The Renovation of the Underground Car Park at the New Town Hall in Büdelsdorf – Challenges and Solution Strategies

The Renovation of the Underground Car Park at the New Town Hall in Büdelsdorf – Challenges and Solution Strategies

The underground car park of the New Town Hall in Büdelsdorf has been the focal point of municipal construction planning since early 2026. Structural examinations have revealed significant damage to reinforced concrete components that could compromise the load-bearing capacity and safety of the building without comprehensive renovation. Decisive steps for the restoration were set in the 2nd meeting of the Steering Committee for the New Town Hall District, which has proven to be a technically and financially demanding project.

Condition and Causes of the Damage

The underground car park shows significant corrosion and chloride damage in the base areas of columns and walls. Investigations revealed that chloride concentrations in some areas exceed 0.5 M-% – a clear indication of a serious threat to the reinforcement. This has led to cracks, spalling concrete, and partial cross-sectional loss of the reinforcement.

The causes of the damage include long-term exposure to de-icing salt and a lack of protective measures against moisture. In addition, structural deficiencies were identified: missing stirrups at connection points, insufficient transverse reinforcement, and structural weaknesses at column bases. These deficiencies contribute to the fact that the load-bearing capacity of the columns is no longer sufficient to meet static requirements.

Renovation Strategy: KKS-System and Conventional Measures

To address the damage and ensure the structural condition for the remaining planned service life of 50 years, Büdelsdorf is combining conventional renovation methods with modern corrosion protection systems.

Cathodic Corrosion Protection (KKS)

A central measure is the application of Cathodic Corrosion Protection (KKS). This method suppresses corrosion on the reinforcement by lowering the potential of the reinforcement through external current polarization. Anodes in the form of titanium grids are mounted on the concrete surface and connected to a direct current source. The reinforcement acts as the cathode, thereby preventing anodic corrosion processes.

The KKS system has the advantage of being less intrusive to the existing load-bearing structure than conventional renovation methods. However, it is associated with higher long-term maintenance costs. For the underground car park, a KKS system with a budget of approximately 130,000 € (including annual maintenance costs of 4,500 €) will be implemented.

Conventional Renovation Measures

In addition to the KKS system, traditional renovation methods are also being used. Chloride-contaminated concrete is being replaced with repointing mortar, corroded reinforcement is being derusted and, if necessary, supplemented. The columns are being reinforced with a statically encircling replacement cross-section made of stainless steel to prevent chloride remigration.

In addition, a surface protection system (OS5b) is being applied to prevent future water ingress. The costs for these measures amount to approximately 290,000 €. Overall, the total budget for the entire renovation is approximately 440,000 €.

Monitoring and Quality Assurance

The restoration work is structurally critical and must therefore be supervised by an approved monitoring body. In addition, qualified planning and execution are required, particularly for the application of the KKS system, which requires certification according to DIN EN 15257.

The city of Büdelsdorf has defined clear procedures, from structural opening and subfloor preparation to the application of the surface protection system under the paving. Important parameters such as surface tensile strength, roughness, and temperature limits during the application of materials are carefully defined to ensure the durability of the restoration.

Outlook and Conclusion

The renovation of the underground car park at the New Town Hall is a central milestone for the future use and safety of the town hall in Büdelsdorf. By combining the KKS system with conventional renovation methods, the city will ensure long-term structural safety and durability of the building. The investment of over 12 million euros underscores the importance of this project for municipal infrastructure.

However, the challenges are not only technical in nature. The city must also ensure that the renovation work is completed on time and in compliance with quality standards. In this regard, close cooperation with engineering firms, approved monitoring bodies, and the executing company will play a decisive role.