Multiscale investigation of fatigue crack growth in high performance concrete based on computer tomography and phase-field fracture modeling

高性能混凝土(HPC)目前被广泛用于建造桥梁、塔楼、海上钻井平台、起重机跑道、高层建筑、风力涡轮机和道路的混凝土构件，所有这些结构都特别容易受到循环荷载的影响。目前，由于过于保守的设计标准，使用高性能混凝土所产生的机会不能得到充分利用。这主要是由于缺乏关于疲劳载荷下材料退化的知识。这个项目的目的是填补这些知识空白。如今，人们普遍认为，通过对混凝土的非均质结构进行显式建模，可以获得对混凝土性能的新见解。在这个项目中，将建立一个高性能混凝土的多尺度计算模型，以阐明疲劳性能的基本原理，特别是损伤、裂纹模式和裂纹扩展速度如何受材料特性和非均匀几何形状的影响。计算模型将集中在所谓的中尺度上，其中高性能混凝土将被认为是由散装水泥砂浆、粗集料、气孔和集料与散装砂浆之间的界面砂浆构成的。模型的多尺度性质将产生于需要适当地将材料特性分配给大块和界面砂浆，而这些砂浆又由较低尺度(微尺度)的水泥浆体、细集料和孔隙组成。细观尺度的转变将建立在砂浆的断裂特性主要取决于局部孔隙率的假设基础上。损伤和开裂将用扩展的相场建模方法来描述，以考虑疲劳效应。将进行全面的实验计划。块体和界面砂浆的断裂特性将被表征为局部孔隙率的函数，从而从实验上证实了微观-细观尺度的转变。中尺度模式的几何形状将完全基于从计算机层析成像(CT)获得的几何形状。为此，将开发创新技术，以增强骨料和砂浆之间的对比度，并实现自动图像分割。将通过试验观察裂纹的萌生和扩展现象，包括在CT舱内进行试验，在单调和循环加载条件下，全场三维测量载荷下的应变和位移。这些实验将使计算模型得到彻底的验证，并将揭示高性能混凝土疲劳破坏的基本机制。

High-performance concrete (HPC) is nowadays widely used for the construction of concrete components for bridges, towers, offshore rigs, crane runways, tall buildings, wind turbines and roads, all structures that are particularly susceptible to cyclic loading. Currently, the opportunities that arise from the use of HPC cannot be fully exploited because of overly conservative design standards. This mainly results from the lack of knowledge regarding the material degradation under fatigue loading. The aim of this project is to fill these knowledge gaps.It is nowadays widely recognized that new insight on the behavior of concrete can be gained through an explicit modeling of its heterogeneous structure. In this project, a multiscale computational model of HPC will be set up to shed light on the fundamentals of the fatigue performance, and in particular on how the damage and crack pattern and crack growth rate are influenced by the material characteristics and by the heterogeneous geometry. The computational model will focus on the so-called mesoscale, where HPC will be considered constituted by bulk cement mortar, coarse aggregates, air voids and interfacial mortar between aggregates and bulk mortar. The multiscale nature of the model will arise from the need to properly assign material properties to the bulk and interfacial mortars, which in turn are constituted by cement paste, fine aggregates and pores at the lower scale (microscale). The micro-mesoscale transition will be based on the assumption that the fracture properties of mortar mainly depend on the local porosity. Damage and cracking will be described with a phase-field modeling approach extended to incorporate fatigue effects.A comprehensive experimental program will be carried out. The fracture properties of the bulk and interfacial mortars will be characterized as functions of the local porosity, thus substantiating the micro-mesoscale transition experimentally. The geometry of the mesoscale model will be entirely based on geometry acquisition from Computer Tomography (CT). For this purpose, innovative techniques will be developed to enhance the contrast between aggregates and mortar and enable automatic image segmentation. Crack initiation and propagation phenomena will be observed through testing, including testing within the CT chamber with full-field three-dimensional measurement of strain and displacement under load, under monotonic and cyclic loading conditions. The experiments will allow for a thorough validation of the computational model and both will shed light on the fundamental mechanisms of fatigue failure in HPC.