Pattern detection of internal tendon rupture on concrete surfaces

• Applicant: Dr.-Ing. David Sanio
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 501774158

Bridges form the backbone of an efficient infrastructure. For many years in service subjected to continuously increasing loads, many bridges now reach the end of their theoretical service life. Sustainability demands to keep them in operation as long as possible while ensuring structural safety. For prestressed concrete bridges it is particularly challenging that internal damage processes such as stress corrosion cracking or fatigue might remain unrecognised from the outside. Sudden rupture of the prestressing steel, not announced by concrete cracking, poses a major risk and concern. Such ruptures might occur everywhere in the structures. Thus, suitable monitoring techniques must make sure global surveillance and at the same time precisely detect individual ruptures. For this purpose, a measuring system based on fibre-optic sensors was developed in the first funding period of the SPP, which measures strains on the concrete surface of structures directly. Despite their quite low intensity it detects the discontinuous strain fields induced by tendon rupture reliably. With a grid of crossed fibre-optic sensors, the characteristic strain patterns of rupture and re-anchoring are recorded over entire surfaces, then automatically processed and separated from external thermal and mechanical interference. Up to now, the measurement system was developed under laboratory conditions and implemented as a prototype on a bridge. In the second funding period, the measurement method will be further developed to work robust under real ambient conditions. The aim is to ensure always reliable detection under the influences of ageing, exposure to environmental conditions, and time, while at the same time the number of sensors and the amount of data is reduced to the most extent. First, the long-term durability of the measurement technology is analysed in laboratory experiments, then in ageing and exposure tests on a real prestressed concrete bridge. Thereby, signal attenuation over time is quantified. From time-dependent, non-linear redistribution of the imprinted strain field due to tendon rupture, a required measurement rate is derived to cover all variable geometric and material boundary conditions. To find the optimal configuration of a measurement grid for distributed tendon layers, a method is developed that combines numerical simulation with two optimisation processes: the Greedy algorithm for the discrete number of sensor strands; heuristics for the continuous spacing of measurement points. The interaction of the influential factors is analysed integrating the effects of durability, time dependence and spatial resolution and a method to design measurement systems is developed. Different designs are evaluated using operational characteristics. For validation, the best will be implemented on a demonstrator bridge.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2388: Hundred plus - Extending the Lifetime of Complex Engineering Structures through Intelligent Digitalization
• Co-Investigators: Dr.-Ing. Mark Alexander Ahrens; Professor Dr.-Ing. Peter Mark