Vladimír Křístek

The cross-section warping effect with the associated creep-induced deformations

Abstract The modern bridge girders are of plated character for which the beamtype analysis is inadequate. This is why the three-dimensional approach applying advanced computational analyses must be used to obtain the true 3D structural performance. The paper is directed to the cross section warping effect and to the warping induced long term prestressing loss due to creep of concrete. It is shown that the severity of cross section warping is of fundamental importance, particularly in the case of unevenly distributed and isolated tendons in cross-section and in the case of short spans. Thus it will be appropriate to introduce a new component of the prestress loss – the prestress loss due to creep of concrete induced by cross section warping.

Torsion of Reinforced Concrete Structural Members

According to the common design methods of calculation of the stress state induced by torsion of massive prismatic concrete structural elements, the structural system is reduced to a simple cage consisting of ties and struts. This model has, however, a number of principal shortcomings, the major of them is the fact that all of simultaneously acting effects like axial forces, bending moments and shear forces are not taken into account – the compressive axial forces increase very significantly the torque capacity of structural members, while due to action of tensile forces, bending moments and shear forces the torque capacity is reduced. These phenomena, applying non-linear approaches, are analysed and assessed.

Effects of Differential Shrinkage in Concrete Bridges

The effects of the external environment significantly affect the distribution of stress in concrete bridges. Diffusion of humidity and temperature fluctuations, as a result of the effects of the external environment, result in the emergence of a very complex state of stress and strain in the elements of concrete bridges and the creation of acceptable or unacceptable cracks. In order to predict these correctly, one must use a creep and shrinkage model that realistically describes the moisture diffusion process, which causes that the shrinkage and drying creep of the bulky parts of the box cross section are greatly delayed compared to the thin parts. This delay cannot be predicted with the classical approach, in which either the shrinkage strain and the creep coefficient (or compliance function) are considered as uniform throughout the cross section, or the thickness effect is simply described by a multiplicative factor on shrinkage strain. With full respect for these phenomena the bridge of a double T section segmental bridge is analyzed.

Precast concrete pavement – systems and performance review

Long-term traffic restrictions belong to the key disadvantages of conventional cast-in-plane concrete pavements which have been used for technical structures such as roads, parking place and airfield pavements. As a consequence, the pressure is put on the development of such systems which have short construction time, low production costs, long-term durability, low maintenance requirements etc.. The paper presents the first step in the development of an entirely new precast concrete pavement (PCP) system applicable to airfield and highway pavements. The main objective of the review of PCP systems is to acquire a better understanding of the current systems and design methods used for transport infrastructure. There is lack of information on using PCP systems for the construction of entirely new pavements. To most extensive experience is dated back to the 20th century when hexagonal slab panels and system PAG were used in the Soviet Union for the military airfields. Since cast-in-situ pavements became more common, the systems based on precast concrete panels have been mainly utilized for the removal of damaged sections of existing structures including roads, highways etc.. Namely, it concerns Fort Miller Super Slab system, Michigan system, Uretek Stitch system and Kwik system. The presented review indicates several issues associated with the listed PCP systems and their applications to the repair and rehabilitation of existing structures. Among others, the type of manufacturing technology, particularly the position of slots for dowel bars, affects the durability and performance of the systems. Gathered information serve for the development of a new system for airfield and highway pavement construction.

Strength Classes of Concrete versus Strength Classes of Fibre Concrete

Basic characteristics of each produced concrete and fibre reinforced concrete are characterized by the compression strength recorded by the standard sets of tests performed on cylinders and cubes. In addition, for the fibre reinforced concrete, the characteristic tensile strength at formation of microcracks and cracks of standard widths is required. Proofs of the referred characteristic tensile strength should be carried out also by the destructive tests on standard specimens including the methodology provided for their implementation.The rapid development of fibre reinforced concrete, accelerated by manufacturers of fibres and their interest to apply the fibre reinforced concrete in structural practice from where the beneficial effects of the tensile strength can be obtained, resulted in conclusion that there is currently no uniform methodology for evaluation of the tensile strength. Tensile strength studies are performed, for example, according to National standardization Committees and research institutes.At present, the two very different methodologies can be applied to test tensile characteristics of fibre reinforced concrete - MODEL CODE and the Czech national standard – ČSN P 73 2452. The results of the destructive tests, obtained in accordance with the mentioned methodologies are so different that the same strength class for the tested fibre reinforced concrete is not possible to be defined.The paper proves the diversity of methodologies to perform destructive testing, by which it is possible to obtain the tensile characteristics of fibre reinforced concrete needed to define the same strength class. Procedures for evaluation of tensile characteristics from results of destructive tests are also assessed. Significance of the obtained strengths from the point of view of objectivity for the practical application of the fibre concrete in the load-carrying structures are discussed.

3D Analysis of Bridges Changing Structural Systems – An Easy Design Tool

Due to increase of requirements on accuracy of structural analyses, practically applicable computational tools for reliable determination of the real structural performance of bridges are needed. A method is proposed for the true full 3D analysis which can be applied to achieve the real spatial behaviour of concrete bridge structures taking into account rheological phenomena and changes of structural systems. Particularly, the effects of shear lag, shear performance of webs of box girders, warping torsion, warping of cross-sections, distortional effects, state of stresses in the singular regions, the real prestress loss, etc., can be correctly determined. The method combines conventional approaches (based on the elementary beamtype assumptions) for calculating the time development of the internal forces due to rheological phenomena and changes in the structural system during construction and routine commercial FEM software intended for calculating spatial shell structures. The method is capable to give the true 3D prediction of structure behaviour by using only commercially available software. The primary advantage of the proposed method is its ease of application which allows the true 3D performance to be determined from simple calculations. The method offers the designers of concrete bridge girders an ideal design tool. The correct 3D simulation can lead to more efficient and economical designs.

Calculating for concrete creep

The authors present an approximate method for calculating creep effects in concrete members and structures without recourse to computers. This ‘effective time’ method has been verified for accuracy by comparing the values thus predicted with results obtained by the step‐by‐step computer solution. Examples are given to illustrate its application. The authors are respectively Chief Scientific Officer and a PhD researcher at Prague Technical University.

Simplified Calculation of the Relaxation of Stress Respecting the Delayed Elasticity

The solution of the relaxation problems that occurs very often in engineering practice when calculating the concrete structures is very complicated unless the calculations are based in the elementary methods as on the rate-of-creep method or perhaps on the principle of delayed elasticity. The more exact calculation of the stresses relaxation regarding the more complex expressions for the creep is rather tedious because the use of an iterative method is unavoidable. So it is when the creep and shrinkage predictions proscribed in the Recommendations of CEB-FIP (1978) are used, and thus the calculation of the stresses in the case of sustained deformations should be done by some iterative way which of course is not too favourite among the engineers.