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Airport Pavements FWD/HWD Testing and Evaluation
By: Frank B. HoltVice President
Dynatest International A/S
Dynamic Testing
• The method of FWD/HWD testing simulates real load conditions
Not to be confused with dynamic analysis
Dynamic Testing• Testing performed on
– Highways– Airfields– Construction sub-base
• Output data used– Strengthening and Maintenance– Pavement Management System– New Design– Airfield parameters– Quality Testing
Theory of Elasticity
• Analytical-empirical method– Calculation of pavement response
• Layered system• Critical stresses, strains or deflections• Loading
• Most widespread method used– Two material parameters needed
• Young’s modulus and Poisson’s Ratio
– Hooke’s law• Ratio of stress over strain is constant• Ratio radial over longitudinal strain = Poisson’s Ratio
Load – Stress - Strain
L
∆L/2
∆D/2
D
Sample in unloaded condition
Sample in loaded condition
µ = εD/εL
εD =∆D/D
εL=∆L/L
Area A
Load Q
σ = Q/Α
Linear Elastic System• ASSUMPTIONS
• LINEAR ELASTIC• HOMOGENEOUS• ISOTROPIC• CONTINUOUS (HORIZONTAL)• HALF SPACE (VERTICAL)
• INPUTS• LOAD• LAYER THICKNESS• LAYER MODULUS• POISSON’S RATIO
Layered System
p - contact pressure
P
Surface E1 , µ1
Base E2 , µ2
Subgrade E3 , µ3
h1
h2
α
Radius r or aTotalLoad
Typical Modulus Values
30-300Subbase
100-1000Granular Sub-base
20000-30000PQ Concrete
8000-15000Lean Concrete
3000-7000Bituminous @200C
Modulus (MPa)Material
Typical Poisson Ratio Values
0.45Soils (fine-grained)
0.4Crushed Stone
0.2Cement Bound
0.35Bituminous Bound
Poisson’s RatioMaterial
Why use a FWD/HWD?• In order to determine layer moduli for
analytical design testing equipment must:– simulate loads similar in magnitude to the
actual loads experienced by the pavement– measure loads to very high degree of
accuracy– measure deflections to a high degree of
accuracy at large radial distance from the load (deflection bowl)
Structural Condition
• HWD survey vital structural component– allows proactive measures– reliable input required
• layer thicknesses• mechanistic models
HWD Approach & Analysis
• Minimum center deflection of 150 microns
• GPR linked input for analysis• Core borings for calibration• Point by point analysis• Linear and non-linear approach• Normal distribution concepts for lateral
wander
Structural Evaluation
• FWD/HWD allows non-destructive testing of pavements
• Detects strength/weakness of all layers
• Enables detection of weakness prior to surface failure
Analytical Pavement Evaluation
1. Back-calculation of deflection bowl
2. Determine pavement life
3. Determine maintenance requirements
Response Models used for Back-calculation
• Radius of Curvature• Method of Equivalent Thicknesses
(MET)– Easy Simplified Model using a Linear
Elastic Model • Layered Elastic Model (LEM)
– Linear subgrade• Finite Element Model (FEM)
Common Back-calculation Software
• Modulus (LET)– American Standard Units– Metric Modulus (no forward calculation)
• ELMOD 5– curvature– FEM/LET/MET models
• PADAL/BISAR/PCASE
Back-calculation
• Input– Material Properties– Layer Thicknesses– Applied Load
• Output– Deflections
• Input – Measured Deflections– Applied Load– Layer thicknesses
• Output– Layer stiffnesses– Calculated Deflections
and %error
Forward Calculation Back-calculation
Response Locations
CL
4 - COMP. STRAIN
3 - COMP. STRAIN2 - TENSILE STRAIN
1 - DEFLECTIONSURFACE
BASE
SUBGRADE
Residual Life and Overlay design
• Inputs– Strains and stresses– Load– Fatigue curves
• Asphalt Strain Criteria – Bottom–up Cracking
• Concrete Stress Criteria – Cracking
• Subgrade Strain Criteria – Permanent Deformation
• Output – No. of load repetitions until failure
Fatigue Curves
• Asphalt strain at the bottom of the layer
• Concrete stress at the bottom of the layer
• CTB stress at the bottom of this layer• Subgrade strain at the top of the
layer
Ullidtz: Pavement Analysis
§ εt = K * (Nf/106)(-1/a) * (E/Eref)b
• ε t = allowable horizontal tensile strain
• Nf = load repetitions to failure
• E = asphalt modulus
• Eref = reference modulus
• K, a, b = material constants
• b = often zero (0)
ELMOD 5
§ Strain = A * (N/106)B * (E/Eref)C
or§ Permissible value = A * Mload
B * (E/Eref)C
• A = µstrain• Mload = N * 106
• C = 0
Asphalt Reference Materials
03000-0.200225NAASRA
03000-0.178195DK (Kirk)
-0.2593000-0.304240AI (Ullidtz)
-0.2596.9-0.3041162AI
03000-0.250538SHELL
03000-0.240251TRL (DBM)
03000-0.231224TRL (HRA)CErefBAReference
Fatigue CurvesASPHALT FATIGUE
10
100
1000
10000
1000 10000 100000 1000000 10000000
LOAD REPETITIONS (N)
TE
NS
ILE
ST
RA
IN (
10^6
)
E = 3450 MPa (500ksi) E = 1380 MPa (200 ksi)
Unbound materials
Reference A ustrain B constant Eref C constant
TRL&Nottingham 451 -0.280 160 0
SHELL 885 -0.250 160 0
Asphalt Institute 484 -0.223 160 0
Reference A B constant Eref C constant
Asphalt Institute 0.1425 -0.307 160 1.16 & 1
Denmark (Kirk) 0.12 -0.307 160 1.16
Vertical strain
Vertical stress
Seasonal Adjustments
• Seasonal variations defined in Parameter Setup file– how is the yearly temperature variation?– how is the yearly variation in unbound
material• Elmod features
– define up to 12 seasons– define climatic constants for each material– Define asphalt modulus/temperature
relationship
Seasonal Adjustments
• Seasonal constants can be:– entered manually
or
– calculated automatically according to user defined sinouisdal or exponential curves
Asphalt Temperature Variation
0
0.5
1
1.5
2
2.5
3
3.5
-10 0 10 20 30 40
Temperature (Celsius)
Stif
fnes
s F
acto
r
Joint Analysis
• Westergaard Theory• Inputs
– FWD Setup changes– As for OB Theory– Joint location
• Outputs– Equivalent foundation stiffness (k-value)– Void Intercept– Joint Condition– Load Transfer– Support conditions
Geophone SetupWhen testing at a joint (or corner) the geophones at distances 8 in. (200 mm) and 12 in.(300 mm) from the load centre must be placed on either side of the joint, as shown below:
Concrete Temperature Variation
• Warping of slabs– Temperature gradient
• Joint expansion• Summary
– Night time slab centre testing– Daytime load transfer testing
Design Loads
• Design loads defined in Parameter Setup file
• Elmod can handle a mix of up to 12 different loads
• Usually for roads all traffic is converted into 1 design load
• For airfield it is advised to base the calculation on a mix of different aircraft types
Design Loads
• A design load is defined by:– wheel load– tire pressure– wheel and axle configuration– percentage of total traffic
Miner’s Law
• THE PRINCIPAL OF LINEAR SUMMATION OF DAMAGE– IF LOAD A FATIGUE LIFE IS NfA AND LOAD B IS NfB THEN
DAMAGE DUE TO 1 PASS OF EACH LOAD IS
• GENERALLY , THIS CAN BE WRITTEN AS
D = 1 N 1 Nf fA fB+
D = (Fatigue)
D = (Rutting)
f
f
1
1
N
N
fii
rii
∑∑
Overlay Design
Calculatestress/strain
Calculateallowable
traffic
Relate to residual life
Does residual life match
design life ?
Adjustoverlay
thickness
Overlay design
Yes
No
PCN according to ICAO
• PCN: Pavement Classification Number
• ICAO: International Civil Aviation Organization
• “A number expressing the bearing strength of a pavement for unrestricted operations”
• Any method may be used
ACN according to ICAO
• ACN: Aircraft Classification Number• “mathematically derived single wheel
load to define the landing gear/pavement interaction”
• ACN = ESWL * 2/1000 kg• Flexible: ACN = f(CBR)• Rigid: ACN = f(k-value)
Rigid pavement ACN
• Reporting stress = 2.75 MPa• Calculate thickness of concrete for
actual gear (to produce 2.75 MPa at bottom)
• Calculate ESWL (1.25 MPa tire pressure) to give same stress with same thickness
Flexible pavement ACN?
• Calculate “t” to give same deflection at subgrade from actual gear and ESWL
• Multiply “t” by “load repetition factor”? (0.9 dual, 0.825 dual tandem)
• Recalculate ESWL from equation
035.325692.0ESWL
CBRESWLt −=
Calculation of PCN
• Moduli are derived from FWD testing (using Elmod3 approach)
• Moduli are modified for seasonal effects• The ESWL, which match the fatigue
relation for the “unrestricted usage” number, is calculated
• Rigid: stress in concrete only• Flexible: stress on subgrade only
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