Lecture 12 TxDOT Flexible Pavement Design Method Texas ME (FPS21)

Background

Design software for the TxDOT method is called Texas Flexible Pavement System (FPS21).

The Triaxial procedure supplements FPS to ensure that pavement design has enough thickness to protect the subgrade against a compressive failure. The new version of the software incorporates the Texas Triaxial design check.

It checks for performance under low frequency but high load magnitudes.

FPS uses multi-layer linear elastic solutions to calculate the pavement responses.

Provides the capability of designing pavement structures with up to six layers over the subgrade (seven layers in total).

The new version of the software (FPS21) provides the capability of designing perpetual pavements where multiple hot-mix asphalt (HMA) layers of different moduli values are required.

Provides the capability of generating user defined pavement structures in addition to the fixed design options.

Provides additional procedures for obtaining estimated Texas Triaxial Class values for the subgrade soils, either based on county-specific soil types or from basic soil properties such as plasticity index.

Incorporates a stress calculation module for analysis of pavement sections.

General Capabilities of FPS21

Pavement Design Concept

The design checks in TxDOT method are principally based on two components:

I. Mechanistic design concepts, which determines the fatigue life and subgrade rutting potential based on typical form of transfer functions, (user has the option to modify the coefficients of the transfer functions for specific HMA thickness, mix volumetrics or geography).

II. The Modified Texas Triaxial criteria, which evaluates the impact of the anticipated heaviest load on the proposed pavement structure.

Design Checks in FPS

Surface Curvature Index (SCI)

The motivation to select the SCI as a measure of combined layers strength was that pavements with smaller SCI value represent smaller differential deflections.

Dual Tire

12”

Deflection

Bowl

W1

W2

Pavement

Surface

SCIdualwheel=

W1-W2

SCI: Surface Curvature Index, the difference of the W1 and W2 deflections.

The rationale behind SCI is that smaller curvature difference corresponds to stronger pavements and therefore longer service life.

2

1

SCIfN

7’’

12”

4,500 lb.

4.23 inch Contact Radius

4,500 lb.

4.23 inch Contact Radius

7’’

SCI dual wheel = W1-W2

W1 = Deflection measured

between dual wheels

W2 = Deflection measured

12” away from dual wheels

Dual Tire Deflection Measurement Plan View

N = Allowable No. of 18 Kip ESALs

PSIf = Terminal Serviceability

PSIi = Ride Score Measurement from PMIS

T = District Temperature Constant (9-38)

SCI= Surface Curvature Index

2)])[(134.0(

]2/1)5(2/1)5[(

wheeldualSCI

Ti

PSIf

PSIN

TxDOT Design Based on the Curvature of the Deflection Basin

Texas Triaxial Check (TTC) Tex-117-E

The Texas Triaxial Classification of soils was developed in the late 1940s and early 1950s by TxDOT as an indexed soil/base classification system related to material shear strength.

Evaluating a material for its Texas Triaxial Classification is covered in “Tex-117-E, Triaxial Compression for Disturbed Soils and Base Materials.”

The Modified Triaxial Design Method requires the use of the subgrade and/or base Texas Triaxial Class as derived from laboratory test results. The results of the analysis are used to determine the minimum amount of material, better than the subgrade, required to prevent shear failure of the subgrade, or to ensure base materials are not over-stressed.

Texas Triaxial Classification (Tex-177-E)

Run triaxial shear test on the soil sample at different confining pressures.

Recall that the strength (stress at failure) was directly related to the confining pressure.

Plot stress-strain curves as shown in the graph.

Determine the deviatoric stress and confining stress for each test.

Texas Triaxial Classification Process

Plot the Mohr circle for each test and graphically plot the tangent that represents the failure envelope.

Texas Triaxial Classification Process

Transfer the failure envelope onto the chart called, 'Chart for Classification of Sub-grade and Flexible Base Material,' and classify the material to the nearest one-tenth of a class.

When the envelope of failure falls between class limits, select the critical point or weakest condition on the failure envelope.

Measure the vertical distance down from a boundary line to the point to obtain the exact classification (3.7) as shown in the 'Chart for Classification of Sub-grade and Flexible Base Material.'

Texas Triaxial Classification Process

Class 1 Class 2 Class 3 Class 4 Class 5 Class 6

High Shear Strength Good Quality Base Material

Low Shear Strength Low Quality Subgrade

Texas Triaxial Classification

Initial Depth of Pavement Table for Texas Triaxial Design Method (HRB, 1949)

Class of Material

General Description of Material

Depth of Pavement (Base and Surfacing, in.)

8,000 lb wheel load 12,000 lb wheel load 16,000 lb wheel load

High E* (20 ksi) Base Course

Low E* (6 ksi) Base Course

High E* Base

Course

Low E* Base

Course

High E* Base

Course

Low E* Base

Course

1 Good

flexible base Good – light bituminous surfacing acceptable

2 Fair flexible

base One to four inches of bituminous surfacing or a stable layer of Class 1 material covered

with a good light surfacing

3 Borderline base and subbase

3 – 8 4 – 10 4 – 10 5 – 12 4 – 12 6 – 14

4 Fair to poor

subgrade 8 – 13 10 – 16 10 – 15 12 – 20 12 – 18 14 – 23

5 Weak base 13 – 17 16 – 21 16 – 21 20 – 26 18 – 24 23 – 30

6 Very weak subgrade

17+ 21+ 21+ 26+ 24+ 30+

Th

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Design Wheel Load in Thousands of PoundsDesign Wheel Load in Thousands of Pounds

55

1010

1515

2020

2525

3030

3535

4040

0000 22 44 66 88 1010 1212

Class 1Class 1

Class 2Class 2

Class 3Class 3

Class 4Class 4

Class 5Class 5

Class 6Class 6

Class 6.5

Class 6.5

1414 1616 1818 2020

FPS Analysis/Design Software

The Flexible Pavement System (FPS) is a Mechanistic-Empirical (M-E) design software routinely used by the Texas Department of Transportation (TxDOT) for:

(1) Pavement structural (thickness) design

(2) Structural overlay design

(3) Stress-strain response analysis

(4) Pavement life prediction (rutting and cracking)

(5) Construction cost analysis

FPS Software Input Parameters

Analysis Period: interval time between reconstruction or major pavement rehabilitation efforts.

Minimum time to first overlay and between overlays.

Minimum serviceability index (higher for major roads).

Design confidence level (reliability) (A: 80%, B: 90%, C: 95%, D: 99%, E: 99.9%).

Interest rate for cost analysis.

Initial Serviceability Index (ISI)

This input depends on the materials used and construction

practices.

Historical initial serviceability indices had a statewide average of

about 4.2, but with the advent of ride quality specifications, the

average should be trending up.

Suggested values are as follows:

Surface Treatments: 4

Thin HMA Surface: 4.5

Thick HMA(>4’’): 4.8

This variable controls the reliability with which the specified quality of pavement service will be satisfied. Its choice should depend on the consequences of failing to provide the specified quality throughout the analysis period.

The designer must specify confidence levels by assigning a letter code A- B- C- D- E. A is the lowest and E is the highest reliability level.

Design Reliability

District Temperature Constants

The original intent of this input was to account for the increased susceptibility of asphalt layer to crack under traffic in cold weather. However, districts in colder winter climates were severely penalized by forcing thicker structures that were still susceptible to thermal cracking.

Beginning ADT (vehicles/day)

This is normally a two direction traffic volume input parameter.

Ending ADT (vehicles/day)

Traffic Volume at the end of a 20-year analysis/design period.

20-Yr. 18-kip ESALs (One Direction)

If the analysis period is other than 20 years, an internal traffic equation will adjust the cumulative ESALs to the correct value for the analysis period used.

Adjustments for lane distribution may be made when at least three lanes exist in both directions.

FPS Traffic Inputs

By clicking on the District input box a map is provided where the user can select a new District and County. Updated default subgrade support values are provided within FPS 21 for every county in Texas. Also, a database of county soil types with average Texas Triaxial Class values is incorporated in the FPS21.

If you don’t want to consider stage rehabilitation, choose Min. Time to First Overlay the same as Length of Analysis Period (Design Life). You’ll end up with the above message. Select Ignore the Check and Continue.

Planned Rehabilitation or Stage Construction

FPS21 Internal Input Parameter Checks

After clicking on the Go back button the following screen will appear that allows access to the layer material parameters table.

Edit the material type description, layer moduli, and thickness ranges as desired.

Editing the Materials and

Thicknesses Table

The program provides five different traffic control methods during overlay operations.

Methods one and two are for two-lane roads (Method one - with and Method two - without shoulders). Methods three- four-and five are for roads with four lanes or more.

The designer must decide which type of traffic control/detour method will be used when asphalt overlays are applied. The model number of the detour method must be coded for this input.

Detour Design for Overlay

The designer is assisted in selecting the correct model by means of a graphical display.

FPS21 Pavement Design Check

Texas Triaxial Design Check Mechanistic Design Check

The mechanistic design check computes and checks the sufficiency of the mechanistic responses in terms of the maximum induced horizontal tensile strain at the bottom of the lowest HMA layer and the maximum vertical compressive strain at the top of the subgrade.

Standard models are available to convert these values into the number of standard 18-kip load applications until either fatigue cracking or subgrade rutting occurs.

Mechanistic Design Check

Mechanistic Check Incremental Thickness variation of 0.5 in.

Mechanistic Check Incremental Thickness Variation of 2 in.

Three options for supplying the subgrade Texas Triaxial Class (TTC):

Option 1 Direct Input:

Requires the designer to input the value based on laboratory tests.

Option 2 Correlations Based on PI:

Allows the user to estimate the TTC based on the soil Plasticity Index (PI). The TTC is automatically calculated.

Option 3 Regional Data:

Recalls a database of soils information for the applicable Texas County and posts it to the Texas Triaxial Design Check screen. When this is selected, the Unified Soil Classification System (USCS) soils type, the percentage of the county that is covered by each soil type, and the TTC of the each soil are displayed. The user clicks on the soil type that best corresponds to the project subgrade.

Texas Triaxial Class (TTC) Check

The Modified Texas Triaxial criteria was developed to prevent the shear failure in the subgrade soil under the heaviest wheel load anticipated for the pavement section.

Results of the analysis will recommend the total combined thickness of granular base, stabilized materials, and HMA surface to prevent shear failures in the subgrade.

Texas Triaxial Check

Texas Triaxial Design Check Texas Triaxial Class (TTC) = 3 Texas Triaxial Class (TTC) = 5

Using Soil PI to Estimate TTC for Design Check

Plasticity Index (PI) = 12 Plasticity Index (PI) = 20

Selection of the Soil Type Based on the Geographic Location

Cohesiometer Value

The cohesiometer is a device that was once used to evaluate relative cohesive strength properties of highway materials.

Allow reduction in overall thickness for stabilized and bonded materials (asphalt concrete, asphalt stabilized base, cement stabilized base, lime stabilized subgrade) using Tex-117-E specification.

Modified TTC Thickness

Using TR (required thickness to protect the subgrade) and cohesiometer value, calculate thickness reduction for stabilized layers.

Determine the allowable reduction in pavement structure thickness due to the cohesive layer (Allowable Reduction-AR).

Calculate the modified thickness (TM) as:

TM=TR-AR

Pavement Response Analysis Tool in (FPS21)