Question:
Does the use of high strength concrete reduce prestress losses in pretensioned bridge beams?
Answer:
Experimental evidence shows that higher strength concrete has a higher modulus of elasticity and a lower creep coefficient than conventional strength concrete. The ultimate shrinkage of high strength concrete is lower than that of conventional strength concrete, even though early age shrinkage may be equal or even higher. It would, therefore, appear that high strength concrete would have lower prestress losses from elastic shortening, shrinkage, and creep. However, this is not always the case. Higher strength concrete allows more prestressing force and thus increased member capacity. Consequently, the total losses may be lower or higher depending on the level of prestress and other factors.
Consider an NU 2000 I-beam with a depth of 78.7 in. (2000 mm), containing fiftysix 0.5-in. diameter strands, on a simple span of 127 ft (38.7 m), and with concrete strengths of 3.5 ksi (24 MPa) at prestress transfer and 5.0 ksi (35 MPa) at service. Using the Detailed Method of NCHRP 18-07,(1) the calculated total prestress loss is 43.3 ksi (299 MPa), or 21.6 percent of the initial prestressing force (Case 1). The same beam with concrete strengths of 7.0 ksi (48 MPa) at transfer and 11.0 ksi (76 MPa) at service has a calculated loss of 27.2 ksi (188 MPa) or 13.6 percent (Case 2). However, the increased concrete strength offers an opportunity for the member to span 147 ft (44.8 m) using fifty-six 0.6-in. (15.2-mm) diameter strands. This change results in a calculated loss of 34.1 ksi (235 MPa) or 17.0 percent of the initial prestressing force (Case 3). The contribution of each of the components to the total loss is given in the table.
The loss calculations for these examples were performed in accordance with the Detailed Method of NCHRP 18-07, which is being considered for incorporation into the AASHTO LRFD Bridge Design Specifications. If the Refined Estimates Method of the 2003 edition of the LRFD Specifications is used, the corresponding total losses for the three examples are 55.6 ksi (383 MPa) or 27.7 percent, 49.6 ksi (342 MPa) or 24.7 percent, and 67.5 ksi (465 MPa) or 33.6 percent. The lower values estimated with the NCHRP method reflect a more precise prediction of the time-dependent properties of high strength concrete and a more precise method of calculating losses.
Reference
- Tadros, M. K., Al-Omaishi, N., Seguirant, S. J., and Gallt, J. G., “Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders,” NCHRP Report 496, Transportation Research Board, Washington, DC, 2003, 63 pp.
Answer contributed by Maher K. Tadros of the University of Nebraska-Lincoln. He may be contacted at [email protected]. edu or 402-554-4842.