Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/620628
EquipmentWorld.com | November 2015 45 Concrete overlay types and applications C oncrete overlays are classifi ed in two categories—bonded and unbonded. Both types can be placed over asphalt, concrete or composite (asphalt over concrete) pavement types, but their application is specifi c to the condition of the existing pave- ment and the traffi c volume of the roadway. Historically, about 55 percent of concrete overlays are used on concrete, followed by 40 percent on asphalt and 5 percent on composite pavements. (source: "Guide Specifi cations for Concrete Overlays" National Concrete Pavement Technology Center) Bonded Bonded overlays are thin and are used more for preven- tive maintenance and minor rehabilitation; the roadway requires detailed surface preparation to be effective. Material thicknesses fall in the 2-to-5-inch range. Bonded overlays over asphalt have been commonly referred to as ultra-thin whitetopping (UTW). The as- phalt needs to be in structurally good condition, with surface defects including rutting, shoving, slippage and thermal cracking, and needs to carry some of the traffi c load. Concrete pavements that need more structural ca- pacity, better rideability, skid resistance and refl ectiv- ity, but are "structurally sound" are good candidates for bonded overlays. Composite pavements require similar conditions to asphalt pavements. Roughly 25 percent of concrete overlays are bonded. Unbonded Unbonded overlays are usually used in major rehabilitation work although it can also be applied on minor rehabilitation jobs. The design life is longer, with less surface preparation required due to the thicker application of material. Material thickness ranges from 4 to 11 inches. An unbonded overlay is the best option for an asphalt pavement that is deteriorating or considered to be in poor condition, with conditions such as severe rutting, alligator cracking, pumping, shov- ing and potholes. This is also known as conventional whitetopping. Depending on the condition of the roadway, spot repairs may be needed prior to resurfacing. Unbonded overlays are desirable when composite pavements are in similar condition. Concrete pavements taking unbonded overlays exhibit structural deterioration, such as scaling, shattering, transverse or longitudinal cracking, pumping, D-cracking (freeze-thaw deterioration) or cracking from alkali-silica reaction (ASR). Spot repairs may also be required. Approximately 75 percent of concrete overlays are unbonded. all aimed at helping people become comfortable with concrete overlays and honestly, dispel some of the myths, such as 'nobody has used it'," Wathne says. The specifi cations he references include the "Guide Specifi cations for Concrete Overlays" published this September by the National Concrete Pavement Technol- ogy Center (cptechcenter.org) at Iowa State University's Institute for Transportation. The 32-page document offers guidance for developing specifi cations to meet the needs of overlay projects and covers details including the type of overlay to be used and the condi- tions in which they can be applied (see sidebar below). Wathne says the concrete pave- ment industry's advocacy efforts in part were an attempt to neutral- ize concerns about using concrete overlays, and highlighting the success of past projects. "FHWA has had an overlay implementation program that's been helpful as well, so we're all kind of "singing off the same sheet of music" in talking about the opportunities with con- crete pavements," Wathne adds. Competition and price parity There is a direct correlation be- tween this thrust and the increase in concrete overlay use by trans- portation agencies, Wathne says, because it has shown another resurfacing option besides tradi- tional asphalt overlays. "We want agencies to be aware that there's another solution out there. Even if the benefi ts of these solutions are identical, just having a competing industry adds pressure to the in- dustries to be more mindful of cost effectiveness," he says. Wathne also points out that many municipalities look at their state DOT to make the fi rst move in adopting new specifi cations and guidelines as well as materials. "When we talk across our chapters, the conversation we hear from counties is that they're waiting for the state," Wathne says, although some large municipalities may lead the way in adoption. Even so, Wathne says that sev- eral county and municipal agen- cies that may not have traditionally looked at concrete overlays in the past 15 to 20 years are now start- ing to consider them for long-term project solutions. "They're looking at projects with the mindset that they want to do projects that they don't have to come back to for longer periods of time. That's what they're planning on doing and they see that they have a long-term view," he adds. Concrete pavement growth Wathne says concrete pavers are recognizing that overlays are a huge market opportunity and repre- sent the future of concrete. "The vast majority of the network has been built, so we recognize that one of the more cost effi cient ways, short of reconstructing, is us- ing concrete overlays," he says.