DESIGN AND PREDICTING PERFORMANCE OF CARBON NANOTUBE REINFORCED CEMENTITIOUS MATERIALS : MECHANICAL PROPERTIES AND DISPERSION CHARACTERISTICS Mahyar Ramezani July 2 4, 2019 Recently , Carbon Nanotubes (CNTs) are drawing considerable attention of researchers for reinforcing cementitious materials due to their excellent mechanical properties and high aspect ratio (length -to-diameter ratio) . However, CNTs might not disperse well within the cement matrix, resulting in little improvement or even degradation of concrete properties. The uncertainty in producing the consistent results in different studies might be attributed to multiple interactions between the experimental variables affecting the nanotube dispersion and the final properties of CNT -cement nanocomposites. Therefore, this research mainly focused on proposing equations that can reliably capture these interactions in order to correlate CNT dispersion with the mechanical properties. The main experimental variables studied included C NT concentration, aspect ratio , ultrasonication energy, ultrasonication amplitude, surfactant -to-CNTs ratio, wat er-to- ceme nt ratio, sand -to-cement ratio , and hydration age of specimen . The study reported in this research was conducted in two parts: experimental program and modeling. vi In the experimental part of this research, a total of 6 3 different mix proportions were used to evaluate the flowability , mechanical properties , and durability characteristics of cement pastes and mortars containing CNTs . Using experimental test results reported in this study and the literature, three critical relations were proposed to consider the CNT dispersion, cement matrix composition, and hydration age of cement. T he proposed critical relations were then added to available theoretical models in the litera ture. The flexural strength and elastic modulus of CNT -cement nanocomposites were predicted through a state-of-the-art probabilistic model using a Bayesian methodology. Finally, the developed probabilistic models were used to identify the optimum ranges of the experimental variables to maximize the mechanical properties. This was do ne through computing the conditional probability of not meeting the specified design requirement. The experimental results indicated that addition of CNTs could significantly improve different properties of cementitious materials, if the optimum range of each variable was used. Also, t o achieve the desired mechanical properties, various combinations of the experimental variables might be use d. The proposed prediction models were shown to capture the interactions between the experimental variables for predi cting the mechanical properties within ±15% and ±18% of the experimental test results for flexural strength and elastic modulus, respectively . Based on the findings of this research , contour plots were developed to provide practical guidelines for future engineers to design CNT -cement nanocomposites. vii TABLE OF CONTENTS ACKNOWLEDGEMENTS ................................ ................................ ............................... iv ABSTRACT ................................ ................................ ................................ ......................... v LIST OF TABLES ................................ ................................ ................................ ........... xiii LIST OF FIGURES ................................ ................................ ................................ ........ xvii CHAPTER 1. INTRODUCTION ................................ ................................ ....................... 1 1.1. General ................................ ................................ ................................ ..................... 1 1.2. Problem Statement ................................ ................................ ................................ ... 1 1.3. Research Objectives and Scope ................................ ................................ ................ 3 1.4. Organization of this Dissertation ................................ ................................ .............. 7 CHAPTER 2. LITERATURE REVIEW ................................ ................................ ............ 9 2.1. General ................................ ................................ ................................ ..................... 9 2.2. Fundamentals of CNTs ................................ ................................ ............................. 9 2.2.1. CNT Structure ................................ ................................ ................................ .....9 2.2.2. CNT Properties ................................ ................................ ................................ .10 2.3. CNT Dispersion Procedure ................................ ................................ .................... 12 2.4. Microstructure of CNT -Cement Nanocomposites ................................ ................. 15 2.4.1. Hydration ................................ ................................ ................................ .......... 16 2.4.2. Porosity ................................ ................................ ................................ ............. 17 2.5. Flow Properties of CNT -Cement Nanocomposites ................................ ................ 19 2.6. Mechanical Properties of CNT -Cement Nanocomposites ................................ ..... 21 2.6.1. Mechanisms of CNTs affecting the Mechanical Properties ............................. 21 2.6.2. Existing Models ................................ ................................ ................................ 24 2.7. Durability of CNT -Cement Nanocomposites ................................ ......................... 34 2.7.1. Freeze and Thaw Action ................................ ................................ ................... 35 2.7.2. Corrosion of Steel Reinforcement Bars ................................ ............................ 36 viii 2.7.3. Thermal Resistivity ................................ ................................ ........................... 37 2.7.4. Alkali-Silica Reaction ................................ ................................ ....................... 38 2.8. Dimensional Stability ................................ ................................ ............................. 39 2.8.1. Shrinkage ................................ ................................ ................................ .......... 40 2.8.2. Creep ................................ ................................ ................................ ................. 41 2.9. Summary ................................ ................................ ................................ ................ 42 CHAPTER 3. MODEL DEVELOPMENT STRATEGY: OVERVIEW ......................... 44 3.1. General ................................ ................................ ................................ ................... 44 3.2. Model Development ................................ ................................ ............................... 45 3.3. Summary ................................ ................................ ................................ ................ 47 CHAPT ER 4. ANALYZING THE MECHANICAL PROPERTIES OF CNT -CEMENT NANOCOMPOSITES: DATABASE AND STATISTICAL ANALYSIS ...................... 48 4.1. General ................................ ................................ ................................ ................... 48 4.2. Effe ct of Dispersion Technique on the Strength ................................ .................... 49 4.3. Important Variables and Data Distribution ................................ ............................ 52 4.3.1. Important Variables ................................ ................................ .......................... 52 4.3.2. Data Distribution ................................ ................................ .............................. 54 4.4. Approach ................................ ................................ ................................ ................ 55 4.5. Effect of CNT Properties on the Strength ................................ .............................. 57 4.5.1. Effect of CNT Length ................................ ................................ ....................... 57 4.5.2. Effect of CNT Diameter ................................ ................................ ................... 59 4.5.3. Effect of CNT Aspect Ratio ................................ ................................ ............. 60 4.5.4. Effect of CNT Concentration ................................ ................................ ........... 61 4.6. Effect of CNT Dispersion Quality on the Strength ................................ ................ 63 4.6.1. Effect of CNT Length ................................ ................................ ....................... 64 4.6.2. Effect of CNT Diameter ................................ ................................ ................... 67 4.6.3. Effect of CNT Aspect Ratio ................................ ................................ ............. 69 4.6.4. Effect of CNT Concentration ................................ ................................ ........... 70 ix 4.7. Detailed Analysis of Identifying Different Ranges ................................ ................ 72 4.7.1. Compressive Strength ................................ ................................ ....................... 73 4.7.2. Flexural Strength ................................ ................................ .............................. 76 4.8. Effect of CNT on the Elastic Modulus ................................ ................................ ... 79 4.9. Effect of CNT on the Toughness ................................ ................................ ............ 81 4.10. Other Important Variables and their Interactions ................................ ................. 83 4.10.1. CNT Concentration and Aspect Ratio (Interaction I) ................................ .....84 4.10.2. Ultrasonication Energy and Amplitude (Interaction II) ................................ .84 4.10.3. Ultrasonication Energy and Surfactant (Interaction III) ................................ .85 4.10.4. w/c and s/c Ratios (Interaction IV) ................................ ................................ .86 4.10.5. CNT Concentration and Hydration Age (Interaction V) ................................ 86 4.11. Summary ................................ ................................ ................................ .............. 87 CHAPTER 5. EXPERIMENTAL PROGRAM ................................ ................................ 89 5.1. General ................................ ................................ ................................ ................... 89 5.2. Raw Materials ................................ ................................ ................................ ........ 89 5.2.1. Cement ................................ ................................ ................................ .............. 89 5.2.2. CNTs ................................ ................................ ................................ ................. 90 5.2.3. Surfactant and Water ................................ ................................ ........................ 90 5.2.4. Sodium Hydroxide ................................ ................................ ............................ 91 5.2.5. Sand ................................ ................................ ................................ .................. 91 5.3. Mix Proportions and Sample Preparations ................................ ............................. 92 5.3.1. Flow Properties (C and M series) ................................ ................................ .....93 5.3.2. Mechanical Properties (I~V and R series) ................................ ........................ 95 5.3.3. Durability: Alkali -Silica Reaction (D series) ................................ ................. 100 5.4. Test Procedures ................................ ................................ ................................ .... 102 5.4.1. Flow Properties ................................ ................................ ............................... 102 5.4.2. Mechanical Properties ................................ ................................ .................... 105 x 5.4.3. Durability: Alkali -Silica Reaction ................................ ................................ ..109 5.5. Summary ................................ ................................ ................................ .............. 110 CHAPTER 6. EXPERIMENTAL TEST RESULTS AND ANALYSIS ....................... 111 6.1. General ................................ ................................ ................................ ................. 111 6.2. Flow Properties ................................ ................................ ................................ .... 111 6.2.1. Cement Paste Test Results ................................ ................................ .............. 111 6.2.2. Cement Mortar Mini -Cone Slump Test ................................ .......................... 119 6.2.3. Summary: Flow Properties ................................ ................................ ............. 121 6.3. Mechanical Properties ................................ ................................ .......................... 123 6.3.1. Interaction between κ and AR (Interaction I) ................................ .................. 123 6.3.2. Interaction between 𝑈𝐸𝑇 and UA (Interaction II) ................................ ........... 127 6.3.3. Interaction between 𝑈𝐸𝑇 and SP/CNTs Ratio (Interaction III) ...................... 130 6.3.4. Interaction between w/c and s/c Ratios (Interaction IV) ................................ .133 6.3.5. Interaction between κ and t (Interaction V) ................................ .................... 135 6.3.6. Effect of CNTs on the Dynamic Elastic Modulus ................................ .......... 137 6.3.7. Summary: Mechanical Prop erties ................................ ................................ ...141 6.4. Durability: Alkali -Silica Reaction ................................ ................................ ........ 143 6.4.1. Effect of CNTs on Expansion and 𝐸𝑑 of ASR -affected Cement Mortars ......143 6.4.2. Effect of CNTs on ASR -affected Cement Mortars ( 𝑓𝐶𝑆, 𝑓𝐹𝑆, 𝐸𝐹) .................. 151 6.4.3. Summary: Durability Property ................................ ................................ .......155 CHAPTER 7. MODELING PROCEDURE: PROBABILISTIC APPROACH ............. 157 7.1. General ................................ ................................ ................................ ................. 157 7.2. Formulation of the Probabilistic Model ................................ ............................... 157 7.3. Bayesian Methodology ................................ ................................ ......................... 158 7.4. Probability of Failure ................................ ................................ ........................... 159 7.5. Sensitivity and Importance Measures ................................ ................................ .. 162 CHAPTER 8. MODELING THE MECHANICAL PROPERTIES: FLEXURAL STRENGTH AND ELASTIC MODULUS ................................ ................................ .... 164 xi 8.1. General ................................ ................................ ................................ ................. 164 8.2. Critical Relations affecting the Mechanical Properties ................................ ........ 164 8.2.1. Dispersio n Relation ( ɳ𝐷) ................................ ................................ ................ 165 8.2.2. Matrix Relation ( ɳ𝑀) ................................ ................................ ...................... 170 8.2.3. Hydration Age Relation ( ɳ𝐻) ................................ ................................ ......... 171 8.3. Regression Model ................................ ................................ ................................ . 173 8.3.1. Applicability of the Proposed Critical Relations ................................ ............ 174 8.3.2. Limitations of the Regression Model ................................ ............................. 181 8.4. Probabilistic Models ................................ ................................ ............................. 183 8.4.1. Database for Constructing Probabilistic Models ................................ ............ 183 8.4.2. Probabilistic Models Assessment ................................ ................................ ...185 8.4.3. Model Selection Criteria ................................ ................................ ................. 190 8.4.4. Prob ability of Failure: Optimization of Variables ................................ .......... 192 8.4.5. Sensitivity and Importance Measures ................................ ............................. 199 8.5. Summary ................................ ................................ ................................ .............. 204 CHAPTER 9. SUMMARY, CONCLUSIONS, RECOMMENDATIONS, AND FUTURE RESRACH ................................ ................................ ................................ ...................... 207 9.1. General ................................ ................................ ................................ ................. 207 9.2. Summary ................................ ................................ ................................ .............. 207 9.3. Conclusions ................................ ................................ ................................ .......... 209 9.3.1. Flow Properties (Track 2) ................................ ................................ ............... 209 9.3.2. Mechanical Properties (Track 1) ................................ ................................ ....210 9.3.3. Durability: Alkali -Silica Reaction (Track 3) ................................ .................. 212 9.4. Recommendations ................................ ................................ ................................ 213 9.5. Future Research ................................ ................................ ................................ .... 214