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CHAPTER 4: THE CONSTRUCTION PROCESS OF SEGMENTAL BRIDGES - page 3 / 47

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Chapter 4: The Construction Process of Segmental Bridges

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Some factors effectively contribute to initial and long-term reduction of the prestressing force. Immediate losses of prestress, also called initial losses, occur once the prestressing force is applied, after the concrete has been placed and cured. Loss of prestress needs to be anticipated during design. Long-term losses in concrete depend on its design mixture, curing, the environmental climate, and the member geometry. Textbooks give information on the reasons for prestress losses and provide many formulas to calculate their effect. The following Table 4-1 based on Barker and Puckett (1997, pp455-466) summarizes these effects:

Table 4-1: Influences Causing Loss of Prestressing Force

Initial loss of prestress Slippage of strands in the anchorages (before wedges or nuts grip firmly) Elastic shortening of concrete member (relieves previously stressed tendons)

Long-term loss of prestress Relaxation of steel strands (loss of stress under constant strain) Creep of concrete member (plastic deformation under constant stress)

Friction between tendon and duct interior (“wobble effect” because of curved ducts)

Shrinkage of concrete member (volume change due to evaporation)

4.1.2 Pre-Tensioning

Prestressing basically can be carried out as pre-tensioning and post-tensioning, referring to the time when the prestressing force is imposed with respect to casting. In pre-tensioning the tendons are anchored to e.g. a stiff frame around the casting bed and are prestressed before the concrete is placed. When the concrete has gained sufficient strength the tendons are relieved from their anchorages and stress the concrete through bond between steel and concrete. Menn (1990) notes that this method is especially feasible for precasting concrete elements because of the solid anchorages required.

4.1.3 Post-Tensioning

Post-tensioning denotes the method of stressing the tendons only after the concrete has reached a specified strength. To allow for the necessary movement of the tendons inside the concrete they

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