Specimen heating as damage indicator for fatigue tests of concrete

• Applicant: Professor Dr.-Ing. Ludger Lohaus
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 284163400

High-strength flowable fine-grain concretes are commonly used in the wind energy industry, where they are exposed to high cyclic stresses. However, there are considerable uncertainties regarding their fatigue behaviour. Preliminary tests show signs of a significantly inconvenient fatigue behaviour com-pared to common high-strength concrete. However, it is so far not clear to what extent the observed differences in the fatigue behaviour are caused by material-technological or primarily technical testing influences. As a part of the preliminary tests, the heating of the test specimen during the fatigue load-ing was identified as a possible indicator of different fatigue behaviour. Increased testing frequencies and increased stress levels, which are significantly influencing the heating, are imperatively required in fatigue tests in order to examine the building life cycle in time-lapse experiments. The research project has several objectives. The focus here is to carry out fatigue tests so that the actual material behaviour can be determined. Concerning this, it is necessary to eliminate technical testing disturbance impacts, which may particularly arise from the increase in temperature of the spec-imens during the experiment. At the same time the fatigue tests should be carried out with the highest possible testing frequency in order to ensure as effective as possible test times. The heating of the test specimen due to the fatigue loading should be used as an indicator for undesired additionally possible damage and therefore as a control parameter for the experimental procedure. Firstly, the consequences of the specimen heating should be captured, parameters influencing the increase in temperature should be identified and initial indications of possible relationships between the respective concrete structure and the temperature development should be determined. The new testing method should be generalizable derived based on these studies and with the help of complementary numerical simulations of the temperature development and possible resulting damage mechanics. Thus, the capacity and the conciseness of statements derived from elaborately fatigue tests can be significantly improved, since only this way the actual occurring material-specific influences on the fatigue behaviour could be identified and pursued scientifically. At the same time, this approach shall clarify if fine-grain concretes in fact have a deviating fatigue behaviour compared to common concrete or not and according to this, whether fine-grain concretes can be designed and applied to the same rules as common concrete or elaborately fatigue verification has to be performed for each material. The research work is also of fundamental importance for future fatigue investigations on both normal concrete as well as on the further increasing variety of special high-performance concretes. Furthermore, they can help clear up existing discrepancies between various literature results.

• DFG Programme: Research Grants