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Pile Restoration of the World's Longest Bridge

World's Longest Bridge- Pinnacle of Durability

Over the course of the last 43 years, the Lake Pontchartrain Causeway has not only held the distinction of being the world's longest bridge, but has been an interesting study of concrete's durability in marine environments. Consisting of parallel spans, the 24 mile (45 km) long bridge carries traffic from New Orleans, Louisiana and Jefferson Parish on the south shore of Lake Pontchartrain to the popular residential communities of St. Tammany Parish on the north shore. The current southbound bridge, opened in 1956, was followed approximately ten years later by a second span that now carries northbound traffic.

During the bridge's impressive history, the condition of the 9,000 54 in. (1.4m) diameter cylinder piles has been carefully monitored as part of a continuing maintenance program. Periodic inspections have followed the requirements of Title 23, Code of Federal Regulations, Part 650, more commonly known as the "National Bridge Inspection Standards" (NBIS).

During 1987, inspections revealed that a small number of the piles were exhibiting longitudinal cracks in the splash zone. The pattern of the cracks corresponded with the location of the prestressing strands. While the cracks appeared to extend to the depth of the strands, there was no evidence of corrosion staining. On some piles, there was also some localized distress to the original epoxy compound used the seal the transverse joints between the pile sections.

The engineers examined several repair option, including patching with epoxy paste, crack injection, several wrap methods, and a relatively new all-polymer encapsulation process. They selected the encapsulation process because it would not only seal the cracks, but would also provide a composite barrier that would completely surround the affected length of the piles and arrest further deterioration. In 1988, 21 piles were encapsulated for a length of 10 ft. (3m) each, with approximately one-half of that length below the water line.

Over the course of the next few years, responsibility for the maintenance program changed hands, but monitoring of the piles continued on a periodic basis. In 1994 and 1995, level II and level III inspections revealed that more piles were exhibiting defects similar to those found in the 1987 study. While longitudinal cracking was still the major concern, there was little or no evidence of steel corrosion.

The new engineers carefully reviewed the repair options. Again, their search revealed several repair products and systems, including wraps and numerous FRP (fiber reinforced polymer) jacketing systems, involving both polymer and portland cement grouts. None, however, appeared to equal the all-polymer encapsulation system used in 1988. To confirm this, they ran extensive tests of the then seven-year-old encapsulations. Several cores were taken through the encapsulations which revealed that a tightly-bonded, composite barrier was still in place (Fig. 1). There was no evidence that any further deterioration to the pile had occurred beneath the encapsulations during the seven-year period.

In-situ bond tests were performed on a number of the older encapsulations (Fig. 2). These tests, performed by coring through the encapsulation materials and applying direct tensile load to an isolated specimen, further confirmed the sound condition of the encapsulations. When tested to failure, the tightly-bonded composite transfers load to the concrete and it fails in tension. In this case, the 8,500 psi (58.4 MPa) concrete, typical of the centrifugally-cast cylinder piles, is the weakest link in the composite.

As a result of these tests and extensive research into other available systems, the engineers again selected the same all-polymer encapsulation system that had been specified and installed seven years earlier.

Phase I of the current Pile Restoration Program began in 1996 and involved encapsulation of 414 piles. Most of the encapsulation work was carried out near or below the waterline, necessitating divers working with surface supplied air. The contractor utilized jack-up vessels to carry out most of the work. Only limited use of floating equipment was allowed within the strict navigation rules set forth by the Causeway Commission (Fig. 3).

The all-polymer encapsulation method required careful surface preparation. Pile cleaning was accomplished by high-pressure water blasting, using 10,000 psi (69 MPa) equipment, operating between 7,000 psi (48 MPa) and 8,000 psi (55 MPa) (Fig.4). All marine growth was removed and a proper concrete surface profile was established. This method was employed both above and below the water line.

In-situ bond tests were performed on older encapsulations. After surface preparation was complete, translucent, marine grade FRP jackets were placed around the piles. Patterns of polymer standoffs, positioned inside the jacket, maintained the minimum 3/8 in. (10 mm) annulus between the pile and the jacket. An aggregate-filled, 100% solids epoxy grout was pumped into the jackets from the bottom up through strategically placed injection ports in the jacket. The progression of grout was carefully monitored through the translucent jackets to ensure that a continuous, void-free encasement was achieved. The aggregate-filled epoxy grout, rising inside the confined space between the pile and the jacket, creates a scouring effect that further enhances the bond between the encapsulation components and the pile. This scouring effect has proven to be very important in achieving maximum bond to underwater surfaces.

Batching, mixing, and pumping of the grout was accomplished with special plural component equipment. With this equipment, the reactive components of the grout are each pre-mixed with aggregate and kept separate until just before entering the jacket. This enables the contractor to work for long periods without concern for the epoxy reaction time. This handling method has the additional advantage of allowing clean-up with water only, greatly reducing the use of solvents and their negative effects on the environment. During winter months, the contractor utilized the temperature-control features of the unit to maintain proper grout consistency.

The encapsulations were topped off with a fillet of marine-grade, water-insensitive epoxy paste, formed as a watershed at the top of each encapsulation. Once completed, the encapsulations are aesthetically pleasing, as they protrude only * in. (12 mm) from the pile surface, and the color of the epoxy grout, as seen through the jackets, is compatible with the color of the piles. More importantly, the piles have been protected by forming a polymer composite that is tightly linked to the pile surface.

Precise application procedures, coupled with equipment designed specifically for the purpose and qualified technical support, allowed the contractor to carry out the work on time, under budget, and with predictable, high quality results.

Note: In addition to the encapsulation work covered here, over 9,000 linear ft. (2,750 m) of small cracks were surface sealed with a marine epoxy paste. Approximately half of the crack sealing was accomplished underwater.

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