Transport processes in concrete at high temperatures

Within the US-European research activity "High temperature effects on cement-based materials" (coordinator: Prof. Willam, University of Colerado, Boulder) the proposed research porject is planned to take over modeling of transport processes and chemical reactions in concrete subjected to high temperatures. Transport processes, e.g., are caused by the evaporation of capillary and chemically-bound water during temperature rise. In case the permeability of concrete is not sufficient to allow the transport of the evaporated water to the free surfaces, the pressure build-up in the pore space may exceed the tensile strenght of concrete and cause spalling. This type of concrete failure was observed during recent fire accidents in European tunnels (Montblanc tunnel, Eurotunnel, Tauern tunnel, ). The spalling depth reached almost 50% of the tunnel lining, requiring long and extensive reconstruction work. Moreover, the reduction of the thickness of the concrete lining by 50% and the dehydration of cement taking place in the remaining 50% of the lining results in a severe reduction of the safety of the tunnel. This was the motivation for the initiation of several, so far experimental, research projects throughout Europe. In Austria, a research project titled "Fire resistance of fibre-, reinforced-, and prestressed concrete" (project leader: Prof. Kusterle, University of Innsbruck) was concerned with the experimental investigation of spalling for different designs of tunnel-linings. The obtained results showed that the increase of the amount of polypropylene (PP) fibers resulted in a decrease of spalling. By adding PP-fibres, spalling was reduced from originally 25 cm to 5 cm for 1.5 kg fibers/m3 concrete and to few millimetres for 3.0 kg fibers/m3 concrete. In the proposed research project, the theoretical basis for transport processes and chemical reactions shall be developed and applied to different types of lining concrete. Starting with the identification of material properties such as the permeability of concrete at different temperatures, a multi-scale model for the estimation of the advection properties shall be developed. The proposed multi-scale approach, however, requires experiments at every scale of observation. Respective experiments such as differential thermal analysis, thermo gravimetry, mercury porosimeter and macroscopic permeability tests will be conducted at the laboratories of the Institute for Strength of Materials (head: Prof. Mang) and of the Institute for Building Materials, Building Physics, and Fire Safety (head: Prof. Schneider). A close cooperation between the proposed research project and the research project "Multi-scale model for concrete subjected to thermochemomechanical loading" (P15912) is planned. Whereas the proposed research project shall provide access to the pressure build-up in the capillary space as a function of the permeability of concrete and the heating rate, project P15912 shall provide the strength criterion in order to assess the risk of spalling. The complexity of cement-based material subjected to high temperatures arising from phase change of the capillary and chemically-bound water, from chemical reactions taking place in the cement paste (dehydration) and the aggregates, and from the influence of admixtures (e.g. PP-fibers) was the motivation for Prof. Willam to form a research team consisting of co-workers with different scientific background such as material science, material modeling, and theoretical and computational mechanics. The findings of the different US and European research groups will be exchanged. For this purpose, annual workshops and research stays at foreign institutes are planned.

Based on the results of the FWF-project "Transport processes in concrete at high temperatures", a realistic safety assessment of reinforced-concrete support structures subjected to fire loading is possible. Support structures made of reinforced concrete (e.g., road or railway tunnels, bridges, underground communication facilities) are severely damaged by fire loading originating from accidents, terror attacks, or military operations. Capillary water evaporates at elevated temperatures, leading to an increase of the pore pressure within the concrete and eventually causing spalling of thermally-damaged near-surface layers of concrete. This can jeopardize the stability and serviceability of the support structure. Fire experiments showed an improved spalling behavior of concrete with additional polypropylene fibers: whereas more than 50% of the cross section spalled off in case of concrete without PP-fibers, spalling decreased to almost zero when a small amount of fibers was added to the concrete mix. This high influence of the amount of PP-fibers on the spalling behavior was attributed to the effect of the fibers on the permeability of concrete. Within the FWF-project "Transport processes in concrete at high temperatures", permeability measurements as well as experiments investigating the pore structure of thermally- damaged concrete were conducted at the Institute for Mechanics of Materials and Structures. The influence of the amount of PP-fibers on the permeability of concrete could be confirmed by the experimental results. In addition, the permeability results served as input into a coupled numerical analysis of the governing transport processes in concrete subjected to fire loading. The developed coupled model was validated by comparison of the numerical results with temperature measurements originating from fire experiments. In addition, stability assessment of support structures made of reinforced concrete subjected to fire loading was conducted using an in-house beam- spring model. Hereby, the influence of different spalling and fire scenarios was investigated. The results allow for an assessment of the safety level of existing as well as new reinforced-concrete support structures under fire.