Superpave Mix Failing VMA and Dust-to-Binder
A 12.5 mm Superpave surface mix is being designed at Ndesign = 100 gyrations. At 5.5% asphalt binder, the trial blend gives Gmm 2.500, Gmb 2.400, air voids 4.0%, VMA 13.6%, VFA 70.6%, and dust-to-binder ratio 1.7. The aggregate has FAA 45%, sand equivalent 48%, and flat-and-elongated 8%. The specification requires VMA at least 14.0%, VFA 65-75%, dust-to-binder 0.8-1.6, FAA at least 45%, SE at least 45%, and F&E no more than 10%. The contractor proposes adding 0.5% binder to raise VMA and lower the dust-to-binder ratio. The plant has capacity to add a coarse intermediate aggregate or to reduce the minus 0.075 mm fraction. The mix already meets all consensus aggregate properties. The design is scheduled for verification tomorrow.
1. Evaluate the contractor's proposal to add 0.5% binder. Will it solve the VMA deficiency?
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Adding binder is not a reliable way to fix low VMA. VMA is controlled mainly by aggregate gradation and packing, not by binder content. If 0.5% binder is added at the same Ndesign, air voids will drop below 4.0% and the mix may become tender or unstable. The dust-to-binder ratio would fall because the denominator increases, so the proposal treats the symptom, not the cause. The VMA deficiency points to a gradation that is too fine or lacks intermediate-size aggregate. The correct first step is to adjust the aggregate structure to increase VMA, then rebalance binder to reach 4.0% air voids at Ndesign. The existing VFA of 70.6% already meets the 65-75% requirement; adding binder would likely raise VFA further and could push it out of range.
2. What gradation adjustment would you recommend, and how would you verify it?
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Increase VMA by making the aggregate skeleton more open. Reduce the minus 0.075 mm fraction, add a coarse intermediate aggregate, or adjust the blend so the gradation has more material in the intermediate sieves and less fine sand. Reducing the fines also lowers the dust-to-binder ratio from 1.7 toward the 1.6 maximum. Do not change the consensus properties in a way that drops FAA below 45% or raises F&E above 10%. Recompact trial blends at Ndesign = 100 gyrations, determine Gmm for each blend, and calculate air voids, VMA, VFA, and dust-to-binder at the binder content that gives 4.0% air voids. Verify that VMA is at least 14.0%, VFA remains 65-75%, and dust-to-binder is 0.8-1.6. If the gradation change alters the optimum binder content, rerun the moisture-susceptibility check before verification.
3. What would you tell the contractor about the verification scheduled for tomorrow?
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The verification should not proceed on the current blend unless the VMA and dust-to-binder failures are resolved. The current mix fails two volumetric requirements, and the proposed binder addition does not address the VMA mechanism. Ask the contractor to submit a revised gradation and a complete volumetric set at 4.0% air voids. The verification should include Gmm, Gmb, air voids, VMA, VFA, dust-to-binder, and the consensus aggregate properties. If the revised blend meets the volumetric requirements, then proceed with the verification and the required moisture-susceptibility testing. If the contractor insists on adding binder only, require a new trial showing that VMA reaches 14.0% at 4.0% air voids without unacceptable VFA or stability loss. Do not accept a verbal adjustment on the day of verification.