@article{oai:repository.naro.go.jp:00002349,
 author = {渡嘉敷, 勝 and TOKASHIKI, Masaru},
 journal = {農村工学研究所報告, Bulletin of the NARO, Rural Engineering},
 month = {Mar},
 note = {Knowledge of the mechanisms of erosion and a method for testing accelerated erosion of the cementitious materials of a concrete irrigation canal are of major importance for maintaining the performance of a canal over the long term. Most previous studies of erosion of concrete in hydraulic structures have examined the effect of high-velocity water flow or the impact intensity of rocks transported in the water flow on erodibility, whereas the long-term effects of low-velocity water flow and calcium leaching on the erodibility of such structures has rarely been examined. Therefore, field investigations, water jet erosion tests, and cellular automata simulations were performed to investigate these phenomena. The first objective of this study was to explain the mechanisms of erosion of concrete irrigation canals. The following results were obtained: (1) Erosion of a concrete irrigation canal results from the complex degradation of the hardened cement paste (hcp) component of the concrete by both chemical and mechanical processes; (2) Calcium hydroxide is leached from the hcp into the water; (3) The microstructure of the hcp is thereby coarsened; (4) Hcp with coarse microstructure has low strength; (5) Collision of flowing water or waterborne sand with the low-strength hcp causes fatigue or abrasive failure, detaching it from the concrete body; (6) Detachment of the hcp from the concrete causes detachment of aggregate that is no longer supported by the hcp matrix from the concrete. These results are based on the following observations. Field measurements of arithmetic surface roughness of concrete irrigation canals in five districts indicate that erosion progressed as the exposure time of hcp to water increased. Moreover, observations of eroded concrete surfaces show that the surfaces of the aggregate did not erode; rather, the hcp around the aggregate was detached from the concrete body. Furthermore, electron probe microanalysis of specimens of concrete from the canals shows that calcium was leached from the surface hcp to a greater depth than the depth of erosion, indicating that leaching, a chemical process preceded mechanical erosion. Despite the occurrence of calcium leaching, no volume loss was observed from the concrete surfaces in contact with water with zero or very slow velocity, indicating that a volume loss of concrete, defined as erosion, requires not only chemical but also mechanical processes. Water jet erosion test results of experimentally leached hcp specimens showed that the maximum erosion rate in leached portions of the specimens was 19.4 to 27.6 times that in non-leached portions. This result indicates that the erosion resistance of leached portions is remarkably deteriorated. Two-dimensional cellular automata simulations were performed to evaluate the contributions of chemical and mechanical processes to the erosion process. The results show that hcp was eroded at nearly the same rate regardless of the diameter or the arrangement of aggregate in the concrete. This finding may indicate that the erosion of hcp is not affected by the presence of aggregate. The second objective of the study was to establish an accelerated erosion test method for the cementitious materials of concrete irrigation canals. As a result, the following method was proposed: (1) The parameters of the accelerated erosion test consisted of nozzle entrance pressure, water jet flow rate, impingement angle of the water jet, distance from nozzle to specimen, nozzle shape, rotation rate of the specimen, water quality, exposure time, and replacement of nozzle; (2) The control specimen was a rectangular solid composed of mortar meeting the JIS R 5201 standard with the dimensions 70 x 70 x 20 mm; (3) The 85th to 95th percentile of measured erosion depth was adopted as the index for determining the erosion front depth; (4) The ratio of the erosion rate of the cementitious materials to that of the control specimen mortar was adopted as the erodibility index of the materials; (5) The reciprocal of the erodibility index was adopted as the erosion resistance index of the cementitious materials; (6) The accelerated rate of erosion of a sound specimen from a concrete canal was estimated as the correlation between the water jet erosion test time and the service time of real concrete canals; (7) A combined test incorporating both chemical and mechanical processes is required to evaluate the erosion resistance of cementitious materials, because in the real environment such materials are affected by both chemical reactions and mechanical actions.},
 pages = {1--57},
 title = {農業用コンクリート水路における摩耗機構および促進摩耗試験に関する研究},
 volume = {52},
 year = {2013},
 yomi = {トカシキ, マサル}
}