WG2 –MODELLING AND BENCHMARKING

Experimental data are necessary but they are not self-sufficient, because gradients of strains due to the ambient conditions/geometry, as well as degree of restraint and external loading need to be tackled by numerical simulations. WG2 shall focus on the numerical simulation of material/structural behaviour both at early age and during the service life and should strongly interact with the results obtained from WG1. The intricate collaboration between the experimental approaches at several material scales (particle, paste, mortar and concrete) and the corresponding simulation approaches based on multi-scale and multi-physics models are considered to be one of the added values of this Action.

The modelling will range from the microscopic level, where the microstructure of CBM is considered, through meso-level simulations, allowing the consideration of the presence of aggregates and/or reinforcement in concrete, up to a structural level, where the material will be seen as influencing the performance of concrete/reinforced concrete structures. The cooperation of several groups with a very strong expertise regarding CBM simulations at all the necessary levels will be a unique opportunity to integrate efforts and provide better understanding and comprehensive description of CBM life cycles.

The associated objectives are threefold. The first one is to support unified approaches for conducting numerical experiments for material properties of CBM. Indeed, it is not possible or/and too much costly to perform all required experiments, for a variety of CBM (mix design: w/c ratio, cement type, supplementary materials), the duration of experiments and various ambient conditions (temperature, drying). With this limitation of the experimental tests in mind, after a proper validation of the models against the experimental data obtained in WG1, the simulations will allow extension of the range of experimental tests to a much broader range of material compositions and environmental conditions, creating grounds for generally accepted models for virtual testing of material properties.

The extended range of inter-laboratory tests available for simulation will provide a solid basis for the comparison and development of several existing and recently proposed simulation models, ranging from simple descriptive empirical formulae, to multi-scale and multi-physics models. The numerical simulation will also encompass structurally relevant effects such as stress relaxation of self-induced stresses.

The second main objective of WG2 is to support unified approaches for macroscopic modelling of CBM behaviour during the life cycle. For such purpose, multi-physics models will be deployed, framed in the context of the thermodynamics of porous reactive media. Several approaches exist for this purpose worldwide, but no wide agreement has been established on the best approaches and modelling strategies to achieve feasible service life simulations.

Macroscopic models capable of describing transport phenomena are also required to predict behaviour of CBM structures regarding their uniqueness: material properties (either obtained through experimental or virtual testing), mechanical boundary conditions, climate conditions, geometry. This is important in order to optimize the materials and construction techniques and to interpret data in monitored structure in a relevant way. It should be remarked that these types of multi-physics macroscopic models may be suitable for use in structural design offices (e.g. embedded into finite element software packages), provided that adequate guidelines are given to the users (scope of WG3). Discussions on both microscopic and macroscopic approaches to modelling of CBM and structures will be held in specially targeted sessions within the workshops and conference of the Action, aimed at reporting recent advances and comparative reviews of models.

A fundamental tool for assisting the development, understanding and comparison of both micro-scale and macro-scale models mentioned above will be the performance of benchmarking tests. It will be possible to evaluate the relative performance of numerical simulation approaches and software (including research and commercial applications) applied to selected real examples of material and structural behaviour. Both blind and a subsequent non-blind stage of benchmarking will be included. The strategy for the establishment of benchmark studies to be performed and their corresponding discussions has some analogies with the strategy described in WG1 for round-robin testing. Nonetheless, because of the intricate interaction of the benchmarking tests in this WG with the experiments of WG1, from which input is necessary, the benchmark milestones in this WG are lagged by approximately 1 year behind the round-robin tests.

Finally, it will be possible to integrate the conclusions from different modelling scales (cement paste, mortar, concrete, structural level) to create a set of general instructions to be used in designing software for CBM and reinforced concrete structures. Thanks to the cooperation of groups developing different modeling approaches and industrial partners using different reinforced concrete simulation software tools, it will be possible to address the necessary development of structural simulation tools to be able to take advantage of nonlinear material simulations, a task that has not been possible until now. It should be noted that the most important outcome of this final integration within WG2 is the incorporation of several levels of complexity in the prediction of service life performance of CBM and structures, with strong emphasis on the possibility of providing improved models that are practical/feasible for design codes. Special attention will be given to the problem of analyzing serviceability under the combined effect of imposed deformations (namely, due to thermal and hygral effects) together with applied loads, with due account for viscoelasticity and cracking effects. In fact, these issues are not being adequately included in existing regulations (namely the structural Eurocodes) and improper service behaviour is often observed, not only in special structures but also in regular residential, industrial and office buildings that represent most of the built environment. The lack of knowledge to these serviceability issues also frequently leads to highly conservative designs in terms of reinforcement distributions, with detrimental consequences at both economical and sustainability levels. Therefore, the final outcomes of WG2 will include a detailed analysis at several levels of complexity of the combined effect of imposed deformations and applied loads in the serviceability performance of a minimum of three main cases, preferably supported by monitoring/testing within the scope of the Action:

  1. a large raft, parking or industrial floor, with associated cracking problems.
  2. a residential or office building aiming to sustained prediction of cracking and deformations.
  3. a retaining wall or reservoir wall, which is highly restrained and prone to severe cracking

The extensive analysis of the case studies will result in specific conclusions on the best feasible and simplified models aimed at the design of ordinary structures for serviceability, with impact on most of CBM and structures.