The bending capacity tests on ten simple supported beams were conducted to investigate the damage characteristics

prestress increment

beading capacity and displacement ductility of unbounded prestressed concrete beams with HRBF500 steel bars. The results show that the HRBF500 steel bars has yielded before the limit state. The concrete in compression zone crushes upon the broken of the beams

which shows a sudden damage mode. The measured ultimate stress increments of unbounded tendons are in linear relationship with the comprehensive reinforcement ratio index

while the values of the ultimate stress increments are obviously higher than those values specified in the code GB50010-2010

and the average ratio of predicted values to testing values is about 0.35. The mid span deflection upon yielding of the beams is large

while the displacement ductility is bad and the average value of the displacement ductility ratio is 1.67. The displacement ductility ratio decreases with the increases of the comprehensive reinforcement ratio index. Based on the experimental results

the proposed calculation formula of ultimate stress increments of unbounded tendons was established

and it is in good agreement with the experimental results when the testing value is less than 450 MPa.