Flexural Behavior and Ductility of Prestressed Beams with High-Strength Concrete


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Title: Flexural Behavior and Ductility of Prestressed Beams with High-Strength Concrete

Author(s): N. Gowripalan and X. W. Zou

Publication: Special Publication

Volume: 172


Appears on pages(s): 573-600

Keywords: Beams (supports); deflection; ductility; elastic properties; energy; high-strength concretes; prestressed concrete; pretensioning

Date: 12/1/1999

High Strength Concrete (HSC) is defined as a concrete with a 28-day characteristic compressive strength of 50 MPa or more. This paper presents the results of an experimental study on pretensioned beams made of HSC with characteristic compressive strengths at 28 days of 55, 75, 85 and 105 MPa and compares with Normal Strength Concrete (NSC) pretensioned beams (in this case 35 MPa concrete). Nine, fully prestressed concrete beams having 8mm diameter plain steel wire or 7.9 mm diameter 7-wire steel strand with stirrups at both ends (to one third of the span) were constructed and tested. The experimental results were compared with the theoretical calculations according to the current Codes of Practice, such as ACI-3 18 and AS3600. The curvature of pretensioned beams tested decreased with increasing concrete strengths from 55 to 105 MPa over the entire loading range from service to ultimate load. The final failure of the beams in all cases occurred by yielding of tendons and conventional reinforcements followed by crushing of the concrete in the constant moment region. The beam reached large curvatures prior to failure and hence appeared to be ductile enough in terms of rotation. However, a comprehensive evaluation of ductility of these beams is still required. With this in mind, the ductility indices, currently used for reinforced and prestressed concrete beams are reviewed and an index based on moment of resistance, total energy and elastic energy, suitable for pretensioned beams is presented. Pretensioned beams made of HSC up to 105 MPa are sufficiently ductile in terms of large deformations, rotations and total energy. However, the inelastic energy absorbed is less than those beams made of NSC.