Ichiro Iwaki

A new look at reliability of reinforced concrete columns

This paper presents an investigation on reliability of reinforced concrete columns. For short columns, the fiber model is used for generating failure surfaces and strain and stress histories of both steel and concrete fibers under proportional and sequential loads. Two failure criteria, one based on the collection of peak-load points, the other based on prescribed maximum concrete strains are presented. For slender columns, failure surfaces are generated using a method proposed in 1991 by Bažant et al. (ACI Structural Journal, 1991, 88, 391–401). The reliability estimation of short and slender columns under random loads is formulated by Monte Carlo simulation in the load space. In this space, isoreliability contours for both deterministic and nondeterministic columns under different load paths and load correlations are plotted. It is demonstrated that these factors may have substantial effects on the reliability of reinforced concrete columns. Therefore, the results of this study can be used to support the consideration of load path and load correlation in the development of improved evaluation and design specifications for reinforced concrete columns.

Strength Development of Concrete Incorporating High Levels of Ground Granulated Blast-Furnace Slag at Low Temperatures

This paper presents the effect of mixture proportions and curing method on the strength development of concrete incorporating high levels of ground granulated blast furnace slag (GGBS) concrete at low temperatures. In the study, the strength development of mortar specimens incorporating GGBS for various mixture proportions and curing methods was investigated. The test parameters included the specific surface area measured by the Blaine method (400 sq m/kg, 600 sq m/kg, and 800 sq m/kg), the level of cement replacement by GGBS (50%, 60%, 70%, and 80% by mass), curing method (water curing, sealed curing, and air curing), and curing temperature (20 deg C and 5 deg C). The results demonstrated that the strength development at early ages of GGBS-mortar cured at 5 deg C was lower than that of plain mortar not containing GGBS. Thus, the effect of heat curing on the strength development of GGBS-concrete was researched. First of all, the periods of heat curing suitable for each mixture proportion were determined. Then strength development after heat curing was examined. The test results indicated that GGBS-concrete with a specific surface area of 400 sq m/kg had serious problems with strength development at early ages and low curing temperature. Heat curing was one way to improve strength development at early ages at lower temperatures. On the other hand, GGBS-concrete with a specific surface area of 800 sq m/kg had no strength development problem at early ages even when cured at 5 deg C, and heat curing seems to have had a bad influence on strength development, particularly at later ages.

THE SEGREGATION TENDENCY IN THE VIBRATION OF HIGH FLUIDITY CONCRETE

The introduction of superplasticizer (Sp) has produced concrete that could flow easily under its own weight. In balancing the concretes ability to flow at the same time maintaining the coarse aggregate distribution, the viscosity of concrete must be enhanced. This study aimed to investigate the effect of vibration on such flowable yet viscous concrete. Concrete flowability and viscosity are quantified by the standard slump cone and V-funnel test, respectively. Despite being flowable, the study concluded that concrete viscosity is an important parameter in determining the tendency for coarse aggregates to segregate. Large-sized coarse aggregates are affected more by the vibration process than the small-sized ones.

Quantitative damage assessment of concrete structures based on 3D laser scanning

Recently scaling damages of concrete structures have become apparent and it is now investigated to develop a method for quantitatively assessing the damages. In this paper, we propose a beneficial method for this purpose based on long-distance 3D laser scanning. In our proposed method, original shapes of the concrete structures before to be damaged are estimated on their laser scanned data by region growing, the distances to the estimated shapes are computed at each scanned point, and then their surface concavity and convexity are quantitatively evaluated based on the computed distance. We demonstrate the effectiveness of our proposed method from various experiments on real scanned data of concrete structures with scaling damages.

INFLUENCE OF AIR CONTENT AND FLY ASH ON FROST RESISTANCE OF CONCRETE EXPOSED TO CHLORIDE

The present study investigates the effects of air content and fly ash on the frost damage resistance of concrete exposed to chloride, focusing on the concrete used in reconstruction after the Great East Japan Earthquake. A scaling test is conducted, and an air void system is measured experimentally. The authors examined a the frost resistance, which are the air content and replacement ratio of the fly ash as well as the combination of the fly ash and ground-granulated blast furnace slag. Concrete having an air content of greater than 3.0% was found to have a high correlation between the air void factor and the amount of scaling. The frost damage resistance of concrete is improved by a water-cement ratio of 45% and an air content of 6.0%. In addition, it is also effective that the fly ash is replaced from the fine aggregate. The results of the present study reveal that to improve the frost resistance by increasing the air content in concrete requires the amount of fine entrained air voids to be increased. 本研究では東日本大震災からの復興に資するコンクリートの提案を念頭に、塩分環境下の耐凍害性について、空気量およびコンクリートの品質変動を補う目的でフライアッシュの混和の影響を検討した。実験では、空気量、フライアッシュの置換方法と置換率、高炉スラグ微粉末の併用を実験水準にし、凍結融解試験と気泡組織測定を実施した。その結果、空気量3％以上は気泡間隔係数とスケーリング量に高い相関があること、水セメント比45％で空気量を6％程度、さらにフライアッシュを外割置換したコンクリートが塩分環境下で優れた耐凍害性を示すこと、また、空気量の増加による耐凍害性の向上は微細な連行空気量の増加が重要であることを明らかにした。

INFLUENCE OF ACCUMULATION OF INVOLVED OR PENETRATED CHLORIDE ION ON SCALING RESISTANCE IN CONCRETE SUBJECTED TO FREEZE-THAW CYCLES

It is known that spraying of NaCl, which is de-icing salt, is accelerated a scaling deterioration to concrete surface. On the other hand, concrete structures are assumed to be subjected under freeze-thaw cycles in condition that is accumulated involved or penetrated chloride. However,. clear explanation has not been made about scaling deterioration under this condition. The aim of this study is to investigate the influence of accumulation of involved or penetrated chloride on scaling resistance in concrete subjected to freeze cycles experimentally. Seven types of specimens that varied the chloride ion concentration near the testing surface were prepared. Then, the scaling deterioration of these specimens under freeze-thaw cycles was investigated according to ASTM C672. The experimental results revealed that the scaling deterioration of Non-AE concrete, which was accumulated of involved or penetrated chloride, was increased than plain Non-AE concrete. Further, it was clarified that frost durability of Non-AE concrete was reduced markedly by the highly concentrated chloride ion near the concrete surface.凍結防止剤としてNaClが散布されるとコンクリートのスケーリングを促進することが知られている。一方、実コンクリート構造物は内在塩あるいは外来塩が蓄積された状態で凍結融解作用を受けることが考えられるが、この種の条件下におけるコンクリートのスケーリング抵抗性については未解明な点が多い。そこで本研究では、凍結融解試験に先立ち内在塩あるいは外来塩を蓄積させた供試体を準備し、試験面表層の塩分濃度がスケーリングに及ぼす影響を評価した。その結果、スケーリングは、凍結防止剤を含む水による凍結融解作用によって、Non-AEコンクリートでかつ内在塩が蓄積する場合と、高濃度の外来塩が蓄積する場合に、塩分が蓄積しない場合に比べ顕在化することを明らかにした。

STRENGTH DEVELOPMENT OF CONCRETE INCORPORATING LIMESTONE POWDER AT LOW CURING TEMPERATURES

著者らの既往の研究により, 石灰石微粉末を混和した高流動コンクリートは低温下において優れた強度発現性を示すことが確認されたため, 石灰石微粉末の産地, 粉末度, 添加量を変化させてモルタル供試体を作製し, 様々な養生条件下における強度発現性を調査した. その結果, 石灰石微粉末の添加により低温下における強度発現が顕著に増進し, その効果は石灰石微粉末の粉末度及び添加量に依存することが明らかになった. そこで, 石灰石微粉末の添加による強度発現機構を解明するため, さらなる検討を行った結果, 低温下における石灰石微粉末の添加による強度発現増進効果は, 主に石灰石微粉末表面が水和生成物の析出サイトとして寄与することに起因し, 石灰石微粉末の多量添加に伴う強度の増進は, 主に石灰石微粉末の空隙充てん効果によるとの結論が導かれた.

EFFECTS OF CURING METHODS IN COLD REGIONS ON STRENGTH OF CONCRETE INCORPORATING GROUND GRANULATED BLAST FURNACE SLAG

This paper investigates the effects of various curing methods in cold weather conditions on the compressive strength of concrete incorporating ground granulated blast-furnace slag (GGBS). The specific surface area of GGBS (4000, 6000, and 8000 sq cm/g) was varied, as was the replacement level of cement by GGBS (50, 60, 70, and 80%). Mortar specimens were prepared for the experiment. The specimens were cured at 5 deg C constant and 20 deg C constant by atmospheric curing, water curing and sealed curing. Compressive strength test results were performed at various ages. Furthermore, the effects of applying heat curing at 30 deg C constant at early age to GGBS-concrete on the strength development were also examined. Results show the concrete incorporating the GGBS with the specific surface area of 8000 sq cm/g would have no strength development problem even though it were cured at 5 deg C by sealed curing. However, heat curing may have a bad influence on the strength development, particularly at later ages. On the other hand, GGBS-concrete with the specific surface area of 4000 sq cm/g would require heat curing in order to improve the compressive strength at early ages. Finally, based on the data derived from this experiment, the relationship between the compressive strength of concrete incorporating GGBS and maturity is discussed.