Experimental Study and Theoretical Prediction on the Flexural Behavior of Unbonded, Post-tensioned Precast Beams Made of Recycled Aggregate Concrete

The use of recycled concrete aggregate instead of natural coarse aggregate is of great interest from a sustainability perspective. The present study investigates the flexural design of unbonded, post-tensioned, precast recycled concrete beams. It also evaluates the application of unbonded prestressed tendons in recycled concrete beams. Flexural behavior tests were performed on 13 beams with different prestressed reinforcement indices, section heights, and comprehensive reinforcement indices. We thoroughly investigated stress changes in prestressed steel strands, beam failure forms, and crack development during loading. The results demonstrate that under the same load level, the average crack width of recycled concrete beams is 36% higher than that of conventional concrete beams. A theoretical analysis indicates that the factors affecting the stress increment of unbonded prestressed tendons are the comprehensive and prestressed reinforcement indexes. A theoretical model of stress increment during loading of prestressed reinforcement was obtained, allowing derivation of an ultimate bearing capacity formula for unbonded, post-tensioned, precast, recycled concrete beams. An influence coefficient for recycled coarse aggregate is introduced and set to 1.14, the experimental data is fitted to derive the calculation formula for the average crack spacing and width. The theoretical model of the stress increment in prestressed steel strands and the proposed crack width model provide a valuable design reference and theoretical support for engineering applications.