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The Inspector’s Job Guide for Construction (11) includes sections on both the
inspection of plant and paving operations.
The Guide assumes that the Inspector will not just be a data or sample taker. The
Inspector should be aware of the whole operation to make sure that a consistent,
uniform quality mixture is produced and constructed.
1.6. Summary and Recommendations.
Chapter 6 presents the summary and recommendations given in the manual. These deal with
the thickness design procedure(s) to use now since the MnPAVE procedure is not documented
fully across Minnesota especially for low volume roads. It is now recommended that either the
Soil Factor or R-Value procedure be used and then the same roadway be designed using
MnPAVE. Comparisons should be made and reported to the MnDOT Research Section. A form
has been developed to report the comparisons.
Traffic is evaluated using 20-year projections of AADT and HCADT for the Soil Factor
design procedure. Equivalent Standard Axle Loads (ESALs) are used for both the R-Value and
MnPAVE design procedures. ESAL predictions over a 20-year design period require an estimate
of AADT, vehicle type distribution, average effect of the various types of vehicles in terms of
ESALs, a growth factor and lane distribution factor for the roadway. Tables and procedures are
presented in Chapter 3 for determining these values both with estimates and using a field
procedure for measuring vehicle type distribution.
The subgrade or embankment is the most important part of a pavement structure. Chapter
presents the methods of evaluating the subgrade strength or stiffness for the three design
procedures. To realize the design parameters obtained for a given soil good construction
practices must be followed. Good construction starts with good specifications that define how the
material is to be constructed and paid for. The MnDOT specifications that are used for subgrade
construction are Nos. 2105, 2111 and 2123. Chapter 4 includes summaries of these specifications
and the field procedures that will most effectively help carry them out. The importance of well-
trained knowledgeable personnel is emphasized.
Chapter 5 presents how the materials used for the pavement section are evaluated for the
three design procedures. The granular equivalent factors are used for the Soil Factor and the R-
Value. The factors are dependent on the specifications which either a granular material or an
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asphalt mixture pass. The GE factors are presented in Chapter 5 and summarized in Chapter 6.
The resilient moduli that are used for the MnPAVE procedure have been related to the other
specification granular and hot mix asphalt materials. Eventually laboratory and non-destructive
field tests (the FWD and DCP) will be used to relate the laboratory tests to the field values. One
big advantage of the mechanistic-empirical design (MnPAVE) is that seasonal variations in
resilient modulus for a material in the pavement section for a given year and from year to year
can eventually be documented.
MnDOT combined 2360 and 2350 (Gyratory/Marshall Design) specifications are
recommended for HMA construction on low volume roads in Minnesota. These specifications
feature the use of volumetrics for field control and quality management (QM) of the team of the
Contractor and the Agency. The Contractor is responsible for Quality Control QC) and the
Agency, Quality Assurance (QA). The specifications include requirements for material quality,
mixture design, mixture variability, density (voids), Voids in the Mineral Aggregate (VMA),
moisture susceptibility, field density and smoothness of the finished surface. Construction
procedures and a checklist for field engineers and inspectors are presented.
One of the major goals of the presentation of design and construction of the subgrade and
pavement section materials is to obtain uniformity, which helps a great deal in the achievement
of good performance.
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THICKNESS DESIGN PROCEDURES
2.1. Background and Introduction
There are three flexible pavement thickness design procedures now used in Minnesota. In
addition some pavements, especially at the local level, are designed by experience based on what
has worked in the past. The three formal thickness design procedures are the Soil Factor Design
found in the MnDOT State Aid Manual (4), the Stabilometer R-Value Design found in the
MnDOT Geotechnical and Design Manual (5) and MnPAVE, which is the mechanistic-empirical
design procedure currently under development. The Soil Factor Procedure was developed in the
1950’s and has been modified somewhat since then. MnDOT adopted the R-Value Procedure in
the early 1970’s. The MnPAVE Procedure is in software form and is being tested against the
other procedures. The Beta version is now available (6). In this Chapter the procedures are
presented along with the factors needed for thickness determination.
The traffic factor for each of the procedures is presented in Chapter 3. The embankment
(subgrade) factors for design and construction specifications and recommended procedures are
given in Chapter 4. The thickness of the pavement section is defined using the Granular
Equivalent for the Soil Factor and R-value design procedures. The Resilient Modulus (M
the thickness of the layers define the structure for the MnPAVE Procedure. The required
specifications and recommended construction procedures to attain the respective pavement
section factors are presented in Chapter 5.
2.2. Soil Factor Design
Since 1954 some pavements in Minnesota have been designed using a table similar to Figure
2.1. This is the 2001 version from the State Aid Manual which uses English and metric units (4).
The chart uses seven traffic categories based on 20-year projected two-way AADT and HCADT
and eight embankment types using the AASHTO classification system. Thickness in terms of
Granular Equivalent (G.E.) is determined for each level of traffic and soil type. Each design also
has a specified maximum spring axle load.
The traffic factors are Average Daily Traffic (ADT) and Heavy Commercial Average Daily
Traffic (HCADT). The ADT and HCADT are both two-way values. The ADT includes all
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vehicles and the HCADT is defined as all trucks with six or more tires; thus HCADT does not
include cars, small pickup and panel-type trucks. The ADT and HCADT normally used for
design are values predicted for 20 years into the future. Local conditions must be considered and
the projected value may either be increased or decreased based on the projected future use of the
road. More specific methods of determining design values are presented in Chapter 3.
As noted in Figure 2.1 a soil factor of 100% represents an A-6 or A-4 soil. Stronger soils
have soil factors less than 100% and weaker soils greater than 100%. The soil factor percentage
represents the percent increase or decrease in the thickness of the subbase (D
). There are ranges
of percentages shown for A-1, A-2, A-4 and A-7 soils. Therefore, it is possible to use some
judgment relative to the capabilities of the soils after evaluating drainage and other design
Minimum Bit. G.E.
Superpave Hot Mix
Plant Mix Asp Pave
(Class 5 & 6) 3138
(Class 3 & 4) 3138
50 - 75
70 - 75
50 - 75
30 - 70
NOTE:If 10 ton (9.1 t) design is to be used, see Road Design Manual 7-3.
For full depth bituminous pavements, see Road Design Manual 7-3.
*Granular Equivalent Factor per MnDOT Technical Memorandum 98-02-MRR-01.
9 TON @ LESS THAN 150 HCADT
9 TON - 600 @ 1100 HCADT
9 TON - MORE THAN 1100 HCADT
7 TON @ 400 - 1000 ADT
9 TON - 300-600 HCADT
7 TON @ LESS THAN 400 ADT
9 TON -150-300 HCADT
FLEXIBLE PAVEMENT DESIGN USING SOIL FACTORS
Required Gravel Equivalency (G.E.) for various Soil Factors (S.F.)
For new construction or reconstruction use projected ADT. For resurfacing or reconditioning use present ADT.
All units of G.E. are in inches with millimeters (mm) in parenthesis.
Figure 2.1 Flexible Pavement Design Using Soil Factors
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considerations. Chapter 4 includes a discussion on the selection of these and other design
parameters for the embankment soils.
The strength and stiffness of the soil supporting the pavement are very dependent on the
density and moisture conditions of the constructed soil. Uniformity is also important to
minimize differential heave during freeze up. The construction specifications and procedures
presented in Chapter 4 must be followed to attain the strength and stiffnesses inferred in the
given soil factors.
The Granular Equivalent (G.E.) defines a pavement section by equating the thickness of each
aggregate or HMA layer to an equivalent thickness of granular base material. Equation 2.1 is
used to calculate the Granular Equivalent. In Minnesota this is a Specification 3139 material,
Class 5 or 6 (9). The relevant specifications for the other pavement materials are listed in Figure
2.1. Minimum bituminous and total granular equivalents are also shown for each traffic category.
The total Granular Equivalent is defined using Equation 2.1.
G.E. = a
= thickness of asphalt mix surface, in. (mm)
= thickness of granular base course, in. (mm)
= thickness of granular subbase course, in. (mm)
= G.E. Factors listed in Figure 2.1.
The required design thicknesses are listed in two categories (minimum bituminous G.E. and
total G.E.). The maximum granular base thickness can be calculated by subtracting the minimum
bituminous G.E. from the total G.E. Other design combinations of bituminous and granular
materials can be determined using the G.E. factors.
The respective specifications and construction procedures necessary to attain the material
characteristics defined for the soil factor design are presented in Section 5.3.2.
2.3. Stabilometer R–Value Design
The Stabilometer R-Value is the current design procedure used by MnDOT to determine the
design thickness of an HMA surfaced pavement. This procedure is based on research done in the
1960’s using results from the AASHO Road Test. The basis of the design is limiting spring
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deflections by increasing the strength (stiffness) of the soil or by increasing the strength
(stiffness) of the pavement layers for a given level of traffic.
Figure 2.2 is the R-Value design chart from the MnDOT Design and Geotechnical and
Pavement Design Manual (5). The embankment R-Value can be measured with a standard
laboratory test (ASTM D-2844) or estimated from the soil type or classification. The R-Value
laboratory procedure used in Minnesota is presented in Chapter 4. An exudation pressure of
1655kPa (240 psi) is used for determining a design R-Value in Minnesota. Predictions of R-
Value from soil classification are also presented in Table 4.5.
The traffic is evaluated in terms of 80-kN (18,000-lb) equivalent standard axle loads
(ESAL’s). For a particular road being designed the ESAL’s are estimated for a design lane in one
direction. Calculated ESAL’s will be different for flexible and rigid pavements for the same
traffic mix. Chapter 3 presents methods for estimating design ESAL’s for flexible pavements in
Figure 2.2 R-Value Design Chart
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The thickness is defined in terms of Granular Equivalent in inches. Granular equivalent
, and a
) for the R-Value design are listed in Section 5.3.2. Equation 2-1 is used to
calculate the total granular equivalent in the same way as for the soil factor design. In addition to
the lines for specific R-Values showing the required GE for a given number of ESAL’s, lines on
the R-Value design chart represent:
The minimum bituminous thickness GE and
Bituminous plus base thickness GE.
The actual thicknesses represented can be calculated using the appropriate G.E. factors.
Examples of designs using the R-Value design chart with minimum thicknesses of
surface and base, plus other combinations are given in Reference 5.
2.4. MnPAVE Design
The Minnesota Department of Transportation and the University of Minnesota have
developed a mechanistic-empirical (M-E) design method for flexible pavements. The
procedure has been developed as a software package (MnPAVE) because of the great
quantities of data and analyses used for the design. A Beta Version of the software is now
available. It is still being fine-tuned somewhat.
MnPAVE predicts the structural performance of pavement sections using calculated
strains in a simulated elastic layered system. To use the elastic layered system moduli and the
thickness of each pavement layer must be determined for the pavement. Up to five (5) layers
can be used for the calculations of:
• The tensile strain in the bottom of the surface layer and
• The compressive strain on the top of the subgrade, which is assumed to be infinite in
Various combinations of material properties (moduli) are used to simulate the seasons
throughout the year. Currently, five seasons are used (winter, early spring, late spring,
summer and fall). MnPAVE calculates the percent of damage that occurs in each season,
maximum stress, strain and displacement at the critical locations, the allowable axle load
repetitions and reliability percentages. The life in years is then predicted using the predicted
traffic in ESAL’s or load spectra.
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Fatigue cracking has been correlated with the tensile strain in the HMA surface layer and
embankment rutting has been correlated with the compressive strain on the embankment. The
performance equations are derived from the development of fatigue cracking and rut depth
on the MnROAD test sections. Moduli of the layers have been measured throughout the year
using backcalculated Falling Weight Deflectometer (FWD) data or estimated from the
Dynamic Cone Penetrometer (DCP) or other standard tests.
The performance equations were also checked using the performance of a number of 40-
year old test sections from Investigation 183 (15). The research to develop the information to
check the performance of these sections was done as part of this project and reported in
Appendix A of this report.
Variability can also be incorporated into MnPAVE. Variations in the following
parameters contribute to the overall variation of the pavement section.
• Layer Moduli
− HMA Surface
− Granular base and subbase
− Subgrade Soil
• Layer Thicknesses
• Load Predictions
− Vehicle class predictions
− Vehicle weight estimates
− Total number of vehicles
The variability of these parameters is used with the predictions equations to calculate the
reliability of the performance predictions. A Monte Carlo simulation is used to calculate the
reliability of the performance predictions (16). With this type of analysis it is possible to
relate the variability of the thickness, material properties and traffic predictions to required
thickness. More uniform construction can therefore be translated into thickness saved or
increased life predictions.
MnPAVE requires that the materials be described by their stiffness (modulus) for the
seasons defined. This requires that the modulus be defined for these seasons either directly or
backcalculated using the FWD or DCP. Correlations with other standard tests as shown in
Table 4.5 can also be used.
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At this time MnPAVE should be used in conjunction with one or both of the current
methods. In this way a city or county can develop confidence in the results of the MnPAVE
design. Without the MnPAVE software it has not been possible to take into account the many
variables that affect the performance of a pavement section.
MnPAVE has the following features:
• Three design levels based on input data quality
• Material properties adjusted seasonally
• Traffic quantified using either ESAL’s or load spectra
• English or System International (S.I.) Units
• HMA modulus temperature adjustment equations that can be modified
• Reliability estimates using Monte Carlo simulations
2.4.2. Set Up
MnPAVE is designed for Windows 95/98/NT operating systems and requires 2 MB of
hard drive space and a 200 MHz processor or higher.
Installation can be accomplished using the following procedure:
1. Create a new folder on the hard drive called “MnPAVE”
2. Copy the *.exe file from the floppy disk to the MnPAVE folder.
3. Run the program.
2.4.3. Start Up
18.104.22.168. Control Panel
The “Control Panel” is the first window to appear when MnPAVE is started. The
control panel includes areas for input data which includes “Climate, Structure and
Traffic” A button to display “Output” also appears on the window. The input must be
entered in order beginning with “Climate” and ending with “Traffic”, because the
seasonal factors used in “Structure” depend on Climate and some of the ESAL
calculations in Traffic depend on Structure. Changes can be made in these input windows
at any time. However, for a given design check, all inputs must be completed before
“Output” can be selected.
22.214.171.124. General Operation
MnPAVE uses the pull-down menu and window selection structures common to most
software packages. The pull-down menu at the top of the screen includes, “File, Edit,
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