Due to the easiness of construction, many maritime structures, principally, foundation piles and earth retaining walls are made of steel. It is then normal that these structures are subject to corrosion that could be severe because of the direct contact with seawater or spray. Usually, there are considered, one or more methods to minimize the effects of corrosion, such as barriers, cathodic protection, or structural over-thicknesses.
For such structures, some inspection and measurements, are performed as quality control during construction. However, in most cases, corrosion assessment of these steel structures, during service life, is negligible. Oversee of service evaluations of structures, normally imply lately discovers of damages and could lead, in the worst scenario, to structural major failures. Afterwards remediation countermeasures, are often costly and done with poor knowledge of which are the critical areas. Evaluation performed during the service life of a structure, or simply maintenance evaluation, could be done visually and include or not discrete measurements along the structures. Nevertheless, there are some advantages in doing measurements for maintenance evaluations. Maybe the most important is to detect differences in the behavior of similar-type elements, and i.e. establish the priorities for maintenance.
This present paper considers some studies cases with evaluation and follow up of structures after construction, including thicknesses and electrical potential measures. The corrosion analysis executed in three maritime projects are explained: a cellular cofferdam breakwater for a marina, an earth-retaining sheet-pile wall for berthing in a commercial port, and a pile-supported trestle in a sugar-pier. These projects are all located at Pacific side of Costa Rica.
First case, is in Quepos. In 2010, it was constructed t he Phase one of a marina, which has two mix breakwaters, bo t h wi t h rubblemound and circular cells of sheet piles, on marine steel, and filled with sand and gravel. Breakwaters are 956 meters long and have 25 circular cells from 12 to 18 meters in diameter, with interconnection arches. An over-thickness was considered for the sheet-piles.
The maintenance plan for the marina, considers tracking t he corrosion experienced by the steel sheet-piles, and comparing 'actual' against expected corrosion rates. An analysis is then required to check that the structural limits for the corrosion additional thickness are not exceeded. The plan includes the rule of implement countermeasures when potential areas of accelerated corrosion are identified. Specific control sections, distributed along the breakwater, both internally and externally of the basin, (or inside the marina), were considered. In each section, thicknesses were measured every meter from the top of the cell to the seabed. These measurements were made using ultrasonic equipment, with special underwater transducer, out of water, and in the submerged sections with divers. Update annual campaigns measurements, are from 2011 to 2013, and 2015 to 2017. From the analysis of the measurements, some recommendations regarding the barrier protection of the sheet-piles for certain sectors of the sheet-pile walls had been issued.
Other case is for the major Port in Caldera. The principal bulkhead of the port, includes three berthing positions from depths -7.5 to -11 m LWL (Low water level). This is a steel sheet-pile retaining wall constructed in 1980. Above water, sheet-piles are protected by a concrete cap, meanwhile, below sacrificial aluminum alloy anodes provide cathodic protection. Evaluations of sheet-pile thicknesses and electrical potential generated by the cathodic system were performed by others in 2003. Additional measurement campaigns, of thicknesses and cathodic protection were executed from 2011 to 2015, and the last one in 2017.
Steel thicknesses measures, with a similar methodology as in the previous case study, are done for control and to verify no important losses are experienced in the sheet-piles. Additionally, electrical voltage assessment is being used as maintenance evaluation to detect areas with low potentials, in comparison with was is required to suppose corrosion inhibition according to standards. On those areas, installation new anodes are welded to regain potential, and then new electrical measurements confirm the protection.
The third case is, is the Punta Morales pier, constructed in 1980, which is a dolphin-type pier with a loading platform, all on piles, fundamentally for sugar export. The sugar is transported from a warehouse on land to the loading platform by means of a conveyor belt. This belt is supported on (9) concrete caps, each one with (4) H-beam steel piles embedded on ground. The piles of the conveyor support are protected from corrosion by painting and by an active cathodic protection.
During routinely inspections, sections losses were viewed on support piles, mostly near low water level (LWL). However, extend of the damages were unknown, especially below water. For assessment, in the five outermost supports, it were measured, steel thicknesses at each pile, every meter from top to sea-bottom, as well as a pile-per-pile electrical potential. Thicknesses measures helped to identify areas with severe losses, i.e with lower measurement thicknesses compared to others. This evaluation did show some piles below water with losses, not necessarily visible. Instead of recommending substitution of the piles, reinforcement of them with steel plates was proposed, according to the distribution of the evaluation measurements. This repair was implemented shortly after the evaluation, and included underwater welding in difficult current and visibility conditions.
In all these study cases, thicknesses and electrical potential determination, helps to differentiate sectors of steel structures, where the phenomenon of corrosion and/or abrasion occurs with varied attack levels. With several campaigns of thicknesses measurements, along the years, it is possible to estimate corrosion rates and useful lives, both general for structures, and specific for each level and section. In turn, this allowed to iden t ify maintenance priorities, defining possible sites where measures of corrosion protection should initiate, wi t h barrier protection, or active or passive, cathodic protection systems, as well in general, t o have confidence in the structural capacity and safety of structures. Moreover, evaluation with discrete measurements along the structures had shown to be cost-effective reducing the costs for repairs and maintenance of the steel elements and cathodic protection systems.