ARDAVAN YAZDANBAKSH, ZACHARY GRASLEY DEPARTMENT OF CIVIL ENGINEERING TEXAS A&M UNIVERSITY High-Performance Stress-Relaxing Cementitious Composites (SRCC) Federal Highway Administration, Exploratory Advanced Research

Cracking and Durability of Concrete Tensile Stress Low Tensile Strength of Concrete External Loading Shrinkage Tensile Stress Crack Moisture Moves Freely in Concrete Corrosion and /or Sulfate Attack alkali-silica reaction Poor ride quality Spalling 2

Methods of Reducing Cracking Techniques for reducing cracking caused by free strain 123 Idea Limit the mechanism by which the free strains occur Relax stress by creating bridged microcracks Relax stress by creating nanoscale damage Technique Shrinkage Reducing Admixtures Using macro or micro fibers Inclusion of micro/nano damage and particles in matrix 3

Objectives To develop a methodology for the design and fabrication of concrete that exhibits remarkable stress-relaxing behavior For this purpose: Different methods of distributing the nano/micro particles in cement paste and controlling particle-matrix bond will be examined Cementitious composites will be produced, mechanically tested for stress relaxation. The results will be compared with the prediction of theoretical models 4

Why choose CNF/Ts? Advantageous properties of CNF/Ts:  Strength 100 times more than that of steel, specific gravity is one sixth of steel  Elastic strain capacity of 12%; 60 times higher than that of steel  Modulus of elasticity: approximately 10 3 GPa, 5 times more than steel  Excellent electrical properties Importance of dispersion 5 (1) (3) (2)

Challenges Effective inclusion of nano-particles in cement paste is difficult because: The nanoparticles tend to strongly attract each other and form clumps or bundles Non-hydrated cement particles are significantly larger than nano- particles, which prevents uniform distribution of nano-particles The bond between cementitious matrix and inclusions like carbon nanoparticles is weak 6

Current State of the Project Inclusion of CNF/Ts in Cement paste

CNFs Dispersed in Water CNF Concentration 1.5% wt – Sonicated for 20 mins with surfactant (2.0% wt) 8

Non-Uniform Dispersion of CNFs Non-dispersed clumps of CNF Reasons: - Short period of sonication - Using inadequate amount of surfactant Longitudinal agglomeration of CNFs Reasons: - Long storage period of dispersion - Drying of dispersion 9

Dispersion in Water vs. Dispersion in Paste Good dispersion in water Poor dispersion in paste 10

Causes and Solutions for Poor Dispersion in Paste Cause: Reagglomeration of CNFs in paste due to  Absorption of water by cement  Consumption of surfactant (superplastisizer) by cement Solution: Using more water and superplastisizer Drawback: Negative effect on durability, strength, and retardation of hydration Cause: Large size of cement particles and clumps Solution: Using fine-grain cement Drawback? 11

SEM Images of Type I Portland Cement Some particles as large as 10 μm Edge of a huge particle (>100 μm) 12

Very Fine Cement Grains and Their Downside Due to the very large surface area, production of a workable paste requires w/c>0.70 and a lot of superplastisizer Ball-milled cement. The average particle size is less than 1 μm. The average particle size is less than the length of a typical CNT. Drawback: 13

Some Important Observations In areas of matrix with CNFs, pullout, fracture, and also fibers parallel to crack surface were observed. 14

The Next Step Functionalization of CNFs with acid Producing fibrous cement paste using treated CNFs and SEM observation of hardened matrix Mechanical testing of CNF-reinforced hardened paste beams for stress-relaxation 15

Concluding Remarks Using nano-particles in cementitious composites may enhance stress relaxation properties Challenges: poor dispersion of fibers and weak CNF/T-matrix bond SEM images of fractured surface of CNF-reinforced hardened paste suggest that manipulation of interfacial bond is possible Acid treatment of CNFs will be examined in order to improve dispersion and bond 16

Jeff Foxworthy says: If you aren’t sure what will happen next… … your research might be exploratory. If people think your ideas are mildly crazy (Mark Sanford crazy, not Unabomber crazy)…. … your research might be exploratory. If you are not only “thinking outside the box” but have to squint a little to see the box… … your research might be exploratory.