7.1 Summary
This chapter presents a summary of the laboratory testing results, the post-tensioning results for the tendons and deck panels, and a summary of the construction documentation. Conclusions are also presented based on the results obtained from testing (laboratory and field) and feedback from the Boone County Engineer’s Office, PCI, Andrews Prestressed Concrete, and the research team.
7.1.1 Laboratory Testing Summary
In total, eight laboratory tests were completed: five single pile abutment tests, two double pile abutment tests, and one pier cap test. Each single pile specimen resisted over four times the unfactored design load without any sign of failure. The double pile specimen supported approximately twice the unfactored design load without failure, even though the CMPs were only 70% full of concrete. Limiting the depth of the concrete was done in an attempt to actually fail the specimen. The experimental cracking moment from the double pile test agreed with the experimental cracking moment determined from the single pile tests. The pier cap resisted over five times the unfactored design load without any signs of failure. It was not possible to completely fail any of the eight laboratory specimens since their strength exceeded the capacity of the laboratory loading system.
7.1.2 Post-Tensioning Summary
Post-tensioning of the Boone County bridge took place on the morning of November 28, 2007. The entire operation took approximately four hours, including the time needed for data collection. Vibrating wire gages were used to measure the force occurring in several of
the post-tensioning strands, to measure the losses during the post-tensioning process, and to measure the distribution of the post-tensioning forces through the deck panels. The forces in the strands, calculated using data from the VWGs, were within two percent of the forces provided by the contractor. After the post-tensioning process was complete, all of the strands were within the tolerances specified by the Iowa DOT. The strains in the deck panels indicated that the post-tensioning force is distributed through the end panels to the interior deck panels almost immediately. Strains (stresses) measured in the panels were relatively constant throughout the deck, regardless of the location of the panel in the bridge, or the distance from the post-tensioning channels.
7.1.3 Construction Documentation Summary
Construction on the Boone County replacement bridge began on July 5, 2007, after demolition of the previous bridge at the site, which began on July 24. There were delays in construction due to heavy rain in August and September; a total 15 working days were lost to due rain and/or wet conditions at the site in August and September alone. In September, piles were driven and erection of the precast substructure began October 4. The pile connections in the abutments and west pier cap were grouted in place October 5 and the east pier cap connections were grouted October 13. Once the grouted concrete reached strength, erection of the superstructure began on October 24.
The bridge girders were set on October 24; on the next day, the interior deck panels were set. The panels were leveled and the CIP diaphragms over the two piers were placed on October 30. End panels were delivered and set in place on November 8, which was followed by the casting of the interior transverse joints. Due to weather conditions, the transverse joints at the end panels (which had to be placed later when the end panels reached the desired
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strength) were slow to reach the required strength for post-tensioning. The post-tensioning operation was delayed until November 28, when the deck was post-tensioned and all of the longitudinal joints were placed.
Once the deck was finished, the contractor formed and cast the CIP portion of the abutments, the wingwalls, and the barrier rails. After the concrete reached the desired strength, the formwork was stripped, the approaches were backfilled and graded to the bridge deck elevation. On December 27, the bridge deck was ground and the bridge was opened to the public. The total construction time for the replacement bridge was 108 working days, with 21 days lost to rain and wet conditions throughout the project, and 9 days lost for miscellaneous reasons (78 actual working days).
7.2 Conclusions
7.2.1 Laboratory Testing Conclusions
The following conclusions are based on the eight laboratory tests of the substructure elements for the Boone County replacement bridge:
• The abutment section capacity is at least 4.5 times greater than the unfactored design load of 80 kip per pile.
• The pier cap section capacity is at least 5.3 times greater than the unfactored design load of 72 kip per pile.
• The CMP affected the behavior and crack pattern in the abutment sections, but had no influence in their capacity.
• The measured cracking moments for the abutment and pier cap, based on observation and strain data, were approximately 264 kip-feet and 312 kip-feet, respectively. • Essentially no differential movement was detected between the precast concrete and
the concrete in the CMP in any of the abutment specimens or pier cap specimen. The movement measured was less than 0.0033 inches in every test.
• No movement was detected in the shear tests; measured movement was less than 0.0005, the LVDT precision.
• Shear failure was not detected between the H-pile in the abutment and the concrete in the CMP.
7.2.2 Post-Tensioning Tendon Forces and Deck Panel Stress Conclusions
The following conclusions are based on the verification of the post-tensioning forces and resulting strains in the precast deck panels:
• Small losses due to seating and elastic shortening of the bridge deck (less than 0.05 percent of the initial force) were measured during the post-tensioning operation. • The forces in the strands provided by the contractor are in agreement with the forces
measured in the four instrumented strands.
• The forces in all of the strands were within the required Iowa DOT tolerances and remained within these tolerances after post-tensioning was completed.
• The force from post-tensioning was distributed evenly through the deck panels, regardless of the panels’ location in the bridge or distance from the post-tensioning channels.
• Stresses calculated from the measured strains in the deck were less than allowable compressive stress due to post-tensioning (AASHTO, 1996). The maximum
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measured stress (1,835 psi) was 45 percent lower than the maximum allowable compressive stress (4,080 psi).
7.2.3 Conclusions from Construction Documentation
The following conclusions were based on the construction documentation of the Boone County replacement bridge and the feed back provided by the Boone County Engineer’s Office, PCI, and Andrews Prestressed Concrete:
• Construction began on July 5 and was completed on December 27, 78 days of actual
construction.
• Delays in construction were caused by heavy rain and wet conditions (21 days). • Setting all of the precast elements went quickly and smoothly with no problems. • It was difficult to meet the slump and entrained air requirements for the specialty
concrete mix used in the CMPs and in the transverse and longitudinal joints of the deck. The admixture to improve the workability of the mix only lasted about twenty minutes in the field.
• The post-tensioning process went smoothly and without any major difficulties. It is important to note that the post-tensioning was performed by the prime contractor (PCI), without any prior post-tensioning experience, with the supervision of a technical expert.
• Deck grinding was difficult; the entire deck could not be ground and low spots due to poor concrete finishing of the transverse joints were left on the deck.