Evaluation of Bonded Concrete Overlays on Asphalt Pavements (2022)

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1   Bonded concrete overlays on asphalt (BCOAs) consist of a 3- to 7-in. concrete layer placed over distressed (e.g., rutted, shoved, cracked) but structurally sound existing asphalt pavement. BCOAs are easy to construct compared with other overlay options because they feature a reduced concrete section compared with conventional concrete pavement and minimal treatment of the existing asphalt pavement (e.g., sweeping to clean surface, mill- ing to remove severe rutting or shoving, repairing localized structural distress). Therefore, BCOAs are a cost-competitive rehabilitation alternative. Although BCOAs have been successfully used since the 1990s, much of the design, material selection, and construction procedures are heavily based on the experience of designers, speci- fication writers, and paving contractors. This approach works well for regions with extensive experience (primarily the midwestern states). Yet, the application of BCOAs has been limited in other regions of the United States, perhaps because the method is not seen as viable outside the wet-freeze zones of the upper Midwest. A regional bias may exist, impeding continued and more widespread implementation. To address potential impediments to implementation, this study documents BCOA prac- tices through a literature review and agency survey; documents performance through site investigations that assessed in-service design, construction, performance, preservation, and rehabilitation; and compares the results of current design methods with actual performance. On the basis of the agency survey, 19 in-service BCOA projects were selected for further evaluation. Projects represented a range of BCOA layer thicknesses, slab sizes (4- × 4-ft to 12- × 12-ft), and truck-traffic volumes. Furthermore, BCOAs with and without synthetic macrofibers and with and without joint sealing were tested. Projects were selected from nine participating state highway agencies, those in Colorado, Illinois, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, and Pennsylvania. In-service performance evaluations were based on automated distress surveys to assess pavement smoothness (using the International Roughness Index, or IRI), faulting, and cracking. Ground-penetrating radar (GPR) surveys were conducted over the same sections as the automated distress surveys to estimate in-place BCOA and existing asphalt layer thicknesses. Detailed site investigations were also conducted on three 0.10-mi segments per project and encompassed • Visual distress surveys and faultmeter testing; • Ultrasonic tomograph testing to estimate layer thickness and bond condition; • Falling weight deflectometer (FWD) testing to assess load transfer and bond condition; and • Coring, dynamic cone penetrometer testing, and unbound material testing. S U M M A R Y Evaluation of Bonded Concrete Overlays on Asphalt Pavements

2 Evaluation of Bonded Concrete Overlays on Asphalt Pavements Laboratory testing of samples extracted from in-service pavements encompassed • Soil classification, the Atterberg limits, and gradation; • Concrete compressive and split tensile strength; • Concrete coefficient of thermal expansion; • Asphalt complex modulus; • Hamburg wheel tracking tests; • Asphalt bulk-specific gravity; and • Composite concrete–asphalt shear testing. Results of the in-service performance evaluations, based on in-service ages ranging from 7 to 26 years and subjected to low, moderate, and high traffic volumes, found most segments performing in good condition. Segments were found to be in good to fair condition with respect to IRI, faulting, and percentage of cracked slabs. BCOA projects with 4- × 4-ft slabs had the most faulting, cracking, and IRI, whereas 6- × 6-ft slabs had the least. Project site assessments yielded additional results: • Evaluation of intersection locations (under stopped conditions only) indicated IRI and faulting are 36% and 17% higher than for nonintersection segments, respectively. Average cracking at intersections was 3%, compared with 1% for the nonintersection segments. • Approximately 84% of measurements of BCOA layers showed no statistical significance between cores and GPR-determined thicknesses. For the asphalt layer thickness, 57% of the measurements showed no significant difference between the two methods. • Automated distress survey results, compared with manual distress results, overestimated faulting and corner breaks on average by 93% and 33%, respectively. Longitudinal, trans- verse, and total cracking were underestimated on average by 35%, 20%, and 26%, respectively. • Slab size, BCOA, and asphalt layer thickness influenced faulting, longitudinal cracking, transverse cracking, total cracking, and longitudinal spalling. Slab size affected transverse and total spalling. For the projects evaluated, joint sealing and synthetic macrofibers had no effect on performance. • The ultrasonic tomography device used in this study more accurately estimated BCOA layer thickness than core measurements. • Calculating effective thickness from FWD testing to estimate bond condition appeared effective for approximately 41% of the cores. However, coring may cause bond separa- tion, resulting in fewer false unbonded conditions than estimated. BCOA design methods include the University of Pittsburgh BCOA-ME, the Colorado Department of Transportation (DOT) Overlay Design Method, the Illinois DOT BCOA Design Spreadsheet Tool, and AASHTOWare® Pavement ME Design. BCOA-ME, Colorado DOT, and Illinois DOT design methods work by recommending an optimal layer thickness for a given slab size as well as for climate and material inputs. On the other hand, Pavement ME Design predicts longitudinal cracking as a measure of performance, on the basis of user- specified thickness, traffic, climate, and material inputs. Project information obtained during the site investigations was used to compare the recommendations of the four BCOA design methods. In general, BCOA-ME and Illinois DOT design methods recommended thinner sections compared with in-place BCOA layer thickness. The Colorado DOT design method, however, recommended a thicker BCOA layer. Pavement ME Design predictions of longitudinal cracking mostly reflected in-service performance.