U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
2023664000
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
REPORT 
This report is an archived publication and may contain dated technical, contact, and link information 

Publication Number: FHWAHRT12023 Date: December 2012 
Publication Number:
FHWAHRT12023
Date: December 2012 
An essential part of this project was to conduct an extensive literature search. While FWD load deflection data have long been used to carry out projectlevel analyses, using these data to conduct networklevel surveys is performed less frequently. Nevertheless, the existing literature shows that several agencies are already using FWD data in their networklevel pavement analysis procedures.
Some agencies have also developed procedures that may assist in the development of a simplified approach to using deflection data in a networklevel analysis that links pavement structural parameters, determined through the use of FWD data to pavement performance.
A review of all available literature was duly conducted, including obtaining information from agencies outside the Unites States, to ascertain which agencies use deflection data, which approach they employ, and whether these particular approaches may be useful in the present study. All deflection parameters (measured or computed) included in this report refer to the Strategic Highway Research Program (SHRP) seven sensor positions.^{(2)}
Relevant literature references were reviewed on the following subjects to achieve the literature review objectives:
It is crucial to establish a reasonable body of knowledge on these topics to address the following:
Table 1 provides a summary of potentially relevant agency practices for FWD data collection and use for networklevel analyses identified during the literature review study (see the complete table of references in appendix A).
Table 1. Summary of useful practices for FWD use in networklevel analyses.
Agency and Publication  Test Point Spacing  FWD Test Frequency  FWD Sensor Positions  Limiting Factors  Basic Details of the PMS Approach 

California Department of Transportation (Caltrans); "Test to Determine Overlay and Maintenance Requirements by Pavement Deflection Measurements"^{(3)} 
N/A 
N/A 
N/A 
None; currently used for flexible and composite pavements only 
Uses center deflection only, plus existing asphalt concrete (AC) or AC + portland cement concrete (PCC) layer thickness. 
Federal Highway Administration (FHWA); Review of the LTPP Backcalculation Results^{(4)} 
N/A 
N/A 
N/A 
None; may be used for flexible, composite, and rigid pavements alike 
Uses Hogg model for subgrade modulus calculation. While it is possible that surface course stiffness can be derived using a related approach reported to FHWA by the authors, this is probably too complex and not worth the effort. 
National Cooperative Highway Research Program (NCHRP); Project 1048: Assessing Pavement Layer Condition Using Deflection Data^{(5)} 
Probably not studied 
Probably not studied 
SHRP positions, generally 
None; all pavement types studied using LTPP database (flexible, rigid, and composite) 
Uses easytoobtain deflection basin parameters and relates these parameters to observed pavement distresses. 
FHWA; Temperature Predictions and Adjustment Factors for Asphalt Pavement 
N/A 
N/A 
N/A 
N/A 
The adjustment for asphalt pavements for temperatureatdepth and center deflection adjustments for pavement temperature may be used to assist in the present study. 
Kansas Department of Transportation (KDOT); "NetworkLevel Pavement Deflection Testing 
A minimum of three tests per mile; five is preferred 
Onethird of network per year; however, onefifth of network may be adequate 
Unknown; probably unimportant 
Flexible pavements only 
Used data from one Kansas district on AC surfaces only (noninterstate), which is a similar approach to the current study. Based on the 1993 American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures.^{(8)} The FWD, especially the center deflection value, added considerably to predictive capability of method used. Based on the concept of pavement structural evaluation (PSE). PSE was found to be twice as important to predict pavement performance as any other observable parameter. Recommended the use of Bayesian statistical models. 
Alaska Department of Transportation (AkDOT); Modeling Flexible Pavement Reponse and Performance(9) 
0.1 mi 
After repaving 
SHRP positions 
No limits 
Deflections are converted to layer moduli, which are then used to obtain stress/strain values under a standard equivalent singleaxle load (ESAL). Transfer functions relate stress/strain to cracking in bound layers and permanent deformation in unbound layers. 
Texas Department of Transportation (TxDOT); Incorporating a Structural Strength Index into the Texas Pavement Evaluation System (FHWA/TX88/4093F)^{(10)} 
0.5 mi 
One recommended per year 
1 ft 
Flexible pavements less than 5.5 inches AC thickness 
Structural strength index (SSI) varies from zero to 100 (weak to strong). Based on normalized basin parameters, such as outer deflections, surface curvature index (SCI), and center deflection under a 9,000lb load. Can characterize subgrades and pavement structure independently in terms of relative stiffness. System based on statistical evaluation of deflections statewide. 
TxDOT; NetworkLevel Deflection Data Collection for Rigid Pavement^{(11)} 
0.5 mi 
N/A 
1 ft 
Rigid pavements 
Not very useful from an analytical standpoint; mostly gives recommendations regarding field test procedures, estimated costs for testing and analysis, and reference to maximum allowable deflections for rigid pavements for network testing. 
Virginia Department of Transportation (VDOT); NetworkLevel Pavement Evaluation of Virginia's Interstate System Using the Falling Weight Deflectometer (Report VTRC 08R18)^{(12)} 
0.2 mi 
45year cycle 
SHRP positions 
None 
Flexible pavements were analyzed by calculating the subgrade resilient modulus, the effective pavement modulus, and the effective structural number (SN). Rigid and composite pavements were analyzed by calculating the area under the deflection basin and the static modulus of subgrade reaction (kvalue). 
National Laboratory for Civil Engineering; Use of Deflections at Network Level in England for Programming and Other Purposes^{(13)} 
N/A; Used deflectograph, complemented by FWD 
35 years 
N/A 
Flexible pavements only 
Used Transport and Road Research Laboratory (TRRL) equations to calculate residual life and strengthening requirements based on deflection. The conclusion was that deflection does not increase with time and in some cases actually decreases due to an increase in pavement materials stiffness due to aging, etc. This approach was used until 2000. 
South Africa; "Benchmarking the Structural Condition of Flexible Pavements with Deflection Bowl Parameters"^{(14)} 
0.2 km 
N/A 
300 mm typical 
Flexible pavements only 
Pavement is divided into three zones based on depth. Uses basin parameters to characterize base, middepth, and subgrade structural condition such as "sound," "warning," or "severe." 
Association of Australian and New Zealand Road Transport and Traffic Authorities; Use of FWD in the Network Level Pavement Condition Survey^{(15)} 
N/A 
N/A 
0, 900, and 1,500 mm 
Flexible and semirigid pavements 
Uses adjusted SN as indicator for pavement bearing capacity. SN is calculated based on d0, d900, and d1500, in which d represents deflection and i is the distance from the center of the plate (0, 900 and 1,500 mm). 
^{N/A = Not available. 1 mi = 1.61 km 1 inch = 25.4 mm 1 ft = 0.305 m 1 lb = 0.454 kg} 
Findings from the literature indicate that only five agencies use, or are planning to use, load deflection data directly within their PMS.
United Kingdom
The United Kingdom uses traveling deflectograph data, not FWD or dynamic load deflection data; therefore, these data are not applicable to the present study.(13,16) The traveling deflectograph does not correlate well with the FWD (or even the rolling wheel deflectometer (RWD)) due to the large differences in loading time and the Benkelman beam style apparatus involved, among other salient factors. While researchers in the United Kingdom have tried to supplement their traveling deflectograph data with FWD data, they discovered that the two do not correlate well, as expected; therefore, the FWD was dropped in favor of the traveling deflectograph. Interestingly, the old Caltrans traveling deflectograph, which was similar to the UK device, was dropped at least 30 years ago, first in favor of the Dynaflect^{®} and later the FWD.
Alaska
AkDOT uses a multiyear cycle for deflection readings based on ongoing preand postrehabilitation projects. Their PMS approach using FWD data was only recently initiated.^{(9)} It cannot be considered a simple process since backcalculation of sorts is currently used to determine remaining life. How well AkDOT's FWD evaluation procedure actually translates to pavement distresses, and therefore to pavement life expectancy, remains to be seen. Alaska's application of load deflection data within their PMS is relatively new. Accordingly, a few more years' worth of data will be needed to evaluate structural versus performance correlations. At the time this report was written, AkDOT gathered over 300 mi (483 km) of FWD data on the Parks Highway. These data were subsequently used as an example of how to incorporate the simplified methodology developed herein and will be shown later in this report.
Conversely, the methodology used to calculate the subgrade "seed" modulus using the AkDOT procedure is straightforward and immediately useable as a standalone parameter. Therefore, it can be used to correlate the subgrade modulus from the LTPP FWD database to see how well that particular approach (i.e., a nonlinear subgrade modulus model) relates to the various performance indicators in the mature LTPP database. The AkDOT procedure currently applies to flexible pavements only; however, rigid pavements may be investigated using the same subgrade modulus closedform backcalculation process.
Texas
TxDOT currently uses deflection data in their PMS for flexible pavements only (district optional).^{ (See references 10, 11, 17, and 18.)} The publication Incorporating a Structural Strength Index into the Texas Pavement Evaluation System has been in use since its publication in 1988 in some of Texas's districts.^{(10)} It is currently used for relatively thin asphaltsurfaced pavements in 4 of 13 districts in Texas  Fort Worth, Houston, San Antonio, and Pharr. It is expected that the entire State will soon use the newer process shown in table 1.^{(10)}
In an older TxDOT method, networklevel deflection basins were collected at selected locations in the State to determine averages and ranges for a variety of pavement thicknesses and subgrade types. Basin parameters were developed and used in lieu of a backcalculation approach due to the lack of pavement structural data (i.e., number of layers, layer thicknesses, and material types). The greatest drawback to this approach is that it does not provide an indication of remaining structural life or bearing capacity. This approach essentially uses two basin parameters: SCI and W7, which is the deflection measured 6 ft (1.83 m) from the FWD load center. SCI is the difference between the deflections measured at the center of the load plate and the deflection measured 12 inches (304.8 mm) from the load plate. From these two parameters, an SSI is determined from one of three tables: one for surface treated pavements, one for thin asphalt pavements, and one for intermediate and thick asphalt pavements. SSI is further adjusted for traffic and rainfall levels to arrive at a final SSI. SSI ranges from 100 for a 100 percent structurally adequate pavement to zero for a 100 percent structurally inadequate pavement (meaning zero percent adequacy). This approach allows the user to determine where the inadequacies lie, whether within the subgrade or somewhere within the pavement structural section itself, above the subgrade.
Virginia
VDOT recently began using deflection data in their PMS, covering the entire network on a 4 to 5year cycle for both flexible and rigid pavements.^{(12)} The procedure is simple and straightforward. However, since it is relatively new, the current study will have to evaluate the relationship of Virginia's structural determination procedures to pavement performance data in the LTPP database.
In developing the VDOT method, a twophase evaluation of a proposed networklevel structural evaluation technique was carried out in 2007 and 2008. Both phases were restricted to interstate pavements. In phase I, only I64 and I77 were included in the study. The results looked promising, so phase II was initiated on the remainder of Virginia's interstate system. A final report was published in 2008, which concluded that networklevel deflection testing could be used to address network needs in terms of rehabilitation strategies and funding management decisions.^{(12)} The author further suggested that the networklevel structural analysis should be expanded to include Virginia's primary network.
Pavement types in the study included flexible, rigid, and composite pavement structures. For flexible pavements, the analysis was conducted according to the 1993 AASHTO Guide for Design of Pavement Structures. ^{(8)} For rigid and composite pavements, the analysis was conducted according to the 1998 Supplement to the AASHTO Guide for Design of Pavement Structures: Part II  Rigid Pavement Design and Rigid Pavement Joint Design.^{(19)}
For flexible pavements, the resilient modulus (M_{r}) is first calculated according to the 1993 AASHTO Guide for Design of Pavement Structures.^{(8)} The effective pavement modulus (E_{p}) is calculated from the center deflection and subgrade modulus, also according to AASHTO procedures. Finally, an effective structural number (SN_{eff}) is determined from E_{p} and the total pavement thickness above the subgrade.^{(8)}
For rigid and composite pavements, the deflection basin AREA and the static modulus of subgrade reaction, k, are used to quantify overall structural condition.
A cumulative distribution curve of SN_{eff}and M_{r} was developed for all flexible pavements in the study. Similar curves were developed for center deflections and deflection basin AREA for rigid pavements. These distribution curves are used to identify structurally weak areas of the network for further projectlevel investigation.
One factor that contributed to the overall success of this approach is the availability of pavement layer data (thicknesses, material types, etc.) in VDOT's Highway Traffic Record Information System database.^{(20)} Also noted were rather high subgrade Mr in the cumulative distribution curves. The 50th percentile value of the subgrade stiffness was around 12,500 psi (86,125 kPa). This may have been due in part to the presence of rigid or semirigid layers in the subgrade, which the AASHTO method does not address, and in part due to the typical nonlinear behavior of cohesive subgrades (with a lower effective modulus under the load than at increasing distances from the load). The 1993 AASHTO Guide for Design of Pavement Structures equations utilize the outer deflection sensors to characterize the subgrade modulus, not the true effective modulus of the subgrade under the load.^{(8)}
South Africa
The Republic of South Africa uses FWD testing at 656ft (200m) intervals to ascertain the bearing capacity of three roadway "zones" or layers (surface, base, and subgrade). The methodology relies on direct deflection basin parameters, not backcalculation.^{(14)} A benchmarking technique was developed to relate direct deflection values and basin parameters to the structural condition of the pavement at both the project and network levels. This benchmarking technique can be used in the absence of detailed information regarding pavement composition.
The South African method correlates FWD deflections to structural layers or zones within the pavement structure. These zones are numbered 1 through 3, where zone 1 corresponds to the materials closest to the load plate within the pavement structure, extending out to 11.7 inches (300 mm) in terms of the deflection basin (and probably somewhat less in terms of depth within the structure). In zone 2, the inflection point in the deflection basin occurs and generally lies from < 1 to 2 ft (< 0.305 to 0.61 m) in depth below the pavement surface. Zone 3 extends from approximately 2 to 6.5 ft (0.61 to 1.98 m) from the center of the load plate and thus represents the deepest layers in the pavement structure  primarily the subgrade.
In the South African method, the following deflection parameters and associated pavement structural zones are represented:
Figure 1. Equation. Calculation of RoC
Figure 2. Equation. Calculation of BLI
Figure 3. Equation. Calculation of MLI
Figure 4. Equation. Calculation of LLI
Where:
L = Diameter of the load plate of the FWD (mm).
D_{0} = Deflection at center of load plate (mm).
D_{200} = Deflection at 8 inches (200 mm) from the center of the load plate (mm).
D_{300} = Deflection at 12 inches (300 mm) from the center of the load plate (mm).
D_{600} = Deflection at 24 inches (600 mm) from the center of the load plate (mm).
D_{900} = Deflection at 36 inches (200 mm) from the center of the load plate (mm).
These deflection bowl parameters are related to a structural condition rating according to the structural condition and the type of base layer, as described in table 2, extracted from Horak's "Benchmarking the Structural Condition of Flexible Pavements with Deflection Bowl Parameters."^{(14)}
The basin parameters LLI, MLI, and BLI are plotted against stationing for a particular roadway to facilitate the structural assessment procedure. This plot of structural condition is often used in conjunction with visual assessments to determine the type of rehabilitation required to restore the pavement to a good condition.
Table 2. Structural condition associated with deflection bowl parameters.(14)
Base Type  Structural Condition Rating  Deflection Bowl Parameters  

D_{0} (μm) 
RoC (m) 
BLI (μm) 
MLI (μm) 
LLI (μm) 

Granular base 
Sound 
< 500 
> 100 
< 200 
< 100 
< 50 
Warning 
500750 
50100 
200400 
100200 
50100 

Severe 
> 750 
< 50 
> 400 
> 200 
> 100 

Cementitious base 
Sound 
< 200 
> 150 
< 100 
< 50 
< 40 
Warning 
200400 
80150 
100300 
50100 
4080 

Severe 
> 400 
< 80 
> 300 
> 100 
> 80 

Bituminous base 
Sound 
< 400 
> 250 
< 200 
< 100 
< 50 
Warning 
400600 
10250 
200400 
100150 
5080 

Severe 
> 600 
< 100 
> 400 
> 150 
> 80 

^{Note: These criteria can be adjusted to improve sensitivity of benchmarking. 1 mil = 25.4 1 ft = 0.305 m} 
The surface modulus is calculated at the various sensors and plotted against distance from the load plate to assess nonlinearity. The standard Boussinesq equations are used for this purpose. Points where the surface modulus increases with increasing distance from the load center are regarded as "stress softening." If the surface modulus is similar for all measured deflections, the subgrade is regarded as linearelastic. Decreasing surface modulus with increasing distance from the load center indicates a "stress stiffening" subgrade.
In addition to the previously listed agencies that already utilize load deflection data within their PMS, the following agencies use deflection data with potential networklevel PMS applications:
The following publications do not describe deflection data applications of specific agencies, but they are relevant to the scope of this project:
Based on the literature review and summary of useful practices presented in table 1, a list of deflection techniques was created. Several techniques were considered to be applicable to the scope of this project and were selected for further analysis. The following techniques and equations were selected when applicable:
Figure 5. Equation. Calculation of CIi
Figure 6. Equation. Calculation of Fi
Figure 7. Equation. Calculation of Si.
Figure 8. Equation. Calculation of Ii
Where i is the sensor 1 through 7.
Figure 9. Equation. Calculation of RA
Where i is the sensor 1 through 7.
Figure 10. Equation. Calculation of TA
Where i is the sensor 1 through 7
Figure 11. Equation. Calculation of SNeff.
Where D is the total pavement layer thickness above the subgrade (mm) and Ep is the effective pavement modulus, computed using the equation in figure 12.
Figure 12. Equation. Calculation of d_{0}.
Where d_{0} is deflection at the center of the load plate (mm), P is load plate pressure (MPa), a is load plate radius (mm), D is pavement layers thickness above subgrade (mm), and Mr is the resilient modulus, given by the following expression:
Figure 13. Equation. Calculation of Mr.
Where Mr is resilient modulus (MPa), P is applied load (kN), dr is deflection at a distance r from the center of the load (mm). r is chosen as the distance to the seventh sensor
Figure 14. Equation. Calculation of E0.
Where E0 is subgrade modulus (effective) (MPa), μ0 is Poisson's ratio for subgrade, S_{0} is the theoretical point load stiffness (MPa), S is the pavement stiffness (MPa), P is applied load (kPa), Δ_{0} is the deflection at center of load plate (mm), L is the characteristic length (mm), and I is the influence factor. ^{(4)}
Figure 15. Equation. Calculation of Esg.
Figure 16. Equation. Calculation of n.
Figure 17. Equation. Calculation of C.
Where:
E_{sg} = Subgrade modulus (kPa).
C = A layer constant, n is a layer constant representing the nonlinearity of the subgrade.
σ = Principle stress (MPa).
σ' = Reference stress (14.5 psi (0.1 MPa)).
E_{0(r)} = Surface modulus at distance r from load center (MPa).
r = Horizontal distance from load center (mm).
z = Vertical depth with respect to the pavement surface (mm).
σ_{(r)} = Principle stress at depth z (assuming z = r).
μ = Poisson's Ratio, d_{(r)} is deflection at distance r from load center (mm).
P = Applied load on pavement surface (MPa).
R = Distance to evaluation point (R^{2} = r^{2} + z^{2}).
θ = Angle with respect to vertical axis through load center.
In addition to the parameters mentioned, the load transfer efficiency (LTE) value was calculated for rigid pavements. There are two ways of calculating LTE  in one method, the FWD test is performed at the slab before the joint (joint approach), and in the second method, the FWD test is performed at the slab after the joint (joint leave). An analysis of both was conducted, and the values were close to each other. Therefore, for simplification, only one LTE value was chosen. The option was for the first method with LTE at the joint approach because it does not require any modification on the configuration of the FWD. The LTE at joint approach is referred to as "LTEA" in this report and was calculated as the ratio of the deflection at the third sensor (12 inches (304.8 mm) from the center of the load) and the deflection at the center of the load.
Several successful techniques for use in networklevel PMS applications were identified through the literature search. A few highway agencies have been using deflections as part of their network condition assessment. Among those, Texas, Virginia, and South African approaches were selected for further investigation in succeeding tasks with little or no modification. In addition, there were several techniques that could be utilized in the present study even though these techniques were not used in a networklevel PMS. Combining all sources available, a list of potential applicable techniques that are relevant to the scope of this project was created as a result of the literature review and is shown in table 1. These deflection techniques formed the basis for the analyses carried out in this project.