Keywords: material optimization, cementitious composites, mesolevel model, fracture simulation, micromechanics of concrete, aggregate-matrix interactions.

Recent advances in cementitious composite materials are particularly related with the mesolevel structure of these materials. Performance enhancements have been obtained by tailoring the material composition in terms of components geometry and contents, as well as improvements on interface bonding between matrix and aggregate or fibre [1,2]. Experimental analyses can, in some cases, be used for qualitatively evaluating effects of component properties but hardly can allow quantitative optimisation of the composite admixture. On the other hand, phenomenological models used to describe the mechanical behaviour of concrete assume the material to be homogeneous, and therefore cannot be used for any type of material optimisation.

This paper presents a framework of a computational tool for simulation of fracture processes of cementitious composites and material optimisations at the mesoscopic level. The tool relies on highly realistic three-dimensional representations of the heterogeneous internal structure of the concrete and corresponding two-dimensional slices for understanding of micro-mechanics in aggregate-matrix and aggregate-matrix interactions. The generation mechanism allows the control of the aggregate volume content, shape and size distribution, resembling the effect of a sieve analysis process. A stochastic-heuristic algorithm was implemented to produce fairly uniform spatial distributions of the aggregates and without overlapping, closely compared to the distribution in the real concrete. The allocation procedure proved capable to produce numerical concrete with dense aggregate distributions of over 80% of the total specimen volume. Two interactive graphical interfaces have been associated to the computer implementation in order to provide support the application of the generation mechanism and assist on the understanding of its functioning. The first graphical interface consists of a Virtual Reality Modelling Language (VRML) plug-in to be used on the Internet browser. The VMRL file allows the visualisation of specimen in 3D graphics and is automatically written by the mechanism. Once the VMRL file has been written, it can be loaded into an Internet browser with a suitable plug-in (VRML viewer) for manipulation of 3D objects. The VRML interface enables the investigation of the complete specimen and global aggregate distribution through different lightening and transparency in rotations and zooms. However, it cannot give comprehensive information on the internal structure of the concrete composite. For such a purpose, a second graphical interface has been developed and implemented using an oriented objected programming (OOP) platform. The latter interface enables the dissection of the specimen through cuts along planes perpendicular to the axes directions. It manipulates three-dimensional data in order to create the impression of a 3D view. In addition, it also allows the display of the specimen slices, which are used in the two-dimensional analyses.

In order to understand problems in spatial distribution, as well as accessing the performance of the proposed generation mechanism, three different tests were carried out with numerical concrete specimens. The performance of the generation mechanism was accessed in terms of aggregate distribution, aggregate content and computational time for different aggregate size and shape [3].

Once the mesoscopic structure of the material is generated, it can be further discretised into 2D or 3D meshes of micro-rod elements as idealisations, which allows for structural analyses. Short fibre reinforcement elements are placed directly into the mesh structure, connecting distant nodes of the matrix. After the idealised structure is available, nonlinear structural analysis is performed into small incremental load/displacement steps in order to simulate the fracture process. Compression, direct tension and wedge-splitting tests have been simulated. Parametrical study was carried out to investigate the effect of different material properties and proportions in the concrete admixtures. Experimental observations could be confirmed by the simulation results.

1

Li, V.C., "Engineered cementitious composites - tailored composites through micromechanical modelling", in Fibre Reinforced Concrete: Present and the Future, N. Banthia, A. Bentur, A. & A. Mufti (Eds.), 64-97, Montreal, 1998.

2

Mihashi, H., Leite, J.P.B., Yamakoshi, S., Kawamata, A.,"Optimizing Fracture Toughness for Desining Ductile Fiber Reinforced Cementitious Composites", The Fifth International Conference on Fracture Mechanics of Concrete and Concrete Structures (Framcos-5), in Fracture Mechanics of Concrete Structures, IA-FraMCos, Vol. 2, pp. 1021-1928, Colorado, USA, 2004.

3

Slowik, V., Leite, J.P.B., "Modellierung des mechanischen Verhaltens von Betonen auf der Ebene des Mesogefüges", Research report, Leipzig University of Applied Sciences, Germany, 1999. (in German)

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