Highway owners and users are demanding more from roadways and bridges. They want infrastructure to be built quickly, at low cost, with minimal impact on the environment, and with low long-term maintenance. They also want long life and aesthetics. Achieving these competing requirements can be difficult, but new technologies are providing solutions.
“We need a major effort to restore, rebuild, or replace our infrastructure, and we struggle with where the resources will come from,” says Byron Lord, Highways for LIFE program coordinator at the Federal Highway Administration (FHWA) in Washington, D.C. “Fortunately, all of the forces are coming together to help us find better ways to deliver safe and effective transportation. We're finding that there are a lot of things out there that work better than what we currently are doing.”
Technologies that have existed for some time have potential that's not been exploited, says Dale Harrington, past director and consultant with the National Concrete Pavement Technology Center and principal engineer with Snyder & Associates in Ankeny, Iowa.
One example is concrete overlays, both bonded and unbonded. As asphalt prices rise, officials are looking for alternatives to traditional pavement, but some overlook this option. “There has been a lot of turnover, and some departments previously familiar with the techniques have no comfort level with them now,” says Harrington. The center has teamed with the FHWA and manufacturers to produce a second edition of “Guide to Concrete Overlays” to fill this gap.
Standardization aids acceptance
In some cases, existing technologies need standardizing. In July FHWA released “Connection Details for Prefabricated Bridge Elements and Systems.” A focus group of bridge engineers noted connection standards as their No. 1 need, says Lord.
The connection details have been proven in the field, and the agency intends to add options to its online database as they are accepted. The manual, downloadable from FHWA's Web site, ensures states won't spend time creating their own details. “It will be great for producing consistent details for the entire U.S., making it easy for contractors to bid them accurately across the country,” says Susan N. Lane, manager for transportation structures at the Washington, D.C., office of the Portland Cement Association (PCA).
New technologies are enhancing pavement options as well, says Wayne Adaska, director of pavements for PCA, Skokie, Ill. One technology is roller-compacted concrete (RCC), which uses conventional concrete materials with a drier mix compacted by vibratory rollers. It requires no forms, joints, finishing, or steel reinforcement, and can be used for heavy-duty applications; it requires minimal maintenance.
RCC, concrete overlays, and other products are being promoted by PCA in a 2009 educational program called Integrated Pavement Solutions. “We're concentrating on ‘orphan materials' outside standard concrete products,” says Adaska. “We want to promote all types of cement and concrete to overcome the industry's fragmentation. The variety of available solutions means that one will fit almost any situation, so decision-makers learn they can look to concrete for any pavement challenge.”
Pervious concrete also is gaining popularity. “It definitely has its place, and its use is growing,” says Adaska. “But there are no standards to help direct its usage, and that's a concern.” Currently used for parking lots, alleys present an ideal opportunity once standards are established, he adds.
Precast concrete pavement panels are getting a closer look, as well. The panels are prestressed in one direction, delivered to the site and set into place, then post-tensioned in the opposite direction to create a stable, immediately available driving surface. Projects have been completed in Texas (2002), California (2004), Missouri (2005), Iowa (2006), and Delaware (2009), with one underway in Virginia.
The most recent project in Newark, Del., was motivated by the panels' ability to be used as soon as they are placed, says Dave Merritt, project manager for The Transtec Group, Austin, Texas, which provides engineering support through FHWA to aid states with post-tensioning construction. “The pavement can be installed quickly, but this project was not about the amount that could be placed, it was about how soon the road could be used,” he says. “They could install pavement each night and open the road to traffic again each morning.”
High-performance concrete grows
A variety of new concrete materials are used more in bridge projects today, and their usage spreads as states learn of their neighbors' success. One of these is high-strength concrete, which helps concrete bridges extend their span lengths. High-strength concrete is defined as providing compressive strength of 10,000 psi instead of the more typical 5000 to 6000 psi, explains engineering consultant Henry Russell.
Many concrete producers can provide 7000 to 8000 psi concrete, but state departments of transportation (DOTs) hesitated to design to that standard due to its shorter curing time. Now they are seeing its benefits. “Higher-strength concrete lets us use smaller member sizes, which means a lighter impact on the environment due to longer spans and fewer foundations,” says Linda Figg, president of FIGG Engineering Group, Tallahassee, Fla. “It allows us to build around and over elements in the landscape to preserve our precious space.”
“High-strength concrete definitely is still on a growing curve,” says Russell. “But its use depends on the state, designers, and producers all coming together and agreeing that it will work.”
Also growing in use is high-performance concrete (HPC), which differs from high-strength concrete because it focuses on durability and low permeability, although high strength often is part of the package. HPC adds durability to bridge components, especially decks, says Lane.
HPC “is more of a process than a product,” says Lord. “It can be engineered to ensure designers get what they want from it.” The mix design typically substitutes fly ash, silica fume, or slag for a portion of cement, which reduces the concrete's permeability and its energy intensity, since cement is the most energy-intensive component of concrete. This enhances its environmental friendliness, a key focus for many states.
HPC usage will grow as designers try to improve durability to lower maintenance costs. “States absolutely are looking for longer service life,” says Figg. “We have the ability, in our material designs and technology, to go beyond 100 years in service life for concrete bridges today, and we must focus on that.”
Some state DOTs are using ultra-high-performance concrete (UHPC), which reaches compressive strengths of 18,000 to 21,000 psi. In April, the Iowa DOT completed construction on the Jakway Park Bridge, the first North American highway bridge to use UHPC girders batched in a ready-mix truck. The concrete had a compressive strength of 21,500 psi.
The bridge was funded by FHWA's TEA-21 Innovative Bridge Construction Program. “The key now is whether states will be able to fund these projects on their own,” Lane says. “But when high strength and durability both are needed to overcome challenges, UHPC will start to shine, and people will use it.”
Self-consolidating concrete also holds great promise. The material consists of a highly flowable concrete mix that doesn't need to be vibrated, eliminating labor, and providing a high-quality surface with no bugholes. Its flowability ensures it won't hang up on tightly packed reinforcement and that it fills oddly shaped corners.
“Some states are nibbling at it, some have jumped in,” says Russell. “States are typically conservative on these innovations and don't move quickly. But producers are pushing for it and are making changes to their production lines to handle it, so states are beginning to permit it—and that means someone will be using it.”
Lightweight concrete is increasing its foothold in bridge design as well, says Lane. It uses lighter aggregates that reduce concrete's density from 150 pcf to 115 to 130 pcf. “I expect we'll see growth in the use of lightweight concrete in both repair and rehabilitation projects,” she adds. States will especially use it to save substructure elements when replacing or expanding superstructure components. The added volume of those expansions can be offset by using lightweight components. The lighter weight also enhances seismic performance by reducing the mass, she notes.
Nanotechnology offers bridge benefits as well, says Figg. Her designers specified a concrete mix for the I-35W bridge in Minneapolis that was built rapidly after the collapse of the original bridge. The mix contained titanium dioxide, which activates with sunlight to serve as a photocatalyzer, encouraging the decomposition of pollutants while maintaining a shiny white appearance.
New techniques thrive
In addition to the array of materials, new techniques are being devised. One that offers great potential for pavement projects involves grooving new or existing pavements to reduce noise. Called the Next Generation Concrete Surface, it consists of a hybrid texture constructed on concrete surfaces that resembles a combination of diamond grinding and longitudinal grooving, explains John H. Roberts, executive director of the International Grooving & Grinding Association (IGGA), West Coxsackie, N.Y. The texture can be constructed, as either a single-pass or two-pass operation, using diamond-tipped saw blades. “These textures, for both new construction and rehabilitation, will have the desirable characteristics of a very smooth profile coupled with good microtexture and excellent macrotexture,” he says.
Tests are underway in several locations with additional ones to come, says Bill Davenport, vice president of communications with the American Concrete Pavement Association, Skokie, Ill. “We're pretty excited about this. The technology is here and now, but we're in the early throes of evaluating the results. So far, they're pretty promising.”
A number of techniques will address the key concern for bridges: speed of construction. Accelerated bridge construction can cut user costs and reduce safety concerns. FHWA promotes techniques to speed construction, using the motto “Get In, Get Out, Stay Out.”
One key method involves segmenting girders into smaller units that are fabricated offsite, transported to the location, and quickly erected. This helps standardize components and cut fabrication costs. Figg says Minneapolis' I-35W bridge used this approach, erecting 120 segments, some weighing 200 tons, across the Mississippi River in 47 days. FHWA estimated user costs at $400,000/day while this critical bridge was shut down.
Segmental construction combines the capability to splice girders together while cantilevering them over the piers to extend span lengths, Russell notes. This minimizes substructure needs and reduces material and erection costs while lessening the environmental impact. “Concrete can compete with steel for long-span applications,” he says.
Speed also is enhanced by combining components into larger assemblies near the site and moving them into place. Utah has been a leader in using self-propelled mobile transports to accomplish this. It also is helping other states understand the technology, Lane notes. “The states are sharing what they've learned to help others get started.”
The girder launcher offers a solution for challenging multispan structures. It pushes girders into place from above the previously constructed span, eliminating the need for falsework or crane positioning that would impact the environment below. It's especially useful for wetlands and congested urban areas.
“We have the techniques today to use concrete to build span-by-span from the top while allowing traffic to continue moving below,” says Figg. The firm is designing an extension of U.S. 280 in Birmingham, Ala., that will place piers in the median to double capacity.
Many states are opening their bidding processes to allow design/build approaches. This procedure lets designers and contractors team up to propose design concepts, taking advantage of their expertise in specialized designs or materials. “Some states are slower to embrace this, because they are concerned that they don't have as much control or aren't as certain they will receive the design that they want,” says Russell. Indeed, in some states, design/build options aren't allowed—but more are using it. “At one point none of them used it, so the trend is growing.”
That trend no doubt will continue, as will the realization among state and local officials and private developers of the benefits that new concrete materials and construction techniques provide. Taking advantage requires careful study. But with expertise growing all the time, new technologies will ensure concrete materials meet any design or construction challenge.
Craig A. Shutt is contributing editor for James O. Ahtes Inc.