Materials Science Forum Vols. 730-732

Abstract: The main aim of this work is the mechanical characterization of a composite material resulting from the combination of three by-products coming from industry, namely, flue gas desulfurization (FGD) gypsum, granulated cork and textile fibers from tire recycling. The material is considered as a green material as the raw material are considered by-products and it is intended to be used as a building material for non-structural purposes in civil engineering construction. The mechanical characterization includes uniaxial compressive tests and bending tests for characterization of the fracture behavior. Additionally, ultrasonic pulse velocity is measured to evaluate its variation with time of curing.

Abstract: With the study of composite materials based on the TiO₂-ZnO pair, we look for better sensitivity and selectivity to the gases, than those of the sensors made out of only one of those metal oxides. This would be due to the fact that some of the interstitial positions that were initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if the single covalent/ionic adsorption is decisive in the observed changes in the materials conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity and selectivity. We will present the results obtained for pelletized sensors of the pair TiO₂-ZnO, with a 85:15 % volume mol ratio of Titanium and Zinc, sintered at 400º and 500°C. The rather involved behaviour of our sensors is understood by measuring their complex impedance subjected to an external sinusoidal varying electric field, which is being applied in the presence of different relative humidities, at various working temperatures. The main goal of this work here described is the study of the relative humidity influence on the sensing properties of the composite sensors, and the development of an electrical model for the sensor electrical response.

Abstract: Cork powder, the most important sub-product of cork processing, combined with thermoplastic matrixes like, high density polyethylene (HDPE), offer a new class of cork-polymer composite (CPC) materials with high added-value. Therefore, reinforcing strategies must be considered to increase the mechanical performance, especially when high content of cork powder is added to the formulation. Coconut fibres have several advantages, such as, low density, renewable source, low cost and biodegradability. The use of these fibres on the reinforcement of CPC materials will not only contribute to improve the mechanical performance but also for increasing the amount of natural component present on the final composition. The main goal of this work was to prepare HDPE/cork (50-50 wt.%) composites reinforced with discontinuous coconut fibres (5 and 10 wt.%) with and without the addition of coupling agent (2 wt.%) by extrusion. The developed reinforced cork based composites were characterized regarding its morphology and mechanical performance. Optical micrographs have shown a homogeneous distribution of the fibres. The coupling agent effect on CPC performance was also investigated. The tensile strength and tensile modulus of the reinforced composites were significantly improved with the addition of coupling agent. The use of 10 wt.% of coconut fibres in the presence of coupling agent promote an increase on maximum tensile strength of around 41 % comparing with the HDPE/cork (50-50 wt.%) composites. Scanning electron microscopy (SEM) micrographs of the tensile fractured specimens confirmed that the use of coupling agent promoted the interfacial adhesion between the fibres and the thermoplastic matrix. Since, like cork powder, coconut fibres have good thermal and acoustic properties, we consider that the novel reinforced CPC herein described have high potential to be used in building and construction systems and other structural 3D applications.

Abstract: Enhancing the performance and lightness of different structures has already been achieved by the employment of fibre reinforced composite materials. Nowadays, a new challenging perspective is being given to these materials by the inclusion of non-metallic conductive components. This emerging technology will lead to multifunctional composites with possible applications in structural health monitoring and traffic monitoring. The aim is to avoid corrosion problems from metallic components, as well as to eliminate the need of expensive equipments used for the health monitoring of large infrastructures. In the present research, the strain-sensing capability of a core-reinforced hybrid carbon fibre/glass fibre braided composite has been investigated in order to develop continuous monitoring system. The characterization of sensing behaviour was performed with the help of an instrumental set-up capable of measuring the change in electrical resistance with mechanical stresses applied to the samples. The effect of core composition (carbon fibre/glass fibre weight ratio) on the strain sensitivity of the braided composites has been studied in order to find out the optimum composition for best sensing capability. Among the three compositions studied (23/77, 47/53 and 100/0), composites with lowest amount of carbon fibre showed the best strain sensitivity with gauge factors up to 23.4 at very low flexural strain (0.55%). Attempts have also been made in this research to develop a piezoresistive matrix for the braided composites in order to further enhance their strain sensitivity. For this purpose, the strain sensing capability of an unsaturated polyester matrix dispersed with chopped carbon fibres (1mm and 3 mm lengths) at various weight % (0.5, 0.75 and 1.25%) was evaluated in order to find out their optimum length and concentration. It was observed that chopped fibres with different lengths showed similar strain sensitivity, which however, improves with the decrease in their concentrations.

Abstract: Several kinds of alloys Ni-based, Fe-based and Al-based oxide dispersion strengthened (ODS) and carbide dispersion strengthened (CDS) have been produced through mechanical alloying. Precipitation-strengthened or dispersion-strengthened steels are a kind of high strengthened steel using fine precipitation or dispersion of carbides. This work present a study of EUROFER97 steel powder reinforced with 3%wt of niobium carbide. The starting materials were wet-milled in a high-energy planetary ball mill for several times up to 5 hours. The ratio of ball to powder weight was 15:1. The milled powders were characterized by SEM, XRD, EDX and laser scattering. The results are presented on base of a microstructure analysis of composite particles of steel-carbide as a function of milling time.

Abstract: Brake pads are composite materials which have been constantly improved by new materials that increase the quality and reduce the non-renewable raw materials. The goal of this work is to study the behavior of brake pads produced with replacement of phenol-formaldehyde resin by lignin up to 40% weight ratio. The Krauss method of characterization and SEM analysis were employed. The results showed an average friction coefficient approximately to μ_m=0.4 and a heterogeneous surface morphology. The satisfactory results are compatible with the current friction materials.

Abstract: The influence of humidity content on the electrical and dielectric properties of a composite made from recycled TetraPak® containers and granulated cork was studied. The material components have been dried before preparation and after the composite was conditioned by keeping the samples in a dry environment (desiccator) or in an oven at high temperature (70°C in air). The differences observed in electrical properties (investigated by isothermal charge and discharge current measurement) and dielectric properties (measured using dielectric relaxation spectroscopy) show that the thermal treatment at high temperature is more efficient on removing water and slows down the re-absorption rate.

Abstract: This article deals with some of preliminary tests to study the adherence mechanism of lime mortars to late XIX^th Century tiles. The bond between them is mainly physical. A water absorption by capillary test and a suction test were performed to study the parameters of mortar penetration in tiles porosity. The old tiles seem to have a higher porosity than new ones. Some of the new specimens can be used to simulate the old tiles in future tests without applying partially destructive tests to historical objects.

Abstract: The alkali-aggregate reaction (AAR) in concrete is a group of chemical reactions that involves the reaction of certain minerals present in the aggregates with alkali and hydroxyl ions in the interstitial solution of cement paste in concrete. These reactions form an alkaline hygroscopic gel that absorbs water and expands causing internal stresses with cracking [1]. The AAR mitigation measures oblige the correct evaluation of the alkali reactivity of the aggregates. This is normally assessed by petrographic, chemical or expansion test methods. Several studies regarding alkali reactivity of aggregates for concrete structures in Portugal, including bridges and dams, indicated that their field performance does not correspond to the previously performed evaluation. Presently, Portuguese methodology is based on the LNEC Specification E461-2007, which shows some limitations regarding rock types such as granitoids [2]. This situation motivated the development of a research project, involving medium and long term expansion tests in different conditions, under accelerated and natural exposure conditions, as well as petrographic evaluation of the main Portuguese aggregates used/to be used in concrete. This paper presents the preliminary results of this research.

Abstract: One of the most worrying degradation mechanisms in hardened concrete is the occurrence of chemical expansive reactions, namely due to alkali-silica reactions (ASR). This pathology involves the formation of expansive products inside the material. This situation is very difficult to deal with, because presently there is no efficient method to repair concrete structures affected by ASR. Hence, there is an urgent need to find preventive methods that may inhibit these reactions in new concrete structures. Nowadays, the use of pozzolanic mineral additions is recommended to mitigate this type of degradation. Moreover, their effect depends on their chemical and mineralogical composition and also on the cement content replacement ratio. Glass is a common material with some environmental problems regarding recycling and landfilling. Some studies exist about its use as fine powder, like a pozzolan, in cement-based materials. However, there are also some problems related to its use as coarse aggregate in cement–based materials due to ASR development. This paper presents the results obtained in mortars exposed to 1M NaOH solution prepared with crushed glass as aggregate and as addition in cement substitution. The results reveal that ASR is reduced with the incorporation of glass as aggregate or addition, and its efficiency depends on its replacement content. This study shows that an incorporation of glass aggregates up to 20% in mass will generate no significant expansion when compared with concrete made with natural aggregates.