Structural Overlay Design

Using NDT Methods

Zhong Wu, Ph.D., P.E.

Louisiana Transportation Research Center

2007 Transportation Engineering Conference

Baton Rouge, February 11-14, 2007

Outline

Background

Objective

Overview of Overlay Design Methods

Research Projects

Summary

Recommendations

Background

Asphalt overlay has been considered as

simplest and fastest means of restoring the distressed surfaces of rigid and flexible pavements

A quality overlay design

improve the roadway’s rideability

restore the pavement structural stability.

Structural overlay thickness design requests

Existing subgrade condition

Existing pavement strength

Engineering judgment

Background (cont…) Current DOTD overlay thickness design

the 1993 AASHTO Pavement design guide software (DARWin)

One required design input, subgrade resilient modulus value, Mr, the pre-assigned parish-map values

not base on in-situ soil properties

Uses component analysis (layer co-efficients) method to determine the existing pavement’s structure number

SNeff = ∑aihi

Such method can lead to design errors (over- or under-estimated overlay thickness) since design values do not represent actual field conditions

Overlay Design Example

-30 -20 -10 0 10 20 30 40 50 60 70 80Change in Mr (MPa)

-150

-100

-50

0

50

100

150

Dif

fere

nce

in o

ver

lay t

hic

kn

ess

(mm

)

-4000 -2000 0 2000 4000 6000 8000 10000Change in Mr (psi)

Hot mix asphaltD1=102 mm, a1=0.0165/mm

Base courseD2=241 mm, a2=0.0063/mm

SubbaseD3=457 mm, a3=0.0040/mm

Subgrade soil

Typical pavement section

W18=5,000,000 ESALs

R=95 %

PSI=1.9S0=0.35

Design Mr=34.5 MPa

Design SN=5

∆Mr = 2,000 psi

=> Underestimated AC

thickness of 1.5 in

∆Mr = -2,000 psi

=> Overestimated AC

thickness of 2.0 in

Objective

to establish a methodology for mechanistic

pavement overlay design, based on

in-situ pavement conditions, and

utilizing non destructive test (NDT) methods,

specifically the FWD and/or Dynaflect.

Dynatest 8002 model Falling Weight Deflectometer Dynaflect

Overview of Overlay Design Methods

Effective Thickness (ET) Approach

1993 AASHTO Pavement Design Guide

Asphalt Institute (AI) ET Method (MS-17)

Deflection-based Approach

AI Benkelman Beam Deflection Method (MS-17)

Caltran Flexible Pavement Rehabilitation

Mechanistic-Empirical (M-E) Approach

EVERPAVE (WsDOT)

New M-E Pavement Design Guide

Effective Thickness (ET) Approach

- Asphalt Institute (MS-17)

Thickness of Overlay = Tn - Te

Tn, new pavement thickness, determined from AI

Design Chart for Full-depth Asphalt Concrete, using

ESALd and Mr

Te, effective thickness of existing pavement

structure

Te=∑ Cihi

where, hi=thickness of the ith layer of the existing

pavement;

Ci=conversion factor associated with the ith existing layer

Effective Thickness (ET) Approach

-1993 AASHTO Pavement Design Guide

Overlay Thickness Equation

SNf from AASHTO pavement design equation

where Mr is a required input, which can be determined from

Laboratory Testing

Backcalculation from NDT measurements

Approximate relationships (used by DOTD)

The effective structure number of existing pavement, SNeff

NDT method

Component analysis method (used by DOTD)

Remaining life method

OL

efff

OL

OLOL

a

SNSN

a

SNh

Deflection-Based Approach

- Asphalt Institute Benkelman Beam (MS-17)

Pavement is modeled as a two-layer system

Layer 1: AC Overlay

Layer 2: Existing pavement

Overlay thickness is determined as following steps:

Determine Representative Rebound Deflection (RRD)

Layer 2’s elastic modulus is determined from RRD.

Compute Design Rebound Deflection (DRD) based on the allowable ESAL: DRD=1.0363 (ESAL)-0.2438

Other Deflections (e.g. FWD, Dynaflect) can be converted into Benkelman beam deflections, such as

Benkelman Beam = 1.61 * FWD

Benkelman Beam = 20.63 * Dynaflect

Mechanistic-Empirical (M-E) Approach

Modeled pavement as multi-layered elastic

or visco-elastic structure

Pavement materials described by their

stiffness and strengths at different times of

the year

Determine the critical stress, strain, or

deflection by mechanistic methods

Predict resulting damages by empirical

failure criteria, e.g. fatigue cracking, rutting.

Mechanistic-Empirical (M-E) Approach

-EVERPAVE

Developed by Washington DOT

Steps:

Backcalculate layer moduli using FWD data

Analyze and determine the two failure criteria parameters.

Fatigue cracking

Rutting

Compute allowable repetitions to failure at each season

Compute damage at each season and sum the seasonal

damage ratio.

Determine the overlay thickness based on the sum of the

damage ratio is less than or equal to one.

)log(854.0)log(291.382.14log actf EN

4843.418 )(10077.1log VfN

EVERPAVE Design Input

New M-E Pavement Design Guide

Developed under

NCHRP 1-37A

New Traffic input

Enhanced Integrated

Climatic Model

(EICM)

Season variations

New Distress Models

Need Calibration

Dr. Matthew W. Witczak (2003)

NDT Overlay Design SurveyState Method Software

Arkansas Equivalent Thickness ROADHOG

Mississippi Equivalent Thickness ELMOD5

Alabama 1993 AASHTO Spreadsheet program

Maryland 1993 AASHTO Spreadsheet program

(VDOT)

Virginia 1993 AASHTO Spreadsheet program

California Deflection-based Design Manual

North Carolina AI Deflection-based Spreadsheet program

South Carolina Deflection-based Spreadsheet program

Idaho M-E WinFlex

Minnesota M-E MNPAVE

Oregon M-E /

Texas M-E FPS-19W

Washington M-E EVERPAVE

Project Selection

Four in-service pavements

I-12 (ESALd=24,400,000, life=15yrs)

LA28 (ESALd=1,513,000, life=10yrs)

LA74 (ESALd=700,590, life=10yrs)

LA44 (ESALd=353,256, life=10yrs)

Each project about 3 to 5 miles long

Design Plan

Based on current DOTD overlay design

method (Mr-parish map, SNeff-estimated)

I-12 4.5” AC overlay + 2” cold planning

LA28 4.5” AC Overlay + 2” cold planning

LA44 3.5” AC overlay + 2” cold planning

LA74 3.5” AC overlay + 2” cold planning

NDT Tests

FWD and Dynaflect tests were performed on

each project site

at 0.1 mile interval

on both traffic directions

8” 4” 6” 6” 12” 12” 12” 12”

d1 d2 d3 d4 d5 d6 d7 d8 d9

FWD Load

Dynaflect Deflection Analysis

Kinchen and Temple

(1980) developed a

“Pavement Evaluation

Chart” for Louisiana

SN of existing pavements

Subgrade Modulus

Routinely use in

pavement research

projects

Pavement Evaluation Chart

FWD Analysis (D0 & D9)

LA 44

0

5

10

15

20

25

1.0

1.2

1.4

1.6

1.8

2.0

3.1

3.3

3.5

3.7

3.9

6.0

6.2

6.4

6.6

6.8

7.0

Station (miles)

FW

D D

efl

ec

tio

n (

mils

)D0 (NB)

D0 (SB)

D9 (NB)

D9 (SB)

LA 74

0

10

20

30

40

50

0.1

0.3

0.5

0.7

0.9

1.1

1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

Station (miles)

FW

D D

efl

ec

tio

n (

mil

s)

D1 (EB)

D0 (W B)

D9 (EB)

D9 (W B)

Dynaflect (SN)

Overlay Thickness Deflection-Based Approach (AI Method)

3.5"3.5"

4.5"4.5"

2"

4"4.5"

3"

2"

3"

2"

3"

0

2

4

6

8

I-12 LA28 LA74 LA44

Project

Ov

erla

y T

hic

kn

ess

(in

)

Current Plan

AI (NB/EB)

AI (SB/WB)

Equivalent Thickness Method

(Arkansas ROADHOG)

3.5"3.5"

4.5"4.5"

2.8"2.7"

4.2"

2"2.6"

2.2"2.9"

2"

0

2

4

6

8

I-12 LA28 LA74 LA44

Project

Ov

erla

y T

hic

kn

ess

(in

)

Current Plan

ROADHOG(NB/EB)

ROADHOG(SB/WB)

Based on M-E Design Approach

Only 0 or 1” overlay thickness required for all

four projects.

Possible explanations:

Backcalculated modulus too high

Default values used in distress models (no

Calibrated)

Not fully understand how to choose a

representative design value

Summary (Deflection-based method)

Simple to use (e.g. AI method)

Needs to verify and calibrate the relationship

between FWD (or Dynaflect) measured

deflections and BB rebound deflections

Relationship between allowable rebound

deflection and ESALd also needs to be

verified and calibrated

Summary (Equivalent Thickness

method)

Simple to use (e.g. AASHTO and ROADHOG)

1993 AASHTO NDT-based method generally

underestimate the overlay thickness, due to over-

estimate the existing pavement SN.

ROADHOG uses its own relationship in estimation

of SNeff.

Such relationships between SNeff and delta(D) may or

may not be applicable to Louisiana condition

Summary (M-E design method)

Complicate to use.

M-E-based overlay design method needs

sophisticate inputs, which usually are not

available directly from in-situ NDT tests

The fatigue and rutting models used in any M-

E base design software must be verified and

calibrated before any locally implementation.

Proposed NDT-based Overlay Design

Procedure for Louisiana Use Effective Thickness approach

The future Structure Number (SNfuture) determined from 1993 AASHTO design equation

Mr determined from in-situ tests (DCP, FWD or Dynaflect)

SNeff determined from FWD or Dynaflect test

If FWD used, SNeff (FWD) needs to be scaled down to SNeff (Dynaflect) for Louisiana Condition

Overlay thickness = (SNfuture-SNeff)/aAC

Future Works

Further validate the Dynaflect deflection

determined SNeff

The proposed overlay design procedure will

be automated into a EXCEL spreadsheet-

based program