Radomir Folić

Experimental Research on Polymer Modified Concrete

Some results from experimental research on polymer modified concrete are presented concerning the testing of fresh and hardened concrete, as well as determination of optimal curing conditions. Latex dispersion on the basis of butadiene styrene rubber was used as a cement modifier. All properties of modified concretes were analyzed based on the quantity of the polymer used. Investigation of the hardened modified concrete behavior was made on the basis of testing concretes modified with 2.5%, 5%, and 7.5% of polymer admixture on the cement mass. Approximately 180 concrete samples of cubical, prismical, and cylindrical shapes were tested. The greatest effects of polymer modification were on the increase of concrete tensile strength, ductility, bond between reinforcement and concrete, and improvement of properties contributing to the durability of reinforcement structures.

The assessment of the structure of Novi Sad Open University damaged in a fire

Abstract The fire which spread over the top six floors of the 54 m tall building of the Open University in Novi Sad, caused severe damage to the load-bearing reinforced (RC) concrete structure and the steel facade structure. This paper presents the recorded data on the damage and the assessment of the structure after the fire. All the damage is described and the most characteristic ones are illustrated with photographs. The damages caused by the fire have been classified according to the criteria of the manner and degree to which they are manifested. The authors separate and describe the defects, which occurred as a result of errors made during the construction of the building. The results of the subsequent testing of the quality of the built-in materials are also presented in the paper. It has been concluded that the damage of the elements of the load-bearing RC structure from the 8 th to the 12 th floor has been so severe that the stability and bearing capacity of this part of the structure has been jeopardized, but with appropriate repair measures it can be restored to its previous condition.

Identification of Modal Parameters of Bridges Using Ambient Vibration Measurements

The paper provides an overview of ambient vibration tests and numerical analysis performed in the framework of Project NATO SfP 983828. The aim of the research is the definition of the dynamic characteristics of bridges on the examples. The paper considers three case studies: two older existing bridges and one newly constructed bridge. A comparative analysis of natural frequencies and mode shapes, obtained by ambient vibration measurements (AVM) and mathematical models (AMs), was carried with the aim to demonstrate the usefulness of ambient vibration tests for identification of the modal parameters of the tested bridge structure. Agreement between AVM and AMs results is very good. The mode shapes are very similar. Some differences between computed and measured frequencies were obtained, which can be attributed to the real nature of the boundary conditions, the uncertainty in the material properties of structure elements, and the mathematical models assumptions.

Durability design of concrete structures, Part 2: Modelling and structural assessment

This paper is a sequel to [19] where the theoretical foundations were given for the analysis of durability of concrete structures. An overview of some approaches to modeling the durability of RC structures is given in the paper. Experiences resulting from experimental and numerical research of some influential parameters leading to corrosion of the reinforcement, cracks and delaminating of the surrounding concrete, which leads to the decrease of service life of the RC structures elements, are described in details. Methodologies of structure evaluation presented in this paper are based on assessment of critical influential parameters and represent a methodology of reasonable maintenance of an engineering construction, i.e. reasonable service life of the structure. The methods are based on analysis of conditions and performance (Condition Rating Method and Performance Rating Method). The results of ratings of concrete structures in the industrial zone of the city of Tuzla are presented in the final part of the paper.

Assessment of Cable-Stayed Pedestrian and Motorway Overpass

The paper summarizes an example of assessment of a cable-stayed pedestrian and motorway overpass in Tuzla city urban environment based on static and dynamic tests. The outlined research included testing of bridges induced by vehicles, ambient vibration testing, modal identification, finite element modelling and assessment of structural behaviour. The field investigations involved measurement of deformations (displacement and dilatation), vibrations induced by vehicles and ambient vibrations. Modes were identified by using Enhanced Frequency Domain Decomposition techniques (frequency domain). Details on the experimental procedures, instrumentation and data analysis techniques are presented and discussed. In the numerical study, deformations forms and vibration modes were determined using a 3D finite element model of the bridge and the information obtained from the field tests. Some conclusions and recommendations for similar bridges were given at the end of the paper.

Uslojeni konačni elemenati u proračunu spregnutih konstrukcija izloženih dugotrajnim djelovanjima

U radu je predložen jedan numerički proračunski model za analizu stanja naprezanja i deformacija zbog puzanja i skupljanja betona i relaksacije čelika za prednapinjanje kod statički neodređenih spregnutih konstrukcija utemeljen na uslojenim konačnim elementima. Algoritam proračuna omogućava da se kroz karakteristične vremenske intervale uvedu sve bitne promjene u konstrukciji u skladu s tehnološkim postupcima pri izvođenju radova. Razvijen je i odgovarajući računalni program prilagođen inženjerskim proračunima u praksi.

Torsional buckling of thin-walled beams in presence of bimoment induced by axial loads

Original scientific paper In this paper the influence of the bimoment induced by external axial loads on the elastic torsional buckling of thin-walled beams with open cross-section is studied. The governing differential equations of a deformed thin-walled beam consistent with Vlasov’s classical assumptions are derived applying the principle of virtual displacements. It is shown that in the cross sections with lack of symmetry the bimoment can considerably affect the torsional buckling load. The obtained results are verified using ANSYS finite element software.

Identifikacija sustava armiranobetonskih grednih mostova utemeljena na terenskim mjerenjima i numeričkim simulacijama

Original scientific paper An overview of research performed in the framework of the NATO Project SfP 983828 is given in the paper. The scope of the research was to identify the parameters affecting the dynamic response of an existing R/C girder bridge, based on ambient vibration measurements and numerical simulations using finite element models (FEM). For this purpose, the bridge across the river Bosnia near Sarajevo and the soil surrounding the bridge were instrumented. Ambient vibration tests and geophysical investigations were performed. The results are studied and a refined three-dimensional (3D) FEM is developed that takes into consideration the soil-structure interaction and superstructure-substructure interaction. The FEM’s with designed parameters and parameters obtained by measurements were developed. The developed FEM models are comparatively assessed and FEM model with congruence between the measured and computationally predicted dynamic characteristics of the structure was defined. The results of the analysis show that the adequate determination of the pier, deck and bearings stiffness is the key parameter for reliable system identification.