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Title:Influence of calcium-(alumino)-silicate-hydrate nanostructure on viscoelastic behavior
Author(s):Hunnicutt, William A
Director of Research:Mondal, Paramita; Struble, Leslie J.
Doctoral Committee Chair(s):Mondal, Paramita
Doctoral Committee Member(s):Popovics, John S.; Lopez-Pamies, Oscar; Jasiuk, Iwona
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Viscoelasticity in concrete has been studied since the early 20th century with continuously advancing technology and methods. Currently, calcium-silicate-hydrate (C-S-H) is thought to be the phase responsible for viscoelasticity in concrete. Advances in small-scale mechanical testing in the past decade have allowed numerous studies to be conducted on both elastic and viscoelastic properties of C-S-H. However, there is still a lack of data on the effect of nanostructure on the viscoelastic behavior of synthesized C-S-H. The nanostructure of C-S-H is modified by adjusting the ratio of calcium and silicon during synthesis. The addition of aluminum to C-S-H forms calcium-(alumino)-silicate-hydrate (C-(A)-S-H) and also modifies the nanostructure. This dissertation aims to identify the effects of the nanostructure on the viscoelastic properties of synthesized C-(A)-S-H. Several mechanisms for viscoelastic behavior in C-(A)-S-H have been presented by other authors, and the research conducted in this dissertation provides clarification on the dominant mechanism. Viscoelastic measurements were conducted on synthesized C-(A)-S-H using multiple nanoindentation methods for the first time. In order to determine the effect of nanostructure on viscoelastic properties methods of data analysis that are independent of porosity were devised. Structure of the aluminosilicate chain and crystallinity of C-(A)-S-H with differing chemistries were measured and related to elastic and viscoelastic properties. These experiments allowed for identification of the principle mechanism for viscoelastic behavior in dry C-(A)-S-H. The research presented supports a viscoelastic mechanism in which time-dependent deformation is a result of movement of C-(A)-S-H sheets relative to each other at interlayer sites. C-(A)-S-H samples with controlled nanostructural properties were produced by synthesis and compaction into a pellet. Crosslinking across the interlayer caused by aluminum substitution for silicon resists sliding in the interlayer. Similarly, increased ionic correlation forces that arise from higher Ca Si resist time-dependent deformation. The effect of forces across the interlayer on time-dependent behavior indicate that this is a critical location in the C-(A)-S-H nanostructure with respect to viscoelasticity. Multiple methods of nanoindentation analysis were applied to synthesized C-(A)-S-H for the first time. Stress relaxation and dynamic nanoindentation measurements have been seldom applied to cementitious materials and not to synthesized C-(A)-S-H; the stress relaxation method was found to be independent of porosity when normalized, but the dynamic method does not appear to be applicable to stiff viscoelastic materials such as C-(A)-S-H due to high error. Creep nanoindentation has been used frequently by other researchers on cementitious materials, but here a method of determining time-dependent solid Young’s modulus of C-(A)-S-H using micromechanics modeling is presented for the first time. The model produced results at zero-time that agree with measurements of elastic properties of solid C-S-H, giving validation to the method.
Issue Date:2018-07-05
Rights Information:Copyright 2018 William A. Hunnicutt
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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