High-dynamic compressive and tensile strength of specimens made of cementitious materials

The strength of specimens made of cementitious materials increases with increasing loading rate. Herein, themodel of Fischer et al. (CCR 58, 2014, 186–200) is revisited in the context of high-dynamic compression andextended to high-dynamic tension. The model is based on the assumptions (i) that cracking will start if the quasistaticmaterial strength is reached, and (ii) that the high-dynamic strength gain refers to the increase of the stressduring the failure process of the tested specimen. The model explains the behavior of cylindrical specimens madeof dry cement paste, mortar, and concrete, subjected to high-dynamic compression. It also elucidates the performanceof cylindrical specimens made of dry mortar and concrete, subjected to high-dynamic tension. It isconcluded that the high-dynamic strength gain is a structural effect and that structures will be damaged if thedynamic stress exceeds the quasi-static strength, no matter how fast the stress is increased and how short thestress pulse lasts.Austrian Science Fund (FWF

Early-Age Evolution of Strength, Stiffness, and Non-Aging Creep of Concretes: Experimental Characterization and Correlation Analysis

Six different concretes are characterized during material ages between 1 and 28 days. Standard tests regarding strength and stiffness are performed 1, 3, 7, 14, and 28 days after production. Innovative three-minute-long creep tests are repeated hourly during material ages between one and seven days. The results from the standard tests are used to assess and to improve formulas of the fib Model Code 2010: the correlation formula between the 28-day values of the strength and the stiffness, and the evolution formulas describing the early-age evolution of the strength and the stiffness during the first four weeks after production. The results from the innovative tests are used to develop a correlation formula between the 28-day values of Young’s modulus and the creep modulus, and an evolution formula describing the early-age evolution of the creep modulus during the first four weeks after production. Particularly, the analyzed CEM I concretes develop stiffness and strength significantly faster than described by the formulas of the Model Code. The creep modulus of the investigated concretes evolves significantly slower than their strength and stiffness. Thus, concrete loaded at early ages is surprisingly creep active, even if the material appears to be quite mature in terms of its strength and stiffness.Austrian Research Promotion Agency (FFG

Rubber Friction on Ice: Experiments and Modeling

Rubber friction on ice is studied both experimentally and theoretically. The friction tests involve three different rubber tread compounds and four ice surfaces exhibiting different roughness characteristics. Tests are carried out at four different ambient air temperatures ranging from −5 to −13∘C, under three different nominal pressures ranging from 0.15 to 0.45MPa, and at the sliding speed 0.65 m/s. The viscoelastic properties of all the rubber compounds are characterized using dynamic mechanical analysis. The surface topography of all ice surfaces is measured optically. This provides access to standard roughness quantities and to the surface roughness power spectra. As for modeling, we consider two important contributions to rubber friction on ice: (1) a contribution from the viscoelasticity of the rubber activated by ice asperities scratching the rubber surface and (2) an adhesive contribution from shearing the area of real contact between rubber and ice. At first, a macroscopic empirical formula for the friction coefficient is fitted to our test results, yielding a satisfactory correlation. In order to get insight into microscopic features of rubber friction on ice, we also apply the Persson rubber friction and contact mechanics theory. We discuss the role of temperature-dependent plastic smoothing of the ice surfaces and of frictional heating-induced formation of a meltwater film between rubber and ice. The elaborate model exhibits very satisfactory predictive capabilities. The study shows the importance of combining advanced testing and state-of-the-art modeling regarding rubber friction on ice

Computational mechanics of materials and structures

<p/>Mechanics of Materials and Structures has become a popular new name of former Institutes for Strength of Materials and/or Structural Analysis at European Universities of Technology. This designation stands for a scientific program aimed at a symbiosis of material and structural mechanics. The adjective “computational” refers to the algorithmic component of Mechanics of Materials and Structures, which is frequently underrated. It was the advent of the digital computer that opened the door to computational mechanics, which has become a scientific discipline with a tremendous influence on our lives. <p/>This survey paper contains a report about a selection of recent research projects carried out at the Institute for Mechanics of Materials and Structures of Vienna University of Technology. Its aim is to demonstrate that the trinity of Computational Mechanics–Materials–Structures has a strong impact on modern life