Beneath the Surface: Comprehensive Condition Assessment Techniques to Fortifying Tankage for the Future

As infrastructure reaches middle age, water reclamation professionals are facing the million-dollar question of should they rehabilitate an existing digester or replace it entirely? In reinforced-concrete structures, the high pH of fresh concrete protects the reinforcing steel and inhibits corrosion. Exposure to chloride ions, carbon dioxide, and low-pH environments degrades concrete's ability to protect reinforcing steel. Chloride ions can overcome the passivating effects of high pH concrete and initiate corrosion of steel reinforcement. Atmospheric carbon dioxide can gradually diffuse through the concrete (traveling through water in the concrete pores) and reduce the concrete's pH, contributing to corrosion of steel reinforcement. Low pH environments, such as highly acidic wastewater, can compromise the integrity of the concrete cement, expediting progression of the acid attack, carbonation, or chloride ions. These phenomena begin at the concrete surface and slowly move inward toward the reinforcing steel as the structures age. Once the concrete's ability to protect reinforcing steel is overcome by one of these phenomena, rehabilitation of the structure becomes costly and time intensive. This paper will present how data-driven assessments of concrete structures can optimize capital funding and minimize downtime. Concrete inspections that rely on only non-destructive visual and aural inspection techniques can at most recommend removal and replacement of concrete near the surface. These inspection methods only scratch the surface without evaluating the corrosion potential of the steel reinforcement. If the corroding steel is left untreated it will continue to expand and the cracks in the concrete will reappear. To truly understand the condition of concrete tanks such as digesters, a more in-depth approach should be taken to justify the decision to rehabilitate or build new infrastructure. The comprehensive condition assessment approach includes visual, aural (or sounding) inspection, concrete pH testing, steel reinforcement location, electrical continuity testing, active corrosion-potential (voltage) testing of steel reinforcement, concrete core collection, and laboratory testing of collected cores for pH and chloride. The Eastern Municipal Water District's (EMWD) acid phase anaerobic digester (APAD) located in Moreno Valley, California will be used as a case study. The APAD has four equal sized cells (labeled A through D) to treat wastewater solids (Figure 1). Each cell was taken offline and inspected independently. Half-cell corrosion-potential (voltage) mapping was performed per ASTM International C876, Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete. Half-cell potential measurements were made on the internal digester walls in an approximate 2-foot by 2-foot square grid pattern, based on reinforcement spacing. Equipotential contour maps (Figure 2) were plotted and used to identify areas where corrosion was more probable. Concrete core locations were selected based on visual condition and corrosion potential. Laboratory testing was performed on selected depths within the concrete cores to establish the profile of chloride concentration and pH as a function of depth from the concrete surface. Water-soluble chloride ion content was measured at half-inch increments (as measured from the concrete surface) per ASTM C1218, Standard Test Method for Water-Soluble Chloride in Mortar and Concrete. Chloride and hydroxide (pH) data for the concrete at reinforcement depth were plotted (Figure 3) to indicate if the concrete is still protecting the steel. Table 1 presents the depth to protective concrete. The assessment completed in 2022 indicated that each of the four cells require some rehabilitation but that each cell requires a different amount. This data-driven approach informs owners when they: 1.can safely defer concrete rehabilitation, 2.should take action to prevent corrosion of the reinforcing steel, and 3.should remove concrete beyond the reinforcing steel to stop corrosion and prevent a costly failure caused by replacing only the top inch of concrete. Additionally, utility owners avoid the following: - Rehabbing structures too often, which is an inefficient use of budgeted dollars - Rehabbing structures too far apart, requiring more extensive rehab and higher costs - Rehabbing structures but enough to remove the root cause of corrosion, leading to misuse of dollars spent. By integrating advanced assessment techniques and proactive maintenance strategies, this approach mitigates the risk of costly failures and enhances the long-term resilience of concrete tankage throughout a treatment plant. Ultimately, these findings empower wastewater utilities to future-proof their infrastructure, safeguarding operations and optimizing asset performance for years to come.