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**Flexible Pavement Thickness Design / Asphalt Institute Method**

Source: Chapter 20: Traffic & Highway Engineering by Nicholas Garber and Lester Hoel, Third Edition, Brooks/Cole. Chapter 16: Highway Engineering, by Paul Wright & Karen Dixon, 7th Edition, Wiley & sons Instructor: Dr. TALEB M. AL-ROUSAN

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**Pavement Types Flexible Pavement:**

Pavement constructed of bituminous & granular materials. A structure that maintains intimate contact with subgrade and distribute loads to it, and depends on aggregate interlock, particle friction, and cohesion for stability. Rigid pavement: Pavement constructed of Portland cement concrete. It is assumed to posses considerable flexural strength that will permit it to act as a beam and allow it to bridge minor irregularities in base and subgrade.

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**Typical Cross Section for Conventional Flexible Pavement**

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**Principles of Flexible Pavement Design**

Pavement structure is considered as a multilayered elastic system. Materials in each layers is characterized by certain physical properties (Mr, E,….). It assumes that subgrade is infinite in both the vertical and horizontal directions. Other layers are finite the vertical direction and infinite in the horizontal direction. The application of the wheel load causes a stress distribution (See Figure 20.2)..

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**Principles of Flexible Pavement Design Cont.**

The maximum vertical stresses are compressive and occur directly under the wheel load. Stresses decrease with increase in depth from the surface. The maximum horizontal stress also occurs directly under the wheel load but can be either tensile or compressive. When the load and pavement thickness are within certain ranges, horizontal compressive stresses will occur above the neutral axis, whereas horizontal tensile stresses will occur below the neutral axis. The temperature distribution within the pavement structure will also have an effect on the magnitude of the stresses

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**Figure 20.2 Typical stresses and temperature distribution in flexible pavements under wheel load**

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**Principles of Flexible Pavement Design**

The design of the pavement is therefore generally based on on strain criteria that limit both horizontal and vertical strain below those that will cause excessive cracking and permanent deformation. These criteria are considered in terms of repeated load applications. Most commonly used methods: Asphalt Institute Method AASHTO method California method

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**Elements of Thickness Design**

Traffic Loading Climate or Environment Material Characteristics Others: Cost, Construction, Maintenance, Design period.

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Traffic Loading Pavement must withstand the large umber of repeated loads of variable magnitudes Primary loading factors: Magnitude of axle loads (controlled by legal load limits). Volume & composition of axle load (Traffic survey, load meters, & growth rate). Tire pressure & contact area. Equivalent Standard Axle Load ESAL (80 kN (18,000 lb or 18 kips) single axle load. The total no. of ESAL is used as a traffic loading input in the design of pavement structure.

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**Climate or Environment**

Climate or environment affect the behavior & performance of materials used in pavements Temperature: high temp. cause asphalt to loose stability, low temp. cause asphalt to become hard & stiff, and frost heave. Moisture: Frost related damage, volume changes due to saturation, chemical stability problems with moisture existence (Stripping).

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**Material Characteristics**

Required materials characteristics: Asphalt surface: Material should be strong & stable to resist repeated loading (fatigue). Granular base & subbase: gradation, stable & strong to resist shears from repeated loading. Subgrade: soil classification, strong & stable. Various standard tests are available for determination of desired properties. CBR, Marshal stability, Resilient Modulus, Shear strength. Mr (psi) = 1500 CBR or Mr (Mpa) = 10.3 CBR

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**Asphalt Institute Method**

Method is based on two assumed stress –strain conditions: Wheel load (W) is transmitted to the pavement surface through the tire at a uniform vertical pressure (Po). The stresses are then spread through the pavement structure to produce a reduced max. vertical stress (P1) at the subgrade surface. (See Figure 20.3 in text ) The wheel load (W) causes the pavement structure to deflect creating both compressive & tensile stresses in the pavement structure. (See Figure 20.4 in text ).

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**Figure 20.3 Spread of wheel load pressure through pavement structure**

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**Figure 20.4 Tensile and compressive stresses in pavement structure**

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**Asphalt Institute Method Cont.**

This method considers the following strains as being responsible for the most common traffic related distresses: Max. Horizontal tensile strains (Et) on the bottom of the asphalt layer (causes fatigue cracking). Max. Vertical compressive strains (Ec) on the top of subgrade (causes permanent deformation). Et & Ec are used as failure criteria

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**Design Procedure for Asphalt Institute Method**

Asphalt Institute thickness design manual was prepared using a computer program and suitable data. The charts have been prepared for a range of traffic load, which are usually adequate for normal traffic volume encountered in practice, when this range is exceeded the computer version should be used. The manual includes charts for six types of pavement structures, and three sets of environmental conditions based on the mean annual air temp. (45o, 60o, and 75o F). Example design chart is shown in the coming slide.

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**Pavement Types in The Asphalt Institute Method**

Full depth asphalt concrete. (see Fig. 20.5) Asphalt concrete surface and emulsified asphalt base. Type I: Emulsified asphalt mixes made with processed dense-graded aggregates. (see Fig. 20.6) Type II: Emulsified asphalt mixes made with semi-processed, crusher-run, pit-run, or bank-run aggregates. (see Fig. 20.7) Type III: Emulsified asphalt mixes made with sands or silty sands. (see Fig. 20.8) Asphalt concrete and untreated aggregate base. Base thickness of 6”. (see Fig. 20.9) Base thickness of 12”. (see Fig )

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**Design Procedure for Asphalt Institute Method**

Select or determine Input data. Select surface and base material. Determine minimum thickness required for input data. Evaluate feasibility of stage construction and prepare stage construction plan. Carry our economic analysis of alternative design and select the best design.

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**Step 1: Determine Design Inputs**

Design Inputs are: Traffic characteristics. Subgrade engineering properties. Subbase and base engineering properties.

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**Traffic Characteristics**

Determined in terms of number of repetitions of an 18,000 lb (80)kN single axle load applied to the pavement on two sets of dual tires (Equivalent Single Axle Load (ESAL)). ٍSee next slides for the determination of the ESAL.

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Traffic Analysis Estimate the number of vehicles of different types (Passenger cars, single unit trucks, multi unit trucks of various sizes) expected to use the pavement over the design period. In case data are not available, estimates can be made from Table 20.4 in text which gives representative values for the united states.

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Traffic Analysis Cont. 2. Estimate the (%) of total truck traffic expected to use the design lane. Design lane: Lane expected to receive the severe service. % of trucks is found by observation In case data are not available, estimates can be made from Table 20.4 which gives representative values for truck distribution in the united states. (see Table 16.1 also in the Reference text).

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**Table 20.4 Distribution of Trucks**

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Traffic Analysis Cont. 3. When the axle load of each vehicle type is known, these can be converted to ESAL using the equivalency factors given in Table 20.3 in text or Table 16.3 in Ref If the axle load is unknown, the ESAL can also be found from the vehicle types by using a truck factor for that vehicle type. Truck Factor (TF): The no. of ESALs contributed by passage of a vehicle. For each weight class, determine the truck factor.

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Traffic Analysis Cont. TF = [SUM (No. of axles in each wt. class X EALF)] / Total No. of vehicles Truck factor can be estimated Using Table 20.5 in text or Table16.2 from ref. Equivalent Axle Load factor or Load equivalency factor (EALF) presented in Table 20.3 in text or Table 16.3 in Ref. EALF: Defines the damage per pass to a pavement by the axle of question relative to the damage per pass of a standard axle load (80 kN or 18-kip) EALF depends on type of pavement, thickness or structural capacity, and failure conditions (based on experience). See Truck Factor Example provided in Figure 16.8 Ref. and example in Table 20.8 in text.

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**Axle & Wheel Configurations**

Single Axle with Single Tire Single Axle with Dual Tires Tandem Axles with Dual Tires Tridem Axles with Dual Tires

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Truck Factor Example

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Traffic Analysis Cont. 4. Multiply (Tf) by the no. of vehicles in each group and get the sum for all groups. ESAL = Sum (TF X No. of vehicles) all groups. See Example provided in next slides.

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**Example on Computation of ESAL**

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**Total ESAL Calculation**

The total ESAL applied on the highway during its design period can be determined only if the following are known: Design period Traffic growth factor Traffic growth factor is estimated using historical records or comparable facilities or obtained from studies made by specialized agencies. It is advisable to determine annual growth rates for trucks and passenger cars separately. Design period: Number of years the pavement will effectively continue to carry the traffic load without requiring an overlay. (usually 20 years).

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**Expected Traffic Volume During Design Period**

See Table 20.6 for growth factors, or calculate it using: Gjt = ( (1 + j)t -1)/ j ) j: Rate of growth. t: Design period (yrs).

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**Table 20.6 for growth factors**

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**Computing Design ESAL (Projected)**

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**Total ESAL Calculation Cont.**

the portion of the ESAL acting on the design lane is used in the determination of pavement thickness. Either lane of a two-lane highway is a design lane. In multilane highways the outer lane is the design lane. See Table 20.7 for percentage of total truck traffic on design lane. The initial daily traffic is in two directions over all traffic lanes. Must be multiplied by direction distribution & Lane distribution to obtain initial traffic on design lane. Traffic to be used in design is the average traffic during design period (i.e. multiply by growth factor).

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**Table 20.7 for percentage of total truck traffic on design lane**

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**Total ESAL Calculation Cont.**

ESALi = (AADTi) (Fd) (Gjt) (Ni) (FEi) (365) ESALi : ESAL for axle category i AADTi: First year annual average daily traffic for axle category i. (Fd): Design lane factor (Gjt): growth rate factor for a given growth rate j and design period t. (Ni): number of axles on each vehicle in category i. (FEi): load equivalency factor for axle category i.

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**Total ESAL Calculation Cont.**

When truck factors are used ESALi = (AADTi) (Fd) (Gjt) (fi) (365) ESALi : ESAL for axle category i AADT: First year annual average daily traffic for axle category i. (Fd): Design lane factor (Gjt): growth rate factor for a given growth rate j and design period t. (Ni): number of axles on each vehicle in category i. (fi): Truck factor for vehicle in truck category i.

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**Total ESAL Calculation Cont.**

When truck factors are used ESAL = SUM [ESALi ] from i =1 to n n= number of truck categories ESAL : ESAL for all vehicles during the design period.

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ESAL Example An 8-lane divided highway is to be constructed on a new alignment. Traffic volume forecast indicates that AADT in both direction during the first year of operation will be 12,000 with the following vehicle mix: Passenger cars (1000 lb/axle) = 50% 2-axle single unit trucks (6000 lb/axle) = 33% 3-axle single unit trucks (10,000 lb/axle) = 17% If the expected annual traffic growth rate is 4% for all vehicles, Determine the design ESAL for a design period of 20 years.

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ESAL Example Solution Growth Factor = Gjt = [(1 + j)t - 1]/ j = [( )20 - 1]/ 0.04 = (or see Table 20.6) % truck volume on design lane = 45 ( assumed, Table 20.7) Load equivalency Factors (Table 20.3) Passenger cars (1000 lb/axle) = (negligible) 2-axle single unit trucks (6000 lb/axle) = 3-axle single unit trucks (10,000 lb/axle) =

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**ESALi = (AADTi) (Fd) (Gjt) (Ni) (FEi) (365)**

ESAL Example Solution ESALi = (AADTi) (Fd) (Gjt) (Ni) (FEi) (365) For passenger cars…… ESAL = 0 or negligible For 2-axle single unit trucks ESAL = (12,000 X 0.33) X 0.45 X X 2 X X 365 = X 106 For 3-axle single unit trucks ESAL = (12,000 X 0.17) X 0.45 X X 3 X X 365 = X 106 Total ESAL = X 106

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**Subgrade Engineering Properties Materials Evaluation**

The main engineering property required for the subgrade is its Resilient Modulud (Mr). The design subgrade (Mr) should be based on expected level of traffic expressed in ESALs. To ensure more conservative design, lower value of (Mr) is used for higher volumes of traffic. It is recommended that (Mr) is found for (6 to 8) samples of subgrade. Arrange Mr values in descending order. Plot as cumulative distribution. Chose design subgrade (Mr) from the curve as follows:

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**Design Subgrade Mr Mr test Value Value >= % values >= 13500 1**

12.5 11900 2 25 11300 3 37.5 10000 4 50 9500 5 62.5 8800 6 75 7800 7 87.5 6200 8 100 Subgrade Design Limits Traffic Level Design Percentile ESAL Value <= 10,000 60 10000 to 1000,000 75 > 1000,000 87.5

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**Resilient Modulus (Mr)**

AASHTO T292 (Resilient Modulus of Subgrade soils). (Mr) can be found using repeated loading procedure test such as (unconfined compression test or triaxial compression test). 0.1 sec loading and 1 to 3 sec. unloading. Linear Variable Displacement Transducers (LVDTs) are used to measure strains. Elastic modulus based on the recoverable strain under repeated loading is called the resilient modulus (Mr) = (Deviator stress/ Recoverable axial strain) Deviator stress = Axial stress – confining pressure Recoverable axial strain = Max strain – permanent strain See Fig in ref. Mr = CBR (MPa) or Mr = 1500 CBR (psi) These formulas can be used when only when Mr < psi….. CBR < 20

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**Subgrade Mr Seasonal Variation**

Freeze Time Recovery Time

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**Subbase & Base engineering Properties**

Certain requirements are needed, which are given in terms of PI % passing sieve # 200 Min. sand equivalent. See Table 20.9 in texts for quality requirements of untreated base and subbase.

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**Table 20.9 Quality Requirements for Base and Subbase Materials**

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**Step 2: Select Surface and Base materials**

Designer is free to select either an asphalt concrete surface or an emulsified asphalt surface along with an asphalt concrete base, an emulsified asphalt base, or an untreated aggregate base for the underlying layer. The choice will depend on the material that is economically available. Asphalt Institute recommend certain grades of asphalt cement that should be used for different temperature conditions (mean annual temp.) See Table in text.

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**Table 20.10 Recommended Asphalt Grades for Different Temperature Conditions**

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**Step 3: Determine Min. Thickness Requirements**

The minimum thickness required for the design ESAL and the type of surface, base, and subbase selected is obtained either by: Computer program DAMA. Charts: using design ESAL and subgrade Mr. See Figure in text See Table for min thicknesses of asphalt concrete over types II & III emulsified asphalt base. See Table for min thicknesses of asphalt concrete over untreated aggregate base.

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**Table 20.11 Min. thicknesses of asphalt concrete over emulsified asphalt bases**

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**Table 20.12 Min. thicknesses of asphalt concrete over Untreated Aggregate Base**

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**Step 4: Feasibility of Planning stage construction**

Planned Stage Construction involves successive application of HMA layers according to a predetermined time schedule. Beneficial when: Funds are insufficient for constructing a pavement with long design life. Great amount of uncertainty in estimating traffic. Concept: Remaining life which implies that the second stage will be constructed before the first stage shows serious signs of distress.

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**Planned Stage Construction Cont.**

Pavement is designed for initial traffic & next stage can be designed using traffic projections based on traffic in service. Stage construction allows weak spots that develop in the first stage to be detected and repaired in the second stage.

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**Planned Stage Construction Cont.**

n1: Actual ESAL for stage 1 N1: Allowable ESAL for initial thickness (h1) selected for stage 1. Then The damage ratio (Dr) at the end of stage 1 is: Dr = n1/ N1 Dr < 1.0 ………. When Dr =1.0 pavement fails. (1-Dr) = Remaining life in the existing pavement at the end of stage 1. h1 is obtained based on Dr =1.0. To keep some life, h1 should be determined based on adjusted ESAL (N1) > ESAL (n1) N1= n1/Dr

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**Planned Stage Construction Cont.**

n2: Design ESAL for stage 2. N2: Allowable or adjusted ESAL to permit selection of (h2) that will carry traffic n2 and use the remaining life in stage 2. Then The damage incurred in stage 2 should not exceed the remaining life. n2/ N2 = (1-Dr) N2 = n2/ (1-Dr) hs = h2 –h1 = Additional thickness required in stage 2 (overlay). MS-1 recommended (5 – 10 yrs) stage 1 with 60% Dr.

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**Planned Stage Construction Example**

Given: Full-depth asphalt pavement subgrade Mr = 10,000 psi Use two stage to construct this pavement Stage 1: 5 yrs, ESAL = 150,000 , Dr = 60% at the end of stage 1. Stage 2: 15 yrs, ESAL = 850,000 Required: Determine thickness of HMA required for first 5yrs. Thickness of overlay required to accommodate the additional traffic expected during the next 15 yrs.

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**Planned Stage Construction Example Cont.**

Solution: n1=150, & Dr = Find N1 = n1/Dr = 150,000/ 0.6 = 250, From design Chart with N1 & Mr find h1=7.3 in use 7.5 in. n2 = 850, & 1-Dr = Find N2 = n2/ (1-Dr) = 850,000/ 0.4 = 2,100, From design chart with N2 & Mr find h2 =11.0 First stage thickness = h1 = 7.5 in Overlay thickness hs = h2 –h1 = 11.0 – 7.5 = 3.5 in.

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**Planned Stage Construction Example Cont.**

Solution: If the design was not divided into 2 stages, the thickness of the pavement using (Mr = 10,000 psi & ESAL = 1000,000) is : 9.8 in use 10 in. The use of stage construction decreased the thickness in first stage by ( = 2.5 in), but increased the total thickness by ( = 1.0 in).

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**Step 5: Economic Analysis & Design Selection**

Find several alternative designs for the same design ESAL and subgrade Mr. Carry out an economic evaluation of these alternatives. Determine best alternative.

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