CORROSION MONITORING IN MARINE ENVIROMENT IN CROATIA

Field: Civil Engineering
Writers: Prof. Dubravka Bjegovic, Ph.D., Faculty of Civil Engineering, University of Zagreb Irina Stipanovic, B.Sc.C.E., Faculty of Civil Engineering, University of Zagreb Marijan Skazlic, M.Sc.C.E., Faculty of Civil Engineering, University of Zagreb Kajo Feric, B.Sc.C.E., Croatian Institute of Civil Engineering, Center Split
Ivo Barbalic, B.Sc.C.E., Croatian Institute of Civil Engineering, Center Split
 
Abstract:
 
In the last decades it has become well known that the corrosion of reinforcement is the most harmful damage that occures in reinforced concrete structures. The ``great`` example of such case happened in the Yacht Marine Split, Croatia, where the reinforced concrete structure was exposed to very aggresive marine enviroment. The old reinforced concrete construction elements had been tested in November 2001 (visual inspection, half-cell potential mapping, mechanical and chemical testing) and as a result of those testings, the experts had concluded that the serviceability and load bearing capacity of the main structure elements are critically damaged. All those testings have showed that the reinforced concrete structure was not properly designed and during building proccess not properly constructed. It has been decided to change main span structure elements, which were permanently in tidel zone and partially to use old columns, which were under water and in a good state.
In the spring 2002 a new marine has been built, with prefabricated prestressed reinforced   concrete elements. On one of the concrete element corrosion protection with migrating corrosion inhibitor has been applied, because of the tiny cracks, that have occured during production. Since the new elements were placed  in  the  position  according  the  project,  the  monitoring  of          the  concrete  elements  with  the galvanostatic pulse technique started. The galvanostatic pulse technique is a non-destructive method, suitable for on-site measuring, which is determining the free corosion potential, corrosion current and concrete resistance.The results of these measurements are presented in the paper.
 
Keywords: case study, concrete, corrosion of reinforcement, deterioration, marine enviroment, monitoring, site measurements.
 
 
1.    Introduction
 
In the last decades it has become well known that the corrosion of reinforcement is the most harmful damage that occures in reinforced concrete structures. The case study described in this paper is an example of the reinforced concrete structure exposed to very aggresive marine enviroment for 30 years. The  analysis  performed  during the  investigations  in November 2001  showed  that  the  structure deterioration degree was in most places at the critical level and the immediate intervention was necessary [1]. In the spring 2002 a new marine has been built, with prefabricated prestressed reinforced concrete elements. Since the new elements were placed in the position according the project, the monitoring of the concrete elements started with the non-destructive galvanostatic pulse technique. This paper is giving the results of the performed tests on the old marine structure and also monitoring results on the new structure.
 
2.    Structure description
 
 
2.1.         Location
 
 
The structure of the marine is located at the Adriatic coast in Dalmatian area, within the city of Split (Fig.1, 2). At this region a typical mediteranean climate is dominating, with very hot summers and mild winters, but during which are blowing strong winds coming either from north (dry and cold), or from south (wet and warm).

 

 

2.2.         Inspectionmethodology ontheoldstructure

 The oldstructurewascomposedofreinforcedconcreteelements(columns)andprestressedreinforced concreteelements(beamsandT-girders),anditwasbuildintheearlyseventies.Duringtheyear 2001the investigation of theoldstructure was performed, whichsummary is following [1].

2.2.1.       Visual structure inspection

On thedockconstructionelements,whichwere demountedandpulledoutfromthesea,primarilyavisual inspectionwasdone(Fig.3-6).Basedonthisvisualinspectionitwasconcludedthatthemainbeamsof thespanstructureare inthecriticalstateof damage(Fig.3-4),andnofuthertestingswereneededtoprove usabilityoftheseelements.Thecolumns ofthetesteddockwerecompletelycoveredwiththeshells (Fig.5-6), and on some places concretecoverwasdestroyedbythereinforcement corrosion (Fig.6).

 

2.2.2.       Compressive strength core testing
 
 
The compressive strength testing was done on six drilled concrete cores from the structure. The results of testing were between 40.1 MPa and 58.1 MPa (recalculated on the characteristic concrete cube strength), which is pointing out that the concrete had higher strength than expected, but this was not enough considering durability criteria.
 
2.2.3.       Concrete chemical analysis
 

On the drilled specimens from the structure chemical analysis were performed, which included chloride and sulphate content, pH value, carbonation. Test results are shown in next diagrams (Fig.7-8).

 

Figure 8. Sulphate profile through the depth of the concrete

The chemical tests on  drilled  concrete cores showed  that  the  concrete was heavily  contamined  by chlorides to a depth much deeper than the reinforcement position. The chloride content was multiple greater than the threshold of 0.4% by weight of cement. Values of pH factor were bigger than 12 on all tested specimens.
On the base of visual inspection and laboratory tests the evaluation of the structure degradation level was given. It was obvious that structure was not enough qualitative designed and built, and above that the structure was exposed to the extremely agressive enviroment. Spalling of the concrete cover was visible on many places on the span beams, and also significant corrosion damages of the reinforcement. This damage type belongs to the critical grade and implies a indispensable intervention.

 

2.3.         New marine construction
 

After the final evaluation of the structure degradation level, the reparation proccess had started. In the spring 2002 a new marine was built, with prefabricated reinforced and prestressed concrete elements. All span structure elements and some columns were changed with new precast elements. The rest of the columns were left in the sea, since they didn`t show high degradation level. During production of beam elements for one of the docks (dock F) tiny cracks occured. Considering the agressive enviroment where they would be placed, a corrosion protection with migrating corrosion inhibitor (MCI® 2000) has been applied on the surface. Migrating Corrosion Inhibitors (MCI®) are used for the protection of reinforcing steel in concrete. They can be incorporated as an admixture or topically applied to the concrete surface. Migratory inhibitors are transmitted through concrete to the reinforcing steel by diffusion [2-3].

3.    Monitoring
3.1.         Monitoring purpose
 
After a new marine construction was finished, the monitoring of the concrete elements started with the galvanostatic pulse technique. Two docks are monitored, dock B1 which is in the most critical position and unprotected, and dock F, which is protected with MCI®2000 inhibitor. Dock B1 is at the beginning of the marine (the lowest position above the sea), and beam elements are in a tidel zone. First measuring with galvanostatic pulse technique was performed in June 2002, second in January 2003, and the third one in June 2003. Monitoring with galvanostatic  pulse method of this marine  construction  will be performed continuosly, at least every six months, to get timely data about any change in corrosion current, potential and concrete resistivity. The purpose of the monitoring is to prove the efficiency of the
MCI inhibitor.
 
3.2.         Method description [4 - 6]
Galvanostatic pulse method is a rapid non-destructive polarization technique, which can be used for evaluation of reinforcement corrosion in laboratory and on site. A short time anodic current pulse is applied  to  reinforcement  galvanostatically  from a  counter  electrode  placed  on  the  concrete  surface together with a reference electrode [Fig.11,12]. The applied current is normally in the range of 5 to 400
µA and the typical pulse duration is up to 10 seconds. The small anodic current results in change of
reinforcement potential, which is recorded as a function of polarization time. Typical potential response is schematically shown in Fig. 10.

 


When the constant current Iappis applied to the system, the polarized potential of reinforcement
Vt, at given time t can be calculated as a function of polarization resistance (Rp), ohmic resistance (Cdl)
double layer capacitance (RΩ). After the polarization resistance Rpis determined by means of this analysis
[6], the corrosion current Icorrcan be calculated from Stern Geary equation:
 
Icorr= B/Rp
 
where B is an empirical constant determined to be 25 mV for actively corroding steel and 50 mV for passive steel. To overcome the problem of the working electrode (reinforcement) and the electrical signal, which  tends to  vanish with increasing  distance,  a  second  concentric  counter  electrode,  so  called Guardring, has been used to confine the current to the area of the central counter electrode (Fig.11.).

 

When the diameter of the reinforcement and the exposed length of the reinforcement (counter electrode diameter) are known the instantaneous corrosion rate can be calculated [4-6].
 
3.3.         Monitoring results
 
3.3.1.       Dock B1
 

Dock B1 was built of the concrete quality C40. Span beams are prestressed prefabricated concrete elements, with U shape in cross section (Fig.9). Concrete surface was done without any protection. On Figure 13 the position of dock B1 is visible and that the first span beam is exposed to sea splashing. Schematic view of test points is are shown on the Fig. 14.

Duringthemonitoringthecorrosioncurrent,corrosionpotentialandconcreteresistivityweremeasured. On nextdiagrams themeasuring results fromdockB1areshown:

 


 

 

3.3.2.       Dock F

Span structure beams for the dock F (Fig. 21) are built with reinforced prefabricated concrete elements. As already described, these elements are treated with MCI inhibitor, after production. Schematic view of test points is shown on Fig. 22.

 

 

 

 

 

The estimation of the remaining service life based on the measured values of corrosion current can be done according Clear [7], as shown in Table 1.

Table 1.

Corrosion current Icorr[µA/cm2]
Corrosion damage
< 0.5 µA/cm2
 no corrosion damage expected
0.5 < Icorr< 2.7 µA/cm2
corrosion damage possible in
10 to 15 years
2.7 < Icorr< 27 µA/cm2
corrosion damage expected
in 2 to 10 years
27 < IcorrµA/cm2
corrosion damage expected in 2 years or less
 
 
 
 
 
 
 
 
 
 
Comparing the monitoring results on two marine docks, performed within 1 year, following remarks can be withdrawn:
•       it is obvious that the MCI ®(dock F) has lowered the corrosion current;
•       according Clear estimation: no corrosion damage is expected on dock F and on dock B1 possible
corrosion damage in next 10 to 15 years;
•       as presumed from previous measurements with galvanostatic pulse method, the potentials are
influenced by inhibitor, so further readings are needed to make conclusion about the influence and the efficiency of inhibitor;
•       concrete resistance will be used as a control measuring; resistance is influenced by weather
conditions, humidity and temperature;
•       the galvanostatic pulse measurements are providing a momentary value of actualy corrosion
stage, which significantly depend on actual concrete conditions (humidity, temperature, pH- value, chloride content) [6];
•       the problem is the real size of reinforcement cross section on which the current is applied, which

has to predicted for on site measuring.

4.    Conclusions
 
Concrete strength for reinforced concrete structures exposed to maritime conditions is just supporting factor, but not the decisive parameter for durability of the construction. For these structures durability criteria is of the bigger importance and should be included in structural design. Concrete durability depends on many parameters and beside structural design, the attention should be directed also on the concrete mix design including corrosion protection, technology of the concrete production, building process and good-quality workmanship. All the structures in the maritime conditions should be monitored during their service life, to avoid high repair costs.
Considering the measuring technique it can be noticed that galvanostatic pulse method is a very suitable technique for on site measuring, since it is rapid and simple. But it is important to underline that the corrosion rate values are very dependent on the environmental conditions and the area on which the current is applied. Different temperature and humidity conditions have a great influence on the measured
corrosion rate values [5]. It is essential to combine the on-site corrosion rate measurements with other
NDT methods to determine the concrete integrity and state of the reinforcement [8]. 

5.    Literature

1.     Croatian Civil Engineering Institute, Center Split: Report on testing of concrete quality in ACI Marine Split, IGH PC Split, No.: 3613 – 2911/01, RN 3600/1-36037, November 2001, (in Croatian);
2.     Bjegovic,  D.,  Miskic,  B.:  Migrating  Corrosion  Inhibitor  Protection  of  Concrete;  Materials
Performance, Vol. 38 (1999), No. 11; pp. 52-56;
3.     Bjegovic, D., Miksic, B., Stehly, R.: Testing Methodology for Migrating Corrosion Inhibitors; Eurocorr 2000 "Past Successes - Future Challanges``, 10.-14. Sep. 2000; London, England, pp.
210-215;
4.     Frølund, T., Jensen, M.F., Bassler, R.: Determination Of Reinforcement Corrosion Rate By Means Of The Galvanostatic Pulse Technique, First International Conference on Bridge Maintenance, Safety and Management IABMAS 2002, Barcelona, 14-17 July, 2002;http://www.germann.org/;
5.     Klinghoffer, O., Frølund, T., Poulsen, E.: Rebar Corrosion Rate Measurements for Service Life Estimates, ACI Fall Convention 2000, Toronto, Canada, Committee 365 “Practical Application of Service Life Models”;http://www.germann.org/;
6.     Bäßler,  R.,  Burkert,  A.,  Frølund,  T.,  Klinghoffer,  O.:  Usage  of  Portable  Equipment  for Determination of  Corrosion Stage of Concrete Structures ;  EUROCORR 2001,http://www.germann.org/;
7.     Clear, K.: Measuring Rate of Corrosion Steel in Field Concrete Structures, paper No. 88-0324,
68th Annual Transportation Research Meeting, 1989;
8.     Bjegovic,  D.,  Mikulic,  D.;  Sekulic,  D.:  Non-Destructive  Corrosion  Rate  Monitoring  for Reinforced Concrete Structures; 15th World
Conference on Non-Destructive Testing; Rome, Italy, October 2000, pp. 642-652
 

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