J. Vantomme

Damage assessment in reinforced concrete beams using eigenfrequencies and mode shape derivatives

Abstract The use of changes in dynamic system characteristics to detect damage has received considerable attention during the last years. This paper presents experimental results obtained within the framework of the development of a health monitoring system for civil engineering structures, based on the changes of dynamic characteristics. As a part of this research, reinforced concrete beams of 6 meters length are subjected to progressing cracking introduced in different steps. The damaged sections are located in symmetrical or asymmetrical positions according to the beam tested. The damage assessment consists in relating the changes observed in the dynamic characteristics and the level of the crack damage introduced in the beams. It appears from this analysis that eigenfrequencies are affected by accumulation of cracks in the beams and that their evolutions are not influenced by the crack damage locations; they decrease with the crack damage accumulation. The MAC factors are less sensitive to crack damage compared to eigenfrequencies, but give an indication of the symmetrical or asymmetrical nature of the induced crack damage. Next to this, the COMAC factors, the strain energy evolution and the changes in flexibility matrices are also examined as to their capability for detection and location of damage in the RC beams, the strain energy method appears to be more precise than the others.

Damage identification in reinforced concrete structures by dynamic stiffness determination

Service loads, environmental and accidental actions may cause damage to constructions. Regular inspection and condition assessment of engineering structures are necessary so that the early detection of any defect can be made and the structures remaining safety and reliability can be determined. When the structural damage is small or it is in the interior of the system, its detection cannot be carried out visually. A useful more elaborate non-destructive evaluation tool is vibration monitoring. It relies on the fact that the occurrence of damage or loss of integrity in a structural system leads to changes in the dynamic properties of the structure. In this paper, different techniques will be presented and compared to derive from experimentally determined modal characteristics of a reinforced concrete beam its dynamic bending stiffness. The degradation of stiffness, due to the cracking of the reinforced concrete, gives information on the position and severity of the damage that has occurred.

Identification of the damping properties of orthotropic composite materials using a mixed numerical experimental method

A mixed numerical experimental approach is the basis of a new method for the identification of the material damping properties of fibre reinforced polymers, which provides an answer to many problems encountered in experimental damping characterization. Experimental modal parameters, measured on a plate specimen, are compared with corresponding results from a numerical calculation, thus allowing to determine the stiffness and damping properties of the material. The relation between the modal parameters (structural parameters) and the material parameters, is obtained by using a numerical model of the specimen in combination with the modal strain energy method.In the first part of this paper, the complex moduli are introduced as measures for both material stiffness and damping and the relation between these complex moduli and the modal parameters of a thin plate specimen is derived. Next, the practical procedure of the mixed numerical experimental method is presented, followed by a procedure for estimating the reliability of the obtained results. Finally, two examples are discussed in which all the independent material damping properties are identified as functions of frequency.

Comparison of techniques for modal analysis of concrete structures

Abstract This paper describes different experimental techniques for obtaining modal parameters of structures. Attention is focused on those techniques that may be applicable to in situ concrete structures (e.g. bridges). In a first stage, experiments are made on reinforced concrete beams of 6 meters length. The beams are excited using three types of excitation methods: impact hammer excitation and two different electromagnetic shaker signals: pseudo-random and swept-sine signals. The modal parameters are determined either by performing curve-fitting procedures on series of measured frequency response functions or by applying the stochastic subspace identification technique to the time response signals of the structure. The influence of the non-linear behaviour of the concrete beams is investigated by performing measurements at different excitation amplitudes. It appears that modal parameter estimates are affected by excitation techniques, data acquisition parameters and processing methods. The main cause of this is the non-linear behaviour which is observed even at very low vibration amplitudes. However, the influence on resonant frequencies and mode shapes is negligible. This is not the case for the modal damping ratios, so that the estimation of these parameters is unreliable.

Analysis of speckle patterns for deformation measurements by digital image correlation

Digital Image Correlation (DIC) - also referred to as white light speckle technique - is an optical-numerical full-field measuring technique, which offers the possibility to determine in-plane displacement fields at the surface of objects under any kind of loading. For an optimal use of the method, the object of interest has to be covered with a speckle pattern. The present paper studies the efficiency of a random speckle pattern and its influence on the measured in-plane displacements with respect to the subset size. First a randomly sprayed speckle pattern is photographed three times. Each picture is taken with a different zoom, yielding three speckle patterns, which are different by the size of the speckles. Secondly a number of speckle patterns are generated numerically using a given speckle size and image coverage. Subsequently, each speckle pattern image undergoes a numerically controlled deformation, which is measured with digital image correlation software. Both imposed and measured displacements are compared and it is shown that the size of the speckles combined with the size of the used pixel subset, clearly influences the accuracy of the measured displacements. Furthermore it is shown that it is possible to create an optimal speckle pattern when a given subset size is chosen.

Modular pedestrian bridge with concrete deck and IPC truss girder

Abstract The paper reports on the design, by means of analytical and numerical tools, the experimental verification of structural elements and the joining procedure of a 13 m span pedestrian bridge, which is composed of a concrete deck and three truss girders made of Inorganic Phosphate Cement (IPC) sandwich panels. The IPC is a cementitious matrix which does allow reinforcement with glass fibres. The bridge project wants to examine the feasibility of building structures with IPC, in view of the advantages of this material: low manufacturing cost, non-inflammable behaviour, chemical resistance and an environmentally friendly composition. The IPC sandwich panels that compose the truss girders, are connected by steel elements, which allow a rapid assembly by pin joints. The design is accompanied by experimental tests on bridge components. These tests reveal that the metal connections are the weak point of the bridge. This is due to assembly problems related to the manufacturing accuracy of both IPC sandwich panels and steel connection elements, and due to the uncertainties as to the connections of the steel inserts to the IPC skins of the sandwich panels. The design shows that in spite of the low stiffness of the glass fibre reinforced IPC, the use of IPC still leads to realistic dimensions of the bridge structure, and to the confirmation that sufficient strength can be obtained for structural applications. The main novelty of the project appears in the bridge concept, which is an act of composite thinking. The project applies classical methodologies for calculations, validation and prototype realization, but shows that a workable structure can be obtained by the synergetic combination of materials, with in this case the IPC as main component.

Numerical analysis and experimental validation for static loads of a composite bridge structure

Abstract The development of a new structure is performed in many different phases. The first phase is the conceptual design, which is necessary to give a preliminary shape to the structure. Afterwards the structural analysis phase follows which is the most important part of the development of a new structure. Analytical methods in complex structures are very demanding and sometimes even impossible to be applied. For this reason the numerical methods are entering the stage and give solutions concerning the structural integrity of the structure that is being examined. In the present work the numerical analysis of an IPC pedestrian bridge and the corresponding experimental results are presented. The aim of this work is to provide the scheme of a numerical modeling procedure that will be applied in the future for the dimensioning of similar structures. In addition different types of elements were investigated as well as different sizes of mesh in order to conclude to a model that gives an exact solution taking into account the computational cost. The model is validated by using real test data extracted from the experimental analysis performed on a real prototype under static loading.

Chloride penetration and carbonation in self-compacting concrete

In this research program, both the steady-state and the non-steady-state migration test are used to determine the chloride diffusion coefficient D (m2/s) of 8 different self-compacting concrete mixes and 1 reference, traditionally vibrated, concrete mix. Simultaneously, the carbonation behaviour of those mixes was investigated. Here fore, the carbonation depth was measured at regular points in time according to NT BUILD 357 (1989) and a carbonation constant A (mm/√year) was deduced. Concerning the chloride diffusion coefficient, test results revealed that the determination of the steady-state migration coefficient according to NT BUILD 355 (1997) is far from easy and question marks could be placed beside the corresponding diffusion coefficient but an explanation for the observed anomalies could not be found yet. The non-steady-state diffusion coefficient according to NT BUILD 492 (1999) was used in order to rank the different concrete mixtures. The carbonation constant could best be measured using an inflated CO2-concentration, resulting in a more linear behaviour of the carbonation depth in function of time. Besides, using this carbonation constant, results reveal that the concrete mixtures could be ranked in the same way as they were by the non-steady-state chloride diffusion coefficient.

Measurement of Complex Moduli of Composite Materials and Discussion of Some Results

This paper describes a new method for the characterization of the engineering constants of linear visco-elastic orthotropic materials.

Parametric Study of an Explosive‐Driven Shock Tube as Blast Loading Tool

The need for efficient blast loading tools is increasing with the development of new protective techniques. Among these tools, one can cite the use of an explosive-driven shock tube (EDST). The purpose of the present study is to define analytical equations to predict incident and reflected pressure and impulse generated at a laboratory scale EDST end in terms of the tube length, the tube diameter, the explosive mass, and the stand-off distance. The formulae are obtained based on a dimensional analysis and a numerical parametric study. The analytical study is supported by a set of experiments, in order to validate the obtained analytical models. The EDSTs discussed in this study are open on both sides with diameters ranging from 0.15 to 0.50m and a length ranging from 0.75 to 3m. Two different tube sections, circular, and square, are examined. Explosive charges from 5 to 50 g of C4 are used. Within these conditions reflected pressures ranging from 0.15 to 11 MPa and reflected impulses ranging from 100 to 3000 Pa·s are obtained at the tube end. The obtained analytical equations are compared to several available results and formulae from the literature. In addition to that, a graphic representation is developed in order to estimate the tube geometry and the explosive mass necessary to generate a given couple of pressure–impulse at the tube end.