Deformabilidad en hormigones con agregados reciclados

Diversos trabajos han demostrado la factibilidad de elaborar hormigones con agregados reciclados, sin embargo sobre algunos aspectos poco explorados aún existen informaciones contrapuestas. Entre ellos se destacan el comportamiento diferido del hormigón (contracción y fluencia) y la capacidad de deformación en tracción (extensibilidad). Estas propiedades afectan directamente el grado de fisuración que puede tener una estructura de hormigón, lo que adquiere una significativa relevancia en la práctica, al considerar su vida en servicio. En este trabajo se presenta un estudio de la deformabilidad de hormigones que contienen 50 o 100 % de agregado grueso obtenido a partir de la trituración de losas de pavimento. Los resultados se comparan con los de otros dos hormigones elaborados con idénticas proporciones de materiales componentes variando el tipo de agregado grueso, piedra partida granítica o piedra partida cuarcítica. Se evaluaron la resistencia a tracción, la extensibilidad en flexión bajo cargas rápidas, y la fluencia en compresión. Bajo cargas de corta duración se encontró una deformabilidad creciente en el hormigón con mayor contenido de agregados reciclados, tanto en flexotracción como en compresión; también se midieron mayores valores de contracción libre y de fluencia. A partir de los resultados surge que es posible estimar la deformabilidad del hormigón con agregados reciclados siguiendo criterios similares a los aplicados en hormigones con agregados naturales; las diferencias de deformabilidad se pueden justificar considerando la menor rigidez del agregado reciclado.Different works have demonstrated the feasibility of elaborating concrete with recycled aggregates, nevertheless there is still opposite information about some aspects that have not been widely studied. The differed behaviour (shrinkage and creep) and the deformation capacity in tension (extensibility) of concrete are among them. These properties have a direct effect over the degree of cracking that can have a concrete structure so, considering its service life, they acquire a significant relevance in practice. This paper presents a study on the deformability of concretes that contain 50 or 100 % of coarse aggregate obtained from crushed pavement slabs. The results are compared with those obtained on other two concretes prepared with the same mixture proportions varying only the type of coarse aggregate, granitic crushed stone or quartzitic crushed stone. The tensile strength, extensibility in flexure under rapid rate of loading and creep in compression were evaluated. It was found that under short term loads the deformability of concrete increases with the content of recycled aggregates, both in flexure and in compression, grater values of free shrinkage and creep were also measured. From the obtained results it appears that the deformability of concrete with recycled aggregates can be estimated following the same criteria applied to concrete with natural aggregate, the differences in deformability can be justified considering the lower stiffness of the recycled aggregate

A compact X-Band ODNP spectrometer towards hyperpolarized 1H spectroscopy

The demand for compact benchtop NMR systems that can resolve chemical shift differences in the ppm to sub-ppm range is growing. However due to material and size restrictions these magnets are limited in field strength and thus in signal intensity and quality. The implementation of standard hyperpolarization techniques is a next step in an effort to boost the signal. Here we present a compact Overhauser Dynamic Nuclear Polarization (ODNP) setup with a permanent magnet that can resolve 1H chemical shift differences in the ppm range. The assembly of the setup and its components are described in detail, and the functionality of the setup is demonstrated experimentally with ODNP enhanced relaxation measurements yielding a maximal enhancement of −140 for an aqueous 4-hydroxy-TEMPO solution. Additionally, 1H spectroscopic resolution and significant enhancements are demonstrated on acetic acid as a solvent

Effect of nitroxide spin probes on the transport properties of Nafion membranes

Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be a useful tool for probing hydration dynamics and interactions within 5–8 Å of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion's inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that commonly used paramagnetic spin probes—here, stable nitroxide radicals—for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity

Noninvasive Quantification of Cell Density in Three-Dimensional Gels by MRI

ObjectiveFor tissue engineering, there is a need for quantitative methods to map cell density inside three-dimensional (3-D) bioreactors to assess tissue growth over time. The current cell mapping methods in 2-D cultures are based on optical microscopy. However, optical methods fail in 3-D due to increased opacity of the tissue. We present an approach for measuring the density of cells embedded in a hydrogel to generate quantitative maps of cell density in a living, 3-D tissue culture sample.MethodsQuantification of cell density was obtained by calibrating the 1H T2, magnetization transfer (MT) and diffusion-weighted nuclear magnetic resonance (NMR) signals to samples of known cell density. Maps of cell density were generated by weighting NMR images by these parameters post-calibration.ResultsThe highest sensitivity weighting arose from MT experiments, which yielded a limit of detection (LOD) of [Formula: see text] cells/mL/ √{Hz} in a 400 MHz (9.4 T) magnet.ConclusionThis mapping technique provides a noninvasive means of visualizing cell growth within optically opaque bioreactors.SignificanceWe anticipate that such readouts of tissue culture growth will provide valuable feedback for controlled cell growth in bioreactors