Asphalt Mixture Volumetrics and Field Compaction Control: 16-Civ-B7 Transportation Planning and Engineering, May 2016, Q6

Question 6: Asphalt Mixture Volumetrics and Field Compaction Control (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.

Quantity	Symbol	Value
Bulk relative density of the compacted mix	G_mb	2366 kg/m³ → 2.366
Maximum (theoretical) relative density	G_mm	2448 kg/m³ → 2.448
Asphalt content, per cent of total mix	P_b	5.5 % (aggregate P_s = 94.5 %)
Bulk relative density of combined aggregate	G_sb	2.65
Relative density of asphalt binder	G_b	1.04
Specified air voids / VFA	V_a / VFA	3–5 % / 65–75 %
Required relative compaction (subgrade)	—	95 %
Laboratory maximum dry density / OMC	γ_d,max / OMC	1800 kg/m³ / 9 %
Field wet density / moisture content	γ_wet / w	1850 kg/m³ / 12 %

Find. (a) the air-void content and voids filled with asphalt of the mixture and whether both fall inside the specified bands, and (b) the relative compaction achieved in the field against the 95 per cent requirement.

Phase diagram of a unit volume of the compacted mixture. VMA is the sum of the air voids and the effective binder; VFA is the fraction of that space the binder occupies.

Approach. Part (a) converts the two densities to relative densities, computes air voids from the gap between bulk and maximum relative density, computes the voids in the mineral aggregate from the aggregate's own volume, and combines them into VFA before comparing with the specification. Part (b) converts the field wet density to a dry density and expresses it as a percentage of the laboratory maximum.

Part (a) — convert densities to relative densities. Dividing by the density of water, 1000 kg/m³, gives $G_{mb}=2.366$ and $G_{mm}=2.448$. The aggregate content of the mixture is $P_s = 100 - P_b = 100 - 5.5 = 94.5$ per cent by mass.
Air voids in the compacted mixture. The maximum relative density describes the same mixture with every air void removed, so the void volume is the fractional shortfall of the bulk value: $$V_a = 100\,\frac{G_{mm}-G_{mb}}{G_{mm}} = 100\,\frac{2.448-2.366}{2.448} = 100\,\frac{0.082}{2.448}$$ $$\boxed{V_a = 3.35\ \text{per cent}}$$ This lies inside the specified 3 to 5 per cent band, though in its lower half.
Voids in the mineral aggregate. VMA is the volume not occupied by the aggregate itself, measured against the aggregate's bulk relative density: $$VMA = 100 - \frac{G_{mb}P_s}{G_{sb}} = 100 - \frac{223.59}{2.65} = 100 - 84.37$$ so $VMA = 15.63$ per cent. For a mixture of typical nominal maximum aggregate size this clears the customary 14 per cent minimum, so the aggregate skeleton itself has adequate void space.
Voids filled with asphalt. VFA is the proportion of the VMA occupied by effective binder rather than air: $$VFA = 100\,\frac{VMA - V_a}{VMA} = 100\,\frac{15.63-3.35}{15.63} = 100\,\frac{12.28}{15.63}$$ $$\boxed{VFA = 78.6\ \text{per cent}}$$
Compare with the specification. Air voids of 3.35 per cent satisfy the 3 to 5 per cent requirement, but VFA of 78.6 per cent lies well above the 65 to 75 per cent band — an exceedance of 3.6 points, far beyond any plausible testing tolerance. The mixture therefore does not meet the specified volumetric requirements.
Diagnose the failure and give the remedy. High VFA with adequate VMA and low-side air voids is the signature of a mixture carrying more binder than its aggregate skeleton has room for: the effective binder occupies 12.28 of the 15.63 units of void space. Holding the VMA constant, the air voids needed to bring VFA down to 75 per cent are $$V_a = 0.25(15.63) = 3.91\ \text{per cent}$$ which is reached by reducing the asphalt content by roughly 0.3 to 0.4 per cent, or by adjusting the gradation to raise the VMA. Left as designed, the mixture is at risk of rutting and bleeding in hot weather, because there is too little air void reserve to accommodate further densification under traffic.
Supporting volumetrics for completeness. The effective relative density of the aggregate follows from the maximum relative density, $$G_{se} = \frac{P_s}{\dfrac{100}{G_{mm}} - \dfrac{P_b}{G_b}} = \frac{94.5}{40.850-5.288} = 2.657$$ so the absorbed asphalt is $P_{ba} = 100(G_{se}-G_{sb})G_b/(G_{sb}G_{se}) = 0.11$ per cent and the effective asphalt content is $P_{be} = 5.5 - 0.11(0.945) = 5.40$ per cent. Absorption is negligible here, which confirms that the high VFA is a mix-proportioning issue and not an artefact of an absorptive aggregate.
Part (b) — convert the field wet density to a dry density. Compaction is always specified on dry density, so the measured water must be removed: $$\gamma_d = \frac{\gamma_{\text{wet}}}{1+w} = \frac{1850}{1+0.12} = \frac{1850}{1.12} = 1651.8\ \text{kg/m}^3$$
Relative compaction. Expressed against the Standard Proctor maximum, $$RC = 100\,\frac{\gamma_d}{\gamma_{d,\max}} = 100\,\frac{1651.8}{1800}$$ $$\boxed{RC = 91.8\ \text{per cent} < 95\ \text{per cent required}}$$ The compaction is not satisfactory.
Quantify what is required and explain why it failed. To reach 95 per cent the dry density must be at least $0.95(1800) = 1710$ kg/m³, which at the measured 12 per cent moisture corresponds to a field wet density of $1710(1.12) = 1915$ kg/m³ — some 65 kg/m³ more than was achieved. The likely cause is visible in the moisture content itself: at 12 per cent the soil is 3 percentage points wet of the 9 per cent optimum, and on the wet side of optimum the pore water resists compaction, so additional roller passes yield little. The correct remedy is to dry the material back toward optimum by aeration before recompacting, rather than simply rolling more.

Quantity	Result	Specification
Air voids, V_a	3.35 %	3–5 % — satisfied
Voids in mineral aggregate, VMA	15.63 %	≥ 14 % — satisfied
Voids filled with asphalt, VFA	78.6 %	65–75 % — NOT satisfied
Effective aggregate relative density, G_se	2.657	—
Absorbed / effective asphalt, P_ba / P_be	0.11 % / 5.40 %	—
(a) Verdict	mix fails on VFA — reduce binder by 0.3–0.4 %	—
Field dry density	1651.8 kg/m³	≥ 1710 kg/m³
Relative compaction	91.8 %	95 % — NOT satisfied
(b) Verdict	unsatisfactory; soil is 3 % wet of optimum	—

Topic. Volumetric proportioning of compacted asphalt mixtures, and relative compaction as a field acceptance test.

Key relations. $V_a = 100(G_{mm}-G_{mb})/G_{mm}$; $VMA = 100 - G_{mb}P_s/G_{sb}$; $VFA = 100(VMA-V_a)/VMA$; $G_{se}=P_s/[100/G_{mm}-P_b/G_b]$; $P_{ba}=100(G_{se}-G_{sb})G_b/(G_{sb}G_{se})$; $\gamma_d = \gamma_{\text{wet}}/(1+w)$; $RC = 100\gamma_d/\gamma_{d,\max}$.

Why this works. A compacted asphalt mixture is a three-phase material, and every specification limit is really a statement about the proportions of those phases. Air voids must be low enough to keep water and air out of the binder, yet high enough to leave room for the further densification that traffic will impose — a mixture placed with too little void space has nowhere to go but sideways, which is rutting. VMA measures whether the aggregate skeleton has left enough space for a durable binder film in the first place, and VFA closes the loop by asking how much of that space the binder actually occupies. The three quantities are not independent: fixing any two fixes the third, which is why a mixture can pass on air voids and still fail on VFA, and why the diagnosis in that case is always about the balance between binder content and aggregate structure. Compaction control follows the same logic in soil: the Proctor test fixes a reference density at a reference energy, and the field result is meaningful only as a ratio to it, on a dry basis, because water is not part of the solid skeleton.

Common pitfall. Using G_mm in place of G_sb in the VMA expression, which inflates VMA and hides a marginal mixture; and computing relative compaction from the wet density, which here would have given a flattering 103 per cent and passed a failing lift. A third error is concluding that more rolling will fix compaction that is failing on the wet side of optimum.

Source. Asphalt Institute, Asphalt Mix Design Methods (MS-2), 7th ed., Ch. 4 (volumetric properties); Mamlouk & Zaniewski, Materials for Civil and Construction Engineers, 4th ed., Ch. 9 (asphalt mixtures) and Ch. 8 (soil compaction); ASTM D698 / AASHTO T99 (Standard Proctor).

Question 6 of 7: Asphalt Mixture Volumetrics and Field Compaction Control

Question 6: Asphalt Mixture Volumetrics and Field Compaction Control (20 marks)