Civil, Architectural, Environmental Engineering

In recent years FRP (Fiber Reinforced Polymer) technology has been developed to repair damaged concrete structures. This work proposes to investigate the complex mechanism of stress-strain evolution at the FRP interface until complete debonding as well as when long-term loads are accounted for. The study has been performed by means of a fully three-dimensional approach within the context of damage mechanics, to appropriately catch transverse effects and normal stresses, developing a realistic and comprehensive study of the delamination process. The adhesion properties have been reconstructed through a contact model incorporating an elastic-damaged constitutive law, relating inter-laminar stresses acting in the sliding direction. A F.E. research code (FRPCON3D) has been developed, including a numerical procedure accounting for Mazars' damage law inside the contact algorithm. The code is also able to describe the delamination process when considering the effects from different concrete surface preparations. Additionally, it has been upgraded to describe the long-term behaviour of composite structures by means of two visco-elastic formulations: (i) Bažant's B3 law has been considered for the concrete component, where creep effect is composed by three different terms, i.e. the elastic part, basic creep and drying creep; (ii) for FRP's fibres and matrix a micromechanical approach has been implemented. The numerical results have been calibrated and validated against available experimental evidences.

The bond behaviour between FRP and concrete elements is investigated, starting from available experimental evidences, through a calibrated and upgraded 3D mathematical-numerical model. The complex mechanism of debonding failure of FRP concrete reinforcement is studied within the context of damage mechanics to appropriately describe transversal effects and developing a realistic study of the delamination process.

Purpose-This paper seeks to analyse 3D growing concrete structures taking into account the phenomenon of body accretion, necessary for the simulation of the construction sequence, and carbon dioxide attack. Design/methodology/approach-A typical 3D segmental bridge made of precast concrete is studied through a fully coupled thermo-hygro-mechanical F.E. model. The durability of the bridge is evaluated and carbonation effects are considered. Creep, relaxation and shrinkage effects are included according to the theory developed in the 1970s by Bažant for concretes and geomaterials; the fluid phases are considered as a unique mixture which interacts with a solid phase. The porous material is modelled using n Maxwell elements in parallel (Maxwell-chain model). Findings-First, calibration analyses are developed to check the VISCO3D model capabilities for predicting carbonation phenomena within concrete and the full 3D structure is modelled to further assess the durability of the bridge under severe conditions of CO 2 attack. Originality/value-The adopted numerical model accounts for the strong coupling mechanisms of CO 2 diffusion in the gas phase, moisture and heat transfer, CaCO 3 formation and the availability of Ca(OH) 2 in the pore solution due to its transport by water movement. Additionally, the phenomenon of a sequential construction is studied and numerically reproduced by a sequence of "births" for the 3D finite elements discretizing the bridge. The fully coupled model is here extended to 3D problems for accreting bodies (as segmental bridges) in order to gather the effects of multi-dimensional attacks of carbon dioxide for such structures.

This paper presents the physical, mathematical and numerical models forming the main structure of the numerical analysis of the thermal, hydral and mechanical behaviour of normal, high-performance concrete (HPC) and ultra-high performance concrete (UHPC) structures subjected to heating. A fully coupled non-linear formulation is designed to predict the behaviour, and potential for spalling, of heated concrete structures for fire and nuclear reactor applications. The physical model is described in more detail, with emphasis being placed upon the real processes occurring in concrete during heating based on tests carried out in several major laboratories around Europe as part of the wider high temperature concrete (HITECO) research programme. A number of experimental and modelling advances are presented in this paper. The stress-strain behaviour of concrete in direct tension, determined experimentally, is input into the model. The hitherto unknown micro-structural, hydral and mechanical behaviour of HPC/UHPC were determined experimentally and the information is also built into the model. Two examples of computer simulations concerning experimental validation of the model, i.e. temperature and gas pressure development in a radiatively heated HPC wall and hydro-thermal and mechanical (damage) performance of a square HPC column during fire, are presented and discussed in the context of full scale fire tests done within the HITECO research programme.

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It is recalled that negative water pressures are of importance in localisation phenomena of fully saturated undrained samples of dilatant geomaterials. A model to simulate cavitation phenomena connected with such pore water tractions is developed and implemented in a simplified form in a dynamics code for partially saturated porous media. A case of localisation is studied from the onset of the instability up to the full developed shear band. The weak mesh dependence of the maximum effective plastic strain, due to the employed physical model, is also shown.

The computational analysis of thermohygrometric and mechanical behaviour of concrete structures is carried out by means of the finite element method. To evaluate the thermal and hygral performance of this material together with the damage and creep effects, the knowledge of the heat and moisture transfer processes taking place inside the medium is first required and then the consequent mechanical behaviour can be analysed. The theoretical approach used to obtain the governing differential equations for heat and moisture transfer is based on the procedure of averaging continuum equations applied to heat and mass transfer and drying processes. According to that model the same set of equations is used to represent both the saturated zone (if present), the unsaturated one and the water-vapour phase changes. The mechanical formulation is based on the virtual work principle and incorporates Prony-Dirichlet series expansion to represent the relaxation or creep functions, avoiding the memorization of the whole strain or stress history. Damage effects are taken into account within a coupled formulation, following a procedure presented in a previous paper. At this preliminary stage the analysis is performed in two stages (first the heat and mass transfer and then the mechanical analysis) using two FEM computer codes in sequence to apply the proposed approach to structures of any shape.

Purpose-The purpose of this paper is to show how to find the regions of dynamic instability of a beam axially loaded and visco-elastically constrained at its ends by Kelvin-Voigt translational and rotational units variously arranged according to different configurations, by using the equation of boundary frequencies. Design/methodology/approach-With respect to visco-elasticity the time variable is present as a parameter so that the above-mentioned exact approach is exploited to draw three-dimensional diagrams of the dynamic component of the periodic load and its frequency, varying with time and with the viscosity parameter m characterizing the restraints. Findings-For not rigidly constrained configurations a peculiar asymptotic tendency is recognizable in both cases. Research limitations/implications-The study allows for identifying the influence of visco-elastic restraints in the response of a beam under a dynamic axial load. Dynamic excitation occurs in several fields of mechanics: dynamic loads are encountered in structural systems subjected to seismic action, aircraft structures under the load of a turbulent flow and industrial machines whose components transmit time-dependant forces. Practical implications-Visco-elasticity accounts for possible vibration control solutions planned to improve the dynamic response of the rod; they can consist of layers of visco-elastic material within the body of the modelled element or local viscous instruments affecting the boundary conditions; the latter is the application this paper focuses on. Originality/value-With this paper a calculation procedure to get an exact solution for particular static configurations of the beam is followed in order to define the influence of visco-elastic restraints under a dynamic axial load; the responses are given in terms of boundary frequencies domains and are supposed to be useful to learn the behaviour in time and in dependence of the intrinsic viscosity of the restraints.