Wagemann, Falk

In natural groundwater and soils, cement-based building materials used in the construction of geotechnical elements can be exposed to chemical attack. In order to ensure the durability of cement-based materials, descriptive provisions are laid down in the current standardisation, which are based on empirical values. For the in situ construction of geotechnical elements, the building materials used and the relationship between load-bearing capacity and material damage, however, the specifications of descriptive requirements show deficits. In this work, the main deterioration mechanisms of concrete-aggressive substances in natural soils and groundwater are presented and concrete technology control options are shown. Furthermore, the provisions and limit values for ensuring the durability of concrete and reinforced concrete components are presented and evaluated in terms of uncertainties when transferring them to geotechnical elements. The techniques of foundation engineering that are essential for the construction of foundations and retaining systems in waterways engineering are characterised with regard to the descriptive durability that can be ensured against chemical attack and afterwards evaluated considering their sensitivity. The work contributes to a better understanding of the special aspects of the material-soil interaction, which has a considerable effect on the properties of the building materials used in geotechnical elements. Among other important aspects, the release of mixing water due to filtration effects during in situ manufacturing and the natural curing environment with low ground temperatures and permanent moisture supply lead to the development of a building material with low porosity and a dense pore structure and thus increase the resistance to chemical attack. Due to the occurrence and uncertainties regarding the effect on the load-bearing capacity, the attack of aggressive carbon dioxide on ground anchors was identified as a relevant and critical scenario and investigated experimentally. During the exposure time of 12 months, various depths of impact – mechanically degraded and chemically physically altered depths – were found for Portland and blast furnace slag cement paste samples. The depths of impact were significantly affected by the buffer capacity due to the calcium hydroxide content of the cement paste and the physical resistance in the period under review. The manufacturing of ground anchors in situ in an environment with aggressive carbon dioxide has the effect of blocking the cement paste of the grout body by adhering soil particles. Predictions for longer exposure times can only be derived with uncertainty on the basis of the available results. Based on a comprehensive research on the issue, concrete technology recommendations are given for increasing the resistance of building materials in a chemically aggressive environment. However, due to the complex interactions between exposure, material resistance and load-bearing capacity of the respective geotechnical elements presented in the work, no general concepts for durability design can be established.

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.

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