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Increasing the concrete cross-section may be appropriate where the existing section has been shown to be deficient, either due to inadequate reinforcement or an insufficient concrete compressive area. The deficiencies may be due either to damage to the structure (e.g. spalling and steel section loss due to reinforcement corrosion) or to a requirement for the structure to carry additional load. Examples of the application of the technique to the strengthening of concrete bridges are given in CBDG Technical Guide 10, Enhancing the capacity of concrete bridges.
The capacity of the increased cross-section should be determined in accordance with the appropriate design code (see ´Standards, codes and regulations - Structural´). The properties of the new concrete (e.g. strength, aggregate type) should be as close as possible to those of the parent concrete, to minimise stresses due to differential strains. Worked examples are given in an appendix to Concrete Society Technical Report 68, Assessment, design and repair of fire-damaged concrete structures.
The new concrete must be adequately connected to the parent concrete, for example by means of drilled and grouted dowels (see also ´Repair and strengthening - Techniques´).
Temporary support should be given to the existing structure while new concrete is being placed and is gaining strength so that when the work is completed and the temporary supports are removed the new section will take up a similar stress pattern as the original concrete .
Acknowledgement: The Concrete Society
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