82 research outputs found
Effect of hot and cold temperature on the mechanical behaviour of macro-synthetic fiber reinforced concretes
No full textFiber reinforced concretes (FRCs) are used in many civil engineering applications, i.e. industrial floors, tunnels, etc., given their ability of reducing concrete cracking. Besides their mechanical characteriza-tion, the effect of the environmental conditions, i.e. temperature, need to be properly understood.
The present paper presents results of experimental tests on the effects of hot and cold temperatures, from -30 °C to + 60 °C, on the mechanical performances of FRCs containing polypropylene fibers. The results show that the temperature affects the compressive, tensile and residual flexural strength of the material
Study of temperature variation effect on MSFRC long term behaviour
No full textThe long-term behaviour of Fiber Reinforced Concrete is a topic still under investigation. Apart from the conditions related to the serviceability states, environmental conditions affect the material behaviour. The present work is focused on macro-synthetic fibres used as reinforcement. In fact, polymers suffer from creep and temperature affects their mechanical characteristics. Therefore, it is important to understand and develop predictive models for these phenomena. The paper presents the results of an experimental campaign on cracked FRC cylinders in uniaxial tensions, under sustained loads at temperatures from 20°C to 40°C. Moreover, a first approach towards a numerical predictive model accounting these phenomena is described. / Il comportamento dif-ferito nel tempo di materiali compositi fibrorinforzati è un tema ancora oggetto d’indagine. Oltre alle condizioni di servizio, le con-dizioni ambientali influiscono sul comportamento del materiale. Il presente lavoro fa riferimento al comportamento di calcestruzzi rinforzati con fibre macro-sintetiche. I polimeri infatti presentano deformazioni viscose e le loro proprietà meccaniche sono influen-zate dalla temperatura. Risulta quindi importante comprendere e prevedere come questi fenomeni possano influenzare il comporta-mento di elementi strutturali e non strutturali. La memoria mostra i risultati di una campagna sperimentale su provini fessurati in cal-cestruzzo fibrorinforzato in regime di trazione, mantenuti sotto carico costante ed esposti a temperature crescenti tra 20 °C e 40° C. Inoltre, è descritto un primo approccio verso la calibrazione di un modello numerico predittivo dell’effetto di queste variabili sul comportamento del materiale
Experimental investigation and numerical modelling of macro-synthetic fibre reinforced concrete materials
No full textFibre Reinforced Concretes are innovative composite materials covering a plenty of structural applica-tions. The mechanical behaviour of these materials has been studied by many researchers, and many official guidelines cover their usage. Nevertherless, many aspects are still under investigation above all its long term performance and time dependent phenomena. The whole research project is aimed at un-derstanding which ascpets, that could be represented by the material properties or external factors, could affect the mechanical behaviour. The investigation is carried on by means of experimental tests on FRCs characterized by different strengths of the matrix, different fibre dosages and different fibre types. The experimental part is the starting point for the elaboration of a numerical predictive model because the prediction of the cracking mechanisms evolution typical of FRCs is the key for the design with fibre reinforced concrete materials. On the other side, the time dependent phenomena affect the behaviour and the serviceability state of the material. The time and the environmental conditions play an important role in the deformations evolution over time. In a first step, the experimental investigation is carried analysing the effect of compressive, tensile and flexural load aimed at studying the separated contribu-tions. The hetereoneneity of the concrete mixed with the discontinuity of the reinforcement need to be accounted. For this reason, based on the experimental results achieved, a numerical predictive model is going to be elaborate. It is straightforward to think that the composite nature makes necessary the char-acterization of the fibrous reinforcement, concrete matrix and their mechanical bond interaction. The present work shows a part of the entire research project. In particular, it discusses the effect of macro-synthetic fibres in cementitious matrixes analysing their short term mechanical performance. Three concrete strength classes, from 33 MPa to 55 MPa are combined with 2 kg/m3 and 4 kg/m3 of fibre dosages. The experimental results are used to calibrate a numerical predictive model based on the Lat-tice Particle Discrete Model constitutive law
High Performance Fibre Reinforced Concrete: optimization of the concrete matrix with porcelain stoneware powder
No full textUHPFRCs feature high performance in terms of compressive strength (> 150 MPa – 21755 psi) and tensile
strength (> 5 MPa – 725 psi). Compared to normal concrete, their matrix is designed with lower water/cement ratios (
0.25), higher cement content and smaller aggregate size. Since their crack pattern is typically characterized by small
cracks, the use of short fibers to reinforce the matrix is more appropriate. In general, UHPFRC production requires plenty
of non-renewable natural raw materials with polluting emissions mainly related to the high dosage of cement. In this
framework, the presented work proposes the usage of porcelain stoneware waste powders from rectification, as part of
fine-grained components. The paper presents preliminary results of an experimental campaign aimed at optimizing the
flexural and compressive performances of concrete admixtures with porcelain stoneware waste materials, using design of
experiments. All the admixtures contained the same quantity of PVA fibers. The performances of the mixes were
evaluated by means of four point bending and compressive tests, performed after 28 and 60 days. Introducing stoneware
waste powders it was possible to obtain high strength materials with a reduced cement content
EFFECT OF FIBER DOSAGE AND MATRIX COMPRESSIVE STRENGTH ON MSFRC PERFORMANCE
No full textThe design of MSFRC structural elements requires a proper knowledge of the relationship between fiber dosage and flexural residual strength. For this reason, the present study analyses the effect of fiber dosage and compressive strength of concrete on MSFRC cracked behavior. In particular, the influence of the aforementioned parameters was investigated through an experimental campaign, based on three-point bending tests, on specimens characterized by four different fiber dosages, ranging from 2 kg/m3 to 8 kg/m3 and three different concrete mixes, with strengths spanning from 33 MPa to 55 MPa.
The analysis of the experimental results has highlighted that, for the macro-synthetic fibers considered, concrete compressive strength has no significant effect on residual flexural tensile strength. Moreover a very high correlation between residual flexural strength and number of fibers across the cracked specimens surface, has been observed. Finally, a strength predictive model was defined using the experimental data
Lattice Discrete Particle Modeling of MSFRC material
No full textFiber Reinforced Concrete (FRC) is an innovative composite material whose important capabilities are currently investigated both experimentally and numerically. The development of a reliable predictive numerical model requires the careful consideration of concrete heterogeneity and random or oriented arrangement of fibres reinforcement. Among the most validated theories, the Lattice Particle Discrete Model (LDPM) effectively simulates the concrete composite behaviour derived from the interaction of aggregates: the meso-structure of the quasi-brittle material is designed with polyhedral particles reflecting the actual coarse aggregate distribution. The LDPM theory was previously extended to account for the fibers crack-bridging effect, typical of FRCs. In this first step of a larger study, the short-term capabilities of the model are studied before investigating the sustained load response. Experimental results from FRC with 8 kg/m3 of polypropylene (PP) macrofibres are used to calibrate the numerical model, presenting many considerations preliminary the model validation
Insights on Lattice Discrete Particle Model Calibration and Validation Procedure to Simulate Polypropylene and Steel Fibre-Reinforced Concrete
No full textThe use of fibre-reinforced concrete (FRC) has been substantially increasing in the last few years, in different fields of the construction industry. Recently, many experiments have been performed to observe the short- and long-term mechanical behaviour of FRC, and several models have been formulated to capture its mechanical response. In this work, the mechanical behaviour is simulated through the Lattice Discrete Particle Model (LDPM) and its extension to fibre-reinforced cementitious composites (LDPM-F). This paper aims to provide insights into the calibration process and potential pitfalls in a case where only limited experimental data are available—in this case, unconfined uniaxial compression and three-point bending tests on different mixes of polypropylene and steel fibre-reinforced concretes. As a first step, a sensitivity analysis is performed to weight the effect of each governing mesoscale parameter on the simulated macroscale behaviour. Then, for each mix at issue, different sets of model parameters are identified as capable of accurately matching the experimental evidence. As a validation, each calibrated set is used to simulate energy absorption tests on round panels. The validation stage shows that one of the identified sets, for the FRC with polypropylene fibres, accurately matches the round panels’ response, while the others result in acceptable predictions. For the mix with steel fibres, instead, none of the sets captures the experimental results, likely due to the different post-cracking behaviour detected in fracture and energy absorption tests. Finally, a parametric study showcases how the LDPM-F might serve as tool to optimise the mix design without extensive experimental investigations
Calibration of the viscoelastic behavior of polypropylene fiber reinforced concrete with the extended Lattice Discrete Particle Model approach
No full textFiber reinforced concrete (FRC) is a composite material whose adoption is becoming widespread. The ability of exhibiting remarkable residual capacity after cracking of the cementitious matrix represents its characteristic feature.
At present, studies dealing with the mechanical characterization of this material are mainly focused on the analysis of the mechanical behavior under short-term conditions. On the other hand, a comprehensive identification of FRC performance requires also the investigation of the rheological properties and the long-term behavior. To this purpose, some researches are making first steps towards the standardization of the creep testing methodologies for FRCs, together with the study of the relevant aspects influencing their time-dependent behavior.
On the other side, the numerical simulation of the FRCs behavior is another challenging topic, due to the composite nature of the material. In particular, the reliability of the model is often driven by the method of introducing the concrete heterogeneity and the fiber distribution. Among the others, the extended lattice discrete particle model (LDPM-F) approach is able to describe the instantaneous composite material behavior through the inclusion of fibers and the simulation of the crack-bridging effect responsible for the FRCs residual strength. At the same time, conventional LPDM model can be coupled with the HTC model and the microprestress solidification theory to fully reproduces the concrete viscoelasticity and aging properties.
In the present paper, a new version of LPDM model has been proposed, where both a concrete viscosity and a fiber viscosity are utilize to describe the long-term behavior of FRC. In the case of plastic fibers (MS), the contribution of fibers to the creep deformation is remarkable. The numerical predictive model has been calibrated with respect to experimental results obtained from instantaneous and creep tests on macro-synthetic fiber reinforced concrete (MSFRC) with polypropylene fibers. In particular, fiber viscoelasticity has been calibrated against tensile creep tests for a single fiber. The validated model has been then used to predict the long-term elongation of pre-cracked notched MSFRC cylinders under the uniaxial tensile stress. Results show that the presented numerical scheme is able to capture this complex behavior
Imsejhin ghall-qadi tal-komunita` Nisranija permezz tal-ministeru sacerdotali
No full textThe article focuses upon the call to service through the exercise of the priestly ministry. The point of departure of the study is the Johannine pericope which describes the washing of the disciples' feet by Jesus (Jn 13:1-20). The article describes how the celebration of the liturgy is a fount of vocations to the ordained ministry. The author then studies three documents of the Second Vatican Council -- Optatam Totius, Presbyterorum Ordinis and Christus Dominus -- in order to depict the centrality of service to priestly ministry. A number of evocative texts are referred to, as well as formation documents from the local Church of Malta. Furthermore, the respective contributions of Bishop Tonino Bello, Pope Benedict XVI and Pope Francis are also given pride of place, as more light is thrown on the theme of the article.peer-reviewe
Experimental and numerical investigation on short and long term performance of macro-synthetic fibre reinforced concrete materials
No full textFibre Reinforced Concretes are innovative composite materials whose applications are growing considerably nowadays. Being composite materials, their performance depends on the mechanical properties of both components, fibre and matrix and, above all, on the interface. The variables to account for the mechanical characterization of the material, could be proper of the material itself, i.e. fibre and concrete type, or external factors, i.e. environmental conditions.
The first part of the research presented is focused on the experimental and numerical characterization of the interface properties and short term response of fibre reinforced concretes with macro-synthetic fibers. The experimental database produced represents the starting point for numerical models calibration and validation with two principal purposes: the calibration of a local constitutive law and calibration and validation of a model predictive of the whole material response. In the perspective of the design of sustainable admixtures, the optimization of the matrix of cement-based fibre reinforced composites is realized with partial substitution of the cement amount.
In the second part of the research, the effect of time dependent phenomena on MSFRCs response is studied. An extended experimental campaign of creep tests is performed analysing the effect of time and temperature variations in different loading conditions. On the results achieved, a numerical model able to account for the viscoelastic nature of both concrete and reinforcement, together with the environmental conditions, is calibrated with the LDPM theory.
Different type of regression models are also elaborated correlating the mechanical properties investigated, bond strength and residual flexural behaviour, regarding the short term analysis and creep coefficient on time, for the time dependent behaviour, with the variable investigated.
The experimental studies carried out emphasize the several aspects influencing the material mechanical performance allowing also the identification of those properties that the numerical approach should consider in order to be reliable
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