In this study freezing characteristics was investigated experimentally on adequately air-entrained cementitious materials (Powers’ spacing factor is between 60-160 µm) with variables including water-binder (w/b) ratio, slag cement addition and a wide range of air content. Results confirm cryogenic suction as the primary transport mode of the external moisture into the surface region to amplify continuous ice growth in capillary pores therein. This leads to pore pressure build-up and eventually surface scaling. Three key factors are identified to govern this cryogenic suction mechanism: (I) the initial ice nucleation in capillary pores and (II) the presence of unfrozen moisture transported through (III) the capillary network. Air entrainment, although needed for protecting internal frost damage, yields no added gain in improving scaling resistance in concrete and is consistent with the capillary suction dominated scaling mechanism. Addition of salt in the pore solution suppresses the ice nucleation and subsequent growth which may explain the pessimum effect.
Calcium-silicate-hydrate/aminobenzoic acid (C-S-H/ABA) composite systems were synthetized and characterized for the first time. Each of 3- or 4-aminobenzoic acid with a concentration of 0.01 mol. per mol. of Ca was added to the C-S-H preparations during their hydration. The C-S-H/ABA systems were filtered and dried after three weeks. These were, then, characterized by X-ray diffraction, Fourier transform infrared spectroscopy and nitrogen adsorption analysis. Porous bodies were also prepared from C-S-H/ABA compacted powders and used for the length-change and mass-change measurements in different test solutions. In addition, the microindentation technique was used to determine the creep modulus and hardness of the compacted samples. It is suggested that the C-S-H/ABA systems had improved durability and enhanced mechanical properties compared to the phase pure C-S-H reference materials. The influence of the 3- and 4-aminobenzoic acid on durability factors was similar. The C-S-H/3-ABA, however, had superior mechanical performance.
Application of blast-furnace slag blended cement is an important option to reduce carbon dioxide emission peculiar to concrete materials in construction, and concrete structures using the blended cement concrete (hereafter denoted as BFS concrete) is in great demand. However, shrinkage cracking resistance of BFS concrete decreases at high temperatures. In this study, mechanisms of the reduction of cracking resistance at high temperatures were investigated aiming at the control of shrinkage cracking of BFS concrete as the final goal. Constitutive material properties necessary for restrained shrinkage stress analysis were derived through experiments including such as restrained cracking test under a temperature of 30, 20 and 10°C, free shrinkage test, and compressive creep test. On the basis of the constitutive material properties obtained with the above tests, applicability of existing restrained shrinkage stress analysis to BSF concrete was examined. Analytical results indicate that the stress analysis was found to reproduce the cracking behavior at an ambient temperature from 10 to 20°C. However, at an ambient temperature of 30°C, the analysis overestimates restrained shrinkage stress, which is considered due to underestimating creep effects. This underestimation may occur due to surface micro-cracks eminently generated on surface.
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