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There are different views in the literature on the relationship between the location of the carbonation front and the onset of reinforcement corrosion. Theoretically, corrosion starts when the carbonation front reaches the reinforcement, but some authors have observed an apparent earlier start of corrosion. In the present study, mortar samples with and without reinforcement were exposed up to 22 weeks to 20 °C, 60% RH and 1.5% CO2. The state of the reinforcement was monitored by potential measurements of potential. The carbonation of the bulk and the mortar-steel interface was detected by spraying a pH indicator on a freshly split or cut surface.Good agreement was found between low potential values (compared to reinforcement in the passive state) and the carbonation of the mortar-steel interface. A difference in the spatial variation of the carbonation depth was observed between plain and reinforced samples. The differences found in the literature between the location of the carbonation front and the corrosion onset can probably be explained by the spatial variation of the carbonation depth in the vicinity of the reinforcement.

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The corrosion of reinforcement in carbonated concrete with high moisture state was measured with and without electrical connection to reinforcement in non-carbonated concrete. The impact of the fly ash content and the cathode-to-anode ratio (C/A) was studied. A model was proposed and applied to quantify the contribution of the anodic, cathodic and ohmic partial processes to the macrocell corrosion and the impact of C/A. The total current density for the high moisture state investigated was high in all cases regardless the amount of fly ash replacement. The governing partial process depended on the cathode-to-anode ratio. Keywords: C. Carbonation C. Corrosion D. Blended cement D. Fly ash E. Concrete 2/22 Nomenclature/definitions:Microcell (uniform) corrosion: the corrosion process in which the anodic and cathodic sites are randomly distributed and continuously alternating. The oxidation of the metal and reduction of oxygen (or hydrogen) take place in adjacent places on the same metal part.Macrocell (galvanic) corrosion: the corrosion process in which the oxidation of the metal and reduction of oxygen (or hydrogen) take place in defined and spatially separated places that can be either on the same metal part or on two different metal parts that are electrically connected. AC-EIS (EIS): alternating electrochemical impedance spectroscopy C/A: cathode-to-anode area ratio [-] CE: counter electrode i: current density [µA/cm 2 ] ig: macrocell (galvanic) current density in the active electrode of the macrocouple [µA/cm 2 ] Ig: galvanic current intensity [µA] imi-A: microcell current density when there is no macrocouple [µA/cm 2 ] imi-A+C: microcell current density in the active electrode of the macrocouple [µA/cm 2 ] itot: total current density (microcell and macrocell) [µA/cm 2 ] LPR: linear polarization resistance mA-an: activation control slope for active reinforcement [mV/Dec] mA-cath: activation control slope for passive reinforcement [mV/Dec] mC-an: activation control slope for sample C anodic reaction [mV/Dec] mC-cath: activation control slope for sample C cathodic reaction [mV/Dec] OCP: open circuit potential [mV] 3/22 PC: Portland cement PCFA: Portland-fly ash cement PDP: potentiodynamic polarization curve RA-C: electrical resistance between reinforcement in carbonated concrete and non-carbonated concrete [Ω] RE: reference electrode RH: relative humidity [%] Rp: polarization resistance [Ω] SCE: external saturated calomel reference electrode w/c: water-to-cement ratio [-] WE: working electrode ZRA: zero resistance ammeter ΔEA: anodic polarization of active steel (anode) when coupled with passive steel [mV] ΔEA-C: difference in potential between active and passive steel when coupled (ohmic drop) [mV] ΔEC: cathodic polarization of passive steel when coupled with active steel [mV] ρ: electrical resistivity of concrete [Ω•m] 4/22

The paper summarizes preliminary results on characterization of the microstructure and phase assemblage of mortar and concrete samples containing Portland and Portland-fly ash cement carbonated at either natural conditions, 60% RH and 1% CO2, 90% RH and 5% CO2 or 60% RH and 100% CO2. Different characterization techniques were used: thermogravimetric analysis to study the solid phases, SEM-EDS point analysis to investigate the chemical composition of the solid phases, optical microscopy to investigate the microstructure, and cold water extraction to characterize the chemical composition of the pore solution. The combined results on microstructure and phase assemblage indicate that carbonation up to 5% CO2 appears representative for natural carbonation. Pore solution analysis revealed similar trends for the three accelerated carbonation conditions.