Successful ASR Prevention in Germany: Influencing Factors and Adequate Measures

Alkali-Silica Reaction (ASR) in concrete pavements has become a real problem in Germany in the end of the 1990s / beginning of the 2000s. In an extensive research project, the background for such ASR-damaging has been examined intensively at the Ruhr University Bochum. ASR in concrete pavements is not only influenced by the reactivity of the aggregate, but rather by a superposition of different influencing factors. For concrete pavements for example, there are specific conditions that increase an ASR significantly. On the one hand, concrete pavements are microstructurally damaged by the superposition of cyclic stresses induced by traffic and climate changes, and on the other hand they are exposed to alkaline de-icing agents during the wintertime. Thereby, an ASR-promoting external alkali supply is given. Three absolute preconditions are necessary for an ASR to occur: potentially reactive aggregates, sufficient supply of alkalis and an adequate degree of moisture. In Germany, there have been numerous measures taken in the last 10 to 15 years in order to prevent ASR-damages in concrete pavements. Already in 2005 the alkali-content (Na₂O-Equivalent) allowed in cements for concrete pavements has been limited to 0.8% by mass. Additionally, in each case the aggregates intended to be used must be assessed beforehand in a special procedure. Since these requirements were established by the highway-authorities in 2005 (with modifications in 2013) there have been no new damages related to ASR observed on concrete pavements, which have been constructed in compliance with these guidelines.

Analysis of the Behaviour of Rocks Under Cyclic Loads

The cyclic load applied to civil and mining structures can lead to a reduction of the materials’ strength, different from that which would occur with a monotonous load. Numerous cases can be found where the decay of the rock parameters subjected to this type of stress leads to progressive or sudden collapse: among them, tunnel walls, pillars and slabs in mining operations, roads with a heavy vehicle transit, abutments of bridges and dams can be quoted. The topic can therefore be fundamental for a correct structural design, to avoid problems during the life of the structure. However, given the heterogeneity of the rock materials and the difficulty of their characterization under this aspect, an unequivocal analysis is hardly achievable. Then, the discussion initially develops through a general historical review of the concept of fatigue, with a synthetic collection of case histories. The laboratory tests on rock samples are then examined and the most important results obtained are discussed. Finally, a comparison between different types of tests is proposed. The experimental data are expressed through the Wӧhler diagram. The goal is to fill the lack of design codes or standards in the field of cyclic stresses applied to rock materials, the understanding of its effect being of great interest in order to apply suitable parameters in the design phase.

Planning an Experiment for Low-Cycle Fatigue under Conditions Deep Cooling

In the elements of structures with a limited resource during operation, significant cyclic stresses can occur, reaching and exceeding the yield strength; the results of an experimental study of the effect of the magnitude of preliminary plastic deformations on the strength and durability of structural alloys under low-cycle loading can be of undoubted interest for practice. The use of experimental planning methods in the study of the influence of the maximum cycle stresses and the magnitude of the preliminary permanent deformation on the low-cycle fatigue of 03Kh13AG19 chromium-manganese steel at T = 4.2 K under pulsating tension showed that these methods can be successfully used.

Reconstructing Stress Resultants in Wind Turbine Towers Based on Strain Measurements

Support structures of offshore wind turbines are subject to cyclic stresses generated by different time-variant random loadings such as wind, waves, and currents in combination with the excitation by the rotor. In the design phase, the cyclic demand on wind turbine support structure is calculated and forecasted with semi or fully probabilistic engineering models. In some cases, additional cyclic stresses may be induced by construction deviations, unbalanced rotor masses and structural dynamic phenomena such as, for example, the Sommerfeld effect. Both, the significant uncertainties in the design and a validation of absence of unforeseen adverse dynamic phenomena necessitate the employment of measurement systems on the support structures. The quality of the measurements of the cyclic demand on the support structures depends on (a) the precision of the measurement system consisting of sensors, amplifier and data normalization and (b) algorithms for analyzing and converting data to structural health information. This paper presents the probabilistic modelling and analysis of uncertainties in strain measurements performed for the purposes of reconstructing stress resultants in wind turbine towers. It is shown how the uncertainties in the strain measurements affect the uncertainty in the individual components of the reconstructed forces and moments. The analysis identifies the components of the vector of stress resultants that can be reconstructed with sufficient precision.

A new Track of Fatigue crack growth in Aluminum Alloy (2219) under Cyclic Stresses

A study of new Track of Fatigue crack growth in aluminum alloy (2219) under cyclic stresses has been made. It was found out that this crack grow and propagate in three phases, the first phase though the grain size (micro-structure short cracks(MSC)), second phase cross the boundary of the grain size to about 1mm in length (physically short cracks (PSC)) and the third phase up to the final fracture (Length cracks(LC)). In addition, two programs were designed on MATLAB to perform the compute calculations to collect the results. The first program to calculate the practical constants and the second to make the calculations required to complete the work schedules. The stress and the parameters effecting the growth of these cracks in each phase were studied. A model consisting of three formulas was established from the experimental results. Each formula describes the behavior of the cracks in the particular phase. The comparison showed that the proposed model is safer than the experimental results for the designed parts of aircraft.

Bicomponent Carbon Fibre within Woven Fabric for Protective Clothing

For the purpose of this research, six types of woven fabrics with different proportions of bicomponent carbon fibres (CF), differently distributed in the fabric, were woven and tested. Fibre composition in the core and sheath was determined with X-ray spectroscopy (EDS). Two types of bicomponent CF were selected which are characterised by different proportions of carbon and other polymers in the fibre core and sheath and different cross-sections of the fibres formed during chemical spinning. Physical-mechanical properties were investigated, as well as deformations of fabrics after 10,000, 20,000 and 30,000 cycles under biaxial cyclic stress on a patented device. Tests of the surface and vertical electrostatic resistance from fabric front to back side and from the back side to the fabric front were conducted. According to the obtained results and statistical analyses, it was concluded that the proportion of CF affects the fabric’s physical and mechanical properties, the electrostatic resistance as well as the deformations caused by biaxial cyclic stresses. A higher proportion of CF in the fabric and a higher proportion of carbon on the fibre surface, gave lower electrostatic resistance, i.e., better conductivity, especially when CFs are woven in the warp and weft direction. The higher presence of CF on the front of the fabric, as a consequence of the weave, resulted in a lower surface electrostatic resistance.

Modeling of Tool for Cold Extrusion of Steel and Tooling with Proportional Bandaging

In the article the results of the finite element modeling of cold extrusion of carbon steel, the stress-strain state of massive dies at high cyclic stresses characteristic of the cold extrusion of steel are presented. The possibility of significant amplification of dies in their working area by creating a variable external pressure from the rims of a special design is shown. Dies banding with variable external pressure, proportional to the internal pressure in the die, is aimed at increasing the technical resource of the tool.