The effects of water on the behaviour of granular materials can be significant. Besides capillary bridges, several other chemo-hydro-mechanical processes can affect the response of hydro-sensitive ...granular assemblies, when water sorption critically alters the individual particles properties (i.e., swelling, deterioration of mechanical properties). It is very common to find such materials in food and pharmaceutical industries, where water sorption can often lead to important resources waste while processing or storing the product. It is therefore necessary to understand the phenomena that affect the material’s functionality, often related to particle agglomeration and degradation. However, despite the relevance of the problem, our knowledge about these phenomena is still relatively poor. With this study we aim to explore the link between water content increase and particle, contacts and assembly scale phenomena. Simultaneous neutron and X-ray tomography allows us to investigate respectively the water uptake and microstructure evolution of two couscous assemblies exposed to high relative humidity while subjected to constant stress, a configuration chosen to simulate the conditions in an industrial silo-storage. We acquire a data-set of images, from which we follow and quantify the variations of water content distribution and the resulting volumetric response of thousands of particles through bespoke algorithms. Despite the abundance of water provided, we observe spatial gradients in water content distribution and consequently in particle swelling. We find that the relation between these two variables can be described as (quasi-)linear. The contact area growth also seems to follow a similar trend.
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The drying of cement-based materials is intimately related to their durability, which has significant economic, social and environmental repercussions. The evolution of the saturation of the pore ...network and the associated drying shrinkage are in fact leading causes of cracking and of the ensuing penetration of aggressive chemicals.
This process is highly heterogeneous, due to the thermo-hydric spatial gradients developing in the material from the exposed surfaces to its core and because of local effects, driven by the intrinsically heterogeneous micro-structure (e.g., by the distribution of pores and aggregates). It follows that macroscopic, sample-scale measurements cannot fully disclose the complexity of the underlying processes. In the last few decades, significant advances in full-field techniques have allowed an unprecedented insight into these local processes. For cement-based materials, x-ray and neutron tomography lend themselves as ideal, and highly complementary, tools for the study of their thermo-hydro-mechanical behavior. Notably, the high sensitivity to density variations of x-ray imaging gives access to the developments of fractures, in 4D (3D + time). On the other hand, neutron tomography allows the study of the evolution of the moisture field in 4D, thanks to its high hydrogen sensitivity. The combination of these two techniques provides a unique insight in thermo-hydro-mechanical couplings, e.g., the effect of cracks on the water content field.
This contribution presents novel 5D datasets (3D tomographies along time, plus truly simultaneous x-ray and Neutron rapid acquisitions) in-operando of a cement paste and of a concrete sample heated at moderate temperatures (up to 140∘C).
The analysis of this 5D data-set (once aligned in time and across modalities) allows for example a quantification of the 4D moisture profiles which were found to predict an overall water loss at hydric equilibrium coherent with the corresponding analytical analysis. In the cement paste sample, the x-ray dataset captures the evolution of an extensive cracking network, opening and propagation toward the core of the sample. A novel analysis procedure is here proposed which allows the extraction of these fractures and the analysis of their interplay with local drying as captured through neutron imaging. This for example reveals the depth of penetration of drying in the vicinity of the fractures along time, which is essential for the assessment and calibration of hydro-mechanical coupled models.
Cemented granular materials are abundant in nature and are often artificially produced. Their macroscopic behaviour is driven by small-scale material processes, which are generally classified as: ...grain breakage, cement damage and fragment rearrangement. This paper presents an experimental analysis of the latter two processes as observed through in-situ X-ray tomography and quantified by a suite of novel image processing approaches. This allows for example all particles and the bonds between them to be identified and their evolution to be individually quantified on a statistically representative volume, in 3D, throughout a loading test. We reveal the high spatial correlation between cement damage and strain rate and their effect on the isotropy of the bonds. Being the second of a two-part contribution, the overarching aim of this paper is to propose a general framework for the micro-inspired study of cemented granular materials. This is developed here, in Part I, in terms of multi-scale experimental quantification at sample and grain-level, while in Part II in terms of a two-way interaction with micro-inspired constitutive modelling.
Structural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive spalling on ...partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in‐operando drying behavior of a high‐alumina refractory castable, proposing a novel experimental layout aimed at a truly one‐dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.
Despite their diverse applications, experimental studies on multi-phase flow with phase change in fractured porous media are rare in the literature. In this study, water condensation of vapor in a ...fractured sandstone is investigated by means of 3D rapid in situ neutron tomography (30 s per tomogram). A water vapor and air mixture is injected at a constant rate, and the accumulation of condensed water is characterized through neutron tomography. Both phase-contrast and absorption-contrast X-ray tomography are jointly used to study the microstructure of the sandstone and measure the morphology of the splitting crack. The interplay between the pore matrix and fissure on the spatiotemporal distribution of water is studied through a combination of these techniques. The condensed water accumulates at the inlet boundary and gradually diffuses into the sample. Once the extracted crack is correctly aligned with with the neutron tomographies (registered), a propensity is seen for water to accumulate in small crack openings close to the cracks and migrate into the porous matrix predominantly due to the capillary effect. When enough liquid water condenses beyond a critical content, the air pressure in the crack transfers it into the porous matrix. This results in higher water content away from the crack. The water front propagates both along and transversely to the crack at a linear rate, albeit at different velocities. The final water distribution is found to be the result of two competing processes: condensation, occurring predominantly in the crack, and diffusion toward the matrix due to the capillary effect, as well as the pressure acting on the bottom of the sample and the crack walls.
Abstract Structural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive ...spalling on partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in‐operando drying behavior of a high‐alumina refractory castable, proposing a novel experimental layout aimed at a truly one‐dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.
The drying of cement-based materials, naturally occurring in most civil engineering contexts, affects their thermal, hydraulic and mechanical properties and is a leading contributor to the loss of ...their durability potential. The techniques conventionally employed to study this moisture transfer, such as gravimetry weight loss and point-wise sensor-based measurements, are often destructive and cannot characterize the local driving phenomena in 4D (3D+time), essential given the highly heterogeneous nature of the involved processes. Conversely, full-field techniques, and notably neutron tomography, are non-invasive and ideal for measuring the moisture transport process due to the high attenuation of neutron by hydrogen.
In this study, the moisture distribution of a set of cylindrical mortar samples was characterized at different hydric states, as imposed through drying in a thermo-hydrically controlled environment (T=20 °C, RH=35%). The lateral surfaces of the samples were sealed to impose a unidirectional moisture flow. The main phases of the mortar (aggregates, cement paste and voids), visible at the 30μm resolution adopted, were separated, and saturation profiles were deduced and validated against the mortar mixture ratios and weight loss measurements.
Besides the intrinsic interest of the spatio-temporal evolution of the local extracted saturation, these hydric gradients are essential to calibrate numerical models, as the commonly used Finite Element model presented here. A minimization algorithm was developed for this purpose to automate, optimize and ensure a more objective numerical–experimental calibration procedure. This has allowed the identification of key hydric parameters such as the convective exchange coefficient and the intrinsic permeability.
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•Vapor penetration in cracked concrete probed with fast neutron radiography.•Full-field analysis revealed moisture clogging and discontinuous moisture fronts.•Moisture accumulation ...was found to precede sudden jumps in moisture profiles.•Pressure driven moisture speed is 4 times the speed of capillary absorption.
This paper presents, for the first time, full-field dynamic measurements of vapour injection and condensation within a fractured concrete, observed in-operando by means of rapid neutron radiography (acquisition rate of 30 Hz with 87 μm pixel size). Time-dependant moisture evolution is analysed in terms of equivalent water thickness measurements. We observe that movement of condensed moisture along and transversely to the crack plane is non-monotonic in both space and time, exhibiting occasional sub-second jumps.
