Keywords: cement paste, nanoindentation, creep, size effect, micromechanical properties.

This paper is concerned with experimental and numerical investigations of cement paste which is taken as a representative of heterogeneous and also time-dependent building material. The main studies are devoted to experimental nanoindentation and its implications to the evaluation of material properties. The paper is also concerned with the appropriateness of conventional methods used for evaluation of micromechanical properties and investigates the possibility of the use of enhanced methods for better description of the nanoindentation process. Limitation of traditional elastic solution was shown on the unique experimental program. Better descriptions of indentation based on analytical visco-elastic solution and a finite element model with general visco-elasto-plastic constitutive relations were proposed. These models were used for the simulation of indentation and for estimation of material parameters at micrometer scale.

An extensive experimental study was performed on cement paste which is a heterogeneous and also creeping material. The work was motivated by the fact that simple evaluation procedures lead to overestimation of elastic properties (size effect on elastic modulus and hardness). Nanoindentation of cement pastes includes some specific procedures compared to standard indentation. Some results obtained on cement pastes by nanoindentation can be found in references [1,2]. First the preparation of samples is a task that requires a lot of experience and precision. Samples need to be polished with the resultant surface roughness less than several tens of nanometres. Second, the indents need to be separated into individual groups corresponding to material phases by ESEM and evaluated statistically. Traditionally, only elastic properties are extracted from the unloading part of indentation loading diagram.

Based on the experimental program, it was found that presence of creep on the unloading branch of the loading diagram disturbs the evaluation of the results and it leads to size effects on the elastic modulus and hardness. Two basic sets of samples were investigated, loaded by cyclic loading with and without dwell period at the peak. The series without dwell period resulted in size effect on elastic properties while for the series with a long dwell period the size effect was substantially decreased.

Several numerical models were constructed for simulation of the indentation process. First the elastic properties were evaluated by standard Oliver and Pharr [3] procedure. Ignoring the creep phenomena lead to the aforementioned size effect in case of the first specimen series. Second, an analytical visco-elastic solution based on the work of Vandamme and Ulm [4] was used. This solution utilizes a holding period with indentation creep where the force is kept constant for fitting of the viscous parameters of the material. Better description of the loading diagram was obtained for one cycle experiment. However, cyclic experiments with general loading histories were not simulated satisfactorily. It led us to the construction of the finite element (FE) model. Several types of constitutive models (elastic-plastic, elastic-creep, elastic-plastic-creep model) were investigated. The FE analyses showed that in description of the micromechanical behavior of the cement paste, both time-independent plastic strains and time-dependent creep stains appear to play an important role. However, parameters of the qualitatively most suitable elastic-plastic-creep model are difficult to obtain. Presently, the possibility of using a more sophisticated method of parameter identification based on genetic algorithms is being researched.

1

Constantinides, G., Ulm F.J., Van Vliet K., "On the use of nanoindentation for cementitious materials", Materials and Structures, 36, 191-196, 2003. doi:10.1007/BF02479557

2

Constantinides, G., Ulm F.J., "The effect of two types of C-S-H on the elasticity of cement-based materials: Results from nanoindentation and micromechanical modeling", Cement and Concrete Research, 34 (1), 67-80, 2004. doi:10.1016/S0008-8846(03)00230-8

3

Oliver W.C, Pharr G.M., "An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments", Journal of Material Research 7, 1564-1583, 1992. doi:10.1557/JMR.1992.1564

4

Vandamme M., Ulm F.J., "Visco-elastic solutions for conical indentation", Int. J. of Solids and Structures, 2005, in press.

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