How do Initial Air Voids and Secondary Compaction Influence Asphalt Mixture Aging?

Abstract: The objective of this study is to evaluate the effect of air voids at construction and traffic-induced secondary compaction on the aging behaviour of asphalt mixtures over a 42-month period starting from construction. The study was conducted on two road sections (R1 and R2) near Hyderabad, India. Both road sections were paved with a 13.2 mm NMSA dense graded bituminous mixture. Monthly core samples were collected from the field, and laboratory investigations were conducted. The investigations included the calculation of bulk specific gravity (Gmb), maximum theoretical specific gravity (Gmm), binder rheological measurements, and sieve analysis to observe the variation in the density, bitumen properties and aggregate gradation, respectively, over time.
Results indicated that Section R1, had 1.3% higher initial air voids than R2 (9.3% vs 8%). This resulted in accelerated oxidative aging in R1 compared to Section R2. It was observed that as the traffic-induced secondary compaction caused air voids to decrease over time the rate of aging also slowed down. However, oxidation was still progressing, albeit at a much slower rate, under the influence of environmental factors such as temperature, atmospheric oxygen and UV radiation. Thereby, it can be concluded that initially, after construction, the air voids play a significant role in driving the oxidation mechanism; however, as secondary compaction takes place and the mixture densifies, oxidation results only from the environmental factors. Regarding the performance implications of these findings, in R1, significant raveling, top-down cracking, and moisture damage were observed within the first three years.
These findings suggest that it is extremely vital to have strict control over the initial air voids and ensure sufficient densification during construction. Leaving significant room for densification during secondary compaction is not a good construction practice and will result in increased oxidative aging and premature pavement failures.