Effects of intermediate drying periods on the scaling behavior of concrete under freeze-thaw deicing salt attack

• Applicants: Professor Dr.-Ing. Christoph Gehlen; Professor Dr.-Ing. Josko Ozbolt
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428338963

Adequate resistance of concrete structures against freeze-thaw deicing salt attack is currently being verified by laboratory tests or is considered fulfilled for certain concrete compositions based on decades of experience. However, numerous cases are known in which concrete compositions, which had failed the laboratory test, showed no damage in practice. A main cause for this is the difference between laboratory and service climate in Germany. While concrete samples in laboratory are exposed to constant moisture supply, field conditions are comparatively dry. Experiments in a previous DFG project showed that intermediate drying periods had a significant effect on the evolution of salt scaling. However, the underlying mechanisms are unresolved. Therefore, the proposed investigations will provide better understanding of the basic mechanisms of salt scaling and improve the evaluation of concrete structures under practical conditions. The experimental work is divided into six work packages. In the first package, the effect of different storage conditions before the first freeze-thaw attack will be quantified. These studies enable the determination of the role of drying periods without pre-damage and salt crystallization. Within the second work package, the causes for the significant effect of intermittent drying periods on scaling evolution are analyzed. In order to detect the specific mechanisms, well-established methods (XRD, EDX, gravimetric measurements, determination of chloride content) as well as new, high-resolution techniques such as single-sided NMR and laser ablation are used. In the third work package, the effect of repeated drying periods is investigated in detail. Herewith, the effectiveness of the positive impact of drying periods can be analyzed. This is very important for the prediction of damage evolution of concrete in practice. The numerical study will be carried out in work package 4 using CHTM models. In the fifth work package, the transferability of the previous results is verified on large-scale concrete components. For this, the applicant has stored concrete samples for ten winters on a highway (exposure condition XF2 and XF4). These samples as well as additional ones will be thoroughly investigated. Through this, a link from laboratory experiments to field conditions is established and the numerical analysis is validated. The results of additional numerical parameter studies (virtual investigation) provide important insights for the development of a simplified engineering model for the prediction of damage development (package 6). Finally, characteristic parameters will be derived from the investigations which enable service life prediction of concrete structures exposed to freeze-thaw deicing salt attack with intermediate drying periods. This enables environmentally and economically optimal design of concrete structures in the exposure classes XF2 and XF4.

• DFG Programme: Research Grants