Jan L. Vítek

Long-term behaviour of concrete structures reinforced with pre-stressed GFRP tendons

Nowadays, composite materials are used more often in every part of industry, including civil engineering. Using these composite materials in civil engineering is innovational and there are many unanswered questions about these materials and the relaxation of the glass fibre reinforced polymers (GFRP) tendon in pre-stressed concrete is one of them. Knowing the long-term behaviour of the pre-stressed GFRP tendons is very important for the right design. Underestimating the long-term changes in the GFRP tendons can lead to serious problems or collapse of a structure. This paper shows two long-term experiments. One of them is the relaxation of pre-stressed GFRP tendons and the second one is creep of a concrete slab reinforced with pre-stressed GFRP tendons. The first experiment shows that relaxation of pre-stressed GFRP tendons is very high. A GFRP tendon was pre-stressed up to 37% (237,9 MPa) of its tensile strength (654,0 MPa). The decrease of tensile stress when the experiment was closed (after 132 days) was about 10,5%. Based on the experimental data, the numerical viscoelastic model consisting of Kelvin links was developed. The modulus of elasticity of the fibres and matrix was determined with the nanoindentation method. Others parameters were fitted from the experimental data. The chosen numerical model corresponds very well with the experimental data, but for the best outcome a longer experiment should be carried out. The numerical model and fitting of parameters were made in MATLAB 2007a software. The creep of the slab shows the long-term behaviour of a structure reinforced with GFRP tendons. The creep test was ended after one year. A concrete slab pre-stressed with GFRP tendons was subjected to a four point loading test with a constant load. During the year deflections and strain were recorded and hence the creep curve is plotted.

Julia Subtyping: A Rational Reconstruction

Programming languages that support multiple dispatch rely on an expressive notion of subtyping to specify method applicability. In these languages, type annotations on method declarations are used to select, out of a potentially large set of methods, the one that is most appropriate for a particular tuple of arguments. Julia is a language for scientific computing built around multiple dispatch and an expressive subtyping relation. This paper provides the first formal definition of Julias subtype relation and motivates its design. We validate our specification empirically with an implementation of our definition that we compare against the existing Julia implementation on a collection of real-world programs. Our subtype implementation differs on 122 subtype tests out of 6,014,476. The first 120 differences are due to a bug in Julia that was fixed once reported; the remaining 2 are under discussion.

Cable-Stayed Footbridge with UHPC Segmental Deck

Ongoing research and development of ultra-high performance concrete (UHPC) in the Czech Republic has been utilized in design and implementation of light-weight segmental deck of the cable-stayed footbridge over the Labe River in Celakovice with main span of 156 m. Detailing, design issues, construction method and efficiency of using UHPC on this large span lightweight bridge is described in the paper. The superstructure of the Celakovice footbridge was completed in the December 2013 and the bridge was opened for the public in April 2014. Main advantages of this project is not only low maintenance and reasonable life cycle cost but also favourable tender price which was achieved by the contractor Metrostav a.s. due to combination of high-strength modern materials steel and UHPC.

New Model for Concrete Creep and Shrinkage Prediction and its Application

At the beginning of this year prof. Z. P. Bažant and his team published a new numerical model for predicting creep and shrinkage in concrete structures. Model, named B4, is conceptually based on the previous version B3. While early prediction models were based mostly on classical concrete composition, the new model allows for observation the variability of modern concrete compositions, i.e. the effects of admixtures, various aggregate types and increasing concrete strength. The model also captures the effects of environment temperature, multi-decade prediction and autogenous shrinkage. This is important for concretes that are produced in Czech Republic and have higher, but not high strength (about 50 MPa). The model also allows to determine internal parameters according to experimental measurements on laboratory specimens or structural members. Therefore it is possible to refine the prediction of the behavior of structures made of this concrete in the long time periods. However, the increased number of input parameters leads to a higher complexity and it is necessary to have computational tools for practical model application. To provide the model to wider engineering community open structure computational program (in MATLAB environment) was created. Software is freely available for download on the internet. Description of the innovations of the model B4 and demonstration of its relatively simple applications using newly developed software products is a subject of this paper.

Application of UHPC Joints in Precast Structures

Acceleration of bridge construction requires the development of new construction methods. In composite steel concrete bridges the cast in situ bridge deck may be replaced by a precast deck. For fast assembly the joints have to be solved adequately. UHPC joints represent an alternative which can satisfy the requirements on mechanical properties of joints, on construction and economy. The function of the joints was experimentally verified using three different experiments. The performance of beams with precast and cast in situ slab was compared.

Analysis of Performance of Local Puzzle Sheet in a Joint Made of UHPC

Nowadays the extraordinary mechanical properties of UHPC can be used for real structures. The exceptionally high bond of reinforcement and UHPC together with significant shortening of lap length are the key features for the simplification of joining of precast elements. The properties of UHPC can be very well used also for providing of subsequent shear connection of composite structures. This paper is focused on analysis of performance of a combined joint where connection of precast elements as well as subsequent composite action between steel and concrete parts of steel-concrete composite structure are carried out in one compact detail. Instead of the traditional headed shear studs there were used local perforated puzzle sheets. More resilient connection of shear sheets to the top flange and a high resistance of UHPC allow for reduction of the extent of shear connectors. In experimental measurements carried out on six beams the behavior of shear connection of conventional monolithic concrete slab with continuous perforated shear sheet and shear connection of precast slab with the joint made of UHPC with local puzzle sheets were compared. During the experiment it has been proven that the performance of the beams with both variants of shear connection is very similar even with the significant reduction of the extent of shear connectors. Numerical analysis which describes the stressing in this detail confirms such behavior.

Long-Term Monitoring of Concrete Building Structure

Long-term deformations of the concrete are important for determination of behavior of concrete structures. For prediction of the long-term deformations rheological models are used. The models are usually derived from measurements on laboratory specimens. There are only few measurements performed on real structures. The objective of the realized experimental program is to compare predicted values of strains and strains measured on a real structure. Vibrating wire strain gauges have been installed in two columns in underground floors of eight-storey office building. Target of this paper is evaluation of measurement from the initial period (7 to 8 months). Measured values have been compared with results of first calculations and with values measured on laboratory specimens placed at the construction site.

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.

Julia: dynamism and performance reconciled by design

Julia is a programming language for the scientific community that combines features of productivity languages, such as Python or MATLAB, with characteristics of performance-oriented languages, such as C++ or Fortran. Julias productivity features include: dynamic typing, automatic memory management, rich type annotations, and multiple dispatch. At the same time, Julia allows programmers to control memory layout and leverages a specializing just-in-time compiler to eliminate much of the overhead of those features. This paper details the design choices made by the creators of Julia and reflects on the implications of those choices for performance and usability.

Self-contained development environments

Operating systems are traditionally implemented in low- level, performance-oriented programming languages. These languages typically rely on minimal runtime support and provide unfettered access to the underlying hardware. Tra- dition has benefits: developers control the resources that the operating system manages and few performance bottle- necks cannot be overcome with clever feats of programming. On the other hand, this makes operating systems harder to understand and maintain. Furthermore, those languages have few built-in barriers against bugs. This paper is an ex- periment in side-stepping operating systems, and pushing functionality into the runtime of high-level programming languages. The question we try to answer is how much sup- port is needed to run an application written in, say, Smalltalk or Python on bare metal, that is, with no underlying oper- ating system. We present a framework named NopSys that allows this, and we validate it with the implementation of CogNos a Smalltalk virtual machine running on bare x86 hardware. Experimental results suggest that this approach is promising.