This thesis is devoted to multiscale analysis of the thermomechanical behavior of cementitious materials, including cement pastes, mortars, and concretes, and of engineering structures of traffic infrastructure, including pavement plates made of plain concrete and tunnel segments made of steel-reinforced concrete. In their service life, concrete structures are subjected to variable ambient conditions in terms of temperature and relative humidity, eventually also to extreme weather events such as hail showers, and occasionally to exceptional load cases such as fires following car accidents. The resulting thermomechanical loading may lead to damage of the structures. Thus, it is a threat to their long-term durability and safety. This raises the interest of civil engineers in quantification of stresses resulting from thermomechanical loading of concrete structures. In the present thesis, this topic is considered in the context of multiscale analysis, bridging all intermediate scales between the scale of nanoscopic calcium-silicate-hydrates and the one of traffic infrastructure. The first part of this thesis is devoted to the thermal expansion of cement pastes, which depends on the internal relative humidity and goes along with the paradox effect that the internal relative humidity increases in case of heating and decreases in case of cooling of the material. The nanoscopic mechanism that controls these material properties is investigated based on multiscale top-down identification. To this end, the microstructure of cement pastes is resolved down to the nanoscopic scale of observation of calcium-silicate-hydrates and gel pores. Pore size distributions of spherical gel and capillary pores are identified on the basis of the Brunauer-Xi-type adsorption isotherms. The Mori-Tanaka scheme provides the scale transition from thermal eigenstrains of solid constituents and changes of effective pore pressures to the macroscopic thermal expansion of the cement paste. Using the described microporomechanics model, it is shown that spontaneous water release/uptake by hydrates governs the complex thermal expansion behavior of cement pastes. The second part of the thesis refers to the extension of the multiscale material model to thermoelastic homogenization of concrete and to the use of this model for macroscopic structural analysis of the steel-reinforced segments of the immersed tunnel of the HongKong-Zhuhai-Macao Bridge. Quantification of thermal stresses, developed in the overhead-slab of the tunnel segments, is based on the assumption that the fire, caused by a car accident, is moderate. To this end, steel-reinforced concrete beams with different support conditions are analyzed. This includes a simply supported beam and a beam with clamped ends. The thermoelastic properties of concrete are quantified by means of the multiscale model. They serve as input for thermomechanical structural simulations performed by means of the Finite Element Method. The computed temperature changes and macroscopic stresses of concrete are finally used as input for top-down quantification of average stresses, experienced by the constituents of concrete: cement paste and aggregates. In this way, microscopic stress fluctuations are quantified. The third part of this thesis deals with the structural response of concrete pavements subjected to a hail shower. At first, the nonstationary heat conduction problem is solved, providing access to the spatially nonlinear temperature fields inside pavement plates. Corresponding thermal eigenstrains are subdivided into a constant part (= eigenstretches of the plate), a linear part (= eigencurvatures of the plate), and a nonlinear part. This split is based on the generalized Bernoulli-Euler hypothesis and on stress resultants obtained by means of the principle of virtual power. The nonlinear iii part of the thermal eigenstrains is prevented at the scale of the generators of the plate. This yields thermal stresses which do not contribute to normal forces and bending moments per unit length. As for the structural analysis, the plate is considered to rest on an elastic Winkler foundation. The described elements of analysis deliver macroscopic stress states in the concrete. Finally, top-down scale transition is used to compute the average stresses in the three concrete constituents: cement paste, fine aggregates, and coarse aggregates, as well as in the interfacial transition zones (ITZs), covering the aggregates. The last part of the thesis refers to the analysis of the structural behavior of a segment of a subway station, subjected to a tunnel fire. The segment is made of steelreinforced concrete. It consists of a top slab, a bottom slab, two lateral walls, and two columns, connecting the two slabs. A large-scale fire test, carried out at Tongji University, Shanghai, China, is described. During the fire experiment, the structural behavior is monitored, based on strain gauges and temperature sensors. Subsequently, the first 30 minutes of the experiment are investigated, based on three-dimensional Finite Element Analyses. This part of the test refers to a moderate tunnel fire. Numerical sensitivity analyses are carried out. Two different sets of thermal boundary conditions and two different models for the thermomechanical behavior of concrete are considered. A temperature-independent linear-elastic material model and a time-dependent elastoplastic model are used. Numerical results are compared with experimental results, indicating the potential and the limitations of the analyses. The following conclusions are drawn from the research on which this thesis is resting: Hydration products appear to release liquid water at heating and to take up water at cooling. This water uptake or release results in the redistribution of water within nanoscopic pores of partially saturated cement paste. This process manifests itself macroscopically by changes of the relative internal humidity and the effective thermal contraction or expansion of cement pastes. The water uptake/release characteristics of the hydration products were identified as a universal function of the internal relative humidity. The term “universal” indicates that these characteristics of the hydration products are independent of the initial composition of the cement paste. Thus, they apply to mature cement pastes, independent of their initial water-to-cement mass ratio. When concrete is subjected to mechanical loading and temperature changes, microscopic stress fluctuations are induced. They are a function of the mismatch of the microscopic thermal expansion coefficients and of the elastic stiffness properties of the constituents of concrete: the cement paste matrix and the aggregate inclusions. Both macroscopic tensile loading and cooling of concrete result in tensile stresses within the interfacial transition zones (ITZs) which cover the aggregates. The ITZs are well-known to be the weakest links of concrete microstructures. Methods of multiscale continuum mechanics allow for deriving mathematical formulae for the analytical quantification of these microscopic stresses. Spatially nonlinear temperature distributions are characteristic features of nonstationary heat conduction problems. Inevitably, they lead to thermal stresses. Engineering approximations, based on “equivalent” linear temperature distributions, appear to be impossible. iv Hail showers are very likely to damage the top surface of pavement plates made of plain concrete. Operational countermeasures are cooling of the exposed top surface of pavement plates right before the start of the hail shower and removal of the hailstones from the top surface of the pavement as quickly as possible. Reliable numerical simulations of tunnel fires are a challenging task. This is the consequence of three types of uncertainties: the possibility of damage of the structure prior to the fire, the actual distribution of the surface temperature during the fire, and the time-dependent high-temperature behavior of concrete. The combination of modern multiscale material models and classical modes of structural analysis is straightforward. This combination leads to multiscale structural analyses. They have the potential to increase the “informative content” of structural simulations.
