Title:
MODELING OF ACOUSTIC NONLINEARITY CHANGE IN CONCRETE DUE TO MICROCRACKS PARTIALLY FILLED WITH ALCALI SILICA REACTION (ASR) GEL
MODELING OF ACOUSTIC NONLINEARITY CHANGE IN CONCRETE DUE TO MICROCRACKS PARTIALLY FILLED WITH ALCALI SILICA REACTION (ASR) GEL
Author(s)
Smajic, Denis
Advisor(s)
Jacobs, Laurence J.
Kim, Jin-Yeon
Kim, Jin-Yeon
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Abstract
Alkali silica reaction (ASR) is a complex, multi-scale chemomechanical process which
can lead to the expansion and damage of concrete structures by forming microcracks, gradually
reducing the structure’s mechanical properties such as strength and stiffness.
This work investigates the effect of the ASR induced gel present within a microcrack
on the material’s nonlinear elastic constants. Therefore, significant changes such as the existence
of lubrication need to be considered in addition to the contact force when deriving
the stress state. The lubrication force depends on the viscosity and transport properties of
the ASR gel and each of these needs to be modeled respectively. The characteristics of this
force lead to the fact that nonlinearity due to microinhomogeneities is altered over time
compared to the pure elastic contact force which is repulsive and tries to open the microcrack.
Firstly, the contact force is approximated by a Hertzian contact between asperities,
characterized by spherical tops located at different heights whereas the introduced lubrication
force is derived in terms of the distance between irregularities and the gel volume
present within a microcrack. As a second step, appropriate asperity distribution functions
are used to derive the internal stress state within the crack which is then superimposed with
the far field stress and the additional acoustic stress in order to express the nonlinear stressstrain
relationship including the overall elastic constants of the solid. The higher order
elastic constants are much more sensitive to microcracks than the Young’s Modulus and
Poisson’s ratio which are altered only slightly due to the presence of microcracks. Nonlinear
ultrasonic measurement techniques are capable of measuring second order waves
which typically occur in nonlinear materials and relating these to the acoustic nonlinearity
parameter . The direct dependence of on the quadratic and cubic nonlinearity parameters
allows the derivation of a nonlinearity parameter relationship to the ASR gel volume
present in a microcrack.
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Date Issued
2020-08-12
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Thesis