576 CEE Lecture Notes Fall 1999 Date: 11/2/99 By: Barry Paye Subject: Rigid Pavement Design Source: Text by Huang Section 12.1
12.1 Calibrated Mechanistic Design Procedure Relates structural models to pavement responses (Figure 12.1) Models Used: Finite Element is used for structural models Regression models for distress (common) Distresses Modeled: Fatigue Cracking Pumping Faulting Joint Deterioration Punchouts Most models shown were developed by University of Illinois in NCHRP Report 1-26. 12.1 Structural Models Minimum Capabilities (from 1-26) 1. Analyze arbitrary slabs. 2. Analyze systems with 2 layers (slab & subbase.) 3. Analyze systems on liquid or solid subgrade. 4. Analyze systems with uniform or nonuniform support. 5. Analyze slabs with load transfer. 6. Consider both warping and curling at the same time. 7. Analyze variable crack spacing. 8. Analyze variable loading conditions. 9. Analyze slabs with variable shoulder conditions. 10.Analyze systems with differing slab or shoulder thicknesses. ILLSLAB and KENSLAB can be used. Finite element analysis requires large computer systems, but not applicable with today's design procedures (and computers). Regression equations make computations easier. Only valid for limited number of variables.
12.1.2
Fatigue Cracking Models
Load repetitions are related to the stress ratio (flexural stress & modulus of rutpure.) Truck Load Placement Concrete Fatigue Transverse Cracking Starts at edge of pavement between joints. Caused by edge loading. Longitudinal Cracking Starts in wheelpath @ edge of joints. Caused by joint loading. Equivalent Damage Ratio (EDR) Traffic applied at the same critical location over all traffic over all locations: 0.05-0.06 for edge loading with asphalt shoulders 0.12-0.34 for edge loading with concrete shoulders 0.25-0.28 for joint loading Curling Stress Loading and curling stresses to be analyzed (modifying equations from Ch. 3.)
Dr =
N
i =1 k =1 j =1
p
3
m
ni , k , j
i, k , j
(12.1)
Dr = accumulated damage ratio i = periods or subgrade support value p = # of periods k = counter for curing conditions (day, night and zero temp. gradient) m = total # of load group ni,j,k = predicted number of load repetitions Ni,j,k = allowable number of load repetitions
Inclusion of 3 different temperature gradients makes calculations confusing. May be easier to look at curling and loading separately and then combine the results. Curling may not significantly affect the fatigue life Curling could be subtracted from loading stress. Edge stress reduced by moisture warping. Reduced by shorter panel Performance lengths. Curve (Figure 12.3) Inclusion/exclusion of curling stresses would change curves, but not %cracking. Difficulties in setting up curves (modulus of rupture) 12.1.3 Pumping and Erosion
Factors Presence of water Rate of water ejection under slab. Erodibility of subbase. Magnitude and # of repeated loads. Amount of deflection. Only encompassing model is by PCA (section 12.2) Not widely used Energy model Relates energy absorbed to erosion. Report 1-26 considered an empirically based pumping model Applicable for JPCP and JRCP. Uses nonlinear regression techniques. COPES Models JPCP (equation 12.2a) and JRCP (equation 12.2b) Pumping index determined by number of axle loadings, soil type, precipitation, slab thickness, subdrainage and freezing index. Pumping prediction model was produced by Purdue. Based on field data.
12.1.4
Faulting Models
Doweled Pavements Caused by erosion under dowels from repeated loading. Can be prevented by keeping bearing stress below 1500 psi. Bearing stress calculated in finite element Regression model (equation 12.3) was determined by COPES. Inches of faulting based on loading, bearing stress, joint spacing, and modulus of subgrade reaction. Climactic information not included, along with many other variables. Figure 12.4 relates predicted to actual faulting. Figure 12.5 relates faulting to bearing stress through joint spacing and subgrade modulus. Undoweled Pavements Regression model (equation 12.4) was determined by COPES. Inches of faulting are based on loading, joint opening, cor...