Multi-phase porous media

Peter Moonen, Bert Sluys, Jan Carmeliet

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From the moment a structure is constructed, it starts deteriorating. Especially for porous building materials, the deterioration process is strongly related to the presence of moisture. Through open pores and micro-cracks, moisture and dissolved particles (e.g. soluble salts) can penetrate into the material, rendering the structure vulnerable to various physical, chemical and biological degradation processes. Frost damage of the wetted material is only one example of a common physical degradation mechanism. In concrete, the leaching of calcium and the alkali-silica reaction are among the most widespread chemical causes of damage. Biological degradation merely affects wood and timber structures, but also concrete and stone-like materials are indirectly exposed: since e.g. vegetation retains water, drying is hindered, and therefore the wetted material becomes more susceptible to other types of moisture-related degradation. As a result of the active degradation processes, the material properties in the wetted region are altered and stresses of various origins develop. In combination with mechanical loading, initial micro-defects may grow and coalesce into macro-cracks, hereby not only reducing the structural strength and stiffness, but also creating new pathways for preferential moisture transport, both leading to an acceleration of the degradation process. It is clear that material degradation usually does not have a single identifiable cause, but results from a complex interaction between several processes. Insight herein can be gained by conducting well-designed laboratory investigations on the one hand, and by developing theoretical models as a basis for numerical simulations on the other hand. Both methods are complementary.

Examples of (a) physical, (b) chemical and (c) biological degradation processes.


Continuous-discontinuous framework

A novel continuous-discontinuous concept to model physical degradation of porous materials was developed. The formulation covers both diffuse damage processes in the bulk material as well as the initiation and propagation of discrete cracks under coupled hygro-thermo-mechanical loading conditions. The cornerstone of the proposed framework is formed by a new type of cohesive zone model, ensuring a gradual transition between two states, namely the continuous state before fracture occurs, and the discontinuous state afterwards. The model essentially describes the influence of the development of a fracture on every relevant field. For example, it expresses how the fluid flow inside the material matrix is affected by the formation of a fracture. The modelling concept is generic, and therefore an extension towards different types of loading conditions is straightforward. Experimental research was conducted to assess the validity of the proposed framework. The continuous-discontinuous predictions compare well with experimental data, demonstrating the potential to successfully tackle a wide range of real-life applications. A more detailed description of the model and its validation can be found in Moonen et al. (2008) and three subsequent papers (2010a, 2010b and 2011). An extension towards multi-phase flows and its validation is described in Derluyn et al. (2013).

Schematic representation of a body Ω consisting of two materials with indication of the cohesive zone and the fracture domain.



  • H. Derluyn, P. Moonen, J. Carmeliet, Deformation and damage due to drying-induced salt crystallization in porous limestone, Journal of Mechanics and Physics of Solids, Vol. 63, pp. 242-255, 2014. (pdf)
  • P. Moonen and J. Carmeliet, Modelling moisture transport in intact and fractured concrete. In: J. Weerheyn (ed.), Understanding the tensile properties of concrete, Woodhead Publishing Limited, Cambridge, UK, 2013. (ISBN: 978-0-85709-045-4) (pdf)
  • P. Moonen, L.J. Sluys and J. Carmeliet, A continuous-discontinuous approach to simulate heat transfer in fractured porous media, Transport in Porous Media, Vol. 89(3), pp. 399-419, 2011. (pdf)
  • J. Alfaiate, P. Moonen, L.J. Sluys and J. Carmeliet, On the use of strong discontinuity formulations for the modeling of preferential moisture uptake in fractured porous media, Computer Methods in Applied Mechanics and Engineering, Vol. 199(45-48), pp. 2828-2839, 2010. (pdf)
  • P. Moonen, L.J. Sluys and J. Carmeliet, A continuous-discontinuous approach to simulate physical degradation processes in porous media, International Journal for Numerical Methods in Engineering, Vol. 84(9), pp. 1009-1037, 2010. (pdf)
  • P. Moonen, J. Carmeliet and L.J. Sluys, A continuous-discontinuous approach to simulate fracture processes in quasi-brittle materials, Philosophical Magazine, Vol. 88(28-29), pp. 3281-3298, 2008. (pdf)
  • S. Roels, P. Moonen, K. De Proft and J. Carmeliet, A coupled discrete-continuum approach to simulate moisture effects on damage processes in porous materials, International Journal for Computer Methods in Applied Mechanics and Engineering, Vol. 195, pp. 7139-7153, 2006. (pdf)

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