Section 3: FPS-19W Design Parameters

The Flexible Pavement Design System (FPS) 19: User’s Manual was developed as a report under project 0-1869 (TTI Research Report TX-02/1869-2) and serves as a preliminary reference. More definitive guidance pertaining to recommended inputs and functionality is given in this section.

Anchor: #i1074456

Program Tools

FPS-19W is equipped with built-in help screens activated by pressing the ‘F1’ key while the cursor is in the applicable field for pages 2 and 3 of the data inputs. More specific guidance is provided here and supersedes the guidance given in the program “Help” screens.

Anchor: #i1070488

Data Input Components

Data input for FPS-19W consists of a Main Menu screen and three pages of inputs. The current screen layout for most design inputs is historical in nature; its origins date back to the FPS-11 system, predecessor to the FPS-19W. When the FPS-11 system was in use, required data input to the program resided on the department mainframe. Note, references in the FPS-19W program to “card” number and the input order were retained to minimize the learning curve for veteran users of earlier versions of the program.


Page 1 of the inputs covers the administrative aspects of the design such as job identification and pavement design type selection.

General Inputs (page 1; cards 3 and 4 on page 2; and cards 7 and 8 on page 3) will be addressed first, followed by Traffic Parameters (card 3), Environmental Parameters (card 4), and Material Parameters (page 3).

See Table 5-1 for a summary of recommended inputs that will have a direct impact on the designed pavement thickness. For low- or medium-volume highways, these recommendations are targeted at accepting higher risk in an environment of increased funding limitations. Exercise caution in selecting input levels at the extreme range of the recommendations for multiple parameters. This strategy may yield structures too thin to perform well and should be weighed against:

  • proven district design strategies and
  • results of the Modified Triaxial design procedure for cited traffic levels
Anchor: #CACFJEGJTable 5-1: FPS-19W Design Input Requirements



Current Value

Suggested Value

Example Impact to Pavement Design


Initial SI1

Surface Treatment



Reduction in flexible base thickness is 1.0”

The lower ISI has the effect of thickening base.

Thin ACP

(1.5” to 4”)



3-5” reduction in flexible base thickness or about 1.5” HMAC

Actual ride is better than previous values. The lower ISI has the effect of thickening base. A 3-4” surface can have two smoothness opportunities.

Thick (>4”)




Reduces HMAC by 1”

Should be reduced to 4.5 if the minimum serviceability value is set at 2.5 or less

Minimum SI1

£1 M ESALs

2.0 - 2.5

2.0 - 2.5

Reduces flexible base by about 1” for lower ESALs up to 1.5 inches at the 1 M ESAL level.

Allowing a lower Min. SI has the effect of thinning the pavement structure. Risk is accepted requiring additional maintenance before termination of pavement life.

Between 1 M and 5 M ESALs

2.5 - 3.0


Reduces flexible base by about 2” for lower ESALs and as much as 3” for higher levels

*May be reduced to 2.5 for thick HMAC when initial SI is 4.5 or less or HMAC thickness exceeds 8”

> 5 M ESALs


2.5 - 3.0*

Confidence Level2

£1 M ESALs

B (90%)

A (80%)

Reduces flexible base thickness by 1”

Recognizes that the department will take more risk on low volume roads and provide maintenance.

Between 1 M and 5 M ESAL

C (95%)

B (90%)

Reduces flexible base thickness by 2”

Reduction in structure will increase risk and will be mitigated by maintenance.

> 5 M ESALs

C (95%) to D (99%)

C (95%)

Minimal, rare use of D

Confidence level D adds too much conservatism to normal pavement design.

District Temperature Constant


District spcific

16 - 38


Reduction of 4” of flexible base for northern districts but adds an additional 1” for southern districts

Temperature constant is a known variable having an undesirable effect on thickness design. This variable is being made a constant in future versions of FPS.

Swelling Potential, PVR, Swelling Rate

High PI Soils

Suggested value is 0

Use 0


Follow existing guidance to not include additional structural thickness as a result of swelling calculated by FPS, but the effects of swelling on performance for the structure designed may be checked. Consider stabilization of the subgrade to address swelling.

Overlay Cost

Future Cost

Use current design practice

Use current design practice

Review the “Detail Cost” sheet in the FPS report and determine if the overlay cost is driving a design for thicker pavements unnecessarily. The district may wish to consider pavement design selection without overlay cost included.

Detour (Road User Cost)

Cost for Detour Non-cash Future Cost

Use the estimated speed through and the appropriate model for the detour

Use approach speed for all traffic speed entries and Model 3 for the detour

Eliminates user costs associated with traffic delays for overlay paving operations.

Does not change thickness but re-sorts so that thinner pavements are brought into considerations for design selection.

Cost per CY


District-specific cost should be used. When the costs exceed the maximum input value, cost of materials should be scaled, proportionally, to each other.

1. Increased difference between initial SI and minimum SI reduces pavement thickness.

2. Confidence is increased when field stiffness test are conducted.

Anchor: #i1003263

General Inputs


Access to the FPS design modules by clicking on the “FPS Pavement Design” button on the main menu. Page 1 of the program allows the designer to enter administrative data on the project and select the design type (combination of layers).

  1. An administrative number may be assigned to each “run” in the “problem” field.
  2. When the designer clicks on the “District” field, a menu appears that will allow the designer to select the district and county number.
  3. The current date is automatically entered in the “date” field.
  4. The applicable highway name is placed by the designer in the “highway” field.
  5. The control-section-job numbers are placed in the appropriate fields.

    CAUTION: Do not leave the control-section-job fields blank as this may cause the program to crash.

  6. Comments may be entered in a five-line field by overwriting any existing comments (no text wrap-around).

The designer must now select which design type will be used for the project. Design types were discussed in “Pavement Design Process.” If more than one type of design is being considered, then each type of design must be run separately and the respective program outputs subsequently compared.


Analysis Period. An analysis period is defined as the interval of time between reconstruction or major pavement rehabilitation efforts. This term is sometimes used synonymously with the pavement design life.

Normally a 20-yr. analysis period is used in flexible pavement design. A 30-yr. analysis period is possible but the designer must input the projected 20-yr. cumulative traffic in the FPS-19W computer program. Adjustments to the traffic are then made internally by the computer program. Similarly, when a very short analysis period (1-2 yrs.) is considered for design of short-term detours, the 20-yr. traffic (ESALs) must be used as the traffic input in FPS-19W.

CAUTION: When using FPS-19W, traffic loading must be entered as the 20-yr. cumulative ESALs. It is only the analysis period that is adjusted to reflect the expected duration. Refer to 20-Yr. 18-kip ESALs (One Direction) design criteria in the Traffic Inputs section for additional guidance.

Minimum Time to First Overlay and Between Overlays. These time intervals are commonly referred to as performance periods and are based on district guidelines, historical trends, and former federal policy. Considerations for performance periods beyond the minimum include minimizing interruption to traffic and avoiding the necessity for mill and inlay operations where constant profile must be maintained.

For flexible pavements, the selected design strategy should provide a minimum initial performance period of 8 yr. before an overlay is required.

Minimum Serviceability Index. This input is also known as the terminal serviceability or serviceability at the end of a performance period. On the indexed scale of pavement performance (0-5), this is the lowest desirable condition before rehabilitative effort is required.

  • For highways of higher importance (cumulative ESALs exceed 5 M), a minimum serviceability index of 3.0 is recommended. A minimum serviceability index of 2.5 can be considered for thick HMAC pavements where the initial SI is 4.5 or less or when the total HMAC thickness exceeds 8.0” (Table 5-1).
  • For highways with moderate traffic (1 M to 5 M ESALs) a minimum serviceability of 2.5 is recommended.
  • For low volume highways where average daily traffic (ADT) is less than 3,000 vehicles per day (vpd) and cumulative ESALs are less than 1,000,000, a terminal serviceability of 2.0 to 2.5 may be used, but should be carefully weighed (proper risk management) where heavy loads or weak soils exist, or speed limits exceed 50 mph.

The minimum serviceability index and other performance related concepts are shown in Figure 5-1. Here performance is defined as a decrease in serviceability over time or traffic loading. Higher initial serviceability may result in longer performance periods, however, the desire (or necessity) to maintain a higher level of minimum serviceability will shorten the performance period. The rate of deterioration in the serviceability index is affected by the overall structural capacity and environment, including severe climatic events. Serviceability can be restored only by performing preventive or standard maintenance, HMAC overlays, or reconstruction.

Pavement Performance Relationships. (click in image to see full-size image) Anchor: #CACBBEAGgrtop

Figure 5-1. Pavement Performance Relationships.

Design Confidence Level. This parameter is meant to address variability in material quality, construction processes, and forecasted traffic as a means of assuring the structure performs as desired. It does not account for defective materials, poor construction, or poor assumptions on material properties. An overall multiplier to the cumulative traffic loading is applied, increasing as the desired level of confidence increases. FPS-19W uses an alphabetic code that is tied to a reliability or confidence level as follows:

A 80%

B 90%

C 95%

D 99%

E 99.9%

A confidence level as low as level ‘A’ can be considered for designs below 1 M ESALs; level ‘B’ is recommended for 1 to 5 M ESALs; and level ‘C’ for above 5 M ESALs.

Examples of designs generated using confidence level A and B on low load level highways and levels B, C, and D on higher load level highways illustrate the impact on initial performance periods and structural layer thicknesses when using alternate confidence levels. Output variation for these sample designs is summarized in Table 5-2.

Anchor: #i1012411Table 5-2: Output Variations

Traffic Volume

Confidence Level

Initial SI

Termin SI




Initial Performance Period (T1 yr.)

Overlay (in. @ T1)

Low (750,000 ESALs)




two-course surface treatment (2CST)

6.0” flex

6.0” Lime Stabilized Subgrade (LSS)







6.0” flex

6.0” LSS



Medium (2.5 M ESALs)





8.0” flex

8.0” LSS







11.0” flex

8.0” LSS



High (7.0 M ESALs)




3.0” HMA

10.0” CSB

8.0” LSS






3.0” HMA

10.0” CSB

8.0” LSS






5.5” HMA

10.0” CSB

8.0” LSS



Interest Rate (%). This parameter is used in the life cycle cost analysis to discount future expenditures for overlay and maintenance costs. A value of 7% is recommended.


Number of Output Pages. This field controls how many pages of designs will be generated. There is a limit of eight designs per page. For 24 designs, up to three pages will be printed. If only 2 or 3 unique designs are generated, then only one page will be printed.

Maximum Funds/SY for Initial Construction. This field can be used to constrain design strategies where funding may be restricted. Generally, this parameter is left at a sufficiently high value ($99.00/SY) to facilitate or encourage maximum program output.

Maximum Thickness of Initial Construction. This field can be used to constrain design strategies to meet profile limitations or limit the number of total designs in the output.

Maximum Thickness of All Overlays. This field can be used to constrain design strategies to meet profile limitations.


Initial Serviceability Index. On the indexed performance scale (0-5), this is the condition of the pavement immediately after construction or rehabilitation. Historically, the statewide average has been about 4.2. With the introduction of ride specifications, this value has been increasing.

Because surface treated pavements tend to be rougher; a value of 4.0 is recommended. For thin-surfaced ACP pavements (1.5 to 4.0”) a value of up to 4.5 may be recommended. For thicker HMAC structures (>4.0” ACP) a value of up to 4.8 may be considered.

Use higher values in these ranges where more smoothness opportunities (e.g., multiple HMA lifts) exist (see Figure 5-1 and Table 5-1).

Serviceability Index after Overlay. This field is intended as a measure of the pavement condition following an overlay predicted by FPS-19W, projected after the initial or subsequent performance period.

Typically, these overlays are thin (2-3 in.) and placed in one lift. Therefore, ride improvement may not necessarily return smoothness to “original” levels; a value of 4.0 - 4.2 is recommended.

If the predicted overlay is thick enough to require more than one lift or district experience dictates otherwise, a value up to 4.5 may be considered, depending on the anticipated final ride condition ( Figure 5-1).

Minimum Overlay Thickness. This parameter is dictated by the nominal maximum aggregate size of the mix typically used for overlays following the initial performance period. A 1/2 in. level-up is automatically included for cost purposes in the program, but does not count for toward an increase in structure.

Overlay Construction Time, Hours/Day. This input is used to evaluate traffic delay costs as a result of overlay operations required at the end of a performance period. Daily construction time typically ranges from 8-12 hrs.

*ACP Compacted Density, Tons/CY. See NOTE for parameter influences. Typically the value ranges between 1.90–2.00.

*ACP Production Rate, Tons/Hour. See NOTE for parameter influences. Typically the value ranges between 150–300 tons/hr.

*Width of Each Lane, Feet. See NOTE for parameter influences. This value should be equal to the typical lane width.

First Year Cost for Routine Maintenance ($/lane-mile). This is a parameter that will affect life-cycle costs. The average cost of routine maintenance for the first year following initial or structural overlay (design option 5) construction should be tracked at the district level. Values have typically ranged from $50-$200/lane-mi.

Annual Incremental Increase in Maintenance Cost. This parameter is an adjustment to the baseline first-year routine maintenance cost where a uniform rate of increase is assumed. This value should again be tracked at the district level. Values have typically ranged from $10–$50/lane-mi.

NOTE: This parameter influences the time required to place the overlay and, as a result, affects the traffic delay costs.


Detour Model during Overlays. There are five different models in the program for handling traffic during overlay operations, each one generating a unique user-delay related cost. Unfortunately, the built-in help screen only addresses three of the five models. The model number (1-5) is entered in this field.

CAUTION: Use of the incorrect detour model can result in excessive user delay costs or cause the program to crash, particularly when insufficient lanes are allotted for very high ADT inputs.

A short description of each model is given here.

  • Model 1. Highway cross section consists of two driving lanes (one each direction) with wide (8-10 ft.) shoulders. Paving operations will block one lane at a time, with traffic in the paving direction using the shoulder or lane in that direction as the detour. Traffic in the non-paving direction is relatively unaffected, although slowing will probably be required.
  • Model 2. Highway cross section consists of two driving lanes (one each direction) with narrow shoulders. Paving operations will block one direction at a time, with traffic in the paving direction being diverted into the on-coming lane using an escort. Traffic in the non-paving direction will be required to stop when traffic is escorted from the opposite direction.
  • Model 3. Highway cross section consists of two or more driving lanes in each direction. Paving operations will block one driving lane at a time, requiring traffic in the paving direction to channel down into fewer lanes. Traffic in the non-paving direction may be completely unaffected if the highway is a divided facility.
  • Model 4. Highway cross section consists of two or more driving lanes in each direction. Directional traffic flow in the paving direction is completely blocked, with traffic diverted to at least one lane in the opposite direction. Traffic in the non-paving direction must be channeled down into fewer lanes to accommodate opposing traffic.
  • Model 5. Highway cross section consists of two or more driving lanes in each direction. Directional traffic flow in the paving direction is completely blocked, with traffic diverted around the overlay zone by special detour, alternate route, or combination of these. Traffic in the non-paving direction may be completely unaffected if the highway is a divided facility.

For low- to medium-volume highways, the Pavement Design Task Force (PDTF, 2009) recommended removing possible cost bias in accounting for these user costs by simply selecting detour model 3 and entering the posted approach speed for all traffic speed entries in the detour area.

Total Number of Lanes. This value includes all driving lanes in both directions. If a facility includes a continuous left turn lane, treat this lane as a shoulder (do not count it as a driving lane).

Number of Open Lanes, Overlay Direction. This value will depend upon the overlay model chosen above and the total number of lanes on the highway. In the case of Model 1, this number would be one (1). In the case of Model 2, this number would be zero (0).

Number of Open Lanes, Non-overlay Direction. This value will depend upon the overlay model chosen above and the total number of lanes on the highway. In the case of Model 2, this number would be one (1).

Distance Traffic Slowed in the Overlay Direction. This input calculates the time delay associated with the detour which is further reduced to a user cost. Input is in units of miles.

Distance Traffic Slowed in Non-overlay Direction. Same as above, except for traffic in the non-overlay direction.

Detour Distance, Overlay Zone. This field is used only if Detour Model 5 is selected. The distance in miles of the alternate route/special detour is input.

Anchor: #i1003623

Traffic Inputs

In the Traffic Data section, average daily traffic (ADT) statistics, cumulative ESAL loading, and percent trucks in the ADT are parameters that must be obtained through the Traffic Analysis Section of the Transportation Planning and Programming Division (TPP) by requesting a “Traffic Analysis for Highway Design.”

Use Form 2124, Request for Traffic Data1, for design traffic requests (see Chapter 2, Pavement Design Process, for guidance). This TPP report will also contain the Average of the Ten Heaviest Wheel Loads, Daily (ATHWLD) and percent tandems in the ATHWLD, parameters that are both required inputs to the Modified Triaxial Check (Chapter 2, Section 5-- “Approved Pavement Design Methods”).

As noted by the Pavement Design Task Force (PDTF)(2009), districts should review the traffic analysis for highway design report to verify data reasonableness. For example, check reported ADTs and percent trucks in the the traffic stream against observations along the project corrider. Also, knowledge of the predominant types of trucks and commodities transported can influence the proportion of fully loaded trucks and the wheel/axle loading imparted to the pavement.

By scrutinizing the reported beginning ADT, percent trucks in the ADT, and backcalculatiing the average ESALs per truck from the TP&P data, a designer can roughly estimate the magnitude of the reported versus observed truck traffic and the damage caused by the average truck. The information gathered can be used as a basis for requesting a re-evaluaton of the forecasted traffic, if necessary.

For multi-lane highways, observation may determine the actual lane distribution of trucks. Determine whether the ATHWLD figure appears reasonable. Compare the reported load against a typical dual tire set on a fully loaded 18-wheeler with a static load of roughly 9,000 lbs.

Beginning ADT (vehicles/day)

This is normally a two direction volume parameter that is required to generate user delay costs during overlays at the end of a performance period and (along with the ending ADT) to determine the distribution of axle loading over time. The beginning and ending ADTs define the composite traffic growth rate over the analysis period. For special situations such as one-way frontage road and ramp analysis, or one-direction analysis in the case of unequal loading, this figure will be one-way volume. This value will be the ADT at the beginning of a 20-yr. analysis period which should correspond to the year the facility is opened to traffic after construction or structural overlay (design option 5) is placed.

Ending ADT (vehicles/day)

Same as above, except this will be the volume at the end of a 20-yr. analysis period. ADT is assumed to increase linearly over time.

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

This figure is entered in terms of decimal millions. If the analysis period is other than 20-yr., an internal traffic equation will adjust the cumulative ESALs to the correct value for the analysis period used. The cumulative 20-yr. traffic MUST ALWAYS be entered in this field. Standard or project specific adjustments for lane distribution factors may be used when at least three lanes exist in the design direction (Chapter 2, Pavement Design Process); lane distribution factors can be applied to highways with two lanes in one direction when there is compelling data to warrant reduction of the 100% ESAL loading.

Average Approach Speed to the Overlay Zone

Figure is entered in miles/hour; typically the posted speed limit is used. This parameter forms part of the equation to determine user delay costs through the overlay zone.

Average Speed in the Overlay Direction

Figure is entered in miles/hour; the estimate will in part depend upon which detour model is used. This parameter is another part of the user delay cost equation. To avoid possible influence on the ordering of recommended thickness options in the FPS output, this value should be set equal to the “Average Approach Speed” to preclude computation of associated user costs in the overall estimate of project costs.

Average Speed, Non-overlay Direction

Same as above, except in the non-overlay direction. Again, this value should be set equal to the “Average Approach Speed” to preclude computation of the associated user costs in the overall estimate of project cost.

Percent of ADT per hour of Construction

Parameter is part of the user delay cost equation. An estimate of the percent of ADT that arrives each hour of overlay construction is needed. If no better information exists, use 5% for urban highways and 6% for rural.

Percent Trucks in the ADT

Parameter is part of the user delay cost equation. The higher the percentage of trucks, the higher the user delay costs will be during overlay operations. Use the percent figure provided by TPP traffic analysis.

Anchor: #i1003705

Environment and Subgrade

The original intent of this module of the program was to compensate for regional 10-yr. average daily temperature differences with decreased performance in regions with cooler climates and allow the designer to account for accelerated deterioration in performance due to swelling soils.

Both of these program features have proven to be ineffective in controlling either thermal cracking or performance degradation caused by swelling soils. As a result, the default will be to neutralize any effect on structural thickness computations.

The designer can run designs using these features to observe the predicted difference in performance life. Use caution when changing design inputs to compensate for the decrease in the predicted performance life.

Mitigation of soil movement is best evaluated with Tex-124-E, Determining Potential Vertical Rise, rather than using FPS-19W. The details of approval requirements for soil movement mitigation is discussed in Chapter 3, Materials Investigation and Selection Information.

District Temperature Constant

Use 31 as the default (corresponds to a Central Texas value). In FPS-19, this will require overwriting the value automatically generated that corresponds to the selected county where the project lies. This input is designed to address the increased susceptibility of hot mix asphaltic concrete (HMAC) to cracking in cold weather. Using the default values keyed to the project county will generate a steeper performance curve (shorter life) for a colder region that in a warmer region. Again, FPS-19 will not generate an effective design that will counter the effects of thermal cracking (based on thickness alone).

Swelling Probability

This parameter is the percentage of the project (expressed as a decimal) that is likely to experience roughness or serviceability degradation due to swelling soils (range 0.0 to 1.0). Use 0 (zero) as a default, unless checking degradation to expected performance life.

Potential Vertical Rise

This input is an estimate of how much the surface of the clay layer can rise if supplied with as much moisture as it can absorb. “Tex-124-E, Determining Potential Rise” may be used, but compensation can be made for any swelling that may have occurred over time in the existing roadbed.

Impact will be nullified if using 0 (zero) as directed in “Swelling Probability.” See Chapter 3, Materials Investigation and Selection Information Geotechnical Investigation for Pavement Structures for deliberate design measures to mitigate potential vertical rise, and guidelines on how and when it should be used.

Swelling Rate Constant

This is an indexed value used to evaluate how rapidly a swelling susceptible soil can swell. A nomograph is included in the help screen with variables on one scale for estimating how tight or fractured the soil is; and on the other scale the relative availability of moisture to cause swelling. A line joining these scales will traverse the swelling rate constant scale, which will be the value entered in this field. The value will range between 0.04 and 0.20. The effect of this field is automatically nullified when zero (0) has been entered for Potential Vertical Rise or for Swelling Probability.

Anchor: #i1003753

Material Parameters

The material modulus will be very influential in layer thickness calculations; the combination of the unit cost and thickness will dictate the structure’s initial construction cost. However, this simplistic approach can be disastrous if the big picture is overlooked.

Properly estimating in-place and proposed new material properties is key to deriving a good performing, yet economical structure. Materials selected must be compatible. Failure to properly consider existing materials, relative stiffnesses, moisture susceptibility, or the adjacent structure (in the case of widening) can lead to early failure and/or very high maintenance costs.

Layer (LYR) Column

The sequence number for each material layer is automatically placed for each pavement design type. The designer should not attempt to add layers to these pre-established sequences. The layer sequence number will appear on the output pages of the program for each design option next to the letter ‘D’ (depth) for each layer.

NOTE: A coding mistake exists for design type 5 (Structural Overlay); the subgrade is labeled “Layer 5” instead of “Layer 4.”

Layer Code (CDE)

This is a one-letter alphabetic code that corresponds to the layer sequence number and appears at the top of the design output to show the material sequence. The designer should not attempt to reorder these characters from the default order.

Material Name

The designer may edit the name of the material to be used in the layer, if preferred.

Cost per Cubic Yard

All new materials proposed in a design must have a cost in terms of dollars/cubic yard. Even existing layers can have costs associated with them, such as reworking a flexible base or milling an existing HMAC layer prior to overlaying. Material costs should be tracked at the district level.

The Average Low Bid Unit Price found on the department’s web site can be used as a basis for these material costs.

Modulus, E (ksi)

Each material layer used in the structure will have a modulus input that should characterize the average stiffness of that material. The construction process, inherent material variability (initially and over time), and effects of environment and traffic loading will typically introduce considerable variance about the average value.

Overestimating this material property can result in a structure with poor permanent deformation performance and may subject the surface to early fatigue, while underestimating can result in an uneconomical pavement.

Additionally, materials that have an average in situ modulus in one circumstance may have a different average modulus if placed in another environment. This is particularly true of unbound base materials’ modulus, which can be significantly influenced by the confinement provided by the layer directly beneath, absence of paved shoulders, or by the amount of moisture infiltrating the structure if the materials are moisture susceptible.

In evaluating a design that consists of layers that were pre-existing (including the subgrade), the Falling Weight Deflectometer ( FWD) is indispensable in determining what stiffnesses (through backcalculation) these layers can contribute to the new structure. Virgin material to be added will require knowledge by the designer (preferably through past use and subsequent evaluation), tempered by the specifics of the current project.

Recent studies indicate that “performance” hot mixes and thick composite HMAC structures, using any type of HMA, have a much stiffer in-place moduli values than conventional thinner surfaced HMAC structures. Laboratory and field testing continues on these mixes to establish stiffness-temperature curves and better define “design” stiffness values by type if necessary. All material modulus values are manually entered into their respective fields (values are not read from the MODULUS summary file).

Below is a partial listing of typical design moduli by material type for newly placed or modified in place materials using FPS-19W.

Anchor: #i1012735Table 5-3: Recommended Material Design Modulus Values

Material Type





Surface Treatment

Item 316, 318

250 ksi


Considered in the structural design only when placed on the surface (not as an underseal).

Limestone Rock Asphalt Pavement

Item 330

200–350 ksi


Material typically placed as asphalt stabilized base or surface for low volume roads.

Hot-mix Cold-laid ACP

Item 334

300–400 ksi



Dense-graded Hot (Warm) Mix Asphalt

Item 340, 341

Combined HMA thickness: £ 8” use 500 ksi

>8.0” use 650 ksi


Permeable Friction Course

Item 342

300 ksi


Thinness of the lift and high air voids do not allow significant contribution to the overall structural capacity.

Performance Design Mixtures

Item 344

Combined HMA thickness:

£6.0” use 650 ksi

6”< T £ 8” use 700 ksi

>8.0” use 850 ksi


Includes SuperPave and CMHB mixes. Rich Bottom Layer (RBL) use 500 ksi.

Stone-Matrix Asphalt

Item 346

Combined HMA thickness:

£6.0” use 650 ksi

6”<T £ 8” use 700 ksi

>8.0” use 850 ksi


Asphalt Treatment (base)

Item 292

250–400 ksi


Use “ Tex-126-E, Molding, Testing, and Evaluating Bituminous Black Base Materials” or alternatively Part III of “ Tex-204-F, Mix Design for Large Stone Mixtures using the SuperPave Gyratory Compactor (SGC)” to establish optimum asphalt content.

Emulsified Asphalt Treatment (Base)

Item 314, various OTU special specs

50–100 ksi


Contact CST-M&P for assistance in establishing optimum emulsion concentration.

Flexible Base

Item 247

If historic data not available, modulus should be from 3–5 times the subgrade modulus or use FPS default. Typical range 40–70 ksi.


In general, a finer graded base will have lower moduli than one that is a coarser gradation. As angularity and soundness of particles decrease, modulus will decrease to the lower end of the scale. Limiting the minus 200 clay fraction will improve resistance to moisture damage.

Lime Stabilized Base

Item 260, 263

60–75 ksi


Use Tex-121-E, “Soil-Lime Testing” to establish optimum lime content. Long-term stiffness improvement will depend on concentration used and affinity of base material to undergo permanent chemical bonding.

Cement Stabilized Base

Item 275, 276

80–150 ksi


Use Tex-120-E, “Soil-Cement Testing” to establish optimum cement content. For Item 276, a minimum 7-day unconfined compressive strength of 300 psi is established for Class L stabilized base. TTI research indicates that higher strengths can lead to detrimental shrinkage cracking. Also, very stiff, stabilized bases are not modeled effectively in FPS-19W. Higher design moduli are not recommended.

Fly Ash or Lime Fly Ash Stabilized Base

Item 265

60-75 ksi


Use “ Tex-127-E, Lime Fly-Ash Compressive Strength Test Methods” to establish optimum fly ash or lime fly ash content.

Lime or Cement Stabilized Subgrade

Item 260, 275

30–45 ksi


Use Tex-121-E or Tex-120-E to establish optimum lime or cement content. Long-term stiffness improvement will depend on concentration used and affinity of subgrade material to undergo permanent chemical bonding. There are cases when a subgrade will be treated (2-3% lime) to provide a working platform for construction equipment and a platform for compactive effort of the overlying layers. This layer should not be accounted for in the structural design.

Emulsified Asphalt Treatment (Subgrade)

Item 314, various special specs

15–25 ksi


Contact CST-M&P for assistance in establishing optimum emulsion concentration.



Priority should be to use the project-specific back-calculated subgrade modulus. Defaults by county are available in the FPS design program. Typical range is 8-20 ksi.


Use of a back-calculated modulus is preferred. FPS-19W defaults to the average county subgrade modulus taken from a limited number of tests. For new highway construction on a new right of way, deflection testing on an adjacent highway, or intersecting highways can provide data for backcalculation. Alternatively, elastic modulus correlations to field or laboratory derived CBR or the program default may be used. Wetter or more highly plastic materials warrant higher Poisson ratios.

Minimum Depth

This is the minimum depth for a given layer that the designer will consider. This may be driven by nominal maximum aggregate size in the case of asphaltic concrete pavement ( ACP) layers, minimum practical layer thickness for flexible bases, or district policy.

  • For existing layers, the average existing layer thickness is input.
  • If a layer is not used, zero is input.
  • For subgrade, use the average depth to bedrock as determined by MODULUS or the default when the subgrade modulus was not backcalculated.

Maximum Depth

This is the maximum depth for a given layer that the designer will consider. This may be driven by the maximum practical compaction thickness for one or more lifts or by district policy.

  • For existing layers, the average existing layer thickness is input (minimum equals maximum).
  • If a layer is not used, zero is input.
  • For subgrade, use the average depth to bedrock as determined by MODULUS or the default when the subgrade modulus was not backcalculated.

Salvage Percentage

The designer enters a percentage of the original cost of the material that may be recovered at the end of the analysis period. Guidelines are given in the built-in help screen for these fields for 10-, 20-, and 30-yr. analysis periods.

Poisson’s Ratio

The designer enters a value typical for the material type used. Recommended values are given in Table 5-3. This input has very little sensitivity. Selection of a value within the ranges identified is sufficient.

Check (CHK) Column

Not used. (Outdated coding from an earlier version of program.)

Once all inputs are entered, the designer clicks the “Go!” button and the output screen appears. If there are no feasible solutions, a message box will appear that will allow the designer to re-run and modify necessary inputs.

When at least one design is produced, further evaluation using the design checks may begin. At a minimum, the Modified Texas Triaxial Design check must be run. Leeway is granted to the project engineer to override the outcome of this check if evidence of past performance can be cited.

Anchor: #i1003871

Modified Texas Triaxial Check

This design check is accessed by using the following procedure

Anchor: #i1069119Table 5-4: Texas Triaxial Design Check




Below the FPS design output display, click on “Check Design.” Once selected, a “pavement plot” window appears with several option buttons.


Click on “design check.” A “FPS Design Check” window will appear.


Select “Texas Triaxial Design Check.” The “Form 1” window appears.


Enter the Modified Texas Triaxial Check data.


The first two parameters (Average of the Ten Heaviest Wheel Loads Daily [ATHWLD] and percentage of tandem axles in the ATHWLD) are provided in the Transportation Planning and Programming Division (TPP) Traffic Analysis for Highway Design report.


The percent tandem axles in the AHTWLD value is used to trigger a multiplier of 1.3 times the ATHWLD for percent tandems at 50% or greater.

  • The Pavement Design Task Force (PDTF, 2009) recommends a factor of 1.0 be used for all designs where traffic loading is below 5 M ESALs.
  • The percent tandem axles must be entered as <50% for these designs.


  • For designs involving more than 5 M ESALs, use the appropriate percent tandem axles provided by TP&P.

Where heavy industry truck traffic, aggregate pits or concrete plants, etc. are common, special design considerations may be required.

Subgrade Triaxial Class Number

The Subgrade Triaxial Class number is derived through actual laboratory testing of soil samples from the project ROW ( Tex-117-E, “Triaxial Compression for Disturbed Soils and Base Materials”), or through use of Soil Conservation Service maps and catalogued results from previous testing ( Soil_Series.xls).

The modified cohesiometer value is the final input. The cohesiometer is a device that was once used to determine a relative index of sheer strength for highway materials as a way to assign greater weight to bound or stabilize materials used in the highway structure. By doing this, the calculated depth of “better granular material” needed to protect the subgrade (or other unbound layers) could be reduced by an amount obtained from a chart (use the Flexible Base Design Chart followed by the Thickness Reduction Chart for Stabilized Layers) found in the “Tex-117-E” test procedure archived versions, August 2002 - December 2009.

These calculations are automatically performed in this design check routine.

Anchor: #i1088067Table 5-5: Application of Cohesiometer and Thickness Reduction for the Triaxial Design Check




When the “reference” button is activated, a table of cohesiometer values appears.

  • Double-click on the material used in the proposed structure that has the highest cohesiometer value.
  • The screen will revert back to the input screen with calculated values for Triaxial Thickness Required, Modified Triaxial Thickness, and the FPS Design Thickness derived earlier.

A “details” button will allow you to review the materials table from your selected FPS design.

The box at the bottom left of the Modified Triaxial input screen will indicate whether the FPS design will meet thickness requirements.

1. Internal access only.

Previous page  Next page   Title page