Rehabilitation of damaged reinforced concrete surfaces of small arch on the Bridge Krčki most
In 1986 the Civil Engineering Institute - GI (today Civil Engineering Institute of Croatia – IGH) engaged in the issue of inspection, defining of state, investigation and design of damage rehabilitation and protection of reinforced concrete structure of the bridge. Initial systematic investigation works on the condition of the bridge were performed. Significant damage of supports of main girders on piers above both arches were observed, as well as penetration of chloride in the concrete of protective coat, only on the lowest bridge elements (arch braces and struts of the long arch). Emergency rehabilitation of all fixed support structures was required, and protection of all external bridge surfaces from further chloride penetration was suggested. By 1987 the Contractor had prepared rehabilitation design for support structures and that same year the construction began. Before the war all damaged support constructions on the long arch and on pier S28 on small arch were rehabilitated, according to the design. At the beginning of the war all works stopped.
At the same time, GI prepared General Technical Conditions for tendering design solutions and protection of bridge reinforced concrete structure from chloride penetration. According to General Technical Conditions, the following year Tenders were invited for rehabilitation and protection of the most endangered and more easily accessible bottom parts of the bridge (five arches, upper surfaces of small arch and long arch braces on Sv. Marko). Several bidders came forward with a number of optional protection systems of well known world producers, but without sufficiently specified features, or proven efficiency in the specific conditions of bridge exploitation. It was decided on the commission level to offer the bidders and all interested to perform test protection surfaces on the long arch brace on Sv. Marko, to test and ascertain their efficiency. For initial works on the bottom, more easily accessible parts, the most probable classical protection system with high quality repair mortar and polymer cement protective coat was selected, based on the offered references.
First, 21 test surfaces were performed (later 5 more were performed) with optional systems of 15 well known European manufacturers of compound materials. After the first winter 18 were eliminated, due to already visible mistakes (cracks, peeling and high permeability which accelerated corrosion in some cases). The remaining three continued to be monitored. However, later on, they didn't prove to be a sufficiently effective nor rational solution, in the first place due to insufficient effectiveness (chloride impermeability) of the finish polymer cement protective coat.
Due to that, concurrently with these negative cognitions, other possibilities were investigated and analysed on test surfaces and on the performed protection and world experts in this field were consulted (Miagava, Tanikava, Swamy, Miller, Rostam, Maag). In the first place, it was related to the present-day increasingly expanding application of cathodic protection and dechlorination. Given that testing of such technologies could not be organized in those conditions, and designing of such protection is too risky, because the main bridge structures (arches) are made of mostly prefabricated RC slabs, connected into structural joints, in the length of about 50 km, with only overlapped and probably electrically poorly connected reinforcement, which is very important for effective implementation of such technologies, rehabilitation solution with cathodic protection was not accepted.
In the meantime (in the last 10 years) inefficient finish polymer cement coatings, used in the world technology (both technical and scientific), were replaced by more effective permanently elastoplastic pure polymer coatings (acrylate and epoxy-polyurethane), which significantly increase chloride impermeability of the protection system (100 to 1000 times), bridge over opening of cracks up to 1 mm, and even, according to some authors, block the already started corrosion process in conditions where chloride concentration is above the critical point in the reinforcement (Tanikawa, Swamy). Based on these cognitions and investigation results, in 1994 IGH prepared a new design solution for rehabilitation and protection of piers of small arch. These were considered the most endangered, according to reports (IGH and HIMK – Croatian Institute for Bridge and Structural Engineering) and the first evaluation of the expert group, founded by the Croatian Road Authority for the purpose of preparing strategy for rehabilitation and protection of RC structure of Krčki Most. Regarding repair mortar (remoulding), the design contains quality conditions of the best from previous investigation. About 100 times stricter condition was introduced for impermeability of the finish coat.
With visibly accelerated deterioration of piers on the small arch, in 1998 the Croatian Road Management invited new Tenders for rehabilitation of the piers. „Viadukt“ d.d. was chosen as Contractor. In the meantime the aforementioned expert group was founded. Based on the results of the repeated tests of chloride concentration in concrete of the protective coat of piers, they corrected IGH's original design solution, and based on the corrected solution, rehabilitation and protection works were performed on the first four piers of the alluvium on the island of Krk.
Support structures on the four rehabilitated and protected piers on Krk were not reconstructed at the time (except pier S28), because design alterations in the inapplicable bridge construction design solution were not solved, nor was the new adequate method of stress removal from the piers.
The required altered solutions were made later and works on rehabilitation and protection of piers continued (S20 and S27 without stress removal and reconstruction of support structures and S21 to S26 with stress removal and reconstruction).