Grade 300 Prestressing Strand and the Effect of Vertical Casting Position

About the report:

The purpose of this investigation was (1) to compare the differences in the transfer length, development length, and flexural strength among Grade 300 strand, the traditional Grade 270 strand, and the predictions of these properties obtained using current code equations for prestressed concrete members, and (2) to determine the effect the as-cast vertical location of the strands (top-strand effect) on these properties.  The current code provisions by the American Association of State Highway and Transportation Officials and the American Concrete Institute are based on years of experimental research on the traditional Grade 270 strand. 

The scope of this project was limited to the fabrication and testing of 20 pretensioned, prestressed beams, 10 of which contained Grade 270 and 10 of which contained Grade 300 strands constructed and tested in the Structures and Materials Laboratory at Virginia Tech. 

The increase in strand strength was found to influence transfer length, development length, and flexural strength; the as-cast vertical location was found to influence only transfer length and, in turn, development length.  Transfer lengths of the Grade 300 strand had an average increase of 10 percent compared to the transfer lengths of the Grade 270 strand.  Development lengths for the Grade 300 strand were also shown to increase compared to the Grade 270 strand.  Flexural bond lengths were found to be relatively the same for both strand strengths, indicating the increase to be primarily dependent on the increase in transfer length.  Minimum flexural bond lengths that resulted in flexural failures were found to be in the range of 45 to 50 in for both strand strengths.  The influence of strand strength on flexural strength was also evaluated.  As expected, members cast with ½ in diameter, Grade 300 strands had about 11 percent higher nominal moment capacities than did those cast with ½ in diameter, Grade 270 strands.  Contrary to the historical definition, the top-bar/strand effect was found to be more dependent on the amount of concrete cast above the strand than the amount below it, with transfer lengths showing a steady increase with a decrease in the amount of concrete cast above the strand.  The current equations for flexural strength were found to give adequate estimates for flexural strength, although a decrease in ductility was noted. 

The study recommends the following:

1. VDOT’s Structure and Bridge Division should use the current AASHTO equation for transfer length and development length for flexural members containing Grade 300 strand cast in non-top strand situations.
2. VDOT’s Structure and Bridge Division should use the current ACI and AASHTO provisions for the calculation of nominal moment capacity for flexural members containing Grade 300 prestressing strands.

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