Henry G. Russell, Henry G. Russell, Inc.
The use of high strength concrete in precast, prestressed concrete beams allows for a higher precompression to be applied to the beams. Consequently, the tensile stress in the bottom flange calculated from the applied bending moment can be higher without exceeding the tensile stress limit. Since the tensile stress limit in the bottom flange at service load usually controls the design for long-span beams, higher compressive strength concrete allows the use of longer span lengths, wider beam spacings, shallower sections, or a combination of these benefits. Articles in previous editions of HPC Bridge Views have illustrated many actual applications, yet few have had specified concrete compressive strengths above 10,000 psi (69 MPa).
For high strength concrete to be used efficiently, it needs to be precompressed to the maximum value allowed by the design specifications. Therefore, as the specified concrete compressive strength increases, the prestressing force also needs to increase. The amount of force depends on the diameter, spacing, and strength of the strand and shape of the bottom flange of the beam. Once the bottom flange is full of strands, additional strands can only be placed in the web, which is less efficient because the strands are closer to the neutral axis.
In a parametric study using concrete compressive strengths from 6,000 to 12,000 psi (41 to 83 MPa), Zia et al.(1) reported that the strength level at which the use of higher strength concrete was not beneficial varied between 8,000 and 12,000 psi (55 and 83 MPa) depending on the beam cross section and the prestressing force.
Russell(2) examined the effect of concrete strengths from 6,000 to 14,000 psi (41 to 97 MPa) on maximum span length for various girder spacings. In most combinations, maximum span lengths increased as concrete compressive strengths increased, although the rate of increase declined. In some combinations, a plateau was reached at concrete compressive strengths of 8,000 to 12,000 psi (55 to 83 MPa) depending on strand diameter, beam cross section, and girder spacing.
A similar study by Kahn and Saber(3) concluded that the maximum effective girder compressive strength with 0.5-in. (12.7-mm) modified strand (area = 0.167 sq. in. or 108 sq. mm) varied from 8,000 to 11,000 psi (55 to 76 MPa), depending on beam spacing and cross section. For 0.6-in. (15.2-mm) diameter strand, the effective strengths ranged from 10,000 to 13,000 psi (69 to 90 MPa).
Based on a cost-efficiency index, Russell et al.(4) concluded that the maximum useful concrete strength was in the range of 9,000 to 10,000 psi (62 to 69 MPa) with 0.5-in. (12.7-mm) diameter strands. With 0.6-in. (15.2-mm) diameter strands, the maximum useful strength increased to about 12,000 psi (83 MPa) for bulb-tee beams. With a Ubeam having a bottom flange with three rows of strands, strengths up to 14,000 psi (97 MPa) were beneficial.
In summary, the maximum effective concrete strengths for readily available pretensioned beam shapes range from 8,000 to 11,000 psi ((55 to 76 MPa) with 0.5-in. (12.7-mm) diameter strands and 10,000 to 14,000 psi (69 to 97 MPa) with 0.6-in. (15.2-mm) diameter strands.
References
- Zia, P., Schemmel, J. J., and Tallman, T. E., “Structural Applications of High-Strength Concrete,” North Carolina Center for Transportation Engineering Studies, Report No. FHWA/NC/89-006, Raleigh, NC, 1989, 330 pp.
- Russell, B. W., “Impact of High Strength Concrete on the Design and Construction of Pretensioned Girder Bridges,” PCI Journal, Vol. 39, No. 4, July/August 1994, pp. 76-89.
- Kahn, L. F. and Saber, A., “Analysis and Structural Benefits of High Performance Concrete for Pretensioned Bridge Girders,” PCI Journal, Vol. 45, No. 4, July/August 2000, pp. 100-107.
- Russell, H. G., Volz, J. S., and Bruce, R. N., “Optimized Sections for High-Strength Concrete Bridge Girders,” FHWA, U. S. Department of Transportation, Report No. FHWA-RD-95-180, 1997, 156 pp.