Adjusting design air void Levels in Superpave mixtures to enhance durability

Abstract

When developed in the early 1990s the Superpave method of asphalt design was structured similar to the Marshall method of asphalt design. In the Marshall method asphalt was designed with 3 to 5% air voids (typically 4%). Compaction specifications typically left the mixture with 8% air voids on the road. It was reasoned that traffic would compact the asphalt mixtures and achieve an ultimate density of 96%, the same as the design. Superpave calls for a design air void content of 4.0% (no range) and compaction specifications allow the mixture to have 7 to 8% air voids. Subsequent research in the early 2000s showed that after traffic densification air voids stabilize at about 6%. This paper discusses proposed changes Superpave asphalt design method to design at 5% air voids. Target air voids after compaction would be the same, 5%. To achieve compaction of 95% (5% air voids) requires changes to the laboratory compactive effort in the Superpave design method. The concept of designing and compacting asphalt at 5% air voids is inspired by the LCPC method of asphalt design used in France. The perceived benefit to Superpave is an improvement in mixture durability expected from lower air permeability leading to a reduced rate of oxidative hardening. This paper discusses the concept of setting air void levels in asphalt design, research to identify changes to the Superpave method and recommendations for design compactive effort to be used.

Relating Ndesign to Field Compaction: A Case Study in Minnesota

“…One obvious, but technically very challenging solution, is to increase the compaction effort in the field to match the design compaction effort required by N design . A much simpler approach is to design more compactable mixtures by choosing an N design value that corresponds to a compaction effort that matches the real compaction effort in the field, as suggested in the Superpave 5 mixture design ( 16 , 17 ). Based on this argument, a method is proposed next to quantify the field compaction effort.…”

“…Previous studies have reported on the effect of N design on the compactability of mixtures, and the authors suggested that N design values be adjusted to improve field densities. The best example is the “Superpave 5” mix design method developed by Purdue University, Heritage Research Group, and Indiana DOT ( 16 , 17 ). The N design levels in Superpave 5 were set at 30 and 50 for traffic levels below and above 3 million equivalent single axel loads (ESAL), respectively, values substantially lower compared with the values in the traditional Superpave design method ( 7 ).…”

“…As a result of the temperature difference, the viscosity of binder differs significantly, which leads to different mechanisms for the two processes (12)(13)(14)(15). Some experimental studies have shown that mixtures can be designed at lower N design levels and be more compactable, and still have high rutting resistance (16,17). These results show that rutting and compaction cannot be equated, and the requirement of N design needs to be further investigated.…”

Field Density Investigation of Asphalt Mixtures in Minnesota

“…%Pass of the sieveÀ j %Pass of the sieve on MDLj ð 2Þ This is based on previous research that showed that mixture compactability is related to how close the gradation curve is to MDL (9,10).…”

“…In this method, N design is related to compaction effort rather than traffic volume, which guarantees consistency between laboratory and field compaction. Superpave 5 mixtures are designed by adjusting aggregate gradation while keeping the effective binder content unchanged, compared with traditional Superpave (9,10).…”