Investigation of Transfer Length, Development Length, Flexural Strength and Prestress Loss Trend in Fully Bonded High Strength Lightweight Prestressed Girders
Nassar, Adil J.
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Encouraged by the performance of high performance normal weight composite girders, Virginia Department of Transportation has sought to exploit the use of high strength lightweight composite concrete (HSLWC) girders to achieve economies brought about by the reduction of dead loads in bridges. Transfer Length measurements conducted on two AASHTO Type IV HSLWC prestressed girders, resulted in an average transfer length of 17 inches, well below the AASHTO and ACI guidance. Two girders composed of HSLWC AASHTO Type II girders and a 48" x 8" normal weight 4000-psi concrete deck were produced. The HSLWC Type II girders were cast of concretes with a compressive strength of 6380 psi and unit weight of 114 pcf. Full scale testing of the girders was conducted to evaluate development length and flexural strength in HSLWC composite girders. Embedment lengths of five, six and eight feet were evaluated. Tests indicated a development length of about 72 inches, marginally below the ACI and AASHTO stipulation. Four of eight strands in the girders showed general bond failure nevertheless, the tested girders exceeded their theoretical flexural capacity by 24 to 30 percent. A third composite girder was cast of a high strength normal weight concrete (HSNWC) Type II girder, and topped with a 48" x 8" normal weight 4000-psi concrete deck. This girder was intended as a control specimen to contrast its test results with the HSLWC composite girders. The targeted compressive strength of both the HSLWC and HSNWC AASHTO beams was 8000 psi. The compressive strength of the HSNWC mixture, however, was about 8990 psi compared to 6380 psi for the HSLWC mixture. Prestress losses in HSLWC AASHTO Type IV girders monitored over a nine-month period were found to be less than those calculated using the ACI and PCI models. Furthermore, the ACI model indicated that the effective prestressess retained in the HSLWC girders in 30 year's time are greater than 50% of the specified tensile strength of the strands.
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