The paper presents a nanoindentation study on five different wood species in which the elastic and creep properties of the S2 cell wall layer and the middle lamella were determined. Measurements were carried out at relative humidities (RH) ranging from 10 to 80% as well as underwater. Indentation moduli were found to decrease by about a third in the S2 layer and by about half in the middle lamella between RH of 10 and 80%. Hardness dropped by 50 to 60% in this humidity range in both the S2 layer and the middle lamella. Creep parameters were almost constant up to a relative humidity of 40%, but they increased considerably at higher RH. The most pronounced change of reduced moduli and creep properties occurred between 60 and 80% RH, which is consistent with the expected softening of hemicellulose and amorphous parts of cellulose in this humidity region. Immersion into water resulted in a further decrease of the reduced moduli to about 20 to 30% of their values at 10% RH and to only about 10 to 20% for the hardness. This can be explained by additional softening of the less ordered regions of cellulose. Technology, Karlsplatz 13/202, 1040 Vienna, Austria; b: Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; c: School of Engineering, University of Glasgow, Glasgow G12 8LT, Scotland, United Kingdom; * Corresponding author: leopold.wagner@tuwien.ac.at Keywords: Nanoindentation; Cell wall; Moisture effects; Creep; Hardwood; Softwood Contact information: a: Institute for Mechanics of Materials and Structures, Vienna University of INTRODUCTIONAdsorbed water significantly reduces the mechanical strength of wood. Individual water molecules are able to diffuse into the wood cell wall ultrastructure and to act as a softening agent. Using nanoindentation, this effect can be studied at the cell wall scale. This eliminates the influences of the cellular structure and the overall mass density of wood, which complicate comparisons between different samples of different wood tissues. Moreover, testing at smaller length scales allows for the probing of different cell wall layers individually. Their different compositions and structures should deliver insight into the effects of water on the material and its mechanical behavior.Nanoindentation was applied to samples of five different wood species, which had undergone extensive microstructural characterization, at four different relative humidities (RH) between 10 and 80% as well as underwater. Both the S2 cell wall layer, which is by far the thickest and stiffest layer, and the middle lamella, which connects neighboring wood cells, were tested. These two layers are crucial for load transfer in wood. The S2 layer withstands axial loads whereas the middle lamella controls the behavior of the wood under shear and in bending. In addition to the reduced modulus, the hardness and creep parameters, in terms of increased deformation under constant loading, were determined. PEER-REVIEWED ARTICLEbi...

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Durability is an essential aspect of the lifetime performance of concrete components. The adequate surface quality and thus the service life of concrete can be achieved, among other things, by appropriate curing during hydration. To measure and control the curing quality, appropriate procedures are required. Gas permeability allows conclusions to be drawn about the porosity of concrete, which has a significant impact on durability. In this contribution, the effect of different curing methods on gas permeability is presented with the help of laboratory and on-site tests, showing that inadequate curing leads to increased permeability in the near-surface area of concrete. The measurement results of concrete samples and components with the same composition but varying curing treatment are compared and evaluated. Influences such as concrete composition and environmental factors on the quality of concrete are observed, and recommendations are made for a reliable assessment of the surface quality as a result of the investigated curing measures.

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