Oct 08, 2015
AASHTO Flexible Design Procedure
Dr. Christos Drakos
University of Florida
Topic 7 AASHTO Flexible Pavement Design
1. Development
1.1 AASHO Road Test
AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALShttp://www.aashto.org/
1.2 Performance MeasurementsEstablishment of performance criteria is critical
Late 50s road test in Illinois Objective was to determine the relationship between the
number of load repetitions with the performance of various pavements
Provided data for the design criteria
AIAASHTO VsFunctional Structural
Topic 7 AASHTO Flexible Pavement Design
AASHO Road Test performance based on user assessment: Difficult to quantify (subjective) Highly variable Present Serviceability Rating (PSR)
1.2 Performance Measurements (cont)
0-1 V. Poor1-2 Poor 2-3 Fair 3-4 Good 4-5 V. Good
A panel of experts drove around in standard vehicles and gave a rating for the pavement
Measurable characteristics (performance indicators): Visible distress (cracking & rutting) Surface friction Roughness (slope variance)
Measure of how much slope varies from horizontal along the direction of traffic
Topic 7 AASHTO Flexible Pavement Design
Establish correlation between user assessment (ride experience) and performance indicators (measurable characteristics)
1.3 AASHTO Performance Relations
0-1 V. Poor1-2 Poor 2-3 Fair 3-4 Good 4-5 V. Good
USER ASSESSMENT PERFORMANCE INDICATORS
Measure of RoughnessMeasure of RuttingMeasure of Cracking
Present Serviceability Index (PSI)PSI = A0 + A1F1 + A2F2 + A3F3A0 A3 = Regression CoefficientsF1 = Measure of roughnessF2 = Measure of ruttingF3 = Measure of cracking
How does the true (user) performance correlate to the measured performance?
calculated the regression coefficients for the PSI equation
Topic 7 AASHTO Flexible Pavement Design
1.3 AASHTO Design Equations1.3.1 Performance Requirements & Design Life
PSI
Time (age)
PSI0
PSIt
PSI = (PSI0 - PSIt)
Terminal PSI (known) Pvt is no longer functional
AASHTO performance requirement = PSI PSI is such that PSIt is NOT reached before end of design life
Design Life
PSI scale: 1 (V. Poor) 5 (V. Good)
Topic 7 AASHTO Flexible Pavement Design
1.3.2 Performance Relation
PSI = fnc (MReff, SN, ESAL) MReff: Accounts for the environment SN: Index relating effectiveness of
PVT structure
PERFORMANCE(PSI)
What are the three factors affecting performance (PSI)?
Structural Efficiency of PVT
ESAL
Structural Number (SN)
MReff
=
known known known
Solve for SN
Topic 7 AASHTO Flexible Pavement Design
1.3.3 Definition of Structural Number
BASE
AC
SUB-BASE
D1
D2
D3
111 aDSN =
222 aDSN =
333 aDSN =
SN
Structural Coefficient (a):a = fnc (E, position in PVT)
a1
a2
a3SN = SN1 + SN2 + SN3
Basic Procedure: Determine the traffic (ESAL) Calculate the effective subgrade modulus (MReff) Select the performance level (PSI) Solve for the required SN needed to protect the subgrade
Topic 7 AASHTO Flexible Pavement Design
1.3.4 Design Notes
i. Different combination of materials & thicknesses may result in the same SN
ii. Your job as a designer is to select the most economical combination, using available materials and considering the following: Geometry requirements (Cut/Fill) Drainage requirements Frost requirements
iii.AASHTO assumes that pavement structural layers will not be overstressed: Must check that individual layers meet structural
requirements
Topic 7 AASHTO Flexible Pavement Design
2. Design Inputs2.1 General Design Variables
Design Life Material Properties Traffic Reliability
Degree of certainty that the pavement will last the design period
Uncertainty in: Traffic prediction Performance prediction Materials & construction
Topic 7 AASHTO Flexible Pavement Design
2.2 AASHTO Reliability Factor (FR)Adjust traffic for reliability:
R1818 FwW =Where:W18 = Design ESALw18 = Predicted ESAL
FR = fnc (R, S0)
Reliability level chosen
Overall Standard Deviation: Traffic Variation Performance prediction
variation Materials (subgrade)Steps:
1. Define functional class (Interstate/Local)2. Select reliability level (R) Table 11.143. Select a standard deviation (S0)
Flexible: No traffic variation: S0=0.35 With traffic variation: S0=0.45
Rigid: No traffic variation: S0=0.25 With traffic variation: S0=0.35
Topic 7 AASHTO Flexible Pavement Design
2.3 Performance CriteriaDesign for serviceability change: PSI = PSI0 PSIt
PSI0 = Initial serviceability index Flexible: 4.2 Rigid: 4.5
PSIt = Terminal serviceability index Major highways: >2.5 Lower volume: 2.0
2.4 Material Properties2.4.1 Effective Subgrade Resilient Modulus Obtain MR values over entire year Separate year into time intervals Compute the relative damage value (uf) for each modulus
2.32R
8f M101.18u
=
Topic 7 AASHTO Flexible Pavement Design
Compute average uffor entire year
Determine effective MR using average uf
2.4.1 Effective Subgrade Resilient Modulus (cont)
2.32R
8f M101.18u
=
Topic 7 AASHTO Flexible Pavement Design
2.4.2 Pavement Structural Layers Layer coefficient ai; relative quality as a structural unit:
2 of material with a=0.2 provides the same protection as 1 material with a=0.4
Initially layer coefficients were derived from AASHO road test results; have subsequently been related to resilient modulus
Hot-Mix Asphalt
AASHTO does not require test to determine HMA modulus; usually assume aHMA=0.44
Topic 7 AASHTO Flexible Pavement Design
2.4.2 Pavement Structural Layers (cont)
Can estimate the base layer coefficient from Figure 7.15 for: Untreated base Bituminous-treated base Cement-treated base
For untreated base can also use the following (instead of interpolating from the figure):
Untreated and Stabilized Bases
( ) 977.0log249.0 22 = EaGranular Sub-bases Can estimate the sub-base layer coefficient from Figure 7.16 Can also use the following (instead of interpolating from the
figure): ( ) 839.0log227.0 33 = Ea
Topic 7 AASHTO Flexible Pavement Design
2.5 Drainage AASHTO guide provides means to adjust layer coefficients
depending on the effectiveness of the drainage Define quality of drainage of each layer based upon:
Time required for drainage Percent time moisture levels approach saturation
Determine drainage modifying factor (m) from Table 11.20 iiii mDaSN =
Topic 7 AASHTO Flexible Pavement Design
2.6 Computation of Required Pavement Thickness
Determine the required SN for design traffic Identify trial designs that meet required SN
2.6.1 Basic Approach
2.6.2 Nomograph to Solve for SN
Topic 7 AASHTO Flexible Pavement Design
2.6 Computation of Required Pavement Thickness (cont)
Declare the known variables W18, ZR, S0, PSI & MR Give an initial estimate for the SN Allow the equation solver (Matlab, Maple, Mathcad, Excel,
etc.) to iterate for the solution
2.6.3 Solving the Equation
log W 18( ) Z R S 0( ) 9.36 log SN 1+( )+ 0.2log
PSI4.2 1.5
0.41094
SN 1+( )5.19+
+ 2.32 log M R( )+ 8.07
Topic 7 AASHTO Flexible Pavement Design
2.6.4 Pavement Structural Layers
SN = a1D1 + a2D2m2 + No Unique Solution! Many design configurations will meet
the required SN Optimize the design; consider the following:
Design constraints drainage, minimum thickness, available materials Construction constraints minimum layer thickness Economics
2.6.5 Layered Design Analysis Nomograph determines the SN required to protect the
subgrade However, each structural layer must be protected against
overstressing Procedure developed using the AASHTO design nomograph
Determine the SN required to protect each layer by entering the nomograph using the MR of the layer in question
Topic 7 AASHTO Flexible Pavement Design
D1
1
11 a
SND =
SNtotal
MReff
First we need to protect the subgrade; use the nomograph to get SN needed to provide adequate protection
BUT, have to protect each layer from overstressing; need to get required SN (level of protection) for each layer
Only top (AC) layer does not need protection
For example: Base needs SN1 protection. BUT, SN1= a1D1
So,
E1, a1
E2, a2, m2
SN1
E3, a3, m3
SN2
Topic 7 AASHTO Flexible Pavement Design
2.6.6 General Procedure1. Using E2 as the MR value, determine from Figure 11.25 the structural
number SN1 required to protect the base and compute the thickness of layer 1 by
2. Using E3 as the MR value, determine from Figure 11.25 the structural number SN2 required to protect the subbase and compute the thickness of layer 2 by
3. Based on the roadbed soil resilient modulus MReff, determine from Figure 11.25 the total structural number SN3 required and compute the thickness of layer 3 by
1a1SN
1D =
2m
2a
*1D1a2
SN2
D
3m
3a
2m*
2D
2a*1D1a3
SN3
D
Topic 7 AASHTO Flexible Pavement Design
2.7 Other Thickness Considerations
64> 7,000,000
63.52,000,000 7,000,000
63500,000 2,000,000
42.5150,000 500,000
4250,000 150,000
41< 50,000
Aggregate BaseAsphalt ConcreteESAL
2.7.1 AASHTO Suggested Minimums
2.7.1 Construction / StabilityLayer must be thick enough to act as a unit:
Thickness > 2* (Maximum Aggregate Size)
Maximize crushed stone thickness minimize AC thickness Can also stabilize base to use less HMA
Use gravel only for fill or frost
Topic 7 AASHTO Flexible Pavement Design
2.8 Cost Considerations Consider:
Different combination of materials Cost of materials Cost of excavation (cut areas)
Express cost as a unit contribution to SN
Asphalt Concrete
Pit-Run Gravel
Crushed Stone
$/unit SNmiai$/sq.yd.-inMaterial
3.120.40 0.800.16
3.370.32 0.950.104.051.60 1.000.37
3.120.800.16
0.4 =
Topic 7 AASHTO Flexible Pavement Design
WORK EXAMPLE ON THE BOARD
2.9 AASHTO Design Example 1
--5,000Roadbed Soil
0.700.1014,000Granular Subbase
0.800.1430,000Crushed Stone
-0.42400,000AC
miaiMRMaterialGiven: Reliability = 90% Overall Std. Dev. = 0.35 W18 = 10 million Design Serviceability Loss = 2.0
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2
0.3212,000Pit-Run Gravel
0.80500,000Cement-Stabilized Base
0.25-Excavation
0.4025,000Crushed Stone
1.20350,000Bituminous-Treated Base
1.70300,000Asphalt Concrete
Cost ($/sq.yd.-in)Modulus (psi)Material
Given: Reliability = 90% Performance period = 20 years Overall Std. Dev. = 0.45 W18 = 5.26 million Design Serviceability Loss = 2.0
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)
0.32SubbasePit-Run Gravel
0.40SubbaseCrushed Stone
0.40BaseCrushed Stone
1.20BaseBituminous-Treated Base
0.80BaseCement-Stabilized Base
1.70BaseAsphalt Concrete
1.70SurfaceAsphalt Concrete
$/Unit SNmiai$/sq.yd-inLayerMaterial
Construct a material information table:
Next step is to fill in the information
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)Asphalt Concrete structural coefficient (a) Figure 7.13:
0.37
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)Bituminous-treated base structural coefficient (a) Figure 7.15:
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)Cement-stabilized base structural coefficient (a) Figure 7.15:
0.118
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)Crushed stone base structural coefficient (a) Figure 7.15:
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)Crushed stone subbase structural coefficient (a) Figure 7.16:
0.16
Topic 7 AASHTO Flexible Pavement Design
2.10 AASHTO Design Example 2 (cont)
4.440.80.0900.32SubbasePit-Run Gravel
2.081.20.1600.40SubbaseCrushed Stone
2.781.20.1200.40BaseCrushed Stone
4.001.00.3001.20BaseBituminous-Treated Base
6.781.00.1180.80BaseCement-Stabilized Base
6.181.00.2751.70BaseAsphalt Concrete
4.591.00.3701.70SurfaceAsphalt Concrete
$/Unit SNmiai$/sq.yd-inLayerMaterial
Are there any obvious conclusions?