Equipment Type

New Efforts to Publicize PCC

Public-relations efforts and new designs combine to help promote benefits of portland cement concrete technologies

February 01, 2004

 

Michigan's Ogemaw County
In Michigan's Ogemaw County, weathered I-75 PCC pavement is overlaid by a 1-inch hot mix asphalt interlayer and topped with a 6-inch, high-performance concrete overlay.
Chicago's Wacker Drive Project
A quarternary, high-performance concrete mix, incorporating cement, ground granulated blast furnace (GGBF) slag, fly ash and microsilica fume, was specified for extreme durability on Chicago's Wacker Drive reconstruction project.
I-75 Ogemaw County, Mich.
High-performance concrete containing 75 percent ground granulated blast furnace slag is placed in 6-inch overlay over 1-inch hot-mix asphalt on I-75 Ogemaw County, Mich.
I-75 Ogemaw County, Mich.
This demonstration project in Michigan's Ogemaw County was intended to explore options that could lower rebuilding costs.

PCC Pavements May Open Faster Through Nuke Tech

 

Nuclear technology may result in faster-opening portland cement pavements, the Federal Highway Administration (FHWA) reported last year, giving PCC yet another encroachment on hot-mix asphalt's significant advantage of fast placement and fast opening to traffic.

For decades, researchers have been trying to determine how water and portland cement powder react during the concrete curing, or setting, process. This reaction plays a vital role in determining concrete set times, thus how quickly drivers can begin using newly placed PCC roads.

Studying this reaction, FHWA's Advanced Research Program was applying, for the first time, an analytical method known as nuclear resonance reaction analysis (NRRA). The results will be used to improve cement industry standards and guidelines for mixing and curing concrete.

The method uses a beam of nitrogen atoms to probe a reacting cement grain to locate hydrogen atoms, a necessary component of water, or its reaction products, FHWA reported. The results of the probe are plotted in a graph called a hydrogen depth profile, which shows the rate of penetration of the water. This also indicates the arrangement of the various surface layers formed during the reaction.

The evolution of the hydrogen profile shows the time of breakdown of the surface layer, which can be used to study the concrete setting process as a function of time, temperature, cement chemistry, and other factors. The NRRA research will continue for at least through 2006 in collaboration with the University of Connecticut.


This year the Federal Highway Administration (FHWA) will launch a bold outreach initiative taking new portland cement concrete pavement technologies out to the state, city and county road agencies and contractors who can use them.

The five-year initiative, dubbed Task 65, will reach public and private sector end-users with new research in whitetopping, weekend reconstruction of intersections, the efficacy of sealing transverse joints, and improvements in smoothness criteria.

But Task 65 is only part of the new dynamic changes in portland cement concrete (PCC) construction that are unfolding in 2004.

FHWA is launching a multifront effort to speed up transfer of PCC technology. The contract for Task 65 is underway by Construction Technology Laboratories in Skokie, Ill., and includes a public relations subcontractor.

"We have the technical engineering expertise combined with the communications expertise in a single contract," says Sam Tyson, concrete pavement engineer, FHWA Office of Pavement Technology. "We have more than 30 concrete research projects, each of which is generating one or more 'products,' each of interest for potential implementation in the field by either state DOTs or paving contractors. Task 65 will get this technology out to the field."

Although not deviating from previous practice, the volume of finished and incoming products in PCC research over the next five years is such that FHWA wanted a mechanism in place for orderly technology transfer. "This research was earmarked under TEA-21 and focuses exclusively on improved quality in concrete pavements that was addressed in that legislation," Tyson says.

In addition to joint sealing, PCC overlays and smoothness, projects include efficacy of elliptical dowel shapes vs. circular dowels, different dowel materials, HPC technology demonstrations, and precast HPC pavements in different states. "Task 65 will take the technology from all of those efforts and deliver it for potential use by the state DOTs and contractors," Tyson said.

Another outreach will be the long-standing FHWA Mobile Concrete Laboratory. Since the late 1980s, the lab—a state-of-the-art concrete testing facility on wheels—has brought innovative concrete technologies to end-users in the field. This 48-foot trailer is equipped with both new and conventional testing equipment. Its skilled engineers and technicians provide state and local road agencies and contractors with equipment evaluations and field demonstrations of specific concrete technologies.

HPC penetrating pavements

Much of this technology transfer will involve high-performance concrete (HPC) mixes, Tyson says. Since the mid-1990s, the building of highway infrastructure using durable HPC has dominated PCC research and application for civil engineering.

HPC is often called "durable" concrete because its strength and impermeability to chloride penetration makes it last much longer than conventional PCC. It's an engineered concrete made up of the classic elements of water, portland cement and fine and coarse aggregates, but with added components.

HPC provides enhanced mechanical properties in precast concrete structural elements, including higher tensile and compressive strengths, and heightened modulus of elasticity. In frost-prone regions, the enhanced durability of HPC helps it resist penetration of chloride-laden snow and ice-melt water.

These durability criteria include air void structure, low permeability, proper water content of fresh concrete, and low susceptibility to cracking. Also important are fast and accurate ways to evaluate properties of curing concrete.

Some industrial "waste" materials of a few decades ago now are integral elements of this new-engineered concrete. These admixtures, such as coal fly ash, silica fume and ground granulated blast furnace (GGBF) slag, add both strength and durability to the concrete, and enhance its marketability as an environmentally friendly product.

Such industrial byproducts figured into a breakthrough PCC overlay project, which used a hot-mix asphalt interlayer between the existing PCC pavement and the PCC overlay, conducted in summer 2003 by the Michigan DOT in conjunction with the Michigan Concrete Paving Association (MCPA).

This overlay was a four-mile stretch of northbound I-75 in Ogemaw County in the north-central Lower Peninsula. Constructed by John Carlo Inc., the project overlaid the existing 30-year-old PCC pavement with a 6-inch concrete overlay, with a 1-inch lift of HMA as a separation layer intended to eliminate reflective cracking. HPC was attained by using conventional Type I cement with 25 percent of cement replaced with GGBF to provide durability.

"The goal of this demonstration is to explore options that could lower costs to the department for concrete overlays," says Bob Risser, MCPA executive director. "Reflective cracking is the Achilles Heel of overlays, but the separator layer stops it."

The MCPA determined that the total cost of the concrete overlay was only 6 percent more per square yard than the rubblize-and-asphalt project on the southbound side, but with the potential of delivering twice the life.

On another thin PCC overlay project, Outer Drive in Wayne County, Mich., a 1-inch HMA separator layer was placed atop a 1920s-era concrete pavement, then topped with a 4-inch PCC overlay.

"We jointed it in 6×6-foot squares, and it's been performing beautifully," Risser says. "They built two more overlays [in 2003] using the same design, and what's really remarkable about all three projects is that Wayne County reported they cost only 15 percent more in initial cost than a 3.5-inch mill-and-fill with HMA, but they're expecting at least twice the life."

Thus, these thin PCC overlays have real potential to cut into HMA market share. "To reconstruct Outer Drive in concrete would have cost over $2 million," Risser says. "We did the PCC overlay for $750,000. On this and the other projects, Wayne County is reporting a 10- to 15-percent cost premium over HMA to obtain twice the life. Now we have a solution that fits into the budget constraints that so many agencies have."

Wireless maturity meters

Also in Michigan last year, wireless portland cement concrete maturity meters were placed in the fresh concrete of a road in Oakland County, Mich., by Tony Angelo Cement Construction, in conjunction with the city of Novi, consultant JCK, and the Michigan DOT.

These wireless maturity meters, only a few inches long, were attached to rebar and concrete was placed around them. Then, a handheld computer permits data to be compiled and analyzed to provide a real-time concrete strength value. Because the wireless meter provides instantaneous data, such as strength gained in the slabs, Michigan DOT was able to monitor and open the pavement without delays due to lab specimen curing and testing.

"You literally walk up to the concrete, tap in the serial number of that maturity meter, and it transmits what the concrete is doing to the palm-held device," MCPA's Risser says. "You can track the progress of the concrete without having to cure and break test cylinders. This also eliminates the hazard of thermocouple wires used to track concrete maturity."

And since the long cure time and testing has been a selling point of asphalt against concrete, the advent of wireless maturity meters may give the "white stuff" a new weapon to use against the "black stuff."

"There were so many businesses along this route, that the county wanted to get the pavement open as quickly as possible," Risser says. "The wireless maturity meters helped this. And because the data are from the slab itself, not the lab specimen, the meters provide a more accurate portrayal of the slab condition."

New costing software

The higher initial material and labor costs of PCC construction always work against it when matched against HMA, when long-term costs are not considered. But new software may help PCC narrow that gap.

Performance-related features of PCC pavements that are taken for granted—such as load-transfer devices like dowel bars between slabs—add cost to the pavement design and have the potential of discouraging PCC use because of the added cost differential versus HMA.

At the 1986 Transportation Research Board meeting, Wisconsin DOT's Steve Shober upset the apple cart when he articulated the case against sealing PCC joints, even in a freeze/thaw climate.

"The need to seal PCC pavement joints/cracks is so ingrained in the United States pavement culture and is so seemingly sound from a theoretic perspective that it has been considered an unchallengeable truth," Shober said in a late-1990s follow-up. "The 'truth' of keeping water and incompressibles out of joints may have had a basis when PCC pavements were built directly on the subgrade, but since the use of base courses the need to seal joints has not been proven."

Shober also linked joint sealing's utility to whether it directly serves the taxpaying customers who use Wisconsin's roads. "Sealing has to somehow enhance pavement performance (ride or longevity) and/or convenience and/or safety," he said. "In addition, joint sealing has to be cost-effective, i.e., the measured benefits have to exceed the costs (which include user delays and safety problems related to closing lanes to reseal joints)."

Now, the FHWA has revisited the issue of the cost-benefit ratios of various PCC pavement features, especially sealed joints. And software that would assist designers and contractors to determine the benefits was to be released in January.

In August 2003, an FHWA expert panel reviewed the new software, which assesses the costs and benefits of PCC design features. When complete, the program will provide planners, project managers and highway engineers with insights into which features provide the biggest return on investment.

The FHWA states that the cost-benefit ratio of sealing transverse joints—which can increase the initial costs of placing a PCC pavement by up to 5 percent—is a major feature that the software will analyze.

What about reclaimed concrete?

Reclaimed asphalt pavement (RAP) is being used increasingly in asphalt base courses, pavement foundations, and driving courses. But until now, crushed, reclaimed concrete aggregate (RCA) from demolition materials has been used only for road foundations.

This began changing in 2003, when the FHWA released a survey of a National Review of Recycled Concrete Aggregate use. This survey determined how state highway agencies are using RCA, established a base line for future RCA adoption, and profiles the more advanced uses of RCA in five states: Minnesota, Utah, Virginia, Texas and Michigan.

RCA is being used throughout the United States as base material, but rarely is used as actual aggregate in PCC. Quite often RCA has large amounts of dirt mixed in and must be further processed, which adds cost. The RCA must be tested to determine its main components. And certain types of RCA may contribute to a malady of concrete, alkali-silica reactivity.

Efforts by FHWA and others to promote new research in portland cement concrete have been launched. As the word moves out, the industry will hear even more about the technologies of PCC.

 

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