A coupled chemo‐thermo‐hygro‐mechanical model of concrete at high temperature and failure analysis

A hierarchical mathematical model for analyses of coupled chemo‐thermo‐hygro‐mechanical behaviour in concretes at high temperature is presented. The concretes are modelled as unsaturated deforming reactive porous media filled with two immiscible pore fluids, i.e. the gas mixture and the liquid mixture, in immiscible–miscible levels. The thermo‐induced desalination process is particularly integrated into the model. The chemical effects of both the desalination and the dehydration processes on the material damage and the degradation of the material strength are taken into account. The mathematical model consists of a set of coupled, partial differential equations governing the mass balance of the dry air, the mass balance of the water species, the mass balance of the matrix components dissolved in the liquid phases, the enthalpy (energy) balance and momentum balance of the whole medium mixture. The governing equations, the state equations for the model and the constitutive laws used in the model are given. A mixed weak form for the finite element solution procedure is formulated for the numerical simulation of chemo‐thermo‐hygro‐mechanical behaviours. Special considerations are given to spatial discretization of hyperbolic equation with non‐self‐adjoint operator nature. Numerical results demonstrate the performance and the effectiveness of the proposed model and its numerical procedure in reproducing coupled chemo‐thermo‐hygro‐mechanical behaviour in concretes subjected to fire and thermal radiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Observations of the flux of very-high-energy gamma rays from the blazar 3C 66A

Measurements of the flux of very-high-energy (>1 TeV) gamma rays from the blazar 3C 66A obtained over four years are presented. The mean flux over the four-year period was (2.8±0.4)×10^?11 cm^?2 s^?1. There is a correlation between the season-averaged flux of very-high-energy gamma rays and the observed optical radiation.     

Dependence of the neutron and gamma-ray emission of Mercury on its surface composition and temperature

Results of computations of the energy spectrum of neutrons radiated by the planet Mercury, generated by the action of cosmic rays, are presented. The dependence of the neutron radiation on both the temperature and composition of the subsurface layers of the planetary regolith are investigated. The fluxes of γ-rays in lines of aluminum (²⁷Al, 7.7240 MeV), silicon (²⁸Si, 3.5390 MeV), calcium (⁴⁰Ca, 1.9427 MeV), and iron (⁵⁶Fe, 7.6312 and 7.6456 MeV) generated during neutron capture by these nuclei are also calculated. The intensity of the radiation in these lines depends on both the composition and temperature of the surface. This must be taken into account when interpreting measurements of γ-ray radiation from nuclear lines produced in neutron capture reactions on the substance of Mercury.     

Instrumentation Development for In Situ 40Ar/39Ar Planetary Geochronology

The chronology of the Solar System, particularly the timing of formation of extra‐terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb‐Sr, K‐Ar), and even applied (K‐Ar), the reliability, accuracy, and applicability of the ⁴⁰Ar/³⁹Ar method makes it by far the most desirable chronometer for dating extra‐terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the ⁴⁰Ar/³⁹Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra‐terrestrial deployment of the ⁴⁰Ar/³⁹Ar method are presented. Although this represents a challenging task, developing the technology to apply the ⁴⁰Ar/³⁹Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution.     

Sensitivity analysis applied to finite element method model for coupled multiphase system

Today multiphysics problems applied to various fields of engineering have become increasingly important. Among these, in the areas of civil, environmental and nuclear engineering, the problems related to the behaviour of porous media under extreme conditions in terms of temperature and/or pressure are particularly relevant. The mathematical models used to solve these problems have an increasing complexity leading to increase of computing times. This problem can be solved by using more effective numerical algorithms, or by trying to reduce the complexity of these models. This can be achieved by using a sensitivity analysis to determine the influence of model parameters on the solution. In this paper, the sensitivity analysis of a mathematical/numerical model for the analysis of concrete as multiphase porous medium exposed to high temperatures is presented. This may lead to a reduction of the number of the model parameters, indicating what parameters should be determined in an accurate way and what can be neglected or found directly from the literature. Moreover, the identification parameters influence may allow to proceeding to a simplification of the mathematical model (i.e. model reduction). The technique adopted in this paper to performing the sensitivity analysis is based on the automatic differentiation (AD), which allowed to develop an efficient tool for the computation of the sensitivity coefficients. The results of the application of AD technique have been compared with the results of the more standard finite difference method, showing the superiority of the AD in terms of numerical accuracy and execution times. From the results of the sensitivity analysis, it follows that a drastic simplification of the model for thermo‐chemo‐hygro‐mechanical behaviour of concrete at high temperature, is not possible. Therefore, it is necessary to use different model reduction techniques in order to obtain a simplified version of the model that can be used at industrial level. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling concrete exposed to high temperature: Impact of dehydration and retention curves on moisture migration

High‐performance concrete is a widely used building material for tunnels, high‐rise buildings, nuclear plants etc. When these structures are exposed to fire, high‐performance concrete is prone to spalling. Moisture migration is believed to be one of the processes directly related to this phenomenon. In this paper, moisture profiles measured experimentally from neutron radiography on heated concrete are compared with results from a numerical model implemented in the finite element code Cast3M. The water loss measured experimentally, and the numerical results suggest that the commonly used constitutive laws for dehydration and water retention curves need to be reconsidered. The influence of these constitutive laws on the moisture migration is investigated. The dehydration constitutive law plays an important role on the dehydration front but has negligible effect on the moisture accumulation behind this front. By contrast, the water retention curves do not influence the dehydration front but affect the quantity and location of water condensation. The role of the permeability is also discussed.     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.     

Method of radial sounding and elemental analysis of formations during well logging using neutron inelastic scattering gamma time-of-flight spectrometry

This paper describes a well logging method and device designed to determine radial inhomogeneities in the elemental content of the borehole environment with high spatial resolution. The sounding factor that determines the spatial resolution is the time elapsed from the moment of neutron emission from the device to the moment the device records the gamma rays from neutron inelastic scattering (inelastic gamma rays, IGRs) in the formation. The time interval characterizes the distance to the point of origin of a gamma ray, and the energy of a gamma ray passing through the formation without interaction determines the chemical element involved in inelastic scattering.Simulations have shown that at each time, the density of inelastic scattering is very well localized in space owing to the small number of fast-neutron scatterings: on average, one to two events. It is the compact localization of inelastic scattering events that provides high radial resolution (and, if necessary, high azimuth resolution) during fast-neutron sounding of formations and measurement of unsteady IGR fluxes. Recording of IGR distributions over time also provides increasing sounding depth because powerful IGR fluxes from nearby regions reach the detector at short times and do not overlap the weaker IGR fluxes from distant regions because the latter reach the detector later.To evaluate the radial resolution of the method, we calculated the response of the sonde for typical models of a borehole environment which include a borehole, an iron casing, cement, an invaded zone, and an uninvaded rock. The boundaries of spatial inhomogeneities and the elemental content in the regions between these boundaries were determined from time dependences of unscattered spectral lines in IGR spectra for the elements Ca, Si, C, O, and Fe. The results of the numerical simulation indicate a high sensitivity of the measurements to the radial boundaries and an adequate spatial resolution: about 1 cm at a 0.1 ns time sampling of logs. The interfaces between the radial zones are clearly marked in the time distributions by steep fronts with a length of 0.1 ns (at a collimation angle of the source of about 30°) to 0.15–0.4 ns (at an angle of 90°).A method of solution was formulated for the inverse problem consisting of determining the boundaries of the radial zones and the elemental content in these zones. The problem is solved using a qualitative model of the borehole environment, for example, a “borehole–casing–cement–invaded zone–uninvaded rock” model. The method is based on searching for approximating model curves to measured time distributions of unscattered IGR fluxes jointly for all components of the model. The search is conducted by spatial optimization of the sought parameters—the distances {rS} from the neutron source to the boundaries of the zones and the concentrations {C} of specified chemical compounds in these zones. The initial approximations for the sought parameters {rS} and {C} are calculated by linear inversion of logs, which proves to be very accurate because the contribution of singly scattered neutrons to the inelastic scattering density at small times (10 ns) is, on average, 50–90%.Model curves are calculated by numerical simulation of the transport of neutrons and gamma rays. An appropriate calculation method is the Monte Carlo technique. Since the multiplicity of neutron scattering is low and, for gamma rays, only the unscattered component is of interest, the numerical simulation is a fast process.The practical implementation of the method requires the use of advanced developments in the design of neutron generators, spectral gamma-ray detectors, and fast analyzers for recording subnanosecond processes. Use of associated-particle neutron generators, Ge semiconductor detectors with electron cooling or LaBr3 (Ce) and BaF2 based fast scintillator blocks of high energy resolution will allow the application of the proposed method to logging measurements.     

The X-ray emission of magnetars

It is shown that cyclotron radiation by electrons near the surface of a neutron star with a magnetic field of ～10¹² G can easily provide the observed quiescent radiation of magnetars (Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters). Pulsed emission is generated by the synchrotron mechanism at the periphery of the magnetosphere. Short-time-scale cataclysms on the neutron star could lead to flares of gamma-ray radiation with powers exceeding the power of the X-ray emission by a factor of 2γ², where γ is the Lorentz factor of the radiating particles. It is shown that an electron cyclotron line with an energy of roughly 1 MeV should be generated in the magnetar model. The detection of this line would serve as confirmation of the correctness of this model.     

Gamma radiation levels in the villages of South Konkan,Maharashtra, India

Ambient gamma radiation study was carried out in South Konkan using thermo luminescent dosimeters (TLDs). A statistical analysis was carried out to understand the distribution of gamma radiation in the study area. The annual effective doses (AEDs) received by the local population from the selected villages were 0.31 and 0.09 mSv year^?1 for indoor and outdoor locations, respectively. For indoor conditions, the maximum dose rate occurred for winter season and the minimum occurred in monsoon season while for outdoor conditions the maximum dose rate occurred in spring season and minimum occurred in the monsoon season. The terrestrial radioactivity in the corresponding villages was measured by a HpGe detector. The radiation hazard indices like absorbed dose rate in air (D), radium equivalent activity (Ra_eq), external hazard index (H _ex) and internal hazard index (H _in), and AED were calculated using soil radioactivity data. The minimum absorbed dose rate in air (33.97 nGy h^?1) corresponds to the Dale village and the maximum (101.86 nGy h^?1) corresponds to the Mithgawane village. Radiation hazard indices as Ra_eq, H _ex, and H _in were found to be within the limit. The average AED from natural radionuclides was found to be lower than the worldwide value. The AEDs of this study were compared with previous studies carried out worldwide. A positive correlation was observed for the absorbed dose rate in air and the activity concentration of U-238, Th-232, and K-40. A positive correlation between activity concentrations of U-238, Th-232, and K-40 was also observed. The comparison between the AEDs calculated using absorbed dose measured by TLDs and the values calculated from soil’s gamma spectrometry showed some variation in the villages of South Konkan.     

Identification of sources of ultrahigh energy cosmic rays

The arrival directions of extensive air showers with energies 4×10¹⁹<E≤3×10²⁰ eV detected by the AGASA, Yakutsk, Haverah Park, and Fly’s Eye arrays are analyzed in order to identify possible sources of cosmic rays with these energies. We searched for active galactic nuclei, radio galaxies, and X-ray pulsars within 3-error boxes around the shower-arrival directions and calculated the probabilities of objects being in the 3 error boxes by chance. These probabilities are small in the case of Seyfert galaxies with redshifts z<0.01 and BL Lac objects, corresponding to P>3σ (σ is the parameter of Gaussian distribution). The Seyfert galaxies are characterized by moderate luminosities (L<10⁴⁶ erg/s) and weak radio and X-ray emission. We also analyzed gamma-ray emission at energies E>10¹⁴ eV recorded by the Bolivian and Tian Shan arrays. The source identifications suggest that the gamma rays could have been produced in interactions of cosmic rays with the microwave background radiation and subsequent electromagnetic cascades in intergalactic space. We estimate the strength of intergalactic magnetic fields outside galaxy clusters to be B≤8.7×10^?10 G.     

A macroscopic damage‐plastic constitutive law for modeling quasi‐brittle fracture and ductile behavior of concrete

A new phenomenological macroscopic constitutive model for the numerical simulation of quasi‐brittle fracture and ductile concrete behavior, under general triaxial stress conditions, is presented. The model is particularly addressed to simulate a wide range of confinement stress states, as also, to capture the strong influence of the mean stress value in the concrete failure mechanisms. The model is based on a two‐surface damage‐plastic formulation. The mechanical behavior in different domains of the stress space is separately described by means of a quasi‐brittle or ductile material response:

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A numerical study on effect of combined wave loadings in concrete slab

This paper focuses on a simple numerical code to investigate the wave propagation and damage effect in a concrete slab. First, the derivations of governing equations and a cross-format central finite difference algorithm are presented. A self-written FORTRAN code is thus developed. Then, the Johnson–Holmquist-Concrete constitutive model is introduced and implemented into this code. Finally, after the code is verified against LS-DYNA, the mutual coupling of combined wave loadings and the induced compression damage in the concrete slab is explored in details. Numerical results show that this code is effective and can conveniently simulate the responses of concrete slab under different combinations of boundary loadings.     

Experimental and numerical investigations of the behaviour of a heat exchanger pile

The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid‐filled pipes cast in concrete and then to building environments. To improve the knowledge in the field of geothermal structures, the behaviour of a pile subjected to thermo‐mechanical loads is studied in situ. The aim is to study the increased loads on pile due to thermal effects. The maximum thermal increment applied to the pile is on the order of 21°C and the mechanical load reached 1300 kN. Coupled multi‐physical finite element modelling is carried out to simulate the observed experimental results. It is shown that the numerical model is able to reproduce the most significant thermo‐mechanical effects. Copyright © 2006 John Wiley & Sons, Ltd.     

Characterization of lattice strain induced by neutron irradiation

Powder X-ray diffraction and Rietveld refinement have been used to study structural damage induced by neutron irradiation of orthoclase (Or₉₄) from Benson Mines, New York. X-ray diffraction profiles of samples exposed to total neutron doses in excess of 6.3×10¹⁸ n/ cm² exhibit small but measurable peak broadening in comparison with unirradiated orthoclase. Rietveld structure refinements and transmission electron microscope observations indicate that the X-ray peak broadening arises primarily from the effect of strain rather than particle size. The results reveal a positive correlation between the neutron dose and the anisotropic strain percentage calculated from the Lorentzian profile parameters. This strain-induced broadening probably stems from numerous point defects created by recoiling atoms during neutron collisions. These observations have important implications for the diffusive behavior of ³⁹Ar and ⁴⁰Ar.     

Identification and description of boron deposits from neutron logging

The reaction of minerals of boron to neutron radiation; is such that neutron logging of boreholes provides a geophysical tool with which to seek their ores.. It is based on the reaction of the isotope of boron B^1O with a thermal neutron: B¹⁰ + n⁹ → Li⁷ + α. A sonde designed specifically for the purpose has been tested. Until they are generally available, another type can easily he modified for neutron logging of boron. Different procedures and interpretation permit initial identification, preliminary assay, and estimates of reserves. —M. Russell     

Induced mutation and in vitro techniques as a method to induce salt tolerance in Basmati rice (Oryza sauva L.)

Embryogenie callus of indica rice (Oryza sativa L.) cv. Basmati 370 induced on MS medium containing 9.05 μM2,4-D was irradiated at 50 Gy of gamma rays of ⁶⁰Co for creating genetic variability against salinity. Irradiated and non-irradiated calluses were screened in vitro through three consecutive proliferation phases at 4.0, 6.0, 8.0 and 10.0 d/Sm electrical conductivity of NaCl. Growth value and number of adapted mutagenized callus was more than that of non-mutagenized callus. Salinity levels beyond 6 d/Sm and 8 d/Sm were lethal to growth and adaptation of non-irradiated and irradiated callus respectively. NaCl adapted irradiated callus showed 2.0%–4.75% regeneration frequency on MS regeneration medium containing 5.37 MNAA and 9.29 μMKinetin. Non-mutagenized salt adapted callus did not show any regeneration. From gamma ray mutagenized cultures, 2 putative lines (M₂ generation) with moderate salt tolerance were obtained at seedling stage. The results suggest that in vitro technique in connection with gamma rays may be used as a versatile approach to improve the level of salt tolerance in Basmati rice for saline environment.     

Experimental study and constitutive modelling of elasto‐plastic damage in heat‐treated mortar

This study investigates the effect of a heat‐treatment upon the thermo‐mechanical behaviour of a model cement‐based material, i.e. a normalized mortar, with a (w/c) ratio of 0.5. First, a whole set of varied experimental results is provided, in order to either identify or validate a thermo‐mechanical constitutive model, presented in the second paper part. Experimental responses of both hydraulic and mechanical behaviour are given after different heating/cooling cycling levels (105, 200, 300, 400^°C). The reference state, used for comparison purposes, is taken after mass stabilization at 60^°C. Typical uniaxial compression tests are provided, and original triaxial deviatoric compressive test responses are also given. Hydraulic behaviour is identified simultaneously to triaxial deviatoric compressive loading through gas permeability K_gas assessment. K_gas is well correlated with volumetric strain evolution: gas permeability increases hugely when ε_v testifies of a dilatant material behaviour, instead of contractile from the test start. Finally, the thermo‐mechanical model, based on a thermodynamics approach, is identified using the experimental results on uniaxial and triaxial deviatoric compression. It is also positively validated at residual state for triaxial deviatoric compression, but also by using a different stress path in lateral extension, which is at the origin of noticeable plasticity. Copyright © 2009 John Wiley & Sons, Ltd.     

Production and Certification of a Unique Set of Isotope and Delta Reference Materials for Boron Isotope Determination in Geochemical,Environmental and Industrial Materials

Isotopic reference materials are essential to enable reliable and comparable isotope data. In the case of boron only a very limited number of such materials is available, thus preventing adequate quality control of measurement results and validation of analytical procedures. To address this situation a unique set of two boron isotope reference materials (ERM‐AE102a and ‐AE104a) and three offset δ¹¹B reference materials (ERM‐AE120, ‐AE121 and ‐AE122) were produced and certified. The present article describes the production and certification procedure in detail. The isotopic composition of all the materials was adjusted by mixing boron parent solutions enriched in ¹⁰B or ¹¹B with a boron parent solution having a natural isotopic composition under full gravimetric control. All parent solutions were analysed for their boron concentration as well as their boron isotopic composition by thermal ionisation mass spectrometry (TIMS) using isotope dilution as the calibration technique. For all five reference materials the isotopic composition obtained on the basis of the gravimetric data agreed very well with the isotopic composition obtained from different TIMS techniques. Stability and homogeneity studies that were performed showed no significant influence on the isotopic composition or on the related uncertainties. The three reference materials ERM‐AE120, ERM‐AE121 and ERM‐AE122 are the first reference materials with natural δ¹¹B values not equal to 0‰. The certified δ¹¹B values are ?20.2‰ for ERM‐AE120, 19.9‰ for ERM‐AE121 and 39.7‰ for ERM‐AE122, each with an expanded uncertainty (k = 2) of 0.6‰. These materials were produced to cover about three‐quarters of the known natural boron isotope variation. The ¹⁰B enriched isotope reference materials ERM‐AE102a and ERM‐AE104a were produced for industrial applications utilising ¹⁰B for neutron shielding purposes. The certified ¹⁰B isotope abundances are 0.29995 for ERM‐AE102a and 0.31488 for ERM‐AE104a with expanded uncertainties (k = 2) of 0.00027 and 0.00028, respectively. Together with the formerly certified ERM‐AE101 and ERM‐AE103 a unique set of four isotope reference materials and three offset δ¹¹B reference materials for boron isotope determination are now available from European Reference Materials.     

Radiation-induced defects in euclase: formation of O<Superscript>−</Superscript> hole and Ti<Superscript>3+</Superscript> electron centers

Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO₄(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O⁻ hole center interacting with one Al neighbor and the second is a Ti³⁺ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O⁻ hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH⁻ ions interconnect distorted Al octahedra and Be tetrahedra in O₅ positions. During irradiation, the electrons are captured by titanium ions (Ti⁴⁺ + e⁻), leading to the formation of paramagnetic Ti³⁺ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O⁻ hole centers and Ti³⁺ electron centers are directly connected through the radiation process.