Micromechanical mechanisms of the time dependent deformation behavior of early-age concrete
Final Report Abstract
The submitter of the stipend carried out a research project at the Massachussetts Institute of Technology, MIT, USA, under the supervision of Prof. Franz-Josef Ulm. The goal of this research project was to identify the mechanisms controlling the creep behavior of early age concrete. Herefore, primarily nano- and microindentation tests were carried out on samples with different degrees of hydration. These tests were accompanied by various bulk analytical tests, with which for example the evolution of the porosity or of the mineralogical composition of the sample were studied. The main idea behind this approach was to determine the mechanical properties and creep behavior of the individual building blocks of cement paste, such as low density and high density calcium silicate hydrate (LD- and HD-CSH), calcium hydroxide (CH) and non hydrated cement, as well as their interaction behavior and to predict the homogenized mechanical response of the paste based on the previous knowledge. Herefore, extensive experimental tests on hardened cement paste were carried out, studying the evolvement of the pastes properties and microstructure as a function of the degree of hydration. Based on the resulting data, the effect of hydration on the mechanical hardness, modulus and creep behavior of cement paste and its individual phases can now be predicted. Further, the mechanical response of hardened cement paste phases measured in nanoindentation experiments was modelled using a micromechanical approach. With the resulting model, pronounced insight into the processes taking place during structure formation of hardened cement paste as well as during cement paste creep could be obtained. The results determined in purely mechanical tests hereby correlate very well with totally independent bulk test results. In doing so it was found that: the creep of hardened cement paste primarily seems to be triggered by a slippage between cement paste particles; the contribution of a deformation of the individual particles to total creep seems to be of minor importance; and the contribution of microcracking to concrete creep on the scale level up to 1 µm seems to be negligible. Further, from the results the following conclusions on the microstructure formation regime could be drawn: Upon percolation, both LD- and HD-CSH form at similar volume fractions. The packing densities of these phases however are not fixed but evolve in time indicating a continuous consolidation of the phases with increasing degree of hydration. The developed micromechanical model shows, that the max. packing density of both phases is approx. 75 vol.-%, corresponding to the value of unimodal spheres. - All performed tests indicate, that at degrees of hydration of approx. 0.7 a shift in formation regime of the CSH-phases seems to occur. This shift is explained by a transition from a dissolution and diffusion driven regime to a regime controlled by the lack of sufficient space to form new CSH particles. Based on these findings, it would be very interesting in the future to closely investigate the influence of microscopic rock powder particles on the evolvement of the packing density and formation regimes. Results obtained in this project hint to the fact, that these powders may alter the formation regime. Further, a key question for future research is the influence of the water content on the deformation behavior of the individual building blocks of hardened cement paste.
