Question 7 of 7: Asphalt Overlay Design by Effective Structural Number

Question 7: Asphalt Overlay Design by Effective Structural Number (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given.

Find. (a) the effective structural number of the existing pavement from the condition survey; (b) the structural number the rehabilitated pavement must provide for the future traffic; (c) the thickness of asphalt overlay that makes up the difference.

Approach. Use the AASHTO-93 condition-survey (component analysis) method: assign each existing layer a reduced layer coefficient from Table 5.2 according to its observed distress, sum to get SN_eff; solve the same AASHTO-93 flexible equation used in Question 3 for the future structural number; and convert the deficiency into an overlay thickness at the overlay's own layer coefficient.

1. Part (a) — assign a layer coefficient to the existing asphalt surface. AASHTO-93 Table 5.2 grades an existing AC surface by the distress observed. The entry that matches "less than 5 percent medium- and high-severity transverse cracking" carries a coefficient of 0.25 to 0.30. Taking the upper end, since the distress described is at the very bottom of that band and the layer shows no alligator cracking at all, $$a_{AC} = 0.30$$
2. Assign a layer coefficient to the existing stabilized base. The same table grades a stabilized base, and "more than 10 percent high-severity alligator cracking" falls in the worst category, 0.08 to 0.15. Alligator cracking of that severity and extent means the layer has already fatigued through, so the conservative end of the band is appropriate: $$a_{SB} = 0.10$$
3. Sum the effective structural number. With thicknesses in inches, as the AASHTO coefficients require, $$SN_{\text{eff}} = a_{AC}D_{AC} + a_{SB}D_{SB} = 0.30\,(1.5) + 0.10\,(6.0) = 0.45 + 0.60$$ $$\boxed{SN_{\text{eff}} = 1.05}$$ The plausible range across the two published bands runs from 0.25(1.5) + 0.08(6.0) = 0.86 to 0.30(1.5) + 0.15(6.0) = 1.35, and the consequences of that spread are quantified at the end.
4. Part (b) — state the design equation. The future structural number comes from the same AASHTO-93 flexible relation the attached nomograph solves, $$\log_{10} W_{18} = Z_{R}S_{o} + 9.36\log_{10}(SN + 1) - 0.20 + \frac{\log_{10}\!\left(\dfrac{\Delta PSI}{2.7}\right)} {0.40 + \dfrac{1094}{(SN + 1)^{5.19}}} + 2.32\log_{10} M_{R} - 8.07$$ with W₁₈ = 1.0 × 10⁵, Z_R = −1.645, S_o = 0.45, M_R = 5000 psi and ΔPSI = 4.5 − 2.5 = 2.0.
5. Solve for the future structural number. Bisection on that equation gives $$\boxed{SN_{f} = 2.89}$$ Substituting SN = 2.89 back into the right-hand side returns log₁₀W₁₈ = 5.000, confirming the root. Read off the supplied chart at W₁₈ = 10⁵ the same value falls between 2.8 and 3.0, so chart and equation agree to the precision the chart can be read.
6. Part (c) — convert the deficiency into an overlay thickness. The overlay must supply the difference between the future requirement and what the existing pavement still contributes: $$D_{ol} = \frac{SN_{f} - SN_{\text{eff}}}{a_{ol}} = \frac{2.89 - 1.05}{0.44} = \frac{1.84}{0.44}$$ $$\boxed{D_{ol} = 4.19\ \text{in} = 106\ \text{mm}}$$
7. Round to a constructible thickness. Adopt 110 mm of asphalt concrete, placed as two lifts (a 60 mm binder course and a 50 mm surface course), which supplies 0.44 × 110/25.4 = 1.91 of structural number and brings the total to 1.05 + 1.91 = 2.96 ≥ 2.89. Two lifts are preferable to one here because a single 110 mm lift compacts poorly and because the lower lift can be used as a levelling course over the cracked surface.
8. Quantify the sensitivity to the layer coefficients. Because part (a) rests on judgement within published bands, the overlay thickness should be reported with that spread attached:

9. Address the distress before overlaying. The whole spread above is only 30 mm, so 110 mm covers the pessimistic case as well and the design is robust to the coefficient choice. The more important engineering point is what the condition survey implies: more than 10 percent high-severity alligator cracking in the stabilized base means that layer has failed structurally, and an overlay placed directly over it will reflect those cracks through within a few seasons. The design should therefore be accompanied by full- depth patching of the worst areas, a crack-relief or reflective-crack-reduction interlayer, and confirmation that the underlying drainage has been corrected — otherwise the 110 mm buys thickness without buying life.

Final Results.