Use of Alternating Current Impedance Spectra as a Supplemental Verification of Rebar Passivation in Two Marine Viaducts

摘要

Cathodic protection (CP) systems were installed on a viaduct in Ketchikan, Alaska, in 2001. Three CP styles were implemented to meet the changing protection needs in several marine environments over varying elevations. The systems installed included galvanic thermal spray zinc metallizing installed on cast-in-place pile caps and precast deck panels in the upper splash and atmospheric zones; fiberglass-reinforced plastic jackets with embedded galvanic zinc mesh anodes installed on the pile caps and piling in the tidal and lower splash zones; and submerged aluminum anodes protecting the piling in the submerged zone. A 2017 inspection found that the systems were still providing protection to most of the cathodically protected areas. In contrast, areas not originally protected with CP showed severe cracking and delamination exacerbated by corrosion of the reinforcing steel. Depolarization testing of CP systems in concrete is often a lengthy process that can take weeks to months to fully achieve. The extreme tidal changes in Ketchikan provided further challenges as measured corrosion potentials in many areas were influenced by tide level. As a result, passivation verification technique (PVT) testing was utilized to provide additional support to the depolarization data. PVT is an alternating current technique that provides a qualitative analysis of the condition of the reinforcing steel by measuring changes in phase angle over a partial spectrum-100, 10, 1, 0.1, and 0.01 Hz. These PVT and depolarization measurements are used to determine if the reinforcing steel is exhibiting passive or active behavior and whether the reinforcement is corroding or protected.