Practical service life assessment and rehabilitation strategy development for hyperbolic shell natural draft cooling towers

Years of service in industrial plant operating environments can cause varying types and degrees of concrete deterioration on reinforced concrete hyperbolic shell natural draft cooling towers. Exposure to plant operating cooling water can potentially result in concrete material erosion and corrosion of embedded reinforcing steel and associated concrete delamination and spalling over time. Such deterioration can affect the integrity of the structure to the degree that repair programs are required to reliably maintain the towers for their required service life. Condition assessment of natural draft cooling towers is challenging due to their massive size, hyperbolic geometry, and plant operational considerations (i.e. schedule, risk and cost). A case history is presented illustrating a rational and targeted investigation approach that included a combination of field condition survey, laboratory materials testing, consideration of operational constraints, probabilistic-based concrete deterioration service life modeling, and structural analysis. Existing degradation as well as predicted conditions from service life modeling informed structural analyses that were performed to identify risk-based limit states over time. The multi-faceted approach provided information critical to understand degradation mechanisms, characterize structural health, and develop prioritized repair strategies to obtain the desired service life extension.