Christopher P. Bobko

Nanoindentation investigation of asphalt binder and mastic viscoelasticity

An exploratory nanoindentation technique for creep testing of two neat asphalt binders and one mastic at room temperature is developed, tested and verified. This work presents a new approach to obtain viscoelastic properties from low-load spherical (blunt) nanoindentation. Interconverted shear relaxation modulus mastercurves are determined from nanoindentation data. The magnitudes and trends of these mastercurves are found to be in reasonable agreement with Dynamic Shear Rheometer (DSR) results in a stiffness range associated with the range of time and temperature used in nanoindentation testing. Nanoindentation creep data is transformed to develop a mastercurve of dynamic modulus. The portion of this mastercurve corresponding to the frequency and temperature range included in nanoindentation testing demonstrates reasonable agreement with DSR results. These initial results suggest the potential to expand nanoindentation testing to forensic investigations involving testing of preserved asphalt binder and mastic components within field-extracted asphalt concrete composites.

Nanoindentation and Atomic Force Microscopy Investigations of Asphalt Binder and Mastic

AbstractNanoindentation techniques were implemented to calculate and interpret linear viscoelastic properties of asphalt binder and mastic through low-load spheroconical (blunt) nanoindentation. Experiments on three rolling thin-film oven (RTFO)–aged binders (two neat and one polymer modified) and 24 RTFO-aged mastics were implemented for reproducible creep indentations at ultra low loads. Creep compliance model parameters were extracted and used to determine dynamic modulus values for each material. Dynamic modulus values from nanoindentation were validated by using macroscopic dynamic shear rheometer (DSR) testing for two binders and two mastics (RTFO-aged). Atomic force microscopy (AFM) images of binder and mastic microstructure were obtained to shed insight on how microstructural phenomena relate to mechanical properties. The new results were combined with previously determined work of cohesion values for three binders and 30 mastics (RTFO-aged) made with the same materials to link microstructural phe...

Improved Schmidt Method for Predicting Temperature Development in Mass Concrete

Designing mass concrete structural elements to avoid early-age thermal cracking requires good predictions of temperatures within the mass concrete. An improved method for predicting temperature in mass concrete structural elements is proposed and validated. The new method combines empirical methods for predicting temperature rise associated with heat of hydration with the Schmidt method, a simplified numerical tool for solving the heat transfer problem. Methods for modeling thermal insulation with the Schmidt method are also discussed. The new method is simple enough to implement in a spreadsheet analysis. Three case studies are modeled with the previous implementation of the Schmidt method and the proposed new implementation. The model predictions are compared with temperature measurements and predictions from detailed finite element modeling. In all cases, the new implementation provides much better predictions than previous versions of the Schmidt method and nearly matches the predictions made by finite element modeling.

Large-Area Nanolattice Film with Enhanced Modulus, Hardness, and Energy Dissipation

We present an engineered nanolattice material with enhanced mechanical properties that can be broadly applied as a thin film over large areas. The nanolattice films consist of ordered, three-dimensional architecture with thin-shell tubular elements, resulting in favorable modulus-density scaling (n ~ 1.1), enhanced energy dissipation, and extremely large material recoverability for strains up to 20% under normal compressive loading. At 95.6% porosity, the nanolattice film has demonstrated modulus of 1.19 GPa and specific energy dissipation of 325.5 kJ/kg, surpassing previously reported values at similar densities. The largest length scale in the reported nanolattice is the 500 nm unit-cell lattice constant, allowing the film to behave more like a continuum material and be visually unobservable. Fabricated using three-dimensional colloidal nanolithography and atomic layer deposition, the process can be scaled for large-area patterning. The proposed nanolattice film can find applications as a robust multifunctional insulating film that can be applied in integrated photonic elements, optoelectronic devices, and microcircuit chips.

Characterizing Strength and Failure of Calcium Silicate Hydrate Aggregates in Cement Paste under Micropillar Compression

AbstractA new methodology is proposed for investigating compressive failure behavior of cement paste at the micrometer scale. Micropillar geometries are fabricated by focused ion-beam milling on po...

Multiscale Modeling of Elastic Properties of Sustainable Concretes by Microstructural-Based Micromechanics

This paper addresses multiscale stiffness homogenization methodology to extract macroscale elastic mechanical properties of four types of sustainable concretes from their nanoscale mechanical properties. Nine different sustainable concrete mixtures were studied. A model based on micromechanics was used to homogenize the elastic properties. The hardened cement pastes were homogenized by three analytical methods based on Self-Consistent and Mori-Tanaka schemes. The proposed multiscale method combines advanced experimental and analytical methods in a systematic way so that the inputs are nanoscale phases properties extracted from statistical nanoindentation technique and mechanical properties of mixture ingredient. Predicted elastic properties were consistent with traditional experimental results. Linking homogenized mechanical properties of sustainable concrete to volume proportions through an analytical approach provides a critical first step towards rational optimization of these materials.