Upload
vicky-ceunfin
View
253
Download
3
Embed Size (px)
Citation preview
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
1/16
AASHTO Flexible Design Procedure
Dr. Christos Drakos
University of Florida
Topic 7 AASHTO Flexible Pavement Design
1. Development
1.1 AASHO Road Test
AMERICANASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS
http://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 variouspavements
Provided data for the design criteria
AIAASHTO VsFunctional Structural
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
2/16
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 Poor2-3 Fair3-4 Good4-5 V. Good
A panel of experts drove around in standardvehicles 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 fromhorizontal 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 Poor2-3 Fair3-4 Good4-5 V. Good
USER ASSESSMENT PERFORMANCE INDICATORS
Measure of RoughnessMeasure of RuttingMeasure of Cracking
Present Serviceability Index (PSI)PSI = A0 + A1F1 + A2F2 + A3F3
A0 A3 = Regression CoefficientsF1 = Measure of roughnessF2 = Measure of ruttingF3 = Measure of cracking
How does the true (user) performancecorrelate to the measured performance?
calculated the regressioncoefficients for the PSI equation
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
3/16
Topic 7 AASHTO Flexible Pavement Design
1.3 AASHTO Design Equations
1.3.1 Performance Requirements & Design Life
PSI
Time (age)
PSI0
PSIt
PSI = (PSI0 - PSIt)
Terminal PSI (known) Pvt isno 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 Efficiencyof PVT
ESAL
Structural Number (SN)
MReff
=
known known known
Solve for SN
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
4/16
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
a3
SN = 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 & thicknessesmay result in the same SN
ii. Your job as a designer is to select the mosteconomical combination, using available materialsand considering the following: Geometry requirements (Cut/Fill) Drainage requirements Frost requirements
iii.AASHTO assumes that pavement structural layerswill not be overstressed: Must check that individual layers meet structural
requirements
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
5/16
Topic 7 AASHTO Flexible Pavement Design
2. Design Inputs
2.1 General Design Variables
Design Life Material Properties Traffic Reliability
Degree of certainty that the pavement will lastthe 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 levelchosen
Overall Standard Deviation: Traffic Variation Performance prediction
variation Materials (subgrade)Steps:
1. Define functional class (Interstate/Local)2. Select reliability level (R) Table 11.14
3. 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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
6/16
Topic 7 AASHTO Flexible Pavement Design
2.3 Performance Criteria
Design 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 Properties
2.4.1 Effective Subgrade Resilient Modulus
Obtain MRvalues over entire year
Separate year into time intervals Compute the relative damage value (uf) for each modulus2.32
R8
f M101.18u
=
Topic 7 AASHTO Flexible Pavement Design
Compute average uffor entire year
Determine effective MRusing average uf
2.4.1 Effective Subgrade Resilient Modulus (cont)
2.32
R8
f M101.18u
=
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
7/16
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 materialwith a=0.4
Initially layer coefficients were derived from AASHO road testresults; 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 ofinterpolating from the figure):
Untreated and Stabilized Bases
( ) 977.0log249.0 22 = Ea
Granular 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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
8/16
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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
9/16
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
PSI
4.2 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 thenomograph using the MRof the layer in question
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
10/16
Topic 7 AASHTO Flexible Pavement Design
D1
1
11
a
SND =
SNtotal
MReff
First we need to protect the subgrade; use the nomographto get SN needed to provide adequate protection
BUT, have to protect each layer from overstressing; needto 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 Procedure
1. Using E2 as the MR value, determine from Figure 11.25 the structuralnumber SN1 required to protect the base and compute the thickness oflayer 1 by
2. Using E3 as the MRvalue, determine from Figure 11.25 the structuralnumber SN2 required to protect the subbase and compute the thickness oflayer 2 by
3. Based on the roadbed soil resilient modulus MReff, determine from Figure11.25 the total structural number SN3 required and compute the thicknessof layer 3 by
1a
1SN
1D =
2m2a
*1
D1
a2
SN
2
D
3m
3a
2m*
2D
2a*
1D
1a
3SN
3D
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
11/16
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 / Stability
Layer 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 0.950.10
4.051.60 1.000.37
3.120.800.16
0.4=
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
12/16
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 Stone1.20350,000Bituminous-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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
13/16
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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
14/16
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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
15/16
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
7/22/2019 Topic 7 - AASHTO Flexible Pavement Design
16/16
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?