Pavement Structural Evaluation: Where we are and … Structural Evaluation: Where we are and where...
Transcript of Pavement Structural Evaluation: Where we are and … Structural Evaluation: Where we are and where...
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
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Mean Max. Temperature (F) Mean Temperature (F)Mean Min. Temperature (F) Mean Precipitation (in.)
Precipitation Minus EvaporationHelena, MT Airport
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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?