Evaluation of the contribution of boundary and initial conditions in the chemo-thermal analysis of a massive concrete structure

The chemo-thermal response at early ages is an important question in the design of massive concrete structures since it affects directly the mechanical response and is related to durability issues such as Delayed Ettringite Formation (DEF). This response is largely dependent on both environmental and initial conditions of the structures. The goal of this paper is then to predict the contribution and the relative importance of the solar radiation, convection, reradiation and conduction on the chemo-thermal response of massive concrete structures at early-age. An approximate solution of the 1D heat equation, with boundary conditions accounting for the solar flux, convection and reradiation, in a semi-infinite domain, is proposed. This solution is used to define scenarios for 3D numerical simulations with more complex geometry and boundary conditions. A particular massive structure for nuclear waste disposal composed of walls and slab having thicknesses ranging from 0.7 to 0.85 m, and for which the evolution of the temperatures was experimentally determined, is studied. The results highlight (i) the competition effects of convection and reradiation with the solar radiation; (ii) the determination of concreting temperatures in order to prevent DEF problems; and (iii) the influence of assembly dates of walls and slabs on the chemo-thermal response. The influence of model definitions such as the use of average or daily varying temperature and daily varying solar fluxes are also studied. It is shown that except in the case of ambient temperatures touching the maximum ambient temperature attained within a year (33 °C for the case studied), the maximum temperature reached within the structure does not exceed the maximum admissible temperature of 70 °C.