TJob-Site Considerations

Because unbonded overlays add significant thickness
to the overall pavement cross section, short sections of
reconstruction may be required at bridge underpasses
to maintain adequate overhead clearance. In such
cases, transition sections are required at both ends to
provide smooth transition from the overlay elevation to
that of the reconstructed section. The recommended
taper length is 90 to 150 m (300 to 500 ft). A similar
transition section is also needed at bridge approaches.

Performance of Unbonded Overlays

In general, the performance of unbonded overlays has
been very good (McGhee 1994; ERES 1999). Where
premature failures have occurred, the failures have
been attributed to poor separator layer design,
excessive joint spacing, or inadequate slab thickness
(McGhee 1994; ERES 1999). Field performance data
suggest that the risk of poor performance is high for
thinner unbonded overlays (less than 152 mm [6 in] for
JPCP and less than 178 mm [7 in] for CRCP).

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).
1990. Guidelines for Unbonded Concrete Overlays.
Technical Bulletin TB-OOSP. American Concrete
Pavement Association, Arlington Heights, IL.

ERES Consultants, Inc. 1999. Evaluation of
Unbonded Portland Cement Concrete Overlays.
NCHRP Report 415. Transportation Research Board,
Washington, DC.

Federal Highway Administration (FHWA). 1990.
“Concrete Pavement Joints.” Technical Advisory
75040.30. Federal Highway Administration,
Washington, DC.

Hall, K. T. 1992. Backeatculation Solutions for
Concrete Pavements. Technical memo prepared for
SHRP Contract P-020, Long-Term Pavement
Performance Analysis. Strategic Highway Research
Program, DC.

Hall, K. T., M. I. Darter, and W. J. Seiler. 1993.
“Improved Design of Unbonded Concrete Overlays.”
Proceedings, Fifth International Conference on
Concrete Pavement Design and Rehabilitation. Purdue
University, West Lafayette, IN.

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.

Tayabji, S. D. and P. A. Okamoto. 1985. “Thickness
Design of Concrete Resurfacing.” Proceedings of the
Third International Conference on Concrete Pavement
Design and Rehabilitation. Purdue University, West
Lafayette, IN.

the Resource Centers and Divisions.

301-577-1421).

rming Agency—This work was performed by Applied Pavement Technology, Inc., 3001 Research Road,
E , I, 61822 under contract number DTFH61-00-P-00507.

: Technical Brief is being distributed according to a standard distribution. Direct distribution

publication from which this Technical Brief was developed—Portland Cement Concrete
the Technology Synthesis—is available from the National Technical Information Service,
‘oad, Springfield, VA 22161 (www.ntis.goy). A limited number of copies are available from
nter, HRD-11, FHWA, 9701 Philadelphia Court, Unit Q, Lanham, MD. 20706 (telephone

nical Brief is disseminated under the sponsorship of the Department of Transportation in the
ation exchange.. This Technical Brief does not establish policies or regulations, nor does it
sement of the conclusions or recommendations. The U.S. Government assumes no liability



JS. Department of Transportation
(“4 | Federal Highway Administration

Technical Brief
Unbonded Portland Cement Concrete Overlays

May 2002

Introduction

An unbonded portland cement concrete (PCC) overlay
is a pavement rehabilitation technique in which a
separator layer (generally hot-mix asphalt [HMA)]) is
placed between the existing PCC pavement and a new
PCC overlay (see figure 1). This separation layer is
placed to ensure independent behavior between the two
slabs, thereby minimizing the potential for reflection
cracking. Unbonded PCC overlays are typically
constructed between 152 and 305 mm (6 and 12 in)
thick.
Separator Layer

(not to scale)

1 PCC Overlay

re eres
— Bales aS

wre. ee
ye Oe ee

Existing

~ gf

7

Subbase

Figure 1, Unbonded PCC overlay (McGhee 1994).

Unbonded PCC overlays are a feasible rehabilitation
alternative for PCC pavements in practically any
condition because the performance of unbonded
overlays is relatively insensitive to the condition of the
existing pavements. However, the most likely
candidate pavements are typically those with extensive
deterioration, including those with material-related
distresses (MRD) such as alkali-aggregate reactivity
(AAR) or D-cracking. Because the performance of the
PCC overlay is less dependent upon the underlying
pavement condition as compared to other overlay
techniques (including HMA overlays and bonded PCC
overlays), minimal preoverlay repairs are required.
Unbonded overlays can be constructed as jointed plain,
jointed reinforced, or continuously reinforced concrete
pavement (JPCP, JRCP, or CRCP), although unbonded
JPCP overlays are by far the most common. Current
pavement practice is away from JRCP designs, and
these are rarely constructed any more.

* “op ,PCC Pavement

Although the performance of unbonded PCC overlays
has generally been good, there are several unresolved
aspects of unbonded PCC overlay design. For
example, the effects of preoverlay repair and separator
layer design on the performance of unbonded overlays
cannot be shown conclusively with the available
performance data, analysis tools, or performance
models. Also, in the past, any bonding between the
pavement layers in unbonded overlays was thought to
cause poor performance; however, current thinking is
that a certain amount of bonding (or friction) between
the separator layer and the overlay is essential for good
performance,

General Design Considerations

The design of unbonded overlays requires
consideration of factors that are applicable to both new
and rehabilitation design, as shown in table 1. In urban
areas, where traffic congestion is already a daily
problem, management of detour traffic during
construction can also be a critical issue. For projects in
congested areas, the use of fast-track paving techniques
may be appropriate to minimize lane closure time.
With fast-track paving techniques, PCC pavement
reconstruction and PCC overlays can be constructed
with weekend lane closures.

Pavement Evaluation

The existing pavement condition is a key input to
overlay design. Field evaluation of candidate
pavements for unbonded overlays typically consists of
a visual distress survey, deflection testing using falling
weight deflectometer (FWD), and coring. If MRD
such as D-cracking or alkali-silica reactivity distress is
present, laboratory testing of the cores may also be
needed to verify the nature of MRD and to avoid
similar problems in the overlay PCC. The structural
integrity of the existing pavement is typically estimated
based on visual distress surveys and FWD testing
results, The condition survey is also important for
identifying the areas that should be repaired prior to
overlaying and for identifying MRD or drainage
problems that may require special design
considerations.



Table 1. Key design factors for unbonded PCC overlays.

Rehabilitation Design

Design Factors Unique to |

¢ Existing pavement condition
« Existing pavement type

© Overlay pavement type

* Preoverlay repair

© Separator layer design

Slab thickness
Joint spacing
Load transfer design
Reinforcement design

Edge support (tied PCC shoulder or
widened slab design)

Subsurface drainage

PCC mix design

FWD testing is also extremely valuable in
characterizing the existing pavement condition. The
information that can be obtained through FWD testing
includes the following:

* — Backcalculated subgrade k-value and PCC
modulus.

e — Subgrade variability.

e Load transfer efficiency.

¢ Presence of voids under joints and cracks.

The backcalculation can be accomplished using any
established procedures, such as those included in the
1993 AASHTO Design Guide (AASHTO 1993) or in
the 1998 AASHTO Supplement (AASHTO 1998).
More detailed information on PCC pavement
backcalculation is provided by Hall (1992).

Preoverlay Repair

As previously mentioned, unbonded PCC overlays
generally require minimal preoverlay repairs. Only
major distresses that cause a major loss of structural
integrity, such as shattered slabs and punchouts, require
repair, The recommended preoverlay repairs for
unbonded overlays include the following (Hutchinson
1982; ACPA 1990; ERES 1999):

«  Full-depth repair of shattered slabs, punchouts
(CRCP), and high-severity transverse cracks.

e Slab stabilization of unstable or rocking slabs.
¢ Diamond grinding or milling if faulting exceeds
6 mm (0.25 in). Alternatively, a thicker separator
layer (50 mm [2 in] HMA) may be used to address
faulting greater than 6 mm (0.25 in).

« Patching of high severity spalling with HMA.

¢ Leveling up of significant settlements with HMA.

In general, if an unbonded CRCP overlay is to be
constructed, more attention must be paid to preoverlay
repair activities to ensure that the existing distresses do
not reflect through the new overlay (ERES 1999).
Depending upon the condition of the existing
pavement, a thicker separator layer, a higher steel
content, or a thicker overlay may be used to address
concerns for reflection cracking.

As an alternative to preoverlay repairs, the existing
pavement may be fractured to provide more uniform
support under the overlay. For pavements with severe
MRD, slab fracturing may be particularly applicable.
If left intact, the continued progression of MRD in the
original pavement can cause premature deterioration of
thin (178 mm [7 in]) unbonded overlays (ERES 1999).

Structural Design

Thickness Design

The required overlay thickness is typically determined
using either AASHTO (1993) or the Portland Cement
Association (PCA) (Tayabji and Okamoto 1985)
design procedures. However, thickness design is an
area of critical deficiency in current practice for
unbonded overlays, as all major design procedures
have significant limitations. Some of the major flaws
include the following:

. Lack of consideration of layer interaction. The
structural contribution of the separator layer and
the effects of friction between the overlay and the
separator layer and between the separator layer and
the underlying PCC pavement are ignored.

* Excessive credit given to existing pavement in
some cases. Design procedures that are based on
structural deficiency (e.g., AASHTO [1993]) tend
to produce unconservative results when the
existing pavement is relatively thick.



¢ Lack of consideration of curling and warping
stresses. This is a particularly critical deficiency
for unbonded JPCP overlays that often leads to
unconseryative overlay thicknesses.

An alternative to the use of the existing overlay design
procedures is to use the 1998 AASHTO Supplement
Procedure, which although for new pavement design
may be used as it accounts for the friction between the
slab and a stiff base as well as the interactive effect of
joint spacing (AASHTO 1998). However, the overlay
thickness alone does not by itself ensure good
performance, and other factors such as preoverlay
repairs, separator layer design, and joint design must be
adequately considered.

Separator Layer Design

The separator layer performs the following key
functions (ERES 1999):

* Isolates the overlay from the underlying
irregularities to allow uninhibited horizontal
movement.

e¢ Provides adequate friction to ensure proper
formation of joints in JCP and cracks in CRCP.
The friction between pavement layers also
contributes to the composite action that is
beneficial to overlay performance.

e Provides a level surface for the overlay
construction.

The recognition that total debonding of the pavement
layers is not essential or desirable for proper
functioning of unbonded overlays is an important
recent development in unbonded overlay design.
However, the modeling of the structural effects of the
separator layer is an area needing further research.

A variety of separator designs have been used in
unbonded PCC overlay construction. The design that
has given the best results are thick (minimum 25-mm
[1-in]) HMA layers (Voigt, Carpenter, and Darter
1989; ACPA 1990; Hail, Darter, and Seiler 1993,
ERES 1999). Although the use of thin layer materials

New
aN an
XI]

Overlay

(such as chip seals or slurry seals) have worked well in
some applications, their use is generally not
recommended because they erode easily near joints and
they do not provide adequate isolation of the overlay
PCC from underlying deterioration if the existing
pavement has significant roughness from faulted joints
and cracks (ACPA 1990; ERES 1999).

Joint Spacing

Because of the concern for high curling stresses, a
shorter joint spacing is typically recommended for
unbonded JPCP overlays. The current AASHTO guide
recommends limiting the maximum joint spacing to 21
times the slab thickness. In general, this
recommendation is reasonable for slab thicknesses up
to about 229 mm (9 in), except that joint spacing less
than 3.7 m (12 ft) is not warranted because that would
make the slabs shorter than the lane width. However,
for thicker slabs (¢.g., more than 240 mm [9.5 in]), the
joint spacing based on 21 times the slab thickness is
excessive, which increases the risk of premature slab
cracking. In general, this risk can be greatly minimized
by limiting the maximum joint spacing to 4.5 m (15 ft),
even for very thick overlays.

Current pavement design practices are away from the use
of JRCP designs, and they are rarely constructed
any more. If used, the recommended maximum joint
spacing for JRCP is 9.1 m (30 ft) (FHWA 1990).

Load Transfer Design

The joint performance is significantly better in
unbonded overlays than in new JPCP because of the
load transfer provided by the underlying pavement
(Hall, Darter, Seiler 1993; ERES 1999). However,
doweled joints are still highly recommended for
pavements that will be subjected to heavy truck traffic
to avoid corner breaks and to minimize faulting.
Without dowels, the risk of corner breaks is high in
unbonded overlays because of the very stiff support
conditions. To maximize the benefits of load transfer
from the underlying pavement, it is recommended that
the joints in the overlay be mismatched from those in

the underlying pavement (see figure 2).

joint

\ ee em

Existing, Pavement

Existing Joints
Figure 2. Illustration of mismatched joints (ACPA 1990).

3