| The islands and reefs in the South China Sea are far away from the Chinese mainland.They are located in a tropical marine environment of high temperature,high humidity,high salt and windy,in which the sands,stones and fresh water resources are rare.Therefore,a kind of "environment-friendly" concrete--coral aggregate seawater concrete(CASC)composed of coral,coral sand,seawater and cement has gradually been applied to the construction of island projects such as ports and airports in the South China Sea.However,the abundant chloride ions in coral and seawater can easily cause extremely reinforcement corrosion in CASC,which is one of the main reasons that the service life of CASC structure in marine environment cannot reach 50a.To explore law of steel corrosion in CASC under marine environment and the service life of CASC structure under the influence of various factors,CASC specimens with different strength grades(C35 and C60)were selected for simulated seawater exposure test.The linear polarization resistance(LPR)method and the electrochemical impedance spectroscopy(EIS)method were applied to test the corrosion behavior of steel in CASC exposed for Od,28d,90d,180d and 270d,and the impact factors includes strength grade,exposure time,concrete cover thickness,type of steel bars and inhibitor(type/content/addmixture method)on the corrosion behavior of steel bars in CASC.At the same time,the durability parameters of CASC with different strength grade were obtained through the chloride ion diffusion test.And the service life of reinforced CASC structures in tropical marine environment were analyzed and predicted by the ChaDuraLife V1.0 life analysis software,which is based on the reliability theory and the modified chloride ion diffusion theory model.The main research contents and results are as follows:(1)By analyzing the EIS diagram of rebear in CASC under different conditions,a unified equivalent circuit model(EEC)was designed to fit it.And the circuit code is Rs(CcRc)(Qdl1Rf)(Qdl2(Rct(CaRa)W)).When inhibitor was admixed without high-frequency effect or not admixed,the EEC code is Rs(CcRc)(Qdl1Rf)(Qdl2(Rct(CaRa)W)).When calcium nitrite(CN)and amino alcohol(AA)inhibitor are admixed in CASC in a pre-admixed way,and the high-frequency effect is observed in EIS figure,the EEC code is Rs(CcRc)(Qdl1iRf)(Qdl2(RctW))and Rs(CcRc)(Qdl(Rct(CaRa)W)),respectively.(2)With the increase of concrete cover thickness and strength grade of CASC,the polarization resistance(Rp),concrete cover thickness resistance(Rc),charge transfer resistance(Rct),corrosion current density(Icorr)and corrosion current(i)of the reinforcement in CASC gradually decreased,indicating that the appropriate improvement of concrete strength grade and cover thickness can effectively reduce the corrosion rate and improve the anti-corrosion property of the reinforcement.According to the electrochemical corrosion law of different rebars in CASC,the anti-corrosion propertyof them is ranked as follows:epoxy resin coated steel(ERCS)>2205 duplex stainless steel(2205S)>316L stainless steel(316L)>organic coated steel(OCS)>ordinary steel(OS).(3)In the CASC specimens without inhibitor,the linear polarization curve of the reinforcement gradually moved negative along Y axis(electrode potential E/V)and positive along X axis(electrode current value Log(I/A)),the Rp gradually decreased,while the Icorrr and i gradually increased with the extension of exposure time(0?270d).The diameter and slope of the high frequency capacitive semicircle in the Nyquist diagram were in a decreasing process,the position of the capacitive resistance semicircle intersection gradually moved to the right,the low frequency capacitive semicircle gradually contracted to the real part of impedance,the absolute value of maximum phase angle,the low frequency impedance module value,the Rc and the Rct were gradually decreasing,while the capacitance(C)and the physical quantity describing the deviation of C showed an obvious upward trend.It indicates that the steel bars in CASC gradually undergo deblunt corrosion,and the corrosion rate gradually increases with time.However,the variation rate of the above parameters in the late stage is smaller than that in the earlier stage,indicating that the corrosion rate of the reinforcement tends to be stable in the late exposure period.(4)When a certain amount of CN(3 wt%and 6 wt%of cementitious system)and AA(2 wt%and 4 wt%of cementitious system)inhibitor were added into CASC,the anti-corrosion property of steel bars will be enhanced with the increase of the amount of inhibitor.The inhibition efficiency of CN is better than that of AA in the initial stage of exposure.With the extension of time,the inhibition efficiency of CN gradually decreased,while the advantage of AA gradually strengthened.When the CN(3 wt%)and AA(2%wt%)inhibitor were pre-admixed in CASC,there were two capacitive resistance semicircle in the high frequency of Nyquist diagram after 28d and 180d,and the second capacitive reactance arc of them represents the oxidation film resistance(Rf)and the deposition film resistance(Ra),respectively.It is indicated that the pre-admixed inhibitor can repair or reconstruct the passivated film which is pitted on the surface of the steel bar at the early stage,that is to say,the effect of the pre-addition method is better than the co-addition method in the working process of inhibitor.The effect of using freshwater as the pre-admixed medium of inhibitor is better than that of seawater.(5)The decreasing relationship between the chloride content and diffusion depth of CASC conforms to the Fick's second diffusion law.The Cf of CASC varies from 0.3 wt%to 0.7 wt%(weight percentage of concrete mass).There was a strong linear relationship between the surface free chloride content(Csf)and the surface total chloride content(Cst)of CASC,which is consistent with that of ordinary high-performance concrete.And the correlation of(1-m)power function boundary condition between Csf and exposure time is more significant than that of 0.5(1-m)power function boundary condition,thus the(1-m)model is used as the boundary condition to calculate the service life of reinforced CASC structures.According to the relationship between corrosion rate measured by LPR and EIS and surface free chloride content of different rebars,the critical chloride content(Ccr)(the percentage of concrete mass)of different types of rebars was obtained.The Ccr values of OS,OCS,316L,2205S and ERCS were as follows:<0.15%,0.21%,0.33%?0.41%,0.41%?0.45%and>0.41%,respectively.(6)The service life of the CASC structures in a tropical marine underwater area was analyzed using the ChaDuraLife V1.0 life analysis software,which is based on the reliability theory and the modified chloride ion diffusion theory.The results show that as the Ccr and the thickness of the protective layer increase,the service life of the CASC structure is gradually extended.With an improvement in the concrete strength grade,the influences of the protective layer thickness and the Ccr on the service life of the CASC structure are more significant,and the effect of the protective layer thickness is slightly more obvious than the effect of the Ccr,while the gap between them is gradually reduced.The desalination of seawater has a significant impact on the life of a CASC structure with a commonly used reinforcing material that easily rusts,while it has a minor effect on the service life of the stainless steel CASC structure.The influence of the initial chloride ion diffusion coefficient(D0)on the service life of the CASC structure is much greater than that of the initial free chloride ion content(C0).Moreover,D0 can still effectively improve the service life of the CASC structure within the high Ccr value range.For the C35 CASC and C60 CASC structure,when the Ccr value ranges from 0.70%to 2.38%(304 austenitic stainless steel,high quality 304L or 2205S stainless steel)and the cover thickness is at least 10.5cm and 10cm,the service life can reach 50 years.In order to meet the service life of 100 years,the C35 CASC with a cover thickness that is larger than 10 cm and high-quality 2205S stainless steel(the Ccr value is larger than 2.38%)should be used,or the C60 CASC with a cover thickness that is larger than 10.5 cm and high-quality 304L stainless steel(the Ccr value is larger than 1.47%)is required to be used. |
PDF Full Text Request