ALACPA-ICAO Seminar on PMS Lima Peru, 19-22 November 2003
Airport Pavements FWD/HWD Testing and Evaluation By: Frank B. Holt Vice 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
Load Distribution Strong Pavement Weak Pavement Load Load Surface Base Subgrade
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 Area A Sample in unloaded condition DL/2 DD/2 L s = Q/A e L =DL/L e D =DD/D m = e D /e L Sample in loaded condition D Load Q
Elastic Modulus STRENGTH S T R E S S ELASTIC RANGE STRAIN E = s/e
Linear Elastic System ASSUMPTIONS LINEAR ELASTIC HOMOGENEOUS ISOTROPIC CONTINUOUS (HORIZONTAL) HALF SPACE (VERTICAL) INPUTS LOAD LAYER THICKNESS LAYER MODULUS POISSON S RATIO
Layered System Total Load Surface E 1, m 1 P Radius r or a p - contact pressure h 1 Base E 2, m 2 h 2 Subgrade E 3, m 3 a
Typical Modulus Values Material Bituminous @20 0 C Lean Concrete PQ Concrete Granular Sub-base Subbase Modulus (MPa) 3000-7000 8000-15000 20000-30000 100-1000 30-300
Typical Poisson Ratio Values Material Bituminous Bound Cement Bound Crushed Stone Poisson s Ratio 0.35 0.2 0.4 Soils (fine-grained) 0.45
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 Forward Calculation Input Material Properties Layer Thicknesses Applied Load Back-calculation Input Measured Deflections Applied Load Layer thicknesses Output Deflections Output Layer stiffnesses Calculated Deflections and %error
Response Locations C L SURFACE BASE SUBGRADE 1 - DEFLECTION 2 - TENSILE STRAIN 3 - COMP. STRAIN 4 - COMP. STRAIN
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 e t = K * (N f /10 6 ) (-1/a) * (E/E ref ) b e t = allowable horizontal tensile strain N f = load repetitions to failure E = asphalt modulus E ref = reference modulus K, a, b = material constants b = often zero (0)
ELMOD 5 Strain = A * (N/10 6 ) B * (E/E ref ) C or Permissible value = A * M loadb * (E/E ref ) C A = mstrain M load = N * 10 6 C = 0
Parameter Screen
Asphalt Reference Materials Reference TRL (HRA) TRL (DBM) A 224 251 B -0.231-0.240 E ref 3000 3000 C 0 0 SHELL 538-0.250 3000 0 AI 1162-0.304 6.9-0.259 AI (Ullidtz) 240-0.304 3000-0.259 DK (Kirk) 195-0.178 3000 0 NAASRA 225-0.200 3000 0
Fatigue Curves ASPHALT FATIGUE 10000 TENSILE STRAIN (10^6) 1000 100 10 1000 10000 100000 1000000 10000000 LOAD REPETITIONS (N) E = 3450 MPa (500ksi) E = 1380 MPa (200 ksi)
Unbound materials Vertical strain 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 Vertical stress 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
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 3.5 3 Stiffness Factor 2.5 2 1.5 1 0.5 0-10 0 10 20 30 40 Temperature (Celsius)
Concrete Pavements
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 Setup When 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 N fa AND LOAD B IS N fb THEN DAMAGE DUE TO 1 PASS OF EACH LOAD IS D = 1 N + 1 N f fa fb GENERALLY, THIS CAN BE WRITTEN AS D = f D = f i i 1 1 N N fi ri (Fatigue) (Rutting)
Overlay Design Calculate stress/strain Calculate allowable traffic Relate to residual life Adjust overlay thickness No Does residual life match design life? Yes Overlay design
ELMOD 5 Output example of responses
ELMOD 5 Output example Life & Overlay
ACN/PCN Method ACN Aircraft Classification Number PCN Pavement Classification Number
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 t = ESWL 0.5692 CBR ESWL 32.035
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
Thank You!