Figure 2 shows positions where CP systems were installed. Places were a slab and a pile cap, and the positions were selected after the macrography. Traces of severe corrosion damage were found in these places.

Figure 2: 
Positions where CP system is installed (Left: Slab, Right: Pile Cap)

Figure 3 is the schematic drawing of the proposed sensor in this study for corrosion monitoring in concrete and all inside space of the sensor was filled with epoxy to prevent concrete specimens from any detrimental factors like sea water. In addition, described earlier, the corrosion monitoring sensors, DMS-100, were installed. The sensor can measure various factors such as potential, current density, corrosion rate, resistivity, and temperature.

Figure 3: 
Schematic drawing of the DMS-100 sensor

The sensor was installed to enhance the accuracy of tests by comparing with the results of steel in concrete. Each two sensors were installed for comparison. The sensor has guaranteed its performance in many laboratory and onsite studies [14][15].

Figure 4 and 5 show the process of construction of CP system. Firstly, the surface treatment of damaged areas was carried out for the purpose of an increase in efficacy of anodes. After the surface treatment, anode and rebar was electrically connected. Lastly, zinc anodes with a mesh type were installed with the consideration for dimensions, and the places were cast in concrete with proper concrete cover depth of 3cm. After construction of CP, damage spaces were recovered. As shown in Figure 4 (d) and 5 (d), structures were well restored with a clean surface.

Figure 4: 
Process of construction of CP system (Slab)

Figure 5: 
Process of Construction of CP System (Pile Cap)

Experiments were carried out for 90 days. During the period, three times of measurements were conducted. First measurement was implemented after installing CP system. Second measurement was conducted after a month from initial measurement. Last measurement was conducted after three month from the initial measurement. Test factors were corrosion & CP potential, 4 hour depolarization potential, current density, and concrete resistivity.

Figure 6 and 7 show the potential variations of rebar for 90 days. As shown in graphs, initial rebar potential of the slab was -457mV vs. SSCE and that of the pile cap was -553mV vs. SSCE. According to ASTM C 876 [20], if potential is lower than -400mV vs. SSCE, rebar is regarded as severely corroded state. The state of rebar was very critical for corrosion. However, in the case of the DMS-100 sensors, sensors are regarded as passive state with a higher potential range of 50~100mV vs. SSCE.

When CP was applied, potential both rebar and sensors was decreased to -750mV ~ -650mV vs. SSCE, which is sustained for more than 90 days. In the case of sensors, higher potential variations were confirmed than rebar because sensors did not be corroded.

Figure 6: 
Potential variations of concrete slab

Figure 7: 
Potential variations of concrete pile cap

Current density can be a key factor in measuring CP state of rebar. Through a number of laboratory and field experiment, the required current densities for CP was 10-20mA/m2, which might extend the service life for the already corroded steel in concrete. It is much higher than that for no corrosion steel in concrete (2mA/m2) [21][22].

Figure 8 shows the CP current density variation of DMS-100 sensors after applying CP for 30 days. The values of current density were 1.8 ~ 2.5mA/m2. Considering the sensor was no corroded, the zinc anode can supply enough CP current. However, it was hard to decide the steel's CP condition by solely checking the result of sensors' current density because steel in concrete has been corroded for a long time, which was different condition with sensors.

In addition, current density has a tendency to be in inverse proportion to resistivity. This is because resistivity plays a key factor to decide the exchanging rate of ions.

Figure 8: 
Current density and resistivity of sensors after applying CP system for 30 days.

Figure 9 and 10 show depolarization potential variation for 90 days. Depolarization potential is normally measured by disconnecting anode from rebar for 4 hours or 24 hours. In this case, the 100mV depolarization criterion for 4 hours was utilized to check the effectiveness of sacrificial anode CP system.

Figure 9: 
Depolarization Potential of concrete pile cap

Figure 10: 
Depolarization Potential of concrete slab

As shown in Figure 9 and 10, the depolarization of rebar was 135mV ~ 180mV, which met the 100mV depolarization criterion.

In the case of DMS-100 sensor, depolarization potential were 200mV ~ 315mV, which was higher than rebar. Depolarization potential of rebar and sensors had a tendency to decrease with time. This was thought that due to the dry of concrete, concrete resistivity was increase with time, which has reduced CP effects.