Abstract

One of the main problems associated with the manufacture of composite materials by liquid moulding (injection, infusion, etc.) is the generation of porosity. The optimization of the properties of the composites passes through a better understanding of the mechanisms of pore formation and transport. This pore formation is associated with a non-homogeneity of resin flow during the manufacturing process as a consequence of the double-scale porosity present in textile preforms. In the channels between bundles of fibers, the resin flows controlled by the pressure gradient and its viscosity. However, within the fiber bundles, the resin flows as a result of the capillary pressure. As a consequence, if the viscous and capillary forces are not balanced, non-homogeneous local flow occurs and the formation of pores, either between the fiber bundles, or internally to the bundles is produced.In this work a systematic study of the spontaneous wetting process of an E glass fiber bundle through wicking tests, using several fluids with different properties, viscosity, surface tension and contact angle is presented. The visualization of the front position of the resin flow at microscopic scale, as well as the formation of the menisci between fibers has been performed using X-ray tomography techniques using synchrotron radiation. The reconstructed volumes have made it possible to obtain detailed quantitative information on the geometry of the menisci which occur as a consequence of the surface tension operating at such scale. These characterization techniques constitute a fundamental tool for the optimization of the processing of composite materials buy liquid moulding.

Abstract

The state-of-the-art additive manufacturing technology of continuous carbon fibre reinforced polymers (CFRP) is based on fused filament fabrication (FFF), which has a great potential for reproducing robust and complex geometries. However, 3D printed CFRP present relatively poor mechanical properties in comparison to traditional composites. This work analyses post-processing temperature effects on the microstructure and on the interlaminar mechanical properties of the printed CFRP. Post-processed pieces at 150°C (exceeding matrix glass transition) showed a reduction of porosity by 80% and improved mechanical properties by 250% with respect to unprocessed pieces.

Abstract

Expansive additives (EAs) have been widely used to reduce the risk of cracking due to shrinkage. Such additives cause early age expansion of concrete due to the formation of Portlandite and Ettringite in the pores of the cement paste, which affects the cement paste's microstructure. The presence of external restraints in the form of reinforcement and support affects the expansion and, ultimately the microstructure of cement paste. In the current study, the microstructure of cement paste with expansive additives is studied using mercury intrusion porosimetry (MIP). A free lime-based EA, with 10 and 15% (by weight) cement replacement ratios, is studied in the current study. The porosity and pore size distribution of the cement paste with EA were studied under free and restrained conditions. Results show that the under the stress-free condition, the porosity of the paste increased, while under the restrained condition, the porosity remained much similar to the reference case of OPC. Further, the pore size distribution of the cement paste with EA under free and restrained conditions is explained, and the effect of EA and restraint on the microstructure is discussed.

Abstract

We derive analytical relationships between the spatial covariance of the (natural) logarithm of hydraulic conductivity (K) and that of representative soil particle sizes and porosity. The latter quantities can be directly measured during routine sedimentological analyses of soil samples and provide a way of incorporating K estimates into groundwater flow models at a relatively modest experimental cost. Here we rely on widely used empirical formulations requiring measurements of representative particle diameters and, in some cases, of medium porosity. We derive exact formulations relating the spatial covariance of these quantities and K and present workable approximations on the basis of perturbation methods. Our formulations provide a direct link between key geostatistical descriptors of sedimentological and hydraulic parameters of heterogeneous aquifers which can be employed in classical estimation and simulation procedures. The approach and theoretical results are tested on an extensive data set comprising 411 particle size curves collected at 12 boreholes in a small-scale alluvial aquifer.

Abstract

Direct Metal Laser Sintering (DMLS) is a complex production process including hosts of parameters and a multitude of physical phenomena, which make the simulation and modeling quite challenging. This work investigates the impact of modified printing parameters (e.g., hatch distance, laser power) on correlating material properties (e.g., Young's modulus, temperature gradient) of hardened aluminum specimens. The ultimate goal is to create a data model that enables data-driven and multi-physical optimization of mechanical components fabricated via DMLS.

Abstract

The current mode plays an important role in the Electrochemical Chloride Extraction (ECE) treatment for concrete. In this study, the experiments were conducted to investigate how the ECE process would be affected by different current modes (continuous and intermittent current modes). The results showed that there were remaining amounts of chloride ions that were hard to be removed by both current modes. Additionally, the porosity under the two current modes showed the same upward trend compared to the initial value, and the chloride diffusion coefficient increased by 1 - 2 times. Also, the chloride-contaminated concrete subjected to the continuous current mode had a higher cumulative chloride extraction content than that subjected to the intermittent mode. The intermittent current mode could not increase the effectiveness of extracting chloride when the current density was not greater than 3 A/m2 and the on-off ratio was not less than 3.2.

Abstract

The diffusion or permeability of ions or gases through the cement matrix is key in the degradation of reinforced concrete for marine applications. The delay in ion diffusion has been an important approach to extend the life of this type of materials. In this work it is detailed how reduce degradation of cement mortar using nanotechnology. The addition of nanoparticles to the cement paste has important implications for the hydration and microstructure, reducing porosity and improving mechanical properties. In this study, different nanoparticles have been used to reduce the porosity of mortars evaluating this improvement by means of electrical resistivity measurements. It is perfectly established the porosity reduction with the increasing of electrical resistivity. Reduction of porosity minimises the intake of aggressive agents into the mortar in a meaningful way, retarding the corrosion of metal reinforcement. Nanoparticles addition (<10%) within cement mortar have allowed to increase the electrical resistivity in up to 112% regarding to control sample, without modifying of mechanical properties.       

Abstract

Abstract

This study explores the behavior of blended mortars of low reactive bottom ashes (BA) and ordinary Portland cement (OPC) in an alkaline solution. Mortar mixtures incorporating OPC with different replacement levels (0% to 30% mass) were studied. Isothermal conduction calorimetric analysis was studied and 28-and 60-day mechanical strength values were found. SEM images was used to identifies porosity structures at 28-day. Water absorption was also investigated. The results revealed different behaviors to low (OPC 2.5 and OPC 5) and moderated (OPC 10 and OPC 30) OPC content. Increase in percentage of Portland accelerated hydration kinetics. There is a second peak formation for moderated OPC content, associated with C-A-S-H gel formation. The partial replacement of bottom ash by OPC tend to reduce the absorption. There is a slower initial water absorption to low OPC content. This behavior is due the higher unreacted BA content, that works as a filler. Otherwise, the increase of water absorption for OPC content is due to the coexistence of Portland cement hydrates and alkali activated reactions.