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May 19, 2011

Road agencies face a critical 21st century challenge: enhance the system’s sustainability.

In the context of pavements, enhancing sustainability means reducing pavements’ environmental footprint, while increasing their contributions to the country’s economic and social well-being.

What if:

Road agencies face a critical 21st century challenge: enhance the system’s sustainability.

In the context of pavements, enhancing sustainability means reducing pavements’ environmental footprint, while increasing their contributions to the country’s economic and social well-being.

What if:

  • New or improved sustainable technologies could be implemented to solve these challenges while saving costs, accomplishing more with the budgets road agencies have?
  • The country’s 1.3 million miles of deteriorated pavements could be rehabilitated quickly for only a fraction of the cost of reconstruction?
  • Agencies could extend the return on their original pavement investments for years, simply by taking advantage of equity remaining in deteriorated pavements?
  • A pavement’s structural capacity could be enhanced to meet changing traffic needs quickly and cost-effectively?
  • Work zones occurred less frequently and for shorter periods, allowing road workers to simply get in, get out and stay out?

“What-ifs” like these are becoming realities through the collaborative efforts of public agencies, industry and universities. At the hub of these efforts is the National Concrete Pavement Technology Center (National CP Tech Center) at Iowa State University. With federal, state and industry support, the center is a vital resource for the concrete-pavement community.

To turn “what-ifs” into realities, the center facilitates strategic short- and long-term partnerships among public and private organizations to advance underused or potential engineered sustainable pavement solutions. Concrete overlays provide one such solution.

This article provides background on concrete overlays and describes a recent research project from the National CP Tech Center that focused on developing methods to improve concrete overlay construction efficiency using existing technologies, investigating innovative materials for use as bond breakers and reducing the inconvenience to the public with improved methods of traffic control and earlier opening times.

Overlay overview

Many deteriorating pavements are good candidates for concrete overlays, which are versatile, sustainable pavement repair/rehabilitation solutions. But concrete overlays have been underused, primarily due to the lack of knowledge about their many applications and benefits:

  • Concrete overlays can be short- or, more important, long-term solutions for existing concrete and asphalt pavements;
  • If a good pavement simply needs to carry heavier loads, a thin bonded overlay creates a thicker, monolithic slab;
  • If a pavement is deteriorating, a thicker unbonded overlay acts like a new pavement, with the old pavement as the base;
  • Every concrete overlay reuses the existing pavement, which continues to contribute to the overall structure;
  • Most concrete overlays are constructed quickly, with minimum traffic disruption; and
  • Concrete overlays generally last longer than oil-based resurfacing solutions, so their life-cycle costs are lower.

To help agencies gain expertise in implementing concrete-overlay solutions, the National CP Tech Center is conducting an intense information and training campaign:

  • In 2006, the center assembled a national team of expert practitioners to develop a straightforward, illustrated guide to concrete-overlay best practices. In 2007, 10,000 copies were distributed;
  • In 2007–08, the center facilitated training for nearly 500 professionals at 20 workshops around the country;
  • Following initial workshops, the team enhanced the guide with suggestions for evaluating current pavements and selecting appropriate overlays, setting up efficient work zones and paving with reduced clearances. In November 2008, 7,500 copies of the revised, second edition of the guide were distributed; and
  • The center has provided technical support to 16 states that were previously not knowledgeable about concrete-overlay technology.

Tying it to a project

To provide agencies with additional knowledge about the use of concrete overlays, the Iowa Highway Research Board (IHRB project TR-600) and the Federal Highway Administration (FHWA) funded a project to develop additional design and construction information to accompany the guide. The objectives of this project were as follows:

1. Evaluate the feasibility, benefits and limitations of GPS mapping of the existing roadway surface prior to construction and the use of machine-control systems for slipform pavers;

2. Develop methods to ensure that the longitudinal joints in the overlay match underlying joints in the existing pavement;

3. Establish methods to determine the level of milling required for existing asphalt surfaces to reduce project overruns;

4. Evaluate the use of innovative materials, such as geotextile layers, for use as bond separators;

5. Evaluate the feasibility of overlaying multilane roads a single lane at a time while maintaining traffic through the work area;

6. Investigate methods to reduce paving train length and width and reduce the time of construction operations; and

7. Determine the opening strength required to allow access by local traffic and construction equipment.

Parts of an investigation

In order to investigate the needs and concerns noted above, four concrete overlay projects were selected. These projects were already under development by the Iowa DOT and Osceola, Poweshiek, Worth and Johnson counties. A portion of the research objectives was implemented on each of the projects. One of the projects, State Rte. 9 in Osceola County, contained the following project details:

  • Existing 18-ft-wide PCC pavement with 4- to 8-ft-wide shoulders; widened to 24-ft-wide roadway with 4.5-in. HMA overlay;
  • Constructed a 5.5-in. PCC overlay, including a 2-ft widening section on each side (final pavement width of 28 ft);
  • 5-ft tie bars were installed over the widening units; and
  • Traffic was detoured off-site during construction.

The following is a summary of key recommendations broken down into key segments:

GPS pavement surface mapping

At a minimum, existing surface profile information should be gathered prior to final design along pavement edges and at the centerline. Additional surveys should be conducted at the quarter points and in each wheel path if the existing surface is badly rutted.

GPS should be utilized in conjunction with lasers, ultrasonic, radar and other technologies to map the surface of existing pavements prior to concrete overlays.

Longitudinal joint formation with GPS-controlled saws

Contractors should consider the use of GPS-guided saws to accurately match new longitudinal joints in concrete overlays to those in the underlying pavement.


Contractors should analyze the additional cost of milling versus the savings in concrete yield during the design phase.

A 12-ft-wide milling machine with closely spaced teeth should be used to improve the overall cross slope of the finished surface and improve the yield of the concrete.

A mill head that is wider than the pavement-widening unit should be used to develop an improved paver pad line, better drainage and improved ride of the final product.

Slipform paver machine control

Contractors should investigate ways to improve concrete yield with prior planning of highway agency goals, mapping of the existing surface and development of tight vertical control point systems along the pavement prior to design and construction.

Contractors should consider the opportunity to demonstrate stringless controls with a combination of GPS, lasers and total stations.

Geotextile bond breaker

Contractors should consider the use of a geotextile bond breaker as an economical alternative to HMA for unbonded concrete overlays.

Contractors should install bond no more than 24 hours before paving to reduce potential for wind and traffic damage.

Concrete opening strength for local traffic use

Contractors should consider using maturity measurements to open local-resident access points less than 24 hours after paving with flexural strengths of 350 psi.

Contractors should consider using maturity measurements to begin shouldering operations less than 48 hours after paving with flexural strengths of 500 psi.

Maturity curves specific to the concrete-mix materials being used should be developed for each project.

Traffic control for one- and two-lane overlay construction

Contractors should weigh the additional costs for maintaining through-traffic and building the overlay in stages against the additional inconvenience of closing the roadway and placing a single- or dual-lane overlay.

Overlay construction operation timing

Perform utility relocations, drainage improvements and pavement-survey activities prior to the overlay project to minimize road-closure times. Maintain through-traffic with flaggers during initial improvements such as sub-drain installation, pavement milling and pavement patching.

A total road closure with detours and two-lane overlay construction should be considered to minimize the overall duration of the project.

FWD testing

FWD testing should be performed before and after the overlay to measure deflection reduction and to verify the adequacy of the overlay depth.

Thickening the overlay research

The National CP Tech Center is currently developing a “Guide for Existing Concrete Overlay Design Methodology” that will highlight best practices for constructing concrete overlays using existing methodologies.

In addition, future research projects on concrete overlays will investigate the following:

Longitudinal joint formation for bonded overlays

The use of manual measurements from hub lines to existing centerlines prior to overlay and re-establishment of those points on the surface prior to the centerline sawing of the overlay.

Surface mapping

The use of LiDAR, slow-moving GPS units or robotic total station work to accurately map the road surface prior to overlay design.

Surface milling

The amount of concrete required to fill the milled surface from a single lane with coarse or widely spaced cutting head and a narrowly spaced cutting head. The yield in overlay concrete of a surface milled from the paving grade line to one that is independent of the paving grade line. The same comparison can be made between a milling unit using only GPS control to one with total-station control.

Machine control

The potential for a survey of the actual pavement overlay surface elevations at the centerline and pavement edges of the finished product to ensure quality compliance with the paving model. The potential of using a combination of lasers, total stations and GPS for machine control of the slipform paver. The use of the ski or moving stringline to achieve overlays and still maintain smoothness specifications.

Opening strength

Effect of shouldering activities at varying concrete flexural strengths (350, 400, 500 psi) to determine the impact on the durability of concrete-overlay-pavement edge.

About The Author: To learn more about concrete overlays, visit www? or contact National CP Tech Center Director Tom Cackler at 515.294.5798; e-mail: [email protected]. For more details about the project described in this article, view the project web

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