What is Whitetopping?

Whitetopping

A whitetopping is a concrete overlay that is placed on an existing asphalt pavement to resurface it. The FHWA describes it as a way to resurface a distressed asphalt pavement.

Types

The FHWA separates whitetopping into three types by thickness and bond.

• Conventional whitetopping is a concrete overlay placed directly over the asphalt. It does not rely on a bond.
• Thin whitetopping (TWT) is 100 to 150 mm (4 to 6 in.) thick. It is bonded to the asphalt. The slabs are square, typically 1.8 m (6 ft).
• Ultra-thin whitetopping (UTW) is 50 to 100 mm (2 to 4 in.) thick. It is bonded to the asphalt. The joints are 0.6 to 1.2 m (2 to 4 ft) apart.

The bonded types use a thinner slab and shorter joint spacing than a conventional overlay.

The bond

The thin slab of a bonded whitetopping depends on the bond to the asphalt. To get it, the asphalt is milled and cleaned before the concrete is placed. CDOT mills 13 to 50 mm (0.5 to 2 in.), and at least 75 mm (3 in.) of asphalt should remain after milling. Concrete does not bond well to new asphalt, whether milled or not. For this reason the FHWA does not recommend thin whitetopping on a newly built asphalt surface.

CDOT ties the longitudinal joints with deformed tie bars, uses no dowel bars across the transverse joints and seals the joints. It assumed a service life of 20 years for thin whitetopping in its life-cycle cost analysis.

Whitetopping on airfields

The FAA does not define whitetopping as a design type. AC 150/5320-6G designs a concrete overlay of an existing flexible pavement in the same way as a new rigid pavement. The FAARFIELD software assumes that the interface between the overlay and the asphalt is unbonded. The AC recommends a bond breaker of fine stone, 5 mm (1/4 in.) or less, between the two layers. The bonded overlay text in the AC covers overlays of existing concrete. It says that the bond is extremely difficult to achieve and that a bonded overlay on a federally funded project needs FAA approval.

For thin panels on a stiff base, such as whitetopping, the engineer’s report must support the proposed joint spacing. The AC sets a minimum thickness for the concrete surface of 150 mm (6 in.), or 125 mm (5 in.) if all aircraft on the pavement weigh less than 30,000 lb (11,520 kg). It lists concrete (item P-501) among the surfaces for locations that need a fuel resistant surface. The AC is mandatory for projects funded through the Airport Improvement Program and the Passenger Facility Charge program.

Inspection

A PCI survey of an airfield pavement follows ASTM D5340, as AC 150/5380-7B sets out. The AC on pavement design lists ground penetrating radar as a tool that can help to detect layer separation.