Graphical representation of transport processes in concrete
Source: BAM, Modelling and Simulation division
Understanding transport processes in concrete makes it possible to determine the influences of fire, shrinkage, penetrating chemicals and changes in thaw and frost on the safety and durability of structures. Numerical simulations play a decisive role here. On the one hand, to reduce the experimental effort and to transfer existing test results to new conditions, and on the other hand, to optimize concrete mixes and the design and dimensioning of construction parts. The paper deals with the modelling of moisture transport at high temperatures. In particular, the effect of moisture accumulation combined with a significant increase in pore pressure is important for high-strength concretes, as this effect can lead to spalling of the upper concrete layer in the event of fire. This represents a major safety risk, especially as it can expose the reinforcement. Modern high-performance concretes are particularly vulnerable to spalling due to their low permeability.
Moisture transport at high temperatures consists of multiple interacting phenomena, such as dehydration, adsorption/desorption, advection, diffusion, heat conduction and transport, as well as evaporation and condensation. The necessary parameters to model these effects come from experiments in which these interactions also take place but are often neglected in the analysis. Furthermore, such results are rarely publicly available. The combination of the large number of processes and the scarcity of data makes accurate and reliable modelling difficult. The finite element model was implemented in the free software FEniCS, and solves the conservation equations for water, air and solid mass, as well as enthalpy.
For this paper, the model was validated using data from computed tomography. CT allows volumetric measurement of the moisture content inside the specimen, in contrast to pointwise measurements with temperature and pressure probes. It turns out that dehydration descriptions based on thermogravimetry are inadequate because the conditions during thermogravimetry do not correspond to those in compact specimens. A dehydration equation based on the CT data is introduced. The reproducibility of the paper including all diagrams was an important concern, that is the source code, an automated execution of all calculations as well as a computational environment with all necessary software are publicly available under DOI 10.5281/zenodo.4452800.
A three-phase transport model for high-temperature concrete simulations validated with X-ray CT data
Christoph Pohl, V. Smilauer, Jörg F. UngerORCiD
published in Materials, Vol. 14, issue 17, article 5047 pages 1- 21, 2021
BAM Modelling and Simulation division