Modelling of Concrete Subjected to Cyclic Loading
Doctoral thesis, 2009
The infrastructure of today depends heavily on concrete structures. Most of these structures
are subjected to repeated loads, known as fatigue or cyclic loads: the loads weaken
the structure. As this phenomenon is of high cost to society, a deeper understanding of
the deterioration process of cyclic loading would be beneficial. The aim of this thesis is
to add to the knowledge of the deterioration phenomenon and to develop models that can
describe the response of concrete subjected to cyclic loading.
In analyses of structures of today, finite element modelling is gaining ground and is being
used more frequently in research and structural design. The investigations here use only
the finite element method.
Many structures incorporate details that are subjected to complicated loading, which results
in complex crack patterns. A suitable tool for describing these crack patterns is
anisotropic damage material models. However, anisotropic models are difficult to implement
and are often computationally inefficient. One of the investigations in this thesis
aims to find out what makes the anisotropic formulation suitable for complex crack patterns.
This is done by implementing a model which can control the amount of coupling
between volumetric- and deviatoric strains. It was found that this coupling is essential
for describing complex crack patterns.
To deepen the understanding of concrete subjected to cyclic loading, the phenomenon
was investigated on the meso-scale level. An interface model was developed and applied
to a three phase representation of concrete that incorporates: aggregates, cement paste,
and interfacial zones around the aggregate. The model in itself does not yield cyclic
behaviour, i.e. no hysteresis loops were generated at the constitutive level. Instead, the
cyclic response was generated by the meso-structure. It was found that the interfacial
transition zones are crucial in amount and strength.
Concrete subjected to cyclic loading was also investigated on the macro-level, with the
ambition to describe the response of concrete structures subjected to cyclic loading. Two
investigations were made: one aims to describe cyclic response in tension and the other
aims to cover tension and the transition to reasonable high states of compression. The
investigations are based on the theory of plasticity and damage mechanics, which are
combined in both a serial and a parallel fashion. In the serial configuration the nominal
stress is computed by adding the damage to the effective stress; for the parallel configuration,
the damage stress and the effective stress are evaluated separately for the same
strain and then added to yield the nominal stress. Furthermore, both models use two
yield surfaces to describe the hysteresis loops. The result of the analyses show an overall
agreement with experimental observations.