The specific surface of hardened cement paste by small-angle X-ray scattering effect of moisture content and chlorides

2010

ACT

The driving force of drying shrinkage of hardened cement paste has been attributed to RTln(h)/v with external relative humidity h and water molar volume v in theories of capillary tension and disjoining pressure. However, these theories fail to explain the considerable hysteresis observed in length-change isotherms. In this study, the sorption isotherm and length-change isotherm of cement pastes were determined with different water-to-cement ratios and cement types, and internal pressure of shrinkage Π was calculated using the measured strain and elastic modulus of the skeleton. This internal pressure is a kind of disjoining pressure originated from hydration force and built up within adsorbed water films as a result of interactions between the hydrophilic solid surface and water molecules. Changes in internal surface energies of hardened cement pastes due to hydration, drying and temperature history result in a different statistical thickness of the adsorbed water layer under the same equilibrium relative humidity. The proposed model gives a rational explanation for the hysteresis in length-change isotherm.

“…14 (Krop 1983, Hilsdorf 1984, Völkl et al 1987. This figure indicates that even in a high relative humidity range, water adsorption sites are affected by the drying process.…”

Section: Sorption Desorption and Hysteresismentioning

confidence: 93%

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Origin of Drying Shrinkage of Hardened Cement Paste: Hydration Pressure

2010

ACT

“…This nature is attributed to calcium silicate hydrate (C-S-H) with low crystallinity which is hydration product of alite and belite. There are many alibi of the colloidal nature of C-S-H. For example, BET surface from nitrogen sorption isotherm (Tomes et al 1957;Hunt et al 1960;Parrott et al 1980;Litvan and Myers 1983), water vapor sorption isotherm (Tomes et al 1957;Maruyama et al 2014a), and surface area obtained by SAXS (Winslow and Diamond 1974;Kropp et al 1985;Völkl et al 1987) are affected by long-term drying Fig. 1 Natural frequency change as a function of time, the relative natural frequency is the ratio of natural frequency to the natural frequency of buildings just after construction.…”

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

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.

“…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.