The main objectives of this analytical and experimental investigation were to study the flexural behavior of partially prestressed concrete beams, and to compare this behavior with those of fully prestressed and ordinary reinforced beams. Eleven concrete beams were manufactured and tested in flexure until failure. Seven of the beams were partially prestressed, containing various combinations of prestressed and non-prestressed reinforcements; two were fully prestressed, and two were ordinary reinforced. All beams were designed to have the same ultimate strength. Six of them had rectangular cross sections, and five had T-cross sections. They were all 21 feet long. The beams were tested as simply supported with a clear span of 20 feet under a two-point loading system. The loads were applied 6 feet apart and symmetrical about the centerline of the beam. They were studied with regard to their ultimate and cracking moments, crack widths and spacings, cambers, deflections, curvatures, losses of prestress, and transfer lengths. This study showed that adding prestressed reinforcement to concrete beams increased the cracking moment and camber, and decreased service load deflections and crack widths. Adding non-prestressed reinforcement to a prestressed concrete beam reduced prestress losses and camber. Cracks in partially prestressed beams were more evenly distributed than in either fully prestressed or ordinary reinforced beams. Concrete beams can be designed by employing different combinations of prestressed and non-prestressed reinforcements. Selection of the mixed reinforcement ratios depends on the special loading characteristics and serviceability and safety requirements of the beam. As a preliminary design, beams may be designed with prestressed reinforcement resisting dead loads and non-prestressed steel carrying live loads. A series of checks will modify the design to obtain the mixed reinforcement ratios that best meet the special requirements of the problem. A comprehensive design procedure is developed for under-reinforced partially prestressed concrete beams. This procedure is based on the ultimate design concept and follows the provisions of the current ACI Building Code. It is also applicable to ordinary reinforced and fully prestressed beams.
