1,720,982 research outputs found
On the effect of curing time and environmental exposure on impregnated Carbon Fabric Reinforced Cementitious Matrix (CFRCM) composite with design considerations
No full textThis paper investigates the effect of curing time and aggressive environmental exposure on the mechanical performance of impregnated Carbon Fabric Reinforced Cementitious Matrix (CFRCM) composite. Following the recently published IIC-ES AC434 guidelines, saltwater, distilled water, alkali and acid resistance are investigated together with freeze-thaw cycles. Mechanical characterization is based on tensile uni-axial tests under deformation control of rectangular-base prismatic specimens. 28- and 60-day curing times are considered for the control environment as well as for saltwater and alkali resistance. Deformation is monitored via digital acquisition. Besides uni-axial tests, experimental results comprise optical and scanning electron microscopy, crack pattern analysis and failure mechanism assessment. Focus is set on the determination of the design limits for the composite system at failure for the tested environments and curing times. In particular, a comparison is drawn with established design criteria already coded for FRP systems, which introduce the concept of safety (or partial) factors. Environmental conversion factors are also defined and calculated on a statistical basis in a twofold manner, as a mean to determine the design strain and strength limits of exposed specimens from the control (unexposed) data. It is found that they provide a convenient method for assessing the composite vulnerability to the aggressive environments at different curing times
Materiali compositi avanzati a matrice inorganica per applicazioni strutturali: studio sperimentale del miglioramento delle proprietà di adesione all'interfaccia
No full textNegli ultimi decenni, la ricerca nel campo dei materiali da costruzione è stata oggetto di uno sviluppo straordinariamente rapido, che ha avuto una ricaduta sostanziale nella concezione e nella progettazione di nuovi edifici, dal design audace, elevate prestazioni meccaniche e basso impatto ambientale. D’altro canto, anche materiali da costruzione tradizionali come cemento e calce, grazie a molteplici interessanti proprietà meccaniche e fisico-chimiche, risultano tuttora non solo di diffuso impiego nella pratica, ma anche oggetto di ricerca avanzata in campo scientifico.
Nel contesto delle tecnologie per il consolidamento ed il miglioramento sismico di edifici esistenti, attualmente largo impiego è riservato ai materiali compositi a fibra continua detti FRP (Fibre-Reinforced Polymer), divenuti tecnologia affidabile e consolidata, grazie ad un ricco background scientifico. Tuttavia, gli FRP presentano limitazioni di utilizzo nei casi in cui siano richieste caratteristiche termiche e chimico-fisiche specifiche. Sono quindi emerse di recente all'interesse delle comunità scientifica e tecnica nuove tipologie di compositi, identificate con gli acronimi FRCM (Fibre-reinforced Cementitious Matrix) and TRM (Textile-reinforced Mortar), che sostituiscono nella matrice il legante polimerico con malte a base calce o cemento. La struttura porosa e la natura idraulica delle malte permette di disporre di materiali ad elevate stabilità termica e permeabilità al vapor d’acqua e spiccata compatibilità con i substrati murari. L’aspetto critico di queste tecnologie innovative risiede nella scarsa adesione a livello di interfaccia fibra-matrice e nella difficoltà di ottenere una corretta impregnazione dei tessuti, a causa della granulometria della matrice. Questo aspetto implica l’innesco di modalità di crisi non controllabili, come lo scivolamento all'interfaccia o il cosiddetto “telescopic failure”, che non permettono di definire valori di progetto affidabili.
Nella prima parte del lavoro, si mettono a punto e si caratterizzano nel dettaglio alcuni rivestimenti, di natura organica e inorganica, depositati su fibre di rinforzo strutturale al fine di migliorare il legame tra le due fasi del composito, oltre che a solidarizzare le fibre interne del multi-filamento per evitare scorrimenti differenziali rispetto alle fibre esterne. Si affrontano inoltre studi sperimentali per determinare la durabilità di tali tecnologie, essendo ad oggi la letteratura tecnica incompleta per quanto concerne la risposta meccanica dei TRM/FRCM in condizioni di esposizione ad ambienti aggressivi. Le campagne sperimentali su TRM sono caratterizzate da prove a trazione del composito e prove a flessione sul laminato applicato su substrato.
Nella seconda parte del lavoro, il ruolo dell’adesione è studiato per una diversa categoria di materiali compositi strutturali a matrice cementizia, ovvero i cosiddetti FRC (Fibre-reinforced Concrete) rinforzati con fibre discontinue in polipropilene (PP). Il PP è caratterizzato da una elevata inerzia chimica, che lo rende refrattario ad instaurare legami chimici con altri materiali, come ad esempio il cemento. Nel presente lavoro si studiano due diversi trattamenti sulle fibre per migliorare l’adesione con la matrice cementizia per via chimica e la risposta meccanica è studiata tramite prove di estrazione (pull-out) e flessione, queste ultime realizzate su diverse scale dimensionali. I due trattamenti proposti e analizzati nel dettaglio (ricoprimento con nano-silice amorfa e etching a mezzo di soluzione aggressiva) contribuiscono in modo sostanziale all'incremento di tenacità dei compositi fibrorinforzati, attivando gruppi funzionali idrofili sulla superficie delle fibre in grado di legarsi con le molecole d’acqua del conglomerato.Building materials have experienced an extraordinarily fast development in the last decades, extending the possibilities for new innovative constructions, with outstanding properties such as high mechanical performance, audacious architecture and low energy consumption. On the other hand, also traditional materials, like cementitious and lime-based systems, are worthy of accurate investigation in order to take advantage of their benefits, which have been exploited for centuries and are still largely employed in contemporary structures.
In the context of seismic retrofitting and structural rehabilitation, nowadays the techniques based on Fibre Reinforced Polymers (FRP) are firmly established and very reliable, and yet they present some critical issues, since some specific physical and thermal requirements are not fully accomplished. Therefore, Textile Reinforced Mortar (TRM) or Fibre Reinforced Cementitious Mortar (FRCM) composites have encountered increasing interest in the scientific community as well as in the technical one. The innovation is the partial or complete substitution of the organic binder with lime-based and/or cementitious mortars, which play the role of embedding medium. The porous texture and the hydraulic nature of these inorganic mortars result in high thermal stability, reversibility, high permeability to water vapour and good compatibility with masonry substrates. The main drawback associated with lime and cement matrices is their intrinsic poor adhesion at the fabric-to-matrix interphase. The poor impregnation quality of the fabrics yarns is responsible for triggering undesirable failure modes (i.e. telescopic failure or interphase sliding), which lead to unreliable design values.
In the first part of the present work, several techniques based on the deposition of engineered coatings for multifilament fabrics are proposed and extensively described and tested in order to improve the interphase adhesion and, at the same time, to strengthen the core filaments bond. Both inorganic and organic coatings on synthetic fibres are discussed and optimized. Special attention is paid to alkali resistant (AR) glass fabrics, which are the prevalent reinforcement for masonry panels due to their good mechanical properties combined with relatively low cost, which make them preferable to carbon or PBO. Besides, some durability issues are investigated for polymer-coated TRM. In fact, although two guidelines have been recently released, no exhaustive indications have been provided about the potential consequences of the exposure to aggressive environments on the mechanical response of TRM. To the aim, tensile and bending tests are performed on TRM composites.
In the second part of the work, the role of interphase adhesion is investigated in a different category of inorganic composite materials, namely in Fibre Reinforced Concrete (FRC), that is commonly employed in industrial pavements. Discontinuous polypropylene (PP) fibres are proposed as dispersed reinforcement. Since PP is characterized by an outstanding chemical inertness, no adhesion is possible with the conglomerate. Thus, the adhesion can be increased through mechanical gripping by using PP fibres with a high surface roughness. Additionally, this research proposes two experimental activities to enhance the interphase adhesion chemically. The mechanical behaviour of the FRC composites is assessed through three-point bending tests at different dimensional scales and through pull-out tests. The two proposed treatments, i.e. deposition of a silica coating and etching with piranha solution, notably improve the toughness of the composite by activating hydrophilic functional groups that are able to bond to the water molecules in the cementitious conglomerate
Durability of fibre-reinforced cementitious composites (FRCC) including recycled synthetic fibres and rubber aggregates
Full text linkWe discuss mechanical performance of fibre-reinforced cementitious composites under exposure to four aggressive environments, namely alkaline, saline, sulphuric acid and distilled water immersion. A standard commercial Portland cement based matrix is considered alongside its lightweight modification wherein quarzitic sand is partially replaced by recycled rubber crumbs. Also, virgin polypropylene fibres are contrasted to PP+PET blended fibres where the PET fraction is obtained from recycling food packaging waste. Performance is assessed in bending as well as in compression. We find that recycled based specimens perform surprisingly well and that exposure to the aggressive environments mainly affects the matrix and it is not necessarily more detrimental to the lightweight partially recycled phase. A one-way analysis of variance (ANOVA) confirms the statistical significance of the results, which fully support the idea that the adoption of a substantial recycled fraction in construction materials still allows for high performance and durability standards
Miscela per massetti alleggeriti contenente aggregati inerti generati dal recupero di campi sportivi
Full text linkLa presente invenzione si riferisce ad una miscela per massetti alleggeriti per la realizzazione di sottofondi non strutturali, sia per nuove costruzioni che per opere di restauro. Tale miscela è caratterizzata da una elevata capacità isolante termica, presentando infatti un basso coefficiente di conducibilità termica e una elevata permeabilità al vapore d’acqua. Sono note miscele per massetti realizzate con inerti naturali o sintetici, vergini o da riciclo. Tuttavia, tali miscele note presentano il problema di non consentire il re-impiego di materiale di recupero di natura polimerica ed elastomerica derivante dallo smaltimento di sottofondi in erba sintetica ad uso sportivo. Allo stato attuale, i suddetti materiali a fine vita, essendo di difficile reimpiego, sono destinati alla termovalorizzazione o al conferimento in discarica. Scopo della presente invenzione è quello di risolvere i suddetti problemi della tecnica anteriore fornendo una miscela per massetti alleggeriti che consenta di recuperare scarti plastici ed elastomerici, nonché sabbia minerale e macrofibre poliolefiniche, da fonti come i sottofondi in erba sintetica ad uso sportivo che, ad oggi, non trovano reimpiego alternativo a fine vita, evitandone il conferimento in discarica o alla valorizzazione termica. Un altro scopo della presente invenzione è quello di evitare trattamenti del rifiuto a monte dell’incorporazione nel premiscelato. Il materiale di riciclo, infatti, essendo di per sé inerte, viene esclusivamente recuperato e parzialmente suddiviso tramite semplice setacciamento, non necessitando di ulteriori trattamenti chimici più complessi e dispendiosi
Strain-Hardening Cement-based Composites (SHCC) for Impact Strengthening of Buildings: Recent Advances in the DFG Research Training Group 2250
No full textConcrete is by far the most widespread construction material worldwide for buildings and infrastructures. While offering wide range of advantages, concrete structures are vulnerable to impact loading such as collisions, rock fall or explosions. This can be traced back to the intrinsically brittle nature of the material. Against this background, the Research Training Group (RTG) 2250 funded by the German Research Foundation (DFG) focuses on the development of strengthening overlays made of strain-hardening cement-based composites (SHCC) and other quasi-ductile mineral based materials capable of drastically enhancing the impact resistance of existing concrete structures. Multidisciplinary collaborative work is carried out by three renowned research institutions in Dresden with nine departments involved. In this contribution, an overview of the recent achievements in the RTG 2250 work are presented, spanning from the design of new sustainable SHCC as high-ductility matrices for textile-reinforced strengthening layers to the structural performance of such layers under impact loading. The latter is assessed by means of customized real-scale test protocols. Furthermore, some insights into the advanced techniques of data acquisition and management, numerical modeling as well as sustainability and resilience assessment are provided
Mechanical Performance of Fiber Reinforced Cement Composites Including Fully-Recycled Plastic Fibers
Full text linkThe use of virgin and recycled plastic macro fibers as reinforcing elements in construction materials has recently gained increasing attention from researchers. Specifically, recycled fibers have become more attractive owing to their large-scale availability, negligible cost, and low environmental footprint. In this work, we investigate the benefits related to the use of fully-recycled synthetic fibers as dispersed reinforcement in Fiber Reinforced Cement Composites (FRCCs). In light of the reference performance of FRCCs including virgin polypropylene (PP) fibers only, the mechanical response of composites reinforced with polyolefin filaments treated with a sol-gel silica coating and polyethylene terephthalate (PET)/polyethylene (PE) cylindrical draw-wire fibers is here assessed through three-point bending tests. Remarkably, recycled polyolefins lead to a notable enhancement in terms of peak strength and post-crack energy dissipation capability. This improvement is ascribed to both the flattened shape of fibers and the surface coating, which turns out to be very effective at strengthening the fiber-to-matrix bond. On the other hand, PET/PE fibrous reinforcement generally leads to a lower toughness, if compared to the virgin fibers. However, no reduction in terms of peak stress is evidenced. Balancing the significance of mechanical performance and environmental sustainability in the framework of a circular economy approach, both fully-recycled fibers at hand can be regarded as promising candidates for innovative structural application
Highly Dissipative Fiber-Reinforced Concrete for Structural Screeds
No full textSynthetic fibers, especially polypropylene (PP) fibers, are emerging as a viable reinforcement for concrete, on account of their excellent durability, affordability, anti-spalling capability, low density, and magnetic transparency. Yet, the chemical nature of PP hinders the development of strong bonds at the fiber-to-matrix interface, with negative effects on the mechanical performance. To overcome this difficulty, in this research fibers are either chemically attacked (etched) or coated through sol-gel nanosilica deposition in order to promote their affinity to the hydration products in the binder. Three-point bending tests at different scales are carried out on unnotched specimens, including large-scale beams consisting of PP-reinforced concrete for structural screeds. Functionalization, especially in the form of silica coating, improves the binder-fiber interaction, which is responsible for a remarkable increment in the specific energy dissipated at failure, with respect to untreated fibers. Most importantly, both surface treatments induce a substantial hardening response as opposed to the softening behavior that is characteristic of low-dosage fiber-reinforced concrete. We conclude that surface functionalization, and especially nanosilica coating, offers significant advantages for better exploiting the reinforcing effect of PP fibers, and these carry over at different scales. In particular, results appear promising for screeds, which advocate optimal mechanical performance and durability while keeping the fiber content to a minimum
Use of Recycled and Virgin Carbon Fibers in Limestone Calcined Clay Cement Composites
No full textThe urgent need to reduce the environmental impact of building materials has led to the recent development of low-clinker binders, such as limestone calcined clay cement (LC3). The possibility of using LC3 to produce cement-based composites with high mechanical properties and reduced environmental impact is certainly of great interest. In this study, two LC3 mixtures with different mechanical performances (low and high strength) were investigated. Chopped carbon fibers coming from end-of-life prepreg carbon textiles (rCF), recovered through pyrolysis, and virgin carbon fibers (vCF) were used to reinforce the matrices. Different fiber dosages were investigated, up to 1.5% by volume. Fiber reinforced LC3 composites were characterized through compression and bending tests. The addition of both rCF and vCF results in a decrease in matrix workability, by increasing fiber volume. On the other hand, rCF were able to increase both the compression and bending performances of the cementitious composites, up to 66% and 53%, respectively. Remarkably, no significant differences were found in using rCF instead of vC
Effectiveness of Sprayed Strain-Hardening Limestone Calcined Clay Cement (SHLC3) Composites in Retrofitting Concrete-Beams
No full textThe attractiveness of strain-hardening cement-based composites (SHCC) in civil engineering applications results from their pseudo-ductile response and multiple fine cracks under tensile loading. Previous research has demonstrated that SHCC show high potential as a repair material for concrete structures. They also hold promise to overcome the current limitations of traditional application techniques, paving the way for advanced automated systems. In this contribution, the effectiveness of a novel SHCC formulation for retrofitting concrete elements is investigated. This formulation is based on limestone calcined clay cement (LC3) and polyethylene fibres. The focus is on the influence of different technological parameters, such as the volume fraction of fibres and the application technique, on the mechanical performance of SHCC. Two different SHCC strengthening systems are developed and applied to the tensile side of concrete beams, either with or without a biaxial carbon textile reinforcement. In total, 18 beams are prepared and subjected to three-point bending (3PB) tests. The influence of the repair systems on the flexural behavior of the retrofitted concrete beams is demonstrated and discussed. The test results reveal that the strengthening layers under investigation significantly improve the flexural performance of concrete beams
Mineral-bonded composites for enhanced structural impact safety: The vision of the DFG GRK 2250
Full text linkExisting reinforced concrete structures feature, as a rule, a relatively low resistance to various sorts of impact loading, such as shock, collision, or explosion. To this aim, the primary goal of the Research Training Group (in German: Graduiertenkolleg, GRK) 2250, funded by the Deutsche Forschungsgemeinschaft (DFG), is to bring substantial improvements in the impact resistance of existing buildings by applying thin layers of strengthening material. By using innovative mineralbonded composites, public safety and reliability of vitally important existing structures and infrastructure should be significantly enhanced. The scientific basis to be developed will additionally enable to build new, impact-resistant structures economically and ecologically. The framework of the GRK 2250 as well as some achievements are herein briefly presented
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