Journal of Advanced Concrete Technology
Online ISSN : 1347-3913
ISSN-L : 1346-8014
Volume 17, Issue 11
Displaying 1-4 of 4 articles from this issue
Technical report
  • Koichi Maekawa, Yasuhisa Okano, Fuyuan Gong
    2019 Volume 17 Issue 11 Pages 616-627
    Published: November 13, 2019
    Released on J-STAGE: November 13, 2019

    For assembly of reinforcing bars embedded in electrolyte like concrete, space-averaged non-local analysis for electric potential is proposed and the anodic and cathodic polarization reactions are formulated in association with the exchange of electric charge between steel reinforcing bars and the electrolyte as matrix. In order to verify the proposed scheme like smeared crack modeling of cracked concrete mechanics, transparent sodium poly-acrylate superabsorbent polymer with calcium hydrate solution was made so that we may identify the location of anodic and cathodic reactions of reinforcement at regular time intervals. The analytically predicted locations of corrosion were compared with the experiments, and its functionality of non-local formulation was verified and its applicability was experimentally examined.

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Scientific paper
  • Marika Eik, Anna Antonova, Jari Puttonen
    2019 Volume 17 Issue 11 Pages 628-638
    Published: November 27, 2019
    Released on J-STAGE: November 27, 2019

    The SEM and EDX methods were used to study both the profile along the perimeter of steel fibres and the composition of the cement paste close to the fibre surface. The paste was made of cement clinker blended with slag and limestone. The results indicate that a wavelength of 31.5 μm and an amplitude of 0.925 μm represent the changes of fibre radius on average, which can be used to estimate the size of empty spaces that particles touching the fibre surface may create and the particles that are able to fill these empty spaces. EDX line analyses in four orthogonal directions from the fibre surface were performed with a step of 0.5 μm along the lines of 60 μm. The distributions of elements indicated that the steel fibre mostly affected the relative amounts of Ca and Si. The comparison between the Si/Ca and Al/Ca ratios pointed out the absence of the AFm type phase close to the fibre surface and that the CSH gel was mostly intermixed with CH, as only a small amount of the AFt type phase was observed. Generally, the counts describing the existence of the CSH gel were larger than those of the CH within a distance of 60 μm from the fibre surface. The counts were closest to each other at about 20 μm from the fibre surface, where the CSH gel had its minimum and CH its maximum value.

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  • Takashi Kawanishi, Takayoshi Hirata, Tsutomu Yashiro, Kenji Kawai
    2019 Volume 17 Issue 11 Pages 639-647
    Published: November 27, 2019
    Released on J-STAGE: November 27, 2019

    A method was developed to estimate the depth of concrete degradation caused by a fire using the rebound number measured with a compact Equotip hardness tester. The method was experimentally verified by measuring a core taken from a concrete specimen heated according to the RABT curve to confirm the distribution of the rebound number from the heating surface. The results were then compared with the distribution of the residual compressive strength estimated from the heated temperature of the concrete specimen and further measured with a core taken from the specimen. This confirmed that the distribution of the damage depth of concrete can be simply estimated by measuring the rebound number of test pieces obtained by cutting a core sample in half. The distribution of the damage depth coincides with the distribution of the measured compressive strength and the residual compressive strength estimated from the temperature data obtained in the fire resistance test.

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  • Keisuke Takahashi, Shingo Asamoto, Mari Kobayashi, Thomas Bier
    2019 Volume 17 Issue 11 Pages 648-658
    Published: November 28, 2019
    Released on J-STAGE: November 28, 2019

    Arid and tropical climates are prone to the occurrence of drying shrinkage cracking at early ages. Shrinkage reducing agents (SRAs) are often used to mitigate drying shrinkage, whose key action on the reduction of early-age shrinkage was seen to be a reduction of capillary tension. However, the roles of SRAs on reduction of shrinkage that have been reported, throw doubt on the idea that SRAs work solely via capillary tension. In order to clarify the mechanisms of early-age shrinkage in the presence of SRAs, the effects of fatty alcohol-based SRAs on shrinkage strain, capillary pore pressure and crack area of cement-based mortars at early ages were investigated. Our results demonstrated clearly that capillary tension does not directly correlate with shrinkage strain at early ages in the presence of SRAs, even under the same profiles of evaporation and hydration. We interpreted the actions of SRAs on early-age shrinkage as follows: SRA micelles adsorb onto hydrates and/or exist in pores, and form the hydrate matrix with incorporated SRA micelles. The matrix could mitigate the capillary tension as stress absorbers, and thereby shrinkage and crack area are reduced. The concept of the matrix could be experimentally demonstrated by the changes in shear creep.

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