ALACPA-ICAO Seminar on PMS. Lima Peru, November 2003

Transcription

1 ALACPA-ICAO Seminar on PMS Lima Peru, November 2003

2 Airport Pavements FWD/HWD Testing and Evaluation By: Frank B. Holt Vice President Dynatest International A/S

3 Dynamic Testing The method of FWD/HWD testing simulates real load conditions Not to be confused with dynamic analysis

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5 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

6 Load Distribution Strong Pavement Weak Pavement Load Load Surface Base Subgrade

7 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

8 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

9 Elastic Modulus STRENGTH S T R E S S ELASTIC RANGE STRAIN E = s/e

10 Linear Elastic System ASSUMPTIONS LINEAR ELASTIC HOMOGENEOUS ISOTROPIC CONTINUOUS (HORIZONTAL) HALF SPACE (VERTICAL) INPUTS LOAD LAYER THICKNESS LAYER MODULUS POISSON S RATIO

11 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

12 Typical Modulus Values Material 0 C Lean Concrete PQ Concrete Granular Sub-base Subbase Modulus (MPa)

13 Typical Poisson Ratio Values Material Bituminous Bound Cement Bound Crushed Stone Poisson s Ratio Soils (fine-grained) 0.45

14 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)

15 Structural Condition HWD survey vital structural component allows proactive measures reliable input required layer thicknesses mechanistic models

16 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

17 Structural Evaluation FWD/HWD allows non-destructive testing of pavements Detects strength/weakness of all layers Enables detection of weakness prior to surface failure

18 Analytical Pavement Evaluation 1. Back-calculation of deflection bowl 2. Determine pavement life 3. Determine maintenance requirements

19 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)

20 Common Back-calculation Software Modulus (LET) American Standard Units Metric Modulus (no forward calculation) ELMOD 5 curvature FEM/LET/MET models PADAL/BISAR/PCASE

21 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

22 Response Locations C L SURFACE BASE SUBGRADE 1 - DEFLECTION 2 - TENSILE STRAIN 3 - COMP. STRAIN 4 - COMP. STRAIN

23 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

24 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

25 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)

26 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

27 Parameter Screen

28 Asphalt Reference Materials Reference TRL (HRA) TRL (DBM) A B E ref C 0 0 SHELL AI AI (Ullidtz) DK (Kirk) NAASRA

29 Fatigue Curves ASPHALT FATIGUE TENSILE STRAIN (10^6) LOAD REPETITIONS (N) E = 3450 MPa (500ksi) E = 1380 MPa (200 ksi)

30 Unbound materials Vertical strain Reference A ustrain B constant Eref C constant TRL&Nottingham SHELL Asphalt Institute Vertical stress Reference A B constant Eref C constant Asphalt Institute & 1 Denmark (Kirk)

31 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

32 Seasonal Adjustments Seasonal constants can be: entered manually or calculated automatically according to user defined sinouisdal or exponential curves

33 Asphalt Temperature Variation Stiffness Factor Temperature (Celsius)

34 Concrete Pavements

35 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

36 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:

37 Concrete Temperature Variation Warping of slabs Temperature gradient Joint expansion Summary Night time slab centre testing Daytime load transfer testing

38 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

39 Design Loads A design load is defined by: wheel load tire pressure wheel and axle configuration percentage of total traffic

40 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)

41 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

42 ELMOD 5 Output example of responses

43 ELMOD 5 Output example Life & Overlay

44 ACN/PCN Method ACN Aircraft Classification Number PCN Pavement Classification Number

45 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

46 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)

47 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

48 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, dual tandem) Recalculate ESWL from equation t = ESWL CBR ESWL

49 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

50 Thank You!