Cathodic protection of precast, prestressed multistory car park slabs, using a surface applied galvanic zinc layer anode.

. A surface applied galvanic Zinc Layer Anode (ZLA) system was installed on a multistory carpark structure to provide cathodic protection to the soffits of the prestressed hollow concrete slabs, which were suffering from chloride induced corrosion deterioration. This paper evaluates the cathodic protection system performance against the international standard for Cathodic Protection of Steel in Concrete, ISO 12696 [13] , 17 months after commissioning. A surface applied galvanic Zinc Layer Anode was selected to provide cathodic protection, as this offered a low risk of exceeding the hydrogen embrittlement potential and provided a unique application approach, which was compatible with the hollow slab construction. During the pilot phase, a total of 40 m 2 of ZLA was applied into two anode zones and monitored for a month, to prove concept and system performance. Following the pilot phase the remainder of the cathodic protection works were completed, extending to a total concrete area of 320 m 2 . The installed system was provided with embedded reference electrodes and wired to enable full performance evaluation as per the requirements of ISO 12696. Three battery powered web-based monitoring devices were used for remote system monitoring and performance assessment. The initial performance data following 17 months of operation, identified that all 12 reference electrodes installed across the 6 anode zones all met the 100mV depolarization criteria within a 24-hour period, as per the requirements of clause 8.6b of ISO 12696. The anode to cathode current was also recorded continuously over this operational period and used to evaluate environmental effects and predicted actual anode service life.


Introduction
The object in question concerns a steel reinforced concrete car park located at Contern in the southern part of Luxembourg. The carpark construction consisted of precast prestressed hollow concrete slabs installed in a transverse beam-column configuration, which resulted in an expansion joint being formed between individual precast slabs. The joints had been leaking for many years, which resulted in chloride contaminated water dripping through the joint onto the slab soffits, leading to corrosion of the prestressed tendons and subsequent concrete deterioration around the joint.
During inspections performed in early 2020 it became clear that the years of leakage at the central transverse beam-column joint had penetrated into the bridge deck initiating corrosion of the prestressed steel tendons. Figure 1 shows a general view of the car park interior.   As an economic remediation solution it was recommended to apply a cathodic protection (CP) system preferably a surface-applied galvanic CP system [2] as this would pose no risk of hydrogen embrittlement of the prestressed tendons, and as specified by the owner would allow for at least 10 years of corrosion control [6] [7] [9] [10] [12] .

Description
The car park, constructed in 2005, consists of a beamcolumn structure with transversely placed prestressed hollow concrete slabs shown in Fig. 8 with a 1,20m width. Two types of slabs were placed of which on one side of the joint in zones 1, 3 and 5, hollow slabs with 12mm diameter tendons as shown in Fig. 9, and on the other side of the joint in zones 2, 4, and 6 hollow slabs with 5mm diameter tendons as shown in Fig. 10. The total steel surface area of the slabs located in zones 1, 3 and 5 having 7x tendons with a diameter of 12mm and 1 tendon with a diameter of 14mm is : 0,31 m 2 steel/m 2 concrete and the total steel surface area of the slabs located in zones 2, 4 and 6 having 12x tendons with a diameter of 5mm and 1 tendon with a diamater of 14mm is : 0,23 m 2 steel/m 2 concrete

Application
The Zinc Layer Anode system used for this project consists of a high purity zinc foil, complete with an ion-conductive, auto moistening, humectant/activator/adhesive layer, which is designed to be surface mounted onto the surface of concrete structures.
Based on previous inspection results it was decided to apply the ZLA on an area on both sides all along the main joint with a width of 1,67m ( Fig.11 and 12) covering a total of 320 m 2 surface area. The 320 m 2 ZLA was devided in 6 equal zones of each appr. 53 m 2 . In total, three battery powered web-based monitoring devices were installed together with 12 ERE reference electrodes.

Results
Parameters like potential decay values and current outputs were measured, recorded and sent through a SIM card to a main server. Every individual with valid login codes had access to these monitoring readings which have been logged since start-up and are presented online through tables and graphs. Instant-off potentials and potential decay scans can be programmed in advance through the website. In total 3 monitoring devices have been installed of which x Device 1 : monitors zone 1 and 2,  The internal channels of the monitoring devices are linked in the following way: Monitoring device nr 1 (zone 1 and zone 2) The monitoring devices can be pre-programmed to switch off the current on any date and time. The tables and graphs below show the current and potential decay readings of all 6 zones and 12 Reference electrodes.      Considering an avarage current ouput for each zone, we are able to calculate the anode and steel current densities : The erratic pattern over time of the current outputs of each zone as seen in the above figures 17, 19 and 21 can be attributed to the seasonal and daily ambient temperature fluctuations. These phenomena have been noticed in several other projects [8] . Fig. 22 shows the ambient temperature vs. current output of ZLA installed on carpark slabs and supports at ARC 2000 in France. The direct relation between the ambient temperature and the current output can be clearly seen.

Conclusions
ISO12696 [13] indicates the 3 criteria which a CP system should meet for satisfactory steel in concrete corrosion protection. Basically ISO12696 says "For any structure, any representative steel in concrete location shall meet any one of the criteria given in items 1 to 3 : 1. an "Instantaneous OFF" potential more negative than −720 mV with respect to Ag/AgCl/0,5 M KCl; or 2. a potential decay over a maximum of 24 h of at least 100 mV from "Instantaneous OFF"; or 3. a potential decay over an extended period (typically 24 h or longer) of at least 150 mV from the instant off subject to a continuing decay and the use of reference electrodes (not potential decay probes) for the measurement extended beyond 24 h.
We can pick out one of the three choices and normally for CP of steel reinforced concrete structures we use criteria nr. 2 which is a potential decay over a maximum of 24hrs from instant-off [1] . Using the readings from the 3 monitorings devices above we will get instant-off potential decay values over a maximum of 24hrs :