ABSTRACT
Cathodic protection (CP) is intended to deliver corrosion protection in concrete structures exposed to aggressive environments, e.g. in deicing salt and marine climates. Although preventative application is possible (called Cathodic Prevention), most cases concern structures that already have developed corrosion and some level of damage to the concrete cover (cracking, spalling). CP involves polarizing the reinforcing steel by a low amount of direct current, originating from a conductor on the surface or in the cross section of the structure connected to a low voltage power source. Spalled areas need to be repaired using normal concrete repair methods. A large difference between CP and the conventional repair approach is that in the latter case, all chloride contaminated concrete has to be removed and new has to be reapplied. With CP, physically sound but chloride contaminated concrete can be left in place. European experience with many hundreds of CP systems shows that they perform well, provided that a minimum of (routine) maintenance is carried out. An inventory in the Netherlands of one hundred well-documented CP systems shows that working lives are at least more than 13 years and probably more than 25 years. Individual cases that needed maintenance are discussed, based on the need to replace electrical connections, reference electrodes, power units and anode materials. All such cases have occurred to a limited extent. It is felt that the quality and life of items needing replacement in the past have now been improved or have become considerably cheaper, allowing some redundancy to be installed. Maintenance is generally carried out by the contractor who installed the system, for a relatively low annual fee. This involves routine checks of power units and testing depolarisation of reinforcement at least twice a year. Regular design of
CP is based on conservative assumptions. Lightweight anode materials and numerical modelling in the design phase may provide more cost effective systems, which can be termed “smart CP systems”. A CP trial on a bridge substructure in Slovenia was carried out in European FP6 research project ARCHES. In the trial, various types of lightweight anodes were applied including conductive coating and titanium strip anodes in mortar dykes on the concrete surface. Numerical Finite Element modelling was applied to the trial CP systems. In the modeling a geometrical analogue is built in two dimensions, as three-dimensional finite element modeling is beyond present computer capabilities. Input parameters are either obtained from the structure (steel potentials, concrete resistivity) or from literature (electrochemical parameters). In our calculations the input parameters had to be fine-tuned to a certain extent in order to obtain realistic current levels. With the adjusted input, local polarisations were calculated in good to reasonable agreement with values measured in the trial. Consequently, it is possible to predict the performance of a CP system in the design phase, which allows for economical optimisation. For this case, life cycle costs were calculated for conventional repair (assuming 25 years life of the repairs and also taking into account that re-repairs would be needed half way the 25 year life), CP with a conductive coating (including re-application of the coating once in a 25 year period) and titanium mesh with a shotcrete overlay (that will last at least 25 years). CP proved to be more economical than conventional repair, taking into account that the life of conventional repairs is limited, as suggested by a European study, and consequently there is a significant probability that re-repairs are needed. For the trial site, it was found that up to 10% lower life cycle costs over a period of 25 years were possible with CP compared to conventional repair.
