Construction Challenges

The following are challenges that were encountered during the construction of the test unit and are presented in order to aid with future construction of this bridge type in the field.

5.1.2.1 Pouring the Bent Cap

Forming and pouring the inverted-T bent cap proved to be somewhat challenging due to the geometry of the bent cap. Prior to pouring the bent cap, the concern was raised that the pressure head of the concrete at the top of the inverted-T would likely be enough to force the concrete in the corbel portion of the bent cap to overflow its formwork. Therefore, in order to remedy this concern, the bent cap was poured in lifts, which necessitated the use of a construction joint. The first lift was poured to the top of the corbel portion and was allowed to set for around 20 minutes. The remainder of the bent cap was then poured in the second lift (Figure 5.8). Though some of the concrete in the corbel still rose slightly above the formwork, pouring the bent cap in lifts seemed to solve the overflow problem. An alternate solution that could be used for bridges of this type in future would be the use of a precast bent cap, instead of cast-in-place. The use of a precast bent cap would also significantly reduce the amount of construction time for the project and result in cost savings.

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Figure 5.8: Casting of the Bent Cap 5.1.2.2 Installing the Ducts through the Cap Beam

Installing the ducts for the untensioned prestressing strand that was placed through the cap and the Northern girders proved to be a fairly significant challenge. First, since the strand did not extend straight through the cap beam and into the Southern girders, the ducts had to be bent as they passed through the cap so that they would terminate at the edge of the corbel and straddle each girder. Therefore, it was decided that the standard corrugated duct used for prestressing applications would be too stiff to accommodate such bends. As a result, a flexible, corrugated, low-grade steel electrical conduit was used instead. This alternative proved to be very effective as it was easily routed within the cap beam (Figure 5.9). It should be noted, however, that this problem is somewhat specific to the test unit and would likely not be encountered in the prototype structure, as the ducts would continue straight into the Southern

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girders, thereby eliminating the need to bend them. However, the rebar in the bent cap should be spaced such that it allows for the accommodation of the duct.

Figure 5.9: Routing of Bent Cap Ducts

Second, it was decided that it would be prudent to make the duct in the cap beam larger than the ducts that were inside the girders. This was done in order to increase the tolerance in the alignment of each section of duct, making it easier to place the ducts in the cap in line with the ducts in the girders to accommodate and grout the strand. Therefore, a 1-½ in. electrical conduit was selected, while 1 in.-diameter sheathing was used in the girders.

Third, the bent cap was highly congested with reinforcement, especially in the vicinity of the column, which made it difficult to place ducts large enough to accommodate the strands (Figure 5.10). This was also true given the fact that a slightly larger diameter conduit was selected within the cap beam. This challenge was solved by routing the ducts around the column instead of passing through it. This problem is not expected if four girders are used instead of five, as a girder would not have had to pass directly through the center of the column.

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Figure 5.10: Placement of Ducts in the Bent Cap around the Column 5.1.2.3 Inserting the Strands through the Ducts in the Cap Beam

Running the strands through the ducts also proved to be difficult, given the bends in the ducts, as there was little clearance within the duct for both strands. Furthermore, grout tubes were mounted on each duct and ran through to the top of the cap to ensure proper grouting. However, in order to mount the grout tubes, a condulet in the shape of a box was placed at the center of the cap beam with a series of connectors that were used to splice on the main duct sections as well as the grout tube. This was a significant obstacle when placing the strands as they had a tendency to get caught in the corners of the conduit, making it difficult to force the strand out and to the other end of the cap beam. This problem would likely not be encountered in the prototype structure as the ducts would not be bent, nor would they likely enter a similar box section. However, in order to remedy the situation, a series of increasingly larger diameter and stiffer objects were fished back and forth across the cap beam. Once a stiff enough wire was pulled through the cap beam, it was attached to one of the strands and was used to pull it through. The first strand was then used to pull the second strand through the cap beam. The

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strands were then fed down the length of the girder (Figure 5.11). Though the process was rather time consuming, it proved to be the most effective.

Figure 5.11: Pushing the Strands through the Girder 5.1.2.4 Partial Pour of the Diaphragm

Safety concerns were raised regarding the stability of the superstructure while transferring the abutments from the falsework to the pinned support system. Initially, the girders were independent of one another at the cap end as they were supported on the corbel of the inverted-T cap beam. This was done to replicate the simply supported condition that the girders would experience as they were placed in the prototype bridge during construction. However, concerns were raised that the girders might fall out of place during the transfer of the abutment support conditions. Therefore, in order to improve the lateral and rotation stability, and hence safety, of the superstructure, it was decided that a partial pour of the diaphragm would be completed. Only the portion of the diaphragm next to the corbel was poured to the full height of the corbel within each bay between girders, as it would provide lateral stability for the girders while still allowing them to remain in a simply supported condition at the bent cap, as required to produce a realistic moment profile along the length of the girder. This would not have been possible had the entire diaphragm been poured. However, this did introduce a construction joint in the diaphragm along the top of the corbel of the bent cap, as shown in Figure 5.12.

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Figure 5.12: Partial Diaphragm Pour between Girders 5.1.2.5 Termination of Untensioned Strands

Unfortunately, due to a miscommunication in the lab as well as the field decision to make a partial pour of the diaphragm, the untensioned strands were not terminated at the face of the inverted-T bent cap on the as-built connection side, as specified. Instead, the strands were extended and grouted all the way to the outside face of the diaphragm. Since the presence of the strands within the effective as-built connection region could have falsely improved the performance of the connection, it was necessary to render the strands ineffective within the as- built connection. Therefore, the grout within the duct on the as-built side of the bent cap was drilled out over the length of the diaphragm, in order to debond the strands, as shown in Figure 5.13. Additionally, as much of the duct was removed as possible, which was somewhat feasible as the electrical conduit that was used could be easily unraveled and fractured over the first couple of inches. It was assumed that any remaining duct would be easily fractured or unraveled as the girder pulled away from the cap.

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Figure 5.13: Untensiond Strand Debonding

5.2 Instrumentation

Given the magnitude of the test specimen, a significant number of both internal strain gauges and external devices were used to capture the response of the structure in its critical regions. A total of 282 strain gauges were used internally, while a combination of 51 string potentiometers, 67 linear potentiometers, and 11 rotation devices were used externally. The following presents the instrumentation plan that was used for both the internal and external instrumentation.