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The provision of additional prestressing is a technique that is used to improve the serviceability performance of concrete structures generally by reducing deflections. Generally the additional tendons will be external to the structure though they may sometimes be embedded in additional concrete if there is a need to increase the concrete cross-section.
It will be necessary to carry out a detailed analysis of the strengthened structure to determine the effect of the additional prestress, both on serviceability performance and ultimate load capacity. One of the disadvantages of providing additional prestressing is the need to provide the necessary anchor blocks and deviators to transmit the loads to the structure.
These will have to be grafted onto the existing structure. The use of fabricated steel elements for anchor blocks and deviators may possibly reduce reinforcement congestion. In addition, access for the stressing equipment may be limited.
Acknowledgement: The Concrete Society
Other references:Highways Agency, ´The design of concrete highway bridges and structures with external and unbonded prestressing´, Departmental Standard (BD58/94) and Advice Note (BA 58/94).