Etude du comportment mécanique des poutres corrodées réparées à l’effort tranchant par l’insertion des joncs de carbone

Cette communication présente les caractéristiques principales d’un modèle par Eléments Finis (EF) développé en utilisant le logiciel FEMIX afin de prévoir le comportement de poutres en béton armé corrodées réparées à l’effort tranchant par l’insertion des joncs de carbone par la technique NSM (Near-Surface Mounted reinforcement). Cette technique consiste à sceller les joncs de carbone dans des engravures déjà préparées sur les surfaces inférieure et latérale de la poutre corrodée à réparer. Les résultats expérimentaux et ceux de la modélisation numérique en mode 3D sont présentés en termes des courbes charge-flèche et des modes de ruine pour quatre poutres courtes: deux poutres corrodées (A1CL3-B et A1CL3-SB) et deux poutres témoins (A1T-B et A1T-SB), les poutres avec « -B » sont réparées seulement en flexion avec un jonc de carbone par la technique NSM, tandis que celles avec « -SB » sont réparées à la fois en flexion et à l’effort tranchant. La corrosion des barres d'acier tendues et son effet sur la résistance à l’effort tranchant des poutres en béton armé sont discutés. Les résultats ont montré une bonne corrélation entre les prédictions par le modèle EF en termes de courbes expérimentales charge-flèche, de la fissuration et des modes de ruine.

Exploring the use of HPFRC and GFRP grids for the production of manhole covers

Corrosion of the steel reinforced concrete elements is one of the common pathologies that limits the long-term performance of urban infrastructures. This problem causes the loss of structural serviceability by decreasing the concrete-steel bond strength and reducing the cross section of the reinforcements. The present study introduces a new system for developing free-corrosion resistance prefabricated manhole covers for applications in the aggressive environments, i.e. wastewater collector systems, sewer systems, stormwater systems, etc. Fibre reinforced cement composites were applied in this system in order to suppress the corrodible steel mesh and maintain the structural ductility as well. Application of fibre reinforced polymer (FRP) system is adopted as the additional solution for increasing the load carrying capacity of these elements without concerns about corrosion. The effectiveness of the applied strategy in developing the manhole covers in terms of load carrying capacity and failure mode is evaluated in this research. Furthermore, this paper discusses a FEM-based simulation, aiming to address the possibility of calibrating the constitutive model parameters related to fracture modes I and II.

Prestress losses in reinforced concrete beams strengthened with NSM-CFRP laminates

Using prestressed near surface mounted fibre reinforced polymers (NSM-FRP) is nowadays regaining the attention from the scientific community for the strengthening of existing reinforced concrete (RC) structures. The application of prestressed internal FRP bars and externally bonded prestressed FRPs has already been deeply investigated and revealed considerable benefits when compared to the corresponding passive solutions. A certain amount of prestress provides benefits mainly associated to structural integrity and material durability. Immediately after prestress transference, it is possible to close some of the existing cracks, decreasing the susceptibility of the element to corrosion and, a certain amount of deflection can be recovered due to the creation of a negative curvature. However, very few studies have been carried out to properly assess the preservation of prestress over time. In this context, several reinforced concrete beams strengthened with prestressed NSM carbon FRP (CFRP) laminates were prestressed and monitored for about 40 days. The data obtained from these experimental programs is in this paper presented and analysed. The observed prestress losses were later modelled using finite elements analysis and, although this topic is not addressed in this paper, the obtained results revealed considerable precision. The largest strain losses in the CFRP laminate were found to be mainly located in the extremities of the bonded length, while in the central zone most of the applied pre-strain was retained over time. The highest CFRP strain losses were observed in the first 6 to 12 days after prestress transfer, suggesting that the application of prestressed NSM-FRP will be very effective over time.

Prestress losses in NSM-CFRP flexurally strengthened RC beams

The use of prestressed near surface mounted fibre reinforced polymers (NSM-FRP) has been long acknowledged to be a suitable approach to strengthen and retrofit existing reinforced concrete structures. The application of a certain amount of prestress to the FRP prior to its installation provides a number of benefits, mainly related to crack width and deflection requisites at serviceability limit state conditions. After transferring the prestress to a structural element, some of the existing cracks can be closed, decreasing the vulnerability of the element to corrosion and, a certain amount of deflection can be recovered due to the introduced negative curvature. However, these benefits can only be assured if the prestress is properly preserved over time. In this context, three series of reinforced concrete beams, in a total of 10 beams, were strengthened with a prestressed carbon FRP laminate (CFRP) and monitored for about 40 days. The data obtained from these tests is in this paper presented and analysed. The observed losses of strain in the CFRP laminate were found to be mainly located in the extremities of the bonded length, while in the central zone most of the initial strain was well-preserved over time. Additionally, the highest CFRP strain losses were observed in the first 6 to 12 days after prestress transfer, suggesting that the benefits of prestressed NSM-FRP will not be considerably lost over time.

A FEM-based model to study the behaviour of corroded RC beams shear repaired by NSM CFRP rods technique

This paper presents the main features of finite element FE numerical model developed using the computer code FEMIX to predict the near-surface mounted NSM carbon-fiber-reinforced polymer CFRP rods shear repair contribution to corroded reinforced concrete RC beams. In the RC beams shear repaired with NSM technique, the Carbon Fibre Reinforced Polymer (CFRP) rods are placed inside pre-cut grooves onto the concrete cover of the RC beam’s lateral faces and are bonded to the concrete with high epoxy adhesive. Experimental and 3D numerical modelling results are presented in this paper in terms of load-deflection curves, failure modes and slip information of the tensile steel bars for 4 short corroded beams: two corroded beams (A1CL3-B and A1CL3-SB) and two control beams (A1T-B and A1T-SB), the beams noted with B were let repaired in bending only with NSM CFRP rods while the ones noted with SB were repaired in both bending and shear with NSM technique. The corrosion of the tensile steel bars and its effect on the shear capacity of the RC beams was discussed. Results showed that the FE model was able to capture the main aspects of the experimental load-deflection curves of the RC beams, moreover it has presented the experimental failure modes and FE numerical modelling crack patterns and both gave similar results for non-shear repaired beams which failed in diagonal tension mode of failure and for shear-repaired beams which failed due to large flexural crack at the middle of the beams along with the concrete crushing, three dimensional crack patterns were produced for shear-repaired beams in order to investigate the splitting cracks occurred at the middle of the beams and near the support.

Biofunctionalization of titanium surfaces for dental implants: osteogenic, anti-microbial and tribocorrosion resistant surfaces

PhD thesis in Biomedical Engineering

Innovative methodologies for the enhancement of the flexural strengthening performance of NSM CFRP technique for RC beams

Tese de Doutoramento em Engenharia Civil

Designing next generation car washing machines with rotary brushes

Tese de Doutoramento - Leaders for Technical Industries (LTI) - MIT Portugal

Influence of design parameters on the mechanical behavior and porosity of braided fibrous stents

In spite of all innovations in stent design, commonly used metallic stents present several problems such as corrosion, infection and restenosis, leading to health complications or even death of patients. In this context, the present paper reports a systematic investigation on designing and development of 100% fiber based stents, which can eliminate or minimize the problems with existing metallic stents. For this purpose, braided stents were produced by varying different materials, structural and process parameters such as mono-filament type and diameter, braiding angle and mandrel diameter. The influence of these design parameters on mechanical behavior as well as stent's porosity was thoroughly investigated, and suitable parameters were selected for developing a stentwith mechanical characteristics and porosity matching with the commercial stents. According to the experimental results, the best performance was achieved with a polyester stent designed with 0.27 mm monofilament diameter, braiding angle of 35° and mandrel diameter of 6 mm, providing similar properties to commercial Nitinol stents.

Development of ductile cementitiuos composites using carbon nanotubes

Concrete is the primary construction material for civil infrastructures and generally consists of cement, coarse aggregates, sand, admixtures and water. Cementitious materials are characterized by quasi-brittle behaviour and susceptible to cracking [1]. The cracking process within concrete begins with isolated nano-cracks, which then conjoin to form micro-cracks and in turn macro-cracks. Formation and growth of cracks lead to loss of mechanical performance with time and also make concrete accessible to water and other degrading agents such as CO2, chlorides, sulfates, etc. leading to strength loss and corrosion of steel rebars. To improve brittleness of concrete, reinforcements such as polymeric as well as glass and carbon fibers have been used and microfibers improved the mechanical properties significantly by delaying (but could not stop) the transformation of micro-cracks into macro forms [2]. This fact encouraged the use of nano-sized fillers in concrete to prevent the growth of nano-cracks transforming in to micro and macro forms. Nanoparticles like SiO2, Fe2O3, and TiO2 led to considerable improvement in mechanical performance and moreover, nano-TiO2 helped to remove organic pollutants from concrete surfaces [3].

Antibacterial properties of fibrous stents coated with silver oxide nanocoatings

Stents are rigid and perforated tubular structures, which are inserted into blood vessels in order to prevent or inhibit the constriction of blood flow, restoring the normal blood flow, when blood vessels are clogged, being used in 70% of angioplasties. These medical devices assume great importance in the treatment of cardiovascular diseases (CVD) which are the leading cause of death worldwide. In the European Union CVD account for 40% of deaths and assume an estimated annual cost of 196 billion euros[1]. Stents must possess certain requirements, in order to, adequately, perform its function, such as biocompatibility (so that its use does not c ause damage on the health of its user), mechanical strength, radiopacity (so that it is easy to view), longitudinal flexibility, ease of handling, corrosion resistance and having high strength and high radial expansion ability to recover. Stents can be made of different materials, but metals, particularly stainless steel, are the most common. However, metallic stents present several dRawbacks such as corrosion and restenosis, leading to health complications for the patient, or even death. In order to minimize these disadvantages, new materials, like fibrous materials, have been used [2]. Monofilaments present high potential for stents development because, in addition to its biocompatibility, these materials allow the application of various surface treatments, such as antibacterial coatings. Furthermore, monofilament exhibit excellent mechanical properties, like greater stiffness and good results when subjected to compression, tensile and bending forces, since these forces will be directly supported by the monofilament [3]. To minimize the reaction of the human body and Limit the adhesion of microorganisms to the stent surface, some coatings have been developed, including the use of novel metals with antimicrobial properties, like silver. The main objective of this study was the development of fibrous stents, incorporation of silver oxide nanocoating. For the development of the stent, polyester monofilaments with 0.27mm of diameter were used in braiding technology, with a mandrel diameter of 6mm and a braiding angle of 35⁰. The mechanical behaviour of the stent were evaluated by mechanical testing under longitudinal and radial compression, bending. The results of compressive strength tests are according with value from literature: 1.13 to 2.9 N for radial compression and 0. 16-5.28N to longitudinal compression. From literature is also possible to verify that stents must present 75% of unchanged diameter during the bending test and must possess a porosity between 70% and 80% [4]. The produced polyester stent presents values of 1.29N for radial compression, 0.23N for longitudinal compression, 80% of porosity and 85.5% of unchanged diameter, during bending tests. For the antibacterial functionalization, silver oxide nanocoatings were prepared, through reactive magnetron g, with an Ag target in an Ar +O2 atmosphere. In order to evaluate the nanostructure and morphology of the coatings, d ifferent technique s like X-ray diffraction (XRD), scanning electron microscopy (SEM) and and X- ray photoelectron spectroscopy (XPS were used. From the analyses of XRD it is possible to verify that the peaks corresponds to planes of Ag2 O and MATERIAIS 2015 Porto, 21-23 June, 2015 characterize a cubic phase. The presence of Ag2 O is corroborated by XPS spectrum, where it is possible to observe silver, not only, in oxide state, but a lso in mettalic state, and it is possible to verify the presence of silver clusters, confirmed by SEM analysis. Films’ roughness and topography, parameters influencing the wettability of the surface and microorganism adhesion, were measured by Atomic Force Microscopy (AFM), and it was observed that the roughness is very low (under 10 nm). Coatings’ hydrophobicity and surface tension parameters were determined by contact angle measurement, and it was verified the hydrophobic behavior of the coatings. For antibacterial tests were used Staphylococcus epidermidis strain (IE186) and Staphylococcus aureus(ATCC 6538), and halo inhibition zone tests were realized. Ag+release rates were studied by means of inductively coupled plasma mass spectrometry (ICP -MS). The obtained results suggest that silver oxide coatings do not modify significantly surface properties of the substrate, like hydrophobicity and roughness, and present antimicrobial properties for both bacteria used.

Wear of PEEK-Ti6Al4V systems under micro-abrasion and linear sliding conditions

Dissertação de mestrado integrado em Engenharia Biomédica

Study of the tightness of the metallization used on top of a thin film capacitor cell

Dissertação de mestrado integrado em Engenharia de Materiais

Tailored deposition of coloured coatings: on the widening of the available colour range

Dissertação de mestrado integrado em Engenharia de Materiais

Comportamento tribológico de biocompósitos de matriz de titânio

Dissertação de mestrado integrado em Engenharia de Materiais