open access

https://doi.org/10.14943/doctoral.k15679

https://hdl.handle.net/2115/93877

混和材を用いて蒸気養生したコンクリートの寒冷地環境における塩害に関する実験的研究

英語

2023-12-25

Hokkaido University

x, 137p.

In the cold regions of Japan, the infrastructure element has to confront the problem of getting attacked by chloride ions from severe external resource such as seawater or deicing salt, resulting in the growing risk of deterioration with service time. Opting to prolong the lifespan of partial deteriorated structures rather than completely demolishing them is a more sustainable, environmentally friendly, and cost-effective approach. Precast concrete as an alternative component can meet all the above requirements, and its versatility has attracted public attention. Steam curing is a widely employed method for producing precast concrete; however, current research on the performance and durability of precast concrete remains relatively limited. It is wellknown that concrete incorporating fly ash and blast furnace slag can enhance the durability and corrosion resistance. A calcium-aluminum additive, CaO·2Al₂O₃, effectively binds chloride ions chemically to stable solids within the concrete, improving chloride resistance. However, whether these benefits from Supplementary Cementitious Materials (SCM) still hold in steam-cured concrete has been the subject of limited study. Combining the advantages of steam curing and SCM to produce high-quality, sustainable, industrialized concrete products for replacing partially deteriorated infrastructure, such as the high-strength concrete bridge slabs in cold regions, would be an ideal prospect worth pursuing. To achieve this noble aspiration, it is essential to first understand the properties of steam cured SCM concrete. This research aims to delve into this subject by two parts of research: The first part of the research focuses on the chloride ion resistance of steam cured SCM concrete in cold marine environments. The concrete types are categorized into four groups, along with a reference specimen (N) that solely employs ordinary Portland cement, fly ash concrete (FA) with a cement replacement rate of 15%, blast furnace slag concrete (SG) with a replacement ratio of 50% and CA concrete with 7% substitute ratio of CaO·2Al₂O₃. In normal-strength concrete with a binder-to-water ratio of 50%, the steam-cured specimens underwent one day of steam curing followed by 13 days of air curing (S) and steam-water curing (SW), which involved water curing for an additional 13 days following the initial day of steam curing, while the standard curing specimens (W) were subjected to 28 days of water curing. Subsequently, all the specimens were relocated to the offshore area of Hakodate, Hokkaido, for exposure to fresh seawater. Mercury Intrusion Porosimetry (MIP) experiments were conducted to analyze the changes in the pore structure of the specimens before and after exposure in steam-cured concrete. The results indicated that pore size distribution was highly correlated with the type of binder used and the curing methods. Furthermore, blast furnace slag concrete exhibited very low chloride ion diffusion coefficients (Dc) under three curing methods, while fly ash concrete tended to benefit from water curing. Notably, steam cured normal-strength CA concrete had Dc of only 0.53 compared to the reference group after one year, despite the MIP showing an increase in porosity around a pore width of 1860nm. This increase in porosity did not significantly affect its strong resistance to chloride ions. In another set of high-strength concrete all adopted materials and curing procedures were the same but with a lower binder water ratio of 0.34 and no SW curing method. The conclusions of this part of study include that water binder ratio is the primary factor that affects chloride ion resistance. In the presence of sufficiently dense high-strength concrete, the influence of binder types and curing methods on chloride ion resistance diminishes. Therefore, the chloride ion resistance of high-strength steam-cured CA concrete is not significantly affected by the addition of CA2. It also indicates that fly ash and blast furnace slag concrete with higher cement replacement ratio are more suitable for practical SCM production of high-strength steam-cured concrete bridge slabs. The next part focuses on the use of high-strength steam-cured concrete as a double-layer reinforced concrete bridge slab, utilizing the fly ash and blast furnace slag as SCM materials, which has been selected in the previous section. The SCM binder in this study is 20% replacement rate fly ash (FB) and 50% blast furnace slag (BB), and high early strength Portland cement is used as cement. The water-cement ratio of SCM concrete is 0.35, and the concrete with pure cement is 0.4. Additionally, specimens with the pessimism ratio of reactive andesite were prepared to investigate the corrosion resistance of high-strength steam-cured fly ash concrete under the combined damage of ASR and chloride ion attack. Each concrete series was cast with 5 specimens to account for the uncertainty of steel corrosion in high-strength steam-cured concrete. All specimens were exposed to 10% NaCl solution at a curing temperature of 40 °C. The experimental results proved that the steel bars in the SCM concrete did not corrode until the end of the three-year chloride exposure experiment and had an extremely low Dc of 0.32 cm²/year for FB concrete and 0.29 cm² /year for BB concrete. In contrast, the control specimens exhibited steel corrosion and abnormally high chloride ion diffusion coefficients due to wide longitudinal cracks caused by steel corrosion. The addition of fly ash plays a role in delaying the onset of ASR and inhibiting chloride penetration, thereby improving the corrosion resistance of concrete. In summary, this research clarified that CA2 additive can effectively restrict chloride penetration in steam-cured concrete under cold marine exposure conditions, despite an increase in porosity within the larger pore width after one year. In addition, highstrength steam-cured concretes with fly ash demonstrated better resistance against the corrosion of steel bars even under combined chloride attack and ASR risk. Moreover, the half-cell potential method can effectively assess the corrosion condition of steel bars within the thick concrete cover of high-strength steam cured concrete, in which the disruption of daily traffic services could be avoided.

2023-12-25

甲第15679号

博士(工学)

(主査) 教授 杉山 隆文, 教授 松本 高志, 准教授 松本 浩嗣, 准教授 橋本 勝文

工学院（環境フィールド工学専攻）

博士論文

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