FINAL REPORT
MBTC-1052
Structural Design of Portland Cement Concrete Overlays for Pavements
"Times New Roman"
"Times New Roman" Nataraj Banihatti
"Times New Roman"Dept. of Civil Engineering
"Times New Roman"University of Arkansas
"Times New Roman"
ABSTRACT
"Times New Roman"
"Times New Roman"'>The most common method used to rehabilitate existing portland cement concrete (PCC) pavements is to place an overlay consisting of asphalt concrete. However, problems with premature rutting of the asphalt overlay and early appearance of reflective cracks in the asphalt overlay have made overlays consisting of portland cement concrete a viable alternative rehabilitation method.Most PCC overlay failures can be attributed to causes other than improper overlay thickness, suggesting that existing design procedures such as the AASHTO procedure provide sufficient overlay thickness to satisfy design requirements. In this study, major factors affecting overlay performance were identified; guidelines for considering those factors in design are presented.In addition, user-friendly computer spreadsheets were developed to aid designers in completing AASHTO-based thickness design for unbonded and bonded PCC overlays.
"Times New Roman"'
INTRODUCTION
cement concrete (PCC) pavements constitute a relatively large percentage of pavements that are designed to carry high volumes of heavy traffic. When designed, constructed and maintained properly, PCC pavements can be expected to provide long service lives. Many factors, however, contribute to the accelerated deterioration of PCC pavements, including construction deficiencies, design loading in excess of that forecasted, material problems, and unanticipated changes in traffic patterns. It is not surprising, therefore, that rehabilitation of PCC pavements is topic receiving much attention. Of the major rehabilitation approaches - resurfacing, recycling, restoration and reconstruction - resurfacing (or overlays) is one of the most commonly performed methods of restoring rideability and improving structural capacity
"Times New Roman"The most frequently constructed type of overlay is made of hot-mix asphalt concrete (HMAC).An HMAC overlay can be placed fairly rapidly, at a very competitive cost, and with little shut down of the facility.However, there are two major problems associated with HMAC overlays: reflection cracking and rutting. These problems contribute to a shorter service life than is desired in many cases for a rehabilitation strategy on high-volume, heavily loaded pavements. Also, a relatively thick HMAC overlay is required to improve the structural capacity of the pavement . Hence, resurfacing with portland cement concrete is gaining popularity.
"Times New Roman"There have been significant improvements in the area of pavement and overlay design procedures.What was once a more-or-less purely empirical method now involves a significant amount of mechanistic procedures.Some of the overlay thickness design procedures used successfully in the recent years are those developed by McCullough et al., Trebig et al., the U.S. Army Corps of Engineers, and the Minnesota DOT Even with numerous options available, many highway agencies use no formal design procedure, but rely on engineering judgment and experience for PCC overlay designs of both rigid and flexible underlying pavements. A few agencies use the AASHTO design procedure, which is a mechanistic-empirical approach.
PROJECT OBJECTIVE
The original overall objective of the project was to develop a rational, consistent method for designing PCC overlays for pavements. The specific objectives were as follows.
Identify the major factors involved in PCC overlay design; determine the method(s) and extent to which these factors are considered in current design procedures, with particular emphasis on the AASHTO procedure.
Conduct an in-depth evaluation of current AASHTO PCC overlay design procedures with respect to roadway and environmental conditions in Arkansas and the surrounding region.
Develop supplemental or companion procedures to current AASHTO procedures that are consistent with sound pavement design principles.
Develop a computer program to design PCC overlays.
A review of the existing literature on PCC overlays suggested that most overlay failures could be attributed to causes other than improper overlay thickness. In light of this, the objectives of the project had to be slightly revised. Instead of developing an entirely new design procedure, major factors that affect overlay performance were identified. Guidelines were given on the AASHTO procedure and user-friendly computer spreadsheets were developed to design bonded and unbonded PCC overlays.
PCC OVERLAYS
Use of portland cement concrete to resurface existing pavements can be traced back to as early as 1913 (2). With increasing axle loads and traffic volumes, PCC resurfacing is becoming even more popular as a rehabilitation alternative with many highway agencies. While there were 375 catalogued projects in 1982, the number increased to more than 700 in 1993 (2). This has been made possible due to the progress in technology, improvements in design procedures, construction guidelines and specifications for all types of PCC overlays.
Concrete resurfacing is used primarily to improve the structural capacity of the existing pavement or to enhance ride quality (functional enhancement) (1) One of the evolving uses of the structural resurfacing involves stage construction, in which future bonded resurfacing may be planned at the time of original pavement design (1) topping”, or PCC resurfacing of existing asphalt pavements, is also becoming very popular.
Types of PCC Overlays
Depending on the type of interface used, concrete overlays can be classified as bonded, unbonded, or partially-bonded overlays. Depending on the presence and type of reinforcement, they can further be classified as jointed plain concrete overlays, jointed reinforced concrete overlays, continuously reinforced concrete overlays or fibrous concrete overlays.
Bonded overlay means that special efforts are employed to enhance bonding between the existing pavement and the overlay. Unbonded means that specific actions are taken to ensure that there is no bond between the concrete layers. Partially bonded means that bonding is not particularly addressed, and as the name itself suggests, some areas may in fact be bonded (1)
Bonded concrete overlays provide two improvements to an existing pavement: increased structural capacity and a new riding surface. Bonded resurfacing is usually thin and hence depends on the existing pavement for structural capacity (3). This means that the existing pavement should be free from distresses if good performance is to be realized.
Bonding a resurfacing to the underlying pavement to achieve monolithic behavior of the two layers is a very efficient means of structural enhancement.A 25 mm (1 in) bonded concrete overlay has approximately the same structural benefit in reducing the stresses as 62.5 mm (2.5 in) of asphalt concrete (4) When the effective slab thickness is increased by a bonded overlay, vertical deflection and subgrade stresses are decreased significantly. When a 75 mm (3 in) bonded overlay is applied to a 225 mm (9 in) concrete pavement, the deflection under a 550 kPa (80 psi) corner wheel load decreases by 31 percent (assuming 90 percent load transfer efficiency at the transverse joint). This reduction in deflection will likely result in reduced pumping, faulting, and loss of support (5)
A bonded PCC overlay when properly constructed, holds the promise of an extended service life, increased structural capacity, and lower life cycle costs, compared with other overlay techniques. Although the initial cost of a bonded PCC overlay may be higher than those of an HMAC overlay, the benefits of longer life and reduced maintenance costs suggest that bonded overlays can be a viable resurfacing alternative (1)
Bonded concrete overlays must be matched by type to the existing concrete slabs. That is, jointed concrete overlays must be used only on jointed concrete pavements, and continuously reinforced overlays can be used only on existing continuously reinforced concrete slabs.Furthermore, for bonded overlays the existing slabs must be distress-free, since most distress in the existing slab will ultimately reflect through the overlay (6)
Early studies showed that bonding between the two layers is principally a mechanical process that depends primarily on the soundness and cleanliness of the underlying pavement (7). However, later work (8) recognized a degree of chemical bonding between the overlay and the underlying pavement. Felt suggested that “a slight degree of roughness is desirable, but an extremely rough surface is not required”. When properly constructed, the bond strength often exceeds the strength of even the strongest layer, so that bond test specimens fail in one of the layers rather than at the interface (10)
Important design and construction considerations for bonded concrete overlays are:
existing slab cracking
pre-overlay repair
surface preparation
overlay thickness
sawing of the joints
curing of the overlay concrete
Following are the advantages and disadvantages of the bonded type of overlays.
Advantages:
Thin overlays can be used. Though 50-125 mm (2-5 in) thick overlays are typical, overlays as thin as 25 mm (1 in) have been successfully used on sound existing concrete pavements (6)
Thin overlays mean lower costs and fewer problems in maintaining minimum overhead clearances and matching existing facilities, which is particularly advantageous in urban areas.
Because of the smaller amount of concrete used with overlay, higher-quality concrete can be used without significant adverse costs.
Disadvantages:
These overlays can be used only on sound, distress-free pavements.
Proper preparation of the existing surface is most critical to achieve bond.
These overlays must be matched by type to the existing concrete slabs. That is, JRCP overlays can be used only on existing JRCP and so on.
Some minor adjustments may be necessary in the concrete mixture to achieve a dense, durable surface
The joints in the overlay must be matched to the joints in the existing slab by both location and type (6)
Unbonded overlays are designed with an interlayer between the new overlay and
the existing slab to isolate the overlay from distress in the underlying
pavement and, thereby, eliminate or reduce reflective cracking (11,
12). This type of overlay has been used effectively over both concrete
and bituminous pavements (12,13,14). Particular economic and
performance advantage is gained when used on existing pavements that have
become significantly deteriorated.
Unbonder overlays are intended for use on existing pavements in which distress is too extensive and too severe to be effectively eliminated before overlaying (6)The bond breaker layer often is composed of HMAC covered with membrane curing compound to impede bonding. With a few special considerations, the resurfacing may then be constructed as if the underlying pavement were a conventional subbase layer (1). Fully unbonded PCC overlays behave eventually as slabs supported by a firm subgrade However, due to the very stiff nature of the existing pavement, thermal curling stresses in unbonded pavement can cause cracks if the joints are not closely spaced. Following are the advantages and disadvantages of the unbonded type of overlays.
Advantages:
A big advantage of this type of overlay is that it is not necessary to match the joints between the existing pavements and overlays or even to clean or seal these joints (6).
Surface preparation is not as critical as in bonded type of resurfacing. However, structural distresses cannot be ignored and uniform support should be ensured.
No special construction techniques are needed for construction.
The major disadvantage of unbonded PCC overlays is the greater thickness required, potentially resulting in higher costs and greater clearance problems. Minimum thickness for unbonded overlays is 150 mm (6 in); typical thickness is likely 175 mm to 200 mm (7 to 8 in), depending on the traffic and the condition of the existing pavement (6). The thickness of the overlay may not be economically feasible for most projects.
When relatively thin unbonded overlays are going to be constructed, it is extremely important that the existing pavement be properly prepared (undersealed, broken slabs replaced, patched, etc.) to ensure good performance.
If the
issue of bonding between the resurfacing and the underlying pavement is of
little importance, such as on thick airfield pavement, the partially bonded
approach may be employed (1). Grout or special additives are not required to promote bond when
partially bonded overlays are used. These overlays are sometimes referred to as
direct overlays (12), implying that little or no surface preparation is
done. The only requirements for
partially bonded overlays are that the surface be free of loose materials and
that the existing concrete surface be sound. Because no particular attention is
paid to cleaning or grinding the base pavement, various degrees of bonding may
occur, but will have little bearing on the performance of the resurfacing.
Recent literature considers partially bonded overlays to be special cases of
the unbonded type, because the evidence shows that the performance is similar (16), Partially bonded overlays should also be
used only on reasonably sound existing pavements, since most cracks in the
existing slab will reflect through the overlay within a short period of time.
Ideally,
the minimum thickness for partially bonded overlays is 150 mm (6 in), although
125 mm (5 in) overlays have been used successfully. Unless joints are closely spaced, however, significant cracking
between joints can be expected when thin partially bonded overlays are used. It
should be noted that partially bonded overlay is not considered a usual
alternative for highway pavement (2). Furthermore, recent airfield pavement related literature makes
little reference to the partially bonded type of overlay. For design purposes,
only the bonded, unbonded and whitetopping types of overlays are considered (2).
Overlays
are constructed to correct two deficiencies namely, functional deficiency and
structural deficiency. In general, structural deficiency will override
functional deficiency because a thicker resurfacing is almost always required -
that is, a resurfacing thick enough to satisfy structural requirements should
be more than thick enough to correct any functional deficiency (2).
The following are some of the basic requirements governing the design of PCC overlays (17).
1.
Thickness must be
sufficient for the anticipated service conditions.
2.
Joints (longitudinal
and transverse) and cracks must have the capacity to transfer applied loads
without loss of surface smoothness. The joint and crack system should minimize
the migration of moisture between it and the underlying pavement.
3.
Reinforcement must
have adequate cover for the exposure conditions and should be of such size and
spacing that all cracks are held tight.
4.
The maximum size
aggregate must be compatible with the resurfacing thickness and spacing of
steel.
5.
Sound, durable
aggregate must be used; air entrainment must also be used if freezing and
thawing or the use of de-icing salts might occur.
6.
Shoulders should be
of concrete, tied to the resurfacing, or another material stabilized for the
full depth of the resurfacing to minimize infiltration of shoulder material
between the underlying pavement and the resurfacing.
An
important consideration in resurfacing design is the condition of the existing
pavement on which the resurfacing is proposed.
Barenberg (6) in 1981 put condition evaluation of the existing pavement
in perspective as one of the most important resurfacing considerations:
Evaluating the true condition of the existing pavement is one of the most critical factors in selecting the best overlay option. This evaluation should reflect how the existing pavement will affect the behavior and performance of the overlaid pavement. Such an evaluation should be based on structural or behavioral considerations rather than serviceability considerations.
It should also be noted that PCC resurfacing shares at least
one design requirement with on-grade PCC pavements: they require uniform
support conditions if satisfactory performance is to be realized. Nearly all
the documented cases of premature overlay failure can be traced to some
violation of this single requirement (2).
Functional
Design
Since a
functional resurfacing needs to be only thick enough to restore the ride
quality or repair surface defects, it may in fact be relatively thin.
Typically, the capability of paving machines, the sizes of the aggregate
particles, and geometric considerations (overpass elevations, guardrail
heights, grades, etc.) will dictate how thick such resurfacing must be (2).
On the other hand, reinforced sections may need to be a minimum of 75 to 100 mm
(3 to 4 in) thick to accommodate the reinforcing steel with sufficient cover to
impede earlier corrosion.
Structural
Design
In general,
structural deficiency will override functional deficiency because a thicker
resurfacing is almost always required - that is, a resurfacing thick enough to
satisfy structural requirements should be more than thick enough to correct any
functional deficiency. While there are
numerous approaches to resurfacing thickness design, all are conceptually
similar and involve the determination of:
(1) the structural capacity required to carry the prevailing and
projected traffic for the design life of the resurfacing; (2) the structural
capacity of the existing pavement; and (3) the difference between (1) and (2).
In the
AASHTO terminology, the structural capacity for a PCC pavement is the slab
thickness (D). The structural capacity of a pavement decreases with time and
accumulated traffic and by the time an overlay is considered, the effective
structural capacity of the existing PCC pavement becomes Deff. The
difference between the structural capacity required to carry the future traffic
(e.g. Df) and effective structural capacity (Deff) will
be the structural capacity of the overlay (DOL) which is shown by
the following general equation.
DOL = Df - Deff
The design
of overlays begins with an evaluation of the existing pavement to determine
thickness, type of load transfer, and type of shoulder. Next, the projected 18-kip equivalent single
axle loads (ESALs) in the design lane for the design period are determined.
A condition
survey is used to determine the types and severity of distress present.
Non-destructive deflection testing is done to evaluate the effective k-value
(subgrade support), slab elastic modulus, and joint load transfer. If a bonded
overlay is planned, it is recommended that the modulus of rupture of the
existing slab be determined by testing pavement cores.
The overlay
thickness is determined as follows (18):
Bonded overlays: ![]()
Unbonded overlays: ![]()
where Dol = thickness of the overlay (in)
Df = Slab thickness required to carry the
future traffic (in)
Deff = Effective slab thickness of the
existing pavement (in)
Effective slab thickness of the existing pavement Deff
is determined from condition survey and is dependent on the amount of distress
such as durability problems, unrepaired transverse joints and cracks, fatigue
cracks, punch-outs, etc.
Slab thickness (Df) to carry the future traffic
is determined from the following AASHTO rigid pavement design equation (18).

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where, W18 = 18 kip ESALs in the design period
ZR =
Standard Normal Deviate (function of reliability R)
So =
Standard Deviation
D =
Slab thickness to carry future traffic (Df)
DPSI =
Loss in Serviceability (Pinitial - Pterminal)
Pt =
Terminal Serviceability
Sc =
PCC Modulus of Rupture (psi)
Cd =
Coefficient of Drainage
J =
Load Transfer Coefficient
Ec =
PCC Elastic Modulus (psi)
k =
Effective Static k-value (pci)
It should be noted that the method for obtaining design inputs such as properties of concrete depends on the type of overlay to be constructed. For an unbonded overlay, PCC properties are representative of the overlay concrete whereas for bonded overlays, the properties are representative of the existing pavement concrete.
Selection of
Overlay Type:
Selection
of the proper type of overlay is very important to ensure good
performance. For example, Bonded
overlays depend entirely on the existing pavement for structural capacity (3),
implying that they should never be constructed on a pavement suffering from
structural damages. Also, bonded overlays should not be constructed on
pavements suffering from durability problems because D-cracking will reflect
through the overlay.
On the other hand, there is not much
restriction for constructing unbonded overlays. In fact, unbonded overlays are most cost-effective when the
existing pavement is badly deteriorated because of reduced need for pre-overlay
repair. However, unbonded overlays are not intended to bridge localized areas
of non-uniform support. Hence, areas of non-uniform support should be
identified and proper repairs should be done to ensure uniform support for the
overlay. Figure 3 illustrates the selection process for the overlay based on
existing pavement condition and pre-overlay repairs. However, the final
selection of the overlay type also depends on other factors such as
availability of equipment, economics, agency experience, etc.
Condition Survey Fjc, Fdur,
Ffat. Input W18, R, So, Pi,
Pt, Cd and J Bonded
Overlay Thickness (Df-Deff) Deff Df PCC
Modulus of Rupture (Sc) PCC Elastic Modulus (Epcc) Effective Static k Value Unbonded
Overlay Thickness D2f
- D2eff Effective Static k Value Condition Survey Fjcu Input
W18, Epcc, Sc, R, Pi, Pt, Cd, and J Deff Df![]()
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UNBONDED
OVERLAYS
Design Inputs:
Determination of Slab Thickness for Future Traffic (Df)
Since
the AASHTO overlay design procedure is basically a structural deficiency
approach, the required slab thickness for the future traffic will have a direct
impact on the overlay thickness obtained.
The inputs that affect Df, in order of significance, are as
follows (19):
1.
PCC modulus of rupture and elastic modulus (Sc
and Epcc)
2.
Reliability level ( R )
3.
Drainage Coefficient (Cd)
4.
Load Transfer Coefficient (J)
5.
Overall Standard Deviation (So)
6.
Design ESALs (W18)
7.
k-value
The PCC modulus of rupture and elastic modulus to determine
Df for unbonded overlay of an existing PCC pavement are
representative of the new PCC overlay to be placed and not those of the
existing slab. Elastic modulus of
overlay concrete can be obtained by the following correlation recommended by
American Concrete Institute for normal weight portland cement concrete:
Ec =57,000 (f’c)0.5
where Ec = PCC Elastic Modulus (in psi)
f’c = PCC
Compressive strength (in psi) as
determined using
AASHTO
T 22, T 140 or ASTM C 39 (see appendix)
The modulus of rupture required by the AASHTO design procedure is the mean value determined after 28 days using third-point loading (AASHTO T97, ASTM C 78). Because of the treatment of reliability in the AASHTO design procedure, the use of the normal construction specification for modulus of rupture is not recommended to be used as input, since it represents a value below which only a small percent of the distribution may lie. If it is desirable to use the construction specification, then some adjustment should be applied, based on the standard deviation of modulus of rupture and the percent (PS) of the strength distribution that normally falls below the specification (18):
S’c (mean) = Sc + z (SDs)
where S’c = estimated mean value for PCC
modulus of rupture (psi)
Sc = construction specification on concrete
modulus of rupture (psi)
SDs
= estimated standard deviation of concrete modulus of rupture (psi)
z = standard normal variate:
=
0.841 for PS = 20 percent*
=
1.037 for PS = 15 percent
=
1.282 for PS = 10 percent
=
1.645 for PS = 5 percent
=
2.327 for PS = 1 percent
*Note: Permissible number of
specimens, expressed as a percentage, that may
have
strengths less than the specification value.
Reliability
The
reliability of a pavement design-performance process is the probability that a
pavement section designed using the process will perform satisfactorily over
the traffic and environmental conditions for the design period.
Reliability
is a means of incorporating some degree of certainty into the design process to
ensure that the various design alternatives will last the analysis period. A
detailed discussion of reliability is beyond the scope of this project. For more information, chapter 4 of the 1993
AASHTO Guide may be consulted. Table 1 gives the suggested levels of
reliability for various facilities.
Recommended
Level Of Reliability
Functional Classification Urban Rural
Interstate
and Other Freeways 85-99.9 80-99.9
Principal
Arterial 80-99 75-95
Collectors 80-95 75-95
Local 50-80 50-80
Table 1. AASHTO Reliability
Levels for Pavement Design (18).
For
a given level of reliability ( R ), the reliability factor is a function of
overall standard deviation (So) that accounts for both chance
variation in the traffic prediction and normal variation is pavement
performance prediction for ESALs. The
AASHTO Guide (18) states:
“It is
important to note that by treating design uncertainty as a separate factor, the
designer should no longer use “conservative” estimates for all the other design
input requirements. Rather than conservative values, the designer should use
his best estimate of the mean or average value for each input value. The selected
level of reliability and overall standard deviation will account for the
combined effect of the variation of all the design variables”.
Drainage Coefficient (Cd)
In the
AASHTO design procedure for rigid pavement, drainage coefficient has a significant
effect on the resulting thickness. Because drainage condition influences slab
support and therefore overall stress condition in the slab, Cd was
introduced into the portion of the AASHTO rigid pavement performance (design)
equation that considers the slab’s strength, stress and support condition (20). As a matter of fact, Cd has the
same relative impact on rigid pavement performance as both concrete modulus of
rupture (Sc) and the load transfer coefficient (J). A 20 percent increase in Cd would have the same effect
as a 20 percent increase in Sc, or 20 percent increase in 1/J.
The selection of drainage coefficient, which ranges from 0.4
to 1.4, is based on the quality of drainage and percent of time during the year
the pavement structure would normally be exposed to moisture levels approaching
saturation. The percent time during which the pavement structure is saturated
depends on the average yearly rainfall and the prevailing drainage
conditions. Table 2 provides the
recommended Cd values.
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Percent
of Time Pavement Structure is Exposed to Moisture
Levels
Approaching Saturation
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Quality
of Less Than Greater
than
Drainage
1% 1-5% 5-25% 25%
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Excellent 1.25-1.20 1.20-1.15 1.15-1.10
1.10
Good 1.20-1.15 1.15-1.10 1.10-1.00
1.00
Fair 1.15-1.10 1.10-1.00 1.00-0.90 0.90
Poor 1.10-1.00 1.00-0.90 0.90-0.80 0.80
Very Poor 1.00-0.90 0.90-0.80 0.80-0.70
0.70
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Table 2. AASHTO Drainage Coefficient for Pavement
Design (18).
While one can spend a
significant amount of time trying to come up with accurate k-values, a small
change in the value of Cd is equivalent to a big change in k-value as
illustrated in Table 3 (20).
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Quality of
Drainage Selected Cd
Value Corresponding k-value (pci)
![]()
Excellent 1.2 942
Good 1.1 501
Fair 1.0 200
Poor 0.9 44
Very Poor 0.8 1
Table 3. Relative
Effect of Cd and k-value for Rigid Pavement Design
Unfortunately,
selecting a proper value for Cd has been a point of concern for
pavement engineers. One of the problems with the methodology is that there are
no well-defined procedures for translating the results of various drainage
design procedures into the rather subjective input (coefficient) used in the
AASHTO design procedure (20).
As a basis for comparison, the value of Cd for
conditions at the AASHO Road Test was 1.0; however, pavements at AASHO Road
Test had very poor subdrainage. Indeed,
the pavements didn’t have a subdrainage facility at all. In light of this, a Cd
value of even 1.0 may be too high and a value of 0.8 to 0.9 may be appropriate
for the AASHO pavements.
As far as modern pavements with better subdrainage facilities
are concerned, a value of 1.0 to 1.15 may be assigned. However, this should be
based on the actual drainage conditions of the pavement.
Joint Load Transfer Coefficient (J)
The joint
load transfer coefficient relates to the ability of a joint to transfer shear
load and this coefficient has a significant effect on the resulting thickness
of the slab. Though the load transfer coefficient appears to be related to joint faulting in a pavement, it has
nothing to do with faulting. Darter, et. al. (21) observe:
It
is very important to remember that the J-factor is an adjustment for slab
stresses that cause corner breaks, and has absolutely nothing to do with joint
faulting. No joint faulting existed at the Road Test. One cannot design a
reduction or an increase in joint faulting by changing the J-factor. This has
been a point of major confusion among pavement engineers for years.
It should
be noted that increasing the thickness of slab cannot prevent faulting (21). Experience has shown that installing dowel
bars is the most effective way of providing load transfer across the joints,
and thus reduces faulting.
For designing an unbonded PCC overlay of an existing concrete pavement, the J factor should be representative of the overlay and NOT the existing pavement. If a bonded PCC overlay or an AC overlay is being constructed on top of an existing PCC pavement, then the J factor should be based on the existing pavement. The load transfer coefficient is obtained from Table 4, based on the type of shoulder.
Asphalt Concrete Shoulder