3 Pavement Purpose Load support Smoothness Drainage DC to Richmond Road in 1919 – from the Asphalt Institute
4 Pavement Significance How much pavement?3.97 million centerline miles in U.S.2.5 million miles (63%) are paved8.30 million lane-miles totalLargest single use of HMA and PCCCosts$20 to $30 billion spent annually on pavementsOver $100 million spent annually in WA
5 Pavement Condition A look at what pavement condition is? Need to ask what is good, what is bad and how do we know?
9 Pavement Condition Defined by users (drivers) Develop methods to relate physical attributes to driver ratingsResult is usually a numerical scaleFrom the AASHO Road Test(1956 – 1961)
10 Present Serviceability Rating (PSR) Picture from: Highway Research Board Special Report 61A-G
11 Present Serviceability Index (PSI) FYI – NOT TESTABLEPresent Serviceability Index (PSI)Values from 0 through 5Calculated value to match PSRSV = mean of the slope variance in the two wheelpaths (measured with the CHLOE profilometer or BPR Roughometer)C, P = measures of cracking and patching in the pavement surfaceC = total linear feet of Class 3 and Class 4 cracks per 1000 ft2 of pavement area A Class 3 crack is defined as opened or spalled (at the surface) to a width of in. or more over a distance equal to at least one-half the crack length A Class 4 is defined as any crack which has been sealed.P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.
12 Typical PSI vs. Timep0Serviceability (PSI)p0 - ptptTime
14 Subgrade Characterized by strength and/or stiffness California Bearing Ratio (CBR)Measures shearing resistanceUnits: percentTypical values: 0 to 20Resilient Modulus (MR)Measures stress-strain relationshipUnits: psi or MPaTypical values: 3,000 to 40,000 psiPicture from University of Tokyo Geotechnical Engineering Lab
15 Subgrade Some Typical Values Classification CBR MR (psi) Typical DescriptionGood≥ 1020,000Gravels, crushed stone and sandy soils. GW, GP, GM, SW, SP, SM soils are often in this category.Fair5 – 910,000Clayey gravel and clayey sand, fine silt soils. GM, GC, SM, SC soils are often in this category.Poor3 – 55,000Fine silty sands, clays, silts, organic soils. CL, CH, ML, MH, CM, OL, OH soils are often in this category.
17 Load Quantification Equivalent Single Axle Load (ESAL) Converts wheel loads of various magnitudes and repetitions ("mixed traffic") to an equivalent number of "standard" or "equivalent" loadsBased on the amount of damage they do to the pavementCommonly used standard load is the 18,000 lb. equivalent single axle loadLoad EquivalencyGeneralized fourth power approximation
18 Typical LEFsNotice that cars are insignificant and thus usually ignored in pavement design.
19 LEF ExampleThe standard axle weights for a standing-room-only loaded Metro articulated bus (60 ft. Flyer) are:Axle Empty Full Steering 13,000 lb. 17,000 lb. Middle 15,000 lb. 20,000 lb. Rear 9,000 lb. 14,000 lb.Using the 4th power approximation, determine the total equivalent damage caused by this bus in terms of ESALs when it is empty. How about when it is full?Empty(13,000/18,000)4 = 0.272(15,000/18,000)4 = 0.482(9,000/18,000)4 = 0.063Total = ESALsFull(17,000/18,000)4 = 0.795(20,000/18,000)4 = 1.524(14,000/18,000)4 = 0.366Total = ESALsIncrease in total weight = 14,000 lb. (about 80 people) or 39%Increase in ESALs is (229%)
20 Environment Temperature extremes Frost action Frost heave Thaw weakening
21 Pavement Types Flexible Pavement Rigid Pavement Hot mix asphalt (HMA) pavementsCalled "flexible" since the total pavement structure bends (or flexes) to accommodate traffic loadsAbout 82.2% of paved U.S. roads use flexible pavementAbout 95.7% of paved U.S. roads are surfaced with HMARigid PavementPortland cement concrete (PCC) pavementsCalled “rigid” since PCC’s high modulus of elasticity does not allow them to flex appreciablyAbout 6.5% of paved U.S. roads use rigid pavement
32 Terms – Flexible W18 (loading) SN (structural number) Predicted number of ESALs over the pavement’s life.SN (structural number)Abstract number expressing structural strengthSN = a1D1 + a2D2m2 + a3D3m3 + …ΔPSI (change in present serviceability index)Change in serviceability index over the useful pavement lifeTypically from 1.5 to 3.0MR (subgrade resilient modulus)Typically from 3,000 to 30,000 psi (10,000 psi is pretty good)
33 Terms – Rigid D (slab depth) S’c (PCC modulus of rupture) Abstract number expressing structural strengthSN = a1D1 + a2D2m2 + a3D3m3 + …S’c (PCC modulus of rupture)A measure of PCC flexural strengthUsually between 600 and 850 psiCd (drainage coefficient)Relative loss of strength due to drainage characteristics and the total time it is exposed to near-saturated conditionsUsually taken as 1.0
34 Terms – Rigid J (load transfer coefficient) Ec (PCC elastic modulus) Accounts for load transfer efficiencyLower J-factors = better load transferBetween 3.8 (undoweled JPCP) and 2.3 (CRCP with tied shoulders)Ec (PCC elastic modulus)4,000,000 psi is a good estimatek (modulus of subgrade reaction)Estimates the support of the PCC slab by the underlying layersUsually between 50 and 1000 psi/inch
35 Reliability Reliability = P [Y > X] Probability Stress/Strength X = Probability distribution of stress (e.g., from loading, environment, etc.)Y = Probability distribution of strength (variations in construction, material, etc.)ProbabilityStress/Strength
38 From the WSDOT Pavement Guide Interactive Design UtilitiesFrom the WSDOT Pavement Guide Interactive
39 New AASHTO Method Mechanistic-empirical Can use load spectra (instead of ESALs)Computationally intensiveRigid design takes about 10 to 20 minutesFlexible design can take several hours
40 Design Example – Part 1A WSDOT traffic count on Interstate 82 in Yakima gives the following numbers:Parameter Data WSDOT Assumptions AADT 18,674 vehicles Singles vehicles ESALs/truck Doubles 1,176 vehicles ESALs/truck Trains vehicles ESALs/truckAssume a 40-year pavement design life with a 1% growth rate compounded annually. How many ESALs do you predict this pavement will by subjected to over its lifetime if its lifetime were to start in the same year as the traffic count?First year ESALsESALs in traffic count year = 0.40(971) (1176) (280) = 2,054.4 ESALs/dayTotal ESALs = 2,054.4 × 365 = 749,856In 40 yearsTotal ESALs = 749,856((1+0.01)40-1)/0.01 = 36,657,740 ESALs
41 Design Example – Part 2Design a flexible pavement for this number of ESALs using (1) the WSDOT table, and (2) the design equation utility in the WSDOT Pavement Guide Interactive. Assume the following:Reliability = 95% (ZR = , S0 = 0.50)ΔPSI = 1.5 (p0 = 4.5, pt = 3.0)2 layers (HMA surface and crushed stone base) HMA coefficient = 0.44, minimum depth = 4 inches Base coefficient = 0.13, minimum depth = 6 inches Base MR = 28,000 psiSubgrade MR = 9,000 psiBy the TableReliability = 95%Design period ESALs = 25 to 50 millionSubgrade condition = AverageHMA surface course = 0.35 ft.HMA base course = 0.75 ft.Crushed stone = 0.45 ft.Total depth of HMA = 1.1 ft = 13 inchesTotal depth of base = 0.45 ft. = 5.4 inches BUT this must be rounded up to the minimum of 6 inchesSN = 0.44(13 inches) (6 inches) = 6.50With the utility:Initial answer is 10 inches HMA, 13 inches baseSN = 0.44(10 inches) (13 inches) = 6.09Differences due to standard table assumptionsBut, if we change HMA to 13 inches we get the same answer
42 Design Example – Part 3Design a doweled JPCP rigid pavement for this number of ESALs using (1) the WSDOT table, and (2) the design equation utility in the WSDOT Pavement Guide Interactive. Assume the following:Reliability = 95% (ZR = , S0 = 0.40)ΔPSI = 1.5 (p0 = 4.5, pt = 3.0)EPCC = 4,000,000 psiS’C = 700 psiDrainage factor (Cd) = 1.0Load transfer coefficient (J) = 2.7Modulus of subgrade reaction (k) = 400 psi/in HMA base materialTable method:Doweled joints, HMA base material25 to 50 million ESALsReliability = 95%0.97 ft. = 12 inchesUtility method11.5 inches rounded up to 12 inches
43 Primary ReferencesMannering, F.L.; Kilareski, W.P. and Washburn, S.S. (2005). Principles of Highway Engineering and Traffic Analysis, Third Edition. Chapter 4Muench, S.T.; Mahoney, J.P. and Pierce, L.M. (2003) The WSDOT Pavement Guide Interactive. WSDOT, Olympia, WA.Muench, S.T. (2002) WAPA Asphalt Pavement Guide. WAPA, Seattle, WA.