Material Law and Model to Estimate Minimum Reinforcement

Massive concrete structures tend to develop cracks over the period of hardening which are caused by stresses. Using existing approaches, the formation of cracks cannot be predicted or controlled satisfactory. Reasons therefore are uncertainties of available material models as well as particular characteristics of massive concrete structures. Furthermore, the influence of concentrated reinforcement positioned near border zones is not taken into account sufficiently by the existing models. Up to now, the material properties of hardening concrete have been described by design models without considering its history of stress. Yet early exposure to loads affects the material very negatively, in particular its tensile strength. To predict the force, time and location at which cracks appear by using the existing material model delivers unreliable results. Therefore, the model will be expanded by the influence of stress-dependent aging under consideration of viscoelasticity. Limitation of crack-width is realised by reinforcing the concrete. Today`s design approach bases on investigations on hardened concrete. The expanded material model which is to be developed will comprise an improved bond model and a new calculation approach for young concrete. The new developed material and bond model will be incorporated in a FE-model. Special characteristics of massive concrete structures are going to be considered; stress and crack formation will be predicted. The model will be validated by on site measurements. Parameter studies and model calculations will build the basis for developing the required design model of massive concrete structures.

Concrete is the most commonly used building material. Our built environment, whether infrastructure or building constructions, would have been inconceivable without concrete, especially due to its robustness and adaptability to specific requirements of several construction tasks. The hardening of concrete, however, causes restraint stressing on structural level which predominantly occurs due to the hydration heat of the exothermal hardening process. This can provoke cracking, especially with increasing thickness of the members but also in case of wide areal dimensions. The serviceability of the construction is therefore ensured by a minimum reinforcement which limits the crack width to a tolerable size. Despite intense efforts, the current standards still do not provide a mechanical consistent design model to determine the minimum reinforcement for crack width control. The reason is the complexity of the problem due to a significant time-dependency of the hardening behaviour of concrete as well as the need of multi-physical simulations. For an appropriate design, most current standards simplify this context by taking up the cracking forces respectively the cracking moment. Without any modification this approach would be on the safe side, however, its application on thick or large members, where minimum reinforcement for crack width control is usually decisive, would require extensive reinforcement amounts, which cannot be verified by practice. To ensure the efficiency of constructions empirical modifications were introduced. Although this concept gives straightforward results, it can only be seen as a pragmatic solution which is generally not on the safe side. Structural damage - especially in form of leakage in watertight constructions - often occurs. In the course of this project the available material models for hardening concrete were improved, especially the influence of initial damage due to the hardening induced stress history itself as well as the viscoelastic behaviour under tensile stressing. Besides, the hardening behaviour on structural level was investigated with numerical parametric studies. Finally, an appropriate design model to determine the minimum reinforcement for crack width control under consideration of the real member behaviour could be developed