Investigation of the Influence of Moisture Content on Fatigue Behaviour of HPC by Using DMA and XRCT

High-performance concrete (HPC) is a topic of current research and construction projects, due to its outstanding compressive strength and durability. In particular, its behaviour under high-cycle fatigue loading is the focus of current investigations, to further pave the way to highly challenging long-lasting constructions; e.g., bridges or offshore buildings. In order to investigate the behaviour of HPC with different moisture contents in more detail, a mixture of silica sand and basalt aggregate with a maximum grain size of 8 mm was investigated with three different moisture contents. For this purpose, cyclic compressive fatigue tests at a loading frequency of 10 Hz and different maximum stress levels were performed. The main focus was the moisture influence on the number of cycles to failure and the development of concrete temperature and strain. In a further step, only the mortar matrix was investigated. For this purpose, the mixture was produced without basalt, and the moisture influence was investigated on smaller-sized test specimens using dynamic mechanical analysis (DMA) and X-ray computed tomography (XRCT). It was shown that the moisture content of HPC had a significant influence on the fatigue damage behaviour due to the number of cycles to failure decreasing significantly with increased moisture. In addition, there was also an influence on the temperature development, as well as on the strain development. It was shown that increasing moisture content was associated with an increase in strain development. XRCT scans, in the course of the damage phases, showed an increase in internal cracks, and made their size visible. With the help of DMA as a new research method in the field of concrete research, we were also able to measure damage development related to a decrease in sample stiffness. Both methods, XRCT and DMA, can be listed as nondestructive methods, and thus can complement the known destructive test methods, such as light microscopy.

Insights about CO Gas-Sensing Mechanism with NiO-Based Gas Sensors—The Influence of Humidity

Polycrystalline NiO thick film-based gas sensors have been exposed to different test gas atmospheres at 250 °C and measured via simultaneous electrical resistance and work function investigations. Accordingly, we decoupled different features manifested toward the potential changes, i.e., work function, band-bending, and electron affinity. The experimental results have shown that the presence of moisture induces an unusual behavior toward carbon monoxide (CO) detection by considering different surface adsorption sites. On this basis, we derived an appropriate detection mechanism capable of explaining the lack of moisture influence over the CO detection with NiO-sensitive materials. As such, CO might have both chemical and dipolar interactions with pre-adsorbed or lattice oxygen species, thus canceling out the effect of moisture. Additionally, morphology, structure, and surface chemistry were addressed, and the results have been linked to the sensing properties envisaging the role played by the porous quasispherical–hollow structures and surface hydration.

Hygromechanical Performance of Polyamide Specimens Made with Fused Filament Fabrication

The material performance of polyamide (PA) samples made with fused filament fabrication (FFF) was analyzed. The authors implemented a well-structured framework to identify the filaments main properties before processing them and characterizing the printed samples. Unfilled and glass-fiber reinforced PA were investigated, focusing on moisture absorption and its effects on dimensional stability and mechanical performance. The properties were collected using differential scanning calorimetry and Fourier-transform infrared spectroscopy, whereas the specimens were characterized by employing compression tests. This framework allowed for the moisture determination, as well as the influence of the moisture absorption. A significant impact was detected for the glass-fiber reinforced PA, with a decrease in the dimensional and mechanical performance. The novelty of this study was to define a well-structured framework for testing the moisture influence of FFF components.

How does the soil moisture influence the surface energy fluxes? An observational study at La Herrería Forest (Central Spain)

<p>This work presents the characterization and comparison of the response of evapotranspiration (ET) to variations in shallow soil moisture (SM) in three years with different precipitation regimes: 2017, 2018 and 2019, through the analysis of tower data from La Herrería site, a forest site in the foothills of the Guadarrama Mountains in Spain. The aim of this work is to improve the comprehension of the relations of these variables (ET and SM) and their dependence on rain regimes in the studied years. To assess this, monthly SM regimes are considered, with three main types: transitional, wet and dry. The study shows the highly variable response of ET to variations in SM, which depends on the three considered SM regimes. In transitional regimes, SM strongly constrains ET variability, in wet regimes, SM does not impact ET variability, and in dry regimes, SM has a small impact in ET variability, due to its small variations. In particular, the months which suit satisfactorily to these regimes are identified, such as July 2018 (transitional, r=0.73), November 2019 (wet, r=-0.27) and August 2018 (dry, r=0.36), being r the coefficient of linear correlation between ET and SM. Some months that do not fit in the proposed scheme are also identified, and they have to be analyzed independently. This research shows the need to take into account different physical processes that affect ET, the complexity in the treatment of observational (tower) data for this type of analysis, and illustrates how the election of the length of the studied period is important for this type of analysis. Hence, it should be carefully chosen, because the interpretation of the results can be different depending on this choice.</p>

STUDY OF THE PLASTICIZING EFFECT OF MOISTURE ON THE PROPERTIES OF VSE-34 BINDER BASED PCMS AFTER 5 YEARS EXPOSURE IN THE DIFFERENT CLIMATE ZONES

The plasticizing effect of moisture on the glass transition temperature of VSE-34 solvent-free epoxy binder based polymer composite materials (PCMs) with different reinforcement was studied. Natural weathering was performed in three climatic regions of Russia: Moscow, Gelendzhik, Sochi. It was shown that the glass transition temperature is significantly reduced after 1 year of exposure and then is gradually increased after 3 and 5 years of exposure. Samples were studied in «as-received» state as well as after drying and saturation by water till mass stabilization in order to investigate the moisture influence on PCMs’ glass transition temperature. Linear relationship between glass transition temperature and moisture content was observed.

Moisture Influence on Compressive Strength of Calcium Silicate Masonry Units–Experimental Assessment and Normative Calculations

This paper primarily assesses the scale of adverse changes to the compressive strength of different types of silicates due to the influence of moisture. The study covers three groups of silicate units of different strength classes—15, 20 and 25—obtained from three different manufacturers. It was demonstrated that in all studied groups, moisture significantly decreased the compressive strength by about 30–40%. In addition, microstructural studies were conducted to analyze the relationship between the specific porosity structure of each group of silicate bricks and their compressive strength. On the basis of SEM (Scanning Electron Microscopy) and EDS (Electron Dispersive Spectroscopy) analysis, the elemental composition of individual silicates was determined and the contact zone between the aggregate and the binder was determined, which largely influenced the obtained compressive strength of each silicates. Next, the study referred to the utility of the normative procedure used to determine the strength class of samples with different geometries and at different moisture concentrations. The results of the calculations showed the high accuracy of the normative-based assessment of strength class, regardless of the manufacturer and the moisture values during examination.

USE OF A PENETRATING ADDITIVE TO INCREASE THE WATER RESISTANCE OF CONCRETE

The penetration of moisture into the capillary-porous structure of concrete leads to the development of corrosion processes, the formation of efflorescence, the decrease in strength and the subsequent destruction of the cement-sand composite. At the same time, the increased humidity of the structure is a favorable environment for the development of various biological damages (mold, fungi, lichens), which, in turn, leads to both decrease in the operational characteristics of the structure and loss of the aesthetic appearance of the building facade. In this regard, the study of possibility of increasing the waterproofing of concrete by compaction of its structure is of great interest. The paper discusses the effect of penetrating additives on the physicomechanical characteristics of fine-grained concrete and fiber-reinforced concrete. It has been established that the introduction of a penetrating additive into the composition of cement-sand composites leads to the filling of the capillary-porous structure of materials with needle-like new formations, which makes it possible to increase their resistance to moisture influence, namely, to increase the waterproof grade, reduce the depth of water penetration into the concrete structure, increase the contact angle of wetting the surface of the material. It was also noted that the simultaneous introduction of a penetrating additive and fiber into the concrete composition allows not only to increase the waterproofness of the composite, but also to improve hydrophobic performance by imparting a hierarchical surface structure to the material.