Experimental study of stress increase of unbonded tendons in continuous post-tensioned concrete beams

The prediction of the stress increase Δσ_p,ULS of an unbonded tendon in a post-tensioned continuous concrete beam at ultimate capacity is more challenging than when bonded tendons are used. Four continuous two-span concrete beams, prestressed with unbonded tendons, were constructed and loaded up to failure in a four-point loading arrangement at the laboratory of Civil Engineering in Tampere University. The test results from the point of moment redistribution were partially published earlier in 2024. This article presents the results of the stress increase in the unbonded tendons of the test beams. Moreover, this article presents a novel nonlinear analysis model, which was employed to examine the influence of the dilatation of the beam's axis of gravity on the magnitude of the additional tendon elongation. The comparison calculations have also been made with a simple beam model from the literature. In all test beams, the stress in the prestressing steel exhibited a significant increase at failure. The effect of beam expansion on the tendon elongation at high loading levels was approximately half of the total additional elongation of the tendon.