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). 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, with its origins dating back to the FPS-11 days when data was input to the program then residing on the department mainframe. References to “card” number and the input order were retained to minimize the learning curve for veteran users of earlier versions. 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. 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).
Anchor: #i1003263General Inputs
PROGRAM MAIN MENU
Access to the FPS design modules is gained 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). An administrative number may be assigned to each “run” in the “problem” field. When the designer clicks on the “District” field, a menu appears that will allow the designer to select the district and county number. The current date is automatically entered in the “date” field. The applicable highway name is placed by the designer in the “highway” field. Finally, the control-section-job numbers are placed in the appropriate fields. 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.
BASIC DESIGN CRITERIA (Card #3)
Analysis Period. An analysis period is defined as the interval of time between reconstruction or major pavement rehabilitation efforts and 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 still 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, a very short analysis period (1-2 yrs.) can be considered for design of short-term detours but again the 20-yr. traffic (ESALs) must be used as the traffic input in FPS-19W.
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 1,000,000) a minimum serviceability of 3.0 is recommended. For highways with moderate traffic (500,000 to 1,000,000 ESALs) a minimum serviceability of 2.5 is recommended. For low volume highways where average daily traffic (ADT) is less than 1,000 vehicles per day (vpd) and cumulative ESALs are less than 500,000, a terminal serviceability of 2.0 may be used, but should be carefully weighed where speed limits exceed 50 mph. The minimum serviceability index and other performance relationships are shown in Figure 5-1.
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%
It is recommended that confidence level ‘B’ be used for low volume (less than 1000 ADT) highways and level ‘C’ for all other projects. Examples of designs generated using confidence level B and C on low volume highways and levels B, C, and D on mid-volume highways illustrate the impact on initial performance periods and structural layer thicknesses when using alternate confidence levels. Output variation for sample low and mid-volume designs is summarized in the table below.
|
Traffic Volume |
Confidence Level |
Surface |
Base |
Subbase |
Initial Performance Period (yr.) |
Overlay (in.) |
|---|---|---|---|---|---|---|
|
Low |
B |
two-course surface treatment |
6.0 in. Flex Base |
12.0 in. Lime Stabilized Subgrade |
26 |
--- |
|
. |
C |
two-course surface treatment |
8.0 in. Flex Base |
12.0 in. Lime Stabilized Subgrade |
15 |
2.5 |
|
Medium |
B |
2.0 in. HMA |
4.0 in. Asphalt Stabilized Base |
6.0 in. Flex Base |
13 |
2.5 |
|
. |
C |
2.0 in. HMA |
6.0 in. Asphalt Stabilized Base |
6.0 in. Flex Base |
12 |
2.5 |
|
. |
D |
2.0 in. HMA |
9.5 in. Asphalt Stabilized Base |
6.0 in. Flex Base |
13 |
2.5 |
Figure 5-1. Performance Relationships.
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.
PROGRAM CONTROLS (Card #4)
Number of Output Pages. This field controls how many pages of designs will be generated, with eight designs per page. Up to three pages (24 designs) will be printed. However, if only a few unique designs are generated (say 2 or 3), 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) so as not to limit the program output.
Maximum Thickness of Initial Construction. This field can be used to constrain design strategies to those that will 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 those that will meet profile limitations.
CONSTRUCTION AND MAINTENANCE DATA (Card #7)
Initial Serviceability Index. On the indexed performance scale (0-5) this is the condition of the pavement immediately after construction or rehabilitation. The statewide average has been about 4.2. Surface treated pavements tend to be rougher; a value of 3.8 is recommended. For thicker HMAC structures that are constructed in multiple lifts, a value of 4.5 is recommended (Figure 5-1).
Serviceability Index after Overlay. This field is intended as a measure of the pavement condition following an overlay predicted by FPS-19W that is needed after the initial or subsequent performance period. Typically these overlays are thin (2-3 in.) and placed in one lift. Therefore, substantial increases in smoothness are not anticipated; a value of 4.0 is recommended. If the predicted overlay is thick enough to require more than one lift, or district experience dictates otherwise, a higher value could be used (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 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. This parameter influences how long it will take to place the overlay, which affects the traffic delay costs. Typically the value ranges between 1.90 – 2.00.
ACP Production Rate, Tons/Hour. This parameter is also related to how fast the overlay can be placed, which again affects the traffic delay costs. Typically the value ranges between 150-300 tons/hr.
Width of Each Lane, Feet. This parameter affects the rate of overlay paving and consequently the total traffic delay costs. 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.
DETOUR DESIGN FOR OVERLAYS (Card #8)
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. Use of the incorrect detour model can result in excessive user delay costs. Unfortunately, the built-in help screen only addresses three of the five models. The model number (1-5) is entered in this field. 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.
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 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 only used if Detour Model 5 is selected. The distance in miles of the alternate route/special detour is input.
Anchor: #i1003623Traffic 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, Transportation Planning and Programming Division (TPP) by requesting a “Traffic Analysis for Highway Design.” In addition, this TPP report will 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).
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. 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 that this will be the volume at the end of a 20-yr. analysis period. ADT is assumed to increase uniformly 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. You must always enter the cumulative 20-yr. traffic in this field. Adjustments for lanal distribution may be made when at least three lanes exist in both directions (see Chapter 2, Pavement Design Process. ).
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.
Average Speed, Non-overlay Direction
Same as above, except in the non-overlay direction.
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: #i1003705Environment and Subgrade
In this section, the program compensates for regional 10-yr. average daily temperature differences with decreased performance in regions with cooler climates and allows the designer to account for accelerated deterioration in performance due to swelling soils. Normally, if a designer suspects a swelling soil to be present in the project limits, a design will initially be run with no probability for swelling damage, followed by another with the appropriate swelling parameters inserted. A comparison can then be made as to the predicted effect on performance of the selected design. FPS-19 will not generate an effective design that will counter the effects of swelling soils (based on thickness alone).
District Temperature Constant
This input is designed to address the increased susceptibility of hot mix asphaltic concrete (HMAC) to cracking in cold weather. The effect is that the performance curve for a similar structure in a colder region of the state will be steeper (shorter life) than 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).
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.
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.
Anchor: #i1003753Material Parameters
Properly estimating in-place and proposed new material properties is key to deriving a good performing, yet economical structure. 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. 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 exorbitant maintenance costs.
Layer (LYR) Column
The sequence number for each material layer is automatically placed for each pavement design type. A coding mistake exists for design type 5 (Structural Overlay) in that the subgrade is labeled “Layer 5” instead of “Layer 4.” 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.
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.
Material Name
The designer may edit the name of the material to be used in the layer if desired.
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 while underestimating can result in over-design or in other cases a structure with poor fatigue properties or a structure that is more prone to environmental cracking. 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 whose modulus can be significantly influenced by the confinement provided by the layer directly beneath, or by 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 back-calculation) 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. Recently, a variety of “performance” hot mixes have been introduced that appear to have much stiffer design moduli values than conventional dense-graded mixes of the past. 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 new materials.
|
Material Type |
2004Specification |
DesignModulus |
|---|