Abstract
In view of the growing demand for sustainable and lightweight concrete structures, the use of ultra-high-performance concrete (UHPC) is becoming increasingly important. However, fatigue loads occur more frequently in nature than static loads. Despite the impressive mechanical properties of UHPC, a reduced tolerance for cyclic loading is known. For this reason, our paper deals with experimental and numerical investigations regarding the main causes for crack initiation on the meso, micro, and nanoscale. After mechanical fatigue tests, we use both scanning (SEM) and transmission electron microscopy (TEM) to characterize microstructural changes. A new rheological model was developed to apply those changes to the mesoscopic scale. The origins of fatigue damaging can be traced back to a transformation of nanoscale ettringite, resulting in a densification of the surrounding binder matrix. Additionally, a higher content of unhydrated cement clinker in the matrix benefits fatigue resistance. On the mesoscale, stress peaks around aggregate grains expand into the surrounding binder with increasing load cycles and lead to higher degradation.
Highlights
IntroductionFatigue-induced damage of cementitious structures, similar to concrete foundations of wind turbines or concrete bridges, is more common than static loading in nature
Fatigue-induced damage of cementitious structures, similar to concrete foundations of wind turbines or concrete bridges, is more common than static loading in nature. When these structures are subjected to cyclic loading, plastic deformation is observed even though the maximum load level is much smaller than the short-term strength under quasi-static loading conditions
We find a positive correlation between the relative content of unhydrated cement clinker in a specimen and its eventual lifetime, the effect being more pronounced for ultrahigh-performance concrete (UHPC) samples due to their lower total binder content
Summary
Fatigue-induced damage of cementitious structures, similar to concrete foundations of wind turbines or concrete bridges, is more common than static loading in nature. When these structures are subjected to cyclic loading, plastic deformation is observed even though the maximum load level is much smaller than the short-term strength under quasi-static loading conditions. UHPC is an almost ideally brittle material with a very dense and high-strength structure. The outstanding mechanical properties are achieved by an optimization on the basis of the maximum packing density theory [1,2] and a very low water to cement ratio (w/c). A high amount of cement, fillers (quartz or limestone powder), and pozzolanic additives such as silica fume lead to the optimized particle size distribution
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