CATHODIC PROTECTION OF THE KRK BRIDGE

Transcription

1 CATHODIC PROTECTION OF THE KRK BRIDGE D. Sekulic, J. Bleiziffer, J. Beslac, M. Ille Summary To extend service life of the Krk Bridge, as one of the most important infrastructure reinforced concrete objects in Croatia, in the sub-structure of the large arch, an active cathodic protection system is installed. Paper describes basic working principles of cathodic protection systems and shows phases of system installation, results of system efficiency monitoring and result analysis. It is shown that, for successful interpretation of measuring parameters, and for efficiency of cathodic protection system assessment, application of statistical methods is necessary. Implementation of this procedure into our practice is of great importance, because we considered that cathodic protection (CP) may be required for service life extension for other Adriatic arch bridges, such as Pag and Šibenik bridge. Keywords: Cathodic protection (CP), reinforced concrete bridges, durability, service life, reinforcement corrosion, chloride ions. 1 Introduction The Krk Bridge was built in 1980 and still has the largest conventional reinforced concrete arch span (390 m) in the world [1]. The bridge is situated in aggressive maritime environment, with its main members like foundation elements and lower parts of the arches being most exposed. After many years of research and testing of various corrosion protection systems [1, 2], impressed current cathodic protection method was selected for the sea level elements and submerged elements that support the larger arch of the Krk Bridge [3]. Cathodic protection (CP) implementation was committed to Institute IGH d.d., which commissioned British company Freyssinet Ltd to undertake pre-design and Installation Survey and design project of a cathodic protection system. Design Project was finished in 2005 and validated by Institute IGH d.d. CP system installation was commissioned to Croatian Company Al-Kor-El d.o.o., which started with works at the end of Quality control of installation works and one year CP system work monitoring was assigned to Institute IGH d.d. 1

2 2 Short CP System description 2.1 Pre-design works Prior to the CP system design, an inspection of corrosion related damage of foundation elements of the large arch of Krk Bridge was performed. Inspection included visual assessment, delamination survey and reinforcement electrical continuity testing. From the results of pre-design survey it was concluded that the sub-structures on the St. Marko and Mainland arch springings are suitable for the installation of an impressed current CP system, but only after welding of discontinuous reinforcement steel and additional continuity testing [4]. 2.2 CP system description Into sub-structures on the St. Marko and Mainland arch springings two identical CP systems, which work independently were installed. Atmospherically exposed zone of substructure have Titanium mesh anode, which is fixed onto concrete surface and covered with repair mortar. In submerged zone disc anodes are fixed onto structure (Figure 1). Ti anode mesh Sea level Ti anode mesh Disc Anodes Disc Anodes Reference electrode Mainland Reference electrodes Fig. 1 Plan of the CP system [1] For monitoring of CP system efficiency, reference electrodes were embedded into the structure at several positions as shown in fig. 1. For atmospherically exposed part of substructure Ag/AgCl/0,5 M KCl reference and MMO-titanium covered electrodes were used. For submerged part Ag/AgCl seawater reference electrodes and Zn pseudo-reference electrodes were used. MMO Ti and Zn electrodes are spare electrodes. 3 CP system installation Specialist supervision of CP system installation works was performed by the Institute IGH, through attendance during key work phases and through control of quality records [5]. First phase of works was electrical continuity of reinforcement testing. For this purpose vertical reinforcement and horizontal reinforcement was exposed by removal of concrete cover as shown in fig. 2. Comprehensive electrical resistance measurements between different parts of reinforcement gave resistance above threshold value, R>1O. 2

3 Therefore, electrical continuity was achieved by connecting reinforcement bars by welding. After welding, continuity testing was done again, until good electrical conductivity between all measurement points, R<1O was achieved. Submerged part of substructure was also tested for electrical continuity. Results were elaborated in a comprehensive test report [6]. After continuity was achieved, uncovered parts of substructure were protected by repair mortar. Second phase was installation of reference electrodes for system efficiency monitoring. After that, Ti anode mesh and connection cables were installed ( fig. 3 and fig. 4), and finally system was protected with a layer of repair mortar ( fig. 5). Fig. 2 Reinforcement continuity testing Fig. 3 Titanium anode mesh installation Fig. 4 Ti mesh on the side surface Fig. 5 Repair mortar on the side surface In submerged zone, at the structure surface several reference electrodes and disc anodes were installed, for which a diver was engaged. All parts were designed for 20 years service life. 4 Quality assurance Quality of performed CP installation works was assured by following the detailed quality plan, prepared by CP system designer [4]. Quality documentation consists of a number of procedures that cover all phases of installation and subsequent operation of installed CP system. During installation, CP engineer should keep records of the materials used and of the installation undertaken. These records include hold points where quality checks should be made, as shown in tab. 1. 3

4 Tab. 1 Quality insurance plan Record Continuity testing and bonding of atmospherically exposed and submerged reinforcing steel DC negative and test connection installation Reference Electrode installation Surface preparation prior mesh anode installation Installation of mesh anode system prior overlay setting during overlay setting after overlay setting Installation of disk anode system Installation of cables, cable management and junction boxes Installation of Transformer - Rectifiers and Remote Control and monitoring systems Energisation and Commissioning System operation Actions Sub-structure reinforcement continuity measurement before and after reinforcement bonding (R<1O) Steel-Seawater-Reference electrode potential measurement (Uon<Uoff) Resistance between connections measurement (Rcon<1O) Control of manufacturer's Calibration, pre-installation check, after installation check of reference electrodes potentials, location and identification of electrodes check. Visual inspection of prepared concrete surface Control of manufacturer's certificate, visual inspections, electrical measurements approval for mortar overlay setting. Surface preparation, check of equipment and materials, electrical control measurements during overlay setting Electrical measurements 72 h after overlay setting Visual inspection of installation by diver, electrical measurements Visual inspection of all cable connections to junction boxes Certificates check, visual inspection, electrical tests at transformerrectifier, cable identification confirmation. Electrical measurements of electrical circuit resistance, potentials and reference electrode potentials. Control of remote measurement and data acquisition equipment Monthly, quarterly system control and Annual system Review 5 Performance monitoring Performance monitoring of CP system is carried out by automatic half-cell data logging. Monitoring results during the first 3 months of CP system operation are given below. Before CP system energisation natural potentials of reference electrodes were measured as shown in tab. 2. Measured natural potentials indicating state of active reinforcement corrosion interpreted according to ASTM C criteria [7]. Tab. 2 Measurement results of reference Ag-AgCl/0,5M KCl for atmospherically exposed anode zone [5]. Reference electrode Natural potentials Initial polarisation 20% Initial polarisation 50% days 15A/8,5V months 5,05A/6,4V R R R R After CP system energisation, half cell potentials were reduced for amount from about -10 mv to -270 mv, in respect to natural potentials, which is one of indications, that in all measurement points reinforcement steel was polarised and CP system is efficient. 4

5 D. Sekulic, J. Bleiziffer, J. Beslac, M. Ille : CATHODIC PROTECTION OF THE KRK BRIDGE Tab. 2 also shows reference electrode potential measurement results for 28 days and for 3 months of system operation. Protective potential of -900 mv is present at the electrode R4, which meets the criteria of CP protection (between -720 mv and mv [4, 9]) but for the remaining electrodes these potentials are higher, which may be due to moisture content, distance of electrodes from the reinforcement or similar. It is expected that better reinforcement polarisation will be achieved over time. 5.1 Statistical approach Continuous measurements of half cell potentials show significant potential fluctuations as shown in fig. 6. Therefore, for the proper assessment of CP system performance it is necessary to include statistical analysis. From the fig. 6 it can be noted that average CP potential value for atmospherically exposed zone during first three months of operation is above the protection criteria, but during the last time period the average value approached to the protection criteria (-720 mv). 0 (a) HalfZaštitni cell potencijal potential st period 3th period 2nd period A no d n a zo na 2 A no d na zo na 1 K rite ri j m V K rite rij m V /2 11/21/21 01 /28/ /4/21 1/18/21 1/21/21 1 /24/21 1/27/21 1 /31/ Date 01/1 /2012 /16/201/2 19/201/22/2012 /6/201/29/ /1 1/17/2 1/21/2012 D a tu m Uaverage =-628 mv (b) (c) Probability density Probability density Uaverage =-513 mv -2s U 2s -2s U 2s Fig. 6 (a) Measurement results of half-cell potentials for the first three months of system operation, (b) Potential distribution for the first time period, (c) potential distribution for the third time period Fig. 6 and fig. 7 show that time fluctuations of potential can be described by normal distribution quite well, which means that these fluctuatio ns are attributed to random process. Normal distribution function for the first time period (fig. 6b) shows that probability for meeting the upper limit of the CP criteria (-720 mv) is quite low (about 50 5

6 %), but for the last time period ( fig. 6c) that probability increased significantly (to about 90 %). The normal distribution for the last time period also shows that probability that measured potentials are below mv is 5 %, i.e. we have 5 % probability that potentials do not meet the lower criteria for CP. Therefore it can be concluded that CP potential shall not be adjusted to lower value, i.e. probability for potential below lower criteria of CP should not be higher than 5 %, because of a risk of reinforcement reaction with hydrogen. 6 Conclusion Paper describes approach to CP implementation to sub-structure of large arch of Krk Bridge. Prior to the cathodic protection design, comprehensive investigation of reinforcement continuity was done to evaluate whether the structure is appropriate for CP installation. During CP protection system installation quality checks were made on check points defined in the quality manual. This is of crucial importance for later satisfactory CP system functioning. Efficiency monitoring for a period of one year after CP system installation is proposed, which is also very important to be able to do necessary corrections of protection potentials for optimal reinforcement protection, and to gain insight into CP efficiency. Furthermore, it is shown that, for successful interpretation of measuring parameters, and for efficiency of CP system assessment, application of statistical methods is necessary. Implementation of this procedure into our practice is of great importance, because we considered that cathodic protection may be required for service life extension for other Adriatic arch bridges, such as Pag and Šibenik Bridge. 7 References [1] J. Beslac, J. Bleiziffer, J. Radic (2010), An overview of Krk Bridge repairs, ARCH'10, SECON, Zagreb [2] J. Beslac, M. Ille, J. Bleiziffer (2011), Repair and protection of krk bridge, CCC2011 The 7th Central European Congress on Concrete Engineering Innovative materials and Technologies for Concrete Structures Budapest, fib Hungary, [3] J. Beslac, D Bjegovic, R. Roskovic (2005), Innovative materials and technologies in the construction and maintenance of concrete structures, Gradevinar, 57, 4; [4] K.P.Woodland (2005), Krk bridge Cathodic Protection of Sub-Structure, Pre-Design & instalation Surway Report, Deteiled Design Document, Quality Plan, Method Statement and Approval to Proceed Documents, Bill of Quantities, Freyssinet Ltd., UK, rev. 0, June [5] Krk Bridge Quality system documentation, Al-Kor-El d.o.o., Pavleka Miškine 1, Zagreb, November [6] Krk Bridge Cathodic protection, Measurements of reinforcing steel continuity, Al- Kor-El d.o.o., Pavleka Miškine 1, Zagreb, January [7] ASTM C Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete [8] Report of Krk bridge sub-structure cathodic protection system control, Al-Kor-El d.o.o., Pavleka Miškine 1, Zagreb, March [9] EN 12696:2000, Cathodic protection of steel in concrete. 6

7 [10] J.P. Broomfield, J.S. Tinnea (1992), Cathodic protection of reinforced concrete bridge components, SHRP-C/UWP , National research council, Washington. Dalibor Sekulic, PhD (Expert) Jelena Bleiziffer, PhD (Director of the Institute for materials and constructions) Jovo Beslac, PhD (Consultant) Mario Ille, PhD ( Director of the Structure management department) * ( 4 J URL Institut IGH, d.d Zagreb Croatia igh@igh.hr 7