--- title: "Asphalt Institute design: 0.95×10³ EAL/day" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:asphalt-institute-design-0-95e3-eal" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/asphalt-institute-design-0-95e3-eal site: BCT Engineering --- # Asphalt Institute design: 0.95×10³ EAL/day Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2074 Asoj, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/asphalt-institute-design-0-95e3-eal - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question Design a flexible pavement by using asphalt institute method from the following data of a stretch of existing two lane road. a) Current traffic of 80KN equivalent single axle load = 0.95 × 10³ EAL/day b) Traffic growth rate = 7.5% c) Design period = 15 yrs d) construction period = 16 months e) CBR of sub-grade to be taken = 5% f) Elastic modulus of asphalt concrete surface course = 2500 MPa g) Elastic modulus of bituminous treated base = 1200 MPa h) Elastic modulus of granular sub base course = 125 MPa Also draw the neat sketches of the pavement layers. ## Short answer Design ESAL = 4.99 × 10⁶. Provide asphalt concrete 90 mm, bituminous treated base 150 mm, granular sub-base 260 mm (total 500 mm) over subgrade CBR 5%. ## Answer #### Given data and assumptions - Existing two-lane road; current traffic 0.95 x 10^3 (950) EAL of 80 kN per day in both directions, growth 7.5 %, design period 15 years, construction period 16 months (1.33 years), subgrade CBR 5 %. - Moduli: asphalt concrete 2500 MPa, bituminous treated base 1200 MPa, granular sub-base 125 MPa. - Two-lane road: design lane carries 50 percent of the total traffic (Asphalt Institute). - Method: Asphalt Institute (AI) thickness design as taught for IOE (and used in the Nepal DoR Pavement Design Guidelines); design reliability 90 percent, terminal serviceability 2.5. #### Step 1 - Subgrade strength Subgrade CBR is given as 5 %. Resilient modulus of subgrade: $M_R = 10\times CBR = 10\times 5.0 = 50$ MPa. #### Step 2 - Design traffic (ESAL) Traffic in design lane at opening: $950\times0.5\times(1.075)^{1.33}=523.1$ EAL/day. $$ ESAL=365\times523.1\times\frac{(1.075)^{15}-1}{0.075}=365\times523.1\times26.118=4.987\times10^6 $$ **Design ESAL = 4.987 $\times 10^6$** #### Step 3 - Thickness required over each layer The AI charts give the thickness of asphalt concrete (full depth) needed to protect a layer of a given modulus for the design ESAL. The same requirement is expressed here as a structural number $SN$ (AASHTO-93 relation with R = 90 percent, $S_0$ = 0.45, $\Delta PSI$ = 1.7), which closely approximates the AI chart readings (if the chart is supplied in the exam, read the thickness from it and use the same layer-coefficient steps below). | Protected layer | Modulus (MPa) | Required SN | Full-depth AC equivalent at $a_1=0.44$ (mm) | |---|---|---|---| | bituminous treated base | 1200 | 1.31 | 76 | | granular sub-base | 125 | 3.23 | 187 | | Subgrade | 50 | 4.53 | 262 | #### Step 4 - Layer coefficients Using $a_1=0.323\log_{10}E-1.42$, $a_2=0.249\log_{10}E-0.977$, $a_3=0.227\log_{10}E-0.839$ with $E$ in psi: - Asphalt concrete (2500 MPa): $a_1 = 0.376$ - bituminous treated base (1200 MPa): $a_2 = 0.328$ - granular sub-base (125 MPa): $a_3 = 0.128$ #### Step 5 - Layer thicknesses - Asphalt concrete: $D_1 \ge SN_{base}/a_1 = 1.31/0.376=88$ mm, so provide **90 mm** (minimum 50 mm). $SN_1 = 1.33$. - Bituminous treated base: $D_2 \ge (SN_{sub}-SN_1)/a_2 = (3.23-1.33)/0.328 = 147$ mm, so provide **150 mm**. $SN_2 = 1.94$. - Granular sub-base: $D_3 \ge (SN_{sg}-SN_1-SN_2)/a_3 = (4.53-1.33-1.94)/0.128 = 251$ mm, so provide **260 mm**. $SN_3 = 1.31$. Check: provided $SN = 1.33+1.94+1.31 = 4.57 \ge 4.53$ (required) - safe. **Answer:** asphalt concrete 90 mm, bituminous treated base 150 mm, granular sub-base 260 mm; total pavement thickness = 500 mm. ```text +-------------------------------------+ | Asphalt concrete 90 mm | +-------------------------------------+ | Bituminous treated base 150 mm | +-------------------------------------+ | Granular sub-base 260 mm | +-------------------------------------+ | Subgrade, CBR 5.0 % | +-------------------------------------+ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the subgrade resilient modulus obtained for the Asphalt Institute design here? A. M_R = 10 x CBR = 50 MPa B. M_R = CBR / 10 = 0.5 MPa C. M_R = 100 x CBR = 500 MPa D. M_R = CBR + 10 = 15 MPa Answer: A. The answer uses M_R = 10 x CBR, so CBR 5 % gives 50 MPa. (Key point: Subgrade resilient modulus) 2. Why is the current 950 EAL/day multiplied by 0.5 in Step 2? A. To allow for the construction period B. To convert from 80 kN axles to standard axles C. To account for 50 % growth D. On a two-lane road the design lane carries half of the two-way traffic Answer: D. The Asphalt Institute takes 50 percent of the total traffic in the design lane of a two-lane road. (Key point: Design lane traffic) 3. What does the factor (1.075)^1.33 do in the design ESAL calculation? A. Grows the traffic from now to the opening of the road after the 16-month construction B. Grows the traffic over the full 15-year design life C. Converts traffic from both directions to one lane D. Converts traffic to 80 kN equivalent axles Answer: A. Traffic keeps growing for the construction period (1.33 years), so it is raised to the opening-day value before summing over the design life. 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