Papers by Author: Vit Šmilauer

Abstract: UHPC (ultra high performance concrete) is used mainly within bridge engineering, due to the demand for subtle but strong material, with high durability against aggressive environment. During the last two years, a series of thin-walled railing panels were made in premises of Skanska a.s. company. They were used for a footbridge in Čeperka village, and this year also for a renovation of a bridge SZ-001 in Sázava municipality. The renovation of a bridge, manufacturing of the railing panels, development of its shape and results of conducted test will be presented.

Abstract: Inorganic foams offer several unique properties such as low thermal conductivity, fire resistance, or UV stability. Inorganic foam specimens were synthesized from fly ash and aluminium powder through an alkali-activation process. Depending on mix proportions, bulk densities ranged between 400 and 800 kg/m³. Thermal treatment at 80°C for 12 hours accelerated curing process. Compressive strength was found in the range 4.5-9.0 MPa, flexural strength 0.6-1.7 MPa, Young's modulus 0.6-1.1 GPa, thermal conductivity 0.14-0.16 W/m/K and thermal capacity around 1100 J/kg/K. Exposing the foams to temperature 800°C led to a small decrease of compressive strength while exposure to 1100°C sintered the foam to higher strength of 13 MPa. Volumetric shrinkage 20% occurred at 1100°C without further disintegration. Residual compressive strength was determined after exposure to NaCl, HCl, Na₂SO₄, MgSO₄, H₂SO₄. The highest reduction to 20% occured in both acids with pH=2 after one year of exposition. Digitized microstructures entered finite element analysis to validate a stress-strain diagram.

Abstract: Research of alkali-activated materials has been a traditional domain of chemists. This paper exploits contribution of micromechanics to the subject. A new model for volumetric evolution of chemical phases is formulated. The first homogenization level identifies elasticity on the scale of N-A-S-H gel. Nanoindentation sensing technique yielded the intrinsic Young's modulus of N-A-S-H gel as ~18 GPa, which was further downscaled to the solid gel particles. Percolation theory had to be introduced to match an early-age elasticity. The second homogenization level takes into account an unreacted fly ash. Homogenization models match well the experimental elasticity and demonstrate stiffening of N-A-S-H gel, induced by increasing packing density of the solid gel particles. The percolation model explains a long setting time of alkali-activated materials.