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).

![]()

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![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
![]()
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.
![]()
Percent
of Time Pavement Structure is Exposed to Moisture
Levels
Approaching Saturation
![]()
Quality
of Less Than Greater
than
Drainage
1% 1-5% 5-25% 25%
![]()
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
![]()
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).
![]()
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
![]()
Load
Transfer Devices à YES NO
Pavement Type
1.
JPCP or JRCP 3.2 3.8 - 4.4
2.
CRCP 2.9-3.2 N/A
Tied PCC Shoulder
![]()
Load Transfer Devices à YES NO
Pavement
Type
1.
JPCP or JRCP 2.5
- 3.1 3.6 - 4.2
2.
CRCP 2.3 - 2.9 N/A
Table 4. Selection of AASHTO Load Transfer
Coefficient (J-Factor) (18).
Tyner et.
al (22) recommend that only tied
concrete shoulders should be used when concrete overlays are constructed. Since AHTD specifications call for tied PCC
shoulders and use of dowel bars, the J-factor may be assigned a value of 2.8
for JPCP or JRCP overlays and 2.6 for CRCP overlay.
As a
general guideline, the dowel diameter should be equal to 1/8th of
the slab thickness in inches. The dowel spacing and length are normally 300 mm
(12 in) and 450 mm (18 in) respectively.
Effective k-value (Modulus of Subgrade Reaction)
Definition:
The modulus of
subgrade reaction is the stress (in lb/in2) that will
cause
one
inch of deflection in the underlying soil. (Units: psi/in, or pci)
The above definition indicates that stiffer the subgrade, higher the k-value. Soils such as clay will have a lower k-value compared to cement treated or asphalt treated bases. Research has shown that the value of k depends on certain soil characteristics such as density, moisture, soil texture and other factors that influence the strength of the soils. The k-value of a particular soil will also vary with size of the loaded area and the amount of deflection. The modulus of subgrade reaction is directly proportional to the loaded area and inversely proportional to the deflection.
Modulus of
subgrade reaction is obtained by conventional plate bearing tests, correlation
with soil properties or other soil tests and also by backcalculation from
deflection testing on concrete pavements.
In overlay design, it is almost always obtained by deflection testing
using the following back-calculation equations (25):



where
d0, 12,24,36 =
deflections @ 0 in, 12 in, 24 in, 36 in. (inches)
lk =
radius of relative stiffness
k = Modulus
of subgrade reaction ( effective dynamic k, pci)
P = FWD
load, pounds
a = load
radius, in. (usually 5.6 in)
g = Euler’s constant,
0.57721
Deflection data should be collected on the outer wheel path along the project at an interval sufficient to adequately assess the conditions. Intervals of 30 m (100 ft) to 300 m (1000 ft) are typical. A load magnitude of 4100 kg (9000 lb) or more is recommended. The k-value should be obtained for each slab tested.
The k-value
backcalculated from NDT data is a dynamic k-value whereas the required input to
the AASHTO design equation is a static k-value. In an analysis of AASHO Road
Test Data, dynamic repeated-load k-values were found to exceed static values by
a factor of 1.77 on the average (23). Research work by Foxworthy involving seven Air Force Base
pavements indicated that dynamic k-values exceeded static k values by a factor
of 2.3 on the average. Reducing backcalculated
k-values by 2 has been found to produce reasonable values for static k-values (25). Hence it is recommended in the overlay
design procedures that backcalculated k-values be divided by 2 to obtain static
k-values for determining Df.
If a single
overlay thickness is being designed for a uniform section a mean k-value may be
used. However, experience has shown
that k-value for edge conditions is two to four times larger than that for
center conditions for the same site.
Uzan et. al. (26) suggest that one should use the values obtained only for
the same conditions that prevailed in their derivations.
However, it
should be noted that k-value can change substantially and have only a small
effect on overlay thickness. Darter et. al (25) concluded in a recent study
that even 50% error in estimating k-value can cause only a 5% error in new
rigid pavement slab thickness (Df).
The error will be even smaller in terms of overlay thickness (DOL). Hence, using an average value for k should
not lead to serious errors.
Also, a previous research at the University of Arkansas
showed that freezing of the subgrade is not a problem in the lower two-thirds
of the Arkansas (27). In addition,
Arkansas doesn’t have a well-defined “rainy” season. In light of this, it is
not recommended that the k-value should be adjusted for seasonal effects.
Though
k-value doesn’t affect the overlay thickness significantly, it does play a
major role in how unbonded overlays perform. A slab built on top of a stiff
base (high k-value) as in the case of an unbonded PCC overlay, can be subjected
to very high curling and warping stresses.
The current AASHTO design procedure doesn’t address this problem in
detail.
Curling and
warping are associated with temperature and moisture differentials in the slab.
During daytime, the top surface of the slab becomes hotter than the bottom and
the slab tends to bend upward, resulting in a void below the middle portion of
the slab. On the other hand, a negative
temperature differential occurs at night and results in the corners and edges
displacing upward, creating the potential for a void near the edges of the
slab. When this happens, traffic load
near the corners or joints can induce very high stress on the surface of the
slab and lead to corner breaks (21).
Warping of
a slab takes place due to moisture gradient (top drier than the bottom) and
this occurs seasonally. Data from Illinois showed that substantial drying
occurs only at the top surface to a depth of less than 50 mm (2 in). The rest
of the slab remains at 80 percent saturation or higher (28). However, in dryer, less humid climates,
greater drying and upward warping of the slab may occur.
Yoder and
Witzak explain the effects of k-value and slab thickness on curling and warping
stresses as follows (29):
Curling
and warping stresses increase as subgrade stiffness (k) increases since for
very stiff subgrades (those with high k values) the subgrade does not yield.
For softer subgrades (clays for example) the subgrade will yield as the slab
warps and the subgrade will assume the general contour of the pavement. For
this case the pavement is supported uniformly over its entire length and
stresses are reduced. For the extreme case, wherein a slab is placed directly
upon another slab, such as in overlays, curling and warping stresses combined
with traffic loading may be so high that the slab will crack. One method of
combating this is to make the overlay slab quite thick.
Experience has shown
that thin concrete overlays built
over existing concrete slabs may crack badly. The material from above paragraph
explains a contributing factor in this, although curling/warping stresses are
not the only factors operating in this situation. Because of the above, thin
concrete overlays should be bonded to the existing pavement.
Regarding thermal curling stresses, Voigt, et. al. (5)
concluded:
Thermal
curling stresses are critical in unbonded concrete overlays because the
temperature gradient through the overlay becomes so large during many days and
nights of the year, and because of the very stiff support from the existing
slab. At these times, curling may cause the overlay slab to lift from the
underlying slabs and create voids between slabs, which (when combined with
traffic load stresses and stiff foundation of the underlying slab) can cause
transverse cracking. It is highly recommended that the overlay joint spacing be
kept short. Maintaining a [slab length (in)/radius of relative stiffness in.]
ratio between 6 and 7 should ensure that cracking will not develop in the
overlay. If, however, longer slabs are used, reinforcement must be included to
keep the cracks tight.
Darter et. al have
concluded (21) as follows about the effect of subgrade and base
stiffness:
The
effect of subgrade and base stiffness on slab stresses is very different when a
temperature differential exists through the slab. When no temperature gradient
exists through a slab, increased subgrade k-value or base modulus value will
always show a reduced tensile stress in the slab under loading and thus design
will require a thinner slab. However, very stiff foundations may actually
increase combined load and temperature curl stresses resulting in thicker slab
requirements. Greatly increased base and subgrade stiffness may not always be
beneficial. Under these conditions it mat be necessary to shorten joint spacing
to avoid premature transverse cracks in the slab.
The above
discussion shows the importance of proper joint spacing to prevent transverse
cracks and corner breaks in unbonded overlays.
Recently, researchers in Germany and Chile have identified a
permanent form of slab curling which is caused by a temperature differential
during construction (30,31,32,33).
Permanent upward corner and edge curling may occur if a high positive
(top warmer than bottom) temperature differential exists through the slab as it
hardens. This occurs especially on sunny days and unless proper measures are
taken to keep the top of the slab cool. If the slab solidifies with a large
positive thermal gradient (~30 F) during construction, the corners and edges
will be permanently curled upward for any lower temperature gradient and such
curling would create a serious loss of support under the corners and edges
leading to corner breaks. This suggests that proper measures are essential to
keep the top of the slab cool and thus avoid construction curling.
Summary: Design Inputs for Df, Unbonded Overlays
·
k-value can be
back-calculated by deflection testing
·
Obtain deflection
data at 100 ft. to 1000 ft. intervals in the outer wheel path
·
Use an FWD load of
9000 lbs. or greater
·
If a single overlay
thickness is being designed, compute the mean k-value for use in design
·
Adjusting k-value for
seasonal effects is not recommended
·
The k-value doesn’t
need to be estimated with great accuracy. An error of 50% or less will not
affect the resulting overlay thickness significantly
·
The stiff nature of
the existing pavement can cause very high curling and warping stresses in the
overlay. This can be prevented by proper joint spacing or increasing the
overlay thickness.
Effective Slab
Thickness of The Existing Pavement (Deff)
The
effective slab thickness or Deff is needed to determine the
thickness of the new overlay. Deff is dependent on the amount of
distress present. The distresses to be
considered include:
§
deteriorated joints
§
deteriorated cracks
§
deteriorated
punchouts
§ expansion joints, exceptionally wide joints (>1 in) or full-depth, full-lane width AC patches.
Since
unbonded concrete overlays are resistant to reflective cracking caused by
surface problems in the existing slab such as durability cracking and fatigue
cracking, these distresses are not considered in determining Deff.
The
effective thickness of the slab is computed from the following equation:
Deff = Fjcu * D
where D = existing PCC slab thickness, in. (10 in
maximum, even if existing D > 10in.)
Fjcu = joints and cracks adjustment factor for
unbonded concrete overlays
The
Fjcu factor adjusts for the extra loss of PSI caused by deteriorated
reflection cracks or punchouts in the overlay that result from any unrepaired
joints, cracks and other discontinuities in the existing slab prior to overlay.
Very little such loss in PSI has been observed for JPCP or JRCP unbonded
overlays. Hence this factor ranges from 0.90 to 1.00, which means that even if
there were significant number of deteriorated joints and cracks, the effective
slab thickness of the existing pavement will not be less than 0.9*(slab
thickness). In light of this, it may not
be necessary to conduct detailed distress surveys if an unbonded overlay is
going to be constructed. Instead, information should be obtained on major
distresses which contribute to non-uniform support such as moving slabs,
punchouts, AC patches, voids beneath slabs, and load transfer. Information about pumping is also essential
to assess the quality of drainage.
If the
existing pavement is very badly deteriorated, a thicker separation layer (³ 50 mm [2in]) should
be used. Also, it should be noted that unbonded overlays are not intended to bridge localized areas
of non-uniform support. Consequently,
all tipping or rocking slabs should be stabilized by slab jacking or sealed by
using heavy rollers to provide uniform support for the overlay (6). For very badly deteriorated pavements, in
addition to placing a thicker separation layer, it may be necessary to break
and seat the slabs to ensure uniform support.
Determination of Joints and Cracks Adjustment Factor (Fjcu):
The following information is needed to determine Fjcu
to adjust overlay thickness for the extra loss in PSI from deteriorated
reflection cracks that are not repaired:
§
Number of unrepaired
deteriorated joints/mile
§
Number of unrepaired
deteriorated cracks/mile
§ Number of expansion joints, exceptionally wide joints (> 25 mm [1 in]) or full depth, full lane width AC patches/mile
The total
number of unrepaired deteriorated joints/cracks and other discontinuities per
mile prior to overlay is used to determine the Fjcu from a figure given
in the AASHTO Guide (18). As an alternative
to extensive full-depth repair for an unbonded overlay to be placed on a badly
deteriorated pavement, a thicker AC interlayer (³ 50 mm [2 in]) should eliminate any reflection cracking
problem, in which case Fjcu = 1.0.
Determination of Existing Slab Thickness:
This is an
important parameter in overlay design. While historic information may be
available, the extreme importance and sensitivity of this variable calls for
the use of destructive testing to verify the available historic information. A
limited amount of coring at randomly selected locations may be used to verify
the historic information (18). These cores can be used for determining the PCC modulus of
rupture and PCC Elastic modulus.
Summary: Design Inputs for Deff, Unbonded
Overlays
·
Distress survey for
unbonded overlays is not as critical as in bonded overlays.
·
Obtain information
about very badly deteriorated areas, non-uniform support, load transfer, and
drainage conditions.
·
A thicker separation
layer means less pre-overlay repair.
·
Consider a thicker
separation layer if existing pavement has many joints and cracks with poor load
transfer.
·
Fjcu=1.0,
if using a thicker separation layer.
·
If the existing
pavement has deteriorated due to poor drainage, proper measures should be taken
to improve drainage.
·
Punchouts in existing
CRCP should be full-depth repaired.
In the case
of bonded overlays, the elastic modulus (E) and modulus of rupture (Sc)
of the existing pavement concrete will be used to design the thickness of the
overlay. Since in this case, the overlay will be bonded to the existing
pavement, the performance life of the overlay depends on the fatigue life of
the existing slab. Among the rigid pavement design inputs, E and Sc
have the most significant effect on the resulting thickness.
Elastic
Modulus can be obtained by (1) backcalculation using FWD data or (2) by
estimation from indirect tensile strength. The following equations are used in
backcalculation (25):



where E pcc = Elastic Modulus of concrete (psi)
D
pcc = Thickness of the existing pavement (inches)
lk
= radius of relative stiffness
dn
= corresponding deflection in inches.
k = Modulus of Subgrade Reaction
(pci)
mpcc= PCC
Poisson’s ratio (0.15)
For
deflection testing, an FWD load of 4100 kg (9000 lb). or more should be
used. Slab deflections should be
obtained on the outer wheel path at an interval sufficient to adequately assess
conditions. Intervals of 30 m to 300 m (100 to 1000 ft) are typical. ASTM D 4694 and D 4695 provide additional
guidance on deflection testing (see appendix).
AREA will
typically range from 29 to 32 for sound concrete. Typical slab E values range
from 20.7 to 55.2 MPa (3 to 8 million psi). Older pavements will have a higher
elastic modulus than newer ones. If a
slab E value is obtained that is out of this range, an error may exist in the
assumed slab thickness, the deflection basin may have been measured over a
crack, or the PCC may be significantly deteriorated.
If a single
overlay thickness is being designed for a uniform section, a mean E value of
the slabs can be used in design. Any E-value that appears to be significantly
out of line with the rest of the data should be discarded.
Like
elastic modulus, PCC modulus of rupture is an important parameter that needs to
be determined as accurately as possible since it has a significant effect on
the slab thickness. It can be estimated from indirect tensile strength or from
backcalculated E. However, it is highly
recommended that this be estimated from indirect tensile strength.
Estimation
from Backcalculated E:
A
correlation between elastic modulus and modulus of rupture was developed by
Foxworthy (24). This correlation can be used for quick determination of
modulus of rupture using backcalculated E- values.
(R2 = 0.71)
where Sc= third-point modulus of
rupture, psi
E =
backcalculated PCC slab modulus, psi
Indirect
Tensile Strength Test:
Cut several
6-inch diameter cores at mid slab and test in indirect tension (ASTM C 496).
Compute the indirect tensile strength (psi) of the cores. Estimate the modulus
of rupture using the following equation (34).
S’c = 210 + 1.02 IT
where
S’c = modulus of rupture, psi
IT =
indirect tensile strength of 6-inch diameter cores, psi.
Effect of the Magnitude of FWD
Load on Backcalculated E :
One of the
objectives of this project was to determine the effects of FWD load on the
backcalculated E values. In an earlier research work at Illinois, Foxworthy et.
al., concluded that consistently higher and often unrealistic E values are
backcalculated for low FWD load levels (24).
Foxworthy’s
work involved airfield pavements that are usually thicker than highway
pavements. However, modern highway
pavements are often built on strong subbases such as lean concrete base that
may affect the backcalculated E-values depending upon the magnitude of the FWD
load.
Data Acquisition. A significant amount of FWD data was needed for this task. Since the AHTD’s Falling Weight Deflectometer was unavailable due to some equipment problems, FWD data from a SHRP Long Term Pavement Performance study was used. Table 5 shows the details of pavement sections on which FWD data was obtained. Data was obtained on outer wheel path and also in the mid-lane region.
![]()
Test Section
ID Pavement Thickness
(Flexural Strength) Base
01 8 in. (550 psi) 6
in. DGAB
02 8 in. (900 psi) 6
in. DGAB
04 11 in. (900
psi) 6
in. DGAB
05 8 in. (550 psi) 6
in. LCB
06 8 in. (900 psi) 6
in. LCB
DGAB: Dense Graded Aggregate Base
LCB: Lean Concrete Base Loads used :
9000 lbs, 13000 lbs and 17500 lbs.
Table 5. Pavement
Sections used in FWD Backcalculation Analysis
Analysis of the data showed some interesting results. Back-calculated E-values for pavements built on aggregate bases didn’t differ much. In some cases, such as Section 050201C1 midlane, slightly higher e-values were back-calculated for higher FWD load range, which contradicts with Foxworthy’s findings. However, for section 050206C1, which is built on Lean Concrete Base, the E-values back-calculated for lower FWD loads exceeded those for higher FWD load by an average of 70,000 psi which agrees with Foxworthy’s findings.
Back-calculation Procedures for Multi-layered PCC Pavements. The above mentioned backcalculation procedures are meant for slabs on grade or slabs built on bases that are not very stiff. In cases of slabs built on stiff bases such as lean concrete base, extremely high elastic moduli will be back-calculated for slabs unless the effect of stiff base is considered in back-calculation. One way to consider this is to use the total thickness i.e. thickness of the slab + thickness of the base in the back-calculation equations.
A different
method has been presented by Ioannides and Khazanovich for back-calculation
elastic moduli for three layered concrete pavements (35). A brief account of
the procedure is given below.
§
The elastic modulus
of the slab is calculated using just the slab thickness.. This will be known as
the effective E-value or Eeff. Now, depending on the type of subbase, a modular
ratio b
has to assumed:
§
After assuming a
proper value for b,
the Elastic Moduli of the two layers are given by the following formulas:

![]()
Since the Modular Ratio b has to be assumed, the accuracy of the E-values depend upon the accuracy of b. Hence, when dealing with slabs built on stiff bases, it is better to determine the E-values by testing cores rather than by back-calculation. When back-calculation methods are used, proper engineering judgment should be used and unrealistic E-values should not be used blindly.
Effect of Temperature on Backcalculated E-values. As far as the effect of temperature on
E-values is concerned, Foxworthy et. al., concluded that only temperature
extremes substantially influence the back-calculated dynamic E values.
Temperature fluctuations between 4 and 32 C (40 and 90 F) are relatively
insignificant, producing little variation in addition to that which is already
inherent in the equipment and pavement materials. However, the overwhelming
temperature effect occurs at the joint, where load transfer plays an important
role in the pavement response to load.
For bonded
JPCP and JRCP overlays, the J-factor should be determined by testing the joint
load transfer efficiency of the existing pavement. Joint load transfer should be determined on the outer wheel path
by using a Falling Weight Deflectometer. The procedure involves loading one
side of the joint and measuring the deflections on either side.
The AASHTO
guide suggests that the load plate be placed on one side of the joint with the
edge of the plate touching the joint. The deflections should be measured at the
center of the plate and at 12 inches from the center. Since the load plate is about 5.9 inches in diameter, this
configuration enables to take deflection readings at about 6 in. on either side
of the joint.
The AHTD
practice is to place the load plate at a distance of 10.5 in. to 11.5 in. on
the leave-side of the joint and measure the deflections on either side of the
joint using sensors #2 (8 in. from load plate) and #3 (12 in. from load plate).
Since these sensors are only 4 inches apart, deflections can be measured very
close to the joint - at 2 in. on each side.
However, it is very important
that the sensors must be located in such a way that the joint is
equidistant from the two sensors.
The
deflection load transfer can be computed from the following equation:

where DLT =
deflection load transfer, %
Dul = deflection of
the unloaded side, inches
Dl = deflection of the loaded side, inches
B = slab bending correction factor.
The slab
bending correction factor, B, is necessary because the deflections measured by
the two sensors would not be equal even if measured in the interior of the
slab. This is due to the bending of the
slab. An appropriate value for the correction factor may be determined from the
ratio of d8 to d12 for typical center slab deflection
basin measurements as shown in the equation below.
![]()
(d8 and d12 measured at the interior)
Analysis of SHRP FWD data has shown that for sensors placed 4 inches apart, the B factor ranges from 1.04 to 1.05. Hence, it is reasonable to assume the value of B as 1.045.
If a single overlay thickness is being designed for a
uniform section, compute the mean deflection load transfer value of the joints
tested in the uniform section. Table 6 gives the value of J for JPCP and JRCP
depending on the mean load transfer efficiency.
![]()
Percent
Load Transfer J
![]()
>70 3.2
50 - 70 3.5
<50 4.0
![]()
Table 6. AASHTO Load
Transfer Coefficients (J-Factor) (18).
If the overlay construction includes a tied PCC shoulder, a
lower J factor may be appropriate.
For CRCP, a
J-value of 2.2 to 2.6 may be used for overlay design, assuming that working
cracks and punchouts are repaired with continuously reinforced PCC.
Effect of
Temperature on Load Transfer Efficiency:
Research
has shown that temperature can affect the load transfer efficiency to a great
extent. Foxworthy et. al., mention that in rigid pavements, temperature changes
influence load transfer efficiency more than any other characteristic of the
system. This temperature effect is composed of both curling effects and
expansion and contraction effects.
However only the combined effect is considered important in joint load
transfer efficiency (24).
Research
has shown that for transverse dummy grove joints and longitudinal key joints,
the load transfer efficiency approaches 100 percent as the temperatures
increase and a minimum value of 20 to 25 percent as the temperature decreases (25). Hence, it is very important that load
transfer tests be done only when the ambient temperature is less than 27 C (80
F).
The
effective static k-value may be obtained by backcalculation using deflection
data. The following equations are used
in back-calculation (25):



where
d0, 12,24,36 =
deflections @ 0 in, 12 in, 24 in, 36 in. (inches)
lk =
radius of relative stiffness
k = Modulus
of subgrade reaction ( effective dynamic k, pci)
P = FWD
load, pounds
a = load
radius, in. (usually 5.6 in)
g = Euler’s constant,
0.57721
Deflection data should be collected on the outer wheel path along the project at an interval sufficient to adequately assess the conditions. Intervals of 30 m to 300 m (100 ft. to 1000 ft) are typical. A load magnitude of 9000 lbs. or more is recommended. The k-value should be obtained for each slab tested.
The k-value
backcalculated from NDT data is a dynamic k-value whereas the required input to
the AASHTO design equation is a static k-value. In an analysis of AASHO Road Test
Data, dynamic repeated-load k-values were found to exceed static values by a
factor of 1.77 on the average (23). Research work by Foxworthy
involving seven Air Force Base pavements indicated that dynamic k-values
exceeded static k values by a factor of 2.3 on the average. Reducing backcalculated k-values by 2 has
been found to produce reasonable values for static k-values (25). Hence it is recommended in the overlay
design procedures that backcalculated k-values be divided by 2 to obtain static
k-values for determining Df.
If a single overlay
thickness is being designed for a uniform section a mean k-value may be
used. However, experience has shown
that k-value for edge conditions is two to four times larger than that for
center conditions for the same site.
Uzan et. al., (26) suggest that one should use the
values obtained only for the same conditions that prevailed in their
derivations.
However, it
should be noted that k-value can change substantially and have only a small
effect on overlay thickness. Darter
concluded in a recent study that even 50% error in estimating k-value can cause
only a 5% error in new rigid pavement slab thickness (Df) (21). The error will be even smaller in terms of
overlay thickness (DOL).
Hence, using an average value for k should not lead to serious errors.
The AASHTO guide provides procedures to adjust the k-value
for seasonal effects and loss of support. However, seasonal adjustment is
inconsistent with lowest springtime gross k-value used in the AASHTO model (21).
Also, an earlier research project at the University of Arkansas showed that
freezing of the subgrade is not a problem in the lower two-thirds of the
Arkansas (27). Also, Arkansas
doesn’t have a defined rainy season. In light of this, it is not recommended
that the k-value should be adjusted for seasonal effects.
As far as adjusting the k-value for loss of support is
concerned, an investigation by Darter revealed that substantial loss of support
is already built into the model from AASHO Road Test and no further adjustment
is needed (21). Additional reduction of k-value for loss of support may
lead to overdesign. Hence engineering judgment should be used when using the
loss of support criteria. If the pavement doesn’t have a strong non-erodible
base it may be necessary to use loss of support when constructing unbonded
overlays. However, as stated earlier, k-value can doesn’t affect the thickness
significantly.
Unlike in unbonded overlays, curling stresses are not severe
in a bonded overlay due to the monolithic action of the overlay and the
existing pavement. Actually, if full bonding is successfully achieved, slab
curling will be reduced due to increased thickness of the resulting slab.
·
k-value can be
back-calculated by deflection testing
·
Obtain deflection
data at 100 ft. to 1000 ft. intervals in the outer wheel path
·
Use an FWD load of
9000 lbs. or greater
·
If a single overlay
thickness is being designed, compute the mean k-value for use in design
·
Adjusting k-value for
seasonal effects and loss of support is not recommended
·
The k-value doesn’t
need to be estimated with great accuracy. An error of 50% or less will not
affect the resulting overlay thickness significantly
Effective Slab
Thickness of The Existing Pavement (Deff)
The
effective thickness of the existing slab (Deff) depends upon the
amount of durability cracking , fatigue cracking and unrepaired joints and
cracks. Deff is computed
from the following equation (18):
Deff = Fjc * Fdur * Ffat
* D
Where Fjc = Joints
and cracks adjustment factor
Fdur =
Durability adjustment factor
Ffat = Fatigue damage adjustment factor
Proper values are
assigned to the various factors after conducting a condition survey. It should be noted, however, that a bonded
overlay is not a feasible option if the amount of distress is severe.
Joints and Cracks Adjustment Factor (Fjc)
Since
unrepaired joints and cracks in the existing slab will reflect through a bonded
concrete overlay, it is recommended that all deteriorated joints and cracks and
any other major discontinuities be full-depth repaired with doweled or tied PCC
repairs prior to the overlay, so that Fjc=1.00.
If it is
not possible to repair all deteriorated areas, then the Fjc factor
is determined as follows depending on the presence of “D” cracking:
Pavements With No “D” Cracking Or Reactive Aggregate
Distress:
For
existing pavements with no “D” cracking or reactive aggregate distress (i.e.
alkalai-silica reaction), obtain the following information:
§
Number of unrepaired
deteriorated joints/mile
§
Number of unrepaired
deteriorated cracks/mile
§
Number of unrepaired
punchouts/mile
§
Number of expansion joints, exceptionally wide joints
(> 25 mm [1 in]) and full depth, full-lane-width AC patches/mile
Tight cracks held together by reinforcement in JRCP or CRCP
should not be included. However, if the crack is spalled and faulted, the crack
should be considered as working. Surface spalling of cracks in CRCP is not an indication that the
crack is working.
The total number of unrepaired deteriorated joints, cracks,
punchouts, and other discontinuities per mile is used to determine the Fjc
factor from a figure in the AASHTO Guide (18).
Pavements with “D” Cracking or Reactive Aggregate
Deterioration:
If the pavement is suffering from D cracking or reactive
aggregate deterioration, Fjc should be determined by considering only those
deteriorated joints and cracks that are not caused by durability problems. Distresses related to durability problems
are considered separately under Fdur. If all the deteriorated joints and cracks are spalling due to “D”
cracking or reactive aggregate, then Fjc should be assigned a value
of 1.0. This will avoid “double-adjusting” the Fjc and Fdur
factors.
Durability Adjustment Factor (Fdur):
Durability cracking or “D” cracking is caused by the use of
non-durable material and/or climatic conditions which results in disintegration
of concrete. This type of cracking is progressive in nature and will gradually
cover increasingly large areas until nearly complete deterioration might result
(29). Hence bonded overlays should not be
constructed on pavements suffering from severe durability problems.
A distress survey should be conducted to obtain information
about durability problems and Fdur is determined as follows:
§
Durability cracking exists, but no spalling 0.96-0.99
§
Cracking and spalling exist (bonded overlay NOT recommended) 0.80-0.95
Fatigue Damage Adjustment Factor
(Ffat):
Ffat
depends on the amount of transverse cracking (JPCP, JRCP) or punchouts (CRCP)
that is caused mainly by repeated loading and is determined as shown in Table
7. Transverse cracks and punchouts caused
mainly by durability problems (D-cracking or reactive aggregates ) should not
be included under fatigue damage.
![]()
Amount of Distress Ffat
Few
transverse cracks/punchouts exist 0.97
- 1.0
JPCP: < 5
percent of slabs are cracked
JRCP:
<25 percent working cracks per mile
CRCP:
< 4 punchouts per mile
A
significant number of transverse cracks/punchouts exist 0.94 - 0.96
JPCP:
5 - 15 percent slabs are cracked
JRCP:
25 - 75 working cracks per mile
CRCP:
4 - 12 punchouts per mile
A
large number of transverse cracks/punchouts exist 0.90 - 0.93
JPCP:
>15 percent slabs are cracked
JRCP:
>75 working cracks per mile
CRCP:
> 12 punchouts per mile
Table 7.
Determination of Ffat Factor for Bonded Overlays (18).
Determination
of Existing Slab Thickness:
This is an
important parameter in overlay design. While historic information may be
available, the extreme importance and sensitivity of this variable calls for
the use of destructive testing to verify the available historic information. A limited
amount of coring at randomly selected locations may be used to verify the
historic information (18). These cores can also be used for determining the PCC modulus of
rupture and PCC Elastic modulus.
CONCLUSIONS
As stated earlier, most overlay failures can be attributed to causes other than proper overlay thickness. Hence it is necessary to know and understand the general causes of overlay failures and the steps that should be taken to avoid them. The listing that follows provides general conclusions for both Bonded and Unbonded overlays, resulting from the studies performed under this project.
·
Nearly all the
documented cases of premature concrete overlay failure are due to lack of
uniform support conditions. (2)
·
All tipping and
rocking slabs must be stabilized by slab jacking or sealed by using heavy
rollers to provide uniform support for the overlay. (6)
·
If the existing
pavement is suffering from extreme distress, a thicker bond breaker (³ 50 mm [2in]) should
be used. (18)
·
In most cases,
minimum thickness for unbonded overlays will be 175 to 200 mm (7 to 8 in). (6)
·
An unbonded overlay
is a good rehabilitation candidate for severely D-cracked pavements. (3)
·
Proper selection of
the interlayer material is critical to the performance of the unbonded overlay.
(3)
·
Due to stiff support
from the existing pavement, curling stresses are very high in unbonded overlays.
·
Short joint spacing
or continuously reinforced design will alleviate high curling tensile stresses
in the overlay caused by curling action. (3)
·
For non-reinforced
unbonded overlay, a joint spacing (in feet) should not exceed 1.75 times the
overlay thickness (in inches). (3)
·
For reinforced
unbonded overlays, a joint spacing of 10 m (30 ft) will result in improved
performance. (3)
·
In unbonded overlays,
deliberate mismatching of the overlay joints has been shown to reduce pumping
action and thus, extend the service life of overlay. It is recommended that the
joints be placed at least 1 m (3 ft) from existing transverse cracks or working
cracks. (5)
·
Transverse and
longitudinal joints must be sawed as soon as possible to relieve initial
stresses.
·
Multiple cracking in
the CRC overlay over the existing joints will occur if moving slabs are not
stabilized. (22)
·
If a relatively thin
unbonded overlay is going to be built, the existing pavement must be properly
prepared (undersealed, broken slabs replaced, patched, etc.). (22)
·
Longitudinal cracking
in unbonded overlays can be attributed mainly due to late sawing or improper
saw depth of the longitudinal centerline joint.
COMPUTER PROGRAM
As
seen in Chapter 2, the AASHTO design procedure involves complex mathematical
equations and cumbersome procedures to determine the thickness of rigid
pavement. The 1993 AASHTO Guide for Design of Pavement Structures contains a
nomograph for designing the thickness of rigid pavement. However, nomographs have
certain limitations- they are prone to errors and are time consuming. Hence,
user friendly spreadsheets were to easily design bonded or unbonded overlays. The paragraphs that follow briefly describe
the operation of the spreadsheets.
Procedure:
The worksheets have been developed in ExcelÔ Version 7.0 for
WINDOWSÔ
‘95. The user just needs to open the required file (Bonded or Unbonded) in
Excel and enter the right data in the right location. A basic knowledge of
WINDOWS and Excel will be quite helpful. However, the worksheets do not require
the designer to do anything other than entering data and using the mouse.
Opening the
Worksheet:
(Before using the program, it is
recommended that a back-up of the worksheets be made on a separate disk and
store it aside)
Open the
required Worksheet as you would any spreadsheet file. You can do this three
ways:
1. By pressing “Ctrl+O”
2. By clicking the “Open” Icon
3. By using the menu on the top
Once the
worksheet is opened, you can adjust the Zoom to suit your convenience. This can
be done by clicking the Zoom icon in the tool bar.
Note: EXCELÔ and WINDOWSÔ are registered trademarks of the Microsoft Corporation.
Entering Data:
Once the worksheet is opened, the user can
click the “Instructions “ button on the top right hand corner which will give
some information on where to enter the data, how to move to different sheets,
etc. The worksheets are protected to prevent accidental deletion of formulas.
The user can enter the data only in certain cells which have been highlighted
yellow.
Once the data is entered in a certain cell, pressing the
“Tab” key moves the cursor forward in sequence to the next data cell. If the
user wants to go back, pressing “Shift+Tab” will move the cursor backward. Of course, the user can also use the arrow
keys to move to different cells on the work sheet. However, using the Tab key
will move the cursor only to those cells which accept the data.
Calculation:
In Excel, calculations can be done either automatically or manually. “Automatic” option means that calculations are done automatically each time the content of any cell is changed. “Manual” option means that the user tells the computer when to do the calculation. In other words, the user can enter all of the data and tell the computer in the end to perform the calculation. The “Manual calculation” feature prevents the computer from performing unnecessary calculations during the intermediate steps.
The user
can chose between the two options by clicking the corresponding buttons at the
top. When the user has chosen the “Manual Calculation” option, the computer
will not perform any calculations until the user clicks the “Calculate” button or presses the F9
key. Hence it is very important to
remember this whenever the user changes data.
ACKNOWLEDGMENT
This project was sponsored by the Mack-Blackwell National Rural Transportation Study Center (MBTC), the U.S. Department of Transportation, Federal Highway Administration, and the Arkansas State Highway and Transportation Department (AHTD). The views and opinions expressed herein are those of the authors, and do not necessarily reflect the official views of MBTC, the U.S. Dept. of Transportation, or AHTD. This report does not constitute a standard, regulation, or specification. The use of commercial product names in this report does not constitute an endorsement or recommendation of the product(s).
REFERENCES
1. Peshkin, D.G. , and A.L., Mueller, “Field
Performance and Evaluation of Thin Bonded Overlays,” Transportation Research Record No. 1286,
1990.
2. National Cooperative Highway Research
Program. “Synthesis
of Highway Practice 204: Portland Cement Concrete Resurfacing,” National
Academy Press, Washington D.C. 1994.
3. Technical Bulletin 007.0-C “Guidelines
for Bonded Concrete Overlays,” American Concrete Pavement
Association. Arlington Heights, Illinois., 1990
4. Darter, M.I., and R.E. Smith, “Evaluation of
the FAA Overlay Design Procedures for Rigid Pavements,” Technical
Report prepared for U.S. Army Corps of Engineers, Waterways Experiment Station,
Vicksburg, Miss., 1981
5. Voigt, G.F., M.I Darter and S.H.
Carpenter, “Field Performance of Bonded Concrete Overlays,” Transportation
Research Record No. 1110, 1987.
6. Barenberg, E.J., “Rehabilitation
of Concrete Pavements by Using Portland Cement Concrete Overlays”, Transportation Research Record No.
814, 198.
7. Felt, E.J., “Resurfacing and Patching
Concrete Pavement with Bonded Concrete.” Highway Research Board
Proceedings. Vol. 35. Highway Research Board, National Research Council,
Washington, D.C. 1956.
8. Tayabji, S.D. and C.D. Ball, “Field
Evaluation of Bonded Concrete Overlays” Transportation Research
Record 1196, 1988.
9. Koesno, K. and B.F. McCullough. “Evaluation of
the Performance of Bonded Concrete Overlay on Interstate Highway 610 North, Houston, Texas” Transportation
Research Record 1196, 1988.
10. McGhee, K.H., and C. Ozyildirim. “Installation
Report, Construction of A Thin -Bonded Portland Cement Overlay Using
Accelerated Paving Techniques,” Virginia Transportation Research
Council, Charlottesville, January 1992.
11.
Darter, M.I., S.H. Carpenter, M. Herrin, E.J. Barrenberg, B.J.
Dempsey, M.R. Thompson, R.E. Smith, and M.B. Snyder. “Techniques
for Pavement Rehabilitation. Participant’s Notebook, revised,
National Highway Institute/FHWA. 1984
12. Transportation Research Board, “Resurfacing
with Portland Cement Concrete,” National Cooperative Highway Research
Program Synthesis of Highway Practice 99, 1982.
13. Gould, V.A., Summarized Committee
Report 1948-1960: Salvaging Old Pavements by Resurfacing. Highway
Research Bulletin 290, HRB, National Research Council, Washington, D.C., 1961
14. Iowa Department of Transportation. PCC Inlay-
Thin Bonded PCC Overlay-Unbonded PCC Overlay. Iowa Highway Project Open House,
1983.
15.
American Concrete Paving
Association and Portland Cement Association. Concrete Resurfacing 1977
Condition Survey, 1977.
16. Marks, V.J., Final Report, Thin Bonded
Portland Cement Concrete Overlay. Project Hr-520, Iowa Department of
Transportation, Ames, 1990
17.
Martin, R. HRB Special Report 116: Design and
Construction of Concrete Resurfacing of Concrete Pavements. National
Research Council, Washington, D.C. 1971.
18. American Association of State Highway
and Transportation Officials, “AASHTO Guide for Design of Pavement Structures,” Washington
D.C., 1993
19. Fogg, J.A., R.L. Baus and R.P Ray, “AASHTO Rigid
Pavement Design Equation Study,” Journal of Transportation Engineering,
American Society of Civil Engineers, Volume 117, Number 1, January/February
1991.
20. Seeds, S.B and R.G. Hicks, “Development
of Drainage Coefficients for the 1986 AASHTO Guide for Design of Pavement
Structures,” Transportation Research Record 1307, 1991.
21. Darter, M.I, K.T. Hall and C.M. Kuo, “Support
Under Portland Cement Concrete Pavements,” Report 372 National Cooperative
Highway Research Program, Transportation Research Board 1995.
22. Tyner, H.L., W. Gulden and D. Brown, “Resurfacing
of Plain Jointed-Concrete Pavements,” Transportation Research Record No.814,
1981
23. Highway Research Board, “The AASHO
Road Test, report 5, Pavement Research,” Special Report 61e, 1962.
24. Foxworthy, P.T., “Concepts for
the Development of a Non-Destructive testing and Evaluation System for Rigid
Airfield Pavements,” Ph.D. Thesis, University of Illinois at Urbana-Champaign,
1985.
25. Transportation Research Board “Revision of
AASHTO Pavement Overlay Design Procedures. Appendix: Documentation of Design
Procedures,” June 1991.
26. Uzan, J, R. Briggs and T. Scullion, “Backcalculation
of Design Parameters for Rigid Pavements,” Transportation Research
Record 1377, 1992.
27. Rao, S., “Analysis of In-Situ Moisture Content Data for Arkansas Subgrades”,
Master’s Thesis, University of Arkansas, Fayetteville, Arkansas, 1997.
28. Janssen, D.J., “Moisture in
Portland Cement Concrete,” Transportation Research Record No. 1121, 1987.
29. Yoder E.J. and M.W. Witzak, “Principles
of Pavement Design,” Second Edition, John Wiley and Sons, Inc. New York, 1975.
30. Poblete, M., A. Garcia, J. David,
P.Ceza and R. Espinosa, “Moisture Effects on the Behavior of PCC Pavements,” Proceedings,
Second International Workshop on the Design and Evaluation of Concrete
Pavements, Siguenza, Spain, October 1990.
31. Eisenmann, J and G. Leykauff., “Effect of
Paving Temperatures on Pavement Performance,” Proceedings, Second
International Workshop on the Design and Evaluation of Concrete Pavements,
Siguenza, Spain, October 1990.
32. Zachlehner, A., “Concrete
Pavement Stress due to the Influence of Hydration and Climatic Conditions,” Ph.D. Thesis,
Technical University of Munich, Germany, 1989.
33. Zachlehner, A., “ Restraint
Stresses in Young Concrete Pavements,” Proceedings, Sixth International
Symposium on Concrete Roads, Madrid, Spain, October 1990.
34. Hammitt, G.M. II, “Concrete
Strength Relationships,” U.S. Army Corps of Engineers Waterways Experiment Station,
Vicksburg, MS, 1971.