Fire Spalling Prevention via Polypropylene Fibres: A Meso-and Macroscale Approach

A deep understanding of concrete at the mesoscale level is essential for a better comprehension of several concrete phenomena, such as creep, damage, and spalling. The latter one specifically corresponds to the separation of pieces of concrete from the surface of a structural element when it is exposed to high and rapidly rising temperatures; for this phenomenon a mesoscopic approach is fundamental since aggregates performance and their thermal properties play a crucial role. To reduce the risk of spalling of a concrete material under fire condition, the inclusion of a low dosage of polypropylene fibres in the mix design of concrete is largely recognized. PP fibres in fact evaporate above certain temperatures, thus increasing the porosity and reducing the internal pressure in the material by an increase of the voids connectivity in the cement paste. In this work, the contribution of polypropylene fibres on concrete behaviour, if subjected to elevated thermal ranges, has been numerically investigated thanks to a coupled hygrothermomechanical finite element formulation. Numerical analyses at the macro- and mesoscale levels have been performed

A THREE-INVARIANT, EXPERIMENTAL BASED, ELASTOPLASTIC FORMULATION FOR STEEL MATERIALS

The pressure sensitivity of metals is addressed in this work. A series of experimental tests, considering different load scenarios able to activate non- deviatoric stress tensor invariants, were performed on steel solid round bars [1, 2]. The load was applied to the specimens in the form of uniaxial tension/compression and torsion combinations which will result in combined tensile and shear stresses that will maintain a constant ratio during the experiment. A digital image correlation system was used to monitor the experiments and provide the displacement field and the strain field distribution at different stages of the tests. This photogrammetric non-contact device was mounted in front of the painted sample, for continuous recording of the relative movement of black dots on the white specimen during the deformation. 3D analyses were carried out and the need to expand the von Mises elastoplastic constitutive model, with an associated flow rule, to a formulation that takes into consideration also the first invariant of the stress tensor and the third invariant of the deviatoric stress tensor was highlighted; so reproducing the main features of the experimental results. A more sophisticated hardening law was proposed as a function of three invariants of the plastic strain tensor. Plastic flow rule was derived by integration of the plastic strain rate through the physical domain of the specimen (elaboration of DIC results) and expressed in terms of the three invariant formulation of the stress tensor to activate the proposed hardening law

Effect of aggregates and ITZ on visco-damaged response of concrete at the meso scale level

A deep knowledge on the behavior of concrete materials at the mesoscale level requires, as a fundamental aspect, to characterize aggregates and specifically, their thermal properties if fire hazards (e.g. spalling) are accounted for. The assessment of aggregates performance (and, correspondingly, concrete materials made of aggregates, cement paste and ITZ –interfacial transition zone-) is crucial for defining a realistic structural response as well as damage scenarios. Particularly, it is assumed that concrete creep is associated to cement paste only and that creep obeys to the B3 model proposed by Bazant and Baweja since it shows good compatibility with experimental results and it is properly justified theoretically. The fully coupled 3D F.E. code NEWCON3D has been adopted to perform meso-scale analyses of concrete characterized by aggregates of different types and different thermal conductivities. Damage maps allows for defining an appropriate concrete mixture for responding to spalling and for characterizing the coupled behaviour of ITZ as well

New results in 3D-mesomechanical coupled analysis of external sulphate attack in concrete

External Sulphate Attack (ESA) is one of the main degradation processes affecting concrete structures. It takes place when the concrete is in an environment rich in sulphate ions and with a high humidity index. Once it has penetrated the concrete, sulphate undergoes chemical reactions that lead to the precipitation of expansive ettringite crystals that cause volumetric expansions of the cement paste/mortar, eventually leading to cracking and damage. The FE analysis is undertaken by representing concrete as composed by aggregate pieces inserted in a cement/mortar matrix. Zero-thickness interface elements are pre-inserted to represent potential fractures along all aggregate-matrix as well as along some selected matrix-matrix element contacts. A fracture-based non-linear constitutive law proposed by Carol et al. (1997) and later developed by Caballero et al. (2006) is used for the interface elements. Concerning the reactive transport problem, the model follows the initial work by Tixier and Mobasher (2003), which was introduced in a mesomechanical analysis with interfaces by Idiart et al. (2011) in 2D, and later in 3D (but no coupling) by Pérez et al (2017). In the present paper, the effects of the coupling between mechanical and diffusion/reaction in 3D, due to the cubic law of the interface diffusivity, are demonstrated. The new results obtained confirm that, also in 3D, penetration of ions, expansive reactions as well as subsequent cracking and degradation, all take place much faster when the coupling effect due to the open interfaces is introduced

Mesoscale modelling of concrete as a multiphase material

In recent years, thanks to upgraded computational resources, concrete has started being modeled as porous medium at 3D meso level, distinguishing in the multiphase system the role of aggregates, cement paste and interfacial transition zone (ITZ). A deep knowledge on the behaviour of concrete materials at the mesoscale level requires, as a fundamental aspect, to characterize aggregates and specifically, their thermal properties if fire hazards (e.g. spalling) are accounted for. The assessment of aggregates performance (and, correspondingly, concrete materials made of aggregates, cement paste and ITZ) is crucial for defining a realistic structural response as well as damage scenarios. A meso-scale approach has been here followed to study concrete behaviour under normal and high temperatures via the 3D fully coupled thermo-hydro-mechanical model developed at Padua University, called NEWCON3D. Particularly, it is assumed that concrete creep and damage are associated to cement paste and ITZ only and that creep of concrete obeys to the B3 model proposed by Bažant and Baweja, instead damage obeys to the Mazars’ damage law with non-local correction. Therefore several numerical analyses at the mesolevel have been carried out: firstly the role of the ITZ and of the aggregates on the hygro-thermal response of concrete have been investigated, highlighting the barrier effect covered by aggregates towards the flux of humidity; subsequently the visco-damaged behaviour of concrete at the meso level is investigated, to understand the influence of ITZ and aggregates on the overall mechanical behaviour at medium temperatures. Indeed, these two components are crucial for defining a realistic structural response as well as damage scenarios allowing to define an appropriate concrete mixture to withstand spalling. Finally, the study of concrete under high temperature conditions, to catch the “shape effect”, comparing columns of different section at the macro level, and the crucial role of the aggregates and the ITZ on the real evolution of cracking, have been performed.Negli ultimi anni, grazie alle attuali risorse di calcolo, si è iniziato a modellare il calcestruzzo come un mezzo poroso al meso livello, distinguendo nel sistema multifase il ruolo degli aggregati, della pasta di cemento e dell’interfacial transition zone (ITZ). Una profonda conoscenza del comportamento del calcestruzzo al mesoscala richiede, come aspetto fondamentale, la caratterizzazione degli aggregati ed, in particolare, delle loro proprietà termiche, nel caso in cui vi siano rischi di incendio (e quindi di spalling). La valutazione delle prestazioni degli aggregati (e conseguentemente, di calcestruzzi come composti da inerti, pasta di cemento ed ITZ) è cruciale per la definizione sia di una risposta realistica strutturale, sia degli scenari di danno. In questo lavoro si è quindi seguito un approccio al mesoscala per studiare il comportamento del calcestruzzo, in condizioni di temperatura normale ed elevata, tramite un modello tridimensionale igro-termo-meccanico totalmente accoppiato sviluppato presso l’Università di Padova, chiamato NEWCON3D. Nello specifico, si è assunto che i fenomeni di viscosità e di danno fossero associati solo alla pasta di cemento e all’ITZ (per gli aggregati si assume un comportamento elastico) e che il creep obbedisse al modello B3 proposto da Bažant e Baweja, invece il danno alla legge di Mazars con la correzione non locale. Si sono pertanto condotte numerose analisi numeriche al meso livello: in primo luogo si è esaminato il ruolo dell’ITZ e degli aggregati sulla risposta igro-termica del calcestruzzo, mettendo in evidenza l'effetto barriera esercitato dagli aggregati sui flussi di umidità; successivamente si è indagato il comportamento visco-danneggiato del calcestruzzo al mesoscala, al fine di comprendere l'influenza dell’ITZ e degli aggregati sulla risposta meccanica globale a temperature medie. In realtà, come già detto precedentemente, queste due componenti sono molto importanti per ottenere una risposta realistica strutturale e per l’individuazione dei possibili scenari di danno, permettendo quindi di definire una miscela di calcestruzzo appropriata, in grado di resistere allo spalling. Infine, vi è uno studio del calcestruzzo in condizioni di temperatura elevata, al fine di catturare l '"effetto forma", confrontando due colonne di sezione differente al macro scala, ed il ruolo cruciale degli aggregati e dell’ITZ sull'evoluzione reale del danno