Problem
The chimney at Käppala wastewater treatment plant on Lidingö showed damage that justified a discussion about reinforcement measures. The chimney is cracked and measurements with inclinometers and accelerometers show that the chimney does not have the theoretical stiffness when dimensioned. In addition, there is a theoretical risk of overturning. It was therefore decided to extend/reinforce the foundation and to reinforce the chimney. Traditional methods such as casting and steel plate reinforcement of the chimney were discussed but have limitations. The task was to investigate the possibility of using carbon fiber composite as a method of reinforcement, especially with regard to fatigue capacity, and to describe the technology, reference projects and a possible workflow for a reinforcement of the chimney.
Results
Research and calculations showed that carbon fiber composite is an effective method to reinforce concrete structures, especially to extend the service life under fatigue loading. Research and laboratory tests from Sweden and internationally show that carbon fiber laminates and NSM bars significantly increase the capacity of a structure without the composite material itself exhibiting fatigue failure. Reference projects such as Kristineheds Waste Heat Plant in Halmstad, where the chimney was reinforced with carbon fiber fabric, show good durability even after 25 years. Reinforcement with carbon fiber is therefore recommended as a long-term and sustainable solution.
Solution
- The report presents reinforcement techniques with:
- NSM bars – recessed bars in sawn grooves, protected in the concrete cover layer.
- Carbon fiber fabric – for curved surfaces like wrapping around the chimney
Furthermore, the reinforcement of the Käppala chimney is presented:
- Assembly of scaffolding and weather protection.
- Complementary condition assessment of concrete and reinforcement.
- Repair of damage and corroded reinforcement (small scale).
- Sawing of grooves for NSM rods
- Grinding/sandblasting of surfaces for carbon fiber fabric
- Assembly/gluing of carbon fiber rods
- Priming of surfaces for bonding carbon fiber fabric
- Application of carbon fiber fabric horizontally in several layers.
- Painting for protection against UV and temperature effects.
Equipment
Reinforcement system from Sto Scandinavia AB (carbon fiber laminate, NSM rods and carbon fiber fabric with associated glue). Concrete slurry as protection outside the carbon fiber and chimney.
Standards
- EN 1990 – Eurocode: Basic rules
- SS-EN 1992-1-1 – Concrete structures
- EN 1504 – Repair and protection of concrete structures
- ISO 16311-1/2 – Condition assessment of concrete
- FIB Bulletin 14 (2009) – External reinforcement with FRP
- ACI 440 – Guide for the Design and Construction of Externally Bonded FRP Systems
Composite reinforcement of concrete, B. Soapstone et al, Handbook of Reinforcement, 2016, ISBN 978-91-7333-763-2
