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Modelling the prestress transfer in pre-tensioned concrete elements

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3. Transfer of prestress

In pre-tensioned concrete elements, when the prestress is released after hardening of concrete, the prestress force transfers gradually into the concrete. The prestressing steel tries to shorten and expands due to Poisson's ratio effect. This results in varying diameter within the transmission length in the shape of a wedge (Fig. 1).

The increment in the bar diameter imposes a normal pressure acting on the surrounding concrete. The pressure starts from high value at the end and continues decreasing along the transmission zone and reaches plateau after that (Fig. 2a). This pressure induces a frictional resistance, which acts against the shortening of the pre-tensioning steel and holds the bar/strand in tension (Fig. 2b). The forming of wedging and frictional forces is known as the Hoyer effect [19]. This phenomenon enhances bonding of the released prestressing steel with the surrounding concrete. As a result, the stress in steel increases gradually until it becomes constant beyond the transmission length (Fig. 2c). The distribution of compressive stress in concrete follows a similar pattern to the stress in steel (Fig. 2d), with an additional shear lag effect.