The Federal Highway Administration's (FHWA's) Accelerated Bridge Construction (ABC) program utilizes innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the onsite construction time that occurs when building new bridges or replacing and rehabilitating existing bridges. The use of Prefabricated Bridge Elements and Systems (PBES) – or the ability to build in a more prefabricated manner, is one strategy of ABC that is gaining popularity at a national level.
PBES can better address the needs of ABC by improving:
- site constructability constraints,
- total project delivery time,
- material quality and product durability, and
- work-zone safety for the traveling public.
Using PBES for ABC can reduce traffic impacts, onsite construction times, and weather-related time delays. PBES also can help minimize environmental impacts, impacts to existing roadway alignment, utility relocations, and right-of-way take.
PBES is defined as the structural elements of the bridge that are built in a prefabricated manner either off site or adjacent to the existing alignment. They include two important features, which is to reduce the onsite construction time and mobility impact time that is associated with conventional construction practices. Prefabricated elements are the single structural components of the bridge as noted in the Frenchtown Brook Bridge project below, while prefabricated systems are entire bridge moves.
The most common reason to use a PBES technology is that it allows for easier and safer accelerated construction to occur for the contractor and traveling public. However, other common and equally viable reasons to use PBES deal with site constructability issues. Long detours, costly use of a temporary structure, remote site locations, and limited construction periods often present opportunities where the use of PBES methods can provide more practical and economical solutions to those offered if conventional construction methods were used.
Why use PBES and other ABC technologies?
Approximately one-fourth of the nation's 600,000 existing bridges require rehabilitation, repair, or total replacement. However, the work that occurs from onsite construction activities can have significant social impacts to mobility and safety. In many cases, the direct and indirect costs of traffic detours that result from the loss of a bridge during construction can exceed the actual cost of the structure itself. For example, full-lane closures in large urban centers, or on highways with heavy traffic volumes, can have a significant economic impact on commercial and industrial activities in the region. Partial lane closures and other bridge activities that occur alongside adjacent traffic can also lead to safety issues. Because of the potential economic and safety impacts, minimizing traffic disruptions is a goal that should be elevated to a higher priority when planning bridge-related construction projects.
Frenchtown Brook Bridge replacement
In East Greenwich, R.I., the 57-year-old, functionally obsolete, cast-in-place Frenchtown Brook highway bridge was in need of replacement. This project was funded in part through the FHWA's Highways for LIFE program, which advances highway infrastructure service life through use of innovation to meet ABC goals when building safe roads and bridges. To minimize traffic disruption, the Rhode Island Department of Transportation (RIDOT) allotted just 65 days for roadway closure from demolition all the way through project completion.
"Any time we work on a road or bridge, we realize it causes disruption for communities and businesses," said Michael P. Lewis, director of RIDOT. "Using accelerated bridge construction techniques will allow the department to do the work as fast as possible while minimizing inconveniences."
Minimizing traffic disruption was critical to the success of this project. RIDOT wanted a low-maintenance, economical, and durable concrete bridge that would utilize precast elements and be installed in days rather than months. Gordon Archibald Inc. and Aetna Bridge Company, the project's engineer and contractor, respectively, worked with Contech Engineered Solutions to determine the best ABC solution for this project to align directly with the FHWA innovation goals. Contech's precast 28-foot by 7-foot CON/SPAN B-series Bridge System, 150 feet in length, with precast EXPRESS Foundations, offered the most advantages, including the speed of precast, the strength of arch action, low maintenance, and lower project cost.
The use of EXPRESS Foundations greatly helped to solve the time crunch of this project by expediting installation of the new structure. The new precast EXPRESS Foundation system blends the speed of precast with the economy of cast-in-place.
The precast portions of the EXPRESS Foundations – the sidewalls and cross-members – were manufactured in a controlled environment resulting in a high-quality product that was easily transported to the site. By using the precast foundation system, the contractor reduced exposure to rain and other poor weather conditions that could have caused delays. The precast trapezoidal wingwall foundations further reduced foundation concrete quantities.
After the contractor excavated and prepared the foundation subgrade, the 16 EXPRESS Foundation precast sections were quickly set in place. Minimal reinforcement at the joints was inserted to provide foundation continuity throughout the length of the structure (see Figure 1). Then, 25 CON/SPAN arches, headwalls, and precast wingwalls were installed on top of the EXPRESS Foundations. Cast-in-place concrete was then poured in the foundation cells, eliminating the keyway grouting. The joints of the structure were sealed, wingwalls grouted, and the backfilling process was completed.
The use of precast bridge components provided RIDOT and the FHWA with an efficiently constructed bridge that saved money and time. In fact, demolition-to-installation of the new structure was completed in just 33 days – just half of the allotted project time – and the bridge was reopened approximately one month ahead of schedule.
With the speed of installation on this project and completion one month early, the contractor was awarded $90,000, the maximum possible amount from RIDOT's $3,000-per-day incentive clause in the contract.
"Going into this project, we knew that it would take only a third of the time to replace this bridge compared with the time it would have taken if we used traditional construction methods," said RIDOT's Lewis. "We are pleased to be able to take this approach with the Frenchtown Brook Bridge and reopen it as quickly as possible for drivers who rely on this bridge."
What is ABC?
From the owner or agency perspective, ABC may require a change in how a project is approached and how it is justified. As such, project planning tools with innovative solutions are provided to help streamline the development process and ensure successful implementation of ABC.
ABC time metrics
Onsite construction time – The period of time from when a contractor alters the project site location until all construction-related activity is removed. This includes the removal of maintenance of traffic, materials, equipment, and personnel.
Mobility impact time – Any period of time the traffic flow of the transportation network is reduced due to onsite construction activities:
Anthony Rotondo, Jr., P.E., is Bridge Program manager, Federal Highway Administration.Philip Creamer, P.E., is director - Precast Engineering, Contech Engineered Solutions.