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CESARE SIGNORINI

DIPENDENTE ALTRA UNIVERSITA
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2024 - Assessing the stress-transfer capability of mineral impregnated PBO yarns in a limestone calcined clay cement-based (LC3) matrix [Articolo su rivista]
Signorini, C.; Nobili, A.; Liebscher, M.; Zhao, J.; Ahmed, A. H.; Koberle, T.; Mechtcherine, V.
abstract

Technical textiles made of poly(p-phenylene-2,6-benzobisoxazole) (PBO) represent attractive candidates for strengthening and repairing damaged concrete and masonry structures, due to the outstanding durability and mechanical performance of PBO fibres. Similarly to their aramid counterparts, PBO fibres have proved very effective against dynamic and impact loading. In this contribution, the pull-out behaviour of PBO multifilament yarns embedded into a blended cement-based matrix is investigated, with particular reference to its stress-transfer capacity. In addition to the as-received PBO yarns, impregnation with a cement-based suspension, which can fully preserve the inorganic nature of the composite system, is also evaluated. Experimental results are presented and interpreted using a one-dimensional mechanical model. The findings indicate that mineral impregnation of the yarns provides a 40% increase in the stress-transfer capacity with the matrix, corresponding to a halving of the anchoring length. These performance gains are also supported by a transition in the failure mechanism which shifts from friction-based pull-out to fibre rupture.

2023 - Ballistic limit and damage assessment of hybrid fibre-reinforced cementitious thin composite plates under impact loading [Articolo su rivista]
Signorini, C.; Bracklow, F.; Hering, M.; Butler, M.; Leicht, L.; Schubert, T.; Beigh, M. A. B.; Beckmann, B.; Curbach, M.; Mechtcherine, V.
abstract

Impact resistance of reinforced concrete (RC) structures can be significantly improved by strengthening RC members with thin composite layers featuring high damage tolerance. Indeed, to limit the well-known vulnerability of cement-based materials against impact loading, the synergistic effects of short fibres and continuous textile meshes as hybrid reinforcement has been proved to be highly beneficial. This paper addresses the characterisation of novel cement-based hybrid composites through accelerated drop-weight impact tests conducted on rectangular plates at different impact energies. Two distinct matrices are assessed, with particular interest in a newly developed limestone calcined clay cement (LC3)-based formulation. Important parameters quantifying energy dissipation capability, load bearing capacity and damage are cross-checked to compute the ballistic limit and estimate the safety-relevant characteristics of the different composites at hand. Although textiles alone can improve the damage tolerance of fine concrete to some extent, the crack-bridging attitude of short, well-dispersed fibres in hybrid composites imparts a certain ductility to the cement-based matrices, allowing a greater portion of the textile to be activated and significantly reducing the amount of matrix spalling under impact.

2023 - Bicomponent PP fibers for sustainable mineral bonded strain hardening composites [Relazione in Atti di Convegno]
Popa, M. -M.; Beigh, M. A. B.; Signorini, C.; Mechtcherine, V.; Scheffler, C.
abstract

2023 - Direct assessment of the shear behavior of strain-hardening cement-based composites under quasi-static and impact loading: Influence of shear span and notch depth [Articolo su rivista]
Tawfik, A.; Signorini, C.; Mechtcherine, V.
abstract

Strain-hardening cement-based composites (SHCC) represent a new frontier for improving the resistance of concrete structures against highly dynamic loading regimes, e.g., in the case of impact. A novel testing device was designed to characterize the shear behavior of such pseudo-ductile cementitious composites, whose dynamic response is extremely complex. The newly developed shear testing device was adapted to investigate the performance of fiber-reinforced, cementitious composites under both quasi-static and impact regimes. In the framework of setup validation and standardization, this article focuses on the investigation of the shear behavior of SHCC specimens and spotlights the influence of two main experimental shear parameters: shear span and notch depth. The purpose-specific shear device was integrated into a hydraulic testing machine and a gravity Split-Hopkinson tension bar (SHTB) for quasi-static and impact shear experiments, respectively. Shear spans of 2 mm and 5 mm were introduced by modifying the test setup. Furthermore, the specimens were shaped through sawn U-notches with varying depths of 3 mm, 5 mm, and 7 mm. The shear response of the SHCC specimens was monitored by means of Digital Image Correlation (DIC), which enabled the accurate derivation of strain fields, cracking behavior, and fracture modes on the specimen surface. The results showed that both shear span length and notch depth regulate the shear/tension fracture propagation. With an appropriate shear specimen shape, the desired dominant shear fracture could be obtained.

2023 - Hierarchical CNT-Coated Basalt Fiber Yarns as Smart and Ultrasensitive Reinforcements of Cementitious Matrices for Crack Detection and Structural Health Monitoring [Capitolo/Saggio]
Liebscher, M.; Tzounis, L.; Signorini, C.; Mechtcherine, V.
abstract

Basalt fiber (BF) yarns were homogeneously coated with single-walled carbon nanotubes (SWCNT) following a versatile and scalable roll coating wet deposition process. The SWCNT layers turned the intrinsically electrical insulating BFs into highly conductive reinforcements, which were deployed as smart and ultrasensitive sensors for the crack detection of cementitious matrices. A subsequent thermal drying process achieved a uniform and dense SWCNT coating confirmed by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The BF-SWCNT yarns introduced in a cementitious matrix exhibited a significant variation of their fractional resistance change (∆R/R0) upon being exposed to in-situ three-point bending experiments. This response renders the BF-SWCNT as novel strain sensors for cement-based elements possessing high sensitivity factor for crack detection. A subsequent analysis of the fractured surfaces via SEM imaging revealed a good interaction between the reinforcements and the cementitious matrices with adhesive failure mechanisms occurring during the fracture process. The developed BF-SWCNT sensors as model composites of single yarns in a cementitious matrix promisingly envisage the use of CNT-coated BFs for sensing applications in cementitious large-scale composite structural parts or strengthening layers for existing structures.

2023 - Hierarchical composite coating for enhancing the tensile behaviour of textile-reinforced mortar (TRM) [Articolo su rivista]
Signorini, Cesare; Sola, Antonella; Nobili, Andrea
abstract

We describe a novel class of interface-functionalised textile-reinforced mortar (TRM) composite materials reinforced with basalt and alkali-resistant glass multifilament textiles embedded in a commercially available hybrid lime-cement mortar, usually applied for masonry retrofitting. Spotlight is set on improving the mechanical (tensile) performance of the system through a scalable and easy-to-apply surface treatment for the dry textiles. The treatment consists in soaking the textiles in highly-diluted epoxy resin, to which rice husk ash (RHA) is later added, acting as pozzolanic filler. The resulting functionalised textiles exhibit remarkable adhesion with the matrix owing to the presence of RHA having high specific surface area and rich amorphous content. Three different RHA powders are assessed and their performance is compared to that of plain silica fume. The role of RHA milling is also discussed. The RHA/epoxy coating significantly improves the ultimate tensile strength and energy dissipation capability of the TRMs. In particular, for basalt-textile reinforced composites, the mean tensile strength is three times as large as that of uncoated specimens, whereas the dissipated energy at failure is nearly four times as much. Furthermore, the surface treatment qualitatively changes the cracking pattern of the TRMs, for many diffused small cracks appear during tensile testing, and this provides evidence of effective stress distribution in the matrix as a result of superior interface adhesion.

2023 - Influence of severe thermal preconditioning on the bond between carbon FRCM and masonry substrate: Effect of textile pre-impregnation [Articolo su rivista]
Bertolli, V.; Signorini, C.; Nobili, A.; D'Antino, T.
abstract

Fabric-reinforced cementitious matrix (FRCM) composites often include polymer-impregnated bundles to improve the exploitation of the textile mechanical properties. However, organic components may degrade when exposed to elevated temperature. In this paper, the bond behavior of a carbon FRCM applied to a masonry substrate and exposed to a thermal preconditioning up to 300 °C for 250 min is investigated. Tensile tests on the textile and flexural and compression tests on the mortar matrix, as well as single-lap direct shear tests of FRCM-masonry joints with bare and impregnated textiles, are performed. Results show that the polymeric impregnation improves the mechanical properties of the FRCM even after thermal preconditioning.

2023 - Strain-Hardening Cement-based Composites (SHCC) for Impact Strengthening of Buildings: Recent Advances in the DFG Research Training Group 2250 [Relazione in Atti di Convegno]
Signorini, Cesare; Mechtcherine, Viktor
abstract

Concrete 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.

2022 - Analytical Approach for Modelling the Pull-Out Mechanism of Recycled Synthetic Fibres in Fibre-Reinforced Concrete (FRC) [Relazione in Atti di Convegno]
Sorzia, A.; Signorini, C.; Volpini, V.; Di Maida, P.
abstract

This study presents a simple one-dimensional analytical model describing the pull-out process of an elastic fibre embedded in a cement matrix, which captures the ductile behaviour of Fibre Reinforced Concrete (FRC) elements. The shear stress arising at the frictional interface between fibre and matrix during the pull-out is assumed to increase with the slippage distance, as a consequence of the growing abrasion of the fibre surface. The equilibrium conditions between the external axial load and the interfacial shear stress are imposed with reference to the undeformed configuration. The model is validated through comparison with both experimental data obtained by testing partially recycled polymeric fibres embedded in a cement matrix, and several datasets available in the literature comprising polypropylene fibres with and without silica coatings. The proposed model can properly describe the response of synthetic fibres that exhibit considerable axial elongation and slip-hardening interface behaviour. However, it may also predict the non-linear relation between the tensile load and the fibre displacement for different kinds of fibre, by setting adequately the constitutive parameters.

2022 - Assessment of the Behaviour of Low-Modulus Polyurethane Foams Subjected to Severe Shear Deformation Conditions [Relazione in Atti di Convegno]
Signorini, C.
abstract

Polymeric materials are broadly employed as buildings materials because of a number of interesting properties for specific applications. Among them, polyurethane (PU) takes advantage of outstanding mechanical properties, such as high deformability and dissipation, as well as remarkable thermal and chemical stability. As a foam, PU is arguably the most popular insulating materials, used as a supplementary layer in precast concrete panels, infill walls and roofs. Experimental assessment of the mechanical behaviour of PU foams is therefore a long-standing issue, which is demanded to validate analytical models and provide reliable parameters in FEM modelling. In particular, reliable experimental assessment to large deformations is still difficult to attain. In the present study, we carry out a preliminary mechanical characterisation of a single low-modulus PU foam by means of a testing machine prototype, which performs simple shear and shear-per-traction deformations of a square-shaped sample, according to the restraining system adopted. Simple curve-fitting of the response leads to different mechanical parameters for the same material. Shear test results are related to compressive tests and microstructural investigation of the PU foam, through Scanning Electron (SEM) microscopy. The proposed polynomial laws for the tangential and normal net forces are applicable for calibrating FEM models aimed to predict the behaviour of soft materials subjected to high deformations.

2022 - Carbon Nanotubes Strengthened Interphase in Textile Reinforced Mortar (TRM) Composites [Relazione in Atti di Convegno]
Signorini, C.
abstract

Performance of inorganic matrix composite materials for structural purposes is strongly dependent on the matrix-to-fabric interphase bond strength. Consequently, owing to lack of congruence between the fabric and the matrix, design performance parameters are strongly penalised. Besides, yarn inner filaments (the core) easily slide over outer filaments (the sleeve) in the so-called telescopic failure. Broad experimental evidence supports the adoption of epoxy coatings to improve matrix-to-fabric strength and prevent telescopic failure, although the presence of the organic phase partially impairs the remarkable advantages associated to the inorganic matrix, such as thermal stability and water vapour permeability. Silica coatings appear as a promising alternative to traditional epoxy, by inducing localised pozzolanic reactivity, firmly linking synthetic fibres and hydraulic lime through the formation of highly cementing products at the interphase. In this work, the effect of a dispersion of multi-walled carbon nanotubes (MWCNT) in a silica nano-coating is assessed in uni-axial traction tests. The silica coating is prepared through sol-gel deposition in which carbon nanotubes are dispersed. The overall amount of carbon nanotubes in the silica sol is fixed at 0.5% wt. Silica-coated AR-glass and carbon fabric composite specimens, embedded in a commercially-available lime mortar matrix, are tested and compared. Carbon nanotubes provide a remarkable enhancement in both ultimate strength and elongation for AR-glass TRM, yielding an impressive two-fold increase in terms of strength. Differently, coated carbon fabrics composites show an increase up to 31% in terms of ductility, in view of an unexpected strength loss.

2022 - Durability of fibre-reinforced cementitious composites (FRCC) including recycled synthetic fibres and rubber aggregates [Articolo su rivista]
Signorini, Cesare; Nobili, Andrea
abstract

We 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.

2022 - Experimental and Theoretical Investigation of the Mechanical Properties of PHBH Biopolymer Parts Produced by Fused Deposition Modeling [Articolo su rivista]
Nobili, A.; Signorini, C.; Volpini, V.
abstract

The mechanical response of printed dog-bone specimens of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) in linear and nonlinear regimes is investigated. Fused filament fabrication (FFF), also regarded as fused deposition modeling (FDM), is adopted to manufacture the PHBH elements. A compressible Mooney–Rivlin constitutive law is back fitted onto digital image correlation (DIC) data, as well as uni-axial tensile test data. Effective mechanical properties are given, which may be used to design and optimize the response of printed structures. Strain values at the verge of failure in tension are also experimentally obtained. Results are especially useful for modeling mechanical response of PHBH, which is an emerging biopolymer with promising application fields in biomedical engineering, through additive manufacturing techniques.

2022 - Highly Dissipative Fiber-Reinforced Concrete for Structural Screeds [Articolo su rivista]
Signorini, Cesare; Sola, Antonella; Malchiodi, Beatrice; Nobili, Andrea
abstract

Synthetic 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.

2022 - Mineral-bonded composites for enhanced structural impact safety: The vision of the DFG GRK 2250 [Relazione in Atti di Convegno]
Signorini, Cesare; Mechtcherine, Viktor
abstract

Existing 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.

2022 - Miscela per massetti alleggeriti contenente aggregati inerti generati dal recupero di campi sportivi [Brevetto]
Signorini, Cesare; Nobili, Andrea; Volpini, Valentina
abstract

La 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.

2022 - New Frontiers in Cementitious and Lime-Based Materials and Composites [Articolo su rivista]
Signorini, C.; Sola, A.; Chakraborty, S.; Volpini, V.
abstract

Cement and lime currently are the most common binders in building materials. However, alternative materials and methods are needed to overcome the functional limitations and environmental footprint of conventional products. This Special Issue is entirely dedicated to “New frontiers in cementitious and lime-based materials and composites” and gathers selected reviews and experimental articles that showcase the most recent trends in this multidisciplinary field. Authoritative contributions from all around the world provide important insights into all areas of research related to cementitious and lime-based materials and composites, spanning from structural engineering to geotechnics, including materials science and processing technology. This topical cross-disciplinary collection is intended to foster innovation and help researchers and developers to identify new solutions for a more sustainable and functional built environment.

2022 - Performance of concrete reinforced with synthetic fibres obtained from recycling end-of-life sport pitches [Articolo su rivista]
Signorini, Cesare; Marinelli, Simona; Volpini, Valentina; Nobili, Andrea; Radi, Enrico; Rimini, Bianca
abstract

Micro-plastics pollution has risen at an alarming pace over the last decades and it is now recognised as a leading environmental emergency. Indeed, only a very small fraction of annual plastic production is successfully reused, while the vast majority is either disposed of (mainly through incineration or landfilling) or dispersed into the environment. In this paper, polyolefins synthetic fibres, obtained from processing disposed artificial turf pitches aimed at paving sport facilities, are studied. Focus is set on assessing their potential for the Fibre Reinforced Concrete (FRC) technology. Mechanical performance is discussed at two fibre volume fractions, namely 3% and 5% vol., alongside environmental impact. The former is assessed in bending and reveals a significant enhancement of the post-crack energy dissipation capability, whose extent is compatible with what is usually obtained by the adoption of virgin fibres. This is especially significant in consideration of the light processing operated on the waste material. Indeed, life cycle assessment is adopted to evaluate the environmental impact of fibre reuse against fibre manufacturing from either virgin materials or plastic waste. It clearly appears that fibre reuse brings a double environmental benefit: on the one side, it decreases the need for new plastics and, on the other, it reduces plastic waste, whose traditional disposal technique, through incineration, entails a considerable footprint.

2022 - Preliminary Assessment of Rice Husk Ash (RHA) as Functional Interphase Agent in Sustainable Composite Systems for Structural Strengthening [Relazione in Atti di Convegno]
Sola, Antonella; Signorini, Cesare; Hanuskova, Miriam; Zapparoli, Mauro
abstract

Over the last few years, the effectiveness of textile-reinforced mortar (TRM) composite systems for structural retrofitting has led to the widespread adoption of these materials in the practice and to the issue of up-to-date design guidelines. Nonetheless, the weak interfacial bonding that is frequently observed between matrix and fibres is likely to cause inconsistent failure modes and, generally speaking, to severely limit the reinforcing potential of the textile. A promising solution to tackle this issue consists in treating the surface of the reinforcing fibres with a functional coating to improve the adhesion at the interphase. In this paper, a pilot study is presented to assess the effectiveness of a fully sustainable polymer coating, consisting in polyvinyl alcohol (PVA) loaded with with rice husk ash (RHA) or with a 50/50 mixture of RHA and silica fume (SF). The coating was applied on basalt fabrics to reinforce TRM coupons that were mechanically tested under uni-axial tensile loads. The mechanical properties of the TRM samples were significantly increased by up to 20%, and the peak load was attained at a higher deformability level, which is a clue of the enhanced ductility of the reinforced elements.

2022 - Veering of Rayleigh–Lamb waves in orthorhombic materials [Articolo su rivista]
Nobili, A.; Erbas, B.; Signorini, C.
abstract

We analyse veering of Rayleigh–Lamb waves propagating in a plane of elastic symmetry for a thin orthotropic plate. We demonstrate that veering results from interference of partial waves in a similar manner as it occurs in systems composed of one-dimensional (1D) structures, such as beams or strings. Indeed, in the neighbourhood of a veering point, the system may be approximated by a pair of interacting tout strings, whose wave speed is the geometric average of the phase and group velocity of the relevant partial wave at the veering point. This complementary pair of partial waves provides the coupling terms in a form compatible with a action–reaction principle. We prove that veering of symmetric waves near the longitudinal bulk wave speed repeats itself indefinitely with the same structure. However, the dispersion behaviour of Rayleigh–Lamb waves are richer than that of 1D systems, and this reflects also on the veering pattern. In fact, the interacting tout string model fails whenever the dispersion branch is not guided by either partial wave. This often occurs when neighbouring veering points interact and partial waves no longer provide guiding curves.

2021 - Antiplane Stoneley waves propagating at the interface between two couple stress elastic materials [Articolo su rivista]
Nobili, A.; Volpini, V.; Signorini, C.
abstract

We investigate antiplane Stoneley waves, localized at the discontinuity surface between two perfectly bonded half-spaces. Both half-spaces are elastic linear isotropic and possess a microstructure that is described within the theory of couple stress materials with micro-inertia. We show that the microstructure deeply affects wave propagation, which is permitted under broad conditions. This outcome stands in marked contrast to classical elasticity, where antiplane Stoneley waves are not supported and in-plane Stoneley waves exist only under very severe conditions on the material properties of the bonded half-spaces. Besides, Stoneley waves may propagate only beyond a threshold frequency (cuton), for which an explicit expression is provided. For a given frequency above cuton, this expression lends the admissible range of material parameters that allows propagation (passband). In particular, significant contrast between the adjoining materials is possible, provided that Stoneley waves propagate at high enough frequency. Therefore, micro-inertia plays an important role in determining the features of propagation. Considerations concerning existence and uniqueness of antiplane Stoneley waves are given: it is found that evanescent and decaying/exploding modes are also admitted. Results may be especially useful when accounting for the microstructure in non-destructive testing (NDT) and seismic propagation.

2021 - Comparing durability of steel reinforced grout (SRG) and textile reinforced mortar (TRM) for structural retrofitting [Articolo su rivista]
Signorini, C.; Nobili, A.
abstract

We assess tensile performance of Steel Reinforced Grout (SRG) and Fabric Reinforced Cementitious Matrix/Textile Reinforced Mortar, upon exposure to aggressive environments. Galvanized and brass-coated Ultra High Tensile Strength Steel fabrics are considered for SRG, while carbon, AR-glass, basalt and PBO fabrics are investigated for TRM, in a common cement mortar. Exposure to the aggressive environments is realized by specimen immersion for 1000 h (41.6 days) at controlled temperature in distilled water as well as alkaline, saline and acid solutions. Mechanical performance of rectangular 1-ply coupons is assessed in uni-axial traction: Ultimate strength and elongation, dissipated energy at failure and environmental conversion factors for design values are calculated and compared. It is found that significant performance difference exists in dependence of the aggressive environment under consideration. As a result, careful selection of the reinforcing fabric leads to substantial advantage in terms of durability, that should be capitalized upon at the design stage. A simple material selection matrix is presented which suggests the best reinforcing textile/aggressive environment combination for design purposes.

2021 - Durable and highly dissipative fibrous composites for strengthening coastal military constructions [Relazione in Atti di Convegno]
Signorini, C.
abstract

Reinforced concrete strategic structures for military purposes are often established in coastal or offshore areas, widely subjected to chemical attacks, mainly due to an aggressive saline and acid environments. Porosity of cementitious conglomerates favour penetration of chlorides, which tend to corrode the internal metallic rebar. The reinforcement of structures with fibrous composite materials is a viable solution to restore the initial requirements of the building, especially when it exerts defence purposes. Among synthetic fibres, polyphenylenebenzobisoxazole (PBO) is an organic fibre based on linked aromatic structures with high elastic modulus and tensile strength and highly dissipative attitudes. In this work, the assessment of durability of continuous fibre-reinforced cementitious mortar (FRCM) composites is carried out comparing the mechanical performance of laminates subjected to uni-axial tensile tests. It is found that PBO-FRCM presents high resistance against aggressive environments and specifically preserve its mechanical strength in the presence of salt-water, where other reinforcing materials undergo a dramatic degradation process.

2021 - Engineered Materials for Sustainable Structures [Monografia/Trattato scientifico]
Frassine, Roberto; Nobili, Andrea; Saccomandi, Giuseppe; Signorini, Cesare
abstract

2021 - Epoxy Resins for Interphase Strengthening of Textile-Reinforced Composites for Structural Applications [Capitolo/Saggio]
Signorini, C.; Nobili, A.
abstract

In the field of structural retrofitting and building rehabilitation, inorganic matrix textile-reinforced composite materials offer many interesting advantages, such as resistance to high temperatures and aggressive environments, water vapor permeability, compatibility with traditional supports and reversibility of intervention. Yet, mechanical performance of the reinforcing system strongly depends on the bond quality at the fabric-to-matrix interphase, which is usually unsatisfactory. Indeed, hydration products of the inorganic binder are incapable of penetrating among the multifilament yarn, thus allowing for the so-called telescopic failure. Therein, inner filaments (the core) slide over outer filaments (the sleeve). This failure mode occurs inconsistently and, although imparting ductility, prevents exploitation of the reinforcing potential of the fabric. In this chapter, we discuss a landscape of coating techniques intended to promote fabric-to-matrix adhesion. Design parameters for the coating, such as formulation, viscosity and thickness, alongside an investigation of the system thermal stability, are discussed based on recent advances in the field.

2021 - Mechanical Performance of Fiber Reinforced Cement Composites Including Fully-Recycled Plastic Fibers [Articolo su rivista]
Signorini, Cesare; Volpini, Valentina
abstract

The 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 applications

2021 - Targeting functionalised carbon nanotubes at the interphase of Textile Reinforced Mortar (TRM) composites [Articolo su rivista]
Signorini, C.; Nobili, A.
abstract

Tensile performance of textile reinforced inorganic matrix composites strongly depends on the matrix-to-fabric bond strength, that is the weak chain in the system. In this work, we investigate the role of multi-walled carbon nanotubes (MWCNT) dispersion in an amorphous silica nano-coating for AR-glass and carbon fabric Textile Reinforced Mortar (TRM) composites. Two lime mortars are considered at 56-day curing. Comparative mechanical testing in uni-axial tension show remarkable enhancements in terms of mean ductility, strength and energy dissipation capabilities. Besides, coating successfully hinders telescopic failure and delamination, which significantly narrows data scattering and benefits design limits. Crack pattern analysis reveals that coating promotes diffuse cracking in the specimen, with gradual and progressive damage buildup. Indeed, mean crack width and mean crack spacing are consistently reduced. BET, optical and E-SEM microscopy supports the action mechanism of the coating, that promotes wettability, surface roughening and imparts a remarkable increase in the specific surface area of the reinforcement.

2020 - A new Rayleigh-like wave in guided propagation of antiplane waves in couple stress materials [Articolo su rivista]
Nobili, A.; Radi, E.; Signorini, C.
abstract

Motivated by the unexpected appearance of shear horizontal Rayleigh surface waves, we investigate the mechanics of antiplane wave reflection and propagation in couple stress (CS) elastic materials. Surface waves arise by mode conversion at a free surface, whereby bulk travelling waves trigger inhomogeneous modes. Indeed, Rayleigh waves are perturbations of the travelling mode and stem from its reflection at grazing incidence. As well known, they correspond to the real zeros of the Rayleigh function. Interestingly, we show that the same generating mechanism sustains a new inhomogeneous wave, corresponding to a purely imaginary zero of the Rayleigh function. This wave emerges from "reflection" of a bulk standing mode: This produces a new type of Rayleigh-like wave that travels away from, as opposed to along, the free surface, with a speed lower than that of bulk shear waves. Besides, a third zero of the Rayleigh function may exist, which represents waves attenuating/exploding both along and away from the surface. Since none of these zeros correspond to leaky waves, a new classification of the Rayleigh zeros is proposed. Furthermore, we extend to CS elasticity Mindlin’s boundary conditions, by which partial waves are identified, whose interference lends Rayleigh-Lamb guided waves. Finally, asymptotic analysis in the thin-plate limit provides equivalent 1-D models.

2020 - Designing epoxy viscosity for optimal mechanical performance of coated Glass Textile Reinforced Mortar (GTRM) composites [Articolo su rivista]
Signorini, C.; Nobili, A.; Sola, A.; Messori, M.
abstract

Preliminary epoxy coating of the reinforcing fabric provides an effective approach for improving matrix-to-fabric strength in inorganic matrix composites. We investigate the effect of epoxy resin dilution in acetone on uni-axial tensile performance of coated alkali-resistant (AR) glass fabric embedded in a lime-based matrix. Remarkably, it is found that dilution has a mixed effect on performance and this trend is consistently retrieved for strength, ductility and energy dissipation. Indeed, performance initially decays and then it suddenly raises to a level close to or even exceeding that of the undiluted specimens. It is postulated that this behaviour is caused by resin viscosity, that falls off exponentially with the dilution degree. Once a viscosity threshold is breached, epoxy is capable of penetrating inside the yarn and thereby prevents telescopic failure, that is the sliding of the outer over the inner glass filaments. Furthermore, the interphase surface area increases dramatically and this enhances performance and narrows scattering. Besides, optimal viscosity is reached at an unexpectedly high dilution degree, whence material cost is significantly reduced. A cost-to-performance comparison of common strengthening technologies is presented, which shows that diluted epoxy composites score comparably to FRPs. It is concluded that epoxy coating optimization plays an important role in designing inorganic matrix composites.

2020 - Failure mechanism of silica coated polypropylene fibres for Fibre Reinforced Concrete (FRC) [Articolo su rivista]
Signorini, C.; Sola, A.; Malchiodi, Beatrice; Nobili, A.; Gatto, A.
abstract

This work investigates the effect of a fast, acid-catalysed sol-gel silica nano-coating on the mechanical performance of draw-wire Polypropylene (PP) fibres used as dispersed reinforcement in Fibre Reinforced Concrete (FRC). The failure mechanism is investigated. To this aim, the role of curing time is also considered. Mechanical performance is assessed in pull-out and three-point bending tests of un-notched beams. Coating deeply affects the post-cracking behaviour of FRC, which shifts from brittle (plain concrete), to softening (uncoated) and finally to plastic-softening (coated fibres). Remarkably, 28-day curing improves over 8-day curing in terms of energy dissipation capability for coated fibres only. This suggests that fibre-to-matrix bond enhancement moves the failure mechanism from delamination at the interface to failure in the interphase zone. In the former case, failure is inconsistent and occurs independently from the curing time while in the latter failure depends on the matrix quality.

2020 - Optimal epoxy dilution for epoxy-coated textile reinforced mortar (Trm): An experimental perspective [Relazione in Atti di Convegno]
Signorini, C.; Nobili, A.; Sola, A.; Messori, M.
abstract

The effect of epoxy dilution with acetone on the mechanical performance of epoxy-coated alkali-resistant glass (ARG) fabric embedded in a lime-based mortar is studied experimentally. The mechanical behaviour of the composite system is assessed in uni-axial tensile tests, according to the ICC guidelines. Epoxy is diluted in acetone and several concentrations, namely 10%, 25%, 50%, 75% and 90%, are considered in an attempt to define a decay law for strength, ductility and dissipated energy at failure. Although epoxy-coating promotes a striking improvement of the mechanical performance with respect to the uncoated specimens, epoxy dilution appears to little affect the global response, even at very low epoxy-to-solvent ratios. Actually, a notable increase in the ultimate strength and strain as well as dissipation capacity are evidenced for 75% dilution. Indeed, epoxy resin is able to uniformly impregnate the bundles of the yarns in a very thin layer, still preserving its contribution to the mechanical performance. In fact, wettability of the yarns plays a fundamental role in the mechanical performance of the laminate for it limitates telescopic failure. A threshold of viscosity is identified. Above this limit, the high quality of impregnation overcomes the issue of the reduction of the amount of epoxy resin in the coating. Besides, dilution strongly promotes the ease of application, as a result of the resin viscosity being sharply reduced. Most remarkably and contrarily to common expectation, the specific energy dissipated at failure exhibits a maximum point, whence an optimal dilution ratio exists which best balances interphase strength and ductility.

2019 - Effect of high temperature exposure on epoxy-coated glass textile reinforced mortar (GTRM) composites [Articolo su rivista]
Messori, M.; Nobili, A.; Signorini, C.; Sola, A.
abstract

An experimental investigation on the mechanical performance of epoxy-coated Alkali-Resistant (AR) glass textile reinforced mortar subjected to elevated temperature is presented. Two epoxy coatings are considered, which differ by the hardening agent alone. After 56 days dry curing, specimens are heated up to four different temperatures. After cooling down to ambient temperature, specimens are assessed in uni-axial tensile test according to Annex A of AC434. First cracking strength and elongation, ultimate tensile strength and elongation, cracked and uncracked moduli, transition point location and energy dissipation capability are evaluated. It is found that, in the explored temperature range, degradation is surprisingly mild and strongly dependent on the resin which is taken as coating agent. Indeed, temperature exposure may lead to strength enhancement. This positive outcome takes place at the expense of ductility and it is traced back, through Differential Scanning Calorimetry (DSC), to a post-curing process. Nonetheless, energy dissipation still decreases with temperature and, remarkably, with the same power-law behaviour for both resins. Such behaviour is compatible with a cumulative Weibull distribution, that is adopted in thermal damage models for resins, and it indicates that the underlying damage mechanism indeed operates on the resin at the fabric-to-matrix interface. (C) 2019 Elsevier Ltd. All rights reserved.

2019 - Lime-cement textile reinforced mortar (TRM) with modified interphase [Articolo su rivista]
Signorini, Cesare; Sola, Antonella; Nobili, Andrea; Siligardi, Cristina
abstract

Background: Lack of interphase compatibility between the fabric and the matrix significantly impairs the load-bearing capacity of textile reinforced mortar (TRM). In this study, we consider the application of two inorganic surface coatings for enhancing the interphase bond properties. Methods: Either of two silica-based coatings, namely nano- and micro-silica, were applied to alkali-resistant glass (ARG) and to hybrid carbon–ARG woven fabric. Mechanical performance of TRM reinforced with the uncoated and the coated fabric was compared in uniaxial tensile tests. Results: Mechanical testing provides evidence of a remarkable enhancement in terms of ultimate strength and deformability for the coated specimens. This effect can be ascribed to the improved hydrophilicity of the fibers’ surface and to the activation of pozzolanic reaction at the interphase. In addition, penetration of nano- and microparticles in the bundle of the textile yarns reduces the occurrence of telescopic failure.

2019 - Sustainable mineral coating of alkali-resistant glass fibres in textile-reinforced mortar composites for structural purposes [Articolo su rivista]
Signorini, Cesare; Nobili, Andrea; Siligardi, Cristina
abstract

The mechanical performance of a silica-based mineral nano-coating applied to alkali-resistant glass textile-reinforced composite materials aimed at structural strengthening is investigated experimentally. The silica nano-film is directly applied to the alkali-resistant glass fabric by sol–gel deposition. Two lime mortars are adopted as embedding matrix, which differ by the ultimate compressive strength and elongation. Uni-axial tensile tests of prismatic coupons are carried out according to the ICC AC434 guidelines. Remarkable strength and ductility enhancements could be observed in the silica-coated group, as compared to the uncoated group, for both mortar types. Digital image correlation, electron scanning and optical microscopy provide evidence of improved interphase strength. X-ray diffraction of the anhydrous mortars brings out the role of the mineralogical composition of the embedding media on the overall bonding properties of the composites. Consideration of design limits and energy dissipation capabilities reveals the crucial role of matrix ductility in bringing the contribution of interphase enhancement to full effect. We conclude that best performance requires optimizing the pairing between fabric-to-matrix adhesion and matrix ductility

2018 - Mechanical performance and crack pattern analysis of aged Carbon Fabric Cementitious Matrix (CFRCM) composites [Articolo su rivista]
Signorini, Cesare; Nobili, Andrea; Falope, Federico O.
abstract

We discuss the effect of environmental exposure on mechanical performance of impregnated Carbon Fabric Reinforced Cementitious Matrix (CFRCM) composite. Following the recently published ICC-ES AC434 guidelines, mechanical performance of prismatic composite specimens is determined on the basis of tensile uni-axial tests. Exposure to saline and alkaline aqueous solutions is considered at 28- as well as 60-day curing time. Special emphasis is placed on crack pattern evaluation as a mean to gain better insight into matrix/fabric bond quality. To this aim, the evolution of the average crack spacing and of the average crack width is determined as a function of strain for all test environments and curing times. It is found that curing time plays a significant role in mitigating the detrimental effect of aggressive environments. Furthermore, the average crack spacing provides a very reliable measure of matrix/fabric bond degradation at all test stages.

2018 - Mechanical performance of epoxy coated AR-glass fabric Textile Reinforced Mortar: Influence of coating thickness and formulation [Articolo su rivista]
Messori, Massimo; Nobili, Andrea; Signorini, Cesare; Sola, Antonella
abstract

The mechanical performance of epoxy coated AR-glass fabric reinforced composite is investigated. A three-stage manufacturing process is considered, which involves fabric surface functionalization, liquid coating deposition and long-term setting and finally fabric embedment in the mortar matrix. Two epoxy coatings are considered, which only differ by the hardening agent. However, coating thickness is significantly diverse as a result of modified viscosity during liquid deposition. Performance is assessed in uni-axial tension as well as in three-point bending and it is expressed in terms of strength curves, data dispersion, crack pattern and failure mechanism. Remarkably, despite being very similar, the analyzed coatings produce a significantly different performance, especially when data dispersion is incorporated and design limits are considered. Indeed, although both coatings are able to consistently deliver fabric rupture at failure, only the thinnest is associated with small data scattering and an almost plastic post-peak behavior in bending. The associated design elongation limit reaches the maximum allowed value according to the ICC guidelines. In fact, it appears that coating thickness plays a crucial role in determining mechanical performance and fabric flexibility. The proposed manufacturing process proves extremely effective at enhancing matrix-to-fabric adhesion and thereby prevent telescopic failure.

2018 - Silica coating for interphase bond enhancement of carbon and AR-glass Textile Reinforced Mortar (TRM) [Articolo su rivista]
Signorini, C.; Nobili, A.; Cedillo González, E. I.; Siligardi, C.
abstract

In this paper, we investigate the effect of silica nano-coating for interphase bond enhancement on the mechanical performance of Textile Reinforced Mortar (TRM) composite materials aimed at structural rehabilitation and strengthening. Alkali-resistant glass (ARG) and carbon fabric reinforcements are preliminarily treated via sol-gel deposition of SiO2coating to promote bond formation capability with the mortar matrix. Optical and electron microscopy provide evidence of interphase bond enhancement. Mechanical performance is assessed both in traction, through uni-axial elongation of prismatic coupons, and in flexure, by three-point bending of laminated masonry bricks. Results are given in terms of mean strength curves, ultimate and design strength and strain values, cracked and uncracked moduli, mean crack spacing, mean crack width and energy dissipation. It is shown that mean absolute performance of silica coating offers a significant improvement over uncoated fabric, yet it is inferior to that of specimens which have been treated with a liquid partially-organic adhesion promoter (polymer coating). However, when design values are considered which incorporate the dispersion of experimental data, silica coating proves superior or at least equivalent to polymer coating, respectively for carbon and ARG fabric. These promising results describe the first application of silica nano-coating to fabric reinforced composite materials.

2017 - On the effect of curing time and environmental exposure on impregnated Carbon Fabric Reinforced Cementitious Matrix (CFRCM) composite with design considerations [Articolo su rivista]
Nobili, Andrea; Signorini, Cesare
abstract

This 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.