Resumen de Multisicale analysis for the design of fiber-reinforced cementitious composites.
Francisco de Paula Montero Chacón
The birth of the so-called fiber-reinforced cementitious composites (FRCC) supposes a major breakthrough in Civil Engineering. This enhanced version of traditional concrete provides excellent mechanical properties by modifying the material structure. The change can be faced at different scales of observation, resulting in different properties. Thus, within this context, interesting questions regarding its internal design and behavior arise. The most important one is how to design FRCC when specific requirements have to be fulfilled.
Traditionally, concrete has been treated as an ordinary homogeneous material. However, being the most used material in construction implies that any small contribution to the enhancement of its performance will promote a deep impact at the structural level. Con-crete is a complex multiscale material that covers different length and time scales, from the nanometers to the meters, and implies different disciplines from Chemistry to Struc-tural Engineering, passing through Materials Science. By modifying the internal structure at these scales it is possible to obtain a completely new material. In the case of FRCC this solution passes through the introduction of natural or artificial fibers into a cementitious matrix. Although the use of FRCC is becoming more extended, there is a lack of numerical models for understanding in details its fracture processes. In this thesis a multiscale methodology based on different numerical models is presented for the design of these types of materials, from the micro- to the macroscale, paying special attention to fracture mechanics.
The first scale studied in this work is the microscale. A fiber-reinforced lattice model is developed for the design of engineered cementitious composites (ECC) at this scale. The original lattice model is enhanced by the inclusion of fibers that interact with the cement paste and this model is used to numerically characterize the effect of fibers on the mesoscopic properties of the resulting ECC. On the other hand, a generator of mortar microstructures is presented. Mortar plays the role of the matrix at larger scales in FRCC; therefore it is important to characterize the effect of different phases such as aggregates, voids, etc.
The next scale observed is the mesoscale. In this sense, a lattice-particle approach is presented for the analysis of fracture mechanics in high-performance fiber-reinforced concrete (HPFRC). In this model, the plain concrete matrix, made of mortar and coarse aggregates, is modeled by means of a lattice particle. The model is extended to the analysis of fiber-reinforced materials by including truss elements representing the fibers and linked to the matrix via special interface elements. The main features and capabilities of this model are discussed through numerical simulations of different test configurations.
The largest scale of observation is the macroscale. At this structural scale, the aforemen-tioned models are computationally expensive. Therefore, continuum-based models, con-taining the information obtained at smaller scales, are implemented for the analysis of fiber-reinforced concrete structures. In this work two models are presented for this pur-pose. The first one is the concrete plasticity model which allows the determination of structural damage. The second one is the fictitious crack model, which is especially suited for the analysis of fracture mechanics. Both models are validated with experimental re-sults.
The way for linking these three scales is not straightforward, and much effort has been devoted to this task for the last years. In this thesis a general multiscale procedure based on numerical homogenization is presented in order to determine equivalent homogeneous properties from a heterogeneous structure. One main issue regarding multiscale analysis in quasi-brittle materials is the definition of the representative volume element (RVE). For this reason, a procedure to determine the RVE size, by means of a three-dimensional lat-tice-particle approach, is presented. Besides, a virtual testing methodology is used to char-acterize the material at different scales.
Finally, the methodology presented in this work is applied in two reference applications dealing with the materials design (i.e. virtual design of a new multifiber-reinforced ce-mentitious composite) and structural analysis (i.e. damage evaluation in a bridge deck). These applications briefly describe the great advantage of multiscale analysis for modeling structural concrete.
