Research On The Anti-corrosion Durability Of Concrete Structure Of Gz Metro Under Multiple Environmental Factors

Being the State key project, metro engineering has a high demand for the durability of concrete structure whose designed service-life is about 100 years. Meanwhile, concrete is the biggest amount component material for subway engineering, its durability has great influence on that of concrete structure. And the particularity of metro environment is that the concrete structure is in a moist and groundwater flowing surroundings, in which it not only suffer, compared with on the ground, more serious carbonization attack and the action of groundwater pressure osmosis, but also was subjected to the attack of various corrosive medium existed in the groundwater and at the same time, the influence of loading, the vibration of running train and the stray current etc, all of which largely increased the concrete corrosion. In comparison with the other construction projects, the problem of concrete anti-corrosion durability for metro engineering is particularly prominent, which directly affects the normal safety operation and service life of metro system. So it is of important theoretical and practical value to comprehensively study the durability of metro concrete on erosion resistance in many factors.Taking GZ metro engineering as the research object, this paper takes the lead of the domestic in comprehensively studying the durability of metro engineering concrete structure in various factors and particularly in corrosion resistance. Firstly, a site investigation on the corrosion condition and situation of metro concrete engineering, an analysis on the raw materials and mix proportion of metro engineering concrete were carried out and a canonical experimental study on the durability of metro concrete was executed. Then,a further research on the resistance of carbonation and corrosive CO₂ attack, the groundwater dissolving corrosion, the corrosion of ions (Cl^-,SO₄^2-), and the erosion under the composite action of stray electric current and loading by using simulative experiment method designed particularly were realized. On the basis of these, a comprehensive evaluation on the concrete structure durability of GZ metro was presented and a set of suggestion for improving the anti-corrosion durability of metro project is completed.From the investigation in site on the corrosion condition and situation of metro concrete structure, it is demonstrated that 7.57%-28.87% of groundwater samples along with the metro line is corrosive to the concrete structure, of which, the corrosive CO₂ has the maximum ratio and the most important effects. Among every line of GZ metro, the average proportion of groundwater samples that have corrosive effect to the steel of reinforcement concrete is all more than 50%, the maximum is about 80%, the single corrosion factor of steel is mainly Cl^- ion, the composite corrosive factor of Cl^-and SO₄^2- consist of 20% of groundwater samples. Carbonation attack is slightly more serious in metro stations halls and platforms than on the ground, and is even much more serious in the tunnels. It is easy to find water leakage in the pre-embeded holes and joints of the prefabrication pipe sections, the construction joints between the track bed and the second lining concrete, the construction joints of the mine tunneling method concrete. There also exist serious leakage in the concrete cracks induced by various reasons, all of these are found to be the most harmful and popular way to destroy the corrosion resistance of the metro concrete.By the quality investigation and experimental durability detection of the practical metro engineering concrete, it was indicated that the quality of the raw materials used in metro engineering was well qualified, except that more than an half of sand samples have the latent alkali activity. The C30P8 concretes which were supplied by the 11 concrete companies varied in their durability performance because of the differences of raw materials, mix proportion and manufactured process: The anti-water-penetration performance of the concretes are all excellent, but the electric flux of 28d concrete samples varies among 650-1360C and the anti-vitriol corrosion coefficient ranges over 0.88-1.18, showing that the mutual differences between different companies are great. Most concretes have the 28d electric flux less than 1000C, the anti-vitriol corrosion coefficient being between 1.0-1.1 and the grade of the anti-vitriol corrosion bein"gmedium". But, there are some concretes with low durability level, of which the 28d electric flux is greater than 1000C, the anti-vitriol corrosion coefficient is less than 1.0, the grade of anti-vitriol corrosion is low.Through a set of accelerated test methods, the resistance to carbonation and groundwater dissolving corrosion of the metro concrete was studied. By the use of model similarity theory, the service-life prediction equation of concrete carbonation based on the data of 28 days was obtained: t_p=λ_tt_m=28(X_p/λ_aX_m)²/365It is showed that the carbonation service-life is related to the thickness of reinforcement cover and the 28d standard carbonation depth of the concrete, practically, strongly relating to the former. The service-life prediction results among 11 concrete companies were quite different, varying from 94 years to 312 years.Taking the CaO leaching amount of 10% as the critical state of service life and the measured leakage amount per day at a certain crack of GZ metro concrete 1000ml as a parameter, the anti-seepage-corrosion safe life of the concretes, deduced by the accelerated test results, would be calculated as follow: T=0.1×VCa/Q(M-M₀)It is showed that the water corrosion service life is related to the percolating water amount and the total CaO content in the concrete. If percolating water speed tend to zero, the water corrosion service life would approach to infinite. With calculating from the experimental results, it can be concluded that the anti-seepage-corrosion performance of the concretes meets basically the requirements of 100-year durability.Through simulating and accelerating test methods, the corrosion resistance of the metro concrete to Cl^- ion, SO₄^2- ion in the groundwater was studied deeply. The fitted equation of penetration amount of Cl^- ion into concrete during 10d¹80d is cj=atb. The test results showed that: the multiplier"a"became larger when temperature rose, the exponent b increased when alternate wetting and drying curing was used, augmenting the concentration of chloride ion would aggrandize the multiplier a and the exponent b simultaneously. Therefore, the alternate wetting and drying and higher concentration of chloride ions curing is a good accelerated test method for studying Cl^- ions corrosion. In the condition of Cl^-+SO₄^2- multiple corrosion, the average penetration amount of Cl^-+SO₄^2-×0.25 (effective Cl^- ions content) was higher than the average penetration amount of Cl^- ion in the condition of Cl^- ion single factor corrosion, which indicates that the existence of SO₄^2- ion accelerated the steel corrosion. Taking 0.1% content of chloride over cement mass as the critical value of steel corrosion, when the measured chloride content in groundwater was 350.58mg/L ("weak"corrosion grade) and the condition of metro was in alternate wetting and drying, the regression equation for chloride ion content, Cj, infiltrating into concrete is: c j = 0 .01356t^0.20962, and attaining this critical value needed only 37.8a, so, the chloride resistance of concrete was far lower than the design requirements. When the measured Cl^- ion and SO₄^2- ions content in groundwater was 7203mg/L ("strong"corrosion grade) and 2805mg/L respectively, and the metro concrete was immersed in groundwater in long-term, the fitted regression equation of Cl^-+SO₄^2-×0.25 was c j = 0 .02608t^0.30291, and attaining this critical value needed only 90d. Apparently, a special anti-corrosion measures should be taken for the individual parts under serious corrosion condition.The influence of stray current on the dissolving corrosion resistance and Cl^- ion and SO₄^2- ion corrosion resistance of the metro concrete was explored. Assuming that the amount of the dissolving CaO was proportional to the current density in concrete and the stray current of the metro was 60mA/m2, it is demonstrated that the service life of metro concrete reduced by 20% compared with that of no stray current only, so the presence of stray current obviously accelerated the ions corrosion. Compared with groundwater corrosion, the stray current not only increased the invasion rate of ion , significantly increased the content of free chloride ions and sulfate ion into concrete, but also changed the distribution state in the concrete of the corrosion ions, Cl^- ion concentrating near the steel, thus significantly increasing the corrosion rate of steel in concrete. Meanwhile, increasing the concentration of the corrosion ions had the greater influence on the steel corrosion than increasing the current density.The anti-corrosion power of metro concrete against the combination action of bending loading and SO₄^2-, Cl^- ion in the groundwater was studied. It is showed that stretching strength caused by the bending stress of loading led micro-fissure to form and extend and outside SO₄^2-, Cl^- ion to penetrate inside more easily and more concentrating in the pure tension stress area, oppositely, the content of Ca2+ ion to decrease, all of which would accelerate the rusting of reinforcement. The mechanic performances of steel bar had been found to descend. The concrete structure was tested to be densified and the strength increased slightly because of the formation of ettringite to fill the fissures, and the existing of Cl^- ion in the structure weakened the densification result. But, this kind of densification and strengthening is insufficient to compensate the effect of attacking of Cl^- and SO₄^2- ions to the reinforcement. The loading action led the stress focusing around the holes and some micro-fissures to extend to become big holes, the most probable bore diameter being increase. Therefore, it is obviously that the important influence of stress corrosion on concrete lies in accelelating the rusting of reinforcement of concrete, which might be milder than that of stray current but graver than that of the erosion of groundwater. Based on the above-mentioned research on durability, a holistic evaluation is made on the durability of concrete structure in GZ metro. The raw materials used for Guangzhou metro concrete are qualified as a whole, the examination of concrete quality all meets the requirements of standard, and the mix proportion is basically reasonable. It is the key point from now on to settle the quality stability of raw materials and the quality uniformity of construction structure, to pursue stricter and scientific mix proportion and the more excellent construction quality aiming to the level of durability. The laboratory research result in this toFig is considered to reflect the durability regulation and possible service life of the concrete main body structure of GZ metro; it is also considered that various defects found through the investigation of built project in site truly reflect the weak link of the durability of GZ metro concrete structure in GZ metro. If they can't be handled carefully and cured strongly, they may become the key problems which influence the durability life and safety of the metro engineering. At the last, some measure suggestions are put forward to improve the durability performance of the concrete structure in GZ metro and certain proposals about the experimental method for concrete durability are also presented.