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.

Negli 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.

Materiali compositi avanzati a matrice inorganica per applicazioni strutturali: studio sperimentale del miglioramento delle proprietà di adesione all'interfaccia / Cesare Signorini , 2020 Mar 05. 32. ciclo, Anno Accademico 2018/2019.

Materiali compositi avanzati a matrice inorganica per applicazioni strutturali: studio sperimentale del miglioramento delle proprietà di adesione all'interfaccia

SIGNORINI, CESARE
2020

Abstract

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.
Advanced inorganic composite materials for structural purposes: enhancement of interphase adhesion
5-mar-2020
RADI, Enrico
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