The Fracture Cut (FraC) approach to mesh three-dimensional (3D) Discrete Fracture Networks (DFN) is presented. The considered DFNs consist of a network of planar two-dimensional (2D) fractures ...sharing intersections that can in turn intersect themselves, resulting in highly complex meshing issues. The key idea of FraC is to decompose each fracture into a set of connected closed contours, with the original intersection traces located at the boundaries of the contours. Thus, intersection segments can be more easily accounted for when building a conforming mesh. Three distinct strategies for intersection points management are also proposed to enhance the quality of resulting meshes. Steady-state single-phase flow simulations are performed to validate the conform meshes obtained using FraC. The results from flow simulations as well as from a mesh quality analysis on a benchmark case show that a flexible AoM strategy (Adding or Moving intersection points) appears to be the best choice to generate ready-to-run meshes for complex DFN. This approach also allows accounting for tiny features within the fracture networks while keeping a good mesh quality and respecting DFN connectivity. Finally, a scalability of the mesh generator is conducted to assess the performance of the approach.
•An original and innovative conforming mesh approach is presented for DFN mesh generation.•Three distinct strategies for intersection points management are proposed.•This approach allows to account for tiny features within the fracture network.•High-accuracy flow simulations are obtained using Cast3M and DuMux open source code.
In the present work, we aim at improving our understanding of the effect of synthetic flexible fibers on the rheology of fresh cement-based suspensions. We first measure the way that synthetic fibers ...do increase the yield stress of various fresh cement pastes and mortars. We then measure, using micro CT, the way these fibers are deformed in a fresh paste. Finally, we propose a simple physical model allowing for the prediction of the fibers conformation as a function of their geometry, their elastic properties and the rheology of the cement-based material, into which they are dispersed. We validate this model on our results and discuss its application at the scale of mortars and concretes.
•Fracture energy of rocks is measured from size effect experiments.•Semi-circular notched specimens of various sizes are used. They easily are obtained from cores.•A fracture energy of ...86 J/m2−90 J/m2 for shales, and of 8 J/m2 to 70 J/m2 is obtained for various limestones.
An experimental technique based on size effect for the measurement of the fracture energy of rocks from core-based specimen is presented. It requires a simple apparatus and specimens may be easily obtained from cores. In order to obtain the fracture energy of the material, the non-dimensional energy release rate appearing in the size effect law, which is not available in the literature, needs to be computed first. This is done with the help of a continuum damage model. Computations have been performed for 4 different geometrically similar specimens and for 3 different values of the fracture energy. The non-dimensional energy release rate is back calculated from the fits of the size effect law. For the shale rocks tested, we obtain a fracture energy in between 86 J/m2 and 90 J/m2. For the limestones, we obtain a wider range, from 8 J/m2 to 70 J/m2, depending on their porosity.
The construction industry's growing demand for earthen materials faces a hurdle due to limited knowledge regarding their fire performance. The present work aims to assess the fire behavior of ...compressed earthen bricks, particularly focusing on spalling. Two influencing parameters were studied: the influence of cement stabilization and water content. To ensure equivalent compressive strength, compaction levels and cement content were thoughtfully chosen, as mechanical properties greatly influence fire behavior. In the material selection phase, unstabilized earthen bricks were compacted across a range of loading levels from 40 to 60 MPa. Subsequently, a loading level of 50 MPa was chosen for fire testing. Cement-stabilized earthen bricks were compacted at a Proctor level pressure with cement content varying from 2.5 to 10%. Additionally, Polypropylene fibers were incorporated into both types of bricks to assess their impact on fire behavior. Various water contents were tested, with samples maintained at hydric equilibrium through different curing conditions. Fire tests with a rapid heating rate (ISO 834-1 standard fire) were performed. Preliminary evaluation of the samples' fire behavior was based on their stability/instability during heating. Unstabilized samples exhibited thermal instability at intermediate water contents (50 and 75% RH), resulting in the detachment of their heated faces accompanied by a sound. However, they remained stable when dried or fully saturated. The inclusion of polypropylene fibers reduced the risk of instability. On the other hand, cement-stabilized bricks exhibited thermal stability, with no surface peeling observed, regardless of water content. The presence of cracks contributed to this stability.
The behaviour of concrete made with recycled concrete aggregates (RCA) at room temperature is well-studied. However, some points still need to be addressed, especially in extreme conditions such as ...durability and high temperature. This paper evaluates the effect of elevated temperatures on the durability of concrete made with RCA. Three concrete mixes were studied: concrete with NA (reference),
100
%
direct replacement (DR) mix (RCA-100-DR) and
100
%
strength-based replacement (SBR) mix (RCA-100-SBR). The latter was designed to achieve the same performance as concrete made with NA. Mixes were exposed to temperatures of
200
∘
C
,
400
∘
C
and
600
∘
C
. After cooling, durability-loss due to thermal exposure was evaluated through water porosity, capillary water absorption, permeability, chloride diffusion and accelerated carbonation tests. At room temperature, the direct addition of RCA decreased all durability parameters. The SBR mix recovered some of the durability properties. Exposure to high temperatures decreases all the properties, but it varies depending on the property. The concrete made with NA and the SBR mix showed similar performance. The durability was also evaluated using a performance-based approach, both at room and high-temperature. The proposed approaches showed potential to evaluate durability indicators, but they should be considered with precaution. Overall, concrete made with RCA reduces the durability of concrete, with or without heat damage, but this decrease can be reduced with proper mix optimization. These evaluations contribute to the post-heating durability of concrete structures made with RCA, which is fundamental to the post-fire assessment of concrete structures.
This paper presents the results of an experimental study carried out on several concretes at temperature up to 600°C. It deals with a high temperature phenomenon of concrete, the transient thermal ...strain (TTS). From reviews of some experimental and numerical works on TTS, we focused our study on the influence of several parameters: load level, compressive strength, heating scenarios, and nature of the aggregates. We also present results of concrete transient mass loss and strains in the radial direction. The study is completed by an assessment of TTS in uniaxial tension and of pure cement paste in compression. The results of the study allow us to better understand the physical origins of TTS. In particular, the results are consistent with the following explanation: TTS is mainly due to the drying and dehydration of the material up to 400°C and, at higher temperatures, the increase of TTS can be explained by the thermomechanical damage of concrete.
An Enhanced Geothermal System (EGS) can be thought of as an underground heat exchanger designed to extract geothermal energy. The performance of these systems can be improved by increasing ...permeability with hydraulic fracturing, following the same technique used for hydrocarbon reservoirs. To understand hydraulic fracturing, whether it is implemented in an EGS or in a hydrocarbon reservoir, it is important to know the fracture parameters of the rock at stake, e.g., the fracture energy. We report here the use of a method based on an energy balance during hydraulic fracture tests. Specimens were prepared and they have been mechanically and hydromechanically characterized at 20 ºC and 100 ºC, a temperature representative of actual reservoir conditions. The fracture energy is obtained from a balance of kinetic, potential and pressure energies involved in the hydraulic fracture tests. The method provides fracture energies that are consistent with the literature data on similar materials. It is also found that the fracture energy increases upon heating.
Experimental analysis of concrete spalling due to fire exposure Mindeguia, Jean-Christophe; Pimienta, Pierre; Carré, Hélène ...
European journal of environmental and civil engineering,
06/2013, Letnik:
17, Številka:
6
Journal Article, Conference Proceeding
Recenzirano
Concrete thermal instability is observed when a concrete structure is totally, or partially, exposed to an accidental fire. This sudden phenomenon, which can appear in the first twenty minutes of a ...fire, is called "concrete spalling". Spalling can seriously jeopardise the stability of a concrete structure during a fire (loss of load-bearing cross section, exposure of the reinforcement to fire) as well as after a fire (decrease in the residual mechanical properties and weaker durability). Moreover, the costs of repairing the damages induced by spalling can be very high. Since the early 1980s, many studies (mainly theoretical) have tried to find the most probable causes for concrete spalling. However, we still do not exactly know the mechanisms of spalling, and this phenomenon still cannot be evaluated by calculations. We present in this paper the results from an experimental research dealing with the behaviour of concrete at high temperatures. The comparison of experimental results, from material characterisation at high temperature to fire tests on medium-sized concrete slabs, allows us to discuss the influence of different parameters of spalling risk. Some important conclusions are drawn on the most probable causes for spalling of concrete when exposed to fire. In particular, we show that pore gas pressure cannot be the only physical origin for concrete spalling.
•An experiment to study nitrogen/steam flowing in concrete cracks in is presented.•Mass flow regulators and sensors accurately monitor fluctuations in the mixture.•Embedded thermocouples register ...fluctuations and phase changes in the crack.
An experimental setup to study nitrogen/steam mixtures flowing through cracks in concrete is presented. Mass flow regulators placed upstream of an evaporator allow the flow rate to be controlled precisely while thermocouples and pressure sensors placed near the entry and exit of the cracks accurately monitor the pressure and temperature fluctuations as the mixture flows. Thermocouples embedded just below the surfaces of the crack provide details of the temperature fluctuations within the crack, which indicate the formation of condensation and transfer of heat to the concrete.
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