By Adam Brodal, P.E.
The U.S. Federal Highway Administration and AASHTO’s Technology and Implementation Group are championing precast concrete pavement systems (PCPSs) as part of their Highways for Life program. PCPSs fit well with the goals of the program, which are to promote construction methods that reduce traffic congestion, reduce project duration, increase safety and produce durable repairs.
About precast concrete pavement systems
PCPSs are long-term pavement repair treatments best suited to heavily trafficked areas that have a high cost for lane closures during construction. All PCPSs include methods for precast production, transportation and ensuring the panel is properly bedded to the subgrade. There are both jointed and unjointed slabs along with prestressed and non-prestressed systems.
The greatest advantage of PCPSs is their ability to meet municipalities’ traffic mitigation goals. Traffic mitigation is the ability to reduce the distress and inefficiencies caused by traffic congestion and delays that the public experiences during road construction. Precast concrete solves traffic mitigation problems on three fronts. First, total time to complete the project is kept to a minimum, because the majority of the work can be performed off-site at the precaster’s facility. Second, the time that the lane is out of service can be scheduled to off-peak traffic hours, because construction can occur at night and the repaired area can be opened to traffic for the morning rush. Finally, the construction footprint is kept to a minimum. PCPSs require only a single lane to be closed on two-lane roads, including the shoulder. When there are more lanes and an interior lane needs work, the interior lane and an adjacent lane need to be closed to safely accommodate equipment and workers. In either case, traffic can continue to flow while the repair is underway.
The second-biggest advantage to using PCPSs is precast concrete’s proven durability. PCPS are designed to last, with the same criteria as new roller-compacted concrete construction that has an expected lifespan in excess of 50 years. The material’s precision and quality are derived from the plant-controlled conditions of the precast facility. Internal quality-control inspection and testing assure a high-quality product. Furthermore, a quality product is achieved through state inspection of the precasting process and field testing of the installed product. A PCPS is further vetted by undergoing prequalification by the state agency, typically with a field test in the precaster’s yard.
A closer look
Most jointed PCPSs use a substance such as grouting or polyurethane injection to support and level or raise the slabs. A typical panel has a double mat of epoxy-coated steel reinforcement. Typical sizes are 12 ft (3.7 m) wide and 8 ft (2.4 m) to 12 ft long. Panels can be made up to 20 ft (6.1 m) long or even longer.
The construction process starts with cutting out the existing pavement repair area. A saw-cut template is typically provided to the contractor for accurate layout.
The construction crew then removes the damaged pavement to accommodate the repair slab. In some PCPS designs, the subgrade is prepared and leveled so that the slab will rest evenly at grade using an engineered grout. In other designs, the existing subgrade soil is left undisturbed and the slab initially rests 1 in. (25 mm) lower than grade, and an engineered foam is used to raise the slab to grade. The repaired section may be opened to traffic in 15 minutes.
In some PCPS designs, Load-Transfer Devices are installed. LTDs are dowel-like mechanical connections between slabs and are required in jointed precast pavements. The time allowed before LTDs are installed is dictated by the municipality and usually dependent on traffic volume. Typically, the municipality requires LTDs to be installed the next business day.
The New York State DOT Dowel Bar Retrofit Method, for example, requires a gang saw to cut four joints in each wheel path. The ribs created by the sawcut in each joint are jack-hammered out and the slot is cleaned. A 1.5-in. (38-mm) diameter epoxy-coated steel-dowel bar assembly (an LTD) with chairs and expansion caps is placed in each slot and grouted for a solid fit.
Roman Stone Construction Co. uses a proprietary polyurethane injection procedure for its Roman Road System. This system allows for the slabs to be placed and set to grade all in one shift. By using the polyurethane injection, almost no subgrade preparation is required. The slab arrives at the site with factory-drilled injection ports to accommodate the polyurethane.
The polyurethane is a two-part polymer that hardens in mere seconds and reaches working strength in 15 minutes. This material loves compression; the greater the pressure required to lift the slab, the greater the final polymer compression strength will be. Road projects typically use a 6 lb/in.3 (0.2 kg/cm3) high-density mixture. The polymer is weather-resistant to cold and rain and is environmentally inert. As the mixture expands, it typically spreads out 3 ft (1 m) in diameter, filling in voids and displacing under-pavement water.
Bidding can be an obstacle with PCPSs due to the use of different, non-customary repair methods. A contractor receiving bids on a PCPS may have items outside of the precaster’s work that affect the cost, such as saw-cutting for joints or sophisticated grading requirements. For instance, NYSDOT tracks PCPS prices with a discreet pay item. This item includes the concrete slab and shipping. Not all items that are required for PCPSs are included in the pay item, and different systems have different required items. This can make cost comparisons difficult and bidding convoluted.
During the bidding process, for example, we have experienced a second obstacle. In our jurisdiction the state is inclined to use the NYSDOT Dowel Bar Retrofit Method for pavement joints. This method has been a sticking point, because it is expensive to saw-cut the large number of joints. A project with 100 slabs could have 2,000 joints, and each joint needs to be cut out with a group of saw blades. Several new jointing methods focused on cost cutting are now under development. One method being researched with PNA Construction Technologies is to develop a plate-doweling system, as opposed to the round-dowel system, with the target of keeping internal slab stress low.
PCPSs are rapidly gaining popularity in several states, thanks to their success in a number of applications. With the Federal Highway Administration and AASHTO turning their attention toward PCPSs, precast concrete can be expected to become mainstream for highway pavement repairs.
Adam Brodal, P.E., is chief engineer with Roman Stone Construction Co. He has 10 years of experience in precast concrete and as an engineering consultant. Educated and trained in Minnesota, Brodal has been with Roman Stone since January 2010. Roman Stone provides the greater New York area with its Roman Road System precast concrete road slabs, conduit, underground utility boxes and traffic barriers (see Precast Inc. Nov.-Dec. 2010).
Sidebar: Project Examples
Sunrise Highway (Route 27) is the major expressway along the south shore of Long Island, New York. It is the main artery for driving to summer recreation destinations in the Hamptons. Roman Stone, with Ahern Contractors, installed the Roman Road System on Sunrise Highway to correct failing joints. Road construction took place in 12 days. A total of 35 slabs were installed on Route 27 in Westhampton between exits 62 and 63, for a total combined concrete volume of more than 78 yd3 (60 m3). Repair locations were isolated and spread over a distance of four miles (6.4 km) of roadway. Slabs were replaced in both lanes, in both the eastbound and westbound roads. Ahern was allotted a six-hour work window each day and was able to successfully install six to seven pieces per shift with only one crew.
The method currently used to verify the performance of pavements is the Falling Weight Deflectometer test (FWD). This test measures the ability of the joint to transfer load. The LTD used on Sunrise Highway was the NYSDOT’s Dowel Bar Retrofit Method. At two separate times, the panels on this project were tested and the slabs performed well. The goal of load transfer efficiencies of 85% or better was achieved.
Long Island Expressway
Roman Stone has begun a high-profile project on the Long Island Expressway. The LIE is the infrastructure backbone to more than 5 million Long Island residents and stretches 70 miles (113 km) from Manhattan to Riverhead. The section being repaired has an average daily traffic volume of about 200,000 vehicles per day.
This job was originally designed for high early-strength cast-in-place concrete. However, this project was characterized by special time constraints, and NYDOT engineers, in consideration of a tighter work window, decided precast concrete was the best option.
The repairs will begin with diamond-grinding on the existing top layer of asphalt down to the existing concrete roadway and then replacing the damaged areas. The project will then receive a new top layer of asphalt. The original survey consists of more than 800 locations to be repaired with primarily 8 ft x 12 ft (2.4 m x 3.7 m) precast slabs. The project will also use 10-ft (3 m) long and 12-ft (3.7 m) long slabs. The extent of road repairs totals eight miles (13 km) of highway, four miles (6.4 km) in each direction.
Roman Stone began the LIE project at the start of 2011 and has ramped up production this spring. Roman Stone is currently producing 10 slabs per day and plans to increase the production rate as the construction season gets underway.