T his lack of confidence inconcrete overlays has beenbased on a number of fac-tors, including the misper-

ception that concrete overlays areexpensive or difficult to build.

The fact is that concrete overlayscan serve as a cost-effective mainte-nance and rehabilitation solution foralmost any combination of pave-ment type and condition, desiredservice life, and anticipated trafficloading, says Dale Harrington, P.E.,one of the authors of the collabora-tive publication, Guide to ConcreteOverlay Solutions, produced by theNational Center for Concrete Pave-

ment Technology, seated at IowaState University.

Experience has shown that well-designed and constructed concreteoverlays can extend the life of exist-ing pavements for an additional 30years or more. Among the manystates using concrete overlays areColorado, Illinois, Iowa, Kansas,Minnesota, North Carolina, Michi-gan, and Oklahoma, to name a few.

Reasons to consider a concreteoverlay solution include:

■ Concrete overlays are not onlya durable rehabilitation tool; theycan also be a cost-effective mainte-nance tool. A wide range of overlay

thicknesses can be used. ■ The existing pavement does

not need to be removed for a con-crete overlay to be placed. This re-duces the cost of a concrete over-lay, compared to reconstruction.The existing pavement can provideadditional structural and load-car-rying capacity.

■ With normal concrete pavingpractices and careful attention to de-tail, concrete overlay projects can becompleted as quickly and efficientlyas any other paving method.

■ Inch for inch, concrete overlaysare one of the most cost-effectivepavement alternatives; typically,

Concrete Overlays: A Proven OptionDespite a successful track record of projects extending back several decades, somepublic agencies and contractors have been hesitant to use concrete overlays.

by the Staff of the American Concrete Pavement Association

Gratiot Avenue in the De-troit metro area received a4-inch-thick concrete over-lay that was cut into 6- by6-foot panels.

they last longer than asphalt of thesame thickness.

■ Concrete overlays are recyclable.Today’s equipment permits over-lays to be removed and reused ashigh-quality drainable base materi-al for a future pavement.

There are a number of additionalbenefits of concrete overlays thatare often not considered, says LeifWathne, P.E., director of highwaysfor the American Concrete Pave-ment Association. For example,since concrete pavements reflectmore light than asphalt, nighttimevisibility is improved and safetycan be significantly enhanced.

Energy savings can also be real-ized along urban roadways and il-luminated interchanges, since few-er/lower-power light standards arerequired to sufficiently illuminatethe roadway. Finally, because ofconcrete’s high reflectivity, concreteoverlays can also be an effectivestrategy to mitigate urban heat-is-land effects, helping to lower thetemperatures in our cities, loweringenergy demand and improving air

quality in the process.

Bonded and unbonded All concrete overlays are divided

into two families: the bondedresurfacing family and the unbond-ed resurfacing family. The key dif-ference is that with bonded resur-facing projects, the overlay de-pends on the underlying pavement— asphalt or concrete — to act inunison with the new pavement.The new pavement is designed tocount heavily on the strength of theunderlying structure. Bonded resur-facing projects require that the exist-ing pavement be in fair to good struc-tural condition.

But unbonded resurfacing proj-ects treat the old pavement as sta-ble base for the new overlay. So, infact, unbonded overlays are oftenused to rehabilitate asphalt or con-crete pavements that are in poorcondition. Unbonded resurfacingprojects do not require a bond be-tween the resurfacing and the un-derlying pavement. Both bondedresurfacing and unbonded concrete

resurfacing can be placed on exist-ing concrete pavements or on com-posite pavements where asphalthas been placed over concrete.

Care in design Harrington says that when plac-

ing a bonded concrete resurfacingon concrete, it’s very important tounderstand the characteristics ofthe existing pavement. “In temper-ature expansion and contraction,the new overlay needs to movewith the underlying concrete,” hesays. “When the joints in the oldpavement open up, you want thejoints in the new pavement to openup. If not, it could debond and sep-arate.

For bonded concrete resurfacingof asphalt pavements (previouslycalled ultra-thin whitetopping), ifthe concrete moves more than theasphalt, the two may debond, Har-rington says. “So we cut the con-crete resurfacing into small panels,and if the concrete wants to move,it won’t move very much, becausethe panels are so small.”

U sually, when a county performs full-depth as-phalt reclamation on a road, the prime con-

tractor will place an asphalt overlay on the re-claimed base material. But in Freeborn County,Minnesota, the county decided to use a concreteoverlay on the reclaimed base.

“We’ve done similar projects with bituminousoverlays, so we figured it would work just as well— if not better — with concrete,” says SusanMiller, P.E., Freeborn county engineer. “Concretewas very competitive on a lifecycle cost basis. Itturned out to be to the county’s advantage to selectconcrete for this project.”

The existing 5.9-mile stretch of County State AidHighway 46 was paved with 9.5 to 10 inches of full-depth asphalt on 2 inches of classified gravel.When the county cored the road, inspectors foundthat the asphalt had stripped from its aggregate inthe bottom. And teepee shaped cracks, 3-incheswide at the bottom, had opened up in the asphalt.

So contractors first milled off 6 inches of asphaltand hauled it away. Full-depth reclamation fol-lowed, by Progressive Contractors Inc. Three inchesof virgin aggregate was added to strengthen thebase. The reclaiming cutter passed through the ex-isting asphalt, mixed it with the new aggregate,and cut into the old subbase.

When the base had been compacted andtrimmed, PCI paved the road with 7.5 inches of

concrete. Dowel bar baskets were placed in thewheel paths only. Joints were spaced at 15 feet onthe 27-foot-wide pavement.

“With the concrete pavement, we could get ourstrength in the concrete and we didn’t need to addas much aggregate base in the section,” says Miller.“If we had used asphalt pavement, we would havehad to add 7 or 8 inches more of aggregate, andthat would have raised our profile — which wouldhave been unacceptable because of right-of-way re-strictions.”

The actual cost of the new concrete road, as bid,was $91,963 per year based on a 50-year design;the estimated cost of the road with a 12-inch aggre-gate base and an 8-inch asphalt overlay was$100,007 per year.

Concrete’s Life-Cycle Cost Wins

This concrete overlay was placed over a reclaimed asphaltbase in Freeborn County, Minnesota.

For an unbonded concrete over-lay on concrete, engineers usuallydesign a thin separation layer ofdebonding material — typically a1-inch layer of asphalt — to beplaced over the existing concrete.“We don’t have to worry about thetwo layers moving together,” saysHarrington. “In Europe, they use afabric as a bond breaker.”

Should you mill old asphalt be-fore placing the concrete overlay?“You can mill it or not, dependingon how bad the surface distortionsare,” says Harrington. “You don’twant a lot of soft spots. Concretedoesn’t need a lot of underlyingstrength but needs uniformity.Milling is probably more importantin bonded overlays, because it pro-vides a more uniform cross sectionand it helps roughen up the sur-face, which improves the bond.”

Pioneering projects Since the early 1970s, Iowa

counties have been placing con-crete overlays on both concrete andasphalt pavements, says ToddHanson, P.E., concrete engineer inthe Materials Department at theIowa DOT. In 1979 and again in1981, the state built 10-inch con-crete overlays over full-depth as-phalt on Interstate 80 in AdairCounty. “Both of those are still per-forming very well,” says Hanson.

Hanson cites a list of 17 bondedconcrete resurfacing projects —most of them done by county agen-cies — between 1973 and 1990.Thicknesses ranged up to 6 incheson the interstate highway. In 1994,Iowa State University researcherJames Cable, P.E., Ph.D., built a

7.1-mile-long research project onIowa 21 for what was called ultra-thin whitetopping, now bondedconcrete resurfacing over asphalt.Featuring 65 test sections, theproject featured concrete thickness-es ranging from 2 to 6 inches andpanels cut in sizes ranging from 2-foot to 6-foot squares.

All 4-inch or thicker sectionswere in excellent condition afterseven years. The practical mini-mum thickness of concrete turnedout to be 4 inches placed in 4-foot-or 6-foot-square panels.

Now, the state of Iowa hassome 8-inch-thick continuously re-inforced concrete pavements thatcould be candidates for unbondedoverlays, Hanson says. “Almost allthe interstates we did from 1967 to1976 were 8-inch CRC,” he notes.

The Michigan experience “We have some overlays on the

freeway system that have been

down for almost 25 years,” saysDan DeGraaf, P.E., executive di-rector of the Michigan ConcretePaving Association. He lists a 1983project on Interstate 96 betweenGrand Rapids and Lansing, one onU.S. 23 near Ann Arbor, and ademonstration section placed inthe early 1990s on U.S. 10 betweenBay City and Midland.

In 1998, the state of Michiganruled that its engineers must run alifecycle cost analysis between con-crete and asphalt for all new repairprojects. “Since then, the concreteoverlay has been the dominantwinner when there is any apprecia-ble traffic involved,” says DeGraaf.“Since that law passed, Michiganhas built at least a dozen concreteoverlay projects. On the freeway,the dominant fix is an unbondedconcrete resurfacing, typically 6.5-to 7.5-inches thick with an asphaltinterlayer.”

DeGraaf says the Detroit metroarea has hundreds of miles of dete-riorated 40-year-old concrete pave-ments and other concrete roadsthat have been overlaid with as-phalt and need work. “So we’vetaken the concept of thinner sec-tions and shorter joint spacingsand applied them as unbondedoverlays,” says DeGraaf. “We’vedone many miles of those since2000. Typically, they’re 4-inchesthick and cut in 6-foot squares.We’ve combined the two designsand it’s working very well.”

Frequently, in Michigan, theagency has placed a 4-inch asphaltoverlay over the concrete. “If you

The state of Illinois paved this concrete overlay on Interstate 70.

Photo courtesy of Randy Riley, Illinois Concrete Paving Association.

A crew from Manatt’s places a concrete overlay on U.S. 71 in Montgomery County, Iowa.

“We have some concrete overlays on the freeway system that have served wellfor almost 25 years,” says Dan DeGraaf, P.E.

try to mill off 3 inches, the wholething comes up, so usually, we justmill off the asphalt, put down thenew asphalt interlayer, and applythe concrete overlay,” says De-Graaf. “Usually, these are curb-and-gutter sections where youwant to go back to the same eleva-tion for the final surface.”

Wayne County, Michigan, getsthe credit for pioneering the thinunbonded resurfacing projects, De-Graaf says. After Wayne Countydid three or four of them, the stateof Michigan observed their successand decided to follow suit with anoverlay on Gratiot Avenue in thesummer of 2005. It was a 3-mile by9-lane-wide project with almost200,000 square yards of concrete,350 manholes, and approximately45 intersecting streets.

Lower cost Local agencies in Michigan have

done eight thin unbonded resurfac-ing projects like Gratiot Avenue.“It’s a combination of the thinbonded overlay with the thickerunbonded technology,” says De-Graaf. “We combined them andmade it a thin unbonded section.”He says the shorter joint spacing of

6-foot squares makes the 4-inch-thick sections work.

“This pavement does handleheavy truck traffic fairly well,”says DeGraaf. “But the best thingis the cost. It’s always been close toasphalt, and this year, our 4-inchconcrete overlay is less expensivethan asphalt when they use Super-pave. We used to say we last twiceas long but cost 10 to 15% morethan asphalt; now we say we cost10 to 15% less and still last twice

as long!” What’s more,

cities and coun-ties in Michiganhave built 32bonded resur-facing projectson asphalt since1996. “The cityof Grand Rapidshas done 17 ofthe 32,” saysDeGraaf.“Wherever theyhave a problemwith rutted as-phalt at inter-sections, theyuse whitetop-ping. They havereally embraced

that technology.”

Proven in Minnesota “We’ve been doing unbonded con-

crete resurfacing projects forslightly more than 20 years, and Iwould say we have more than 20projects that are performing well,”says Doug Schwartz, P.E., concreteengineer with the Minnesota DOT.“Concrete overlays are our stan-dard way of doing things.”

Schwartz points out that overlayconstruction never exposes the un-derlying subbase soil to rainstormsthat can delay construction whendoing remove-and-replace projects.Most of Minnesota’s concreteoverlays are placed over concretewith a 1-inch interlayer of open-graded asphalt to act as a bondbreaker.

And Minnesota now expects toget the same life from a concreteoverlay as from a new concretepavement. “Our first thought wasthat we’ll get 20 years out of theseoverlays, and that will be doingwell,” says Schwartz. “But now, wethink we’ll get the same number ofyears as new construction. Wewant to use methods that are mostcost-effective. That’s why concreteoverlays have been a Godsend.” BR

Placing a concrete overlay on Interstate 70 in Illinois.

Photo courtesy of Randy Riley, Illinois Concrete Paving Association.

This project in Delaware County, Iowa, is called a “reverse Oreocookie” because asphalt is sandwiched between two layers ofconcrete.

North Dakota Chapter

Reprinted with permission from Better Roads, September 2007, by M&D Printing (630) 260-1718 (171783).

“Over more than 20 years, we have more than 20 projects that are performingwell,” says Doug Schwartz, P.E., Minnesota DOT.