Transverse reinforcement optimization of a precast special roof element through an experimental and numerical procedure

The transverse behavior of a long span three-plate precast roof element is investigated by means of an experimental and numerical research. The performed study highlights that the failure mode of this folded-plate element is strongly influenced by the amount of transverse reinforcement in the wings. This latter is usually designed through simplified methods, which often lead to over-dimensioning in terms of steel welded mesh. To avoid excessive costs for the producers, transverse reinforcement optimization should be required. In this work, a non-linear FE modelling was applied for this purpose. The reliability of the followed numerical procedure was first verified by an initial type testing (i.e. experimental load test up to failure). The agreement between numerical and experimental results showed the efficiency of the model in simulating all the main sources of non-linearity related to both material behavior and element geometry. Numerical analyses were so used to perform a parametric study as a function of transverse reinforcement amount, aimed at determining a coefficient of “model inaccuracy”. This coefficient should be used as a correction factor for the element design in routine calculations based on beam theory

Experimental investigation on the mechanical behaviour of AAC blocks for sustainable concrete masonry.

To satisfy the increasing demand of energy efficient buildings, AAC manufacturers are nowadays encouraged to produce blocks with ever lower densities. However, a compromise between energy-saving requirements and mechanical performances is needed to ensure structural safety, as well as an adequate structural durability. This paper reports a comprehensive experimental study on AAC mechanical properties (compressive and tensile strengths, as well as fracture energy), and on their dependency from material density and moisture content. The collected data are compared with some well-known analytical relations taken from the literature, which are often used for the calibration of mechanical parameters required for mathematical and/or finite element modelling of AAC load-bearing masonry, as well as of AAC masonry-infilled framed structures. These comparisons highlight some critical issues in the formulation of analytical relations having a general applicability; however, it was found that RILEM suggestions are appropriate for the considered AAC productions, at least for densities greater than 400 kg/m3

Experimental investigation on the mechanical behaviour of AAC blocks for sustainable concrete masonry

To satisfy the increasing demand of energy efficient buildings, AAC manufacturers are nowadays encouraged to produce blocks with ever lower densities. However, a compromise between energy-saving requirements and mechanical performances is needed to ensure structural safety, as well as an adequate structural durability. This paper reports a comprehensive experimental study on AAC mechanical properties (compressive and tensile strengths, as well as fracture energy), and on their dependency from material density and moisture content. The collected data are compared with some well-known analytical relations taken from the literature, which are often used for the calibration of mechanical parameters required for mathematical and/or finite element modelling of AAC load-bearing masonry, as well as of AAC masonry-infilled framed structures. These comparisons highlight some critical issues in the formulation of analytical relations having a general applicability; however, it was found that RILEM suggestions are appropriate for the considered AAC productions, at least for densities greater than 400 kg/m3

EVALUATION OF CRACK WIDTH IN RC TIES THROUGH A NUMERICAL "RANGE" MODEL

The problem of cracking in reinforced concrete (RC) tensile members has been studied extensively in the past, not only for the analysis of tension zones, but also for understanding and modeling the behavior of beams in bending. Despite the large number of published studies, there is still no agreement on the relative importance of the most critical parameters influencing crack width and spacing (especially bond-slip and stress diffusion in concrete cover), as proved by the development of more than twenty different formulae available in technical literature [1]. Aim of this work is to investigate if a model based exclusively on bond-slip is able to predict correctly crack width and spacing or if the contribution of stress diffusion in concrete cover - which is included in several design Codes and in some numerical or analytical approaches – must be considered. To this purpose, a one-dimensional numerical model based on bond between steel and concrete is here developed for analyzing the behavior of RC tension ties, by also taking into account the influence of bond deterioration near crack surfaces. To consider the uncertainty of crack pattern evolution, the model provides a range of crack widths and spacing that, according to bond theory, are possible for a given load. The effectiveness of the proposed procedure is verified through comparisons with significant experimental results on RC tension members available in the technical literature [2-3], both in terms of global behavior and in terms of crack width and crack spacing evolution as loading increases. These comparisons prove that bond deterioration improves the results and that the proposed approach can be successfully adopted for design purposes, since it provides a correct estimate of maximum crack width. The obtained results are also compared with Codes provisions and the effectiveness of different approaches for predicting crack width is analyzed and discussed. References [1] Borosnyoi A, Balazs GL. Models for flexural cracking in concrete: the state of the art. Struct Concr, 2005; 6(2): 53-62. [2] Wu HQ, Gilbert RI. An experimental study of tension stiffening in reinforced concrete members under short-term and long-term loads. In: UNICIV Report No. R-449, 2008, The University of New South Wales, Sidney, Australia. [3] Gijsbers FBJ, Hehemann AA. Enige trekproven op gewapend beton (Some tensile tests on reinforced concrete). In: Report BI-77-61, 1977, TNO Inst for Building Mat and Struct, Delft, The Netherlands

fracture toughness of fibre reinforced concrete determined by means of numerical analysis

Abstract As is well-known, the addition of fibres to concrete mix (Fibre Reinforced Concrete, FRC) produces a positive effect on cracking behaviour. In this work, the results of an experimental campaign on FRC specimens with randomly distributed micro-synthetic polypropylene fibrillated fibres are examined. The tests concern single-notched beams under three-point bending, where the fibre content varies. Such an experimental testing is numerically analysed through a non-linear finite element model, named 2D-PARC, where a proper constitutive law for fibre-reinforced concrete is implemented. The load-crack mouth opening displacement (CMOD) curves numerically obtained are employed to determine the critical stress-intensity factor (fracture toughness) for different values of fibre content, according to the two-parameter model. The comparison between such numerical results and those obtained by applying the two-parameter model to the experimental load-CMOD curves is performed

Fracture behavior of concretes containing MSWI vitrified bottom ash

The incorporation of waste materials into concrete allows responding to some of the most significant issues of our society: waste management and climate change. Experimental studies carried out in last decades have shown that municipal solid waste incineration (MSWI) ash, and particularly bottom ash, which constitutes the major solid by-product of incineration process, can be adopted to produce building materials. However, several issues are related to the safety and the environmental impact of MSWI ash utilization for concrete production, mainly linked with the leaching of heavy metals and toxic organic components. To solve these problems, several treatments for MSWI ash can be adopted and, among them, in this work the attention was focused on vitrification technology, which enables to convert the ash in a glassy inert solid material. The aim of the present paper is to study the feasibility of developing a “green concrete” that incorporates vitrified MSWI bottom ash as partial cement replacement, so reducing the cement content and consequently the carbon dioxide emissions as well as the raw materials consumption related to its production. The vitrified MSWI bottom ash, ground at micrometer size, was inserted into the admixtures by considering two percentages of cement substitution (10% and 20% by weight of cement). The flexural behavior of concrete containing vitrified MSWI ash was investigated through three-point bending tests under crack mouth opening displacement control. The crack path evolution was further explored by adopting the Digital Image Correlation technique. By analyzing the obtained results, it can be concluded that the use into concrete of vitrified MSWI bottom ash as cement replacement up to a percentage of 20% by weight of cement, allows reaching comparable flexural resistances with respect to the reference concrete. So, the proposed approach can represent a viable solution for the development of environmental-friendly concretes able to reduce the environmental impact of the concrete industry, which is mostly related to cement production, as known

Mechanical characterization of different biochar-based cement composites

Abstract The attention on the use of raw materials, the energy consumption as well as carbon dioxide production of cement factories are boosting the experimentation on innovative and sustainable materials in concrete technology. In recent years, biochar has become an emblematic material with a thousand facets. Mainly investigated up to now as amending in the agricultural field, biochar can be explored as a building material due to its innumerable properties. Indeed, several applications have been studied to use it as a filler to modify the nanogranular nature of the cement matrix, or as a substitute for clinker, aggregates and clay, reducing the carbon footprint and the emissions of greenhouse gases linked to the production processes of cementitious materials. In this paper, nano/micro-particles of biochar, the solid by-product from the gasification process of biomass derived from wood waste, has been used in different cement composites aiming at determining the optimal percentage of addition while trying to guarantee an improvement of mechanical properties. The results showed that an optimized percentage of biochar nano/micro-particles can increase the strength and toughness of the composites

Biochar-based cement pastes and mortars with enhanced mechanical properties

Nowadays, the environmental impact of cementitious material industry and more generally of building activities is matter of concern, especially in terms of their effects on climate change and consumption of natural resources. Within this context, the aim of this paper is the investigation of the role of biochar, a solid carbonaceous by-product material resulting from biomass pyrolysis/gasification of residual biomass, as a sustainable ingredient for the production of cementitious materials, combining carbon sink properties with enhanced mechanical properties. Although biochar is mainly investigated as agricultural amendment, there is also evidence that biochar may be a eco-friendly material to enhance the sustainable performance of cementitious materials. As outlined in literature, biochar can be used as filler to modify the nanogranular nature of cement matrix, or as substitute of clinker to reduce the emissions of greenhouse gases related to cement production. In this work, biochar is added as micro-nano particles in different cementitious composites, i.e. cement pastes and mortars, as a function of filler or partial substitute of cement. The main mechanical properties of biochar-based materials are then investigated to determine the optimal percentage of biochar addition

Sviluppo di un modello non lineare per elementi in C.A. ordinario e fibrorinforzato finalizzato allo studio di rivestimenti di gallerie in presenza di incendio

Nel presente lavoro di tesi è stato sviluppato un modello costitutivo non lineare esistente per l’analisi fino a rottura del comportamento di elementi in c.a. ordinario e fibrorinforzato. In particolare, tale modello è stato qui esteso ed applicato allo studio dei rivestimenti di gallerie in calcestruzzo in condizioni di incendio. Il modello, che segue un approccio di tipo “smeared” e prevede l’applicazione del metodo di decomposizione delle deformazioni in fase fessurata, è formulato in termini di matrice di rigidezza secante, per consentirne l’implementazione all’interno del codice ad elementi finiti adottato. Il modello, denominato 2D-PARC, è stato inizialmente rivisto per quanto concerne la modellazione del contributo resistente offerto dal calcestruzzo, in modo da descrivere correttamente il comportamento del materiale sotto stati di sforzo pluriassiali, sia in fase pre che post-fessurata. A tal fine è stato adottato un approccio basato sull’elasticità isotropa non lineare, che, seppur caratterizzato da un’elevata efficienza computazionale, è al contempo in grado di fornire una rappresentazione molto accurata del comportamento del calcestruzzo. Tale formulazione permette inoltre di simulare gli importanti fenomeni di dilatazione e crushing del materiale in compressione. Successivamente il modello è stato esteso per poter mettere in conto le deformazioni impresse del calcestruzzo, a partire da quelle dovute al ritiro. A tal fine le equazioni di equilibrio e di congruenza che governano il problema sono state opportunamente riscritte, riformulando di conseguenza la matrice di rigidezza del materiale e aggiornando le procedure richieste per il soddisfacimento della convergenza interna dell’algoritmo. Le deformazioni da ritiro sono state valutate in funzione del gradiente di umidità presente nell’elemento. In particolare, quest’ultimo è stato ottenuto eseguendo un’analisi termica equivalente, che sfrutta il parallelismo tra le equazioni di diffusione che governano il problema termico e quello dell’umidità. Al fine di validare sia la nuova modellazione adottata per la descrizione del comportamento del calcestruzzo che l’inserimento delle deformazioni da ritiro, sono stati effettuati numerosi confronti con le evidenze sperimentali disponibili in letteratura tecnica relative a svariate tipologie strutturali in c.a., sia ordinario che fibrorinforzato. Da ultimo, il modello è stato applicato allo studio dei rivestimenti in calcestruzzo di gallerie in condizioni di incendio. Il problema è stato studiato mediante analisi numeriche tridimensionali agli elementi finiti, in cui il comportamento del rivestimento è stato simulato attraverso il legame costitutivo 2D-PARC. A tal fine si è resa necessaria un’ulteriore estensione del modello, in modo da tenere debitamente in conto sia l’influenza delle deformazioni termiche (inserite nell’algoritmo in modo del tutto analogo a quanto già effettuato per il ritiro), quanto il degrado delle caratteristiche del materiale alle alte temperature. Lo stato tenso-deformativo nel rivestimento è stato quindi valutato simulando in maniera realistica tanto le fasi di scavo e successiva installazione del rivestimento – attraverso l’adozione di una procedura step-by-step – quanto la successiva fase di incendio, operando un’analisi non lineare termo-meccanica sequenziale accoppiata. I risultati della modellazione sono stati quindi confrontati con quelli forniti da soluzioni in forma chiusa disponibili in letteratura; in particolare, per validare la modellazione durante l’incendio, è stato sviluppato un opportuno modello analitico ad hoc. E’ stato inoltre condotto uno studio parametrico finalizzato ad investigare l’influenza, sul comportamento strutturale del rivestimento in fase di incendio, di alcuni parametri significativi, quali ad esempio le proprietà meccaniche del terreno, la profondità della galleria e il tipo di incendio considerato.This work deals with the development of an existing nonlinear model for the analysis up to failure of reinforced concrete and fiber reinforced concrete (RC and FRC) elements, as well as its extension to the study of concrete lining behavior under fire conditions. The model, which belongs to the smeared approaches and applies a strain decomposition procedure in the cracked stage, is formulated in terms of a secant stiffness matrix to be implemented into a finite element (FE) code. In more detail, the model, named 2D-PARC, is firstly revised with regard to the modeling of concrete contribution, so as to correctly describe the behavior of the material under a general biaxial state of stress both before and after cracking development. The adopted formulation, based on isotropic non-linear elasticity, is able to provide a sophisticated representation of concrete behavior, being at the same time characterized by a high numerical feasibility. In this way, the computational efficiency of 2D-PARC is highly improved. Moreover, the significant phenomena of crushing and dilatation of concrete in compression are inserted in the algorithm. Subsequently, the model is extended to include initial deformations of concrete, which were not considered in the original formulation. At first, shrinkage strains are inserted in the algorithm. To this aim, a new set of equilibrium and compatibility equations is written and the material secant stiffness matrix is consequently rearranged and implemented into the adopted FE code, by properly updating the internal convergence procedures. Shrinkage strains are computed as a function of the moisture field in the element, which is in turn obtained by performing an “equivalent” thermal analysis, by exploiting the analogy between the equations governing the moisture and the thermal problem. Extensive comparisons with data from the literature are provided to validate first the enhanced concrete modeling and subsequently the inclusion of shrinkage strains. Plain concrete specimens, as well as RC and SFRC elements belonging to different structural typologies are considered, so as to validate the proposed model over a wide range of conditions. Finally, the model is further extended and applied to the structural assessment of concrete linings under fire conditions. Concrete lining behavior is then analyzed by performing 3D FE simulations where mechanical nonlinearity is taken into account through 2D-PARC constitutive law. To this aim, thermal strains are inserted into the algorithm by following the same procedure adopted for shrinkage deformations. At the same time, the decay of strength and stiffness of concrete during heating is considered, by inserting the dependence of the main mechanical properties on temperature. In order to realistically represent the soil excavation phase and the subsequent lining installation, a step-by-step procedure is followed; whereas a sequentially coupled thermo-mechanical procedure is applied for the fire stage. A heat-transfer analysis is carried out so as to obtain the temperature field under an assigned fire scenario; then the simulation of tunnel behavior under fire conditions is performed on the basis of the obtained results. The stress-strain field is then evaluated by taking into account both the temperature-induced effects and the ground-structure interaction. The effectiveness of the proposed model is verified through comparisons with analytical closed-form solutions. The accuracy of the proposed approach in simulating the excavation and lining installation phases is verified by comparing FE results to well-known analytical relations available in the literature. Moreover, a simplified analytical model is developed on purpose, so as to validate the simulation of concrete lining under fire conditions. Finally, a parametric study is also performed in order to better highlight the influence exerted by different parameters on the structural response of the lining during fire

Experimental research on mechanical properties of biochar-added cementitious mortars

In recent years, biochar, the solid by-product resulting from biomass pyrolysis or gasification, has been mainly studied and applied as soil amendment, while research on its application as a building material is still scanty. The rising interest in this context is mainly related to the chance of reducing the emission of greenhouse gases that have serious environmental impacts and are responsible for the climate change. Since biochar is mainly composed by carbon, the aim is to obtain smart materials able to capture and store carbon in buildings for decades in a stable form. This paper aims to prove the feasibility of using biochar derived from agro-forestry waste residues as carbon sequestrating additive in cement mortar, by adding it, during mixing, at 1% by weight of cement. In order to assess the efficiency of biochar-added mortar as building material, its mechanical properties have been properly investigated; the results show that the addition of char in the admixture leads to comparable compressive strength, flexural strength, toughness and ductility with respect to traditional cement mortars