Pavement Structural Evaluation:Where we are and where we are oing

Prof. Lynne H IrwinCornell Local Roads Program

Points for discussion

• How did we get where we are?• Where are we?• Where do we need to go?

Does E = 1500 x CBR?

If so, why are we here?

How good is "good enough?"

Once Upon a Time …

• Visual evaluation was the main tool

• Test pits helped to gather information at a point

• We had no reliable method to predict how long an overlay would last

Field Test Methods

• 1955: Benkelman beam. WASHO Road Test.

• 1955: Hveem, Pavement deflections and fatigue failures. HRB Bulletin 114.

Buffalo, NY - 1966

Field Test Methods

• 1975–1985: Falling weight deflectometer

• 1990: Seismic pavement tester

Dubai, 1978

Laboratory Test Methods

• 1966: Repeated-load triaxial test for unbound materials

• 1980: Indirect tensile test for asphalt concrete

Analytical Tools

• 1885: Boussinesq. One-layer theory• 1934: Miner. Cumulative damage theory

1.0 N

n Dk

1 i i allow ,

i Where ---

D = cumulative damage due to all loads

k = number of load levels considered

ni = number of load repetitions at load level i

N ll bl b f l d titi t l d l l i

Analytical Tools

• 1945: Burmister. General theory of stresses and displacements in layered systems. Jour. of Applied Physics

• 1962: Hveem, Zube, Bridges, and Forsyth. The effect of resilience-deflection relationship on the structural design of asphaltic pavements. Proc. 1st

Ann Arbor Conference

Analytical Tools

• 1963: AASHTO interim guide (empirical)• 1965–1972: Layered elastic computer

programs (BISTRO, BISAR, CHEVLAY)• 1977: Irwin. Determination of pavement

layer moduli from surface deflection data for pavement performance evaluation. Proc. 3rd Ann Arbor Conference

Analytical Tools

• 1982–1986: Backcalculation computer programs (MODCOMP, EVERCALC)

• 1988: 1st conf. on nondestructive testing of pavements and backcalculation of moduli. ASTM STP 1026.

• 2007–2010: Mechanistic-empirical pavement design guide. NCHRP 1-37a.

Interest Groups

• 1980–Today: TRB committees• 1992: FWD users group• 1993: RPUG• 2009: GPR team

Where Are We?

• Multitude of pulse loading devices• Portable computers that get faster and

more capable every day• Software that enables us to analyze the

capabilities and limitations of existing pavements

• Interest groups, conferences, and smart people

Where Are We?

• Mechanistic-empirical design methods are advancing rapidly

• We know how to backcalculate layer moduli (with some difficulty)

• We know how to estimate depth to hard bottom

• We know how to use mechanistic-empirical methods to design pavements

Basic Assumptions of Elastic Layer Theory (Quick Review)

Surface load - uniformly distributed over a circular areaVertical stress continuity at interfaces Horizontal strain continuity at layer interfaces (no slip)Layers extend horizontally to infinityBottom layer is a semi-infinite half-spaceMaterials - homogenous, isotropic, and linearly elastic

Additional Tools Available

• Finite element analysis methods• Neural network analysis methods• Seismic analysis of surface waves• Ground penetrating radar• Cross-anisotropic test methods for

unbound materials• Dynamic test methods for asphalt

concrete

"We measure with a micrometer, mark with a grease pencil, and cut with a chainsaw" … (Marshall Thompson)

Where Are We Deficient?

• Field testing protocols– ASTM and AASHTO offer guidance, but

most agencies do their own thing• Lab testing methods

– Stress-dependency of unbound materials– Constitutive models– Visco-elastic properties of asphalt concrete

• Seasonal variation of materials• Spatial variability of materials

Considering seasonality

• The various regions of the county belong to one of four climatological zones– Wet-freeze (Wisconsin)– Wet-no freeze (Georgia)– Dry-freeze (Montana)– Dry-no freeze (Arizona)

• Pavement materials properties (i.e., moduli) change during the seasons

Effect of seasonality

The modulus of asphalt concrete is temperature sensitive– Not greatly affected by moisture

The moduli of unbound base and upper subgrade materials are moisture sensitive– Not greatly affected by temperatures above

freezingDeeper subgrade is not seasonally affected

Helena, MT Airport Weather Averages30-year monthly normals, 1971-2000

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Month

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Prec

ipita

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(in.)

Mean Max. Temperature (F) Mean Temperature (F)Mean Min. Temperature (F) Mean Precipitation (in.)

Precipitation Minus EvaporationHelena, MT Airport

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-1

0

1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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p(

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Asphalt Concrete in a Hot Summer -Cool Winter Area

J F M A M J J A S O N D

odulus

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Granular Base in a Wet-Freeze Area

J F M A M J J A S O N D

odulus

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0%

anular Base in a Wet-No Freeze Area

J F M A M J J A S O N D

Modulus

100%

60%

Where Do We Need To Go?

• Work on our understanding of …– Seasonality– Spatial variability– Use of probabilistic methods in design– Stress-dependency of materials– More accurate constitutive models – The role of visco-elasticity in pavement

evaluation and design

Where Do We Need To Go?

• Improve field testing protocols– Test early in the life of a road– Seasonal testing for project level design

• Improve lab testing methods• Combine FWD, SASW and GPR testing

into a single pass• Head toward real-time backcalculation

in the field

Too Good to Test?

Where Do We Need To Go?

• Data screening in the field and in the office– Utilize the SLIC transform– Account for the weather-affected upper

subgrade layer– Avoid wasting time analyzing bad data

• Avoid "black box" methods that no one can understand

We Have Come a Long WayIn 30 Years

Let's talk about it

How Good Is Good Enough?