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

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.

Disadvantages

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.

Partially Bonded Overlays

            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).

DESIGN OF PCC OVERLAYS

            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 D_eff. The difference between the structural capacity required to carry the future traffic (e.g. D_f) and effective structural capacity (D_eff) will be the structural capacity of the overlay (D_OL) which is shown by the following general equation.

D_OL = D_f - D_eff

            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    D_ol        = thickness of the overlay (in)

                        D_f         = Slab thickness required to carry the future traffic (in)

                        D_eff        = Effective slab thickness of the existing pavement (in)

Effective slab thickness of the existing pavement D_eff 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 (D_f) 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

                        Z_R         = Standard Normal Deviate (function of reliability R)

S_o         = Standard Deviation

D          = Slab thickness to carry future traffic (D_f)

                                  DPSI       = Loss in Serviceability (P_initial - P_terminal)

                                    Pt         = Terminal Serviceability

                                    S_c         = PCC Modulus of Rupture (psi)

                                    C_d                  = Coefficient of Drainage

                                    J           = Load Transfer Coefficient

                                    E_c         = 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 (D_f)

            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 D_f, in order of significance, are as follows (19):

1.    PCC modulus of rupture and elastic modulus (S_c and E_pcc)

2.    Reliability level ( R )

3.    Drainage Coefficient (C_d)

4.    Load Transfer Coefficient (J)

5.    Overall Standard Deviation (S_o)

6.    Design ESALs (W₁₈)

7.    k-value

PCC Modulus of Rupture and Elastic Modulus

The PCC modulus of rupture and elastic modulus to determine D_f 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:

E_c =57,000 (f’_c)^0.5

            where    E_c = 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) = S_c + z (SD_s)

                        where    S’_c = estimated mean value for PCC modulus of rupture (psi)

                                    S_c  = construction specification on concrete modulus of rupture (psi)

                                    SD_s = 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 (S_o) 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, C_d 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, C_d 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 C_d 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 C_d 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 C_d 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 C_d and k-value for Rigid Pavement Design

Unfortunately, selecting a proper value for C_d 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 C_d 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