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Item type:Publication, Access status: without files , Impact of sulfate on the release of genotoxic metals from hardened cement pastes(Elsevier, 2025-07-15); ; ; ; We investigated the effects of environmentally relevant sulfate concentrations on the leaching behavior of certain metalloids in hardened cement pastes. In our study, different cement pastes made of Portland cement (CEM I), blast furnace slag cement (CEM III/A) and a sulfate-resistant cement (CEM I SR0) were cured for 28 days and leached with ultrapure water and with sulfate-containing water. The released concentrations of the most metals and metalloids were independent of the presence of environmentally relevant sulfate concentrations below 1 µg/L or even below the limit of quantification (LOQ). However, the contact to sulfate-containing water led to an increased chromium release from CEM I, compared to leaching in ultrapure water. Under the same conditions an increased release of vanadium was observed from CEM III/A. A micronucleus test of the selected eluates revealed genotoxic effects which can be very likely attributed to the presence of vanadate. We were further able to connect the different leaching behavior of cement in sulfate-containing water compared to ultrapure water to changes of the specific surface area of the hardened cement pastes.Publicationtype: Journal Article4 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A rectified unidirectional rotary PTO for two-body wave energy converters(2023-07-01); ; ; ; This work presents a new mechanism for a mechanical power take-off (PTO) system that can be used in a variety of wave energy converters (WECs). This mechanism uses two ball screws to convert the relative linear motion of the WEC bodies into unidirectional rotary motion. The resulting rotary motion finally causes a generator to rotate and generate electricity. Due to the oscillatory nature of the wave, the relative velocity of the WEC bodies will have a sinusoidal motion; therefore, the angular velocity of the ball screw becomes zero at two moments during one period of movement. The designed mechanism prevents the generator shaft from changing its direction of rotation at these moments. This mechanism also decouples the generator shaft from the ball screw when the angular velocity of the generator shaft exceeds the angular velocity of the ball screw due to its rotational moment of inertia. As the relative velocity of the bodies increases, the generator shaft is re-coupled to the ball screw and, in consequence, rotates at the angular velocity of the ball screw. This article examines the effect of using the present mechanism on increasing the electrical output power of the WEC at different wave frequencies.Publicationtype: Journal Article41 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of Grouting on the Resistance of Portland and Blast Furnace Slag Cement Paste Against Chemical Attack by Aggressive Carbon Dioxide(2023-06); ; Chemical attack by aggressive carbon dioxide on cement-based materials is of major concern for foundations of engineering structures with long service life. The exposure in natural groundwaters leads to dissolving attack, where all hydration products are deteriorated. The resistance of cement-based materials increases therefore with a low porosity and a high content of inert aggregates. For ground anchors cement suspensions are grouted during the manufacturing process. In non-cohesive soils the mixing water is filtered by the applied pressure. This leads to an improved and dense microstructure. Due to their small dimensions and high tensile load capacity ground anchors can be subjected to a loss of load bearing capacity already in short-term periods because of the attack by aggressive carbon dioxide. This paper investigates the effect of grouting on the resistance of cement paste against the chemical attack by aggressive carbon dioxide. Three cement types CEM I, CEM III/A and CEM III/B are investigated. The object of the investigations are grouted and neat cement paste samples which are prepared in the laboratory and afterwards stored for 3 and 6 months in an aggressive solution with ~ 90 mg CO2 /l. The depth of impact is assessed by polarized light microscopy on thin sections. The results conclude in an assessment of the resistance of the different cement types and an evaluation of the durability for issues in building practice.Publicationtype: Conference Paper20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Quellverhalten von Portland- und Hochofenzementstein bei unterschiedlichen TemperaturenZementgebundene Baustoffe quellen bei permanenter Wasserlagerung, d. h., sie erfahren eine zeitabhängige Volumenzunahme. Wesentlich für das Quellverhalten zementgebundener Baustoffe ist der volumetrische Anteil an Zementstein. Für Bereiche, in denen Betonbauteile permanent einer Wasserumgebung ausgesetzt sind, wie zum Beispiel bei Gründungen im Grundwasser, kann das Quellverhalten ggf. die Tragwirkung verbessern, da sich Bauteile im Boden infolge des Quelldruckes verspannen. Dieser Beitrag untersucht das Quellverhalten von Zementstein aus CEM I und CEM III/B mit w/z-Werten von 0,3 und 0,5 bei 10 °C und 20 °C betrachtet. Zusätzlich wurden im Rahmen der Untersuchungen Zementsuspensionen verpresst, um Filtrationseffekte abzubilden. Neben dem Quellmaß zu verschiedenen Zeitpunkten (1, 3, 5, 7, 14, 28 und 56 Tage) wurde der Hydratationsgrad und Gehalt an Calciumhydroxid mit Hilfe der Thermogravimetrie und die Porosität mittels Quecksilberdruckporosimetrie als Materialkennwerte bestimmt. Die Untersuchungsmethoden erlauben einen Rückschluss von Porosität und Hydratphasenausbildung auf das Quellverhalten der untersuchten Zementsteine zu ziehen. Bei beiden Zementarten führen niedrigere Lagerungstemperaturen zu höheren Quellmaßen.Publicationtype: Conference Paper19 33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Attack of aggressive carbon dioxide on hardened Portland and blast furnace slag cement pasteNatural groundwater and soil can contain substances which are aggressive to concrete and cement-based structures. Especially aggressive carbon dioxide occurs frequently. Chemical attack by aggressive carbon dioxide occurs in several phases in commercial cements used in foundation engineering. After a period of carbonation a leading front where calcium hydroxide is dissolved, depending on the availability in the binder matrix, is generated. As the attack progresses, the calcium carbonate formed is dissolved, leaving a layer rich in amorphous silicon dioxide, which has lower mechanical strength. This can result in problems in the long-term load-bearing behavior of more sensitive components with smaller dimensions and under tensile stress, in particular ground anchors and micro piles, which are manufactured using cement suspensions in the subsoil. This study evaluates the mechanisms induced by aggressive carbon dioxide in CEM III/A and CEM III/B compared with CEM I paste. Cement paste specimen with water-cement ratios of 0.4 and 0.5 stored in about 90 mg/l (exposure class XA2) aggressive CO2 for 3, 6 and 12 months are investigated. The methods include X-Ray fluorescence analysis as well as polarized light microscopy on thin sections. The depth of impact is measured and evaluated in terms of durability of geotechnical elements.Publicationtype: Conference Paper20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical and experimental study of the hydrodynamic coefficients and power absorption of a two-body point absorber wave energy converterThis study compares the hydrodynamic coefficients of a two-body point absorber wave energy converter (WEC) model obtained by three methods, namely the experimental method, the finite volume method (FVM), and the boundary element method (BEM). The BEM and FVM numerical simulations of the WEC motion are done using ANSYS AQWA and OpenFOAM, respectively. An experimental method for the accurate calculation of the added mass, damping coefficient, and wave excitation force for the WEC is introduced. The experiments are carried out in a wave tank with a scaled-down WEC model. A comparison of the results shows that – although the BEM is computationally fast, inexpensive, and able to estimate the added mass of the WEC with a reasonable accuracy – it is not able to accurately calculate the damping coefficients. The power absorbed by the WEC is calculated using the hydrodynamic coefficients obtained by all methods. The results show that the viscous damping dramatically decreases the absorbed power. A practical range of dimensionless damping coefficients is proposed to estimate the viscous damping of a two-body point absorber WEC, based on the experimental and FVM results. The upper bound of the absorbed power is also discussed in this paper.Publicationtype: Journal Article51 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Dimensional optimization of a two-body Wave energy converter using response surface methodology(2022-10-01); ; ; ; This paper proposes a dimensional optimization procedure for a laboratory scale two-body point absorber wave energy converter (WEC) using the design of experiment (DoE) methodology. Response surface methodology (RSM) is utilized to estimate a second order polynomial function correlating the average absorbed power, as the objective function, to five geometric parameters. Optimum values of parameters correspond to the peak of the surface fitted to the absorbed powers calculated for different sets of input parameters selected by the Box-Behnken design (BBD). The sensitivity of the objective function with respect to each parameter is investigated. The WEC is assumed to operate under regular waves in a specified range of frequency, 0.5–1 Hz. The amplitude and complex-conjugate controls are applied to keep the power take-off (PTO) system in optimum conditions. ANSYS-AQWA is used to calculate hydrodynamic parameters of the WEC required to solve the equations governing the absorbed power.Publicationtype: Journal Article30 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effect of grouting on the hydration and microstructure of portland cement and release of environmentally relevant substancesGround anchors are frequently used in foundation engineering. During manufacturing cement suspension is filled in a borehole and subsequently grouted. In non-cohesive soils, grouting leads to a filtration process, where the mixing water is pressed out from the cement suspension and a so-called “filter cake” is formed. The scope of the experimental investigations are ground anchors from a large-scale experiment and grouted and neat cement pastes from laboratory tests. The hydration process and microstructure of the hardened grouted cement paste have been investigated in comparison to grout bodies from the large-scale experiment. Elution tests were carried out to determine the leaching behaviour of both the neat cement paste and the grout bodies. Furthermore, filtered mixing water from laboratory test samples were analysed. The porosity was determined by water absorption under atmospheric pressure and mercury intrusion porosimetry (MIP). The hydration process was investigated by using thermogravimetric analysis (TGA). The composition of the filtered mixing water and the results of the leaching tests allowed to assess the impact of the grouting process on the surrounding soil and groundwater from an environmental point of view. This paper concludes with an evaluation of the effect of the grouting process. The results from the experimental programme are compared between grouted and neat cement paste as well as for grout bodies. The conclusion is completed by consideration of environmental aspects.Publicationtype: Conference Paper44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Cement-based geotechnical elements exposed to chemical attack—provisions for durability in standardsGeotechnical elements made of concrete, mortar and cement slurry, such as bored piles and grouted anchors, are widely used for the foundations of civil engineering structures. Depending on the environmental conditions and properties of the subsoil they can be exposed to chemical attack by groundwater and/or soil. The current system in standards for specifying and ensuring the durability of geotechnical structures is of a prescriptive nature, giving limiting values and provisions for composition and construction details. This paper examines the viability of the provisions in the standards for the durability design and assessment of various geotechnical elements against different types of chemical attack. The existing deficits and uncertainties are indicated. © 2019 Taylor & Francis Group, London.Publicationtype: Conference Paper332