Superthin Concrete Overlays
| October 09, 2012 |
Spurred forward by some brave new thinking, the concrete pavement industry is working toward thinner concrete overlays these days – down to 3 inches, even 2 inches.
Some Iowa counties have paved a lot of miles of 4-inch overlays that have stood up well for years. And Illinois, Tennessee, and other states have paved many miles of thin concrete overlays (formerly called ultra-thin whitetopping). These applications, as well a stubborn economy is leading the American Concrete Pavement Association (ACPA) to advance overlay technology in new directions.
“Very thin overlays are the next frontier for the concrete pavement industry,” says Jerry Voigt, P.E., ACPA president and CEO. “This is where we have to go,” he says with conviction.
Voigt proposes that the industry reverse-engineer a new overlay material that would have durability, uniform thickness, compressive strength, and all the qualities of conventional concrete overlays. But instead of trying to fit conventional concrete into a thinner section by adjusting panel sizes and joint spacings, Voigt says the industry should rethink and fit the material to thin applications.
The first thinking should be to design the needed characteristics of a good-quality overlay, he says. With specific engineered qualities like durability, flexibility and toughness in mind, the next effort would be to design – or reverse engineer – a version of concrete to fit those qualities. Voigt says the new overlay may be a shorter-term solution – it may not last 30 or 40 years, as people have come to expect from conventional concrete.
Thinner Can Work
“I think we need to strive to achieve at least 3 inches,” says Voigt. “A 2-inch layer would really be pushing the envelope. But if we can get to a 3-inch layer by modifying our material and looking at the concrete differently, then the number of places that could apply that solution will grow much more than it would with a 4-inch overlay.”
Jeffery R. Roesler, PhD, P.E., is an associate engineering professor at University of Illinois, and worked with Amanda C. Bordelon, Ph.D., to develop a new thin concrete wearing surface material. Called flowable fibrous concrete (FFC), the mixture incorporates a hybrid of synthetic fibers to give the concrete toughness and limit the size of cracks. “An objective of the wearing surface was to construct reasonable slab sizes and crack widths while ensuring economic feasibility,” according to a paper by Roesler and Bordelon for the American Society of Civil Engineers.
At the University of Illinois, Roesler and his students cast a 400-foot-long, 2-inch-thick demonstration overlay with FFC to evaluate constructability and concrete material performance including placement issues, crack spacing and width development, and interface bonding conditions. “I think we have something here that we believe works,” says Roesler.
The demonstration overlay was placed with conventional ready-mix concrete trucks using available materials. “We demonstrated that it’s easy to do,” says Roesler. We used all conventional concrete paving equipment that you would normally see on a paving project.” He says the FFC was placed with a vibrating screed, and flows like honey, so placement is a simple matter.
“It basically flows like self-consolidating concrete that has been used over the past ten years in structures to compact under its own weight and flow around rebar,” says Roesler. “You need very little vibration to get it to consolidate.”
At ACPA, Voigt has reviewed the flowable fibrous concrete, and says it shows promise. “I think we could try it very soon,” Voigt says. “We haven’t gone through the reverse engineering analysis to work our way back to it, but I think there are some concepts there that we could try, straight-away. We need to find an agency to take that step.”
Roesler says that FFC is designed to last 10 years on city streets and low-volume roads. The fibers can hold cracks tightly together, “but you have to come with the mindset that we’re going to get cracks eventually,” he says. “Someone has to take the risk and that’s the biggest thing to overcome,” he notes.
Roesler says the cost of FFC probably slightly exceeds that of asphalt in the same thickness, because of the fibers and extra cement in the mixture design. “But we’re thinking that this is a 10-year surfacing and that usually exceeds the life of a city overlay of asphalt,” he says.
Bordelon is now an assistant engineering professor at the University of Utah. For her, one of the challenges facing thin concrete overlays is to accurately determine the strength of the underlying material.
“If you get an asphalt that is too severely deteriorated and has a lot of cracking, or if the subgrade is unstable, there’s not a lot you can do,” she says. “If you put an overlay on it, the overlay may not last long. To have a good asphalt in terms of moderate to no cracking is probably the best.” Moreover, she says, developing a good bond between the asphalt and concrete overlay is important. One way to do that is by milling the asphalt down to leave a roughened surface.
E. Tom Cackler is director of the National Concrete Pavement Technology Center at Iowa State University. He sees three challenges facing very thin overlays. For one, current design methodologies, such as DARWin ME, don’t address thin sections. “And Dr. Roesler has been very involved with that in his research,” says Cackler. “Are there any differences we need to do with the mix design? So we’re very interested in the topic, and over the next few years we’ll be focusing on thinner sections.”
Laboratory work is needed to overcome the challenge of a mix design methodology, says Cackler. “We need to look at how the stresses are actually distributed in these thinner sections. What are the failure mechanisms? When we learn those things, then we need to correlate that with practice – what we actually observe in the field.”
Secondly, Cackler says better tools are needed to assess the support value of the existing pavement, whether it’s an existing concrete, or asphalt, or a composite. “That’s another area that needs to be quantified, so that you have a consistent approach to your inputs for your support conditions for these overlays,” says Cackler.
A third challenge is to consider what may help to enhance the concrete mixture. “Dr. Roesler is looking at flowable concretes, the use of fibers, and the strength of the mixes – those are things we need to look at to see if these thinner sections perform better with some modifications to the engineering properties of the mix itself,” says Cackler.
Letter From Iowa
Jim Cable, PhD, P.E., is president of Cable Concrete Consultation, and works with the CP Tech Center on the use of thin overlays in Iowa. In the past two years, the Iowa Department of Transportation has asked the Center to consider ways to build thin concrete overlays one lane at a time, under traffic, on a two-lane roadway.
Cable reports success in doing just that. The 4.5-inch-thick overlay was built last year on a 19-mile stretch of U.S. 18 in northeast Iowa. “It was the first official one-lane paving project that the DOT had done,” says Cable. “We came away from it with a report and about 40 recommendations of how we can do it better in the future.
Some years ago Cable led an effort to experiment with ultra-thin overlays on Highway 21 in Iowa. “Some of that pavement is still uncovered,” says Cable. “But when you get down to 2 inches, you have to be very cognizant of depth, because you want to get a uniform thickness of 2 inches. For anything less than 4 inches in depth, I will tell you that for success you need to add fibers to the concrete.”
Roller-Compacted Concrete Comes of Age
By Dan Brown, Contributing Editor
As a result of the strong interest and increased use, the American Concrete Pavement Association (ACPA) in December approved an RCC task force to refine standards and specifications, provide RCC-related training, and promote the technology. By April, ACPA had identified some 20 industry professionals who desired to serve on the task force. By June, that number had almost doubled, which is an indication of the strong interest in both the technology and the goals of the task force.
“In the United States, we’re seeing RCC being used for roadway shoulders, industrial facilities, distribution centers, haul roads – and increasingly, for streets and roads,” says Jerry Voigt, PE, president and CEO of ACPA. “We’ve also seen it used as a base course for highway and other roadway applications.”
RCC is a special type of concrete that has the same ingredients as conventional concrete but in different proportions. RCC mixtures are drier than conventional paving concrete, so it has a lower water/cement ratio; RCC mixtures typically have zero slump. As is true with conventional concrete paving, RCC mixtures sometimes include more fly ash in place of some portland cement content, and mixtures very often contain fibers for added strength.
The technology has advanced somewhat along regional lines in North America. The Southeastern United States and Western Canada have had a relatively longer history of using RCC; however, the technology is rapidly gaining acceptance in other areas. In Canada, Rico Fung, P.Eng., LEED AP, director, Markets & Technical Affairs-Ontario for the Cement Association of Canada, Toronto, reports most applications in Canada are in Alberta, B.C. and Ontario.
“RCC is very cost effective, which is a key advantage for various applications,” says Voigt. “We’re seeing owners trying RCC instead of asphalt based on its cost. RCC can be placed with various types of equipment, with best results typically coming with high-density paving equipment. RCC is placed without forms and typically does not require finishing.”
Compaction is also a key factor in RCC paving. High-density pavers equipped with tamping bars can impart much of the required density (up to about 95 percent) directly behind the paver. Heavy-duty and/or medium-duty vibratory rollers follow immediately behind the paver to help increase compaction.
RCC does not use steel (rebar, dowels, or tie bars). Aggregate interlock provides shear resistance at joints and prevents displacement or faulting. In many instances, sawed joints are not required, but for some applications, longitudinal and transverse joints are cut with concrete saws.
Set up is generally fast and relatively simple for trained crews. The material also can be placed directly on a good quality subgrade, and generally requires no finishing work. RCC is cured, but it does not typically need to be covered with curing blankets, plastic, or the like.
As one might expect, roller-compacted concrete is a very strong material. The Portland Cement Association (PCA) says it has high flexural strength – from 500 to 1,000 psi. That means it can support heavy, repetitive loads without failure, and can span localized soft subgrade areas, which reduces maintenance costs and pavement downtime.
Moreover, RCC has high compressive strength – 4,000 to 10,000 psi, so it can withstand high concentrated loads and impacts from heavy industrial, military, and mining applications. RCC is a high-density, low-water-absorption material. That gives it excellent durability, even under freeze-thaw conditions. Water does not seep through RCC.
The low water content and low water/cement ratio of RCC provide its strength, low permeability, and durability. These properties also help RCC to be resistant to chemical attack. RCC also resists abrasion, making it a great surface for large vehicles.
A Brief History
By most accounts, the U.S. Army Corps of Engineers researched RCC use in the early 1970s for dam construction.
RCC was also reportedly used in the 1970s by the Canadian logging industry, which switched to environmentally cleaner, land-based log-sorting methods.
A test plot of RCC was built at Fort Stewart, Georgia, in 1983. The plot consisted of a short road for tracked vehicles; along with the test road, two more RCC test plots were built that same year. The question of freeze/thaw durability of RCC was addressed in 1984 when a full scale test pavement at the Cold Regions Research Engineering Laboratory completed a range of climate controlled tests. Viewed as a success, the testing prompted a memorandum to field offices encouraging the use of RCC paving for “horizontal construction” where applicable for all facilities administered by the Corps of Engineers in early 1985.
Since then, the technology has expanded into other applications, including parking areas, industrial facilities, roadways, shoulders, etc. The lingering efforts of the 2007 economic slump, coupled with the volatility of asphalt supply and pricing has led to a steady increase in number and scale of RCC projects.
Task Force Focus
“RCC is not new technology, but our position is that it is new to many owner/agencies, contractors, and other stakeholders,” says Voigt. “We saw the strong interest in RCC among our members and customers, so that’s why we created an ACPA task force.”
A conference call in April set the broad direction for the task force, and also served as a forum to discuss an RCC Task Force Meeting and Workshop, held during ACPA’s mid-year meeting in June. Co-Chairs Jim Mack (CEMEX) and John Edwards (Interstate Highway Construction) presided over the workshop and meeting, which featured presentations and panel discussions that covered contractors’ perspectives, agency perspectives, and research associated with RCC.
The discussions also covered workmanship, mixture designs, equipment considerations, and specifications. At the end of the full-day event, the task force was divided into smaller groups that will focus on education and training (primarily to arm contractors with as much information as possible); specifications (to look at existing specs and evaluate gaps and other needs); and technology transfer (with the aim of creating an RCC database and other resources to communicate RCC technology and best practices).
Notwithstanding enthusiasm and a strong interest in roller-compacted concrete, ACPA says it is not a replacement for conventional forms of concrete paving (notably jointed-plain and continuously reinforced concrete pavements) but a complementary technology that should be considered seriously for certain roadway, industrial, and parking lot applications, among others.