Concrete Products

MAR 2018

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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56 • March 2018 TECHNICAL TALK BY TOM KUENNEN NEARLY THREE DECADES ON, WHITETOPPING STILL PERFORMS Portland cement concrete unbonded overlay test sections are still doing their job after nearly 30 years, say James Greene, Ohhoon Kwon, Abdenour Nazef, and Bouzid Choubane, State Materials Office, Florida Department of Transportation, in their TRB paper, A Long-Term Performance Evaluation of an Experimental Concrete Overlay. The standard response to a distressed asphalt pavement is a mill and overlay with hot mix asphalt. Wishing to compare performance of a concrete overlay to an asphalt overlay in a same-type situation, Florida DOT designed and constructed a 1.9-mile unbonded concrete overlay on U.S. 1 between Daytona Beach and Titusville in 1988. The concrete overlay was part of a larger eight-mile milling and resurfac- ing of a deteriorated asphalt pavement. "[T]here are many reported benefits to concrete overlays as a rehabilitation and preservation technique," the authors write. "In addition to being cost-effective, properly designed concrete overlays can be constructed quickly, can be easy to maintain, and extend the life of the original pavement for 30 years or more." Concrete overlays may be bonded or unbonded, with the design dictated by the condition of the existing asphalt pavement. "In the United States, more than 200 concrete overlay projects have been constructed each decade since the 1980s," say Greene, Kwon, Nazef and Choubane. "Of these projects, nearly 80 percent were unbonded concrete overlays and approximately 30 percent of these unbonded concrete overlays were constructed over an existing asphalt pavement rather than a concrete or composite pavement." The DOT took the opportunity to evaluate different designs. Test sections were divided into three groups based on 6-, 7- and 8-in. design thicknesses. Each of these groups included six 500-ft. subsec- tions with three joint spacing levels, and two dowel bar configurations consisting of standard 12-in. spacing and wheel path only. "This project successfully demonstrated the effectiveness of a concrete overlay as a rehabilitation method for distressed asphalt pavements," the authors say. "After 28 years of service and more than 2.25 million trucks, the experimental concrete overlay sections on U.S. 1 have outlived the design life by almost 20 years without the need for major repair or rehabilitation. While all of the sections performed well over the original design life, thicker sections, as one would expect, were found to have less cracked slabs at the end of the study." Researchers found: • The asphalt base in the concrete overlay sections served as a non-erod- ible base which reduced faulting and slowed the increase in pavement roughness. • Despite being designed as an unbonded overlay, a bond between the concrete overlay and asphalt was confirmed after construction. After 28 years of service, the bond was still evident in nine of 19 cores. Debonding occurred at the corners more often than the slab center. Joint spacing likely played a role in bond retention. • Corner deflections may be controlled through a combination of slab thickness and dowel configuration. Corner deflections for the 8-in. over- lay with standard dowels were reduced by nearly 50 percent compared to the 8-in. overlay with dowels only in the wheel paths. • Sections with a standard dowel configuration had average joint load transfer efficiency (LTE) values of approximately 80 percent and dif- ferential deflections of 2 mils, while the sections with dowels in the wheel paths only had average LTE values of 46 percent and differential deflections of 7 mils. • In general, most sections exhibited good cracking performance. The section with 20-ft. joint spacing had the most overall cracked slabs (20 percent), and 20-ft. joint spacing no longer is recommended. Of the 25 slabs within this section, two were found with transverse and longitudinal cracks and one was found with a corner crack. No corner cracks were observed in any 8-in. overlay sections. • Dowel configuration and base type had the biggest impact on pavement smoothness. International Roughness Index readings increased by more than 60 percent for sections with dowels in the wheel path only, and more than 15 percent for sections with standard dowels. IRI increased approximately 75 percent in the control section, which included stan- dard dowels and a lime rock base. Distresses observed in the Florida overlay test sections. UNBONDED ILLINOIS PCC OVERLAYS PERFORM BEST Portland cement concrete overlays on Illinois interstates have mixed results over a period approaching 50 years, say Laura Heckel, P.E., Applied Research Associates in Champaign, Ill., and Charles Wien- rank, P.E., Illinois Department of Transportation, in their paper, Performance of Concrete Overlays on Illinois Interstates 1967 through 2016. IDOT has been constructing concrete overlays of existing Interstate concrete pavements for nearly five decades, including two bonded and eight unbonded installations. The agency also constructed one thin unbonded concrete overlay as an alternative to rehabilitation with a hot mix asphalt overlay. An evaluation of performance history of the 11 pavement sections was undertaken. A bonded concrete overlay involves placing new concrete directly on the existing concrete, creating one thicker monolithic pavement slab capable of handling traffic for a normal design life, typically 20 years from the placement of the overlay, Heckel and Wienrank write. "Bonded concrete overlays are thin, in the range of 3 to 4 in.," they note. "The distresses of the existing concrete pavement will reflect through a bonded concrete overlay. Because of this, pavements in good condition but with insufficient structural capacity for the anticipated traffic levels are the best choices for bonded concrete overlays. A typical application would be in areas with an existing bare concrete pavement where truck traffic increased dramatically, such as in areas of new commercial or manufacturing development." An unbonded concrete overlay involves placing a fully designed new pavement over the top of an existing pavement. "The design assigns little or no structural value to the existing pavement," observe Heckel and Wienrank. "An interlayer is used between the existing concrete pavement and the new concrete pavement to cause the existing and new pavements to act independently, thereby reducing stresses between the two layers. An unbonded concrete overlay is a good choice for deteriorated pavements at the point of needing reconstruction. An unbonded concrete overlay eliminates the need MONTAGE: Greene, Kwon, Nazef and Choubane

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