By Peter Smith, P.E. and Evan Gurley
The Brooklyn Bridge, designed by John Roebling, has been a symbol of American ingenuity since it opened in 1883 (1). When it opened, the bridge provided passage for horses and buggies, elevated trains, bicycles and pedestrians. Over time, it evolved to also carry trolleys and cars.
By 1954, major renovations of the inner and outer trusses eliminated the elevated train structures and the roadways on each side were widened from two lanes to three, a configuration that remains today. Ramps to intersecting roadways were also added to provide access to and from the bridge.
In terms of complexity and technological innovation, the suspension spans and approaches on both ends are some of the most remarkable components of the Brooklyn Bridge. The 1,525-ft Manhattan approach includes two conventional structures and two arch structures and the 2,100-ft Brooklyn approach includes five conventional structures and three arch structures over the streets below.
An innovative upgrade
In the late 1990s, it became apparent the asphalted concrete pavement on the approaches needed to be replaced. Measures also needed to be taken to protect the masonry arch block structures below. Design consultant Weidlinger Associates Inc., a subcontractor to New York City-based URS Corp., was faced with many daunting challenges. The work could not impede the flow of more than 105,000 vehicles, 2,661 commuter bicycles and thousands of pedestrians per day and had to be done with equipment that would not damage the 127-year-old arches and arch block structures below. Work also had to be completed in strict accordance with the city’s noise abatement requirements and with minimal pollution. But perhaps the most difficult challenge was that the new pavement had to be impervious to water, which would prevent damage to the classic masonry structures below.
WAI continued the Brooklyn Bridge tradition of ingenious design by developing a waterproof pavement consisting of precast pavement slabs fabricated off site in a controlled environment. Precasting the slabs off site helped to satisfy the specified noise and air pollution requirements. The slabs were manufactured to exacting specifications, allowing them to fit the complex geometry between the existing bridges.
Precast pavement was chosen primarily because it can be placed overnight, allowing the road to reopen to traffic the next morning. The system was made waterproof by sandwiching it between two membranes. The first membrane was sprayed on the graded concrete fill below, while the second was sprayed directly on top of the precast slabs, creating a “belt and suspenders” waterproofing system. Water collected by each membrane is directed to specially-designed drainage systems.
Getting to work
Work on the precast approaches began in 2010 when general contractor Skanska Koch Inc. partnered with Ferreira Construction Co. Inc. to install the precast pavement slabs as part of the New York City Department of Transportation’s $508 million Brooklyn Bridge rehabilitation project. Project surveyors 50 States Engineering Co. performed a 3-D laser scan and a conventional total station survey to gather correct existing data and proceed with ramp and slab construction.
Using a master file developed by Skanska Koch, project precaster The Fort Miller Co. Inc. developed shop drawings for each of the approximately 950 mark-numbered slabs on the project. Because of the many skewed bridge abutments, existing underground vaults, new catch basins and water valves, and changes in cross slope – especially adjacent intersecting access ramps – the majority of the slabs were unique in plan-view dimensions, thickness and surface planarity.
Slab installation began in July 2011. Ferreira, an experienced precast pavement installer, had a six-hour overnight window to work. Each night, Skanska Koch rerouted traffic to one side of the bridge and the nearby Manhattan Bridge to provide Ferreira with access to all three lanes of either the Brooklyn- or Manhattan-bound roadways. This complex, almost instantaneous modification of traffic flow required careful and expert coordination.
Placement and installation
The installation process consisted of removing the existing AC pavement, milling the cinder concrete fill below the pavement to the correct elevation and cross slope, installing a waterproof membrane on top of the cinder fill, and placing bedding material and the new precast slabs on top. Application of the upper waterproof membrane and final AC overlay was delayed until all of the precast slabs were in place.
Because the entire process could not be completed in one night, Ferreira developed an innovative removable and replaceable roadway system consisting of top-textured steel plates supported by variable-depth timbers. This provided a smooth ride over the working area during daylight hours.
The subgrade preparation work was the most uncertain of all because of the many unexpected obstacles encountered, including steel columns and framing steel from the old railroad structure, reinforcing bars and underground cavities. While the designer made every effort to locate such items, many had not been recorded correctly – if recorded at all – during the original construction.
When unexpected obstacles were encountered, milling ceased until an acceptable solution could be determined. The time required to resolve these issues and to complete the subgrade process varied widely from area to area, sometimes taking more than three nights.
Once the subbase surface was fully prepared, the next step involved installing the waterproofing membrane on the cinder concrete surface. After the surface was thoroughly cleaned, workers applied an 80-mil layer of polyurea waterproofing membrane. To meet application specs, the moisture content of the cinder concrete surface had to fall between 5-10% and the air temperature had to be at least 5 degrees above the dew point. These exacting conditions added limitations to the times work could be done.
The precast slabs were typically placed within one or two nights of the waterproof membrane installation. Once the temporary roadway was removed, the bedding material was placed, compacted and graded to a surface accuracy of approximately 1/8 in. in accordance with the approved Super-Slab installation procedure. The slabs were placed directly on this surface, each to their specified location using a vertical right angle laser.
On a typical installation cycle, Ferreira installed 12-14 slabs, each approximately 10 ft wide by 12 ft long, although many slabs were much larger. Many slabs were skewed or otherwise specially-shaped to fit between bridges and other structures. While the majority of the slabs were 9-in. thick, other slabs varied from 8-12 in. depending on the location. Some of the perimeter and bridge approach slabs were haunched or otherwise specially-shaped to accommodate adjoining structures. For areas where few or no steel obstacles were encountered during the removal process, Ferreira placed 16-18 slabs per cycle. Grouting of the slabs typically occurred following slab placement. The complete cycle of pavement removal, milling, waterproofing and placement of slabs varied between 5-10 nights, so slabs were not placed every night. During the occasional full or partial weekend closures, all of the previously-described operations were performed consecutively so that many more slabs could be placed.
Traffic had to be maintained on the newly-placed precast slabs until all the slabs on each approach were completed. To accomplish this, temporary AC ramps (transitions) were placed between bridge joint assemblies at finish grade and the precast slabs at 2 in. below. Traffic used this pavement arrangement of precast slabs and ramps for months until all of the slabs on one side of the bridge were completed.
After all the precast slabs were in place, the top waterproofing membrane material was installed over a period of approximately two weeks. The precast slabs were first cleaned by shot-blasting, followed by an application of concrete primer and a subsequent application of two coats of waterproof membrane material. The top coat of membrane received a broadcast coating of aggregate material to provide a skid-resistant surface until the final AC overlay was installed a few days later. The temporary AC ramps were removed just prior to installation of the AC overlay in those areas. Overall, the crisp, clean finished pavement surface belies the sophistication of the waterproof pavement structure lying below it.
Flexible, powerful precast
The final result is impressive. The project team was able to install the waterproof pavement under the tires of New York City traffic without negatively impacting everyday users of the bridge.
Precast pavement has frequently been viewed as standard, flat slabs to be used in all locations, and designers have counted on casting non-standard pavement sections in place and grinding away the tops of slabs until they conform to required non-planar surfaces. This project proves that non-standard precast panels, uniquely shaped in cross section, plan-view and surface planarity, can be routinely designed, fabricated and installed on very complex and heavily traveled projects in a manner that the traveling public is not significantly inconvenienced. That is what precast concrete pavement is all about.
Peter Smith, P.E., is the vice president of market development and product engineering for The Fort Miller Co. Inc.
Evan Gurley is a technical services engineer with NPCA.