Discrete Element Modeling and Electron Microscopy Investigation of Fatigue-Induced Microstructural Changes in Ultra-High-Performance Concrete

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.

Monitoring concept for the propagation of compressive fatigue in externally prestressed concrete beams using digital image correlation and fiber optic sensors

“…The developed test setup and accompanying measuring concept provide the foundation for a deeper analysis of fatigue-induced stress redistribution processes. The monitored strain histories can be used as input for constitutive material models that capture the compressive fatigue degradation of concrete, for example, earlier research 31,33,35 which can thus calculate the stress distribution and capture the process of redistribution. This allows quantifying the significance of influencing factors such as load duration, amplitude, and concrete strength and also sequence effect, cyclic load frequency, or moisture content.…”

“…This can contribute to a better understanding of the fatigue stress redistribution phenomenon and a more effective transformation of the fatigue characterization knowledge gained from material tests into the structural response. Additionally, the detailed view of such a phenomenon will provide a valuable input for the validation of advanced numerical models for concrete fatigue, for example, earlier studies 31–36 …”

“…Additionally, the detailed view of such a phenomenon will provide a valuable input for the validation of advanced numerical models for concrete fatigue, for example, earlier studies. [31][32][33][34][35][36] The paper starts with the description of the experimental campaign in Section 2, including the material properties, specimen geometry and test setup, the load introduction concept to achieve a compressive fatigue degradation process over the cross-section, and the applied measurement techniques. The obtained experimental results are presented in Section 4, comparing the evaluation of the compressive strains obtained using DIC and FOS.…”

Cyclic Deterioration of High‐performance Concrete – Findings of the Priority Programme SPP 2020

“…Considering the fact that every single load cycle during fatigue testing introduces a permanent strain into the material, it must be connected to irreversible changes on the micro-or nanoscale. Scanning and transmission electron microscopy were conducted on UHPC and pure binder samples from various states of compressive fatigue damage to investigate these changes down to their origins [24].…”

“…Based on the experimental results, a rheological bonded particle model was developed to apply and investigate the changes to the mesoscopic scale [24]. [24] and a spatially resolved x-ray spectroscopy map of aluminium distribution (right) [24] surrounding a nanoscale crack. Scale markers are 500 nm.…”