Lecture 3: Stresses in Rigid Pavements

Transcription

1 Lecture 3: Stresses in Rigid Pavements

2 Nature of Responses under Flexible and Rigid Plates Flexible plate: Uniform Contact Pressure Variable Deflection Profile Flexible Plate Rigid Plate plate: Non-Uniform Contact Pressure Equal Deflection Rigid Plate

3 Comparison of Deflections at the Surface Rigid vs. Flexible Plate The deflection under a rigid plate is 79% of that under a flexible plate.

4 Contact Pressure and Deflection Calculation under the Rigid plate Ullidtz (1987) gave the distribution of contact pressure under a rigid plate as: Notice that the distribution of the contact pressure is a function of radial offset from the load centerline. By integrating a point load over the contact area of the plate, the deflection can be calculated as: Assuming m=0.5, the surface deflection at centerline can be calculated as:

5 Stresses in Rigid Pavements A. Stresses due to Environment Warping (or Curling) Stresses Stresses due to temperature differential or change in humidity (temperature or moisture gradient) Location: Interior Loading Edge loading Corner Loading Shrinkage/Expansion Stresses B. Stresses due to External Loading Such as stresses induced by traffic loads Location: Interior Loading (sufficiently away from the joints- no discontinuity effect) Edge loading Corner Loading C. Other Stresses Such as frictional forces between the slab and foundation

6 Warping Stress - Day Time Slab Surface Temperature > Slab Bottom Temperature

7 Warping Stress - Night Time Slab Bottom Temperature > Slab Surface Temperature

8 Constrained Transverse Joints Slab Surface Temperature > Slab Bottom Temperature

9 Temperature (F) Temperature Variation for Precast Concrete Panel Installed using HDP Deep Injection (Ashtiani, 2011) 115 Slab #1 Temperatures :57 8:09 9:21 10:33 11:45 12:57 14:09 15:21 16:33 Time of day Surface temperature Top thermocouple Mid slab thermocouple Slab / Grade Interface thermocouple Below grade thermocouple

10 Typical Day Time Warping Typical Night Time Warpingg Top of the PCC Layer is Warmer Bottom of the PCC Layer is Cooler Top of the PCC Layer is Cooler Bottom of the PCC Layer is Warmer

11 Calculation of Warping Stresses Edge Stress t CEeDT 2 t = E = e = DT = C = Slab edge warping stress (psi) Modulus of elasticity of PCC (psi) Thermal coefficient of PCC (approximately /F) Temperature differential between the top and bottom of the slab (F) Coefficient, function of slab length and the radius of relative stiffness, l

12 Radius of Relative Stiffness Definition l 4 3 Eh 2 12k(1 ) n = E = l = k = h = Poisson Ratio Modulus of Elasticity (psi) Radius of Relative Stiffness (in) Modulus of Subgrade Reaction (pci) Slab Thickness (in)

13 Westergaard s Model of Subgrade Reaction Elastic layered theory can t be applied for jointed rigid pavements due to the fact that one of the assumptions of the layered theory was that layers are infinitely long in horizontal direction ( no effect of discontinuity at joints). To solve this problem Westergaard (1925) assumed that a rigid pavement could be considered as a slab on a Winkler foundation. In other words, the foundation reaction (or vertical stress) equals to the deflection times a constant (k) called modulus of subgrade reaction. Note that the modulus of subgrade reaction (k) has unit of (pci).

14 Determination of the Warping Stresses, Coefficient (C)

15 Calculation of Warping Stresses, Cont. Interior Stress t EeDT C1 C t = E = e = n = C 1 = Slab interior warping stress (psi) Modulus of elasticity of PCC (psi) Thermal coefficient of PCC (~ /F) Poisson s ratio for PCC Coefficient in direction of calculation C 2 = Coefficient in direction perpendicular to C 1

16 Calculation of Warping Stresses, Cont. Corner Stress t EeDT 3(1 ) a l t = E = e = DT = n = a = l = Corner Warping Stress (psi) Modulus of Elasticity of PCC (psi) Thermal Coefficient of PCC (~ /F) Temperature Differential between the Top and Bottom of the Slab (F) Poisson s Ratio for PCC Contact Radius for Corner Load Radius of Relative Stiffness

17 Stresses due to External Loads Westergaard Equations Interior loading (tensile stress at the bottom of the slab) Edge loading loading (tensile stress at the bottom of the slab) Corner loading (tensile stress on the top of the slab) W = h = a = l = b = Wheel load (lb) Slab thickness (in.) Radius of Wheel Contact Area (in.) Radius of Relative Stiffness (in.) Radius of Resisting Section (in.)

18 Slab Expansion/Contraction z CL e Dt d z = C = L = Joint opening (or change in slab length, in.) Base/slab frictional restrain factor (0.65 for stabilized bases; 0.80 for granular bases) Slab length (in.) e = PCC coefficient of thermal expansion by aggregate type (e.g., 6.0x10-6 /F for gravel; 3.8x10-6 /F for limestone) Dt = d = Maximum temperature range Shrinkage coefficient of concrete (e.g., in./in. for indirect tensile strength of 500 psi)

19 Load Transfer Efficiency (LTE) in Rigid Pavements

20 Loss of Foundation Support Loaded Slab Unloaded Slab ( v ) l ( v ) ul (e (e p ) p ) ul l Unbound Granular Base Bonding Agent Subgrade

21 Load Transfer Efficiency (LTE) in Rigid Pavements Load Transfer Efficiency (LTE) Deflection based, LTE d LTE d Deflection Deflection unloaded slab loaded slab LT Stress based, LTE FAA Criteria (stress based), LT Stress 1 Stress free edge Stress Stress loaded Stress slab free edge free edge LTE loaded d slab a 2 a LTE 393 LTE l l a a LTE LTE l l LTE LTE LT 1 LTE 3 2 Stress Stress unloaded slab loaded slab

22 Falling Weight Deflectometer (FWD) A falling weight deflectometer (FWD) is a device designed to simulate deflection of a pavement surface caused by a fast-moving truck. The FWD generates a load pulse by dropping a weight. This load pulse is transmitted to the pavement through a 300-millimeter (mm) diameter circular load plate. The load pulse generated by the FWD momentarily deforms the pavement under the load plate into a dish or bowl shape. Based on the force imparted to the pavement and the shape of the deflection basin, it is possible to estimate the stiffness of the pavement layers by using various computational methods.

23 Falling Weight Deflectometer (FWD) General Definitions Non-destructive test equipment for pavements. Imparts an impact load to a pavement structure. Measures deflection of the pavement surface at different radial offsets. Different types based on the application : Light Weight Deflectometer (LWD), Falling weight Deflectometer (FWD), and Heavy Weight Deflectometer (HWD). LWD HWD FWD

24 Components of FWD Load Cell LVDT, Geophones, Accelerometers Displacement measurement Infrared temperature gages Pavement Surface Temperature Air Temperature Electronic Distance Measurement Control/Data Acquisition Unit

25 FWD Details E 1, n 1 HMAC E 2, n 2 Unbound Aggregate Base E 3, n 3 Stabilized Subbase E 4, n 4 Natural Subgrade Impact Load created from dropping weights from specified height. Load cell used to measure the impulse loading. Heavy-duty load cell required to support in excess of 60 kips in magnitude. Deflection profile is key output. Temperature and load data used with deflections to back-calculate pavement structure characteristics.

26 Temperature Dependency of Deflection Profile in a Flexible Pavement

27 Application of FWD/HWD in Pavement Engineering Determination of the in-situ layer moduli. Estimation of the structural capacity and analysis of the remaining life. Determination of the load transfer efficiency of joints in concrete pavements. Pavement management.

28 Application of Falling Weight Deflectometer (FWD) for calculations of Load Transfer Efficiency (LTE) HWD Load Loaded Slab Geophones Unloaded Slab (e p ) l (e p ) ul Deflection based LTE (ratio of plastic strains at the two sides of the joints) is the most common method to determine LTE and joint stiffness of PCC slabs in pavement industry. LTE d Deflection Deflection unloadedslab loadedslab Deflection Basin

29 Case Study (I) Superposition of Thermal Stresses and HWD Load Rapid Damage Repair using Precast Concrete Slabs Tyndall AFB, (Ashtiani,2010)

30 Day Time Super Position of Thermal and Load induced Stresses for FWD Loading (Ashtiani,2011)

31 Distribution of Vertical Stresses at the Top of the Subgrade (Day Time)

32 Night Time Super Position of Thermal and Load Induced Stresses for HWD Loading

33 Distribution of Vertical Stresses at the Top of the Subgrade (Night Time)

34 Case Study (II) Superposition of Thermal Stresses and Stresses due to C17 Aircraft Landing Gear Rapid Damage Repair using Precast Concrete Slabs Tyndall AFB, (Ashtiani,2010)

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37 1 2 3 Corner Loading Mid-Slab Loading Edge Loading 4 5 Landing in the direction of dowel bars Direction of Landing Landing perpendicular to the direction of dowel bars

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39 Y-direction Distribution of Vertical Stresses at the Top of Stresses in Z-direction the Subgrade (Corner/Edge Loading for Slab #1)

40 Sensitivity Analysis of Parameters of Different Measures of Load Transfer Efficiency (LTE)

41 Sensitivity Analysis of the parameters of the LTE model. LTE d a 2 a LTE 393 LTE l l a a LTE LTE l l 3 2 E (psi) 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 4,000,000 4,500,000 5,000,000 l a/l LTE a/l

42 Sensitivity of the Load Transfer Efficiency to the Parameters of the Model, effect of (a/l) d LTE l a LTE l a LTE l a LTE l a LTE

43 E (psi) 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 4,000,000 4,500,000 5,000,000 l a/l For plate loading with r=6 in k=100 pci and h=9 in, LTE is not very sensitive to modulus of the PCC slab l 4 3 Eh 2 12k(1 ) 0.23 LTE d LTE

44 Table 1. Variables of the Finite Element Simulations Simulation ID LTE x (%) E pcc (ksi) FE Calculated e p

45 Impact of Orthogonal Load Transfer Efficiency On Deformed Mesh.

46 Traffic Considerations in Rigid Airfield Pavements

47 Traffic Mix for Airfield Pavement Design = Must use the entire traffic mix, no more Design Aircraft. Traffic Model - Cumulative Damage Factor (CDF) sums damage from each aircraft based upon its unique pavement loading characteristics and Location of the main gear from the runway centerline. DOES NOT use the design aircraft method of condensing all aircraft into one design aircraft.

48 Sample Aircraft Traffic Mix CDF Contribution Annual CDF CDF Max Aircraft Name Gross Weight Departures Contribution For Aircraft Sngl Whl-30 30,000 1, Dual Whl-30 30,000 1, Dual Whl-45 45,000 1, RegionalJet ,450 1, RegionalJet ,500 1, Dual Whl ,000 1, DC ,000 1, MD ,000 1, B ,500 1, B ,000 1, B ,000 1, A300-B2 304,000 1, B ,000 1, A , B , B , Condition specific and not a general representation of noted aircraft

49 Sample Aircraft Traffic Mix CDF Contribution

50 Runway Centerline. Large Aircraft Gear Locations B B A-330 B A-300-B2 B-757 B-727 B MD-83 MD DC-9-50 DW 100,000 Regional Jet 700 Regional Jet 200 DW 45,000 DW 30,000 SW 30, Distance From Centerline (in)

51 Cumulative Damage Factor (CDF) Graph

52 Sample Cumulative Damage Factor (CDF) Graph

53 Design of Airfield Pavement Shoulders Shoulder must provide sufficient support for unintentional or emergency operation of any airplane in the traffic mix. Shoulders are primarily intended to provide: Protection from erosion and generation of debris from jet blast (no loose materials close to the runway). Enhanced drainage.

54 Gear Naming Conventions Single S Dual D Triple T Quadruple Q 2 Singles in Tandem 2S 2 Duals in Tandem 2D 2 Triples in Tandem 2T 2 Quadruples in Tandem 2Q 3 Singles in Tandem 3S 3 Duals in Tandem 3D 3 Triples in Tandem 3T 3 Quadruples in Tandem 3Q

55 Gear Naming Conventions- Examples S Single Wheel D Dual Wheel 2D Dual Tandem 3D B777 2D/D1 DC-10 2D/2D1 A

56 Aircraft Gear- Examples 2D/2D2 B747 2D/3D2 A380 C5 Lockheed C5

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