@article{shahrin_bobko_2019, title={Micropillar compression investigation of size effect on microscale strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste}, volume={125}, ISSN={["1873-3948"]}, DOI={10.1016/j.cemconres.2019.105863}, abstractNote={The compressive strength and failure of concrete and cement exhibit strong size effect over various length scales. To investigate possible size effect on compressive strength and failure mechanism of Calcium-Silicate-Hydrates (C-S-H) in cement paste, micropillar compression experiments were performed on micropillar geometries fabricated by focused ion beam milling on potential C-S-H locations identified through coupled backscatter electron imaging (BSE) and energy dispersive spectroscopy (EDS) analysis. The compressive strength of C-S-H (181–1145 MPa) measured from C-S-H micropillars of varying diameters indicated presence of a size effect with strong increase in strength with decreasing diameter. The deformation mode at failure also exhibited size effect: the dominant failure mode changed from axial splitting to plastic crushing as the pillar diameter was decreased. The observed relationship between strength and pillar diameter can be modeled by an inverse square root dependency which closely corresponds to Bazant's scaling law of quasi-brittle failure.}, journal={CEMENT AND CONCRETE RESEARCH}, author={Shahrin, Rahnuma and Bobko, Christopher P.}, year={2019}, month={Nov} }
 @article{shahrin_bobko_2017, title={Characterizing Strength and Failure of Calcium Silicate Hydrate Aggregates in Cement Paste under Micropillar Compression}, volume={7}, ISSN={["2153-5477"]}, DOI={10.1061/(asce)nm.2153-5477.0000137}, abstractNote={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...}, number={4}, journal={JOURNAL OF NANOMECHANICS AND MICROMECHANICS}, author={Shahrin, Rahnuma and Bobko, Christopher P.}, year={2017}, month={Dec} }
 @article{bagal_zhang_shahrin_dandley_zhao_poblete_oldham_zhu_parsons_bobko_et al._2017, title={Large-area nanolattice film with enhanced modulus, hardness, and energy dissipation}, volume={7}, journal={Scientific Reports}, author={Bagal, A. and Zhang, X. A. and Shahrin, R. and Dandley, E. C. and Zhao, J. J. and Poblete, F. R. and Oldham, C. J. and Zhu, Y. and Parsons, G. N. and Bobko, C. and et al.}, year={2017} }
 @article{veytskin_tammina_bobko_hartley_clennell_dewhurst_dagastine_2017, title={Micromechanical characterization of shales through nanoindentation and energy dispersive x-ray spectrometry}, volume={9}, ISSN={["2352-3808"]}, DOI={10.1016/j.gete.2016.10.004}, abstractNote={Shales are heterogeneous sedimentary rocks which typically comprise a variable mineralogy (including compacted clay particles sub-micrometer in size), silt grains, and nanometer sized pores collectively arranged with transversely isotropic symmetry. A detailed understanding of the micro- and sub-microscale geomechanics of these minerals is required to improve models of shale strength and stiffness properties. In this paper, we propose a linked experimental–computational approach and validate a combination of grid nanoindentation and Scanning Electron Microscopy (SEM) with Energy and Wavelength Dispersive X-ray Spectrometry (EDS/WDS) at the same spatial locations to identify both the nano-mechanical morphology and local mineralogy of these nanocomposites. The experimental parameters of each method are chosen to assess a similar volume of material. By considering three different shales of varying mineralogy and mechanical diversity, we show through the EMMIX statistical iterative technique that the constituent phases, including highly compacted plate- or sheet-like clay particles, carbonates, silicates, and sulfides, have distinct nano-mechanical morphologies and associated indentation moduli and hardness. Nanoindentation-based strength homogenization analysis determines an average clay packing density, friction coefficient, and solid cohesion for each tested shale sample. Comparison of bulk to microscale geomechanical properties, through bulk porosimetry measurements, reveals a close correspondence between bulk and microscale clay packing densities. The determination of mechanical microstructure and material properties is useful for predictive microporomechanical models of the stiffness and strength properties of shale. The experimental and computational approaches presented here also apply to other chemically and mechanically complex materials exhibiting nanogranular, composite behavior.}, journal={GEOMECHANICS FOR ENERGY AND THE ENVIRONMENT}, author={Veytskin, Yuriy B. and Tammina, Vamsi K. and Bobko, Christopher P. and Hartley, Patrick G. and Clennell, Michael B. and Dewhurst, David N. and Dagastine, Raymond R.}, year={2017}, month={Mar}, pages={21–35} }
 @article{veytskin_bobko_castorena_2016, title={Nanoindentation and Atomic Force Microscopy Investigations of Asphalt Binder and Mastic}, volume={28}, ISSN={["1943-5533"]}, DOI={10.1061/(asce)mt.1943-5533.0001532}, abstractNote={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...}, number={6}, journal={JOURNAL OF MATERIALS IN CIVIL ENGINEERING}, author={Veytskin, Yuriy and Bobko, Christopher and Castorena, Cassie}, year={2016}, month={Jun} }
 @article{veytskin_bobko_castorena_2016, title={Nanoindentation investigation of asphalt binder and mastic viscoelasticity}, volume={17}, ISSN={["1477-268X"]}, DOI={10.1080/10298436.2014.993393}, abstractNote={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.}, number={4}, journal={INTERNATIONAL JOURNAL OF PAVEMENT ENGINEERING}, author={Veytskin, Yuriy and Bobko, Christopher and Castorena, Cassie}, year={2016}, month={Apr}, pages={363–376} }
 @article{bobko_zadeh_seracino_2015, title={Improved schmidt method for predicting temperature development in mass concrete}, volume={112}, number={4}, journal={ACI Materials Journal}, author={Bobko, C. R. and Zadeh, V. Z. and Seracino, R.}, year={2015}, pages={579–586} }
 @article{veytskin_bobko_castorena_kim_2015, title={Nanoindentation investigation of asphalt binder and mastic cohesion}, volume={100}, ISSN={["1879-0526"]}, DOI={10.1016/j.conbuildmat.2015.09.053}, abstractNote={A nanoindentation technique for determining the cohesive properties of neat, modified, and aged asphalt binders and mastics with varying filler volumetric concentrations is developed, tested, and verified. Cohesive properties of binder and mastic are critically important to the fracture resistance of asphalt concrete. A new approach to calculate and interpret important cohesive properties from nanoindentation data through low-load sphero-conical (blunt) nanoindentation is presented. Work of effective cohesion values are determined as the average response over multiple possible microstructures for three asphalt binders and 30 different mastics of varying filler volumetric concentrations. Results point to evidence of a critical filler volume fraction beyond which further addition of filler does not affect work of effective cohesion. This plateau in work of effective cohesion values is speculated to be related to the combined effects of volume-filling, particle interactions, and physicochemical interactions. The critical filler volumetric concentrations corresponding to the plateau in work of effective cohesion range between 0.20 and 0.30, which is within the range from literature of 0.15–0.30. Testing of binder and mastic through nanoindentation is an important step toward in situ testing of mastic within asphalt concrete, which is inaccessible using conventional macroscopic experimental methods.}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Veytskin, Yuriy and Bobko, Christopher and Castorena, Cassie and Kim, Y. Richard}, year={2015}, month={Dec}, pages={163–171} }
 @article{bobko_edwards_seracino_zia_2015, title={Thermal cracking of mass concrete bridge footings in coastal environments}, volume={29}, number={6}, journal={Journal of Performance of Constructed Facilities}, author={Bobko, C. P. and Edwards, A. J. and Seracino, R. and Zia, P.}, year={2015} }
 @article{zadeh_bobko_2014, title={Nano-mechanical properties of internally cured kenaf fiber reinforced concrete using nanoindentation}, volume={52}, ISSN={["1873-393X"]}, DOI={10.1016/j.cemconcomp.2014.04.002}, abstractNote={Kenaf fiber reinforced concrete (KFRC) is being considered as a more sustainable fiber reinforced concrete. The effect of water-absorbing kenaf fibers on the microstructure of concrete was studied using nanoindentation. Results showed that cement hydration products found in KFRC are similar to those in conventional concrete, but relative volume fractions of these hydration products differ in KFRC. KFRC samples have more CH/CSH and less LD CSH in bulk than would be expected for plain concretes with similar water–cement ratios. Further nanoindentation experiments were performed in the interfacial transition zones around individual kenaf fibers, revealing a porous phase, a high percentage of LD CSH, and a lack of HD CSH in the fiber interfacial zones. The water absorbance of kenaf fibers and associated internal curing effects explain both results. Enhanced production of higher density hydration products in KFRC may provide a pathway for optimal design of these materials.}, journal={CEMENT & CONCRETE COMPOSITES}, author={Zadeh, Vahid Zanjani and Bobko, Christopher P.}, year={2014}, month={Sep}, pages={9–17} }
 @article{zadeh_bobko_2014, title={Nanomechanical characteristics of lightweight aggregate concrete containing supplementary cementitious materials exposed to elevated temperature}, volume={51}, ISSN={["1879-0526"]}, DOI={10.1016/j.conbuildmat.2013.10.034}, abstractNote={In order to determine the effect of elevated temperature on hydration products of lightweight aggregate concrete containing fly ash and ground granulated blast-furnace slag, nanoindentation characteristics of three different mixtures were investigated. The results indicated existence of porosity and two types of primary hydration products, Low Density and High Density Calcium–Silicate Hydrates (LD C–S–H and HD C–S–H), in the bulk cement paste. It was revealed that there is less degradation of mechanical properties of C–S–H phases in samples of lightweight aggregate concrete exposed to elevated temperature compared to those with conventional aggregate. Additionally, the interfacial transition zone of lightweight aggregate was investigated. It was revealed that because of internal curing caused by lightweight aggregate, mechanical properties of the interfacial transition zone in these samples were very similar to bulk paste. The effect of elevated temperature, however, was more pronounced in the interfacial transition zone of lightweight aggregate than in bulk paste. A dissipated energy parameter was introduced to use in the deconvolution method which demonstrated good correlation with indentation modulus and hardness as well as packing density of the C–S–H phases in general.}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Zadeh, V. Zanjani and Bobko, C. P.}, year={2014}, month={Jan}, pages={198–206} }
 @article{zadeh_bobko_2013, title={Nanoscale mechanical properties of concrete containing blast furnace slag and fly ash before and after thermal damage}, volume={37}, DOI={10.1016/j.cemconcomp.2012.09.003}, abstractNote={Portland cement blended with waste products such as blast furnace slag and fly ash are frequently used to create more sustainable concrete, but their nanoscale mechanical behavior, particularly after thermal damage, has not been well-studied. Here, nanoindentation experiments confirm that concrete produced with blended cements contains hydration products with nearly identical nanoscale mechanical properties to the hydration products found in concretes produced with ordinary Portland cement. The volume fractions of the hydration products, particularly calcium-silicate-hydrate (C-S-H) phases, are formed in different proportions with the addition of fly ash and blast furnace slag. After exposure to fire damage, the nanoscale behavior of concretes produced with fly ash and slag also matches the nanoscale behavior of conventional concretes. This suggests that any macroscopic differences between fire damage behavior of blended cement concrete and ordinary Portland cement concrete must have origins in a larger length scale.}, journal={Cement & Concrete Composites}, author={Zadeh, V. Z. and Bobko, C. P.}, year={2013}, pages={215–221} }
 @article{elsaid_dawood_seracino_bobko_2011, title={Mechanical properties of kenaf fiber reinforced concrete}, volume={25}, ISSN={["1879-0526"]}, DOI={10.1016/j.conbuildmat.2010.11.052}, abstractNote={This paper presents the findings of an experimental research program that was conducted to study the mechanical properties of a natural fiber reinforced concrete (FRC) which is made using the bast fibers of the kenaf plant. The kenaf plant is quickly developing as a replacement crop for the dwindling tobacco industry in the south-eastern United States. Appropriate mixture proportions and mixing procedures are recommended to produce kenaf FRC (KFRC) with fiber volume contents of 1.2% and 2.4%. The compressive strength, compressive modulus, splitting tensile strength and modulus of rupture of KFRC specimens are presented and compared to the properties of plain concrete control specimens. The experimental results indicate that the mechanical properties of KFRC are comparable to those of plain concrete control specimens, particularly when accounting for the effect of the increased w/c ratio required to produce workable KFRC. Further, the results indicate that KFRC generally exhibits more distributed cracking and higher toughness than plain concrete. Scanning electron micrographs (SEM’s) indicate that a good bond between the kenaf fibers and the surrounding matrix is achieved. The SEM’s also provide interesting information regarding the mechanisms which contribute to the failure and post-peak behavior of the KFRC which may be beneficial to future modeling efforts. The research findings indicate that KFRC is a promising ‘green’ construction material which could potentially be used in a number of different structural applications.}, number={4}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Elsaid, A. and Dawood, M. and Seracino, R. and Bobko, C.}, year={2011}, month={Apr}, pages={1991–2001} }
 @article{bobko_gathier_ortega_borges_ulm_abousleiman_2011, title={The nanogranular origin of friction and cohesion in shale - a strength homogenization approach to interpretation of nanoindentation results}, DOI={10.1002/nag.984}, abstractNote={Abstract}, journal={International Journal for Numerical and Analytical Methods in Geomechanics}, author={Bobko, C. and Gathier, B. and Ortega, J.A. and Borges, L. and Ulm, F.-J and Abousleiman, Y.}, year={2011} }
 @article{ulm_vandamme_jennings_vanzo_bentivegna_krakowiak_constantinides_bobko_van vliet_2010, title={Does microstructure matter for statistical nanoindentation techniques?}, volume={32}, ISSN={["0958-9465"]}, DOI={10.1016/j.cemconcomp.2009.08.007}, abstractNote={In their paper, Trtik et al. (2009) identify spurious peaks in the application of statistical nanoindentation technique as a critical obstacle for mechanical phase identification. In this discussion, we show that Trtik et al.’s finding is a consequence of an unrealistic virtual 3-D checkerboard microstructure considered by the authors. These peaks are not a general feature of indentation on multiphase materials, nor can the presence of such peaks be attributed to an intrinsic shortcoming of the grid-indentation technique. We also show that the authors’ assertion of the absence of homogeneous material regions extending beyond 3 μm in cementitious materials is groundless.}, number={1}, journal={CEMENT & CONCRETE COMPOSITES}, author={Ulm, Franz-Josef and Vandamme, Matthieu and Jennings, Hamlin M. and Vanzo, James and Bentivegna, Michelle and Krakowiak, Konrad J. and Constantinides, Georgios and Bobko, Christopher P. and Van Vliet, Krystyn J.}, year={2010}, month={Jan}, pages={92–99} }
 @inproceedings{bobko_elsaid_dawood_zanjanizedah_seracino_2010, title={Natural fiber reinforced concrete: Microstructure, internal curing, and mechanical properties from nanoindentation and macroscopic testing}, booktitle={ASCE Engineering Mechanics Institute Conference}, author={Bobko, C. and Elsaid, A.H and Dawood, M. and Zanjanizedah, V. and Seracino, R.}, year={2010} }
 @article{bobko_ortega_ulm_2009, title={Comment on "Elastic modulus and hardness of muscovite and rectorite determined by nanoindentation" by G. Zhang, Z. Wei and RE Ferrell [Applied Clay Science 43 (2009) 271-281]}, volume={46}, ISSN={["1872-9053"]}, DOI={10.1016/j.clay.2009.08.008}, abstractNote={In their paper, Zhang et al. (2009) report on a series of nanoindentation experiments on muscovite and rectorite, two clay minerals with very different interlayer structures. Their work raises a fundamental question regarding the mechanical definition of a single clay mineral. Furthermore, if such a definition is possible, is nanoindentation capable of assessing the mechanical behavior of a single clay mineral? Comparison of Zhang et al.'s results with previously published nanoindentation results on shale, and with results of other methods of assessing the mechanical behavior of single clay minerals, provides an avenue to address these questions. In a final analysis, it may be impossible to strictly define the mechanical behavior of a single clay mineral without the additional context of specific applications.}, number={4}, journal={APPLIED CLAY SCIENCE}, author={Bobko, Christopher P. and Ortega, J. Alberto and Ulm, Franz-Josef}, year={2009}, month={Dec}, pages={425–428} }