THESIS
2020
xxi, 256 pages : illustrations ; 30 cm
Abstract
The world is filled with concrete structures. Deterioration of concrete structures is a major
problem, leading to structural deficiency and eventual failure of civil infrastructures if problems
are not properly dealt with. Such degradation is related to the transport properties of the concrete.
The formation and opening of cracks are found to significantly increase the penetration of water
and chemicals leading to accelerated degradation process. Research on the fracture of composites
has led to the development of strain-hardening cementitious composites (SHCCs, also known as
bendable concrete). SHCCs are known to exhibit tensile strain-hardening behavior up to several
percents, intrinsic crack width control through multiple cracking (rather than the formation of a
small numbe...[
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The world is filled with concrete structures. Deterioration of concrete structures is a major
problem, leading to structural deficiency and eventual failure of civil infrastructures if problems
are not properly dealt with. Such degradation is related to the transport properties of the concrete.
The formation and opening of cracks are found to significantly increase the penetration of water
and chemicals leading to accelerated degradation process. Research on the fracture of composites
has led to the development of strain-hardening cementitious composites (SHCCs, also known as
bendable concrete). SHCCs are known to exhibit tensile strain-hardening behavior up to several
percents, intrinsic crack width control through multiple cracking (rather than the formation of a
small number of wide cracks)and autogenous healing capability. SHCCs have therefore been
seriously contemplated for practical applications to build resilient and durable structures. For
strategic and ubiquitous application of SHCCs, the utilization of these advantageous properties in
current practice requires validation of design assumptions on the macro scale.
The design and behavior of SHCC are dictated by composite theory based on micromechanics
of the failure mechanisms which involve the various constituents (i.e. fibers, matrix, and fiber-matrix
interface). The theory indicates the occurrence of multiple cracking under steady-state
cracking conditions if the fibers bridging the crack can sustain higher stress than the matrix
cracking strength of the SHCC composite. After first cracking, crack bridging fibers debond due
to the formation of tunneling crack (breaking of the chemical bond at the interface). Until the
fibers are eventually pulled out (slippage) or ruptured, the increase of this bridging stress with
pull out displacement leads to sequential matrix cracking. Due to the limitation of current
research, comprehensive validation of these micromechanical behaviors at the macro scale
(structural elements) remains inadequate. In this study, macro-scale investigation on SHCCs is
performed to segregate cracking events based on source mechanisms. As physical cracking
mechanisms generate characteristic stress waveforms, acoustic Emission technology was used to
collect the waves from accelerated damage tests. These waveforms from a single SHCC
composite were clustered to compute the relative concentration of each mechanism.
This process of sequential matrix cracking continues until a crack is localized (composite has
failed). The composite behavior can hence be divided into 3 regimes a) elastic b) strain
hardening and c) softening (crack localization). Structure elements made with SHCCs are
deisgned to operate in both linear and hardening regime but crack localization should be avoided.
Experiments were performed to study crack events generated during accelerated damage tests to
characterize these regimes. A new damage indicator, power spectral entropy (PSE) of AE wave
was developed to remove the interference of epistemic uncertainty. A computational framework
using PSE to inversely estimate the operational regime was successfully developed and validated
in laboratory test results.
In strain-hardening cementitious composites (SHCCs), the bridging effect of fibers can control
crack openings to very small values so the cracks can be self-healed in the presence of
water/moisture. To rely on autogenous self-healing to enhance the durability of structures, a
method to assess the state of self-healing is required. A strategy is developed for in-situ
measurement of the healing effect with limited measurement and without pre-knowledge of the
environmental condition (such as moisture). In the strategy, the effective macroscopic cracking
effect is first distinguished by frequency-dependent (dielectric) impedance spectroscopy
measurements both along (parallel to) and across the cracks. Thereafter, the approximated
resistance vs moisture curve is found to reflect the evolution of autogenous self-healing, and a
curve indicating almost complete healing agrees well with the recovery in member stiffness
measured from loading test. The recovery due to autogenous healing leads to the recovery of
both mechanical and transport properties. To determine the component of mechanical recovery
due to autogenous healing, a new methodology based on AE is explored. Specifically, a new
parameter reflecting the mechanical recovery is proposed and it is consistent with the expected
healing of tested specimens placed under various environmental conditions.
Crack widths control the durability of structural elements made with SHCCs. Both designing
SHCC for target application and validating the durability in practical condition require
knowledge of (the statistics of) surface cracks such as average crack widths (ACW) and crack
density (CD). A novel approach employing a deep convolutional neural network was explored
for automated identification and characterization of surface cracks. With our trained and tailored
deep neural network (TDCCN), it is possible to characterize cracks under a practical condition,
as images do not require a prior bias on the optimal experimental settings (such as polishing the
surface and painting it white/yellow, artificially enhancing the contrast and/or ensuring proper
camera angle with appropriate lighting condition to only focus on the preselected region). The
development of ACW and CD can be tracked. More importantly, this can help
researchers/engineers screen through a large number of images of SHCC members efficiently
and the method could be potentially applied to monitor the safety of real SHCC structures.
In summary, a number of monitoring methods for SHCC members have been developed in
this thesis. With the use of this methods to validate the design assumptions behind SHCC
members in the field, the wider application of this relatively new construction material can be
facilitated.
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