University of Structural Engineering and Architecture Lyuben Karavelov

Abstract The objects of the study were cement mortars prepared from white Portland cement, natural river sand, marble powder and a high range water reducing admixture. Microstructural analysis was carried out by powder X-ray diffraction, scanning electron microscopy and differential thermal analyses, enhanced with evolved gas analysis. Analysis of the results showed that stable structure formed at early stages, with minimal changes in phase composition and quantity during subsequent aging in water. The mortars, in which the sand was completely replaced by marble powder with fine additive, was characterized by stability of composition, which gives reason to conclude for the preparation of cement mortars with high durability.

This report examines the behavior of a reinforced concrete slab, part of an experimental reinforced concrete structure, under loads derived from the casting of the formwork system of the subsequent plate and from the laid concrete mix in the slab above it. The deflections in the individual stages were measured with the help of dial gauges. The full strength and deformation characteristics of the concretes used in the construction were researched. The behavior at all stages of loading of the slab was studied, including each phase of formwork of the slab above it and the laying of the concrete in the formwork itself. Conclusions have been made about the behavior of the slabs during production using the method for Early striking of reinforced concrete slabs.

The report considers two variants of a building with a special purpose of the roof slab. Option 1 is a model of the building - without a helicopter pad, and Option 2 has a helicopter pad. For basic structural elements, the values of: displacements, shear forces and load-bearing reinforcements are shown. Specialized software for computational models is used. Combinations of impacts are proposed, including dynamic loading. The solutions of the two options are compared and a conclusion is made about the efficiency of the solutions with the helicopter service. The maximum values of the internal forces are obtained by taking into account together the horizontal and vertical components of the seismic impact.

This study is devoted to the comparison of shear wall computational models and the juxtaposition of displacement and stress fields. Two types of numerical models are compared. The first type concerns discretization of the geometry by using the beam finite elements model, and the second type-with plane finite elements. The displacements and the internal forces are used as the key indicators for the comparison of the mechanical behaviour. Some conclusions have been drawn, as well as recommendations have been made on the application of the juxtaposed type computational models and their practical application in real life.

The report shows a continuation of analyses of reinforced concrete shear walls solved by horizontal forces [14]. In certain places in the walls the stiffnesses are reduced, thus comparing the obtained results between the groups of models. Solutions for wall and mixed structural systems are shown. The decisions were made with specialized Tower software based on work by the Finite Element Method. Interesting conclusions can be seen for the level between the rigid basement and the first floor. Decreases in bending moments and increases in shear forces are observed. Motivation – quick analysis of walls subjected to the action of horizontal forces (hand analysis, without program system); the goals-proving the idea of the influence of the supports from the basement levels; the methodology-solutions with specialized software-only for a certain speed in the decisions. The solutions have characteristic geometry and loads and can be easily parameterized with correction factors.

In the process of the structural design of the working and façade scaffolds, the self-weight of the elements, the service load and the wind load should be taken into account. The wind load on a covered façade scaffold is determined by many factors, including the wind velocity, the solidity ratio of the building’s façade, the geometric characteristics of the scaffold’ components and the type of cladding – netting or sheeting. The paper investigates the effects of wind loads on both types of façade scaffolding – with and without claddings. A computational finite element model was developed to study the specifics of the scaffoldings in the function of the nets permeability and the solidity ratio of the façade. Load combinations, according to the requirements in EN 12811-1, were considered to obtain the internal forces and stresses of the scaffold elements subject to wind actions. Based on the numerical analyses, it was found that the porosity and air permeability of the nets have a significant effect on the internal forces and stresses of the façade scaffolds elements, which also depend on the anchorage patterns and solidity ratio of the façade. In conclusion, recommendations are given regarding the application of specific structural schemes and anchoring patterns, depending on the permeability of the different façade scaffold nets.

Objects of this research are white cement mortars, characterized by both of high content of addition of marble powder and reduced water-cement ratio. The hardened cement mortars, formed after 28-and 120-day curing under standard conditions, are studied. The newly formed phases, containing [SO4]2-, [CO3]2-, [OH]-, etc., are identified using X-ray powder diffraction, infrared spectroscopy and thermal analysis (coupled with analysis of the outgoing gas mixture by mass spectrometry). Based on of the formed calcium silicate hydrates, calcium aluminate hydrates, the influence of sulphate ions is analysed and the mechanism of the thermal decomposition reaction at high temperatures in an oxidizing gas environment is studied. This allows us to establish that the hydration of Portland cement depends on the addition of marble powder (technically calcium carbonate CaCO3), as well as mono- and hemicarboaluminates are formed instead of monosulphoaluminates.

This article describes the experimental studies to determine the compressive strength and modulus of elasticity of concrete in 12 reinforced concrete beams at the age of 5 years using a non-destructive ultrasonic pulse velocity method (NDUPVM). The probable compressive strength and modulus of elasticity were determined by a portable ultrasonic device Proceq - TICO, according to EN 12504-4: 2004. The results were evaluated for three different ways of positioning the transmitter and receiver - direct transition, semi-direct transition and indirect or surface transition. The compressive strength and modulus of elasticity were calculated accordingly the experimental results. The results were compared with theoretically calculated according to EN 1992-1-1. Experimental beams are made by concrete class C25/30, fine fraction of coarse aggregate (dmax=12 mm), consistency S3.

This paper describes the construction and behavior of a large steel double twin truss launching gantry with total length 168m, designed by Dr. Eng. Borislav Bankov (1912÷1992), used for erection of prestressed concrete bridge girders with span up to 60m (weight up to 220ton) along the Hemus highway, Republic of Bulgaria. It consists of two twin frame steel girders and rails on the top and on the bottom flanges for moving the special girder carrying devices. Experimental investigations are performed by the Road Agency and vertical deflections of the mentioned twin truss under different loads and support conditions are measured. 3D FEM of the existing steel twin truss launching gantry considering and neglecting the the increased axial stiffness of the lower chord at truss joint zones are developed by the authors to investigate its behavior under different load and support conditions. Results from these FEM are analyzed, compared with the test results and discussed.

Durch die durch die DBU geforderte und vor kurzem positiv abgeschlossene zweijahrige umfassende Machbarkeitsstudie bzgl. der Ubertragung und Realisierung des TYPHABOARD-Konzepts auf die regionalen, baulichen sowie soziookonomischen Gegebenheiten in Bulgarien wurde eine fundierte wissenschaftliche Grundlage geschaffen. Das Projekt hat Werner Theuerkorn, gemeinsam mit dem Fraunhofer IBP, geleitet, und das Fraunhofer IMW, die VSU Lyuben Karavelkov in Bulgarien, sowie Sienit Holding haben aktiv mitgewirkt.