Equipment World Digital Magazine
Issue link: https://read.dmtmag.com/i/473140
'Thin' remains 'in' Thinner overlays of portland ce- ment concrete remain a high prior- ity for the concrete paving industry, but while hyperthin PCC overlays have been placed as pavement preservation measures, they are uncommon and most overlays don't approach the thinness of as little as 1 or 2 inches the National Asphalt Pavement Association is promoting with Thinlay. "There are no hard and fast rules as to what's thin or not, but I would say 4 to 6 inches constitutes the thinner side of what we do," said Jerry Voigt, president and CEO, American Con- crete Pavement Association. He added the volume of these thin- ner concrete overlays has increased in recent years. "Our volume of paving fluctuates every year, and we monitor that," Voigt told Equipment World. "We also monitor overlays, and we now are between 10 to 15 percent of our total volume of paving now in concrete overlays. Eight to 10 years ago it was less than 5 percent, so we are excited about that growth. And half of today's overlay volume is 6 inches or less." Also, ACPA has seen a dramatic change in application of overlays. "We've seen a shift in concrete overlay placement," Voigt said. "They used to take place mostly over concrete. Now they're mostly over asphalt. Some two-thirds of the con- crete overlays being placed are going down on asphalt pavements." Why the growth in concrete over- lays? "Agencies can't reconstruct ev- erything that they might want to, so they are looking at different options for pavements," Voigt said. "And they have been turning to the option of concrete overlays more than they have before." This growth in thinner concrete overlays takes place as the industry refines designs and terminology. "In the day, we used the term ultrathin whitetopping, which generally was used for concrete on asphalt," Voigt said. "People still use those terms, but we have de-emphasized them. We prefer to talk about concrete overlays being bonded or unbond- ed, or on concrete or on asphalt." This has led to new terms describ- ing concrete overlays: s "#/! BONDED CONCRETE OVERLAY on asphalt, formerly called thin or ultrathin whitetopping s "#/# BONDED CONCRETE OVERLAY on concrete s 5#/! UNBONDED CONCRETE OVER- lay on asphalt s 5#/# UNBONDED CONCRETE OVER- lay on concrete, and s "ONDED OR UNBONDED CONCRETE overlay on composite pavement (usually asphalt cap over concrete). In general, bonded concrete over- lays over asphalt will be thinner, and unbonded overlays over asphalt will be thicker. Composite pavements offer their own advantages, ACPA says. "You can take advantage of the asphalt cap," Voigt said. "Typically, if an unbonded overlay is being placed, an interlayer of fabric or thin asphalt overlay will be required. With composite pavements you al- ready have the asphalt layer there." The interlayer is required to bridge imperfections in the existing pave- ment to be paved over. "If your exist- March 2015 | EquipmentWorld.com 50 road science | continued Photo: ISTHA Photo: James K. Cable, Ph.D., P.E. Illinois State Toll Highway Authority places test of two-lift, wet-on-wet con- crete overlay on I-88 in 2012. Section of very thin 2-inch textured concrete overlay on Iowa State Route 2.