33 research outputs found
Direct Experimental Evidence for Differing Reactivity Alterations of Minerals following Irradiation: The Case of Calcite and Quartz
Concrete, a mixture formed by mixing cement, water, and fine and coarse
mineral aggregates is used in the construction of nuclear power plants (NPPs),
e.g., to construct the reactor cavity concrete that encases the reactor
pressure vessel, etc. In such environments, concrete may be exposed to
radiation (e.g., neutrons) emanating from the reactor core. Until recently,
concrete has been assumed relatively immune to radiation exposure. Direct
evidence acquired on Ar-ion irradiated calcite and quartz indicates, on the
contrary, that, such minerals, which constitute aggregates in concrete, may be
significantly altered by irradiation. Specifically, while quartz undergoes
disordering of its atomic structure resulting in a near complete lack of
periodicity, i.e., similar to glassy silica, calcite only experiences random
rotations, and distortions of its carbonate groups. As a result, irradiated
quartz shows a reduction in density of around 15%, and an increase in chemical
reactivity, described by its dissolution rate, similar to a glassy silica;
i.e., an increase of around 3 orders of magnitude. Calcite however, shows
little change in dissolution rates - although its density noted to reduce by
around 9%. These differences are correlated with the nature of bonds in these
minerals, i.e., being dominantly ionic or covalent, and the rigidity of the
mineral's atomic network that is characterized by the number of topological
constraints (n) that are imposed on the atoms in the network. The outcomes
are discussed within the context of the durability of concrete structural
elements formed with calcitic/quartzitic aggregates in nuclear power plants
A Dissolution-Precipitation Mechanism is at the Origin of Concrete Creep in Moist Environments
Long-term creep (i.e., deformation under sustained load) is a significant material response that needs to be accounted for in concrete structural design. However, the nature and origin of concrete creep remain poorly understood and controversial. Here, we propose that concrete creep at relative humidity â„ 50%, but fixed moisture content (i.e., basic creep), arises from a dissolution-precipitation mechanism, active at nanoscale grain contacts, as has been extensively observed in a geological context, e.g., when rocks are exposed to sustained loads, in liquid-bearing environments. Based on micro-indentation and vertical scanning interferometry data and molecular dynamics simulations carried out on calcium-silicate-hydrate (C-S-H), the major binding phase in concrete, of different compositions, we show that creep rates are correlated with dissolution rates - an observation which suggests a dissolution-precipitation mechanism as being at the origin of concrete creep. C-S-H compositions featuring high resistance to dissolution, and, hence, creep are identified. Analyses of the atomic networks of such C-S-H compositions using topological constraint theory indicate that these compositions present limited relaxation modes on account of their optimally connected (i.e., constrained) atomic networks
Restrained shrinkage cracking of cementitious composites containing soft PCM inclusions: A paste (matrix) controlled response
The addition of phase change materials (PCMs) has been proposed as a means to mitigate thermal cracking in cementitious materials. However, the addition of PCMs, i.e., soft inclusions, degrades the compressive strength of cementitious composites. From a strength-of-materials viewpoint, such reductions in strength are suspected to increase the tendency of cementitious materials containing PCMs to crack under load (e.g., volume instability-induced stresses resulting from thermal and/or hygral deformations). Based on detailed assessments of free and restrained shrinkage, elastic modulus, and tensile strength, this study shows that the addition of PCMs does not alter the cracking sensitivity of the material. In fact, the addition of PCMs (or other soft inclusions) enhances the cracking resistance as compared to a plain cement paste or composites containing equivalent dosages of (stiff) quartz inclusions. This is because composites containing soft inclusions demonstrate benefits resulting from crack blunting and deflection, and improved stress relaxation. As a result, although the tensile stress at failure remains similar, the time to failure (i.e., macroscopic cracking) of PCM-containing composites is considerably extended. More generally, the outcomes indicate that dosages of soft(er) inclusions, and the resulting decrease in compressive strength does not amplify the cracking risk of cementitious composites
Quantitative evaluation of carbonation in concrete using nonlinear ultrasound
A new nonlinear ultrasonic technique for nondestructive evaluation of concrete components is developed and implemented to characterize the effects of carbonation on concrete. The physical principle of this method is the second harmonic generation (SHG) in propagating Rayleigh surface waves which are detected by a non-contact air-coupled transducer. The nonlinearity parameter, as an indicator of material properties, is experimentally obtained from measured Rayleigh wave signals and is used to quantitatively evaluate the progress of carbonation under accelerated conditions. The experimental results show that there is a significant decrease in the measured nonlinearity parameter, most likely originated from the deposit of the carbonation product, CaCO3, in pre-existing voids and microcracks. The sensitivity of the nonlinearity parameter is also verified by comparing with the measured Rayleigh wave velocity. The results in this paper demonstrate that the SHG technique using Rayleigh surface waves can be used to monitor carbonation in concrete
Does a dissolution-precipitation mechanism explain concrete creep in moist environments?
Long-term creep (i.e., deformation under sustained load) is a significant
material response that needs to be accounted for in concrete structural design.
However, the nature and origin of creep remains poorly understood, and
controversial. Here, we propose that concrete creep at RH (relative humidity) >
50%, but fixed moisture-contents (i.e., basic creep), arises from a
dissolution-precipitation mechanism, active at nanoscale grain contacts, as is
often observed in a geological context, e.g., when rocks are exposed to
sustained loads, in moist environments. Based on micro-indentation and vertical
scanning interferometry experiments, and molecular dynamics simulations carried
out on calcium-silicate-hydrates (C-S-H's), the major binding phase in
concrete, of different compositions, we show that creep rates are well
correlated to dissolution rates - an observation which supports the
dissolution-precipitation mechanism as the origin of concrete creep. C-S-H
compositions featuring high resistance to dissolution, and hence creep are
identified - analysis of which, using topological constraint theory, indicates
that these compositions present limited relaxation modes on account of their
optimally connected (i.e., constrained) atomic networks
Apport de la simulation âmatĂ©riauâ Ă la maĂźtrise des propriĂ©tĂ©s dâusage des bĂ©tons
Le besoin dâestimer les propriĂ©tĂ©s dâusage des bĂ©tons en phase de prescription ou Ă des fins de maĂźtrise de la durĂ©e de fonctionnement des ouvrages de production en service a poussĂ© EDF R&D Ă investiguer la piste de la simulation numĂ©rique afin de relier de maniĂšre thĂ©orique la formulation du matĂ©riau aux caractĂ©ristiques du matĂ©riau durci. La maturitĂ© des connaissances scientifiques actuelles en matiĂšre de thĂ©orie de lâhomogĂ©nĂ©isation et de chimie de lâhydratation des bĂ©tons, bien quâil reste encore des travaux Ă accomplir sur ces deux sujets, a plaidĂ© pour lâintĂ©gration de ces connaissances dans un outil unique baptisĂ© Vi(CA)2T dĂ©veloppĂ© par EDF. Cet outil permet notamment dâestimer les propriĂ©tĂ©s dâĂ©lasticitĂ© au cours de lâhydratation et de fluage, caractĂ©ristiques qui sont de premiĂšre importance pour le comportement structurel des ouvrages tels que les enceintes de confinement
Modélisation micro-macro du fluage propre du béton
On s'intéresse au fluage propre d'un béton.
Les expériences de nano-indentation montrent que seuls les C-S-H présentent un fluage
significatif. Ainsi, des changements d'échelle successifs sont réalisés par
homogénéisation des milieux aléatoires pour passer de la particule de C-S-H (mécanisme
couramment admis : glissement relatif des feuillets) au béton. Les fonctions de fluage
obtenues sont comparées à des résultats expérimentaux sur plusieurs formules, et à des
calculs sur microstructures 3D