r - The Hong Kong Polytechnic University

spectrum is obtained in an approximate manner by assuming that the displacement response is harmonic and
hence that the velocity at each circular frequency is equal to the frequency time the displacement. The
acceleration (or pseudo-acceleration) response spectrum is obtained from the displacement response spectrum
by multiplying by the circular frequencies squared. The pseudo-acceleration represents the total acceleration in
the bridge while the pseudo-velocity and the displacement are relative quantities. Upon obtaining the response
spectrum diagrams, the seismic forces in the bridge can be obtained by the CQC (complete quadratic
combination) method in combining the effects from different vibration mode shapes considered. The flow
diagram of the software tools for seismic monitoring is shown in Figure 14.
D. Customized Software Tools for Corrosion Status Monitoring [Ref. 1, 9, 15, 22, 26, 27 and 30]
Corrosion of reinforcement in structural concrete is normally caused by two major factors, carbonation of the
concrete cover and the penetration of chlorides providing from the marine atmosphere or from chemicals in
contact with concrete. The former generally results in uniform corrosion of reinforcement while the latter
induces localized corrosion. Both types of corrosion are of electrochemical nature. Therefore electrochemical
techniques, or corrosion cells, are used to monitor the electrochemical corrosion activity of the metallic
reinforcement with the four aims: (i) to identify the initiation period or the time until the steel reinforcement
becoming depassivation either by the presence of chloride salts or by carbonation, (ii) to estimate crack
initiation and propagation, and (iii) to estimate the time to corrosion crack of concrete cover.
The corrosion cells used for corrosion monitoring are the anode-ladder system from S+R Sensortec GmBH. A
typical corrosion cell is composed of seven components, i.e., one anode ladder assembly, one platinized titanium
cathode, one PT 1000 (Platinum-type) temperature sensor, one manganese dioxide reference electrode, one
reinforcement connection bracket and two relative humidity sensors. A picture of corrosion cell is shown in
Figure 15. The corrosion cells are used to measure six types of parameters, i.e., open circuit potential, macrocell
current, concrete resistivity, linear polarization resistance, concrete temperature, and concrete relative humidity
in tower-bases, pier-base and concrete cross-girders in side-span.
The first and second parameters provide the information on the time-to-corrosion, i.e., the time required for the
reinforcement changing from the passive state to the active state. The critical values of voltage and electric
current for no corrosion, as advised by sensor supplier, are >-150 mV and 100 kΩ-cm. The fourth parameter is the only electrochemical parameter
with quantitative ability regarding the corrosion rate of reinforcement. In the measurement of this parameter, a
small perturbation potential is applied to the anode or reinforcement (by means of the counter electrode or the
platinized titanium cathode and the reference electrode or the manganese dioxide reference electrode), and the
resulting current response is measured. This small potential perturbation potential should be applied step-wise,
starting below the free corrosion potential and terminating above the free corrosion potential. The linear
polarization resistance is the ratio of the applied potential and the resulting current response and is inversely
related to the uniform corrosion rate. The critical value of linear polarization resistance for no corrosion is > 250
kΩ-cm 2 . The fifth and sixth parameters are used to provide respective concrete temperature and relative
humidity in concrete for references in concrete resistivity measurement. The Camur II System equipped with
Gamry G300 and DC105 Corrosion Techniques Software is deployed for the measurement of these six types of
parameters.
Based on the measurement results of linear polarization resistance and concrete resistivity, the parameter of
corrosion rate is derived basing on the Stern Geary equation. The critical value of corrosion rate for no corrosion
is < 0.1 μA per cm 2 . The derived corrosion rate will then be used to estimate the reinforcement deterioration
(including crack formation and crack width opening) and the time to corrosion cracking of concrete cover. The
software tools for the report of corrosion monitoring results are customized to output the measured/derived
results in the standardized format required by the monitoring report and to derive the corrosion rate, the
reinforcement deterioration and the time to corrosion cracking of concrete cover. The flow diagram of the
software tools for corrosion status monitoring is shown in Figure 16.
E. Customized Software Tools for Highway Traffics Monitoring [Ref. 4, 15 and 25].
In highway traffics monitoring, two major traffic events of daily flow traffics (which result in cyclic load-effects)
and jammed traffics (which result in maximum static load-effects) are considered. The former will induce
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