Problem
At the beam bridge over Fröviforsån (Bridge 18-186-1), built in 1957, horizontal cracks were observed in the lives of the post-tensioned concrete beams. The cracks follow the route of the tension cables. The Swedish Transport Administration wanted a survey of the tension cables and cracks and an analysis of possible causes of the damage. The assignment included non-destructive testing, measurement of movements and vibrations and a discussion of probable causes of the cracking.
Results
The survey showed that several casings and tension cables deviated from the theoretical locations shown in the drawings. Ultrasonic crack depths ranged from 50-100 mm, and it could not be ruled out that some cracks reached all the way into the casings. Measurements with accelerometers and crack sensors showed that the natural frequency of the bridge was in the range of 12.5-15.5 Hz, in good agreement with calculated values, indicating that the cracks do not affect the global stiffness of the structure. Crack openings and strains were stable over time and were not affected by temperature variations. It was concluded that the cracks are mainly caused by a combination of thermal stresses, effect of the tension reinforcement and small concrete cover layers.
Solution
The project was implemented in several stages:
- Geo-radar scanning (GPR) for locating tension cables and checking their positions against drawings.
- Ultrasonic (UPV) crack depth measurement, where depths of 50-100 mm were measured.
- Crack mapping to document the location and width of cracks.
- Cover layer measurement to determine the thickness of the concrete layer over reinforcement and casing.
- Installation of accelerometers, LVDT crack sensors and strain gauges for vibration and motion measurement.
- Vibration analysis (RMS, PSD) to determine natural frequencies and compare with analytical calculations.
This methodology provided a reliable picture of both the extent of the cracks and the dynamic behavior of the structure.
Equipment
- Georadar (Proceq GP8000/8800)
- Ultrasonic equipment (Pundit PL200 UPV)
- Coating thickness gauge (Profoscope)
- Accelerometer
- LVDT crack detector
- Strain gauges
- Acquisition system with cloud connectivity
Standards
EN 1990 – Eurocode: Basic design rules
EN 1991-1-1 – Loads on structures
SS-EN 1992-1-1 – Concrete structures
ISO 16311-1/2 – Condition assessment of concrete structures
The Swedish Transport Administration’s regulations and the Swedish Road Administration’s inspection manual
