LeJeune Road Flyover
Serving more than 29.6 million passengers in a year, the Miami International Airport is certainly one of the world’s busiest and most heavily trafficked transportation centers. With that many people, it is not surprising that traffic congestion leaving the airport is often a problem. To help ease the congestion leaving the airport going westbound to the City of Hialeah, officials opted to construct the LeJeune Flyover, which connects LeJeune Road northbound to Okeechobee Road westbound. During construction of the Flyover, the bridge began to experience cracking from the construction loads – prompting officials to be concerned that the problem would only worsen after it opened to traffic. It was a challenge to find a solution that would minimize disruption to traffic, yet also ensure a long life for the structure. However, the solution came in the form of an innovative turnkey repair and strengthening solution for the pier caps and columns.
Anatomy of the Structure
Approximately 33-feet wide and 262-feet long, the Flyover is designed to carry two lanes of vehicular traffic. The superstructure is supported on two piers and consists of a twin, steel box girder bridge with an 8-inch cast-in-place composite deck. One of the piers, Pier Two, has a cantilevered arm that gives the pier an L-shape and is set on a footing that is below grade. The second pier, Pier Three, is a hammerhead, T-shaped pier.
A few months after the bridge deck was constructed, an inspector discovered cracks in the pier resulting from construction loads. The backside of Pier Two had a series of horizontal cracks that were uniformly spaced 12 to 18 inches apart that started at the pier cap and went down the pier to the footing. Additional cracks that formed a V-shape were found at the pier cap. Although these cracks were barely within the criteria set forth by the FDOT guidelines, officials believed that it was too close for comfort and horizontal strengthening was necessary. It appeared that some form of post-tension strengthening in a vertical and horizontal direction now would be required on the pier and the pier cap. Upon review of the details in the as-built construction documents, it was determined that the horizontal cracks on the backside of Pier Two were caused by a rebar detailing error — the vertical reinforcing steel in the column was not properly lap-spliced with the top steel in the pier cap. This also was the case for Pier Three, which had insufficient reinforcing steel in the pier cap. Pier Two also experienced “V” shaped cracks in the pier cap due to insufficient top reinforcing bars.
Only minimal repairs were necessary for Pier Three, which had cracks at the pier cap. In the middle of the T-shape, hammerhead pier cap, the same V-shaped crack pattern was discovered. As such, the pier cap needed strengthening horizontally. Further, hairline cracks that propagated were visible at both piers. The engineers were able to determine that the cracks were growing in size by tracing the original crack and evaluating it over a period of a few weeks. Investigation revealed that the cracks were still propagating on the structure because it was trying to relieve stress from the superstructure above.
Repair System Selection
Since the FDOT wanted to ensure that the structure would have a service life of at least 75 years, they required a solution to the cracks that would be durable, cost-effective and aesthetically pleasing. Additionally, the bridge was located over a canal, which created an environment that was classified as moderately aggressive. Because of this corrosive environment, the repair solution also had to be extremely resistant to corrosion. Based on the need to combine constructability and engineering, the engineer-of-record for the project contacted STRUCTURAL/VSL to develop a turnkey repair and strengthening strategy. VSL was tasked with developing a solution for the repair of Pier Two and Pier Three in a manner that would have the least amount of impact on traffic. Although the initial repair strategy required excavating and dewatering at Pier Two, this solution was cost-prohibitive, time-consuming and would have dramatically impacted the traffic pattern. As such, VSL was charged with developing an alternative solution to this perplexing problem.
In order to meet the myriad of challenges for the project, STRUCTURAL worked with the owner and designer to select a repair solution with post-tensioning tendons that was fully encapsulated and would be encased in concrete. According to Jay Thomas, vice president of sales at STRUCTURAL, encapsulated tendons were chosen for several reasons. First, it was crucial that the repair method for the horizontal cracks did not require enlarging the pier as this would have required the repair team to excavate around Pier Two and pour concrete below the water table. Also, adding post-tensioning tendons in a secondary enlarged concrete section below the water table would cause the steel to become susceptible to a corrosive environment because of the cold joint between the new concrete and existing concrete. The FDOT felt this option would have reduced the service life of the structure. Further, excavating at Pier Two would have dramatically added to the time needed to complete the repairs and would have significantly increased the cost. As such, all parties decided that encasing the post-tensioning tendons by placing them into drilled vertical holes would be the optimal solution. Having experience with post-tensioning systems for decades, the FDOT had a comfort level with the method and supported the repair solution.
Preparing the Site to Ensure Safety
Since the site required working 20 feet above the ground, the first step was erecting scaffolding at both piers. The repair solution also necessitated special access for core drilling equipment. As such, a four-foot-by-six-foot work platform was created specifically for the core drilling machinery. Further, since Pier Two was over a canal, the repair contractor had to have a special platform for the scaffolding to set on as it bridged the canal below. This scaffolding was tested to ensure that it was properly designed for construction loads. To provide minimal impact to the existing landscape, protective tarps were installed around the work area so construction debris resulting from the concrete chipping process would not fall into the canal or onto surrounding traffic.
One of the primary concerns for all involved parties on the project was ensuring the safety of the workers, the structure and its surroundings. As such, the first day on the job consisted of safety training. The first half of the day was spent at the site doing hands-on training and the second half of the day was spent reviewing the repair procedures. Life rescue issues were put in place at the site in case they were needed in an emergency situation. Special considerations also were taken to ensure safety since work would be occurring over water.
Yet another challenge was the extraordinarily tight schedule for the project required, which resulted in two eight-hour shifts, six days per week. To accommodate this schedule, light towers were erected onsite and security personnel patrolled the area at night. Ensuring the safety of the workers, the structure and citizens was critical so police maintained a presence at the jobsite to help direct traffic and to prevent vehicles from driving on the bridge until the repairs were complete.
At both pier caps, bush hammers were used to prepare the concrete surface to a quarter-inch amplitude. However, because of the nature of the aggregate often used in Florida ready-mix concrete, the surface preparation also required the use of 15-pound pneumatic chipping guns to help achieve the desired quarter-inch profile. This aggressive profile allowed for a good mechanical bond between the old and new concrete.
To open the pores of the concrete, the structure was then water-blasted. This ensured that the pores of the old concrete were open and ready to receive the new concrete. Maximum compatibility was created by using the same concrete mix design that was used in the original construction of the bridge piers.
In order to meet the latest FDOT post-tensioning specifications regarding the durability of structures, advanced technology was utilized to provide the most optimal and durable solution. This technology came in the form of a prepackaged grout with zero bleed characteristics and plastic corrugated duct sheathing for enhanced corrosion protection. Around the post-tensioning anchorage blockouts, an elastomeric coating was applied to ensure there was no penetration of water.
Executing the Repair
The STRUCTURAL team selected post-tensioned concrete anchors to ensure a cost-effective and durable solution for repairing the horizontal cracks at Pier Two. By using vertical post-tensioning tendons, compressive forces are introduced into the concrete and thus, cracking is reduced. The repair team opted to core two holes, 5.5 inches in diameter and 40 feet in depth, into the top of the pier cap – stopping only a few inches from the bottom of the footing. The core was extracted in 12-inch depths until the maximum depth was reached. Made of high strength prestressing steel, the post-tensioning tendon was lowered into the hole and grouted in two stages. The first grout stage was at 15 feet from the bottom of the cored hole and created a ‘bond zone” for the anchorage. The hydraulic jack stressing operation was then performed at the top of the hole and the second stage of grout was pumped into the remainder of the hole. This stage created corrosion protection by enveloping the strands in a layer of cementitious grout.
Repair of the “V” shaped cracks at the pier cap and column connection on both Pier Two and Pier Three required casting an enlarged section of concrete on both sides of the pier cap. High-strength, horizontal post-tensioned bars were cast in the concrete section. Rebar dowels were used to connect the new concrete to the old concrete. The post-tensioned bars were then stressed using a hydraulic jack to compress the old section. The compressive force was then transferred from the new concrete to the old concrete through the rebar dowel bars and high-strength Spin-Lock® anchors causing the pier cap to squeeze together and reduce the crack-widths.
As with most concrete repair projects that require deep, precise holes to be cored, many unforeseen challenges arose. For example, the original design called for two 5.5-inch diameter holes to be cored simultaneously. However, the first hole began to experience problems at 10 feet. At this depth, the drill bit seized and stopped turning. When the core driller attempted to extract the core, it would not budge. As such, the decision was made to revise the original strategy and create a 5-inch hole instead of a 5.5-inch hole. Essentially, this adaptation resulted in a smaller hole cored inside the original core barrel stuck in the hole. The necessary modification created significant challenges for the repair team because all designs had to be revised including the post-tensioning anchor hardware to fit the smaller hole. Even with this adaptation, the coring operation was complete in 10 days.
The Road Frequently Traveled
The LeJeune Flyover demonstrates the success that can occur when all parties on the team work together to develop innovative solutions to complex problems. Before this project began, the road had been under construction for nearly two years, so it was critical that the project was completed in a timely fashion with minimal impact. By developing an open working relationship with the owner, innovative solutions were developed and the contractor was able to provide a turnkey solution that included repair design services, labor resources and shop drawings. Repairs were completed two weeks ahead of schedule and were delivered using the most cost-effective solution. The field portion of this extremely fast-track project was completed in a mere four weeks – allowing the Flyover to be opened ahead of schedule and ensuring a long life for the structure.