From failure to recovery: CFRP strengthening of a near-collapse reinforced concrete wastewater structure with root cause diagnosis, failure mechanisms, and structural implications under aggressive environmental conditions

Reinforced concrete structures in wastewater treatment plants (WWTPs) operate under uniquely aggressive conditions, including biogenic sulphuric acid attack, sustained moisture, elevated temperatures, and partial submergence. Although Fiber Reinforced Polymer (FRP) strengthening is well established for bridges and other concrete infrastructure, its application within WWTPs raises concerns regarding bond durability and long-term performance, and field case studies in such environments remain scarce. This paper presents the structural assessment and Carbon Fiber Reinforced Polymer (CFRP) rehabilitation of an industrial WWTP equalisation tank in Sri Lanka, in which a defective construction joint within an interior column had triggered progressive deterioration after 15 years of service, producing slab deflections of over 250 mm (span/32) far exceeding the BS 8110 serviceability limit of span/250. A multistage investigation combining visual crack survey, scanning electron microscopy (SEM-EDX), and three-dimensional finite element analysis identified two coexisting deterioration mechanisms: wastewater ingress through the defective joint, causing reinforcement corrosion and column fracture; and biogenic sulphuric acid attack within the tank headspace, producing ettringite formation in the slab and beam soffits. Rehabilitation was carried out under operational constraints, comprising column reconstruction, epoxy crack injection, externally bonded CFRP strengthening with elevated temperature post curing at 55–65°C to enhance the cured adhesive Tg, and protective finishing. An environmental reduction factor of 0.80, supported by accelerated ageing and lap shear bond data, was adopted in the strengthening design. Post rehabilitation analysis confirmed restoration of the vertical load path, reduction of peak bending moments, and re-establishment of structural continuity. The paper provides a complete diagnostic to rehabilitation workflow and practical guidance on material selection, surface preparation, post curing, and protective detailing for FRP applications in confined and chemically aggressive service environments.