Determining the Condition of Reinforced and Prestressed Concrete Structures Damaged by Elevated Temperatures

This paper focuses on the effect that high temperatures generated during a fire have on reinforced and prestressed concrete structures. Fire is one of the common reasons for rehabilitations and reconstructions during the service life of a building. Before these can be started, however, it is necessary to assess the structure's technical condition. It is therefore necessary to know the physico-chemical changes occurring in the concrete due to fire. Rising temperatures effect the evaporation of all water contained and cause the dehydration of CSH compounds. Transformations in the crystalline structures of siliceous aggregate minerals result in an increase of volume and crack formation. The decarbonation of limestone aggregate produces escaping gases which damage the concrete as well. Additional mechanical stress caused by the sum of the volume changes is another important factor. In addition, the different thermal expansion of steel and concrete severs the bond between the two. Changes in the material structure are then detrimental to the mechanical parameters. Concrete sees significant changes at temperatures as low as 300 °C. A structure's load-bearing capacity is often determined by reinforcement bars or prestressing tendons, which reach their critical limits at temperatures lower than concrete. The paper draws on knowledge about the condition of materials damaged by elevated temperatures and discusses the assessment of a concrete skeleton structure after a fire. The main problem was presented by the ceiling, which was built from Spiroll prestressed ceiling slabs. The grouting between the slabs had disintegrated, the bottom faces of the slabs fell off and the prestressing tendons often lost tension. A renovation procedure for the structure was suggested.