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    Item type:Publication, Access status: with files ,
    Quellverhalten von Portland- und Hochofenzementstein bei unterschiedlichen Temperaturen
    (Ernst & Sohn, 2023)
    Wagemann, Falk
    ;
    Schmidt-Döhl, Frank
    ;
    Rahimi, Amir
    Zementgebundene 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 Paper
      19  33
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    Item type:Publication, Access status: without files ,
    Attack of aggressive carbon dioxide on hardened Portland and blast furnace slag cement paste
    (CRC Press, 2023)
    Wagemann, Falk
    ;
    Schmidt-Döhl, Frank
    ;
    Rahimi, Amir
    Natural 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 Paper
      20
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    Item type:Publication, Access status: without files ,
    Effect of grouting on the hydration and microstructure of portland cement and release of environmentally relevant substances
    (2022-06)
    Wagemann, Falk
    ;
    Rahimi, Amir
    ;
    Amann, Steffen
    ;
    Schmidt-Döhl, Frank
    ;
    Heidenreich, Fabian
    Ground 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 Paper
      44
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    Influence of cement grouting on the microstructure of cement paste
    (2021-06)
    Wagemann, Falk
    ;
    Wehrmann, Lucas
    ;
    Schmidt-Döhl, Frank
    In foundation engineering, ground anchors are a frequently used type of foundation element. The manufacturing method of ground anchors with grouting of cement slurry has a high impact on the load-bearing capacity of those elements especially in non-cohesive soils. In subsoils like these, a filtration of the cement grout occurs depending on the grouting pressure and duration. Due to this filtration, the initial water-cement ratio is reduced in the grouted body especially in the edge area. This paper will investigate the influence of grouting on the microstructure of various cement suspensions. Cement suspensions of different cement types (CEM I and CEM III) have been examined in laboratory tests with a filter press. Influencing factors on groutability are besides the water retention of the cement type also the building up of a filter cake in the transition respectively filtration area. The microstructure is characterized by porosity investigations (e.g. mercury intrusion porosimetry). With the aim to improve the microstructure in the contact zone additives are used and their influence on the groutability and the hardened cement paste are outlined. The results will be discussed under the aspect of providing a dense binder matrix to increase the resistance to aggressive groundwater.
    Publicationtype: Conference Paper
      102
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    Item type:Publication, Access status: without files ,
    Cement-based geotechnical elements exposed to chemical attack—provisions for durability in standards
    (CRC Press, 2018)
    Wagemann, Falk
    ;
    Schmidt-Döhl, Frank Michael
    ;
    Rahimi, Amir
    Geotechnical 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 Paper
      332