Deterioration and Rehabilitation of A Newly Built Pre-Cast Reinforced Concrete Seawater Cooling Tower

The support structure of the seawater cooling tower (CT) comprised pre-cast columns, beams, and slab panels and in-situ cast foundations and pedestals. The CT was commissioned in 2004 and within a few months several leaks appeared across the entire support structure in the HDPE liner that was laid over the slab panels to contain seawater. About 40% of the slab panels, beams, and columns were contaminated with seawater. Thick salt deposits covering large areas were visible at several locations.  
    A diagnostic survey was conducted in 2005 to determine risk of the reinforcing steel corrosion and concrete deterioration in contaminated columns, beams and slab panels. Chemical analysis test results  showed that chloride ion concentration at the rebar depth was in well excess of the threshold limit. Overall, 60% and 30% of the half-cell potential mapping readings showed 95% and 50% corrosion probability respectively. Similarly, 60% corrosion rates measurements showed value >1 uA/cm²,   and about 10 to 25% of the readings showed moderate to high corrosion rate, i.e. between 0.5 to 1.0 uA/cm². But no sign of concrete cracking were visible or noted in 2005 survey, however by 2007, cracks appeared on almost all the slab panels, beams, and columns that were seawater contaminated.     Rehabilitation of the CT has been in progress, which included complete replacement of the slab panels with new cast in-situ slab constructed with built-in cathodic prevention system. All deteriorated columns and beams have been patch repaired and then cathodic protection system installed onto them to protect the reinforcing steel from corrosion.     The paper will describe and discuss the condition survey results, cathodic prevention and protection system design, installation, and initial commissioning and monitoring results.

Key words: Reinforced concrete, Concrete deterioration, Condition survey, Cathodic Protection