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Current research interest in multi-scale modeling of cement paste requires accurate characterization of the time-dependent mechanical properties of the material, particularly the C-S-H phase. Nanoindentation is evaluated as a tool for measuring both the instantaneous and the short-term viscoelastic properties of cement paste. Atomic force microscopy (AFM) based indentation is compared to conventional nanoindentaion in measuring mechanical properties of cement pastes. Time-dependent solutions are derived to characterize creep indentation tests performed on hardened cement paste and to extract the time-dependent properties. The effect of approximating C-S-H viscoelastic properties with a time-independent Poisson's ratio is discussed, and arguments for utilizing a time-independent Poisson's ratio for short-term response are presented. In evaluating AFM as a mechanical characterization tool, various analytical and numerical modeling approaches are compared. The disparities between the numerical self-consistent approach and analytical solutions are determined and reported. The measured elastic Young's modulus values acquired by AFM indentation tests are compared to Young's modulus values from nanoindentation measurements from cement paste. These results show that the calcium silicate hydrate (C-S-H) phase of hydrated portland cement has different properties on the nanometric scale than on the micron scale. Packing density of C-S-H particles is proposed as an explanation for the disparity in the measured results. The AFM measured uniaxial viscoelastic compliance values are compared to similar values obtained with traditional nanoindentation for the same material. The comparison of these results shows that the calcium silicate hydrate (C-S-H) phase of portland cement has similar but distinct properties on the sub micron scale than on the micron scale. Additionally, the effect of moisture is evaluated by controlling the relative humidity (RH) of the testing environment between 40% and 100% plus, or wet. The viscoelastic compliance appears to be highest at 40% RH and the material appears to be less compliant at higher relative humidity levels. Possible mechanisms controlling the viscoelastic deformation are presented and evaluated in conjunction with the moisture related poromechanical effect.

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The focus of this research was to investigate the use of functionalized/non-functionalized multi walled carbon nanotubes (MWCNTs) as reinforcements for the Portland cement paste. The unique geometrical characteristics of the carbon nanotubes (CNTs), as well as its unique mechanical properties such as high strength, ductility and stiffness, were the vital motivation for this study. In this research, we combined this unique material (CNTs) with concrete which is the most used man-made material. When compared to other composite materials, a limited amount of research has been conducted on the CNTs/cement composites. In order to investigate how the aspect ratio of functionalized/non-functionalized MWCNTs affects the mechanical properties of cementitious composites, ten different mixes of the MWCNTs/cement composites were prepared and tested. The different batches had a fixed water/cement ratio of 0.4, and variations of MWCNTs length, concentration and surface treatment. The cement nanocomposites were cast in small-scale specimens (beams) for the three-point flexural testing. Four major mechanical properties were evaluated at ages of 7, 14, and 28 days from the casting day: the maximum flexural strength, ultimate strain capacity (ductility), modulus of elasticity, and modulus of toughness. The results for the different nanocomposite batches were compared with the plain cement (reference) batch. The mechanical testing results showed that at 28 days almost all of the MWCNTs composites increased the flexural strength of the cement nanocomposites. At 28 days, the long MWCNTs increased the flexural strength more than the short MWCNTs. In general, the ultimate strain (ductility) of the short MWCNTs nanocomposites was higher than the ultimate strain of the long MWCNTs nanocomposites. The flexural strength of short 0.2 percent MWNT and long 0.04 percent MWNT (OH) increased by 269 percent and 83 percent, respectively, compared to the plain cement sample at 28 days. The highest ductility at 28 days for the short 0.1 percent MWNT and the short 0.2 percent MWNT was 86 percent and 81 percent, respectively. Clear evidence was obtained from the SEM images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks. In conclusion, CNTs as nano reinforcements, can effectively improve certain mechanical properties of the cement paste composites.

Cement - within - Cement (C-C) revision techniques have been demonstrated to reduce the complications associated with removal of secure cement from the femoral canal during revision hip joint arthroplasty. Material failure at the interface between new and old cement mantles represents a theoretical limitation of this technique. The objectives of this thesis are to describe the variability in material properties of uniform and bilaminar polymethylmethacrylate (PMMA) cement mantles in shear with respect to duration of post-cure and the influence of commercial inclusion of antibiotics on bilaminar cement mantle interfacial shear strength. Uniform mantles of Surgical Simplex P and Antibiotic Simplex PMMA cements demonstrated variability in ultimate shear stress to failure with respect to duration of post-cure (p < 0.001), however the variations were quantitatively small and unlikely to be of clinical relevance. Bilaminar cement mantles were 15 - 20 percent weaker than uniform mantles (p < 0.001) and demonstrated similar time dependant material property variations in shear (p < 0.001). Bilaminar PMMA test specimens manufactured using Antibiotic Simplex cement demonstrated equivalent ultimate shear stress to failure as bilaminar specimens manufactured from Surgical Simplex (p=0.52). High C-C interfacial strengths are demonstrated as early as one hour after cement application. Interfacial adhesion by mechanisms other than mechanical interlock significantly influence the bond formed between layered PMMA cements, with an important contribution by diffusion based molecular interdigitation. In the presence of a secure cement-bone interface, C-C femoral revision can be recommended as a viable technique on the basis of the strong interfacial bond formed between new and old cement mantles. The use of Antibiotic Simplex in C-C revision is recommended as detrimental effects on the interfacial shear properties have not been demonstrated with the commercial addition of Tobramycin.

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Thesis (MScIng)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Thesis (Ph.D.)--Texas A & M University, 1986. / Includes bibliographical references (leaves 115-120).