Pavement Design

CEE 320Steve Muench

Al-Balqa’ Applied UniversityFaculty of Engineering Technology

Department of Civil EngineeringM. KharabshehFall 2013-2014

04/21/23 1

Empirical

1993 AASHTO Flexible Equation

1993 AASHTO Rigid Equation

07.8log32.2

1

109440.0

5.15.4log

20.01log36.9log 10

19.5

10

101810

RoR M

SN

PSI

SNSZW

25.075.0

75.0

10

46.8

7

10

101810

42.1863.215

132.1log32.022.4

1

10624.11

5.15.4log

06.01log35.7log

kE

DJ

DCSp

D

PSI

DSZW

c

dctoR

Terms – Flexible

The ZR and So variables account for reliability

W18 (loading) Predicted number of 80 kN (18,000 lb.) ESALs over the pavement’s life.

SN (structural number) Abstract number expressing structural strength SN = a1D1 + a2D2m2 + a3D3m3 + …

ΔPSI (change in present serviceability index) Change in serviceability index over the useful pavement life Typically from 1.5 to 3.0

MR (subgrade resilient modulus) (Measure Soil Stiffness) Typically from 3,000 to 30,000 psi (10,000 psi is good)

Recommended Values of ZR and So

Functional Classification Recommended Level of Reliability

Urban Rural

Interstate and Other Freeways

85 – 99.9 80 – 99.9

Principal Arterials 80 – 99 75 – 95

Collectors 80 – 95 75 – 95

Local 50 – 80 50 – 80

Typical values of So used are 0.40 to 0.50 for flexible pavements and 0.35 to 0.40 for rigid pavements.

Resilient Modulus MR

Typical values for fine-grained soils are 2,000 to 10,000 psi.

Typical values for course-grained soils are 10,000 to 20,000 psi.

If resilient modulus results are not available, then use the following correlations:For fine-grained soils with a soaked CBR between 5 and 10 , use the following equation to correlate CBR to resilient modulus (MR):

Design MR (psi) = 1,500 x CBRFor non fine-grained soils with a soaked CBR greater than 10, use the following equation:

MR = 3,000 x CBR 0.65

Terms – Rigid

D (slab depth)S’c (PCC modulus of rupture)

A measure of PCC flexural strengthUsually between 600 and 850 psi

Cd (drainage coefficient)Relative loss of strength due to drainage

characteristics and the total time it is exposed to near-saturated conditions

Usually taken as 1.0

Drainage Coefficient

Quality of Drainage Water Removal Within

Excellent 2 hours

Good 1 day

Fair 1 week

Poor 1 month

Very Poor no drainage

Terms – Rigid

J (load transfer coefficient) Accounts for load transfer efficiency Lower J-factors = better load transfer Between 3.8 (undoweled JPCP) and 2.3 (CRCP with tied

shoulders)

Ec (PCC elastic modulus) 4,000,000 psi is a good estimate

k (modulus of subgrade reaction) Estimates the support of the PCC slab by the underlying

layers Usually between 50 and 1000 psi/inch

Mean Effective k-value (psi/inch)

Typical Range – 50 to 500 Typical Value for Pavement Design 200 – 250

If the subgrade resilient modulus is known or obtained from the CBR, then use the following

equation:

k-value = MR / 19.4

Reliability

X = Probability distribution of stress(e.g., from loading, environment, etc.)

Y = Probability distribution of strength(variations in construction, material, etc.)

Pro

babi

lity

Stress/Strength

Reliability = P [Y > X] dxdyyfxfXYPx

yx

Typical PSI vs. Time

Time

Ser

vice

abil

ity

(PS

I) p0

pt

p0 - pt

Subgrade Characterized by strength and/or

stiffness

California Bearing Ratio (CBR)Measures shearing resistanceUnits: percentTypical values: 0 to 20 Resilient Modulus (MR)

Measures stress-strain relationshipUnits: psi or MPaTypical values: 3,000 to 40,000 psi

Subgrade

Some Typical Values MR

Classification CBR MR (psi) Typical Description

Good ≥ 10 20,000

Gravels, crushed stone and sandy soils. GW, GP, GM, SW, SP, SM soils are often in this category.

Fair 5 – 9 10,000

Clayey gravel and clayey sand, fine silt soils.  GM, GC, SM, SC soils are often in this category.

Poor 3 – 5 5,000

Fine silty sands, clays, silts, organic soils.  CL, CH, ML, MH, CM, OL, OH soils are often in this category.

Flexible Pavement Design for Highways (Nomograph to Solve for SN)