| October 03, 2011 |
What About Overlays?
Agencies looking for dollar value balanced with road quality are looking at concrete overlays
By Daniel C. Brown
More than ever before, state departments of transportation (DOTs) and other roadbuilding agencies are choosing concrete overlays as a way to rehabilitate and preserve their existing pavements. Concrete overlays allow agencies to save the cost of removing an existing pavement. And significantly, concrete is generally more competitive these days with asphalt on a first-cost basis, as well as a lifecycle basis.
“Concrete overlays allow us to take the substantial equity that we have in our existing pavement system, which generations before us have invested in, and to hold onto that investment,” says Dale Harrington, P.E., principal senior engineer, Snyder & Associates, on behalf of the National Concrete Pavement Technology Center. “We can hold that investment by utilizing the strength of that existing pavement, be it in concrete or asphalt, and enhance it with a concrete overlay. That way, we bring the pavement into a whole new cycle without having to destroy the existing slab.
“The other major benefit of concrete overlays is sustainability,” says Harrington. “By leaving the existing pavement in place, and not wasting it, we are making the pavement even more sustainable. That is a huge plus!”
What’s more, concrete overlay technology has matured, which helps spur growth, says Leif Wathne, P.E., vice president of highways and federal affairs, American Concrete Pavement Association (ACPA). “In large parts of the country, there has been phenomenal growth over the past five years in concrete overlays,” says Wathne.
“Concrete overlays give states and other agencies an opportunity to seize a long-term solution,” says Gerald F. Voigt, P.E., ACPA president and CEO. “Most of the DOTs are operating under tight budgets these days, and we don’t have a new highway bill. A concrete overlay does not require an expenditure to tear out the existing pavement. Overlays are quite adaptable to a tighter budget scenario.”
Concrete overlays come in two primary categories: bonded and unbonded. The key difference is that with bonded overlays, the new structure depends on the underlying pavement – asphalt or concrete – to act in unison with the overlay. The new pavement is designed to count heavily on the strength of the underlying structure. Bonded overlays require that the existing pavement be in fair to good structural condition, and they are generally thinner than unbonded overlays.
Unbonded overlays treat the old pavement as a stable base for the new overlay. Unbonded concrete overlays, which are generally thicker than bonded overlays, are often placed to rehabilitate asphalt or concrete pavements that are in poor condition. Unbonded overlays do not require a bond between the new overlay and the existing pavement.
Both bonded and unbonded concrete overlays can be placed on three types of pavements: concrete, asphalt, or composite structures. Composite pavements are generally those where asphalt has been placed over concrete.
“The adoption of concrete overlays by DOTs has enabled them to introduce competition in a market segment where they traditionally have not seen much competition between the two industries,” says Wathne. “That competition enhances the ability of agencies to stretch their highway dollars.”
From 2004 to 2009, concrete overlays of 6 inches or less have grown from about 1.2 million square yards to about 5.4 million square yards, ACPA data shows. And according to bid-letting information, a total of 17 million square yards of overlays were built in 2009 and 2010 – and about half of that was composed of thinner overlays. “Most of it was built on existing asphalt pavement,” says Voigt. “So that is just a huge jump from what we have seen in the past.”
Wathne says that when you introduce the concrete pavement industry into the rehabilitation market segment, that not only lowers prices, it stimulates innovation. “You have another industry in there challenging the existing industry, and that is a good thing for the customer, which in this case is the DOT or the public sector, actually,” says Wathne.
In terms of innovation, ACPA’s Voigt points to a concrete overlay project happening this year on U.S. Highway 18 in Chickasaw and Fayette Counties in Iowa. There, contractor Manatt’s, of Brooklyn, Iowa, is building an unbonded overlay on 19 miles of two-lane roadway. This is the first such concrete overlay to be built one lane at a time, under traffic. Stringless concrete paving, by which automated machine controls guide the paver, opens up the project and allows concrete trucks access from the centerline side. A 24-hour pilot car will lead a group of vehicles first in one direction, then in the other.
The existing roadway is a 7-inch-thick concrete pavement topped by a 6-inch asphalt overlay. The asphalt overlay will be milled down by 1 to 1.5 inches, and a 4.5-inch concrete overlay will be applied, says Todd Hanson, P.E., a PCC engineer in the Materials Division of the Iowa DOT.
As evidence of the growth of concrete overlays, Voigt points to Kansas, where the DOT has recently let approximately 1.5 million square yards of concrete overlays for Interstate 70. “Those overlays are what we call a ‘6-by-6-by-6’ overlay – 6 inches thick and 6-foot-by-6-foot panels,” says Voigt.
“There is an upcoming national open house on that,” Voigt says. “It is essentially a mill-and-fill using concrete instead of asphalt. They are milling down and placing concrete to re-establish the grade that was there.”
In Missouri, MoDOT has built some 25 concrete overlay projects over the past 12 years, says John Donahue, P.E, construction and materials liaison engineer. Most of those projects have been built on Interstate highways, but some have been paved on lower-volume roads as well. Donahue says Missouri has placed several unbonded concrete overlays that use a geotextile as the bond breaker between the old concrete and the new.
“And we have done, to a lesser extent, a number of thin bonded concrete overlays,” says Donahue. “Typically those are about 4 inches thick. In that case, we actually try to get a bond with the asphalt below. We have typically used those at intersection locations where we have had historical rutting occurring on the asphalt.”
A third type of overlay in Missouri is what the state calls its “big block design.” It is an unbonded overlay – thinner than the state’s conventional concrete overlay, which is 8 inches thick. “We would construct it at 5 or 6 inches thick, and place it down on an asphalt surface, or even a concrete surface if we use the geotextile interlayer,” says Donahue. “Then we saw the pavement into 6-foot-by-6-foot panels.”
ACPA and the National Concrete Pavement Technology Center (CP Tech Center) provide instructional information and provide training related to concrete overlays. ACPA’s education and training program, for example, focuses on delivering training on various aspects of design, construction and pavement rehabilitation using concrete overlays.
The CP Tech Center offers workshops to state agencies on concrete overlays through the National Concrete Consortium. The consortium is a group of 21 states that have pooled their resources and joined together to work on concrete pavement issues and solutions, Harrington says. This NC2 group meets annually to hear speakers and discuss concrete pavement issues.
The CP Tech Center, in partnership with the Federal Highway Administration (FHWA), also provides technical support to DOTs that are considering such overlays. “We have been providing technical experts to assist the DOTs with identifying suitable candidate pavements for the overlays, helping them with the specifications and assisting with construction support,” says Tom Cackler, P.E., director of the CP Tech Center.
Harrington, who works full time for the CP Tech Center and is its former director, says concrete overlay expert teams have visited 22 different sites across the nation over the past two-and-a-half years. An example comes from South Dakota, where the DOT requested an expert team’s services.
The project was constructed on South Dakota Route 50 in 2009. “It was an existing 8-inch concrete pavement with 10-inch shoulders,” says Harrington. “The South Dakota DOT placed an asphalt interlayer as a bond breaker, then a 7-inch unbonded concrete overlay.
“The Center’s team, including FHWA, completed a presentation on overlays to the South Dakota DOT and gave them a site evaluation field review of Highway 50. The team provided comments on design and specifications as they were being developed. Finally, the team provided guidance during the construction and sent the Center’s mobile laboratory to the project for a week. The lab completed testing of material properties for South Dakota DOT. The Center’s team assistance was provided at no cost. When the project was complete, we sent them a final report and evaluation.”
Harrington says when the expert team program concludes in mid-2012, the CP Tech Center will publish a report on all the sites that have been visited. FHWA has requested that the full report include lessons learned about concrete overlays during the expert team consultations. That full report will be shared with all state transportation departments, Harrington says. States visited to date include Delaware, Georgia, Illinois, Louisiana, New Mexico and Virginia. Others involved include Maryland, Minnesota, Nevada, North and South Dakota, Pennsylvania, Texas, Washington state and West Virginia.
“So far, four states have done the actual construction,” says Harrington. “Some of them, such as Virginia, are working on them right now. Virginia built one this year; the others will build projects as they get the funds to do so.”
In North Dakota, the state has completed two concrete-on-asphalt overlays, says Clayton Schumaker, an assistant materials engineer with the DOT. The most recent one was finished this summer – a 5-mile project on ND Highway 200 near Hillsboro. A portion of the existing pavement consisted of asphalt with an aggregate base, and the remainder was a composite pavement with a concrete base paved in 1948.
The roadway consists of several segments, so various overlay thicknesses were used – ranging from 5 to 7.5 inches – to account for the different existing pavements and varying traffic loadings. Two major industrial facilities bring heavy truck traffic on ND 200 in the project area – and both facilities requested concrete, based on its performance since 1948, Schumaker says.
“One consideration was cost, and our engineer’s estimate says the concrete overlay is less expensive than doing full-depth asphalt reclamation then placing an asphalt overlay,” Schumaker says. “We’re looking at concrete overlays for a number of other projects where we have high-traffic volumes and the rehabilitation strategies are limited to full-depth reclamation, total reconstruction or the concrete overlay.”
In addition to technical support from the expert teams, the CP Tech Center has published “A Technical Summary of the Design of Concrete Overlays Using Existing Methodologies,” dated May 2011. The document provides an overview of the concrete overlay design process and identifies some of the more sensitive variables inherent with three different software procedures: (1) the 1993 AASHTO Guide for Design of Pavement Structures; (2) the Mechanistic-Empirical Pavement Design Guide (MEPDG); and (3) the ACPA method for bonded concrete overlays on asphalt pavements (BCOA).
The first method, the 1993 AASHTO Guide, is the procedure most commonly used today for concrete overlay thickness design. The MEPDG, and more precisely, the recently released DARWin-ME program, is being implemented by numerous states. Finally, ACPA’s BCOA method is presented to address the unique behavior of thinner-bonded concrete on asphalt. The technical summary is expected to be released later this year.
As state transportation agencies and roadbuilders grapple with the persistent economic concerns, a growing number of them are increasingly turning to concrete overlays to provide high-quality, durable, sustainable and economically-viable solutions. For more information on concrete overlays, visit acpa.org.
ACPA Apps can do a lot of work for you
By Robert Rodden, P.E.
Director of Technical Service and Product Development
American Concrete Pavement Association
It is the era of technological wonders in communication. Consider, for example, how smartphones are becoming dominant features of the communication landscape.
Keeping pace with this important technology wave, the American Concrete Pavement Association (ACPA) introduced its first web-based applications last November, and has since introduced more than 40 web-based tools, or web applications, via a special web portal, http://www.apps.acpa.org. ACPA has released new iPhone and iPad applications.
The web apps are divided into several categories including some general interest apps, design apps, and construction and pavement analysis tools, as well as interfaces to design software. Although these apps are germane to concrete pavement placement, repair or preservation, many of these web-based tools are more general and will help people with broader construction interests.
One very popular web app is ACPA’s National Concrete Overlays Explorer at http://www.apps.acpa.org/apps/OverlayPass.html. The exploration begins with a map featuring push pins, and adjacent to it, a sidebar menu of project details that serve as filters. By activating a push pin or selecting from the sidebar menu, the user can view project construction details, photos, performance information and much more. The app is designed to answer questions agencies, consultants and contractors have about key project details, for example where concrete overlays have been used and how they have performed.
The general-interest apps include a database of state agency practices, a glossary of concrete pavement industry and general transportation-construction terms, a units converter with many industry-specific conversions not included in other online converters, and other useful tools that can benefit construction professionals on the grade or in the office.
Design apps include a bonded concrete overlay on asphalt thickness designer. Developed by ACPA with support of the Federal Highway Administration (FHWA), the National Concrete Pavement Technology Center (CP Tech Center) and the Illinois Center for Transportation (ICT) at the University of Illinois at Urbana-Champaign (UIUC), this application is based primarily on the results of FHWA-ICT-08-016, “Design and Concrete Material Requirements for Ultra-Thin Whitetopping.” This is the formal name of a research project conducted in cooperation with the ICT, the Illinois Department of Transportation (IDOT) and FHWA.
The online thickness designer allows pavement design professionals or others with engineering expertise to enter general design factors, existing pavement structure details, concrete material details and factors related to the concrete overlay to determine the necessary bonded concrete overlay thickness.
Among the 18 design apps are a Westergaard stress and deflection solver, a compression seal joint width calculator and an online thickness designer based on ACPA’s popular StreetPave software for conventional jointed plain concrete pavements. Also included is an online k-value calculator, as well as an equivalent single axle load (ESAL) calculator that allows users to estimate future or historic traffic counts.
Construction and pavement analysis tools include an extremely popular evaporation rate calculator; a strength converter that converts between compressive, flexural, split tensile strengths and modulus of elasticity; a pavement joint noise calculator, and a concrete temperature calculator. Users also can find highway specifications used in airport pavement applications, calculate area and volume, determine staking intervals and the maximum recommended joint spacing, and perform more than a dozen different construction and pavement analysis operations using ACPA’s web apps.
In the pavement design software section, there are interfaces to evaluation versions of ACPA’s StreetPave roadway pavement design tool, WinPAS roadway pavement thickness design and evaluation tool (based on the 1993 AASHTO Design Guide for Pavement Structures), AirPave airport pavement design tool, and the newest design tool in ACPA’s software suite, PerviousPave, a pervious concrete pavement design tool. The pavement design software section also includes links to:
• COMPASS: computer-based guidelines for job-specific optimization of paving concrete. This tool is currently under review by the FHWA and is not an official set of guidelines, yet. When accepted by FHWA, it will result in a truly performance-driven mixture design system.
• HIPERPAV III (HIgh PERformance Concrete PAVing) software, developed by the Transtec Group for the FHWA, and used to analyze the early age behavior of jointed concrete pavements continuously reinforced concrete pavements and bonded concrete overlays.
• EverFE 2.24, a 3-D finite-element analysis tool used for simulating the response of jointed plain concrete pavement (JPCP) systems to axle loads and environmental effects. EverFE was jointly developed by the Universities of Maine and Washington with funding from the Washington and California State Departments of Transportation.
• And DowelCAD 2.0, a software program created collaboratively for optimization of dowel bar design. The software also allows engineers to determine joint responses to varying dowel sizes or investigate the impact of various alternate dowel bar configurations and shapes.
Keeping pace with web-based technology also means meeting the growing demand among smartphone users. ACPA has developed a number of iPhone and iPad apps, and continues to monitor use of other platforms, including Android, with an eye on further development to meet demand. The current list of ACPA’s iPhone and iPad apps includes:
Area and Volume Calculator: An app that allows users to quickly calculate plan area and volume of material based on a pavement or subbase/subgrade layer’s thickness, width and length.
Evaporation Rate Calculator: This app uses the evaporation rate equations in Paul Uno’s ACI Materials Journal article, “Plastic Shrinkage Cracking and Evaporation Formulas,” (July/August 1998). The equations are based on the popular evaporation rate nomograph from ACI 305R, Weather Concreting. Using this tool, users can quickly calculate the evaporation rate at hourly intervals based on concrete temperature, air temperature, ambient relative humidity and wind velocity.
Concrete Mixture Proportioner: Provides a method of proportioning a concrete mixture using the absolute volume method in substantial conformance with ACI 211.1-91, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete.”
Maximum Joint Space Calculator: Allows users to calculate joint spacing in jointed plain (unreinforced) concrete pavement.
Staking Interval Calculator: Calculates the rate of change of a vertical curve and, based on this value, provides maximum staking interval recommendations.
Subgrade Resilient Modulus Calculator: Based on the conversion factors included in NCHRP Report 128, “Evaluation of AASHTO Interim Guide for the Design of Pavement Structures,” this tool allows users to quickly estimate the Subgrade Resilient Modulus (MRSG) from either a California Bearing Ration (CBR) or Resistance Value (R-value) measurement.
Joint Noise Estimator: An app based on the work of acoustics expert Dr. Paul Donavan, Sc.D., a senior scientist at Illingworth and Rodkin. This tool allows designers to estimate the impact of various joint geometries and condition on the overall tire-pavement noise level. It may also be used to guide pavement maintenance efforts in terms of the noise-related benefits attainable from sealing joints.
These mobile applications are available in Apple’s iTunes App Store store (http://www.store.apple.com/) for a nominal fee. Users of iPhones and iPads who click on the iTunes App Store link on the ACPA App Library site will be taken directly to an application purchase site; all other users will be directed to landing pages that describe the product and include ordering information.
For additional information about the ACPA web apps and links to software programs, contact the author at 847.423.8706 or firstname.lastname@example.org. v