r - The Hong Kong Polytechnic University

structural health monitoring function, the smart aggregate can be used to perform early-age
concrete strength monitoring (Gu et al. 2006), impact detection (Song et al. 2007a) and dynamic
seismic detection (Gu et al. 2010). In previous studies, smart aggregates have been successfully
used in health monitoring of various concrete structures under different loading cases. The
proposed smart aggregate-based approach has been used to perform health monitoring for the
following cases: a concrete bridge bent-cap under static loading (Song et al. 2007b), a concrete
frame under static loading (Laskar et al. 2009, Zhao et al. 2006), a shear wall under reversed
loading (Yan et al. 2009), circular columns under seismic loading (Liao et al. 2008), and circular
columns under reversed cyclic loading (Moslehy et al. 2010). The purpose of this project was to
verify the effectiveness of the smart aggregate in health monitoring of an FRP-strengthened RC
column under reversed cyclic loading.
In the experiments, an active-sensing approach and an impact response-based approach were used
to perform the health monitoring. In the active-sensing approach, one smart aggregate was used to
generate the sweep sine signals to propagate through the concrete structure; the other distributed
smart aggregates were used as sensors to detect the responses. In the impact response-based
approach, an impact hammer is used to strike on the concrete column on marked pre-determined
location to generate stress waves to propagate through the concrete structure; the other distributed
smart aggregates were used as sensors to detect the impact responses. In both approaches, transfer
functions of the sensor signals were obtained during the testing process. The existence of cracks
attenuated the wave propagation and affected the frequency response curves. A damage index
matrix was formed based on the change of frequency response curves to demonstrate the health
status at different locations. Furthermore, a weighted damage index was developed to
comprehensively evaluate the overall damage status.
To verify the effectiveness of the proposed piezoceramic-based health monitoring approach,
structural health monitoring tests were performed on a shear-critical RC column in the Thomas
T.C. Hsu Structural Research Laboratory at the University of Houston (UH). A reversed cyclic
loading protocol was used to gradually load the column to failure, while the smart aggregates were
used as transducers to perform the structural health monitoring during the loading procedure. After
the column failed, it was repaired using an FRP fabric, which was wrapped around the plastic
hinge region of the damaged column. Wrapping a column with FRP confines the concrete in the
column, allowing the confined concrete to carry significant stresses at higher strains. Confined
concrete demonstrates an increase in both compressive strength and ultimate strain. In an
FRP-wrapped column, the transverse strains greatly increase at high load values because of
internal cracking. Concrete bares out against the confining FRP, which eventually ruptures,
causing failure of the column. In this research, the FRP-strengthened column was again loaded
until failure by a reversed-cyclic loading protocol, and the smart aggregates were once again used
to evaluate the health status of the column during the loading procedure. It was observed that
although the failure mode was different from a typical RC column, the smart-aggregate based
health monitoring system could successfully detect both the failure occurrence and its severity.
EXPERIMENTAL SETUP
The smart aggregates, as shown in Figure 1, were formed by embedding a water-proof coated
piezoceramic patch with lead wires into a small concrete block before casting. A RC column was
fabricated at UH, and six smart aggregates were installed at pre-determined distributive locations
in the shear-critical column before casting. These locations are shown in Figures 2 and 3.
Water-proof coating
Electric wires
Piezoceramic patch
Figure 1 Illustration of a smart aggregate
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