Final Report Evaluation of I-Pave Low Volume Road Design Software

Transcription

1 Final Report Evaluation of I-Pave Low Volume Road Design Software May 2015 Michael I Darter, William R. Vavrik, Dinesh Ayyala Applied Research Associates, Inc. 1

2 Objectives 1. Obtain all documentation for I-Pave as well as 1993 AASHTO procedure 2. Review I-Pave procedure does it follow 1993 AASHTO procedure? 3. Examine inputs and program built-in default values for new and rehabilitation design and unit costs for both flexible and rigid pavements 4. Examine equations used for calculation of ESALs, design thickness and life cycle costs 5. Evaluate whether inputs and default values are appropriate and comment 2

3 1. Introduction Methodology Tasks 1. Proper use of 1993 AASHTO design equations for calculating ESALs for new flexible and rigid pavement design and rehabilitation. 2. List of user-defined inputs and default or built-in values for design variables, source of data for each variable and their correct use in design equations. 3. Subgrade characterization resilient modulus (M r ) for flexible pavement design and modulus of subgrade reaction (k-value) for rigid pavement design. 4. Procedure used to calculate the design thickness of HMA and PCC layers for new and rehabilitation design. 3

4 1. Introduction Methodology Tasks 5. Layer coefficient adjustment and other design considerations for different distress levels entered by user for all types of rehabilitation design. 6. Basis for proposition of rehabilitation alternatives and thickness calculation procedure. 7. Calculations involved in life-cycle cost analysis and an assessment of default unit costs and other variables provide an unbiased and reasonable life-cycle cost estimate. 4

5 1. Introduction Methodology Approach Terminology Default variables are those which are built into the software, shown to or hidden from the user and are assigned fixed values and cannot be changed (hard-coded) 1. Identify user-entered and default variables used in each I-Pave calculation procedure 2. Develop a detailed sensitivity analysis matrix using a range of values for each user-entered variable 3. Reduce the complete matrix for I-Pave runs reduced sensitivity matrix, representative of the entire range of selected values 4. Compare results from I-Pave output and 1993 AASHTO design equations to achieve objectives 5

6 2. I-Pave Overview Overview of I-Pave Software New Pavement Rehabilitation Design Design Flexible Flexible & Rigid& Rigid Life Cycle Life Cost Cycle Cost New Flexible New Design Rigid Design Analysis Output Analysis Output Output Output Summary Summary New Flexible New Design Rigid Design 6

7 Methodology Complete Sensitivity Matrix Traffic variables were used to develop a full factorial matrix Design life 15, 20, 25, 30 years Initial ADT 500, 1000, 1500, 2000 Percentage trucks 5, 10, 15, 20, 25% Growth rate 1, 2, 3, 4, 5% (compound) Total = 4 X 4 X 5 X 5 = 400 sets of input values Other variables were randomly assigned values from a selected range. Reliability 50, 60, 70 or 80% Terminal serviceability, p t 1.9, 2.0, 2.1, 2.2 or 2.3 Base and stabilized subgrade thicknesses Allowable Subgrade Type Suitable, Select or Unsuitable 7

8 2. Methodology Methodology Reduced Sensitivity Matrix Complete sensitivity analysis matrix 400 sets of inputs Reduced sensitivity analysis matrix 50 sets of inputs Complete sensitivity matrix used to calculate HMA and PCC thickness using 1993 AASHTO design equations, and life cycle costs Calculations using1993 AASHTO equations were performed in Excel Reduced sensitivity matrix used as inputs in I-Pave and output values were recorded for comparison with the 1993 AASHTO calculations 8

9 2. Methodology Methodology Reduced Sensitivity Matrix Index Number Refers to specific set of input values Index No. Y (years) ADT 0 T (%) G R (%) p t Base St.SG Subgrade Type Suitable Select Unsuitable RANDOMLY SELECTED Unsuitable Unsuitable Suitable Unsuitable Unsuitable Select 9

10 3. New Pavement Design Traffic Inputs and Calculation of ESALs Load equivalency factor (LEF) is the average number of ESALs per truck. LEF used in flexible pavement ESALs calculation was found to be equal to (Iowa SUDAS Ch. 5 Pavement Thickness Design, Table 5F-1.06) LEF used in rigid pavement ESALs was not equal to recommended value from same reference Iowa SUDAS Ch. 5, Table 5F-1.06 recommends LEF of for rigid pavements, whereas the value calculated from I-Pave output was Higher LEF Higher ESALs (~ 45% higher), therefore greater design PCC thickness 10

11 3. New Pavement Design Iowa SUDAS Load Equivalency Factors 11

12 3. New Pavement Design Structural Variables Default Values Elastic modulus of crushed stone base, layer coefficients and drainage coefficient are recommended values in 1993 AASHTO guide (Part II, Section 2.1.3) and SUDAS specifications (Table 5F-1.05). S c 646 psi is mean value of Iowa DOT QMC samples calculated by MEPDG study for Iowa (Wang et al., 2008) Load transfer coefficient, J is fixed at 3.2, whereas AASHTO design guide uses different values for slabs with and without dowels. Iowa SUDAS recommends J = 3.1 for PCC thickness < 8" and J = 2.7 for PCC thickness > 8" 12

13 3. Subgrade Structural Variables - Subgrade Three types of soil in I-Pave as per Iowa DOT specifications (Iowa DOT Standard Specifications, Section 2102) Select soil CBR = 7 Class 10 soil (suitable) CBR = 5 Unsuitable soil CBR = 3 13

14 3. Subgrade Structural Variables Subgrade Types Select Soil Cohesive soils: A-6 or A-7-6 soils of glacial origin, Proctor density > 110 pcf, Plasticity index > 10 Granular soils: A-1, A-2 or A-3, Proctor density > 110 pcf, P.I < 3.0 Class 10 Soil Normal earth materials like loam, silt, peat, clay, sand and gravel Suitable Density > 95 pcf, AASHTO M Group Index < 30 Unsuitable Not satisfying above requirements 14

15 3. Subgrade Examination of I-Pave Subgrade Strength Resilient modulus M r for Iowa subgrade soils were derived from MEPDG study by Ceylan et al. (2008) M r values from the study were determined from laboratory testing of specimens at optimum moisture content, which is a requirement for MEPDG M r for select (CBR = 7) and suitable (CBR = 5) soils in I- Pave are obtained from Ceylan (2008) M r for unsuitable soils (CBR = 3) is assumed as 4,500 psi Soil Type CBR R-value M R (psi) k (pci) Select Suitable Unsuitable

16 3. Subgrade I-Pave Subgrade Strength M r values for subgrade soils with no stabilized layer Source: Ceylan et al. for Iowa MEPDG Implementation Characteristics of Unbound Materials for MEPDG, MEPDG Work Plan Task 5, Iowa DOT,

17 3. Subgrade I-Pave Subgrade Strength When stabilized subgrade is present, a composite Mr value is used which depends on the thickness of stabilized layer does not depend on type of natural subgrade 17

18 6. Discussion Discussion Calculation of k-value from M r Equation for converting Mr to k-value for rigid pavement design from 1993 AASHTO design guide is k k (pci) (pci) = Equation for converting Mr to k-value for rigid pavement design as used (also shown in soil data output) in I-Pave is = MR (psi) 19.4 M R (psi) 30 Current I-Pave relationship leads to lower subgrade strength by 35%, thereby leading to higher PCC thickness. Example: For M r = 5000 psi, k-value is reduced from 258 to 167 psi/in. 18

19 3. Design Procedure Design Thickness Calculation Procedure Design thickness was calculated using 1993 AASHTO equations and compared to I-Pave output for all sets of input values in reduced sensitivity matrix Both flexible and rigid pavement design thickness calculation follow 1993 AASHTO procedure Flexible Design: Thickness from I-Pave is identical to that calculated from 1993 AASHTO design equation Rigid Design: Thickness from I-Pave is slightly LOWER than that calculated from 1993 AASHTO design equation Final design thickness in I-Pave is rounded to nearest higher 0.5 inches The minimum design thickness for HMA is 3 inches and the minimum design thickness for PCC is 6 inches 19

20 3. Design Procedure AASHTO and I-Pave Design Thickness Data Flexible Pavement Rigid Pavement Index No Actual Thickness, in Rounded Thickness, in Actual Thickness, in Rounded Thickness, in AASHTO I-Pave AASHTO I-Pave AASHTO I-Pave AASHTO I-Pave Shaded cells show different final design thicknesses for PCC 20

21 4. Rehabilitation Design Rehabilitation Design Procedure in I-Pave Rehabilitation design can be performed in I-Pave for three pavement types: 1. Flexible pavement (AC) 2. Rigid pavements (PCC) 3. Composite pavements (AC over PCC) Inputs for new pavement design are also required for rehab design. Additional inputs required are Pavement age in years Length of section to be rehabilitated, miles Severity of distresses, specific to existing pavement type 21

22 4. Rehabilitation Design Rehabilitation Design Procedure Overview 22

23 4. Rehabilitation Design Rehabilitation Design I-Pave Output 23

24 4. Rehabilitation Design I-Pave Rehabilitation Alternatives Flexible Pavements 1. AC overlay of existing flexible pavement 2. Milling of existing AC surface with AC overlay 3. Milling of existing AC surface with PCC overlay (only available for low severity of any distresses) 4. Cold in-place recycling of existing AC surface with AC overlay 5. Full-depth reclamation of existing AC surface with AC overlay 6. Remove and replace existing pavement through reconstruction Composite Pavements All options as above, along with complete removal and replacement of existing AC surface 24

25 4. Rehabilitation Design I-Pave Rehabilitation Alternatives Rigid Pavements 1. AC overlay of existing rigid pavement 2. AC overlay of existing rigid pavement with longitudinal joint patching 3. Crack and seat existing PCC slab with AC overlay 4. AC interlayer as bond breaker with PCC overlay (only available for low severity of any distresses) 5. Rock interlayer with AC overlay 6. Rubblization of existing PCC with AC overlay 7. Remove and replace existing pavement through reconstruction 25

26 4. Rehabilitation Design Rehabilitation Design Procedure Findings I-Pave shows three possible rehabilitation alternatives for every design run ESALs for all three pavement types are calculated using an LEF of (same as new flexible design) No effective SN for existing pavements in output summary No life cycle cost analysis is provided for rehabilitation design, hence it is not possible to compare alternatives No output is obtained when all five distresses are selected simultaneously for flexible pavements (with any level for each distress) PCC overlay only selected if low severity distress exists! 26

27 4. Rehabilitation Design Rehabilitation Design Procedure The rehabilitation design procedure used in I-Pave could not be verified using the data collected. Problems faced during the analysis were: Existing distress types and severity have some impact on rehabilitation alternatives and design thickness No reference document cited in I-Pave Equation used to calculate required thickness of the overlay and layer coefficients are not given, and could not be determined from available information Major obstacle for analysis was precedence order of distresses: Which distress predominantly controls what alternatives are shown in the output? 27

28 5. Life Cycle Cost Analysis Flexible Pavement LCCA Asphalt binder unit price displayed in output and that used in calculation are different, resulting in lower unit price of mix 28

29 5. Life Cycle Cost Analysis Flexible Pavement LCCA Maintenance Cost Output shows maintenance cost as $2,700/year Calculations showed that maintenance for flexible pavement is fixed at $55,000 for entire design period, irrespective of design period length Salvage Value Calculating initial cost and salvage value per lane mile Average initial cost of HMA = $60/ton, or $23,760/in Salvage value used in I-Pave = $1.65/yd 2, or $11,616/in Observation: Salvage value as benefit is about 50% of the initial cost, resulting in low EUAC for flexible pavements 29

30 5. Life Cycle Cost Analysis Rigid Pavement LCCA PCC Initial Cost Default unit prices are shown only for integer values of PCC slab thickness Final design thickness is rounded to nearest highest integer for calculating PCC initial cost A pavement whose design thickness is 6.5" therefore has an initial cost corresponding to that for a 7" pavement. Does unit price include dowel bars? Aggregate Base and Subgrade Stabilization Cost Material unit costs and use in calculation are same as those for flexible pavements 30

31 5. Life Cycle Cost Analysis Rigid Pavement LCCA No salvage value for PCC PCC demolition cost of $2/yd 2 is also used in EUAC, i.e. reconstruction is assumed after 20 years Maintenance cost is calculated as $1,900 X Design Life (in years). For 20 years, maintenance cost is $38,000 Maintenance cost is used in calculation as a benefit rather than as a cost. This lowers the EUAC for rigid pavements. EUAC Calculated by I Pave Total EUAC = EUAC Initial - EUAC Maint. + EUAC Demolition = $19, $2, $ = $17,

32 5. Life Cycle Cost Analysis Maintenance Cost Comparison Design Life (years) Flexible Pavements Total Cost PW, $* EUAC, $ Rigid Pavements Total Cost PW, $* EUAC, $ 15 55,000 4, ,500 2, ,000 3, ,000 2, ,000 3, ,500 2, ,000 2, ,000 2, * Values calculated using I-Pave output data, other values are shown in I-Pave LCCA output Calculation of total maintenance costs for flexible and rigid pavements are inconsistent 32

33 5. Life Cycle Cost Analysis I-Pave Life Cycle Cost Analysis? LCCA is performed only over the design life and does not consider any future costs. This violates FHWA policy in that LCCA should be over at least 35 years Life Cycle Cost Analysis in Pavement Design, FHWA 1998 Guide for Pavement Type Selection, NCHRP Report 703 I-Pave does not conduct LCCA 33

34 6. Discussion Discussion Subgrade Resilient Modulus M r values used in I-Pave were derived from MEPDG study (Ceylan et al.) In the MEPDG study, resilient modulus testing was conducted on specimens at optimum moisture content, which is the required input for MEPDG For 1993 AASHTO flexible pavement design, M r must be an "effective" roadbed soil modulus (wet of optimum) which accounts for seasonal variation in moisture conditions (1993 AASHTO, Part II, Section 2.3.1) Von Quintus and Killingsworth (1993): M r should be representative of the more critical moisture condition measured during the year, i.e. higher moisture content 34

35 Discussion AASHO Road Test M r Resilient modulus of 3,000 psi was used to develop the AASHTO flexible pavement design equation, as explained by the last two terms of the design equation 2.32 log 10 M r 8.07 = 2.32* log 10 (3,000) 8.06 = 0 This value is consistent with laboratory tests conducted on subgrade soil from the AASHO road test (1993 AASHTO, Part III, Section 5.3.4) The use of a value greater than 3,000 psi is an indication that the soil is stiffer than the silty-clay A-6 at the Road Test site, and consequently will provide increased support and extend pavement life. (1993 AASHTO, Part III, Section 5.4.5) 35

36 Summary Factors that affect bias in I-Pave design procedure 1. Subgrade resilient modulus, M r for flexible design is unrealistically high for I-Pave/1993 AASHTO design 2. Minimum PCC thickness of 6" for low volume roads whereas minimum HMA thickness is 3" 3. No salvage value for PCC 4. Load equivalency factor (LEF) for rigid pavements is higher than Iowa SUDAS recommendation 5. Rounding off PCC thickness to next highest integer value for initial cost calculation uses unit price of 7 slab for a 6.1 design 6. Methods for calculating total maintenance cost for flexible and rigid pavements are inconsistent 7. Rigid pavement EUAC equation is incorrect maintenance should be treated as a cost instead of benefit 8. I-Pave LCCA does not meet the requirements of FHWA or NCHRP for pavement type selection 36