Specimen heating as damage indicator for fatigue tests of concrete

高强度可流动细粒混凝土通常用于风能行业，在那里他们暴露于高循环应力。然而，它们的疲劳行为有相当大的不确定性。初步试验表明，与普通高强度混凝土相比，它的疲劳性能明显不方便。然而，迄今为止尚不清楚观察到的疲劳行为差异在多大程度上是由材料技术或主要技术测试影响引起的。作为初步试验的一部分，试样在疲劳载荷期间的加热被确定为不同疲劳行为的可能指标。在疲劳试验中，必须增加试验频率和提高应力水平，这对加热有重大影响，以便在延时试验中检查建筑物的生命周期。这个研究项目有几个目标。这里的重点是进行疲劳试验，以便确定材料的实际性能。对此，有必要消除技术测试干扰的影响，特别是在实验过程中试样温度的升高。同时，疲劳试验应以尽可能高的试验频率进行，以确保尽可能有效的试验次数。由于疲劳载荷引起的试样加热应作为不期望的额外可能损坏的指示，因此作为实验过程的控制参数。首先，应捕获试件加热的后果，确定影响温度升高的参数，并确定各自混凝土结构与温度发展之间可能关系的初步迹象。新的测试方法应该在这些研究的基础上，借助对温度发展和可能产生的损伤力学的补充数值模拟，推导出可推广的方法。因此，通过精心设计的疲劳试验得出的结论的能力和简明性可以大大提高，因为只有这样才能科学地确定和追求实际发生的材料对疲劳行为的特定影响。同时，该方法应明确细粒混凝土是否确实与普通混凝土相比有偏离疲劳行为，并据此确定细粒混凝土是否可以按照与普通混凝土相同的规则进行设计和应用，或者是否需要对每种材料进行详细的疲劳验证。该研究工作对今后普通混凝土的疲劳研究以及今后越来越多的特种高性能混凝土的疲劳研究具有重要意义。此外，它们可以帮助澄清各种文献结果之间存在的差异。

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.