Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

The curing temperature is known to influence the strength and durability of concrete. In general, curing at elevated temperature results in high early strength gain, but impairs the long-term strength and transport properties. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. This work has adopted a combination of macro and micro characterisation to further a comprehensive and quantitative understanding of the property development under different curing conditions, through approaches which include both qualitative and quantitative microstructural analysis with SEM, EDS, XRD, NMR and TG. A systematic study on several mix designs and curing regimes has been carried out, to relate microstructural properties such as degree of hydration, C-S-H composition, C-S-H relative density and capillary porosity to compressive strength and water sorptivity. The results show that the Ca/(Si+Al) ratio of C-S-H is constant with temperature, and the increase in relative C-S-H brightness is thus attributed to the higher C-S-H density and lower chemically bound water at higher temperatures. The early hydration of concretes is accelerated by high temperatures, but the ultimate value is not limited, as opposed to widely reported lower values in the literature. This discrepancy is explained by the fact that previous workers have assumed that the bound water is constant for different curing temperatures whereas this is not the case. Linear relationships were established between the degree of hydration and the compressive strength of concretes, which depend on curing temperatures and mix design. Through their influence on the density and distribution of hydration products, elevated temperatures increase the ultimate capillary porosity of concretes. The relationship between capillary porosity and compressive strength seems independent of temperature to a first approximation, but no unique relationship is shown between sorptivity and compressive strength. Materials which were exposed to higher temperatures for only short periods of time, such as might be experienced in prefabrication or in the curing of large masses, showed some recovery in both microstructure and mechanical properties from the detrimental effects of high temperature.

Thermo-hydro-mechanical behavior of energy tunnels

Matthias Timothee Stanislas Wojnarowicz

Subsurface geothermal in combination with Ground Heat Exchanger (GHE) and heat pumps systems had proved over the past decade to be an effective way in space heating and cooling. GHE’s embedded in geostructures such as tunnels has become more and more attractive in the current state where we try to mitigate greenhouse gas effects. The scope of this paper is to provide a fully coupled numerical model to assess the different physical aspects of an energy tunnel such as the kinetic and thermal boundaries layers development, thermally induce stress, and the heat generation. To do so, a finite element model is used to assess the heat exchange between, the GHE, the concrete lining, the air within the tunnel, the surrounding ground, and groundwater flow. The numerical model is then confronted with data from real-scale experimentation that was performed in Torino in 2018. The predictions from the numerical model show a quite good match with the data from the experimental prototype. Later on, similar models are generated to assess the effect of the heat exchange on air velocity and temperature within the tunnel as well as the induced thermal stress. The numerical results highlight the formation of both the kinematic and thermal boundary layer and shows that those boundaries are affected by the GHE’s activation. For the kinematic aspect, the lower temperatures observed in the vicinity of the heat exchangers tend to lower the velocity magnitude especially close to the wall surface. This change in velocity may be linked to a buoyancy effect cause by the lower temperature of the air in this regions. Indeed natural convection occurs along the tunnel, cooler fluid is pushed down while the warmer fluid rise; locally a lower temperature tends to decrease the viscosity of the fluid and increase its volumic mass. It was observed that the activation of GHE’s enhances this phenomenon in their vicinity due to the temperature gradient that they produce. Moreover, GHE’s activation tends to increase the overall turbulence of the flow and thus the eddies’ formation. The locations of those disturbance are difficult to predict due to the fact that small disturbance may generate large vorticity within the flow. However, it is shown that after passing the GHE zone, the velocity profile tends to stabilize to the undisturbed case. The kinematic entry length seems to happen further down the stream in the case where GHE’s are activated. This is due to the temperature disturbance generated by GHE’s activation. This phenomenon is increased with respect to the number of GHE loops activated. Furthermore, some probing was performed in the vicinity of the heat exchanger to assess the thermal induce stress due to the heating operation. The recorded stress shows peaks at early stages due to the higher temperature gradient that is generated between the GHE’s and concrete at beginning. Finally, after some time, the temperature reaches a steady state hence the stabilized thermal response recorded.

2020

Fire performance evaluation of GFRP-balsa sandwich bridge decks

Niloufar Vahedi

Bridges are a critical component of national infrastructures and economies. A significant percentage of bridges built all around the world are made of reinforced concrete which according to building codes have a life span between 80-100 years. However, structural inspections demonstrate that deterioration can begin in as little as 10 years due to the limited durability of steel-reinforced concrete. Exposure to humidity, freeze-thaw cycles and de-icing salts during winter result in corrosion of the steel reinforcement and spalling of the surface concrete which subsequently lead to partial or full bridge closure for required repair. Maintenance, upgrading and replacement of bridges may have significant economic and social impact due to disturbances in traffic flow. Mentioned reasons motivated scientists and engineers to develop and use more durable alternative materials with lower life-cycle cost. Composite sandwich bridge decks composed of glass fiber-reinforced (GFRP) face sheets and a balsa wood core, which are subject of this projects, provide the possibility of a more durable, lightweight and easily installed alternative. Balsa wood is a lightweight wood species exhibiting high strength- and stiffness-to-weight ratios, and therefore it is a favorable core material in bridge decks. However, this new type of sandwich bridge decks has to meet the same requirements as traditional concrete decks do. In particular, their behavior during a fire incident has to be known and predictable. Although fire is an accidental action according to Eurocodes, it has to be taken into account in the design of bridges and should not lead to a premature collapse. The knowledge about the combined thermo-mechanical behavior during a fire of these new sandwich decks is therefore crucial, in particular since the polymer matrix of the GFRP material and the balsa core are sensitive to high temperatures and combustible.GFRP sandwich structures are also used in other high-risk fire applications, e.g. in the automotive, naval and aerospace fields and numerous works about their thermo-mechanical behavior have been performed. However, these results cannot be simply transferred to GFRP-balsa sandwich bridge decks which are subjected to much more severe loads and their life span is much longer. The aim of this project is thus to investigate the thermo-mechanical behavior of GFRP-Balsa sandwich bridge decks in order to overcome this barrier of insufficient knowledge.Three level of investigations are planned, a) on the constituent material level, b) on the sandwich panel level, and c) on the bridge deck and superstructure level. Experimental investigations and modeling are conducted on each level from small- to full-scale. Since the performance of GFRP laminates under elevated temperatures have been investigated by many researchers, only thermo-physical and thermo-mechanical properties of balsa wood are investigated by proper experiments. At the second level, thermo-mechanical sandwich response models are developed and validated by experiments. The final level is conducted in order to investigate the thermo-mechanical behavior of the bridge deck at the structural system level which includes adhesively-bonded joints between the individual balsa panels of the deck and between the deck and the bridge girders. Furthermore, post-fire recovery models are established in order to assess the remaining structural capacity after a fire incident.

EPFL2022

Fire performance evaluation of GFRP-balsa sandwich bridge decks

Niloufar Vahedi

EPFL2022

Thermo-hydro-mechanical behavior of energy tunnels

Matthias Timothee Stanislas Wojnarowicz

2020