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AASHTO 1993 Flexible Pavement Design Equation Date goes here Outline AASHO Road Test Present Serviceability Index (PSI) Equation and terms Example AASHO Road Test AASHO Road Test (1) 1958 - 1961 Picture from: Highway Research Board Special Report 61A-G AASHO Road Test AASHO Road Test (2) • Construction: August 1956 - September 1958 • Test Traffic: October 1958 - November 1960 • Special Studies: Spring and early summer 1961 AASHO Road Test Test Loops (1) Picture from: Highway Research Board Special Report 61A-G AASHO Road Test Test Loops (2) Picture from: Highway Research Board Special Report 61A-G AASHO Road Test Environment • • • • Mean Temperature (July) Mean Temperature (January) Annual Average Rainfall Average Frost Depth (for fine-grained soil) 76°F 27°F 34 inches 28 inches AASHO Road Test Flexible Materials • HMA – Dense-graded – 85-100 pen asphalt • Base Course – Crushed limestone – 10% passing No 200 – Average CBR = 107.7 • Subbase Course – Sand/gravel mixture – 6.5% passing No 200 – CBR = 28 – 51 • Subgrade – – – – A-6 soil (silt/clay) 82% passing No 200 Average CBR = 2.9 Optimum wc = 13% AASHO Road Test Flexible Sections • HMA – to inches thick • Base Course – to inches thick • Subbase Course – to 16 inches thick • Thickest section – – – – – inches HMA inches base 16 inches subbase Used for heavy loads 2.6 to 3.6 PSI at test end • Thinnest section – inch HMA – Used for light loads – to 25 ESALs to failure AASHO Road Test Flexible Performance • Majority failed • Even thickest sections sustained appreciable damage • Most failed during spring thaw – Frost action was a major contributor – Thicker base & subbase helped to mitigate frost action Reliability (ZR, S0) Reliability = P [Y > X] PY X f x x f y y dy dx x Y = Probability distribution of strength (variations in construction, material, etc.) Probability X = Probability distribution of stress (e.g., from loading, environment, etc.) Stress/Strength Reliability (ZR, S0) Reliability ZR 99.9 -3.090 99 -2.327 95 -1.645 90 -1.282 80 -0.841 75 -0.674 70 -0.524 50 S0 Typical values for flexible pavement are 0.40 to 0.50 S0 cannot be calculated from actual traffic or construction numbers so it is almost always assumed to be 0.50 Solving the Equation • Iterative process – Both ESAL and structural equation have SN • Often solved assuming ESAL values 1993 AASHTO Structural Design Step-by-Step Date goes here Step 1: Traffic Calculation • Total ESALs – Buses + Trucks – 2.13 million + 1.33 million = 3.46 million Step 2: Get MR Value • CBR tests along Kailua Road show: – CBR ≈ • MR conversion AASHTO Conversion M R 1500CBR 15008 12,000 psi NCHRP 1-37A Conversion M R 2555CBR 0.64 25558 0.64 9,669 psi Step 3: Choose Reliability • Arterial Road – AASHTO Recommendations Functional Classification Recommended Reliability Urban Rural WSDOT 85 – 99.9 85 – 99.9 95 Principal arterials 80 – 99 75 – 95 85 Collectors 80 – 95 75 – 95 Local 50 – 80 50 – 80 Interstate/freeways Choose 85% 75 75 Step 3: Choose Reliability Reliability ZR 99.9 -3.090 99 -2.327 95 -1.645 90 -1.282 85 -1.037 80 -0.841 75 -0.674 70 -0.524 50 Choose S0 = 0.50 Step 4: Choose ΔPSI • Somewhat arbitrary – Typical p0 = 4.5 – Typical pt = 1.5 to 3.0 – Typical ΔPSI = 3.0 down to 1.5 Step 5: Calculate Design • Decide on basic structure Resilient Modulus (psi) Layer a Typical Chosen HMA 0.44 500,000 at 70°F 500,000 ACB 0.44 500,000 at 70°F 500,000 UTB 0.13 20,000 to 30,000 25,000 Aggregate 0.13 20,000 to 30,000 25,000 • Note: AASHTO doesn’t differentiate between types of HMA and base but many agencies – Differentiation may not based on any testing Step 5: Calculate Design • Solve equation for layers – HMA and ACB is one layer – UTB and aggregate is the other • Solve for each layer using the MR of the layer directly underneath • Divide up HMA and ACB • Divide up UTB and aggregate Step 5: Calculate Design • Preliminary Results – Total Required SN = 3.995 – HMA/ACB • Required SN = 2.74 • Required depth = 6.5 inches – UTB and aggregate • Required SN = 1.13 • Required depth = inches Step 5: Calculate Design • Apply HDOT rules and common sense – HMA/ACB • Required depth = 6.5 inches 2.5 inches Mix IV (ẵ inch Superpave) inches ACB (ắ inch Superpave) UTB and aggregate • Required depth = inches • Minimum depths = inches each – inches UTB – inches aggregate subbase Comparison Layer California AASHTO HMA Surface 2.5 inches 2.5 inches ACB 7.0 inches 4.0 inches UTB 6.0 inches 6.0 inches Aggregate subbase 6.0 inches 6.0 inches