• Tuesday, May 21st, 2013

(BCO, formerly known as whitetopping and bonded overlays)

What:  2 to 5” of bonded Portland Cement Concrete is placed over asphalt, composite or existing concrete pavements that are in good to fair structural condition for long lasting and cost effective resurfacing and minor rehabilitation.  The pavement is first analyzed, and then prepared with any necessary repairs.  A critical step is roughening the substrate surface so that the new concrete will firmly bond to the old surface.  This may be done by shotblasting, sandblasting or cold milling.  The overlay is placed, and saw cuts are placed in the overlay over underlying joints, patches, and working cracks in order to accommodate movements and prevent reflective cracking.  Over asphalt pavements, transverse joints are saw cut, typically at 2 to 6 ft intervals.  Bonded concrete overlays are constructed using conventional concrete paving practices and procedures.  Bonded together, the overlay and the existing pavement perform as one monolithic pavement, with the existing pavement continuing to carry a significant portion of the load.

Where:  BCOs are appropriate for busy intersections and asphalt roads rutted by heavy truck and bus traffic can be rejuvenated with a rigid surface that won’t rut or shove.  Whitetopping overlays have been successfully used on interstate highways, state primary and secondary roads, and intersections, as well as major airport and general aviation runways, taxiways, and aprons.  The existing pavement (concrete or asphalt) must be in good condition with no significant stresses.  Rutting greater than 2” on asphalt pavements should be milled to correct the profile.  Milling the asphalt surface also helps improve the bond between the concrete overlay and asphalt interface.  Asphalt pavements to be overlaid should be at least 3-4” thick.  On concrete pavements, random cracks should be repaired, slabs should be stabilized where there is faulting and/or pumping, asphalt patches should be replaced with concrete patches to ensure bonding, joint spalling should have partial depth repair, and scaling should be removed and cleaned.

Why (advantages):  On asphalt pavements, bonded concrete overlays eliminate surface distresses such as rutting and shoving.  On concrete pavements, BCOs eliminate extensive scaling or surface cracking and improve friction, noise, and rideability.  Concrete overlays also increase structural capacity, even if that is not the primary objective of the maintenance activity.  When properly constructed over properly prepared pavements, bonded concrete overlays are one of the longest lasting treatments.

Why not (disadvantages):  Bonded concrete overlays are not recommended when the existing pavement is badly deteriorated and a substantial amount of removal and replacement of existing layers is necessary during rehabilitation.  Bonded concrete overlays are also not appropriate if there is significant deterioration from material durability problems such as D-cracking or alkali-silica reaction.  The process may be more expensive and time consuming than other treatments.  New technologies, though, have been developed to speed traffic return.  Overhead structures and embankment slopes may require expensive adjustments to meet design standards.  A comparison of the life-cycle costs for each alternative should be performed.

When:  Bonded concrete overlays are designed for good-performing pavements to extend their life.  They are not intended for use on a pavement at the end of its service life, but can increase the structural capacity of a relatively new concrete pavement that was under-designed for in-service loading.  They restore the riding surface of severely spalled pavements and pavements with high steel, that exhibit otherwise good performance.

Estimated life extension:  3-17 years.

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