Performance of precast, prestressed concrete I-girders employing 0.7-in. diameter prestressing strands under shear-critical loading conditions
The majority of precast, pretensioned concrete elements are currently fabricated using 0.5- or 0.6-in. diameter prestressing strands. However, in recent years, potential benefits such as reduced fabrication costs and extended span capabilities have led to an interest in using larger-diameter 0.7-in. strands in the pretensioning industry. Such an increase in the diameter of strands might impact the shear strength of pretensioned girders due to the possibility of atypical failure modes that are not considered in current design provisions. An experimental program was conducted to study the effects of using 0.7-in. prestressing strands on the performance of precast, prestressed concrete I-girders under shear-critical loading conditions. Four full-scale pretensioned Texas bulb-tee girders (Tx-girders) employing 0.7-in. strands were fabricated and tested at Ferguson Structural Engineering Laboratory at the University of Texas at Austin. The mild steel reinforcement in the specimens was detailed according to standard drawings by the Texas Department of Transportation for girders employing 0.6-in. strands. The test program investigated the shear failure in girders with different concrete release strengths, overall member depths, shear span-to-depth ratios, and strand patterns. Analysis of the results revealed clear signs of atypical shear failure mechanisms in all specimens. Considerable strand slip was recorded at both ends of the specimens prior to peak load. In three of the specimens, the shear failure resulted in prominent horizontal cracks at the interface between the web and the bottom flange. However, all specimens demonstrated significant diagonal cracking prior to failure. Yielding of the stirrups was also confirmed in all specimens, indicating a shear-tension failure. The capacities of all specimens were conservatively estimated using the general procedure in AASHTO LRFD Bridge Design Specifications and the detailed method in ACI 318-14. The findings of this study reveal no concerns regarding the performance of existing design provisions in predicting the shear strength of Tx-girders that employ 0.7-in. diameter prestressing strands.