Effetti di scala sulla resistenza a trazione dei materiali

The dissertation analyses the scale effects on the tensile strength of materials. By the term scale effects it is meant the variation in a mechanical property as a function of structural size. In particular, it has been observed by numerous investigators that the nominal tensile strength of many materials decreases with increasing size of the specimen tested. This phenomenon is more evident in disordered materials, that is, materials that are macroscopically heterogeneous and damaged. On the basis of Weibull's statistical theory and the principles of Linear Elastic Fracture Mechanics, a self-similarity distribution for defect size is presented (Chapter 4). With this distribution the length of the most critical defect is taken to be proportional to the linear size of the specimen. It is shown that the assumption of self-similarity represents the instance of maximum disorder that can be encountered in real materials, and it supplies, in a strength-size bilogarithmic plane, a linear scaling law with an inclination of -1/2, corresponding to the power of the stress singularity envisaged by LEFM. This formulation contains the fractal concept of self-similarity, even though it is limited to maximum defect dimension. In order to consider the real nature of the micro-structure of the materials, a more complex fractal model is presented (Chapter 5) in which the property of self-similarity is extended to the entire population of defects. This topological law, based on fractal theory and on the so-called renormalisation procedure, states that in order to obtain a nominal constant strength for the material it is necessary to refer to surface areas with non integer physical dimensions. For disordered materials, such as for instance concrete and rocks, renormalised tensile strength is given by a force acting on a surface having a fractal dimension lower than 2. The dimensional decrease, always comprised in the [0, 1/2] range, represents self-similar vacancies in the undamaged section associated with the presence of pores, voids, defects, cracks, aggregate and inclusions, and it approaches the 1/2 limit only for extremely brittle and disordered materials, as is assumed, incidentally, in statistical approaches. As a rule, the scale variation taken into consideration in experimental investigations does not exceed one order of magnitude. In such circumstances, it is only possible to determine a single tangential inclination in the bilogarithmic diagram. Only by taking into account scale variations higher than one order of magnitude it proves possible to detect the transition from disordered to ordered conditions, and a continuous transition from -1/2 to zero inclination may be seen to appear. In physical reality, the peak load resistant section can be viewed as multifractal, of dimension 1.5 on small scales and dimension 2 at large scales. This clearly shows a transition from the extreme disorder that is associated with small scales, where a self-similar distribution of Griffith cracks predominates, to the extreme order of large scales, where the disorder of the microstructure is no longer visible, on account of the limited dimensions of the defects and heterogeneities. The assumption of multifractality for the microstructure of the damaged material (Chapter 7) is the basis of the so-called Multifractal Scaling Law (MFSL). Such law consists of an approximation method which imposes the concavity of the bilogarithmic curve facing upward, which contradicts Bazant's size effect law (SEL). To verify this scaling law and to determine experimentally the variation in nominal tensile strength and fracture energy, a totally innovative testing set-up has been created, involving the use of three servo-controlled jacks (Chapter 5). The interaction of the three jacks, arranged in L formation, makes it possible to centre instant by instant the resultant of the load with the respect to the undamaged section even in the presence of cracks which make the latter asymmetrical. The main goal of this instrumentation is to determine the parameters of the concrete subjected to uniform tension, eliminating any secondary bending effect which may affect the results and lead to erroneous explanations of the scale effect

feasibility and effectiveness of exoskeleton structures for seismic protection

Abstract In this study, a self-supporting structure, namely an exoskeleton, is considered as set outside a main structure and suitably connected to it. From the structural point of view, the exoskeleton is conceived as a "sacrificial" appendage, called to absorb seismic loads in order to increase the performance of the main structure. From the architectural and technological point of view, additional functions may be associated through an integrated design approach, combining seismic with urban and energy retrofitting. Particular and attractive applications can therefore be envisaged for existing buildings. A reduced-order dynamic model is introduced, in which two coupled linear viscoelastic oscillators represent the main structure and the exoskeleton structure, respectively, while either a rigid link or a dissipative viscoelastic connection is considered for the coupling. The equations of motion are set in non-dimensional form and a parametric study is carried out in the frequency domain to confirm that exoskeleton structures can be feasible and effective in reducing earthquake-induced dynamic responses

Considerations over the Italian road bridge infrastructure safety after the Polcevera viaduct collapse: past errors and future perspectives

In the last four years, Italy experienced the collapse of five road bridge: Petrulla viaduct (2014), Annone (2016) and Ancona (2017) overpasses, Fossano viaduct (2017) and Polcevera (2018) bridge. Although for deeply different reasons, the collapses occurred can all been gathered into the same common cause: the (lack of) knowledge of the effective structural condition, a serious problem that affects existing constructions. As it will be shown in the paper, different problems such as missing of the as-built designs, an appropriate construction and movement precautions, a heavy vehicle checking, and a material decay monitoring can nevertheless be addressed as an inadequate knowledge of what is happening to/in the structure. In the first section, the paper will report a short description of the failures for the five bridges, while in the second part a main set of problems involved in bridge safety and maintenance will be discussed. Finally, in the third part, a review on innovative and peculiar investigation and monitoring techniques will be illustrated. The collected results can shed new light on future perspectives for the Civil Engineering sector, sector that has to be ready for facing the challenges of preservation, restoration and/or replacement of the existing infrastructural constructions, worldwide

Biochar addition for 3DCP: a preliminary study

This contribution presents the first results of an ongoing research aimed at highlighting the possible reduction in the environmental impact of concrete through the synergy between two interconnected strategies: the exploitation of by-products, in this case biochar, for the realization of 3D printable cementitious conglomerates. Thanks to the use of biochar, the mixes presented are characterized by an excellent dimensional stability in the fresh state, evaluated through the extrusion test. Regarding the hardened state properties, the contribution highlights the effects of biochar-to-cement ratio, water-to-cement ratio (in combination with biochar content) and sand-to-cement ratio on the flexural and compressive strength of the mixes. The evaluation of CO2 emissions shows that a proper mix design could result in a significant reduction in CO2 emissions (up to 43%) while maintaining good mechanical performance (compressive strength of at least 60 MPa)

An experimental set-up for cyclic loading of concrete

Abstract Innovative cementitious composite materials are drawing considerable interest due to their substantially improved mechanical properties as compared to ordinary cement-based materials. Their enhanced ductility is promising and particularly suited to structural applications under severe dynamic loading conditions. Cyclic response is essential to understand the effects of loading and unloading on the material, as well as to understanding how it behaves in the transition from tension to compression. It is also fundamental to identify its properties in terms of energy dissipation and strain-rate sensitivity. This paper presents the first part of an ongoing research project which aims to develop the constitutive relationship in innovative cementitious composites and its numerical implementation. Results from this research will facilitate the investigation of the ductility and durability of existing buildings. In this paper, an experimental set-up for uniaxial cyclic loading is described. It was developed to study reversed cyclic compression/tension loadings of innovative cementitious composites. To set the cyclic loading process, cylindrical specimens of concrete were tested. All the tests were performed on a Zwick testing machine with 50 kN load cell. The machine was customised with accessories specifically designed to meet test requirements, avoiding instability and bending moments during the alternating phases of uniaxial compression and tension. Strain gauges were used to measure lateral deformations. The customized machine has shown good performance so far. In order to test specimens with a higher number of cycles and a higher loading rate, improvements to the machine are currently under development. These tests will allow greater insight into the ductility of innovative cementitious composite materials

Increase the fracture energy of foamed concrete: Two possible solutions

The aim of the present paper is to investigate the influence of the curing conditions and the addition of an eco-friendly filler, biochar, on the flexural strength and fracture energy of a "green" special concrete characterized by lightness, high thermal and acoustic insulation properties and excellent fire resistance: foamed concrete. The study aims to highlight the properties of this promising material that, depending on its density, can be used for both structural and non-structural purposes. In fact, if the material is designed with a density not exceeding 800 kg/m3, it can be employed in interior partitions or in high energy-efficiency building envelopes; on the other hand, if the material is designed with a density greater than 1400 kg/m3, it can be used for structural purposes. All this makes it legitimate to state that it is a material that can be engineered according to specific needs. In this contribution the possibility to improve the fracture energy through biochar addition in this special concrete is also analyzed and presented. In particular, two different dry density were investigated: 800 kg/m3, and 1600 kg/m3. The first one for non-structural applications, the second for structural purposes. With regard to the biochar, used for 1600 kg/m3density, two different percentages, 2% and 4%, were investigated. Two different curing conditions were analyzed, namely in air at 20°C, wrapped in cellophane at the same room temperature and cured in water at 20 °C. Three-point bending tests in CMOD (crack mouth opening displacement) mode and compressive tests on the two-halves of the broken specimens have shown interesting results. Curing conditions significantly affect the fracture energy and the addition of biochar at 2% concentration seems to be beneficial in improving the fracture behavior of foamed concrete

Stochastic Multi-objective Optimisation of Exoskeleton Structures

In this study, a structural optimisation problem, addressed through a stochastic multi-objective approach, is formulated and solved. The problem deals with the optimal design of exoskeleton structures, conceived as vibration control systems under seismic loading. The exoskeleton structure is assumed to be coupled to an existing primary inner structure for seismic retrofit: the aim is to limit the dynamic response of the primary structure to prevent structural damage. A non-stationary filtered Gaussian white noise stochastic process is taken as the seismic input. Design variables pertain to the mechanical properties (stiffness, damping) of the exoskeleton structure. Two concurrent and competing objective functions are introduced, in order to take into account not only safety performance but also economic cost considerations. The resulting trade-off is solved searching the Pareto front by way of a controlled elitist genetic algorithm, derived from the Non-dominated Sorting Genetic Algorithm-II. Sensitivities of Pareto fronts and Pareto optimal sets to different system parameters are finally investigated by way of a numerical application

The use of Biochar to reduce the carbon footprint of cement-based materials

Abstract The organic waste management is a most current topic, because its processing and degradation it is responsible for emissions of methane and other greenhouse gases, leading to serious environmental problems. Limited oxygen thermochemical processes, such as pyrolysis or gasification, have demonstrated the energy recovery potential of the treated biomass and its environmental benefits. However, the solid part of the process -Biochar- it is considered as a waste, as only its coarse ash can be used as soil improvers. Nevertheless, several researchers have explored its potential application as green filler in order to reduce the carbon footprint both of cement production and cement-based construction materials. In this work, Biochar microparticles were used both as a filler inside the cement paste and mortar composites and as a substitute for the cement powder inside the mixes. Based on previous work, this investigation has a twofold objective: to understand the full influence of the use of an optimized percentage of Biochar (2% with respect to the weight of the cement) either as a filler in the mixture or as a substitute for cement, while guaranteeing an improvement in the strength without losing ductility. The results showed that 2 wt% of Biochar's particles are sufficient to increase the strength and toughness of the cement and mortar composites and, in place of the cement in the mixture, can maintain the mechanical properties equal to those of the reference samples

influence of pyrolysis parameters on the efficiency of the biochar as nanoparticles into cement based composites

Abstract In this research, a particular kind of biochar provided by UK Biochar Centre has been added as nanoparticles into cementitious composites. Its principle characteristic lies in the standardization of its process production, that makes it suitable to been used as filler in cement-matrix composites, ensuring the reproducibility of the cement mix (I. Cosentino "The use of Bio-char for sustainable and durable concrete", 2017). The pyrolysis parameters and the content of carbon in the standardized biochar influenced its efficiency to enhance the mechanical properties of the cement composites: the results, in terms of flexural strength and fracture energy, have been worse than those obtained in previous studies (L. Restuccia "Re-think, Re-use: agro-food and C&D waste for high-performance sustainable cementitious composites", 2016), in which particles have been produced with higher temperature. However, also with standardized biochar a general enhancement of mechanical properties has been recorded, a sign that they can be used to create new green building materials

new self healing techniques for cement based materials

Abstract: In recent years, researches concerning cement-based materials has been focused not only on the strength and the toughness but also on the durability. In fact, the interest on concrete's self-healing process is increasing, due to the rapidly deterioration of that material which tends to crack and thus quickly deteriorate. In this paper, a new self-healing technology for cement-based materials is proposed. This technology is based on the encapsulation method of repairing agent inserted in randomly distributed shell inside the material during its preparation. Two different kind of shells were used: glass spheres and pharmaceutical capsules. The material the shells are made of has to be endowed with a series of fundamental characteristics. That material has to be inert with respect to the repair agent so that it doesn't react with it, resisting to the severe stress condition that the shells undergo during the mixing, and at the same time being capable of breaking down when the crack intercept them, having a good compatibility with the cement mixture. The results demonstrate that it is possible to use this kind of shell to encapsulate the repairing agent: the crack breaks them and they release the healing agent, which allows patching up the crack