Takafumi Sugiyama

Evaluation of Microstructure and Transport Properties of Deteriorated Cementitious Materials from Their X-ray Computed Tomography (CT) Images

Pore structure, tortuosity and permeability are considered key properties of porous materials such as cement pastes to understand their long-term durability performance. Three-dimensional image analysis techniques were used in this study to quantify pore size, effective porosity, tortuosity, and permeability from the X-ray computed tomography (CT) images of deteriorated pastes that were subjected to accelerated leaching test. X-ray microtomography is a noninvasive three-dimensional (3D) imaging technique which has been recently gaining attention for material characterization. Coupled with 3D image analysis, the digitized pore can be extracted and computational simulation can be applied to the pore network to measure relevant microstructure and transport properties. At a spatial resolution of 0.50 μm, the effective porosity (φe) was found to be in the range of 0.04 to 0.33. The characteristic pore size (d) using a local thickness algorithm was found to be in the range of 3 to 7 μm. The geometric tortuosity (τg) based on a 3D random walk simulation in the percolating pore space was found to be in the range of 2.00 to 7.45. The water permeability values (K) using US NIST Permeability Stokes Solver range from an order of magnitudes of 10−14 to 10−17 m2. Indications suggest that as effective porosity increases, the geometric tortuosity increases and the permeability decreases. Correlation among these microstructure and transport parameters is also presented in this study.

DETERMINATION OF CHLORIDE DIFFUSION COEFFICIENT OF HIGH-PERFORMANCE CONCRETE BY ELECTRICAL POTENTIAL TECHNIQUE

Electrical potential technique was used as an accelerated testing method to determine the chloride diffusion coefficient of high performance concrete. An electrical potential difference of 15 volts was applied to a disc-shaped concrete specimen of 50 mm in thickness. This direct current voltage increased the flux of chloride ions migrating through the concrete and permitted the determination of the chloride diffusion coefficient (D cl) within 3 to 4 weeks of the testing period.

Computation of Crack Tortuosity from Microtomographic Images of Cement‐based Materials

Cracking in cement‐based materials such as mortar and concrete plays a major factor in the material’s strength and durability. The crack tortuosity is therefore important to understand and model the effect of microstructure attributed to cracking on the transport properties of the material. This paper describes a method to identify the crack pattern from the microtomographic images and quantify its tortuosity by 3D image analysis and random walk simulation, respectively. The tortuosity is defined as the ratio of the self‐diffusion coefficient of non‐sorbing walkers in free space (no solids) to the long‐time self‐diffusion coefficient of these walkers in crack space. X‐ray microtomography, a 3D nondestructive imaging technique, was used to examine the cracks in damaged mortar at a spatial resolution of the order of 10 micrometers. Results of the crack measurement in a mortar that was subjected to cyclic freezing‐thawing environment were presented.

STRENGTHENING EFFECTS USING STEEL PLATE ON THE STRUCTURAL PERFORMANCE OF REINFORCED CONCRETE BEAM WITH VERTICAL CONSTRUCTION JOINT

せん断スパン比 (a/d) が2.35, 引張鉄筋比 (pw) が3.37%の小型RCはりおよびa/dが3.80, pwが1.71%の大型RCはりを作製し, せん断スパン内にそれぞれ鉛直打継目を設けた. この打継目を鋼板被覆で補強して曲げ強度試験を行い, ひび割れ性状, 曲げひび割れ発生荷重, 斜めひび割れ発生荷重, 最大荷重, 破壊形式, スターラップおよび鋼板のひずみをそれぞれ測定した. 鋼板補強には, 斜めひび割れ発生荷重の増加や鉛直打継目によって局所的にスターラップの分担せん断力が増加するのを, 鋼板が分担して防止する効果があることを明らかにした. また, スターラップの配筋量やコンクリート強度が小さいほど, 鋼板が負担するせん断力が大きくなることも明らかにした.

AN EXPERIMENTAL STUDY ON SEGREGATION OF MODEL CONCRETE DURING PULSATILE FLOW OF CONCRETE FLOWING IN PIPE

コンクリートポンプ圧送時の管内脈動の抑制には, 脈動による波形状の変動に対してこれを打ち消すための逆位相を有する脈動 (補流) を外部より加えて両脈動流を重ね合わせることで可能となる. この考え方に基づき管内脈動流を適切に制御することを目的として, 実機コンクリートポンプを模擬した小型ポンプ圧送装置を用い, 管内流速に着目し, 実験的に検討した. 外部より加える脈動流の波形および吐出量は, 質量保存に基づき容易に決定できること, 2つの脈動流の合流により懸念されるコンクリートの材料分離は, 適切な引込み速度および合流角度を与えることで最小限に抑えられることを明らかにした.