Studies And Application Of The Technology To Prevent Danger Of Concrete Structure Subjected To Alkaline Saline Corrosions

Three aspects of the durability problems of concrete subjected to alkaline saline corrosion were investigated to meet the construction and durability requirement of the casting pier of the Dongying Yellow River Bridge, namely the chloride diffusion, sulfate corrosion and alkali-aggregate reactions. As the depth of the casting piers amounts to 120m, vibrating free and self-condensing concrete without bleeding and segregation was required, to satisfy which, superfine mineral powder was employed. As the Dongying underground water was rich in chloride ion, sulfate ion and alkaline ions, the durability effect of superfine mineral powders, including slag powder, class â…  fly ash, natural zeolite and metakaolin powders were studied. On the basis of the experimental results, composite superfine mineral powder was developed and used in the engineering to prevent danger of concrete structure subjected to alkaline saline corrosions.The dissertation firstly investigated the problem of chloride diffusion. AASHTO T277(ASTM C1202) was employed to measure the 6h charge passed of the pier concrete, through which the service life of the structure was predicated. As the charge passed data were influenced by many parameters that have no relation with chloride diffusion properties, conductivity ratio of concrete saturated with KOH solution to that of the KOH solution's was measured to reveal the chloride diffusion properties of concrete determined by porosity, tortuousity (connectivity) of the capillary pores. As the diffusion of chloride ion was accompanied with chemical reactions by which chloride ion was bound, concentration of the free and bound chloride ion in the pastes incorporating different mineral admixtures was analyzed after immersion of the specimens in 6% NaCl solution for 300 days. It is shown that the diffusion of chloride was effectively inhibited by composite superfine mineral powders, among which, metakaolin and slag powder share the best results.The second part of the thesis studied the effect of the composite powders on the sulfate corrosion of the cement and concrete, the composite powder consists of metakaolin as necessary component to guarantee the chloride binding ability of the cement and concrete. The results showed that the composite superfine powders decreased the alkalinity and content of the concrete greatly, and the expansion due to gypsum and ettringite formation was considerably mitigated as the reaction was reversed to the left. The concept of "gypsum saturation coefficient", was raised in an attempt to explain the possibility of ettringite formation, with which, and together with the decrease of content and alkalinity, the phenomenon that low C3A or high C3S cement with poor anti-sulfate corrosion property, while sulfoaluminate cement exhibited good behavior was elucidated. The last part of the study deals with the inhibiting effect of superfine mineral powders on the alkali-aggregate reactions, and the alkaline release of the aggregate. Besides the introduction from cement, concrete alkaline also released from aggregate, while the expansion due to alkali-aggregate reactions was effectively inhibited by addition of zeolite and fly ash, as the alkalinity of the concrete was greatly reduced through dilution and secondary hydrations. NH4+ type zeolite with higher alkali ions absorption ability was produced through modification of the nztural zeolite, by which the alkali-silica reaction was inhibited with small dosage...