Re-alkalization

Article Blindernveien: a realkalisation case study In 1993, a survey of two 10-story apartment buildings in Blindernveien, Oslo, revealed extensive carbonation and rebar corrosion. A conventional break-out and repair method was advised in the Spring of 1995. However, after removing the paint by light grit blasting, it became apparent that far more extensive rebar corrosion had occured, indicating that five or six times the original amount of concrete would need to be replaced. Constructed in 1958, the buildings had undergone conventional concrete repairs in 1970 and the mid-1980s. It now became evident that the repair work completed in the 1980s had not succeeded for two reasons; a failure to determine the extent of the areas requiring repair, compounded by an incomplete understanding of the mechanisms of concrete deterioration. In the light of the more extensive corrosion problem discovered, the repair strategy had to be re-examined. The break-out and replace method became significantly more expensive due to the larger volume of concrete in need of replacement and because of external support needed during the repair in order to ensure structural stability and safety. Since the cover zone was carbonated, resulting in a general rebar corrossion problem, it became evident that realkalisation would provide a more appropriate repair, both from a technical and an economical point of view. Realkalisation is an electrochemical treatment that enables repair of a structure as a whole. This means, repair of all reinforcement bars and all carbonated concrete. Thereby the danger of leaving corroding areas undetected and unrepaired is avoided. This is of utmost importance to ensure a long-lasting repair. In Norway, the technique has an eight-year track record and it was duly recommended as the most cost-effective repair for these structures. Because realkalisation is an electrochemical method, rebar continuity is vital. At Blindernveien the continuity was poor due to inappropriate placement and tying of the rebars. This had to be improved by making a number of electrical connections to the rebars. The repair procedure can be summarized as follows: All cracked and spalled concrete was patch repaired and full rebar continuity was established. The realkalisation anode system was installed. The anode system consisted of a steel mesh embedded in an alkaline reservoir temporarily applied on the concrete surface. The mesh was fixed to the vertical faces using plastic plugs and plastic spacers. On the balcony soffits, the mesh was fixed to wooden battens fastened to the concrete. When the mesh was in place, specially formulated cellulose fibre was sprayed directly onto the concrete surface together with a liquid electrolyte. The cellulose fibre acts as a reservoir for the electrolyte which is an alkaline solution. The anode system may be applied to all types of concrete surfaces - horizontal, inclined or vertical, regardless of shape, profile, surface condition or architectural detail. Individual treatment sections of up to 500m were installed. Installation time of a 500m treatment section was 3-4 days using a team of three men. Leads to the rebars were connected to the negative pole of an AC/DC rectifier. Leads to the anode meshes were connected to the positive pole. A voltage was applied and adjusted to give a current flow equivalent to approximately 1A per m of concrete surface. The rectifiers automatically adjust the voltage to maintain the required current output. Concrete cores were obtained and examined on-site using the pH indicator phenolphthalein to determine when carbonated concrete was sufficiently realkalised. Voltage and current flow were checked once or twice a day and fresh electrolyte sprayed onto the fibre reservoir when necessary. Once the treatment was completed the rectifier was switched off, cables dismantled, and the anode system removed. The concrete surfaces were washed with high pressure warm water and left to dry before application of a protective coating. Realkalisation of the first building was completed successfully and on schedule.

Updated by Martin R. Beaudette