Journal of Advanced Concrete Technology
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Volume 14 , Issue 6
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Scientific paper
  • Esayas Gebreyouhannes, Koichi Maekawa
    Volume 14 (2016) Issue 6 Pages 271-286
    Released: June 11, 2016
    JOURNALS FREE ACCESS
    The initiation and subsequent propagation of corrosion-induced cracks is simulated by multi-phase mechanics where the coupled corrosion gel migration and crack-based fracture can be coherently taken into consideration. A key focus is given to the nature of corrosion products and their dependency on the corrosion rate and the anisotropic stiffness and permeability caused by cracking. Migration of corrosion gel to both micro-pores of cementitious composites and crack gaps are explicitly considered under the gradient of gel pressure, which is newly defined on the corrosion gel phase. The effects of cover depth, corrosion rate and the mix-proportion of concrete on the crack initiation and propagation are discussed with experimental facts. The broken symmetry of corrosion profile around reinforcing bars is successfully simulated as a nature even though the geometry of analysis domains would be in perfect symmetry. This scheme which allows kinematics of produced gel is indispensable for a versatile framework of durability mechanics.
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  • Reza Rashetnia, Farnam Ghasemzadeh, Mohammad Pour-Ghaz
    Volume 14 (2016) Issue 6 Pages 287-298
    Released: June 23, 2016
    JOURNALS FREE ACCESS
    Elastic modulus measurements are often required to evaluate the structural performance or to estimate the damage in cement-based materials. The axisymmetric flexural vibration (AFV) of a thick circular disk is a test method that can be used to estimate the dynamic elastic modulus of cement-based materials. Cement-based materials are inherently nonlinear materials and their nonlinearity increases with damage. The main objectives of this paper are to investigate the consequences of material nonlinearity on the AFV measurements, and to understand the implications of the change of material nonlinearity with damage on these measurements. In this work, experimental measurements are performed on mortar specimens damaged by freeze-thaw. Acoustic emission is used as a benchmark method to monitor and quantify the damage.
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  • Arezou Attar, Bora Gencturk, Mohammad Hanifehzadeh, Kaspar Willam
    Volume 14 (2016) Issue 6 Pages 299-310
    Released: July 01, 2016
    JOURNALS FREE ACCESS
    Dry cask storage systems (DCSS) are widely used worldwide for storage of spent nuclear fuel (SNF). Particularly, in the United States, other than the SNF pools, DCSS are the only means for storage of SNF. In the United States, the DCSS are licensed for an initial 20 years (with a possible extension of 40 years). The absence of a long-term (or perma-nent) storage facility has brought up concerns regarding the long-term performance of DCSS, which may now have to be used for extended durations reaching over 100 years. The DCSS with an exposed concrete overpack account for ap-proximately 61% of the DCSS inventory in the United States. The corrosion of the steel reinforcing bars (rebar) and the alkali-silica reactivity (ASR) of concrete have been identified as two of the main degradation mechanisms. In this paper, the accelerated aging of reinforced concrete (RC) overpacks of a vertical DCSS is evaluated experimentally at the struc-tural scale. Three 1/3-scale specimens were fabricated. The first specimen was built using a conventional self-consolidating concrete to serve as a control. The second and third specimens were prepared using special concrete mix-tures, designed to accelerate the corrosion of rebar and ASR. All three casks were observed for 2 years for aging-induced deterioration using various non-destructive approaches including visual inspection, half-cell potential, Schmidt hammer, and ultra-sonic pulse velocity (UPV) measurements. The RC overpacks have been observed to exhibit signifi-cant distress due to these aging mechanisms. The overall conclusion is that accelerating ASR and corrosion through use of reactive aggregates and/or addition of chemicals (NaOH and CaCl2 in this particular case) is a viable and practical approach for large-scale studies. Although accelerated aging of concrete structures have been extensively studied in the literature, this is one of the first studies on the long-term degradation in DCSS due to corrosion and ASR.
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  • Ippei Maruyama, Katsutoshi Beppu, Ryo Kurihara, Akihiro Furuta
    Volume 14 (2016) Issue 6 Pages 311-323
    Released: June 29, 2016
    JOURNALS FREE ACCESS
    Here we investigate the mechanism by which shrinkage reducing admixture (SRA) affects hardened cement paste (hcp). The first desorption process for hcp is always accompanied by irreversible shrinkage. Initially we demonstrate the well-known mechanism of SRA acting on capillary force using Vycor glass. Additionally, sorption isotherms and length-change isotherms are measured for both saturated hcp as well as hcp aged at 11% RH for two years. SRA was concluded to be present on the surface of the concave meniscus of the pore solution in Vycor glass, and that the inclusion of SRA reduces the surface tension of the pore solution, the equilibrium Kelvin radius, and the shrinkage due to capillary force. However, a comparison of long-term and short-term length-change isotherms and water vapor sorption isotherms of hcp suggests the possibility of partial evaporation of SRA molecules during the 2-year drying process, as well as the presence of immobile SRA in cement paste. Moreover, the immobile SRA is still active, and is found to reduce the amount of water sorption and shrinkage strain. It is thought that the secondary role of the SRA, which is related to immobile SRA in the cement paste, becomes active at room temperature, at below 80% RH, and only occurs in the irreversible shrinkage component of hcp produced by the initial desorption process.
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