By: Michael Ayers, Ph.D., and Dale Harrington, P.E. Contributing Authors
It’s no secret that the world economy has descended into what some say is the worst recessionary cycle since the Great Depression some eight decades ago.
There have been few safe harbors from the downturn, which has had a significant effect on the road-building community.
All of this has required states, counties and municipalities, as well as the industry, to meet the critical needs of highways, roadways and airports with fewer resources. It is not surprising that overlays have become a mainstay of pavement preservation and rehabilitation, but what many may not realize is that concrete overlays are becoming used increasingly throughout the nation.
The life-cycle cost benefits of concrete pavements have long been understood, but more recently concrete has been specified more because it has been competitive on first costs in many applications.
Bonded separately
Concrete overlays fall into two basic categories: bonded or unbonded. Bonded or unbonded concrete overlays can be used for asphalt, concrete or composite pavements.
The decision to use either a bonded or unbonded concrete overlay depends on the objective. For resurfacing or minor rehabilitation, bonded overlays are appropriate, because they can eliminate certain surface distresses and/or add structure capacity, providing the underlying pavement is in good structural condition. A bonded overlay is typically 2 to 4 in. thick and is part of the asphalt and concrete pavement minor rehabilitation toolbox.
For minor to major pavement rehabilitation, particularly when there is structural deterioration in the existing pavement, unbonded concrete overlays are the appropriate choice. An unbonded concrete overlay, typically 4 in. to 11 in. thick, is essentially a new pavement placed on an existing, stable platform.
With both bonded and unbonded, the paving operation is the same as for conventional concrete pavement, and overlay placement can be done using slipform or fixed-form methods for hand pours.
Effective sawing and curing also is critical, in terms of both the timing and amount to be accomplished. For sawing, planning the saw cuts and having an adequate number of saws are important, particularly with bonded overlays of asphalt. Proper curing is not unique to overlays, but is a common issue on providing a uniform and fully coated surface.
Joint accountability
In addition to the preparation and curing considerations noted, there also are some additional considerations that are specific to the various types of concrete overlays. Technical documentation, such as the “Guide to Concrete Overlays,” second edition, September 2008, by the National Concrete Pavement Technology Center, should be consulted for more details, as well as other key considerations, but here are a few examples:
For bonded concrete overlays of concrete pavements, joint design and layout are very important considerations. All overlay joints must match those of the underlying pavement. Additional load-transfer devices are typically not required in the overlay, as they are already in the existing pavement. When transverse sawing, the saw cuts must be made through the entire overlay thickness, plus about ½ in. extra beyond the as-constructed (and not planned) overlay thickness. Longitudinal joints should be cut to at least half the thickness (T) of the constructed overlay, written more simply as T/2. Some states also cut the longitudinal joint to full depth of the overlay.
For bonded concrete overlays of asphalt pavements or composite pavements, transverse joints should be saw-cut to at least T/4 and longitudinal joints to at least T/3. Whenever possible, joints in the overlay should not be placed in wheel paths. As a general rule, slabs are square and the thickness correlates to the slab size. Typically the smallest panel is 3 ft square, with increases on the basis of 1 ft panel width and length per 1 in. of overlay thickness between 4 and 6 in. thick. It is recommended that the width and length of the panels in feet should not exceed 1.5 times the overlay thickness in inches. These small, square panels reduce curling, warping and shear stresses at the bond.
For unbonded concrete overlays of concrete pavements, a separator layer is required to isolate the overlay from the existing pavement (thereby preventing reflective cracking and bonding/mechanical interlocking) and to provide a level surface for overlay construction. Typically, 1 in. of asphalt is used, although a new generation of geotextile was used on a demonstration project in Missouri in September 2008 and is being evaluated for future projects.
Joint spacing for unbonded concrete overlays of concrete is somewhat shorter than conventional concrete pavement joints to help minimize curling and warping stresses. Joint spacing is typically based on thickness of the overlay and for overlays between 4 and 5 in., 6- x 6-ft panels are used; with overlays equal to or greater than 5 in. thick, the spacing, in feet, is two times the overlay thickness up to 15 ft. Some agencies have required transverse joints in the overlay to be offset from those in the existing slab, whereas others do not and have not experienced any load-transfer issues. Tiebars are normally used in overlays of 5 in. or greater.
Dowels normally are not used for thicknesses less than 8 in., but for thicker sections, particularly those carrying heavy truck traffic, dowels might be necessary in transverse joints. Placement and finishing are essentially the same as conventional concrete paving, but bridge approaches and shoulders may require special attention. The asphalt separation layer, if used, should be kept below 120°F, using surface watering to reduce the temperature. No standing water should remain on the surface at the time of overlay placement.
For unbonded concrete overlays of asphalt and composite pavements, thickness of the overlay is variable (typically between 4 and 11 in.), and placement of the concrete overlay is the same as for conventional concrete paving on an asphalt base. Load-transfer requirements will be the same as for a new concrete pavement, although the depth of the saw cuts should be adjusted to account for any increase in the concrete pavement thickness resulting from distortions in the existing asphalt pavement surface so that the required T/3 or T/4 is maintained. Finishing is the same as for concrete paving, but as mentioned throughout this article, proper curing and sawing are always important considerations.
As with all unbonded concrete overlays, the joint spacing is somewhat smaller than for conventional concrete pavements. For overlays less than 6 in. thick, the joint spacing in feet is typically 1.5 times the slab thickness in inches. For overlays equal to or greater than 6 in., the joint spacing in feet of two times the thickness in inches is often recommend, with a maximum spacing of 15 ft. Tiebars typically are included in unbonded concrete overlays of asphalt and composite pavements that are 5 in. or greater.
Over-the-top overlays
Of the many overlay projects that have been constructed in recent years, three that have captured considerable attention can be found in North Carolina, Michigan and Oklahoma.
The I-77 Reconstruction Design-Build project in Yadkin County, N.C., was constructed by Lane Construction Corp.
The use of overlay technology, and more specifically, an unbonded concrete overlay on the existing concrete roadway, helped the contractor address the challenges presented by a very tight schedule and some stringent traffic-control requirements. The project was a total of 26.1 lane-miles and used 11-in.-thick concrete for mainline sections, 13 in. in full-depth reconstruction (FDR) areas and 9 in. on ramps and loops. The overlay was placed on existing continuously reinforced concrete with 11?2 in. of surface-course bond breaker, while the FDR areas used 6 in. of stone and 4 in. of asphalt base course.
The concrete pavement portion of this design-build project was scheduled to begin in mid-September 2007 and was scheduled for completion in May 2009. The contractor completed the project in November 2008, almost six months ahead of schedule.
Because of holiday and seasonal restrictions, Lane had very short time windows to complete the work on various sections before they needed to be reopened to traffic. To address the challenge, the projects were separated into multiple sections that allowed completion of each section over a span of two to three months.
Traffic had to be in a two-lane pattern, and the contractor could not close or narrow a lane during holidays, summer weekends or during NASCAR sporting events taking place in nearby Charlotte, N.C. The design-build team solved the challenge by placing a detour the entire length of the project, allowing for a two-lane crossover pattern in the median. The challenge also was met by completing the concrete paving in phases. Because the ramps, loops and bridge tie-ins could only be closed for 11 days, the construction team worked around the clock to complete the work during these closures.
The majority of the concrete was placed directly in front of a Guntert & Zimmerman S850 paver. The paver was equipped with a spreading plow that spread the concrete before it entered the slipforms, thereby increasing productivity by eliminating a placing machine. In areas where dowel baskets were used, a GOMACO 9500 concrete spreader/placer was used. Triaxle dump trucks deposited the concrete directly onto the 9500 placer, and with the use of conveyor belts, the concrete was spread evenly in front of the paving machine.
Another innovation used on the project was wireless maturity testing. The process, which involved a wireless, handheld device that collected data from microprobes in the concrete, helped estimate the time at which the new concrete obtained sufficient strength to allow construction equipment onto the slabs.
In Oklahoma, a concrete overlay was used on project NHY-033N (012). U.S. 59 passes through some very scenic views in Sequoyah County. Motorists travel along steep, 7% grades past Wildhorse Mountain and along Robert S. Kerr Lake.
A one-year-old, 10-in. asphalt pavement placed on 8 in. of lime-treated subgrade in this location was shoving, heaving and rutting severely. Facing imminent and complete failure of the pavement, the Oklahoma Department of Transportation (ODOT) decided to correct the problem with a 7-in. doweled, jointed plain concrete pavement. Like many agencies, ODOT was facing a shortage of roadway funds and had to use maintenance funds to repair the roadway. Luckily, bids for the project came in 22.5% below the engineer’s estimate.
Time was critical, as only 30 calendar days were allotted for the project, which involved 1.61 miles of four-lane divided highway, for a total of 7.05 lane-miles. The work was done in two phases and involved moving daily traffic (estimated at 7,120 vehicles/day, of which 21% was truck traffic) in a two-lane, head-to-head configuration while the adjacent side was repaired.
When Duit Construction Co. arrived on the scene, the northbound lanes had to be closed before they failed completely. The contractor crews repaired and overlayed the northbound lanes first, then switched traffic to complete the southbound lanes, ultimately finishing the job within the 30 days.
During the planning stage, typical sections were drawn by hand in a set of contract plans just three pages long. ODOT District I Engineer Darren Saliba and his staff were credited with the idea of sketching roadway plans in an accurate and timely manner. This allowed ODOT to bid the project quickly to make the repairs before the roadway failed.
The project involved a 14-ft truck (right) lane and a 12-ft passing (left) lane, with an 8-ft shoulder outside the truck lane.
An optimized gradation mixture design using the Shilstone method was credited with creating a concrete pavement that is strong, long lasting and cost effective. The average compressive strength of the concrete was 4,460 psi for the mainline sections. ODOT uses what is called a Class AP concrete on the shoulders. The mixture is about 1,200 psi less than mainline sections.
Although there were some concerns about using dowels on such thin slabs, as well as with using an automated dowel-bar inserter, Duit credited proper stringline practices and proper maintenance of the automated dowel-bar inserter with a problem-free approach to the concerns.
One other challenge was that, just to the north of this project, there was another reconstruction project going on, making coordination of the projects challenging and adding complexity to the lane switches and overall traffic management.
In Michigan, the Michigan Department of Transportation’s (MDOT) final link in the improvements on U.S. 131 from Ann Street in Grand Rapids to 17 Mile Road in Cedar Springs was a 6.5-in. concrete overlay project of 26 lane-miles of the highway.
The U.S. 131 corridor north of Grand Rapids has experienced tremendous growth, and following that growth, traffic volumes have increased significantly.
One of the challenges of this project was a bridge over 6 Mile Road on southbound U.S. 131. The bridge is narrow and was not able to handle conventional construction traffic configurations. The project used MDOT’s split/merge traffic plan, which enabled two lanes of traffic to be maintained in each direction.
There was a considerable effort to correct the old roadway’s parabolic surface, superelevation and crown- cross slope to bring the road up to current standards.
Ajax Paving Industries Inc. placed the 6.5-in. unbonded concrete overlay on concrete, using a 1-in. asphalt separator layer prior to placement of the concrete surface course over the existing 9-in. jointed reinforced concrete pavement. The original pavement was on a 6-in. aggregate sub-base on 12 in. of sand sub-base. Some full-depth reconstruction was required at areas under bridges where the roadway passed over the freeway or transitioned down to bridge decks on the freeway and over streams or local roads. (The areas requiring reconstruction used 10.5 in. of concrete.)
A Shilstone-type well-graded concrete mixture, with three aggregates, was used. The mixture, which contained 40% slag cement, used only 294 lb/cu yd of portland cement.
Prior to beginning the paving, the contractor identified areas on the existing pavement that had “tented” because of incompressibles in the joint and improper expansion-joint maintenance of the original jointed reinforced concrete pavement, which was more than 40 years old. The repairs had to be made in such a manner that the additional traffic from the traffic-control switch would not degrade the roadway during construction.
Another challenge was the peak-hour traffic, due to the highway’s use as a commuter route and a weekend destination route to northern areas of Michigan.
Editor’s Note:
Portions of this article were based on the “Guide to Concrete Overlays,” by Dale Harrington, P.E., et al. (2nd Edition, September 2008). This technical resource is available as a commercially printed publication from the American Concrete Pavement Association, as well as an electronic (PDF) version available online.
Printed copies are available for $7.50 for members of ACPA and $30 for non-members. To order printed copies, please contact ACPA, 5420 Old Orchard Road, Skokie, IL 60077-1059, or call 847/966-2272. Be sure to specify the desired quantity and the publication number (TB021.02P). Alternatively, you may view or download free electronic copies (PDFs) at www.cptechcenter.org.
About The Author: Ayers is with the American Concrete Pavement Association, Skokie, Ill. Harrington is with Snyder & Associates, representing the National Concrete Pavement Technology Center, Iowa State University.
