Formation And Diffusion Of Steel Corrosion Products In Concrete And Corrosion-induced Concrete Cracking Process

Chloride-induced steel corrosion is one of the major reasons of deterioration of reinforced concrete structures(RCSs).It takes huge maintenance and repair costs caused by steel corrosion each year for every country in the world.Steel corrosion can lead to cross-section loss,reduce bond strength between steel and concrete and result in concrete cracking.In practical engineering,corrosion-induced concrete cracking has been identified as the ultimate state of RCSs.The distribution of steel corrosion products at the steel-concrete interface due to the formation and diffusion of corrosion products is an important factor affecting the corrosion-induced concrete cracking.Therefore,investigation on the steel corrosion and corrosion-induced concrete cracking process is very important for the safety and durability of RCSs,and thus it has important theoretical significance and engineering application value.Recently,a new type of low-alloy(LA)reinforcing steel is becoming a hot research topic in the field of the durability of RCSs in marine environment due to its improved corrosion resistance and relatively low cost.So far,the research and development of LA steel is still at the initial stage in China,and the long-term corrosion behavior and the mechanism of corrosion resistance for LA steel in concrete needs to be further studied.Therefore,LA steel has not yet been widely applied in RCSs by replacement of the traditional low-carbon(LC)steel.In this thesis,both LC steel and LA steel with 0.86 wt.%Cr were adopted to investigate the performance of steel corrosion resistance and the process of corrosion-induced concrete cracking.The influence of steel types,steel surface conditions(with and without mill scale)and corrosion-inducing methods(long-term natural chloride-induced corrosion,impressed current-induced accelerated corrosion,and chloride electromigration-induced accelerated corrosion)on the steel corrosion resistance and the distribution of corrosion products at the steel-concrete interface was systematically studied.The characterization of the passivation capability and corrosion behavior of steels in concrete was conducted by electrochemical methods,such as corrosion potential(E_corr),linear polarization resistance(LPR)and electrochemical impedance spectroscopy(EIS).Moreover,the microstructure and chemical composition of corrosion products,the formation and diffusion of corrosion products at the steel-concrete interface and the corrosion-induced concrete cracking process were extensively investigated by X-ray computed tomography(X-CT),scanning electron microscopy/backscattered electron(SEM/BSE),X-ray energy-dispersion spectroscopy(EDS)and Raman spectroscopy.The research of this thesis can not only further optimize the mechanism of corrosion-induced concrete cracking,but also supplied a theoretical foundation for the wide application of LA steel in RCSs.The main original contents and results are as follows:(1)The passivation behavior and chloride-induced long-term corrosion behavior of steels in concrete were analyzed by electrochemical techniques.The results indicated that with progressive passivation in concrete,higher passivation capability was highlighted for LA steel compared with LC steel.In addition,the pickling process for removing mill scale may result in surface imperfections,which could in turn reduce the passivation performance of steels in the early age.Therefore,in consideration of the durability of RCSs,the curing time should be properly extended to provide sufficient time for the passivation of steels.During the early time of chloride-induced corrosion,the steel surface conditions had a remarkable influence on steel corrosion performance.However,after long-term exposure to chloride solution,the physical and chemical properties of corrosion products played a pronounced role on the corrosion resistance of steels.(2)The distribution rules of steel corrosion products at the steel-concrete interface under different corrosion-inducing methods were clarified by microstructural analysis.Corrosion products can accumulate on steel surface as the corrosion layer(CL)and also penetrate into the adjacent concrete as the corrosion-filled paste(CP).When using the impressed current-induced accelerated corrosion method,corrosion occurred over the whole steel surface and extensive homogeneous CL can be observed,while under long-term natural chloride-induced corrosion and chloride electromigration-induced accelerated corrosion conditions,the steel corrosion was confined to localized regions and pitting corrosion can be observed.In addition,decalcification of cement paste also occurred in the corrosion pits of steels.(3)Combined with electrochemical methods and microstructural characterization technologies,it was confirmed that the mill scale can influence corrosion resistance and corrosion morphology of steels as well as the corrosion-induced concrete cracking process.Under long-term natural chloride-induced corrosion condition,as received steels had higher corrosion resistance than pickled steels at the early corrosion stage.However,due to the surface imperfections and Cr depletion in the mill scale,pickled LA steel exhibited higher corrosion resistance than other steels with increasing exposure time.After long-term exposure to chloride solution,typical pitting corrosion was highlighted for pickled LC and LA steels,while only evident CL can be found for as-received steels.As for impressed current-induced accelerated corrosion,the presence of mill scale can delay the penetration of steel corrosion products into adjacent concrete,thus resulting in earlier cracking of concrete cover.(4)The anticorrosion mechanism of LA steel in concrete was revealed based on the results of electrochemical measurements,EDS elemental mapping and chemical composition analysis of corrosion products.The alloying element Cr contained in LA steel involved in two processes of the formation of protective passive film and the formation and diffusion of steel corrosion products.The inner rust layer of LA steel was primarily composed of stable and protective maghemite(?-Fe₂O₃)due to the enrichment of Cr,which can not only act as a good physical barrier to the mass transfer of chlorides and oxygen,but also generate less expansion stress to the surrounding concrete.(5)Based on the influence of different corrosion-inducing methods,different properties of mill scale and types of steel,a phenomenological model for the distribution of corrosion products at the steel-concrete interface was proposed.According to the phenomenological model,the corrosion-induced concrete cracking model was further improved in view of the formation rate of steel corrosion products and the diffusion of corrosion products into surrounding porous concrete.