CHARACTERIZING SATURATED MASS TRANSPORT IN FRACTURED CEMENTITIOUS MATERIALS

ABSTRACT

Concrete, when designed and constructed properly, is a durable material. However in aggressive environments concrete is prone to gradual deterioration which is due to penetration of water and aggressive agents (e.g., chloride ions) into concrete. As such, the rate of mass transport is the primary factor, controlling the durability of cementitious materials. Some level of cracking is inevitable in concrete due to brittle nature of the material. While mass transport can occur through concrete’s porous matrix, cracks can significantly accelerate the rate of mass transport and effectively influence the service life of concrete structures. To allow concrete service life prediction models to correctly account for the effect of cracks on concrete durability, mass transport thru cracks must be characterized. In this study, transport properties of cracks are measured to quantify the saturated hydraulic permeability and diffusion coefficient of cracks as a function of crack geometry (i.e.; crack width, crack tortuosity and crack wall roughness). Saturated permeability and diffusion coefficient of cracks are measured by constant head permeability test, electrical migration test, and electrical impedance spectroscopy. Plain and fiber reinforced cement paste and mortar as well as simulated crack samples are tested. The results of permeability test showed that the permeability of a crack is a function of crack width squared and can be predicted using Louis formula when crack tortuosity and surface roughness of the crack walls are accounted for. The results of the migration and impedance tests showed that the diffusion coefficient of the crack is not dependent on the crack width, but is primarily a function of volume fraction of cracks. The only parameter that is changing with the crack width is the crack connectivity. Crack connectivity was found to be linearly dependent on crack width for small crack and constant for large cracks (i.e.; approximately larger than 80 μm). The results of this study can be used to predict diffusion and permeability coefficients of fractured concrete.

 

TABLE OF CONTENTS

 

List of Figures …………………………………………………………………………………………………………….. viii

List of Tables ……………………………………………………………………………………………………………… xiii

Chapter 1: Introduction ………………………………………………………………………………………………….1

1-1 Introduction …………………………………………………………………………………………………………………. 1

1-2 Research objectives ……………………………………………………………………………………………………… 2

1-3 Organization of contents ……………………………………………………………………………………………… 3

1-4 References …………………………………………………………………………………………………………………… 4

Chapter 2: Mechanisms of Deterioration and Mass Transport in concrete …………………………. 6

2-1 Concrete durability problems……………………………………………………………………………………….. 7 2-1-1 Corrosion of steel reinforcement ………………………………………………………………………….. 7

2-1-1-1 Carbonation …………………………………………………………………………………………. 11

2-1-1-2 Chloride attack …………………………………………………………………………………….. 12

2-1-2 Freeze/thaw damage …………………………………………………………………………………………… 14

2-1-3 Alkali silica reaction (ASR) ……………………………………………………………………………….. 17

2-1-4 Sulfate attack ……………………………………………………………………………………………………… 20

2-2 Mechanisms of mass transport in concrete  ………………………………………………………………… 22

2-2-1 Saturated permeation  …………………………………………………………………………………………. 23

2-2-2 Unsaturated permeation ……………………………………………………………………………………… 26

2-2-3 Diffusion ……………………………………………………………………………………………………………. 30

2-2-4 Other transport mechanisms ……………………………………………………………………………….. 34

2-3 Service-life prediction models ……………………………………………………………………………………. 34

2-3-1 Life-365 Service Life Prediction Model ……………………………………………………………… 34

2-3-2 STADIUM …………………………………………………………………………………………………………. 37

2-3-3 4SIGHT ……………………………………………………………………………………………………………… 38

2-4 Summary ……………………………………………………………………………………………………………………. 39

2-5 References …………………………………………………………………………………………………………………. 40

Chapter 3: Quantifying the Effects of Crack Width, Tortuosity, and Roughness on Water

Permeability of Cracked Mortars  ……………………………………………………………………………………….. 44

3-1 Introduction ……………………………………………………………………………………………………………….. 44

3-2 Quantifying the geometric properties of cracks ………………………………………………………….. 48

3-2-1 Effective crack width …………………………………………………………………………………………. 48

3-2-2 Crack tortuosity and surface roughness ………………………………………………………………. 53

3-3 Materials and experiments  ………………………………………………………………………………………… 56

3-3-1 Sample preparation …………………………………………………………………………………………….. 56

3-3-2 Permeability measurement …………………………………………………………………………………. 57

3-3-3 Measuring crack dimensions ………………………………………………………………………………. 59

3-4 Results and discussion ……………………………………………………………………………………………….. 60

3-4-1 Comparison between average, effective, and LVDT crack measurements…………… 60

3-4-2 Saturated permeability as a function of crack width ……………………………………………. 62

3-4-3 Crack tortuosity and surface roughness ………………………………………………………………. 63

3-4-4 Effect of tortuosity and roughness on crack permeability ……………………………………. 67

3-5 Conclusions ……………………………………………………………………………………………………………….. 69

3-6 References …………………………………………………………………………………………………………………. 70

Chapter 4: Evaluating Ion Diffusivity of Cracked Cement Paste Using Electrical

Impedance Spectroscopy  ………………………………………………………………………………………………75

4-1 Introduction ……………………………………………………………………………………………………………….. 75

4-2 Methods for Measuring the Diffusion Coefficient of Concrete …………………………………… 78

4-3 Theory ……………………………………………………………………………………………………………………….. 84

4-4 Materials and Experiments …………………………………………………………………………………………. 94

4-5 Results and Discussion ………………………………………………………………………………………………. 99

4-6 Conclusion ……………………………………………………………………………………………………………….. 103

4-7 References ……………………………………………………………………………………………………………….. 103

Chapter 5: Permeability, Electrical Conductivity, and Diffusion Coefficient of Simulated

Cracks ………………………………………………………………………………………………………………………..110

5-1 Introduction ……………………………………………………………………………………………………………… 110

5-2 Methods …………………………………………………………………………………………………………………… 112

5-3 Theory ……………………………………………………………………………………………………………………… 112 5-3-1 Hydraulic Permeability of Cracks …………………………………………………………………….. 112

5-3-2 Ion Diffusivity of Cracks ………………………………………………………………………………….. 114

5-4 Experimental Methods ……………………………………………………………………………………………… 115

5-5 Results and Discussion …………………………………………………………………………………………….. 125

5-5-1 Hydraulic Permeability …………………………………………………………………………………….. 125

5-5-2 Ion Diffusivity ………………………………………………………………………………………………….. 127

5-6 Conclusion ……………………………………………………………………………………………………………….. 130

5-7 References ……………………………………………………………………………………………………………….. 131

Chapter 6: Summary and Conclusion ………………………………………………………………………….134

6-1 Summary of Research Approach ………………………………………………………………………………. 134

6-2 Conclusion ……………………………………………………………………………………………………………….. 135

6-3 Suggested Future Research ………………………………………………………………………………………. 136

Appendix A  ………………………………………………………………………………………………………………..137 Appendix B  ………………………………………………………………………………………………………………..157

CHAPTER 1: INTRODUCTION

 

1-1 Introduction

Concrete is the most widely used man made material in the world. The United States uses about 400 million cubic yards of ready mixed concrete each year [1] . Worldwide, 12 billion tones (≈6.5 billion cubic yards) of concrete are manufactured annually [2] . Most of the transportation infrastructure is made of concrete with a design service life of 50 to 100 years. Long lasting materials play a major role in building durable and cost effective structures. Durability is a problem especially when concrete is exposed to aggressive environment such as deicing salts, marine structures, or severe freezing and thawing environment. The need to design long lasting concrete structures requires knowledge of parameters affecting the durability and service life of concrete and steel reinforcement. Some of the most common durability problems of concrete are freeze/thaw damage, alkali-silica reaction (ASR), sulfate attack and corrosion of reinforcing steel [3] . The primarily factor governing the durability of concrete is mass transport. Deterioration of concrete due to the previously mentioned mechanisms is significantly influenced by the rate of moisture, ion, and gas/vapor transport in concrete[4] . This is further discussed in chapter 2.

 

A number of durability models have been developed that can predict service life of concrete structures by considering the physical and chemical phenomena that influence concrete’s longterm performance and service life expectancy. Most of the existing service life models (e.g., STADIUM, Life-365) consider concrete as a continuum porous media and do not account for the presence of localized or distributed cracks. Cracking on the other hand, is inevitable in concrete.

Humidity and temperature changes and the resulting volume changes can cause tensile stress development and cracking if concrete is restrained against such movements [4] . Load induced cracking also occurs when tensile stress (e.g., at negative moment regions in a bridge deck) exceeds the tensile strength of concrete[5] . Such cracks can widen over time due to creep and further cracks could develop by fatigue (e.g., due to repeated traffic load). Cracking can increase the deterioration rate of concrete significantly by accelerating transport of moisture and aggressive agents into concrete and to the level of reinforcement. Figure 1-1 shows a submarine pile that was cracked during driving. Signs of rust are visible on the surface of concrete only 6 months after installation.

 

Figure 1-1: Rapid corrosion of steel due to cracking

 

1-2 Research objectives

The goal of the presented study is to characterize mass transport in saturated fractured concrete. The results will provide the much needed material/crack transport property inputs that can be incorporated into service-life prediction models to allow simulation of the effect of cracks on durability of concrete. This is especially significant for prediction of the remaining life of structures in service and selection of the best maintenance strategies for concretes that have experienced some level of cracking (e.g., early age shrinkage cracking). The results will quantify saturated transport properties (permeability, diffusivity) as a function of crack geometry (width, length, tortuosity, surface roughness). This will allow one to determine if there is a safe crack width that has negligible impact on durability of concrete. Safe crack width can be prescribed as the maximum allowable crack width in codes and specifications such as ACI-318 or AASHTO Bridge Design Manual. This will further enable weighing the benefits of crack mitigation strategies (e.g., use of fiber reinforcement or shrinkage reducing admixtures) against their costs.

 

1-3 Organization of contents

The following provides a brief description of the contents of this thesis. Chapter 2 addresses the most common durability problems of concrete. The mechanism of each problem is explained and the theory behind it is briefly discusses. Various modes of mass transport in concrete are reviewed with focus on fluid permeation and ion diffusion. Finally, some of the existing service life prediction models are introduced.

 

In chapter 3, water permeability of cracked mortars in saturated conditions is studied. Effect of cracking on permeation rate of water into concrete is experimentally determined. Geometry of cracks is characterized with the use of digital image analysis and relationships between crack geometry parameters (e.g., width, roughness, tortuosity) and permeability are established. These relationships are evaluated against the theory of laminar flow inside parallel-plate gaps.

Chapter 4 uses electrical impedance spectroscopy to measure electrical conductivity and saturated diffusion coefficient of cracked cement paste samples. The relation between diffusion coefficient and crack geometry is studied. Crack connectivity (e.g., inverse tortuosity) is also measured by electrical impedance spectroscopy.

 

Chapter 5 introduces a Plexiglas setup that was designed to simulate cracks in concrete. Saturated permeability, diffusion coefficient (using electrical migration test) and electrical connectivity (using electrical impedance spectroscopy) are measured on sample cracks with a broader range of crack widths. Using this setup allows simulation of parallel-plate cracks with desired width and surface roughness. The results are used to evaluate four hypotheses regarding permeability, diffusivity, connectivity and surface effects of cracks in concrete.

 

Finally, chapter 6 provides a summary of the findings in this study and discusses the main conclusions. Suggestions for future work are also provided.

 

1-4References

• Portland Cement Association., Design and control of concrete mixtures. Engineering bulletin, Skokie, Ill. etc.: Portland Cement Association, 1988.
• P. Broomfield, Corrosion of steel in concrete : understanding, investigation and repair. 2nd ed., London ; New York: Taylor & Francis. xvi, 277 p., 2007.
• G. Richardson, Fundamentals of durable reinforced concrete. Modern concrete technology., London ; New York: Spon Press. xii, 260 p., 2002.
• Mindess, J.F. Young, D. Darwin, Concrete, 2nd Ed., Prentice Hall, Upper Saddle

River, New Jersey, 2003.

• N. Hassoun, A.A. Al-Manaseer, Structural concrete : theory and design. 4th ed.,

Hoboken, NJ: J. Wiley, 2008.

CHARACTERIZING SATURATED MASS TRANSPORT IN FRACTURED CEMENTITIOUS MATERIALS

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