curing temperature and the concentration of microcracks in the sampled concrete.

• Statistical inference analysis was implemented to further investigate the nature of the findings. The analysis demonstrated that no significant differences exist between the mean crack densities for room temperature cured samples (RT) and high temperature cured samples (HT).

Based on the aforementioned findings, it can be concluded that the temperature levels reached during mass concrete curing do not significantly influence the formation of microcracks in the concrete structure.

Some noteworthy factors that may have influenced the obtained results include:

• Exposure to heat during epoxy-ethanol replacement and impregnation.

• Manual control of grinding and polishing activities.

• Operator bias in microcrack feature identification.

• Variations in image grayscale qualities possibly resulting from the golden colored epoxy solution used for this study.

• Percentage of specimen surface area studied (1% of total available surface).

Recommendations

The results of this study showed that current specifications by the Florida Department of

Transportation for monitoring temperature differentials in mass concrete are not susceptible to

microcrack formation due to the lack of control of high temperature conditions. The results of this study did not reveal significant differences in microcrack formation for curing temperatures up to 185°F. Therefore, a recommendation to modify FDOT mass concrete structure design standards could not be justified on the basis of microcrack formation due to elevated temperature levels during curing.

The following recommendations are provided for conducting future work in this field of study:

• During grinding of specimen a method to control applied pressure should be implemented. This can be achieved by upgrading to a grinder/polisher machine with programmable polishing steps, a sample holder, and sample force controller. (Equal or similar to MetPrep3 by Allied High Tech Products, Inc.)

• For the sectioning step a diamond wafer saw with a lower cutting speed would also reduce the potential for secondary crack formation.

• The ultra low viscosity kit proved to be very difficult to work with, particularly in achieving full polymerization of the epoxy. A simpler resin-hardener based epoxy product with sufficient low viscosity characteristics could be utilized.

• With the use of a vacuum pump chamber, the epoxy impregnation process could be accelerated and improved, without the need of dehydration with ethanol. A product equal or similar to the Epovac vacuum impregnation apparatus by Struers, Inc. could facilitate this process.

• A study of specimens from the samples with different pozzolanic material content would be an interesting topic of research. Future research could be developed to evaluate the effects of the combination of high temperatures during curing and pozzolanic content on microcrack formation.

APPENDIX A IMAGE ANALYSIS DATA RESULTS AND ANOVA STATISTICS

Table A-1. Results of one-way ANOVA for density values of oven-dried (OD) and standard specimens. Summary: OD OD OD OD HT-1 HT-2 RT-1 RT-2 HT-1 HT-2 RT-1 RT-2 Sample sizes 31 28 44 36 43 23 44 36 Sample means 0.79% 2.69% 0.83% 1.60% 2.03% 4.20% 2.00% 3.43% Sample std. dev. 0.33% 0.94% 0.38% 0.96% 0.86% 1.75% 0.74% 1.46% Sample variances 0.001% 0.009% 0.001% 0.009% 0.007% 0.030% 0.005% 0.021% Weights for pooled variances 10.8% 9.7% 15.5% 12.6% 15.2% 7.9% 15.5% 12.6%

No. of samples 8 Total sample size 285 Grand mean 2.07% Pooled variance 0.01% Pooled std. dev. 0.97%

One-Way ANOVA Deg. of Source SS freed. MS F pvalue Between variation 3.09% 7 0.44% 46.731 0.0000 Within variation 2.62% 277 0.01% Total variation 5.71% 284

Conf. intervals for mean diff. Confidence level 95.0% Tukey method Difference Mean diff Lower Upper Significant? HT-1 - OD HT-1 -1.24% -1.94% -0.54% Yes HT-1 - OD HT-2 -3.41% -4.23% -2.59% Yes HT-1 - OD RT-1 -1.21% -1.91% -0.51% Yes HT-1 - OD RT-2 -2.64% -3.37% -1.91% Yes HT-2 - OD HT-1 0.66% -0.06% 1.39% No HT-2 - OD HT-2 -1.51% -2.35% -0.67% Yes HT-2 - OD RT-1 0.70% -0.02% 1.42% No HT-2 - OD RT-2 -0.74% -1.49% 0.01% No RT-1 - OD HT-1 -1.20% -1.84% -0.57% Yes RT-1 - OD HT-2 -3.38% -4.14% -2.61% Yes RT-1 - OD RT-1 -1.17% -1.81% -0.54% Yes RT-1 - OD RT-2 -2.60% -3.27% -1.94% Yes RT-2 - OD HT-1 -0.43% -1.11% 0.24% No RT-2 - OD HT-2 -2.61% -3.40% -1.81% Yes RT-2 - OD RT-1 -0.40% -1.07% 0.27% No RT-2 - OD RT-2 -1.83% -2.54% -1.13% Yes

Table A-2. Results of one-way ANOVA for density values for room temperature (RT) and high temperature (HT) sample specimens. Summary: HT-1 HT-2 HT-3 RT-1 RT-2 RT-3 Sample sizes 30 27 30 43 35 21 Sample means 0.79% 2.69% 0.46% 0.83% 1.60% 3.02% Sample std. dev. 0.34% 0.95% 0.31% 0.39% 0.97% 0.88% Sample variances 0.0011% 0.0091% 0.0010% 0.0015% 0.0095% 0.0077% Weights for pooled variances 16.11% 14.44% 16.11% 23.33% 18.89% 11.11%

No. of samples 6 Total sample size 186 Grand mean 1.426% Pooled variance 0.005% Pooled std. dev. 0.682%

One-Way ANOVA Deg. of Source SS freed. MS F pvalue Between variation 1.535% 5 0.307% 65.996 0.0000 Within variation 0.837% 180 0.005% Total variation 2.372% 185

Conf. intervals for mean difference Confidence level 95.0% Tukey method Difference Mean diff Lower Upper Significant? HT-1 - HT-2 -1.90% -2.43% -1.38% Yes HT-1 - HT-3 0.33% -0.18% 0.84% No HT-1 - RT-1 -0.04% -0.51% 0.43% No HT-1 - RT-2 -0.81% -1.30% -0.32% Yes HT-1 - RT-3 -2.23% -2.79% -1.67% Yes HT-2 - HT-3 2.23% 1.71% 2.76% Yes HT-2 - RT-1 1.87% 1.38% 2.35% Yes HT-2 - RT-2 1.10% 0.59% 1.60% Yes HT-2 - RT-3 -0.33% -0.90% 0.25% No HT-3 - RT-1 -0.37% -0.84% 0.10% No HT-3 - RT-2 -1.14% -1.63% -0.65% Yes HT-3 - RT-3 -2.56% -3.12% -2.00% Yes RT-1 - RT-2 -0.77% -1.22% -0.32% Yes RT-1 - RT-3 -2.20% -2.72% -1.67% Yes RT-2 - RT-3 -1.42% -1.97% -0.88% Yes

Table A-3. Specimen image analysis database. SAMPLE 95B00PHT-1 SAMPLE 95B00PHT-2 SAMPLE 95B00PHT-3 SAMPLE 95B00PRT-1 IMAGE TOTAL FIELD TOTAL FIELD TOTAL FIELD TOTAL FIELD NO. LENGTH AREA Density LENGTH AREA Density LENGTH AREA Density LENGTH AREA Density μm/μm2 μm/μm2 μm/μm2 μm/μm2 μm μm2 μm μm2 μm μm2 μm μm2 (%) (%) (%) (%) 1 7,491.77 565,500 1.32% 23,467.8 565,500 4.15% 4,826.36 565,500 0.85% 10,677.7 565,500 1.89% 2 5,983.83 565,500 1.06% 15,224.9 565,500 2.69% 7,101.48 565,500 1.26% 8,037.01 565,500 1.42% 3 10,470.9 565,500 1.85% 9,405.5 565,500 1.66% 5,596.59 565,500 0.99% 7,662.43 565,500 1.35% 4 1,689.90 565,500 0.30% 13,244.0 565,500 2.34% 7,317.64 565,500 1.29% 7,843.53 565,500 1.39% 5 * * * 15,067.1 565,500 2.66% * * * 4,864.01 565,500 0.86% 6 4,027.73 565,500 0.71% 17,955.6 565,500 3.18% 1,366.88 565,500 0.24% 4,460.26 565,500 0.79% 7 5,131.49 565,500 0.91% 15,565.1 565,500 2.75% 2,084.95 565,500 0.37% 5,307.58 565,500 0.94% 8 4,548.57 565,500 0.80% 23,055.1 565,500 4.08% 5,406.50 565,500 0.96% 4,740.08 565,500 0.84% 9 5,261.23 565,500 0.93% 16,543.7 565,500 2.93% 1,322.60 565,500 0.23% 3,957.26 565,500 0.70% 10 5,568.56 565,500 0.98% 14,729.4 565,500 2.60% 2,099.34 565,500 0.37% 2,305.18 565,500 0.41% 11 2,421.67 565,500 0.43% 11,579.7 565,500 2.05% 2,511.63 565,500 0.44% 1,414.15 565,500 0.25% 12 3,484.91 565,500 0.62% 13,282.2 565,500 2.35% 2,350.72 565,500 0.42% 3,000.55 565,500 0.53% 13 7,491.77 565,500 1.32% 13,929.9 565,500 2.46% 4,826.36 565,500 0.85% 10,677.7 565,500 1.89% 14 5,213.27 565,500 0.92% 11,125.0 565,500 1.97% * * * 2,193.10 565,500 0.39% 15 3,445.32 565,500 0.61% 12,155.2 565,500 2.15% 1,736.71 565,500 0.31% 927.84 565,500 0.16% 16 2,303.45 565,500 0.41% 9,296.56 565,500 1.64% 1,760.96 565,500 0.31% 5,707.95 565,500 1.01% 17 4,777.85 565,500 0.84% 13,066.6 565,500 2.31% 1,488.34 565,500 0.26% 2,645.15 565,500 0.47% 18 1,860.06 565,500 0.33% 6,915.8 565,500 1.22% 2,613.02 565,500 0.46% 3,161.14 565,500 0.56% 19 4,217.37 565,500 0.75% 18,251.3 565,500 3.23% 2,084.28 565,500 0.37% 3,559.77 565,500 0.63% 20 3,882.76 565,500 0.69% 13,349.5 565,500 2.36% 1,050.66 565,500 0.19% 4,523.52 565,500 0.80% 21 4,234.28 565,500 0.75% 8,303.56 565,500 1.47% * * * 2,315.60 565,500 0.41% 22 4,810.63 565,500 0.85% 12,627.8 565,500 2.23% * * * 3,484.24 565,500 0.62% 23 6,451.88 565,500 1.14% 15,814.2 565,500 2.80% * * * 7,553.78 565,500 1.34% 24 1,847.11 565,500 0.33% 26,697.7 565,500 4.72% 2,073.01 565,500 0.37% 4,941.52 565,500 0.87% 25 2,740.36 565,500 0.48% 30,149.8 565,500 5.33% 1,633.38 565,500 0.29% 4,040.99 565,500 0.71% 26 4,084.78 565,500 0.72% 13,925.3 565,500 2.46% 1,017.60 565,500 0.18% 5,291.40 565,500 0.94% 27 3,847.41 565,500 0.68% 16,701.1 565,500 2.95% 1,126.98 565,500 0.20% 5,646.08 565,500 1.00% 28 3,830.48 565,500 0.68% * * * 852.33 565,500 0.15% 4,701.32 565,500 0.83% 29 4,221.85 565,500 0.75% * * * 1,613.22 565,500 0.29% 5,441.09 565,500 0.96% 30 6,181.49 565,500 1.09% * * * 1,328.64 565,500 0.23% 4,958.20 565,500 0.88% 31 * * * * * * 2,241.99 565,500 0.40% 3,777.75 565,500 0.67% 32 2,740.36 565,500 0.48% * * * 1,633.38 565,500 0.29% 4,040.99 565,500 0.71% 33 * * * * * * 1,938.08 565,500 0.34% 6,234.84 565,500 1.10% 34 * * * * * * 3,003.31 565,500 0.53% 3,821.89 565,500 0.68% 35 * * * * * * 2,068.63 565,500 0.37% 7,609.57 565,500 1.35% 36 * * * * * * * * * 6,275.83 565,500 1.11% 37 * * * * * * * * * 3,746.51 565,500 0.66% 38 * * * * * * * * * 3,534.84 565,500 0.63% 39 * * * * * * * * * 2,596.82 565,500 0.46% 40 * * * * * * * * * 2,391.93 565,500 0.42% 41 * * * * * * * * * 3,648.79 565,500 0.65% 42 * * * * * * * * * 3,090.04 565,500 0.55% 43 * * * * * * * * * 4,487.18 565,500 0.79% Ave. 0.79% 2.69% 0.46% 0.83% Std. Dev. 0.34% 0.95% 0.31% 0.39%

Table A-3. Continued. SAMPLE 95B00PRT-2 SAMPLE 95B00PRT-3 SAMPLE 95B00PODHT-1 SAMPLE 95B00PODHT-2 IMAGE TOTAL FIELD TOTAL FIELD TOTAL FIELD TOTAL FIELD NO. LENGTH AREA Density LENGTH AREA Density LENGTH AREA Density LENGTH AREA Density μm/μm2 μm/μm2 μm/μm2 μm/μm2 μm μm2 μm μm2 μm μm2 μm μm2 (%) (%) (%) (%) 1 6,200.0 565,500 1.10% 13,046.4 565,500 2.31% 2,946.7 565,500 2.29% 13,240.7 565,500 2.34% 2 3,130.0 565,500 0.55% 20,037.0 565,500 3.54% 14,147.5 565,500 2.50% 28,556.1 565,500 5.05% 3 2,734.0 565,500 0.48% 13,751.5 565,500 2.43% 5,553.1 565,500 0.98% 21,081.0 565,500 3.73% 4 3,066.0 565,500 0.54% 8,069.4 565,500 3.20% 6,738.2 565,500 1.19% 13,768.0 565,500 2.43% 5 3,535.0 565,500 0.63% * * * 3,921.2 565,500 0.69% * * * 6 2,941.0 565,500 0.52% 25,207.3 565,500 4.46% 1,486.4 565,500 0.26% 15,808.4 565,500 2.80% 7 6,938.0 565,500 1.23% 14,350.4 565,500 2.54% 24,294.5 565,500 4.30% 23,529.1 565,500 4.16% 8 4,607.0 565,500 0.81% 12,385.9 565,500 2.19% 24,294.5 565,500 4.30% 24,070.9 565,500 4.26% 9 5,340.0 565,500 0.94% 17,159.2 565,500 3.03% 5,498.8 565,500 0.97% 35,958.9 565,500 6.36% 10 8,500.0 565,500 1.50% 16,620.3 565,500 2.94% 4,061.6 565,500 0.72% 40,569.6 565,500 7.17% 11 8,500.0 565,500 1.50% 13,105.5 565,500 2.32% 5,490.6 565,500 0.97% 28,304.8 565,500 5.01% 12 5,800.0 565,500 1.03% 14,598.5 565,500 2.58% 4,118.3 565,500 0.73% 51,556.2 565,500 9.12% 13 6,200.0 565,500 1.10% 22,819.7 565,500 4.04% 12,946.7 565,500 2.29% 3,240.7 565,500 2.34% 14 5,851.4 565,500 1.03% 20,047.4 565,500 3.55% 10,008.8 565,500 1.77% 28,254.2 565,500 5.00% 15 10,291.6 565,500 1.82% 16,108.5 565,500 2.85% 14,666.1 565,500 2.59% 13,479.0 565,500 2.38% 16 4,244.0 565,500 0.75% 18,212.9 565,500 3.22% 14,154.3 565,500 2.50% 24,954.5 565,500 4.41% 17 7,785.2 565,500 1.38% 16,838.7 565,500 2.98% 9,170.6 565,500 1.62% 16,194.1 565,500 2.86% 18 5,768.9 565,500 1.02% 28,603.0 565,500 5.06% 15,813.4 565,500 2.80% 36,802.4 565,500 6.51% 19 5,348.5 565,500 0.95% 11,560.2 565,500 2.04% 11,286.6 565,500 2.00% 18,755.1 565,500 3.32% 20 9,562.2 565,500 1.69% 22,714.2 565,500 4.02% 7,009.0 565,500 1.24% 18,611.6 565,500 3.29% 21 5,424.4 565,500 0.96% 16,569.2 565,500 2.93% 10,886.8 565,500 1.93% 22,517.2 565,500 3.98% 22 18,090.8 565,500 3.20% 7,181.7 565,500 1.27% 11,999.9 565,500 2.12% 17,412.4 565,500 3.08% 23 13,235.2 565,500 2.34% * * * 17,190.0 565,500 3.04% 6,293.6 565,500 2.88% 24 12,084.7 565,500 2.14% * * * 10,735.6 565,500 1.90% * * * 25 0,144.4 565,500 1.79% * * * 13,934.2 565,500 2.46% * * * 26 17,788.4 565,500 3.15% * * * 11,993.1 565,500 2.12% * * * 27 14,877.6 565,500 2.63% * * * 18,957.5 565,500 3.35% * * * 28 25,651.8 565,500 4.54% * * * 16,944.4 565,500 3.00% * * * 29 18,796.7 565,500 3.32% * * * 12,580.9 565,500 2.22% * * * 30 15,202.7 565,500 2.69% * * * 11,998.6 565,500 2.12% * * * 31 7,650.7 565,500 1.35% * * * 13,170.5 565,500 2.33% * * * 32 0,144.4 565,500 1.79% * * * 13,934.2 565,500 2.46% * * * 33 8,198.6 565,500 1.45% * * * 10,912.5 565,500 1.93% * * * 34 5,442.0 565,500 0.96% * * * 16,625.6 565,500 2.94% * * * 35 17,262.0 565,500 3.05% * * * 10,498.8 565,500 1.86% * * * 36 * * * * * * 13,065.8 565,500 2.31% * * * 37 * * * * * * 8,175.6 565,500 1.45% * * * 38 * * * * * * 8,716.7 565,500 1.54% * * * 39 * * * * * * 7,625.7 565,500 1.35% * * * 40 * * * * * * 12,605.9 565,500 2.23% * * * 41 * * * * * * 11,052.3 565,500 1.95% * * * 42 * * * * * * 11,450.1 565,500 2.02% * * * 43 * * * * * * * * * * * * Ave. 1.60% 3.02% 2.03% 4.20% Std. Dev. 0.97% 0.88% 0.87% 1.79%

Table A-3. Continued SAMPLE 95B00PODRT-1 SAMPLE 95B00PODRT-2 IMAGE TOTAL FIELD TOTAL FIELD NO. LENGTH AREA Density LENGTH AREA Density μm/μm2 μm/μm2 μm μm2 μm μm2 (%) (%) 1 16,022.1 565,500 2.83% 9,063.1 565,500 3.37% 2 4,868.0 565,500 2.63% 16,995.3 565,500 3.01% 3 14,915.6 565,500 2.64% 21,693.3 565,500 3.84% 4 11,914.9 565,500 2.11% 29,033.5 565,500 5.13% 5 12,911.0 565,500 2.28% 20,954.3 565,500 3.71% 6 13,599.2 565,500 2.40% 23,183.8 565,500 4.10% 7 17,684.2 565,500 3.13% 33,686.8 565,500 5.96% 8 6,440.8 565,500 1.14% 33,460.8 565,500 5.92% 9 19,629.9 565,500 3.47% 26,789.9 565,500 4.74% 10 17,686.9 565,500 3.13% 29,312.5 565,500 5.18% 11 16,602.6 565,500 2.94% 20,389.6 565,500 3.61% 12 16,801.9 565,500 2.97% 6,132.0 565,500 1.08% 13 16,022.1 565,500 2.83% 19,063.1 565,500 3.37% 14 14,646.2 565,500 2.59% 36,788.0 565,500 6.51% 15 7,921.4 565,500 1.40% 19,581.5 565,500 3.46% 16 15,625.8 565,500 2.76% 30,206.5 565,500 5.34% 17 10,977.4 565,500 1.94% 29,701.0 565,500 5.25% 18 5,094.8 565,500 0.90% 14,721.3 565,500 2.60% 19 8,813.4 565,500 1.56% 14,492.9 565,500 2.56% 20 8,812.6 565,500 1.56% 13,285.6 565,500 2.35% 21 7,361.0 565,500 1.30% 22,469.4 565,500 3.97% 22 6,243.7 565,500 1.10% 14,025.6 565,500 2.48% 23 18,389.3 565,500 3.25% 24,781.7 565,500 4.38% 24 9,731.2 565,500 1.72% 22,514.4 565,500 3.98% 25 13,976.9 565,500 2.47% 22,007.7 565,500 3.89% 26 9,714.9 565,500 1.72% 14,588.4 565,500 2.58% 27 14,217.9 565,500 2.51% 9,630.9 565,500 1.70% 28 12,968.1 565,500 2.29% 11,222.2 565,500 1.98% 29 5,813.5 565,500 1.03% 8,364.4 565,500 1.48% 30 6,125.7 565,500 1.08% 8,676.3 565,500 1.53% 31 9,224.6 565,500 1.63% 11,467.5 565,500 2.03% 32 13,976.9 565,500 2.47% 22,007.7 565,500 3.89% 33 5,660.4 565,500 1.00% 6,730.0 565,500 1.19% 34 9,850.4 565,500 1.74% 5,882.0 565,500 1.04% 35 4,711.1 565,500 0.83% 16,520.8 565,500 2.92% 36 6,196.7 565,500 1.10% * * * 37 9,288.7 565,500 1.64% * * * 38 12,203.4 565,500 2.16% * * * 39 10,218.8 565,500 1.81% * * * 40 9,131.2 565,500 1.61% * * * 41 4,853.4 565,500 0.86% * * * 42 8,487.1 565,500 1.50% * * * 43 10,537.5 565,500 1.86% * * * Ave. 2.00% 3.43% Std. Dev. 0.75% 1.48%

APPENDIX B GLOSSARY

Fly ash: The finely divided residue that results form the combustion of ground or powdered coal and that is transported by flue gases from the combustion zone to the particle removal system. (American Concrete Institute, ACI 116-R00)

Map cracking: Intersecting cracks that extend below the surface of hardened concrete; caused by shrinkage of the drying surface concrete that is restrained by concrete at greater depths where either little or no shrinkage occurs; vary in width from fine and barely visible to open and well-defined. (American Concrete Institute, ACI 116-R00)

Mass concrete: Any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change, to minimize cracking. (American Concrete Institute, ACI 116- R00)

Microcrack*: Microscopic fracture with dimensions greater than 20 microns in length and less than 10 microns in width. (*Definition developed during this research project as part of the image analysis methodology)

Pozzolan: A siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but that will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties; there are both natural and artificial pozzolans. (American Concrete Institute, ACI 116-R00)

Pozzolan, artificial: Materials such as fly ash and silica fume. (American Concrete Institute, ACI 116-R00)

Pozzolan, natural: A raw or calcined natural material that has pozzolanic properties (for example, volcanic tuffs or pumicites, opaline cherts and shales, clays, and diatomaceous earths). (American Concrete Institute, ACI 116-R00)

Scanning electron microscope (SEM): An electron microscope in which the image is formed by a beam operating in synchronism with an electron probe scanning the object; the intensity of the image-forming beam is proportional to the scattering or secondary emission of electrons by the specimen where the probe beam strikes it. (American Concrete Institute, ACI 116-R00)

Thermal cracking: Cracking due to tensile failure, caused by a temperature drop in members subjected to external restraints or by a temperature differential in members subjected to internal restraints. (American Concrete Institute, ACI 116-R00)

LIST OF REFERENCES

American Concrete Institute (ACI) Committee Manual of Concrete Practice, “Cement and concrete terminology (ACI 116R-00)”, Farmington Hills, Michigan, (2001).

American Concrete Institute (ACI) Committee Manual of Concrete Practice, Report 207.1R-96, “Mass Concrete”, Farmington Hills, Michigan, (2003).

American Concrete Institute (ACI) Committee Manual of Concrete Practice, Report 207.2R-95, “Effect of restraint, volume change, and reinforcement on cracking of mass concrete”, Farmington Hills, Michigan, (2003).

American Concrete Institute (ACI) Committee Manual of Concrete Practice, Report 224R-01, “Control of cracking in concrete structures”, Farmington Hills, Michigan, (2001).

American Society for Testing and Materials, ASTM C 856, “Standard practice for petrographic examination of hardened concrete”, 1998 Annual Book of ASTM Standards, V. 04.02, Concrete and Aggregates, pp.410-424, (1998).

Ammouche, A., Riss, J., Breysse, D., and Marchand, J., “Image analysis for the automated study of microcracks in concrete”, Cement and Concrete Composites, Vol. 23, pp. 267-278, (2001).

Bunday, B., Godwin, M., Lipscomb, P., Patel, D., and Bishop, M., International Sematech Manufacturing Initiative, Manufacturing Effectiveness Series - “Meeting manufacturing metrology challenges at 90nm and beyond”, (August 2005).

Chini, A. and Parham, A., “Adiabatic temperature rise of mass concrete in Florida”, Final Report to the Florida Department of Transportation, Gainesville, Florida, (February 2005).

Darwin, D., Abou-Zeid, M.N., and Ketchman, K.W., “Automated crack identification for cement paste”, Cement and Concrete Research, Vol. 25, No. 3, pp. 605-616, (1995).

Hornain, H., Marchand, J., Ammouche, A., Commène, J.P., and Moranville, M., “Microscopic observation of cracks in concrete – A new sample preparation technique using dye impregnation”, Cement and Concrete Research, Vol. 26, No. 4, pp. 573-583, (1996).

Hurd, M.K., American Concrete Institute (ACI) Special Publication Number 4, Formwork for Concrete, Chapter 13: “Mass Concrete”, Sixth Edition, Farmington Hills, Michigan, (1995).

Detwiler, R.J., Powers, L.J., Hjorth, U., Ahmed, W.U., Srivener, K.L., and Kjellsen, K.O., “Preparing specimens for microscopy”, Concrete International, Vol. 23, No. 11, pp. 51-58, (November 2001).

Florida Department of Transportation, Structural Design Guide, Section 3.9: “Mass Concrete”, Tallahassee, Florida, (2002).

Marusin, S., “Sample preparation – the key to SEM studies of failed concrete”, Cement and Concrete Composites, Vol.17, pp. 311-318, (1995).

Nemati, K.M., and Stroeven, P., “Stereological analysis of micromechanical behavior of concrete”, Materials and Structures, RILEM, Vol. 34, No. 242, pp. 486-494, (October 2001).

Patel, H.H., Bland, C.H., and Poole, A.B., “The microstructure of concrete cured at elevated temperatures”, Cement and Concrete Research, Vol. 25, No. 3, pp. 485-490, (April 1995).

Roy, D.M., Cady, P.D., Sabol, S.A., and Licastro, P.H. “Concrete microstructure: Recommended revisions to test methods”, Strategic Highway Research Program, National Research Council, Washington, DC (1993).

Shah, V., “Detection of microcracks in concrete cured at elevated temperatures”, Thesis for University of Florida, (2004).

Soroushian, P., Elzafraney, M., and Nossoni, A., “Specimen preparation and image processing and analysis techniques for automated quantification of concrete microcracks and voids”, Cement and Concrete Research, Vol. 33, No. 12, pp. 1917-2128, (December 2003).

Struble, L., and Stutzman, P.E., “Epoxy impregnation of hardened cement for microstructural characterization”, Journal of Materials Science Letters, Vol. 8, pp. 632-634, (1989).

Stutzman, P.E., and Clifton, J.R., “Specimen preparation for scanning electron microscopy”, Proceedings from the Twenty-First International Conference on Cement Microscopy, April 25-29, 1999, Las Vegas, NV, pp. 10-22, (1999).

BIOGRAPHICAL SKETCH

Enrique Javier Villavicencio Camacho was born in Río Piedras, Puerto Rico on August 22,

1978 to Dr. Rafael Villavicencio Jimenez and Dr. Nilda Camacho Arroyo. Enrique is married to

Bárbara Alonso Vila and has one daughter, Pía Cecilia.

Enrique was raised in Guaynabo, Puerto Rico along with two brothers and one sister. He graduated from high school in 1996 from Colegio San Ignacio de Loyola in Río Piedras, Puerto

Rico. In 2000 he obtained his Bachelor of Science degree in Mechanical Engineering from the

Massachusetts Institute of Technology in Cambridge, Massachusetts. Prior to commencing his graduate studies at the University of Florida, he worked for A² Group, Inc., an engineering and construction firm in of Miami, Florida. During his graduate career at the University of Florida,

Enrique worked as graduate research assistant to Dr. Abdol Chini at the M.E. Rinker, Sr. School of Building Construction. Upon completion of his graduate studies he plans to start up his own construction company in San Juan, Puerto Rico.