over major distortions

Consider increased sawcut depth |

+50 mm (2 in)

DiS typical

am

Figure 2, Consideration of HMA rut depth in determining appropriate sawcut depth (ACPA 1991).

Where the performance of whitetopping overlays has
not been satisfactory, the problem can often be traced
to a design flaw or inadequacy. For example, the
Wyoming DOT reported that nondoweled whitetopping
overlays developed significant faulting after a few
years of Interstate highway traffic (McGhee 1994).
Similar findings were made on nondoweled
whitetopping overlays in Utah (McGhee 1994).

References

American Association of State Highway and
Transportation Officials (AASHTO). 1993. Guide for
Design of Pavement Structures. American Association
of State Highway and Transportation Officials,
Washington, DC.

American Association of State Highway and
Transportation Officials (AASHTO). 1998.
Supplement to the Guide for Design of Pavement
Structures. American Association of State Highway
and Transportation Officials, Washington, DC.

American Concrete Pavement Association (ACPA).
1991. Guidelines for Concrete Overlays of Existing
Asphalt Pavements (Whitetopping). Technical Bulletin
TB-009P. American Concrete Pavement Association,
Arlington Heights, IL.

American Concrete Pavement Association (ACPA).
1998. Whitetopping—State of the Practice.
Engineering Bulletin EB210P. American Concrete
Pavement Association, Skokie, IL.

American Concrete Pavement Association (ACPA).

2000. “Iowa Whitetopping—20 Years and Still Going
Strong.” Concrete Pavement Progress, Vol. 36, No. 2.
American Concrete Pavement Association, Skokie, IL.

Darter, M. I., K. T. Hall, and C. M. Kuo. 1994,
Support Under Portland Cement Concrete Pavements.
NCHRP Report 372. Transportation Research Board,
Washington, DC.

Federal Highway Administration (FHWA). 1990.
Concrete Pavement Joints. Technical Advisory T
5040.30. Federal Highway Administration, Washing-
ton, D.C.

Hutchinson, R. L. 1982. Resurfacing With Portland
Cement Concrete. NCHRP Synthesis of Highway
Practice No. 99. Transportation Research Board,
Washington, DC.

McGhee, K. H. 1994. Portland Cement Concrete
Resurfacing. NCHRP Synthesis of Highway Practice
204. Transportation Research Board, Washington, DC.
Portland Cement Association (PCA). 1984. Thickness
Design for Concrete Highway and Street Pavements.

Engineering Bulletin EB109P. Portland Cement
Association, Skokie, IL.

71-1421).

rechnology Synthesis—is available

pringfield, VA 22161 (www.ntis. gov).
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By

03-008

Technical Brief
Conventional Whitetopping Overlays

May 2002

Introduction

Conventional whitetopping is the construction of a new
portland cement concrete (PCC) pavement over an
existing hot-mix asphalt (HMA) pavement (see figure
1). These PCC overlays are defined as being greater
than 102 mm (4 in) thick, but typically range from 203
to 305 mm (8 to 12 in) when placed on primary
interstate highways, and from 127 to 178 mm (5 to 7
in) when placed on secondary roads (ACPA 1998).

[~ Interface

PCC Overlay

Existing

Subbase

Figure 1. Conventional whitetopping overlay
(McGhee 1994).

Whitetopping is an appropriate rehabilitation
alternative for badly deteriorated HMA pavements,
especially those that exhibit distresses such as rutting,
shoving, and alligator cracking (ACPA 1998). Because
the PCC surface is capable of bridging a significant
amount of deterioration in the underlying HMA
pavement, minimal preoverlay repairs are required.

Structurally, conventional whitetopping overlays are
similar to new PCC pavements, and they are designed
as such. All recommended design and construction
practices for new PCC pavement are directly applicable
to conventional whitetopping. The only significant
difference is that whitetopping overlays require a
deeper sawcut to ensure proper formation of joints.
Whitetopping overlays can be constructed as jointed
plain, jointed reinforced, or continuously reinforced
concrete pavement (JPCP, JRCP, or CRCP), with the
use of JPCP designs being most common. Current
practices are away from JRCP designs, and they are
rarely constructed any more.

; HMA Pavement

Conventional whitetopping is contrasted from thin and
ultra-thin whitetopping overlays in that these latter
designs employ a milled HMA surface to develop a
mechanical bond between the PCC overlay and the
existing HMA pavement; conventional whitetopping
assumes an unbonded condition, although some
bonding does occur. Ultra-thin whitetopping is
constructed between 50 and 102 mm (2 and 4 in) thick
whereas thin whitetopping is constructed between 102
and 203 mm (4 and 8 in) thick.

General Design Considerations

The design of whitetopping overlays involves
consideration of factors that apply to rehabilitation
projects, as well as those that apply to new pavements,
as shown in table 1. In urban areas, where traffic
congestion is already a daily problem, management of
detour traffic during construction can be a critical
issue. For projects in congested areas, the use of fast-
track paving techniques may be appropriate to
minimize lane closure times.

Pavement Evaluation

The evaluation of the existing pavement is an essential
part of any overlay design, and typically consists of a
visual distress survey, deflection testing using a falling
weight deflectometer (FWD), and coring. Coring is
important in order to characterize the thickness and
condition of the HMA layers.

The FWD testing results will provide information on
the foundation support value. However, one problem
in determining this value for whitetopping is that for
existing HMA pavements the subgrade support is
generally characterized by the resilient modulus (M,),
whereas the input needed for PCC pavement design is
the modulus of subgrade reaction (k-value). Although
the 1993 AASHTO Guide contains an approximate
correlation between M, and k, the more appropriate
approach is to consider the contribution of the HMA
layer in improving the bending stiffness of the PCC
surface (Darter, Hall, and Kuo 1994), This approach is
adopted in the 1998 AASHTO Supplement for the
consideration of stabilized bases in new PCC pavement
design (AASHTO 1998).


Table 1. Design factors for whitetopping overlays.

Design Factors Unique to
: Rehabilitation Design

Design Factors Common to |

New and Rehabilitation Design

« Existing pavement condition
© Overlay pavement type
¢  Preoverlay repair

Slab thickness
Joint spacing
Load transfer design

Reinforcement design
Edge support (tied PCC shoulder or
widened slab design)

Subsurface drainage

PCC mix design

Preoverlay Repair

The most critical issue in considering repairs to the
existing HMA pavement is to ensure that uniform
support is provided for the PCC surface. To obtain the
desired performance, areas of subgrade or base failure
must be removed and replaced with a stable material
(McGhee 1994). In addition, the repair of badly
deteriorated areas is recommended; these include
severe rutting, shoving, and potholes (ACPA 1998).
Guidelines for preoverlay repairs for conventional
whitetopping are given in table 2.

Surface Preparation

For conventional whitetopping, no special efforts are
made to encourage bonding between the overlay and
the underlying HMA surface; however, a surface
preparation step may be required to address distortions
in the existing HMA pavement surface or to correct
surface profile. Three common methods of surface
preparation are used for conventional whitetopping
overlays:

* Direct placement. In this approach, the PCC
overlay is placed directly on the existing HMA
surface after sweeping. Any ruts in the existing
pavement are filled with PCC, resulting in a
thicker PCC pavement in rutted areas. The direct
placement method is recommended when the
rutting on the existing HMA pavement does not
exceed about 25 mm (1 in).

© Milling. In this approach, the existing HMA
surface is milled to obtain a uniform surface.
Milling thicknesses typically range from 25 to 76
mm (1 to 3 in) and can be used to remove all
surface distortions and adjust cross slopes if
needed (ACPA 1991). This approach requires less
surveying time and cost than direct placement, but
results in additional costs for milling (and disposal,
in some cases). Milling can also be used in
combination with direct placement.

* Placement of leveling course. In this approach, a
leveling course of HMA is used to produce a
uniform surface for paving (ACPA 1991). The
leveling course typically consists of 25 to 50 mm
(1 to 2 in) of HMA. Because this method involves
the additional expense of HMA work, this option
is usually not considered when the distortion
depths exceed about 50 mm (2 in) (McGhee 1994).
In such cases, milling is typically less expensive
(ACPA 1991).

A minimum HMA thickness of 50 mm (2 in) (after any
milling) is recommended for conventional
whitetopping overlays (ACPA 1998).

Structural Design

Thickness Design

Conventional whitetopping is designed as a new PCC
pavement, treating the existing HMA pavement as a
stabilized base course. An unbonded condition is
assumed, and the required overlay thickness is
determined using any established PCC pavement
design procedure, such as the AASHTO procedures
(AASHTO 1993; AASHTO 1998) or the Portland
Cement Association (PCA) procedure (PCA 1984).
The overlay thickness is taken as the new PCC slab
thickness required to handle future traffic for the given
design conditions.

in addition to the general pavement design procedures,
simple design charts have been developed for selecting
PCC thicknesses for whitetopping overlays (ACPA
1998). In these charts, the slab thickness is selected
based on the number of trucks per day, the design PCC
flexural strength, and the subgrade k-value. The
ACPA (1998) recommends a minimum overlay
thickness of 152 mm (6 in) for conventional
whitetopping of primary and interstate roads.


Table 2. Guidelines for repair of existing HMA pavement distresses prior to PCC overlay (ACPA

1998).

General Pavement Condition]

Recommended Repair* |

Rutting (< 50 mm [2 in])

None or milling***

Rutting (> 50 mm [2 in])

Milling or leveling

Shoving Milling

Potholes Fill with crushed stone cold mix or hot mix
Subgrade failure Remove and replace or repair

Alligator cracking None

Block cracking None

Transverse cracking None

Longitudinal cracking None

Raveling None

Bleeding None

* Other factors to consider: adding edgedrains,

milling or leveling.

** Consider increasing the joint sawing depth

Joint Spacing

The maximum joint spacing recommended for
conventional whitetopping constructed as JPCP is 21
times the slab thickness (ACPA 1998). For example,
the recommended maximum joint spacing for a 203-
mm (8-in) whitetopping is 4.2 m (14 ft). As a basic
tule of thumb, this guideline is adequate, although it
may provide excessive joint spacings for thicker slabs.
If used, JRCP designs should have a maximum joint
spacing of 9.1 m (30 ft) (FHWA 1990).

Load Transfer Design

Load transfer designs for conventional whitetopping
are identical to those for new PCC pavements. In
general, doweled joints are recommended for all
pavements that will be subjected to significant truck
traffic. Experiences in Wyoming and Utah showed that
whitetopping projects built without dowels develop
significant faulting in a few years under interstate
traffic (McGhee 1994).

Job-Site Considerations

Several job-site factors require special consideration,
including bridge approaches, overhead clearances, and
shoulders. At bridge underpasses, reconstruction of a
short section may be necessary to satisfy the vertical
clearance requirement. Reconstruction requires
sections at both ends to provide a smooth transition
between the overlay and the reconstructed section. The
recommended taper length for the transition is 90 to
150 m (300 to 500 ft) (ACPA 1991). A similar
transition section is also needed at bridge approaches

costs of direct placement vs.

During paving, if the HMA pavement surface becomes
uncomfortable to touch with an open palm, water
fogging or whitewashing is recommended to reduce the
surface temperature (ACPA 1998). Although the use
of water fogging has worked well in reducing the
surface temperature, the use of whitewashing should be
used cautiously as it can reduce the friction between
the PCC overlay and the HMA.

Toint Sawing

The sawcut depth is a concern for conventional
whitetopping overlays because the distortions in the
underlying HMA pavement can effectively increase the
slab thickness, especially in rutted areas (see figure 2).
A minimum sawcut depth of one-third the PCC
thickness is recommended (ACPA 1998). A deeper cut
should be made where the overlay thickness varies
more than 25 mm (1 in) over the nominal thickness.

Performance of Whitetopping Overlays

The majority of whitetopping overlays have provided
good to excellent performance (McGhee 1994). The
success of this design is often attributed to the uniform
support and bond provided by the underlying HMA
pavement (McGhee 1994),

California has used whitetopping extensively since the
1960s, and has enjoyed considerable success with the
treatment (Hutchinson 1982). Towa also has had
outstanding performance from whitetopping overlays,
many of which have been placed on their county
highway system (ACPA 2000). Both agencies report
performance in excess of 20 years.