Asphalt Institute redesign of a Northern Ontario pavement: 16-Civ-A6 Highway Design, Construction, and Maintenance, May 2018, Q5

Question 5: Asphalt Institute redesign of a Northern Ontario pavement (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. An Asphalt Institute design already completed as a full-depth section, together with the constraints the supervisor has added:

Design inputs and constraints
Quantity	Value
Mean annual air temperature	7 °C (Design Charts A-1 to A-6)
Subgrade resilient modulus	$M_{r}=70\ \text{MPa}$
Initial full-depth asphalt concrete design	225 mm
Frost depth	1.2 m
Minimum structure thickness	70 % of the frost depth
Base course material	cold mix from local silty sand, mobile plant
Layer order (Figure 2)	asphalt concrete / cold asphalt mix / granular base / subgrade
Cost drivers	asphalt plant 70 km away; good aggregates scarce

Find. A revised three-layer section — asphalt concrete, cold mix and unbound granular base — that carries the same traffic, satisfies the frost-depth rule, and minimises both the asphalt concrete and the granular base.

Figure 5.1 — the revised three-layer section. The total thickness is set by frost protection; the cheap on-site cold mix carries the structure.

Approach. Recover the design traffic from the full-depth chart, identify the correct emulsified-mix type for a local silty sand, read the equivalent thickness from its design chart, set the asphalt concrete to the tabulated minimum over that base, and let the granular layer make up the balance to the frost-protection thickness.

Recover the design traffic. The full-depth design fixes the traffic implicitly. Entering Design Chart A-1 (Full Depth Asphalt Concrete, MAAT 7 °C) at $M_{r}=70\ \text{MPa}$ and following the 225 mm curve gives$$EAL\approx 2.3\times 10^{6}\ \text{equivalent }80\ \text{kN single axle loads}$$Every subsequent chart is entered at this same traffic level, which is what makes the layers comparable.
Frost protection sets the total thickness. The supervisor’s rule is explicit:$$T_{\text{total}}\ge 0.70\times 1\,200\ \text{mm}=\boxed{840\ \text{mm}}$$The initial 225 mm full-depth section fails this test by a factor of nearly four — which is precisely the omission the supervisor identified. Structural adequacy and frost adequacy are separate requirements, and here frost governs the total by a wide margin.
Classify the cold mix. The Asphalt Institute divides emulsified asphalt mixes by the aggregate they are made from: Type I from processed, dense-graded aggregate; Type II from semi-processed crusher-run, pit-run or bank-run aggregate; Type III from sands and silty sands. The material here is local silty sand from the excavation, so it is an emulsified asphalt mix Type III and Design Chart A-4 governs. This is also the economically correct choice: it is made on site with a mobile plant and consumes the excavation spoil rather than imported aggregate.
Thickness of the emulsified-asphalt pavement. Entering Chart A-4 at $M_{r}=70\ \text{MPa}$ and $EAL=2.3\times 10^{6}$ gives a required total thickness of about 335 mm, and rounding up to the chart’s 25 mm increment,$$T_{\text{III}}=\boxed{350\ \text{mm}}$$The implied substitution ratio is $350/225=1.56$, i.e. about 1.6 mm of Type III cold mix replaces 1 mm of hot-mix asphalt concrete — the expected penalty for a sand-based, emulsion-bound material.
Minimum asphalt concrete over the cold mix. A Type III base cannot be exposed to traffic, so Table VI-2 requires an asphalt concrete surface: 75 mm at $10^{6}$ EAL and 100 mm at $10^{7}$. Interpolating logarithmically at $2.3\times 10^{6}$,$$t_{AC,\min}=75+25\log_{10}\!\left(\dfrac{2.3\times 10^{6}}{10^{6}}\right)=75+25(0.362)=84\ \text{mm}$$Because the supervisor wants the hot mix minimised, specify the smallest practical thickness that clears this: $$t_{AC}=\boxed{90\ \text{mm}}$$placed as a 50 mm binder and a 40 mm surface course, which is also the minimum that allows two compacted lifts.
Cold-mix base thickness. The asphalt concrete is part of the 350 mm bound structure, so the balance is made up in cold mix:$$t_{\text{cold mix}}=T_{\text{III}}-t_{AC}=350-90=\boxed{260\ \text{mm}}$$placed in two lifts of 130 mm, cured before the surfacing is laid.
Granular base thickness. The unbound layer is now needed only to complete the frost-protection thickness and to give a working platform and drainage blanket over the subgrade:$$t_{\text{granular}}=T_{\text{total}}-T_{\text{III}}=840-350=\boxed{490\ \text{mm}}$$This is the minimum consistent with the frost rule, which is exactly what the supervisor asked for. Ignoring its structural contribution is conservative: Charts A-5 and A-6 show that 150 mm and 300 mm of untreated aggregate base each reduce the required bound thickness, so the 490 mm provided is a genuine reserve rather than dead thickness.
Assemble and check.$$90+260+490=\boxed{840\ \text{mm}}=0.70\times 1\,200\ \text{mm}\quad\checkmark$$The section satisfies the traffic through the 350 mm bound structure, satisfies frost through the 840 mm total, uses only 90 mm of imported hot mix, and takes 490 mm of the total from an unbound layer whose thickness is the minimum the frost rule permits.

Check: chart reads and the thickness-accounting convention. The two design charts were read from the printed figure of appendix pages 5 and 6; a reasonable reading tolerance is about ±10 % on EAL and one 25 mm curve interval on thickness, neither of which changes the design because frost, not traffic, sets the total. The asphalt concrete is taken to be part of the 350 mm emulsified-pavement thickness, following the Asphalt Institute convention that hot mix substitutes for an equal thickness of the emulsified mix; if instead it were placed on top of a full 350 mm of cold mix the bound structure would be 440 mm and the granular layer would shrink to 400 mm, with the same 840 mm total.

Question 5 — proposed section
Layer	Material	Thickness	Basis
Surface	Asphalt concrete (hot mix)	90 mm	Table VI-2 minimum (84 mm interpolated)
Base	Cold mix, Type III emulsified asphalt	260 mm	Chart A-4 total 350 mm less the 90 mm surface
Subbase	Unbound granular base	490 mm	balance to the frost-protection thickness
Total structure	—	840 mm	70 % of the 1.2 m frost depth
Design traffic	—	2.3 × 10⁶ EAL	Chart A-1 at $M_r=70$ MPa, 225 mm
Substitution ratio, Type III to hot mix	—	1.56	350 mm / 225 mm

Topic. The Asphalt Institute (MS-1) thickness design method, emulsified-asphalt mix types, and frost-depth control of total pavement thickness.

Key relations. Design Charts A-1 (full-depth), A-2 to A-4 (emulsified mixes Types I, II, III) and A-5 to A-6 (untreated aggregate base), all entered with $M_{r}$ and $EAL$ at the site MAAT; Table VI-2 (minimum asphalt concrete over emulsified bases); Table VI-3 (minimum asphalt concrete over untreated aggregate base); frost rule $T\ge k\times$ frost depth.

Why this works. The Asphalt Institute charts are families of equal-thickness curves in the $(EAL,\ M_{r})$ plane, generated by limiting two mechanistic criteria at the design traffic: the horizontal tensile strain at the bottom of the bound layer, which controls fatigue cracking, and the vertical compressive strain at the top of the subgrade, which controls rutting. Because every chart in a set is generated for the same criteria and the same climate, a design point can be carried from one chart to another — which is what makes the back-calculation in step 1 legitimate: the full-depth thickness is a proxy for the traffic, and once the traffic is known any other material family can be entered at the same point. The mix-type distinction is not cosmetic. Type III mixes are made from sands and silty sands, have little aggregate interlock and rely almost entirely on the emulsion for stiffness, so they need substantially more thickness than hot mix and cannot be left as a running surface at all — hence the Table VI-2 minimum cover. Finally, frost depth enters the design from a completely different direction: it is not a load-carrying requirement but a means of keeping the frost-susceptible subgrade below the freezing front, so it sets a floor on the total thickness that structural adequacy can never relax.

Common pitfall. Designing only for traffic and reporting a 350 mm section in a region with a 1.2 m frost depth — the frost rule is the whole point of the supervisor’s note. The second pitfall is choosing Type I or Type II because they are thinner, even though a silty sand can only be a Type III mix; the cheap material is the one the site produces, and the design must absorb its lower stiffness in thickness.

Source. Asphalt Institute, Thickness Design — Asphalt Pavements for Highways and Streets (MS-1), Ch. IV–VI and Design Charts A-1 to A-6; Garber & Hoel, 5th ed., §20.6 (Asphalt Institute method).

Question 5 of 7: Asphalt Institute redesign of a Northern Ontario pavement

Question 5: Asphalt Institute redesign of a Northern Ontario pavement (20 marks)