The creep properties of calcium silicate hydrates (C-S-H) are assessed by means of nanoindentation creep experiments on a wide range of substoichiometric cement pastes. We observe that, after a few ...seconds, the measured creep compliance of C-S-H is very well captured by a logarithmic time function. The rate of the logarithmic creep is found to scale in a unique manner with indentation modulus, indentation hardness, and packing density, independent of processing, mix proportions, indenter geometry and load history. The comparison with macroscopic creep experiments on concrete shows that minutes-long nanoindentations enable a quantitative assessment of the long-term creep properties of cementitious materials, orders of magnitude faster than macroscopic testing. Finally, we show that a strong analogy exists between this logarithmic creep behavior of C-S-H and that of soils, which suggests a granular origin of creep of geomaterials.
An urban heat island (UHI) is a climate phenomenon that results in an increased air temperature in cities when compared to their rural surroundings. In this Letter, the dependence of an UHI on urban ...geometry is studied. Multiyear urban-rural temperature differences and building footprints data combined with a heat radiation scaling model are used to demonstrate for more than 50 cities worldwide that city texture-measured by a building distribution function and the sky view factor-explains city-to-city variations in nocturnal UHIs. Our results show a strong correlation between nocturnal UHIs and the city texture.
Concrete is the most widely manufactured material in the world. Its binding phase, calcium–silicate–hydrate (C–S–H), is responsible for its mechanical properties and has an atomic structure fairly ...similar to that of usual calcium silicate glasses, which makes it appealing to study this material with tools and theories traditionally used for non-crystalline solids. Here, following this idea, we use molecular dynamics simulations to evaluate the fracture toughness of C–S–H, inaccessible experimentally. This allows us to discuss the brittleness of the material at the atomic scale. We show that, at this scale, C–S–H breaks in a ductile way, which prevents one from using methods based on linear elastic fracture mechanics. Knowledge of the fracture properties of C–S–H at the atomic scale opens the way for an upscaling approach to the design of tougher cement paste, which would allow for the design of slender environment-friendly infrastructures, requiring less material.
•Calcium–silicate–hydrate (C–S–H) is shown to have a disordered nanostructure.•Our atomistic model of C–S–H is in agreement with the total X-ray scattering data.•Fracture toughness and energy of glassy silica and C–S–H are computed.•As opposed to glassy silica, C–S–H breaks in a ductile way.
Cement setting and cohesion are governed by the precipitation and growth of calcium-silicate-hydrate, through a complex evolution of microstructure. A colloidal model to describe nucleation, packing, ...and rigidity of calcium-silicate-hydrate aggregates is proposed. Polydispersity and particle size dependent cohesion strength combine to produce a spectrum of packing fractions and of corresponding elastic properties that can be tested against nanoindentation experiments. Implications regarding plastic deformations and reconciling current structural characterizations are discussed.
Several composites comprise material phases that cannot be recapitulated
ex situ, including calcium silicate hydrates in cementitous materials, hydroxyapatite in bone, and clay agglomerates in ...geomaterials. This requirement for
in situ synthesis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. Current advances in experimental micro and nanomechanics have afforded new opportunities to explore and understand the effect of thermochemical environments on the microstructural and mechanical characteristics of naturally occurring material composites. Here, we propose a straightforward application of instrumented indentation to extract the
in situ elastic properties of individual components and to image the connectivity among these phases in composites. This approach relies on a large array of nano to microscale contact experiments and the statistical analysis of the resulting data. Provided that the maximum indentation depth is chosen carefully, this method has the potential of extracting elastic properties of the indented phase which are minimally affected by the surrounding medium. An estimate of the limiting indentation depth is provided by asssuming a layered, thin film geometry. The proposed methodology is tested on a “model” composite material, a titanium-titanium monoboride (Ti–TiB) of various volumetric proportions. The elastic properties, volume fractions, and morphological arrangement of the two phases are recovered. These results demonstrate the information required for any micromechanical model that would predict composition-based mechanical performance of a given composite material.
Despite its ubiquitous presence in the built environment, concrete's molecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal ...concerns about concrete's environmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.
Urban heat island (UHI) is a climate effect that magnifies air temperature in cities. In the US it affects over 80% of the population and in general is considered an adverse phenomenon with ...externalities ranging from increased air pollution to higher energy demand and deteriorated human comfort. Therefore, UHI and its mitigation strategies have been studied extensively to focus on hot summer months and demand for cooling energy. However, current approaches fail to recognize that for regions in cold climates UHI may be a positive phenomenon with benefits from decreased heating energy demand exceeding downsides of higher cooling energy demand. Here, for the period of 12 years we analyze for 48 US states the cost that UHI imposes on the $120B residential market with CO2 emissions that exceed 550 M tons each year. While for states situated in warm climates UHI significantly increases the energy bill, due to varying heating and cooling costs and emissions associated with generation of energy, for some regions located in cold climates, UHI significantly reduces energy demand and carbon emissions. This information will help legislators and policy makers understand better energy demand of buildings and subsequently reduce their carbon footprint at city and state levels.
We present results of a hybrid experimental, theoretical, and simulation-based investigation of the postbuckling behavior of thin elastic rods axially impacted by a projectile. We find a new ...postbuckling mechanism: mode coarsening. Much akin to inverse energy cascade phenomena in other nonlinear dynamic systems, energy is transferred during mode coarsening from higher to lower wave numbers-unless the rod breaks, abruptly dissipating in the course of fracture the rod's strain energy. We derive a model that provides a predictive means to capture mode coarsening in the form of a nondissipative, purely geometric force relaxation mechanism, and validate the model by means of molecular dynamics (MD) based structural dynamics simulations for rods of wood and pasta considering different thermodynamic ensembles. The scalability of theory and simulation for engineering applications opens new venues toward safe design of engineering structures subject to impact-induced risks of buckling, ranging from skyscrapers, to aerospace structures, to the crashworthiness of vehicles, for example.
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