Experimental Study Of Preconditioning By CO₂ To Improve Reinforcement Corrosion Performance In Recycled Aggregate Concrete

Recycled aggregate concrete（RAC）,as a new environmental building material,was limited used in practical engineering applications because of the properties of recycled aggregate（RA）worse than natural aggregate.The resistance of chloride ion penetration of RAC is poor,resulting in the steel bars in RAC prone to corrosion.Corrosion of reinforcement lead of chloride ion seriously affects the service life and security of concrete structure.The paper is based on enhancement of properties of RA and improvement of properties of RAC by CO₂ curing in order to improve reinforcement corrosion performance in RAC.The major comments of this paper are summarized as follows.1.Carbon dioxide was used to enhance the properties of RA by using a special pressurized reaction kettle,and the optimal conditions for CO₂ curing were investigated.It turned out that the apparent density of recycled coarse aggregate was increased and the water absorption and crushing index were decreased.Pressure value for 0.4MPa in reaction kettle and curing duration for 24 hour were more reasonable.2.Experimental work was conducted for carbon-conditioned recycled aggregate with varying chamber duration（0%,50%,and 100%）and replacement percentages（0%,50%and 100%）for concrete production.36 specimens（Φ100mm×50mm）for experiment of resistance against chlorine penetration of RAC and 36 specimens（100mm×100mm×400mm）for experiment of reinforcement corrosion in RAC were made depending on 12 RAC mix proportions.Electric flux of RAC was tested according to the ASTM C1202 specification and the extent of corrosion of every steel bar was performed by electrochemical impedance spectroscopy（EIS）with PASTAT4000 electrochemical work station after different corrosion duration（0d,3d,7d,14d,28d）.The results of experiment showed that resistance chloride ion permeability and corrosion resistance of RAC produced with carbon-conditioned recycled aggregate were improved.3.Experimental work was conducted for RA replacement percentages（0%,50%and 100%）for concrete production.36 specimens（Φ100mm×50mm）for experiment of resistance against chlorine penetration of RAC and 36 specimens（100mm×100mm×400mm）for experiment of reinforcement corrosion in RAC were made depending on 3 RAC mix proportions,and then cured in reaction kettle.The process of CO₂ curing refers to the pressurized exposure（0.5MPa）of RAC to CO₂ for a certain period of time（6h,12h,24h and 48h）.This paper investigated the effect of CO₂ curing on resistance chloride ion permeability and corrosion resistance of RAC.The results of experiment showed the electric flux of RAC decreased significantly after CO₂ curing.When the corrosion age was short,the passivation effect of reinforcement achieved the best after CO₂ curing for 24 hour.But corrosion potential and polarization resistance will be significantly reduced after CO₂ curing for 48 hour,still better than those of the specimen without CO₂ curing.When corrosion ages reach 28 day,the longer the specimens were cured,the more serious the ability of concrete cover protecting the reinforcement deteriorated.Rebar corrosion rate is growing faster than that of the specimens without CO₂ curing.