Part I of this paper introduces an experimental programme carried out on RC members with thick-walled hollow circular crosssections to study their behaviour under combined bending and shear. The study looked at ultimate resistance and propagation of characteristic crack pattern as well as the shape and behaviour of the failure sections as a function of wall thickness, amount of longitudinal and transverse reinforcement, shear span and axial force. Test results were used to verify a newly developed calculation model describing the behaviour of the members investigated at failure under combined bending and shear. This model will be presented in Part II.

The aim of the research was to characterize the concrete mixtures for spun-cast concrete and to determine the relationship between the compacting ratio and the manner of segregation. In the research 23 specimens of 9 different mixtures with low w/c ratio were tested. Investigated parameters included w/c, spinning speed, duration, and properties of the aggregate. The strength of the concrete in the outer, middle and inner region of the spun-cast element and of the vibrated cube was assessed by testing drilled cores. The optimal compacting energy caused by spinning was defined for different mixtures. Statistical regression functions depending on the parameters above were defined for the change of strength in the wall of the element.Keywords spun-cast concrete · strength · experimental test AcknowledgementThe authors wish to express their gratitude to BVM Épelem LTD, SW Umwelttechnik Hungary LTD, Railone LTD for the research materials and for sponsoring the research. Thanks to the Department of Structural Materials and Engineering Geology for their assistance in the laboratory work. Special thanks to Mr András Eipl. István VölgyiDepartment of Structural Engineering, BME, Bertalan L. u.

The aim of the research was to determine the relationship between the Schmidt N rebound index and the compressive strength of spun-cast concrete elements with low water-cement (w/c) ratio. In the research 22 specimens of 8 different mixtures were tested. The investigated mixture parameters included w/c ratio, compaction intensity and type of aggregate. The compressive strength of the concrete in the outer, middle and inner region of the spun-cast element and of the vibrated cube were assessed by testing drilled cores after non-destructive testing of the specimens. Statistical regression functions have been defined for the determination of the relationship between the rebound index and the compressive strength. A special new method has been developed for the quality control of spun-cast concrete elements using a rebound hammer.Keywords spun-cast concrete · strength · parametric experimental study · rebound index · quality control Acknowledgement The authors wish to express their gratitude to BVM Épelem Kft., SW Umwelttechnik Hungary Kft. for the research materials and for sponsoring the research. Thanks to the Department of Structural Materials and Engineering Geology for their assistance in the laboratory work. Special thanks to Dr. Salem Georges Nehme and Mr András Eipl.This work is connected to the scientific program of the "Development of quality-oriented and harmonized R+D+I strategy and functional model at BME" project. This project is supported by the New Hungary Development Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). István Völgyi

Part II analyses the applicability of current shear design models for RC members with a hollow circular cross-section on the basis of experimental results introduced in Part I of this paper. A new calculation model is proposed which assigns the contribution of the concrete zone in compression to shear resistance. The proposed model takes into account how the flexural and shear reinforcement, the load-to-support distance and the shape of the cross-section affect the shear resistance. The model is based on the analysis of potential failure sections subjected to bending and shear, and applies a compatibility criterion that considers how the member carries the load. The analogy between the failure of the concrete compression zone and the failure of the soil along a sliding surface are presented as well as the conditions for the development of the failure section.