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Abstract: Standard methods for the dynamic identification of civil engineering structures and mechanical systems generally rely on the determination of eigenfrequencies/eigenvectors and damping. A basic assumption for the use of these methods is the linear elastic behaviour of the tested structures. Nevertheless many structures exhibit a non-linear behaviour even at low levels of external excitation. For instance, reinforced concrete structures cracked by shrinkage or by the overcoming of the concrete tensile strength, exhibit different values of the flexural stiffness depending on the opening of the cracks. In the present paper a new methodology is presented to identify the characteristics of a non-linear mechanical system using the Fokker–Plank Equation (FPE) that allows evaluating the probability density function of the response of structural systems loaded with Gaussian white noise. [Copyright &y& Elsevier]

Abstract: Softening and time-dependence of fracture are two complex and coupled phenomena that have to be taken into account in order to simulate realistic concrete or rock behaviour. Understanding the interaction between these two phenomena is important to design reliable civil engineering structures subjected to high level loading during a long time. The aim of this paper is to develop a simple time-dependent softening model applied to quasi-brittle materials such as rock or concrete. A three-dimensional constitutive visco-damage model describes phenomena like relaxation, creep and rate-dependent loading using a unified framework. The model could be viewed as a generalisation of a time-independent damage model and is based on strong thermodynamical arguments. A general material stability analysis is proposed in case of uniaxial monotonic loading. Phenomena as creep failure under high-sustained load are explained quite simply within stability theory. Creep failure appears as the manifestation of a bifurcation phenomenon. Consequently, this model is able to predict creep failure for various stress level. Conversely, for low-sustained load, the motion asymptotically converges towards an equilibrium configuration, also called equilibrium curve. As for endochronic models, no initial threshold is assumed. Nevertheless, an apparent elastic domain is identified for constant strain rate tests, which reveals the competition between the internal kinetics of damage and the strain rate imposed on the material. [Copyright &y& Elsevier]

Non-destructive evaluation appears more and more important in the civil engineering economic stakes. In this context electrical resistivity measurements get sensitivity to parameters allowing to assess concrete structures conditions. This article analyses the ability of the resistivity measurement to study cracks in concrete. Its ability to detect and to locate cracks and spalling is shown with on site measurements on a damaged slab. Then specific studies on such disorders allow to distinguish the influence of their characteristics.The sensitivity of the method to cracks depth, according their moisture conditions, is assessed by computation. Experimental works on a reinforced concrete beam, ideally cracked, confirmed these results.Some assumptions based on measurements realised on a size-one structural component allow to assess the general influence of crack opening and bridging degree between crack lips.Qualitative results show the similar effects on measurement of various cracking parameters. Prospective works presented in the paper lead us to say that electrical resistivity method applied to civil engineering structures is a relevant tool for the assessment of structural damage. [Copyright &y& Elsevier]