M.E. Stavroulaki

A numerical investigation of the effect of the interfacial zone in concrete mixtures under uniaxial compression: The case of the dilute limit

A two-dimensional numerical investigation concerning the effects of the interfacial transition zone (ITZ) on the mechanical behavior of concrete is presented in this paper. The ITZ lies between the cement paste and the aggregates. Considering elastic two-dimensional models of the cement paste, the aggregate material and the ITZ, which is modeled as a thin band around each grain of aggregate, the response of the composite material was calculated using the finite element method. A parametric analysis was conducted for different model geometries as well as varying model parameters for the dilute limit, i.e. when the aggregate volume fraction is very small. Results indicate that the maximum tensile stresses which could lead to the development of microcracks, as well as larger displacements, tend to develop mainly in the ITZ. The distribution of stresses and displacements along appropriate sections of these models are also presented.

Damage detection in concrete structures using a simultaneously activated multi-mode PZT active sensing system: numerical modelling

In this study, a structural health monitoring approach that integrates both electromechanical admittance (EMA) and guided wave (GW) techniques is presented. More specifically, the EMA technique is used for local damage identification, by employing a piezoelectric transducer (PZT) as admittance sensor. Simultaneously, the same admittance sensor is disturbed by selected elastic GWs launched by another PZT to monitor the damages located beyond the sensing area of the admittance sensor. The validation of the integrated approach is achieved by identifying the changes in electrical admittance signatures as measured on the surface electrodes of PZTs. These changes occur when damage alters the mechanical impedance of the examined concrete structure and when propagating GWs encounter structural damage. Finite element models of damages occurring in conventional unreinforced, steel-reinforced or fiber reinforced plastics-reinforced concrete specimens are investigated. Results illustrate that the proposed integrated technique is an efficient approach for damage identification of concrete structures.

Modelling and strength evaluation of masonry bridges using terrestrial photogrammetry and finite elements

Several numerical models are presented in this article, for the study of the ultimate behaviour of a real stone arch bridge. For the exact representation of the geometry an integral and comprehensive survey involving Terrestrial Photogrammetry and Ground Penetrating Radar is in order to provide a realistic 3D geometric model for the subsequent mechanical analysis of the bridge. The accuracy of the photogrammetric method permitted detecting cracks in different areas and the GPR completed the geometric model with information of hidden parts such as backfill, arch ring thickness, etc. Finite element analysis models, incorporating damage, elastoplasticity and contact, are then developed. Comparison between these models is considered in a single arch of the structure. The classical four hinges mechanism appears in the arch. A model of the whole structure, where the arch and the fill are taken into account, is finally developed. Results show how damage is developed in the body of the arch, for loadings that include forces, or vertical and transverse displacements in the supports.

Modelling prestress restoration of buildings by general purpose structural analysis and optimization software, the optimization module of MSC/NASTRAN

Abstract General purpose structural analysis and optimization techniques within the MSC/NASTRAN software are used in this paper for the analysis of optimal prestress restoration of buildings. To alleviate admissible stress violations a minimum prestress reinforcement is sought along with the position of the prestressed elements. Prestressing is modelled by fictitious thermal loading on the linear (rod) elements which model the prestressing cables (tendons). A quadratic cost function for the prestressing cost with a penalty term that counts for stress violations is assumed for the optimal prestressing problem. Certain aspects of the computer implementation, including the use of mathematical programming and structural optimization tools for large-scale structures, are included. The theory is illustrated by numerical examples concerning the prestress restoration of a masonry wall subjected to static loading.

Developing a multi-mode PZT sensing solution for active SHM in concrete structures

Two damage detection and identification techniques are integrated in this work, including electromechanical impedance and guided waves methodologies, by using piezoelectric (PZT) patches bonded or embedded to the host structure. In particular, the electromechanical impedance has been used for identifying local damaged regions by monitoring the changes in structural mechanical properties, which are coupled with the electrical impedance of a PZT patch. In the guided wave propagation, one PZT patch acting as an actuator launches an elastic wave through the structure, which is can be measured by another PZT patch. The integration approach of these two methodologies is then straightforward because the same piezoelectric patch can be used for both methods. This integration approach is experimentally validated in this work by developing a reused and “multi-mode” sensing system for damage detection of concrete structures.

Dynamic Response of Masonry Walls Connected with a Reinforced Concrete Frame

In some cases of rehabilitation only the perimeter walls from the an old unreinforced masonry building are kept and a reinforced concrete frame with horizontal reinforced concrete slabs, is placed inside the initial structure. Proper connections are done in order to have cooperation between these two different structural systems. In order to investigate the influence of the connectivity between these systems on the dynamic response of masonry walls, different finite element models of a simple two-story stone masonry building are analyzed in this work. The model of unilateral contact with friction is used for the interface. Dynamic base excitation is applied on the three-dimensional finite element models considering elastic material for the concrete and elastoplastic material model for the masonry. From the results it is shown that the contribution of a reinforced concrete system to a system of structural masonry walls depends on the way of connection between the two systems, the specific characteristics of the dynamic loads like the earthquakes and the direction of loading, in relation with the geometry of the structure (like the existence of openings to the walls).

Unilateral contact analysis and failure prediction in stone bridges

Unilateral contact analysis with friction can be performed by general purpose finite element programs, provided that the user has basic theoretical background knowledge. These mechanisms are of great importance for the safety evaluation and structural rehabilitation of monuments. This is demonstrated by the investigation of some damage scenarios for a stone bridge.

Optimal prestress in modal analysis via induced temperature modelling

The optimal prestress reinforcement of civil engineering structures is studied here. A modal analysis framework, which is widely adopted for the dynamic, aseismic design, is assumed. The prestress action is modelled by means of induced temperature fields in the prestress cables, an option which facilitates the numerical treatment by means of general purpose finite element software. The optimal choice and the optimal placement of the tendons are treated by a ground (sub) structure technique. The proposed method is illustrated by means of numerical examples.

Dynamic Criteria For The Reinforcement OfOld Buildings

The influence of reinforcement methods in the structural life of masonry walls during the restoration of old buildings is the subject of this work. Comparison of the stress distributions due to earthquake loadings before and after reinforcements using the method of finite elements, permits us to find the critical areas of overloading and to determine the effectiveness of the reinforcements. The prestressed cable method of reinforcement is shown in some detail. Given the materials and geometry of an old building, our main purpose is to improve the prestressed cable method so that the structural wall can have a longer life under consecutive dynamic loadings.

Investigation of the structural behaviour of a masonry castle by considering the actual damage

The structural behaviour of Frangokastello, a mediaeval masonry castle located in Crete, Greece is studied. The structure presents several damaged areas, consisting of cracks and local failure of masonry. The finite element method (FEM) is used to investigate how the existing failure of the structure affects its mechanical response. First, an eigenvalue analysis of the structure without the cracks and a dynamic modal analysis are done. Then, a non-linear constitutive model using a smear crack law is used to investigate the limit state of the structure under static and dynamic loading. Finally, unilateral contact interfaces are introduced, to simulate the cracks which appear in the structure. This model, which consists of several non-linearities, is tested under non-linear time history analysis. Comparison of the results demonstrates how the pathology of the structure affects its response. This procedure is necessary towards taking actions for the reinforcement of the structure.