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Publication Number: FHWA-HRT-12-023
Date: December 2012

 

Simplified Techniques for Evaluation and Interpretation of Pavement Deflections for Network-Level Analysis

APPENDIX A

Table 48. Useful practices for FWD use in network-level analyses.

Agency and Publication Use of Deflection Data in PMS FWD Test Point Spacing FWD Test Frequency FWD Sensor Positions Limiting Specifics on PMS Use Applicability of Current Analysis Approach in Streamlined and Simple PMS Would Unmodified Result be Useful Relating Observed Rate of Deterioration to Deflection? Possibility
of this Approach Combined with Others
Summary

Caltrans;
Caltrans Test Method No. 356(3)

No

N/A

N/A

N/A

N/A

Yes

No

Probably

Uses center deflection only plus existing AC or AC + PCC layer thickness. Since it only applies to the DNF regions of California, the criteria would need shifting, and the time of year or day of testing would have to be better defined. Although it is generally used for rehabilitation design, it could be tiered into three or more categories (e.g., immediate project-level design, cosmetic or pavement preservation improvements, and no rehab or cosmetic improvements at this time).

FHWA;
Review of the LTPP Backcalculation Results(4)

No

N/A

N/A

N/A

N/A

Yes

No

No

If the subgrade modulus or indicator is regressed against various performance indicators, the Hogg model can be regressed at the same time. Furthermore, the entire LTPP database contains all available Hogg model subgrade stiffness values. While it is possible that surface course stiffness can also be derived using a related approach reported to FHWA, this is probably not worth the effort (subject to review by other team members).

NCHRP;
"Assessing Pavement Layer Condition Using Deflection Data"(5)

No; feasibility study only

Probably not studied

Probably not studied

SHRP positions, generally

No limits pavement types were studied using LTPP database

Yes

Yes

Probably

This project used easy-to-obtain deflection basin parameters and related these parameters to observed pavement distresses. As far as performance indicators are concerned, this study fits well into the current study. More information on the specifics should be obtained from the final report.

Swedish Road and Transport;
Strategic Highway Research Program and Traffic Safety on Two Continents(33)

No; feasibility study only

N/A

N/A

N/A

N/A

No

No

No

The method uses a kind of forward-calculation using modulus matrices similar to how the MODULUS program works. It was only proposed for a single province in Sweden, and it is not practical or easy to understand for a typical transportation department. Mechanistic concepts are employed, which are unlikely to be met with widespread success in the United States or Canada. AC pavements only.

South Carolina Department of Transportation;
Feasibility of Including Structural Adequacy Index as Indicator of Overall Pavement Quality in the SCDOT Pavement Management System(34)

No

N/A

N/A

N/A

N/A

No

No

No

Conclusions were that FWD data may be useful for flexible pavements only; recommended pilot study for this purpose. Not useful for the present study.

FHWA;
Temperature Predictions and Adjustment Factors for Asphalt Pavement (FHWA-RD-98-085)(6)

No

N/A

N/A

N/A

N/A

Yes

No

Yes

The adjustment for asphalt pavements for temperature-at-depth and center deflection adjustments for pavement temperature may be used to assist in the present study.

FHWA; Backcalculation of Layer Moduli of LTPP General Pavement Study (GPS) Sites, FHWA-RD-97-086(35)

No

N/A

N/A

N/A

N/A

No

No

No

Not useful for present study (back-calculation only).

Kansas Department of Transportation;
" Network-Level Pavement Deflection Testing and Structural Evaluation(7)

No; study only based on 3 years of data from one district

Minimum of three tests per mile; five tests is preferred.

One-third of network per year; however,
one-fifth of network may be adequate

Unknown; probably unimportant

AC only

Yes

Yes

Probably

Data were used from one Kansas district on AC surfaces only (non-interstate). For the technology and information available at the time, it used a similar approach to the current study. The study was based on the 1993 AASHTO Guide for Design of Pavement Structures. FWD, especially center deflection, added considerably to predictive capability of method used.(8) Based on the concept of PSE; PSE is twice as important as a pavement performance predictor than any other observable parameter. Bayesian statistical approaches were recommended; however, Kansas did not implement it on a statewide basis thereafter.

Taiwan;
"Evaluating the Structural Strength of Flexible Pavements in Taiwan Using the Falling Weight Deflectometer"(21)

No

N/A

N/A

N/A

N/A

Yes

No

Probably as it uses deflection-based parameters to a structural index

The report elaborates on using a combination of deflection-based parameters into a structural index. The index was empirically developed and relates to poor or good pavement condition assessed by experienced engineers and surveys. The method employed is not clearly described, but it involves binary assessment (0 and 1) of pavement conditions. This assessment is then related to the structural index through a linear equation. The binary analysis is similar in concept to what the research team is proposing using the probabilistic model.

Independent study in the United Kingdom; "Assessing Variability of In Situ Pavement Material Stiffness Moduli"(36)

No

N/A

N/A

N/A

N/A

N/A

N/A

N/A

The report discusses variability, and conclusions are made regarding spacing of FWD testing.

Independent study in Minnesota;
Spatial Variability of Falling Weight Deflectometer Data: A Geostatistical Analysis (29)

No

N/A

N/A

N/A

N/A

Yes

No

Probably

This report discusses variability in FWD measurements using the geostatistical concept. Geostatistics are used when a continuous measure is sampled at discrete points in space, like deflections. They use a variogram to assess the variance of measurements and infer the optimum spacing that will results in maximum coverage of pavement sections with different structural behavior. This approach is similar to what the research team is proposing for phase II.

Netherlands;
Application of FWD Measurements at the Network Level (37)

No

N/A

N/A

N/A

N/A

Possibly

No

Probably

The report describes development of a structural adequacy indicator based on SCI600 in non-trafficked area, and structural distress indicator based on SCI600 in trafficked and non-trafficked area and cracking.

United Kingdom;
Use of Deflections at Network Level in England for Programming and Other Purposes (13)

Yes

N/A; used deflectograph, complemented by FWD

3–5 years

N/A

Flexible pavements only

No

N/A

N/A

Used 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, decreases due to increases in pavement materials stiffness. This approach was used in United Kingdom until 2000.

Department of Main Roads, Queensland, Australia;
Reliability of Optimal Intervals for Pavement Strength Data Collection at the Network Level(38)

No; feasibility study only

200–1,000 m, 1,000 m selected

N/A

Unknown; probably unimportant

Flexible pavements only, wet and non-reactive soil

No

No

No

Budget estimates were developed for different FWD spacings. SNs were calculated from deflections. The information obtained is not useful in the present project.

Department of Main Roads, Queensland, Australia
A Probability-Based Analysis for Identifying Pavement Deflection Test Intervals for Road Data Collection(39)

No; feasibility study only

200–1,000 m, 1,000 m selected

N/A

Unknown; probably unimportant

Flexible pavements only, wet and non-reactive soil

Possibly

No

Probably

Kolmogorov-Smirnov probability was used based on goodness-of-fit test to identify optimal interval for pavement deflection data collection. Based on limited amount of data on a specific pavement type (surface treated pavement with granular base) and subgrade. Not applicable to present project.

Hong Kong;
"Pavement Management - Development of a Pilot PMS" (40)

Pilot project

50 m

N/A

N/A

N/A

No

No

No

Only the abstract was available, and it provided little useful information.

Europe;
The Way Forward for Pavement Performance Indicators Across Europe, Final Report - COST Action 354 Performance Indicators for Road Pavements(41)

N/A. European project that collected data from different countries/ agencies

N/A

N/A

N/A

N/A

Possibly

No

Yes

The proposed approach uses residual life or SCI300 for calculation of bearing capacity performance index. There are no details how remaining life is calculated and that part is probably not useful for current project. The use of parameters of deflection basin (like SCI300) can be considered in present project.

Europe;
Use of Falling Weight Deflectometers in Pavement Evaluation(42)

Yes

200–500 m

3–5 years

0, 300, 600, 900,1,200, and 1,500 mm for d0 > 1.0 mm; 0, 300, 600, 900, 1,500, and 2,100 mm for d0 < 0.50 mm

Not specified

Possibly

No

Yes

The proposed approach uses central deflection or SCI (eventually SN calculated from deflections) at the first two levels. At the third level, it proposes the use of back-calculation, which is not feasible for the present project.

Australia;
Comparison of Project-Level and Network-Level Pavement Strength Assessment (43)

Yes, some members of
Austroads

N/A

N/A

0, 900, and 1,500 mm

Flexible and semi-rigid pavements

Possibly

No

Yes

Uses adjusted SN (SNP) as an indicator for pavement bearing capacity. SNP is calculated based on d0, d900, and d1500.

United Kingdom; Structural Assessment of the English Strategic Road Network - Latest Developments(16)

Yes, but rolling deflectometer only.

N/A

N/A

N/A

N/A

No

No

No

The report discusses future use of traffic speed deflectometer instead of deflectometer or FWD at the network level. A modified BELLS equation was used for pavement temperature estimate based on air temperatures. Not useful for present project.

AkDOT;
Modeling Flexible Pavement Response and Performance(9)

Yes

0.1 mi

After repaving

SHRP positions

No limits

Possibly, for "standardized" pavement structures

No

Yes

Deflections are converted to layer moduli, which are then used to obtain stress/strain values under a standard ESAL. Transfer functions relate stress/strain to cracking in bound layers and permanent deformation in unbound layers.

TxDOT; Incorporating a Structural Strength Index into the Texas Pavement Evaluation System(10)

Yes, but optional by district

0.5 mi

Recommended one per year

1 ft

Flexible pavements less than 5.5 inches AC thickness

Yes

Yes

Yes

SSI varies from zero (weak) to 100 (strong). Based on normalized basin parameters such as outer deflections, SCI, and center deflection under a 9,000 lb (4,086 kg) load. Can characterize subgrades and pavement structure independently in terms of relative stiffness. The system is based on statistical evaluation of deflections statewide.

TxDOT; "Development of Structural Condition Index to Support Pavement Maintenance and Rehabilitation Decisions at Network Level"(17)

Yes

0.25 mi

N/A

1 ft

Flexible pavements

Yes

Yes

Yes

SNeff and Mr are calculated using deflection parameters and then compared to the structural number required (SNreq) using traffic data. Structural condition index (SCI) is the ratio of SNeff and SNreq. Novel approach uses simplifications arrive at SNeff directly from deflections.

TxDOT; Development of a New Methodology for Characterizing Pavement Structural Condition for Network-Level Applications(18)

Yes

0.25 mi

N/A

1 ft

Flexible pavements

Yes

Yes

Yes

SNeff and Mr are calculated using deflection parameters, then compared to SNreq using traffic data. SCI is the ratio of SNeff and SNreq. Novel approach uses simplifications to arrive at SNeff directly from deflections.

TxDOT;
Network-Level Deflection Data Collection for Rigid Pavement(11)

Yes

0.5 mi

N/A

1 ft

Rigid Pavements

N/A

N/A

N/A

From an analytical standpoint, the report mostly provides recommendations regarding field test procedures. It refers to an earlier document regarding maximum allowable deflections for rigid pavement network testing.(11)

VDOT;
Network-Level Pavement Evaluation of Virginia's Interstate System Using the Falling Weight Deflectometer(12)

Yes

0.2 mi

4–5 year cycle

SHRP positions

No

Yes

Yes

Yes

Flexible pavements were analyzed by calculating the subgrade resilient modulus, the effective pavement modulus, and the effective SN. Rigid and composite pavements were analyzed by calculating the area under the deflection basin and the static modulus of subgrade reaction.

United States;
Modern Pavement Management (44)

No; propose method only

Unspecified

Unspecified

Uses center def only

Flexible pavements

Yes

Yes

Yes

Uses traffic loading info to establish a maximum tolerable Benkelman beam deflection. Not useful for purposes at-hand.

South Africa;
"Benchmarking the Structural Condition of Flexible Pavements with Deflection Bowl Parameters" (14)

Yes

0.2 km

N/A

300 mm typical

Flexible only

Yes

Yes

Yes

Pavement is divided into three zones based on depth. The report uses basin parameters to characterize base, mid-depth, and subgrade properties as sound, warning, or severe.

N/A = Not available.
1 mi = 1.61 km
1 inch = 25.4 mm
1 ft = 0.305 m
1 lb = 0.454 kg

 


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