Constitutive Parameters
The numerous parameters which appear in the advanced models described in the previous chapter are in general dependent of some of the state variables, such as temperature. In this chapter, many expressions used in practice for the evolution of the hygral,
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Constitutive Parameters Fekri Meftah, Colin Davie, Stefano Dal Pont and Alain Millard
Abstract The numerous parameters which appear in the advanced models described in the previous chapter are in general dependent of some of the state variables, such as temperature. In this chapter, many expressions used in practice for the evolution of the hygral, thermal and mechanical parameters are presented.
5.1 5.1.1
Hygral Parameters Evolution of Pore Structure During Heating
The loss of stability of solid phases in concrete, paste and aggregates, when exposed to high temperatures, affects its pore structure. The physical and chemical changes occurring in solid phases induce changes in pore size distribution and total porosity. Noumowe (1995) has given the variation of porosity with different levels of temperatures for an ordinary concrete and a high performance concrete heated up to 600 °C. The total pore volume of a heated concrete increases non-linearly with the increase of temperature. On heating up to 300 °C, a relatively small increase in pore volume is observed in comparison to the weight losses. This was considered to F. Meftah (&) Institut National des Sciences Appliquées, Rennes, France e-mail: [email protected] C. Davie School of Engineering, Newcastle University, Newcastle upon Tyne, UK e-mail: [email protected] S. D. Pont Laboratoire 3SR, Université Grenoble-Alpes, Grenoble, France e-mail: [email protected] A. Millard Service d’Études Mécaniques et Thermiques, Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA), Saclay, France e-mail: [email protected] © RILEM 2019 A. Millard and P. Pimienta (eds.), Modelling of Concrete Behaviour at High Temperature, RILEM State-of-the-Art Reports 30, https://doi.org/10.1007/978-3-030-11995-9_5
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be due to structural changes resulting from the desiccation and dehydration processes occurring mainly in pores of radii less than 40 Å (Andrade et al. 2003). Heating up to 600 °C will increase the total pore volume. This increase is higher than would be expected in comparison with weight losses (Fig. 5.1). On the other hand Bažant and Thonguthai (1978, 1979) observed an upward jump in concrete permeability of two orders of magnitude. This was related to the fact that the pore volume available for capillary water must increase significantly when temperature and pore pressure increase. Then, it may therefore be suspected that either an expansion of pores has occurred by break down of partition walls, or micro-cracks are formed (Andrade et al. 2003). Many authors have investigated micro-cracking (Dougill 1968; Blundell et al. 1976). They have indicated that the differential strain between the aggregate (which expands) and the cement paste (which shrinks) at different temperatures will initially induced a small compressive stress in the paste. As the temperature increases, the compressive stress is reduced and changes to a larger tensile stress. The changes of porosity with the increase of temperature were
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