Civil engineering structures are designed and built for use according to plan during life-time. The design is usually performed under the assumption of an ideal state of structures and building materials during the total life-time. Actually the state of materials and thus the properties of buildings are changing over the period of use, what may influence the reliability and quality of the building during life-time and may lead to a reduction of safety with respect to loads and usage. The evolution of the properties of building materials and structures may be caused by physical or chemical reasons and takes place on different spatial and temporal scales.
Goal of the research programme is the development of scientific approaches to describe and to evaluate the changing of properties and quality of buildings and infra-structures with respect to physical and chemical effects. The multi-coupled processes, which are responsable for the evolution of building materials, will be described by model equations on different spatial and temporal scales within the theory of continuum mechanics and the theory of porous media. The models will be the basis for prognoses of the development of building materials and structures.
The multi-scale models describe the different phenomena of aging as a coupled process, what needs a direct coupling of the single processes in space and time, in order to consider the interaction of processes and to integrate all information for an evaluation of the quality of buildings. In an extension of the current approaches on the macroscopical scale, the mechanisms of transport as well as physical and chemical damage shall be investigated experimentally and shall be modelled on different scales of materials and buildings. Based on experimental and numerical investigations simplified engineering approaches shall be taken into account, which can be realized to practice.
The scientific training of the doctoral-students follows a structured programme of workshops, seminars and publications, whereas the individual work will be honoured by credits. The programme contains scientific experiences in the field of experimental work, mathematical modelling of processes and structural analysis, and the training of key skills in the field of scientific work and scientific management. Based on the broad education regarding different building materials the doctoral-students will get the possibility to understand and to evaluate completely different phenomena and to describe them by advanced models for life-time-prognosis of civil engineering structures.
Further information on the Research Training Group: https://www.tu-braunschweig.de/en/grk-2075
Spokesperson: Prof. Dr.-Ing. Manfred Krafczyk
Institutes involved: Institutes of the TU Braunschweig of the Faculty of Architecture, Civil Engineering and Environmental Sciences as well as the Carl-Friedrich-Gauß-Faculty, the Fraunhofer Institute for Wood Research and the LU Hannover
Funding period / duration: 4.5 years
Funded by: German Research Foundation (DFG)
Ageing influences on the interaction of embedded reinforcement and externally bonded carbon fibre lamellae in reinforced concrete components
Ageing, material fatigue and the increased traffic load on many concrete bridges on federal trunk roads in Germany necessitate restoration or strengthening measures. An economical method of strengthening reinforced concrete components is to use bonded reinforcement made of carbon fibre reinforced polymer (CFRP). The externally bonded reinforcement increases the load-bearing capacity of the aged concrete components and thus causes an extension of the service life. The load-bearing effect of this rehabilitation measure is mainly influenced by the adhesive bond between the bonded carbon fibre polymer and the concrete. However, the adhesive bond is considerably affected by environmental influences, ageing phenomena and fatigue damage over the period of use. To ensure the stability of these reinforced components, the load-bearing capacity of the adhesive bond must be ensured. The investigation and modelling of the composite load-bearing behaviour are crucial for concrete components reinforced with bonded CFRP lamellae.
The aim of the research project is to determine the internal forces in concrete, steel and bonded reinforcement of a reinforced concrete component under steady states of stresses. In this context, approaches have to be developed to determine the force distribution between inserted and bonded reinforcement, considering different bond ratios.
Experimental investigations on mixed reinforced concrete beams will serve as the basis for modelling the time-dependent composite load-bearing behaviour of different reinforcement strands as well as the degradation behaviour of the concrete. Furthermore, state-of-the-art fibre-optic measuring systems will be used to record the strain states of the reinforced component throughout the period of investigation.
With the description of the time-dependent material behaviour and the degradation models of the composite load-bearing behaviour, it will be possible to create a prognosis model that is able to make a statement about the residual load-bearing capacity of the reinforced structure.
Contact person: Zhuo Chen M. Sc.
Influence of mechanical properties of materials on the durability of repaired reinforced concrete components
In a classic concrete repair, the damaged concrete is removed and re-profiled with a repair mortar or concrete. After such a repair, a structural element is usually considered to be as good as new. A detailed statement on the remaining service life of a repaired reinforced concrete component was not possible until recently. However, this is of great interest to the owner or user of a reinforced concrete structure from an economic point of view.
As a result of two research projects at the Technical University of Munich, it has now been possible to transfer the approach to probabilistic lifetime design according to Gehlen into a semi-probabilistic, and thus strongly practice-oriented approach. It is now possible to calculate the remaining service life of a reinforced concrete component with regard to carbonation-induced and chloride-induced corrosion, depending, among other things, on the weather conditions, the applied mortar layer thicknesses and the material-specific resistance to chloride penetration and carbonation.
The basis of the calculation approaches is Fick's 2nd law, which describes the diffusion mechanisms and allows a material-specific estimation of the penetration behaviour of liquids and gases into the concrete on a chemical level. The ageing of a component and its material resistance to chemical attack is taken into account in the case of carbonation using the root-time approach and in the case of chloride penetration by adjusting the chloride diffusion coefficient using an experimentally determined age exponent.
The influence of mechanics on the description of aging processes in a repair layer has not been integrated in the current models for lifetime design so far. The need for research in this area becomes clear when considering the controllability of stress distributions between repair material and old concrete via the variation of the Young's modulus in the repair layer. A soft system avoids the load, while a stiffer system relieves the old concrete and takes over the load transfer itself. As a result of deformations, micro-cracking of the hardened cement paste matrix occurs long before the formation of cracks visible to the eye. This is accompanied by a change in the material resistances relevant to durability.
The aim of the work is therefore to determine ideal combinations of mechanical parameters between old concrete and repair material, initially on the basis of laboratory tests and depending on numerous variables (e.g. class of old concrete, geometry of concrete repair, load scenario, modulus of elasticity, strength) in order to achieve the longest possible service life. The stress distribution in the cross section of the component and also on the composite joint is to be considered. In the following, these considerations will be linked to investigations on the influence of mechanical ageing on the chloride diffusion coefficient.
Contact person: Dipl.-Ing. Stefan Ullmann
Crack initiation along the bonded joint between concrete and CFRP laminates
The rehabilitation of aged buildings and infrastructure projects has become increasingly important in civil engineering. Engineers often have to deal with old buildings that require retrofitting. A material that meets the requirements for reinforcement, is easy to process and has good mechanical properties is carbon fibre reinforcement plastic (CFRP). An important point of reinforcement is to check the adhesive bond between the concrete and the slats because the bond failure, called decoupling, is brittle and occurs without notice. In the course of previous investigations, a possible correlation between cracking and a coefficient of the bond that depends on the fracture energy was found. Cracking has so far been represented by roughness parameters, but the factors influencing cracking are still the subject of research.
The aim of the project is to investigate the properties of concrete that can be decisive for the formation of cracks. The parameters to be investigated are the mechanical properties of the aggregate and the cement paste as well as the shape and grain distribution (grading curve) of the aggregate. The research programme will be divided into two parts. First, attention will be paid to the small scale (mesoscale) to better understand the mechanisms. In this part of the research, the focus will be on the examination of the samples using micro CT and the subsequent analysis (segmentation and reconstruction) of the images. The knowledge gained will be validated in further experiments in the second phase of the project. Here, experiments on the macro level are planned in particular, in which samples are subjected to cyclic loading. Based on the results of the experimental programme, an engineering model applicable in practice will be developed.
Contact person: Matteo Lunardelli M. Sc.
Mesoskalen-Modellierung der Riss-induzierten Durchlässigkeit von Beton
Cracking due to tension in concrete is characteristic for reinforced concrete construction. In the case of macro cracks even small crack widths are able to impair the durability of struc- tures. In particular, the prediction of water transport focus great interest in practice. The influence of crack width on flow rates has been investigated in several researches.
Nowadays the fluid transport coul d be estimated considering crack opening and roughness of the crack surface. A flow law, which is often used because of its simplicity is the cubic law (q = (ξ∙g∙I∙b∙w³)/(12∙ν) [m³/s]).
The goal of the project is to improve the approach taking into account other important parameters of the material like concrete strength and concrete mixture. The degree of reinforce- ment and the element thickness are import ant factors as well.
Several experiments are planned. At first a permeation test of reinforced c oncrete samples with a specific crack width at the sample surface will be performed. Wedges will be pushed into the concrete sample in order to induce crack between 0,1 and 0,3 mm. In the first step the fluid will be water, in further steps also corrosive fluids will be investigated. Furthermore, several values like the used aggregates and w/c-values and concrete mixtures will be varied. To determine the effect of selfhealing of cracks the permeation test will be carried out as a short term and as a long term experiment. Micro-CT investigations will be carried out on cores drill ed from the permeation test samples. The crack will be filled with epoxy resin before x-rays measure ments to fix the crack pattern and get detailed information of the crack geometry. To estimate the roughness, the samples fractured surfaces will be analyzed with a digital microscope.
Approaches to develop the cubic law are:
- Stochastic description of cr ack path and roughness of the crack surface,
- Stochastic description of the crack width through the component thickness,
- Description of the concrete permeability by taking into account the influence of macro cracks. A computational description model is plan ned b esides the mentioned experiments,
- The experimental and computational results will be used to express a finer and more extensive description of the fluid transport for cracked porous structures for the practical applications.
Publications within the framework of the RTG:
Conference contribution with publication in conference proceedings:
L. Mengel, D. Köhnke and H. Budelmann. Radiation effects on concrete. Research on Radioactive Waste Management, Ethics - Society - Technology. Final ENTRIA Conference, Braunschweig, September 2017.
Contact person: Lena Mengel M. Sc.
