Dimensional change and elastic behavior of layered silicates and Portland cement paste

2016

ACT

Based on a review from a view point of multi-scale perspective, it is concluded that possible mechanisms of natural frequency change of concrete structures under an ordinary drying condition are reduction of Young's modulus of concrete and stiffness reduction of reinforced concrete member due to shrinkage-induced cracking. Both two mechanisms can be provoked by drying, which is natural environmental condition for all the building structures. This natural frequency change is important for the integrity analysis of nuclear power plants because seismic response change and sympathetic vibration between structure and installed implementations are expected. In addition, to understand the mechanism this mechanism should contributed to post-earthquake integrity evaluation which is necessary to efficient and continuous operation of nuclear power plants. The lost parts of scientific knowledge and needs of experimental data for whole picture of this natural frequency change have been pointed out.

Section: Cement Paste Properties Under the First Dryingmentioning

confidence: 99%

Multi-scale Review for Possible Mechanisms of Natural Frequency Change of Reinforced Concrete Structures under an Ordinary Drying Condition

2016

ACT

“…However, studies have reported that hcp undergoes irreversible shrinkage after the first desorption process (Helmuth and Turk 1967;, indicating the occurrence of an irreversible reaction accompanied by a physical property change after drying. This irreversible reaction has been studied using various procedures, including nitrogen BET surface-area measurements (Hunt et al 1960;Parrott et al 1980;Litvan and Myers 1983;Maruyama et al 2014a), water-vapor BET surface-area measurements (Tomes et al 1957;Maruyama et al 2014a), and surface area and C-S-H structures obtained by small-angle X-ray or neutron scattering (Bayliss 1973;Winslow and Diamond 1974;Kropp et al 1985;Völkl et al 1987;Thomas et al 1998, Chiang et al 2012, 2013, among other techniques and interpretations (Feldman and Sereda 1968;Wittmann 1968, Parrott et al 1980Parrott and Young 1981;Beaudoin 1982;Jennings 2008;Beaudoin et al 2010;Maruyama 2010;Muller et al 2013). However, research efforts have largely ignored the strength of hcp and concrete during the initial drying and heating process.…”

Section: Introductionmentioning

confidence: 99%

Development of Soundness Assessment Procedure for Concrete Members Affected by Neutron and Gamma-Ray Irradiation

Kontani

Takizawa

et al. 2017

ACT

In 2008, Nuclear and Industrial Safety Agency (NISA) (currently integrated to the Nuclear Regulatory Authority) launched a project to develop a soundness assessment method for concrete members subject to a radiation environment. Presently, the soundness of concrete members subject to radiation is evaluated based on whether the predicted fast neutron fluence and gamma-ray dose values are lower than specific reference values in Japan, which are 1×10 20 n/cm 2 and 2×10 5 kGy, respectively. These reference values were determined based on report by Hilsdorf et al. This project begins by reviewing Hilsdorf et al.'s report, and we find that the scientific evidence for the current reference values is weak. We thus conclude that new experimental research is required to assess the current reference values and to propose a new alternative soundness assessment procedure if needed. We quantitatively evaluated the influence of neutrons, gammarays, and the resultant heating and drying processes on the strength of concrete as well as their underlying mechanisms. The irradiation experiments confirmed the degradation mechanism of concrete due to neutron irradiation. The main reason for this degradation is the metamictization of rock-forming minerals, which, in turn, leads to aggregate expansion. Due to aggregate expansion, cracks around aggregates form, which reduce the compressive strength and Young's modulus of concrete. Among the rock-forming minerals, α-quartz is the most sensitive to neutron radiation.60 Co gamma-ray irradiation experiments demonstrated that concrete strength increased as the gamma-ray dose and gammaray flux does not have a dose-rate impact on the first radiolysis of evaporable water in cement paste within the present study. The effect of gamma-ray irradiation on the properties of concrete is equivalent to that of heating and drying. Concrete strength alteration due to heating and drying is attributed to the colloidal and porous nature of hardened cement paste and crack formation around the aggregate due to a mismatch in the volume changes of the mortar and aggregate. In addition, a numerical analysis code called DEVICE (Damage EValuation for Irradiated ConcretE) is developed to harness knowledge obtained from concrete samples to predict the distribution of the physical properties in concrete members and their changes over time. From these fundamental studies, we propose a new soundness assessment procedure for concrete members subject to radiation. We also recommend a new radiation-induced strength-degradation reference value of 1×10 19 n/cm 2 for fast neutron.

“…The shrinkage mechanism of hardened cement paste is still under discussion, and several theories such as the surface energy theory, capillary tension theory, and disjoining (or hydration pressure) theory exist, with a combination of them having been proposed thus far (Powers 1965;Feldman and Sereda 1968;Wittmann 1968;Hansen 1987;Beaudoin et al 2010;Maruyama 2010). In addition to these theories, hardened cement paste under the first desorption process shows irreversible shrinkage due to colloidal alternation of C-S-H (Helmuth and Turk 1967;Maruyama et al 2015).…”

Section: Modulus Of Elasticity Shrinkage and Thermal Expansion Coefmentioning

confidence: 99%

Numerical Approach towards Aging Management of Concrete Structures: Material Strength Evaluation in a Massive Concrete Structure under One-Sided Heating

Igarashi

2015

ACT

For the purpose of performance evaluation of an existing reinforced concrete member, a chemo-thermo-hygro model to predict the spatial and temporal changes of physical properties of concrete in the member, named the "Computational Cement-Based Material Model (CCBM)", was proposed. This proposed simulation model includes models of rate of hydration of cement minerals, phase composition, and resultant properties of cement paste (i.e., compressive strength, Young's modulus, Poisson's ratio, thermal expansion coefficient, autogenous shrinkage, drying shrinkage, heat capacity, heat transfer coefficient, water vapor sorption isotherms, and water transfer coefficient). Furthermore, the model for compressive strength of concrete considered the variation in cement paste strength due to its colloidal features as well as micro-defects produced around aggregate due to differences in volume between aggregates and mortar upon heating and drying. The concrete properties of spatial distribution and temporal changes were evaluated by coupling these models with heat and water transport. Validation of these models was achieved by using existing experimental data. Using this CCBM, a thick concrete wall made with moderate Portland cement with a water-to-cement ratio of 0.55 under one-sided heating was simulated and potential problems that can arise during an integrity evaluation were discussed. If the required compressive strength, which was assessed within 91 days of placement, remains unchanged, an additional hydration process can build an adequate strength margin to overcome the risk of strength reduction due to heat and drying. However, in the case that the required strength is increased due to a re-evaluated risk, such as the magnitude of an earthquake, performance evaluation is not trivial as the core sample taken from the side where the execution of sampling is possible could exhibit a greater strength than the average strength of the target concrete member. Therefore, numerical evaluation might aid in this kind of situation.