78 research outputs found
A 3D porous media liver lobule model: the importance of vascular septa and anisotropic permeability for homogeneous perfusion
No full textThe hepatic blood circulation is complex, particularly at the microcirculatory level. Previously, 2D liver lobule models using porous media and a 3D model using real sinusoidal geometries have been developed. We extended these models to investigate the role of vascular septa (VS) and anisotropic permeability. The lobule was modelled as a hexagonal prism (with or without VS) and the tissue was treated as a porous medium (isotropic or anisotropic permeability). Models were solved using computational fluid dynamics. VS inclusion resulted in more spatially homogeneous perfusion. Anisotropic permeability resulted in a larger axial velocity component than isotropic permeability. A parameter study revealed that results are most sensitive to the lobule size and radial pressure drop. Our model provides insight into hepatic microhaemodynamics, and suggests that inclusion of VS in the model leads to perfusion patterns that are likely to reflect physiological reality. The model has potential for applications to unphysiological and pathological conditions
Design of polymeric capsules for autonomous healing of cracks in cementitious materials
No full textNow, most of the capsules used to contain polymeric healing agents in self-healing concrete, are made of glass. However, glass capsules cannot be mixed in concrete and are therefore placed manually into the moulds during concrete casting in laboratory tests. This represents a major drawback for an eventual industrialisation.
In this study, polymeric capsules were designed to meet three requirements: breakage upon crack appearance, compatibility with the polymeric healing agent and survival during concrete mixing. Three different polymers with a low glass transition temperature (Tg) were selected (PLA – PS – P(MMA-n-BMA)). These polymers are brittle at 20°C, and consequently have the possibility to break upon crack appearance, but are rubbery above their glass transition temperature and, consequently, can survive mixing upon heating.
Differential Scanning Calorimetry and Dynamic Mechanical Analysis were performed to define the glass transition temperature of the selected polymers and to quantify the evolution of their mechanical properties with increasing temperature.
Concrete mixing tests were performed both at 20°C and at a temperature above the Tg of the capsules. Mixing at increased temperature was done by previously heating the capsules and the concrete components. The survival rates increased drastically when the capsules and the concrete components were heated. Even capsules with a thin wall (thickness 0.4 mm) resisted a 2 minute concrete mixing process, whereas none of them survived at 20°C.
In addition, the compatibility of the capsules with a two-component polyurethane healing agent was studied. The pre-polymer hardened after some days.
This research revealed that suitable design of polymeric capsules can help to meet the requirements for self-healing concrete even though further research is needed before a possible use in industry
Self-healing of thermal cracks in sandwich panels
No full textSandwich panels are prefabricated, insulated concrete wall elements, which are sensitive to thermal cracking due to their composition (concrete outer cladding – insulation – concrete inner cladding). During hot days, the temperature of the outer concrete layer can rise up to ~ 60°C and the temperature difference between inner and outer layer causes crack formation in the outer concrete layer. Since cracking impairs the durability of concrete (e.g. accelerated corrosion of reinforcement steel by carbonation or chloride ingress), the aim of this research project is to regain impermeability and prevent esthetical damage through incorporation of self-healing capabilities.
At first, different healing agents (polyurethane (PU) and water repellent agent (WRA)) were screened based on their ability to regain impermeability and their behaviour upon reloading of cracked and healed samples. Two types of PU and three types of WRA were then selected to be applied in a real scale test.
For the real-scale test, the different healing agents were encapsulated by glass capsules and embedded in different zones in the outer layer of a sandwich panel (7.59 m x 1.20 m). After about 14 days, the test setup was built and the outer layer of the self-healing sandwich panel was thermally loaded up to temperatures of ~ 60°C for 9 hours per day. The temperature at the inner layer was kept constant at ~ 21°C. Due to the temperature difference, the panel bended, cracking occurred in the outer cladding, capsules broke and the healing agent was released. Some healing agents leaked out of the crack and left stains behind. Adaptation of the capsule volume, viscosity of the healing agent or concrete cover thickness over the capsules could solve this problem. PU and WRA were able to reduce the water permeability of cracks. Cracks treated with WRA remained water tight upon reloading, while PU can lose their bond with the crack surface resulting in an increased water absorption. In future research, more elastic polyurethanes, with a high bond strength to the concrete matrix, will be tested in order to solve this problem
Towards the biomimetic design of hollow fiber membrane bioreactors for bioartificial organs and tissue engineering: A micro-computed tomography (μCT) approach
No full textHollow fiber membrane bioreactors (HFMBs) with cells cultured in the extracapillary space (ECS) have been proposed for bioartificial organs, to assist patients with failing organs, or to produce in vitro engineered biological substitutes of tissues and organs. They have not gained clinical acceptance yet. One factor limiting therapeutic application is the irregular membrane distribution in the HFMB shell, often considered a typical feature of clinical-scale HFMBs. Such distribution does not permit good control of shell spaces, prevents from offering cells a template structure mimicking the tissue-specific extracellular matrix (ECM) and an adequate supply of oxygen and nutrients, and limits control over cell migration, organization, and differentiation in the ECS. In this study, micro-computed tomography and image analysis techniques were used to characterize the space distribution in the shell of HFMBs varying for membrane packing density and bundling technique, and to investigate whether and how it is possible to manufacture HFMBs in which the distribution of intermembrane spaces in the ECS is uniform and biomimetic. Results suggest that the arrangement in HFMBs of hollow fiber membranes bundled in rolled cross-woven mats at high packing density permits to obtain a uniform shell-side membrane distribution with pore size distribution favoring cells migration around the membranes, and mimicking the ECM structure of bone tissue
Non-destructive testing techniques for the observation of healing effects in cementitious materials: an introduction
No full textTo develop an appropriate method of self-healing for cementitious materials including the right composition and amount of suitable healing agents it is required to investigate the healing efficiency for certain material mixtures. While some researchers evaluate the regain in compressive strength by means of destructive load tests, this method is obviously second best in particular for field applications. In a large EU project the best candidates among the non-destructive testing methods are investigated to be applied in small and large laboratory experiments as well as at real structures in-situ. The paper is giving an introduction to these techniques and addresses also issues of structural health monitoring used for example to monitor the healing effects on a long term basis and to assess the condition of the structure, where self-healing techniques are applied
On the development of secondary motion induced by the free surface in the rod climbing flow
No full textThe author investigates the development of secondary motions in the rod climbing flow as a function of the rod angular velocity. He compares the free surface flow with a similar flow in which the upper free surface is replaced by a slip wall. It is shown that the complex secondary activities result from the free surface boundary condition. He also focuses on the streamline pattern in the secondary flow generated under the bulge.Anglai
Investigation of the fracture cracking behavior of self-healing systems by use of optical and acoustic experimental methods
No full textNowadays the self-healing process efficiency in loaded structural materials is evaluated by studying the damage mechanisms. Based on fracture mechanics theories, the resistance to damage and the cracking recovery can be an indication of healing performance.
Experimentally, the cracking behavior is quantified by measuring the fracture energy of the material during cracking and the fracture process zone area at which the damage is expanded. In literature, damage detection at loading stage of testing and damage recovery due to healing mechanisms at the reloading stage is monitored by application of several experimental (Non-) Destructive Methods.
In this study, the Fracture Process Zone (FPZ) in different heterogeneous materials (polymer and cementitious composites) is visualized in strain and deformation (crack opening-close-reopening) profiles of the crack tip area by application of Digital Image Correlation (DIC) and the fracture energy released in different stages of cracking is quantified and located by Acoustic Emission (AE). The combination of the aforementioned optical and acoustic techniques can confirm the recovery of cracked specimens in which healing mechanisms are applied
Most recent advances in the field of self-healing cementitious materials
No full textWhile the Japanese researchers Ohama et al. [1] already mentioned in 1992 that a self-healing effect was noticed when polymer-modified concrete without hardener was made, the real pioneer in the research on self-healing concrete is Carolyn Dry from Illinois. The first time she proposed the use of encapsulated polymers to obtain self-healing concrete dates back to 1994 [2] and based on her publication output, she remained active within this field until 2003 [3, 4]. Within this timeframe, Victor Li started his research on fiber-reinforced self-healing concrete in Michigan [5]. From 2000 onwards other researchers in Japan (Mihashi, Nishiwaki et al.) [6-8], France (Granger et al.) [9], the United Kingdom (Joseph et al.) [10] and the Netherlands (ter Heide et al.) [11] started their research on self-healing cementitious materials. However, it was only in 2007, when the Dutch IOP program on self-healing was granted and the first international conference on self-healing materials was organized in the Netherlands, that self-healing concrete gained world-wide attention and all over the world research groups started working on this topic. One year later, in Belgium or more specifically at the Magnel Laboratory for Concrete Research of Ghent University, research on self-healing concrete started. In this keynote, an overview of the most recent developments within the Magnel Laboratory will be given
Microfibres and hydrogels to promote autogenous healing in cementitious materials
No full textCementitious materials are sensitive to crack formation and it would be beneficial if the material could stop the crack propagation, repair the damage and reach again the original liquid-tightness and/or strength. Therefore, a cementitious material with synthetic microfibres and superabsorbent polymers (SAPs) is proposed. Upon crack formation, the microfibres will become active and due to the bridging action, they will stop the opening of a crack, forcing the cementitious material to crack somewhere else. There, other fibres will become active. In this way, not one large crack, but several small healable cracks are formed.
Further cement hydration and calcium carbonate precipitation will seal the crack if sufficient building blocks and water are present. The building blocks are available through the well-designed mixture with a low water-to-binder (W/B) ratio and water is available through the inclusion of SAPs. These polymers are able to extract moisture from the environment and to provide it to the cementitious matrix for autogenous healing. This healing will lead to the regain in mechanical properties.
In this paper, the formed products are studied by means of optical and scanning electron microscopy. The healing efficiency was evaluated by reloading cracked and healed specimens and by comparing the new mechanical properties with the original properties.
The crack width was limited to 50 μm at 1% strain. While specimens without SAPs showed a regain of mechanical properties of 40-55% in wet/dry cycles, specimens with SAPs showed a total regain of 80-95%. Even in humid air, those specimens show partial healing of 35-55%. SAP B, a cross-linked potassium salt polyacrylate, showed better healing properties compared to SAP A, a copolymer of acrylamide and sodium acrylate. The smart mate with SAP B thus is an excellent material to use in future building applications
Resistance of cracked concrete healed by means of polyurethane against chloride penetration
No full textA lot of damage is reported for constructions in marine environments. Marine environments are very aggressive, because of the high chloride concentration in sea water. Chlorides affect durability by initiating corrosion of the reinforcement steel. When cracks appear in the concrete structures, chlorides will penetrate faster and will initiate corrosion. A possible solution is self-healing concrete. Self-healing concrete has the ability to recover without external intervention. From the literature concerning self-healing concrete, it is clear that research focuses on the general concept, the mechanical properties and water permeability. Based on the water permeability it is concluded whether harmful substances will penetrate. Specific data on degradation of self-healing concrete in aggressive environments are not available. Nevertheless, these data are important to ensure a good estimation of the service life extension. In this research, the effect of the healed cracks on the resistance against chlorides was investigated for two concrete types, namely ordinary Portland cement concrete and blast-furnace slag concrete with 50 % cement replacement. Non-steady state migration tests, based on NT Build 492, were performed with uncracked, cracked and healed concrete. In our previous research, autonomous crack healing was obtained by encapsulating polyurethane healing agents. To release the healing agents, realistic cracks were formed by means of a controlled splitting test. In the current work, as a first step, cracks (notches) were manually healed with a two-component healing agent based on polyurethane. These cracks (notches) were formed by means of steel plates with a width of 0.1 and 0.3 mm. The migration tests were performed at constant setup parameters, namely 30 V and 8h. The chloride penetration front was visualized by means of the colorimetric method. By comparing the penetration depths, it seemed that concrete with a healed crack of 0.1 mm can fully regain its resistance against chloride penetration
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