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 Trends in Designing Parabolic trough Solar Concentrators and Heat Storage Concrete Systems in Solar Power Plants

Energy availability has always been an essential component of human civilization and the energetic consumption is directly linked to the produced wealth. In many depressed countries the level of solar radiation is considerably high and it could be the primary energy source under conditions that low cost, simple-to-be-used technologies are employed. Then, it is responsibility of the most advanced countries to develop new equipments to allow this progress for taking place. A large part of the energetic forecast, based on economic projection for the next decades, ensure us that fossil fuel supplies will be largely enough to cover the demand. The predicted and consistent increase in the energetic demand will be more and more covered by a larger use of fossil fuels, without great technology innovations. A series of worrying consequences are involved in the above scenario: important climatic changes are linked to strong CO2 emissions; sustainable development is hindered by some problems linked to certainty of oil and natural gas supply; problems of global poverty are not solved but amplified by the unavoidable increase in fossil fuel prices caused by an increase in demand. These negative aspects can be avoided only if a really innovative and more acceptable technology will be available in the next decades at a suitable level to impress a substantial effect on the society. Solar energy is the ideal candidate to break this vicious circle between economic progress and consequent greenhouse effect. The low penetration on the market shown today by the existent renewable technologies, solar energy included, is explained by well-known reasons: the still high costs of the produced energy and the \u201cdiscontinuity\u201d of both solar and wind energies. These limitations must be removed in reasonable short times, with the support of innovative technologies, in view of such an urgent scenario. On this purpose ENEA, on the basis of the Italian law n. 388/2000, has started an R&D program addressed to the development of CSP (Concentrated Solar Power) systems able to take advantage of solar energy as heat source at high temperature. One of the most relevant objectives of this research program (Rubbia, 2001) is the study of CSP systems operating in the field of medium temperatures (about 550\ub0C), directed towards the development of a new and low-cost technology to concentrate the direct radiation and efficiently convert solar energy into high temperature heat; another aspect is focused on the production of hydrogen by means of thermo-chemical processes at temperatures above 800\ub0C. As well as cost reductions, the current innovative ENEA conception aims to introduce a set of innovations, concerning: i) The parabolic-trough solar collector: an innovative design to reduce production costs, installation and maintenance and to improve thermal efficiency is defined in collaboration with some Italian industries; ii) The heat transfer fluid: the synthetic hydrocarbon oil, which is flammable, expensive and unusable beyond 400\ub0C, is substituted by a mixture of molten salts (sodium and potassium nitrate), widely used in the industrial field and chemically stable up to 600\ub0C; iii) The thermal storage (TES): it allows for the storage of solar energy, which is then used when energy is not directly available from the sun (night and covered sky) (Pilkington, 2000). After some years of R&D activities, ENEA has built an experimental facility (defined within the Italian context as PCS, \u201cProva Collettori Solari\u201d) at the Research Centre of Casaccia in Rome (ENEA, 2003), which incorporates the main proposed innovative elements. The next step is to test these innovations at full scale by means of a demonstration plant, as envisioned by the \u201cArchimede\u201d ENEA/ENEL Project in Sicily. Such a project is designed to upgrade the ENEL thermo-electrical combined-cycle power plant by about 5 MW, using solar thermal energy from concentrating parabolic-trough collectors. Particularly, the Chapter will focus on points i) and iii) above: - loads, actions, and more generally, the whole design procedure for steel components of parabolic-trough solar concentrators will be considered in agreement with the Limit State method, as well as a new approach will be critically and carefully proposed to use this method in designing and testing \u201cspecial structures\u201d such as the one considered here; - concrete tanks durability under prolonged thermal loads and temperature variations will be estimated by means of an upgraded F.E. coupled model for heat and mass transport (plus mechanical balance). The presence of a surrounding soil volume will be additionally accounted for to evaluate environmental risk scenarios. Specific technological innovations will be considered, such as: -higher structural safety related to the reduced settlements coming from the chosen shape of the tank (a below-grade cone shape storage); - employment of HPC containment structures and foundations characterized by lower costs with respect to stainless steel structures; - substitution of highly expensive corrugated steel liners with plane liners taking advantage of the geometric compensation of thermal dilations due to the conical shape of the tank; - possibility of employing freezing passive systems for the concrete basement made of HPC, able to sustain temperature levels higher than those for OPC; - fewer problems when the tank is located on low-strength soils

Conceptual Study of a Thermal Storage Module for Solar Power Plants with Parabolic Trough Concentrators

The thermal storage technology (TSE) has a relevant strategic importance for the success of solar plants devoted to electric energy and heat production. The major benefits in the use of storage include higher efficiency and reduction in the mean levelled cost of the electric energy unit (LEC). Sensible heat storage systems within solid media have been identified, both technically and economically, as a very promising solution. The development of such a storage technology, adopting concrete, could reduce the specific cost to less than 20\u20ac per kWh of thermal capacity; additionally, such a solution is suitable for small-medium size plants with a power ranging from 1 MW to 5 MW, to be easily introduced in the Italian territory and with reduced operational and maintenance needs. In large size CSP systems, as the ARCHIMEDE plant built by ENEL with ENEA technology, a high temperature fluid storage (between 400 and 500\ub0C) is required. Such a temperature seems at present not adequate to allow for adopting concrete, whereas the production of concrete able to sustain 250-300\ub0C appears as a reachable objective. It is supposed to study a storage system characterised by a parallelepiped structure with appropriate section, selfbearing and supported on its major axis, as well as by a piping system directing the thermovector fluid within the cemented matrix

Dynamic Stability of Elastic Rectangular Plates with Viscoelasto-Damaged Constraints

The problem of dynamic instability of rectangular plates with complex constraints at the edges is here solved starting from a theory in the literature; the suggested approach allows for the definition of generic, non perfect constraint configurations for the regions of instability due to in-plane compression loads as diagrams that are expressed in function of the applied dynamic force and its frequency. The novelty of the work stands in (1) the study of viscoelasticity, combined with damage, at the constraints, which allows for the discussion of the performance of a rectangular plate with regards to dynamic instability at varying degrees of damping and damage, and (2) the analytical approach developed to solve the associated eigenvalue problem for rectangular plates with generic viscoelasto-damaged constraints at two opposite edges and simply supported at the other edges. Regarding the second point, the differential equation of motion of such a system is derived and, via the variable separation method, a solution for the transverse displacements is sought among those satisfying the generic boundary conditions at the four edges. Namely, the constraints are assigned in a homogeneous way, which has particularly convenient results for computational purposes. By imposing that the determinant of the resulting system vanishes, a closed-form solution is found for the free vibration problem. The associated boundaries of the unstable regions for specific constraint configurations are discussed

Non-linear modelling, design and production of steel blast-resistant doors and windows

Numerical-experimental results are here described, derived from an innovative experience at both national and international level, related to modelling, designing and producing steel blast-resistant doors and windows. Their capability to sustain thermal loads due to fire hazards is additionally accounted for. The activity has been developed within a collaboration between Wellco S.p.A. and some researchers of the Department of Structural and Transportation Engineering of the University of Padua, Italy. The study has been conducted to define and characterize the non-linear response of a large number of doors and steel framed windows, with the objective of sustaining dynamic loads from explosive hazards of fixed magnitude, variable design and clearing times. The local overcome in the strength limit (with correspondent plastic response) and possible formation of plastic hinges has been critically discussed. Numerical models have allowed for refining first design sketches and subsequently understanding the real thermo-mechanical behaviour for the investigated structures. Experimental tests on typical steel doors at 1:1 scale have been performed at the Laboratory of Construction Materials of the same Department above. Such tests had the objective of “a-posteriori” verifying the correctness of the already available numerical results, validating the adopted procedures and correspondingly guaranteeing the doors’ structural efficiency even under dynamic loads higher than design ones