How to make asphalt mixture have better performance?

June 23, 2022 air voids,asphalt mixture,RAP,RAS,mineral interstitial ratio
We can start from the following three aspects to improve the durability, rutting and crack resistance of asphalt mixtures:
1. Adjust the grading to ensure that there is enough mineral interstitial ratio (VMA) to ensure that the asphalt mixture has enough asphalt;
2. Select the appropriate void ratio;
3. Control the impact of recycled asphalt mixture (RAP) and recycled asphalt shingle (RAS) on asphalt mixture.

Among them, VMA is the abbreviation of "Voids in Mineral Aggregate", which means the space between aggregates that can be filled with asphalt. The space filled with asphalt is called VFA (Voids Filled with Asphalt), and the remaining space is called air voids.


The VMA index is very important to the durability and crack resistance of the mixture. When the void ratio is fixed (the design void ratio in the Superpave mixture design method is 4%), reducing the VMA will reduce the amount of asphalt. Although this reduces the cost, it will make the durability of the asphalt mixture worse.
When the void fraction is fixed, lowering the VMA results in a lower amount of bitumen. In order to reduce costs, asphalt mix designers and contractors usually make the VMA just equal to or slightly exceed the minimum specification requirements through grading design and ultimately reduce the amount of asphalt, which will make the asphalt mixture more economical but will affect its performance. Durability. Superpave and other specifications specify the minimum VMA based on this.
When the asphalt mixture changes from the target mix design stage to the production mix design stage, its VMA will decrease. This is because the asphalt mixture is subjected to the stirring action of the mixing blades in the mixing pot compared to the design stage. The violent tumbling of the aggregate in the mixing drum will cause more aggregate particles to break up. The reduction value of VMA depends on the hardness of the aggregate, but it is mostly between 0.2 – 0.5 %. In order to avoid the reduction of VMA in the production process, the mixture designer should design the VMA to be smaller than the minimum value specified in the specification during the production mix design stage. The VMA is slightly larger or the amount of mineral powder is reduced to consider the impact of aggregate fragmentation.
Asphalt acts as a binder, and reducing the amount of asphalt used in production usually means that the asphalt mixture is dry and can lead to premature loosening and cracking of the pavement. In addition, the lower the amount of asphalt used, the more difficult it is to compact the mixture.
The choice of lower VMAs, higher design void ratio, and the use of more RAP and RAS will reduce the amount of asphalt; the combination of these factors can reduce the amount of asphalt binder in the asphalt mixture by an average of 0.2-0.6% (depending on the mix total weight).
However, we can design a better asphalt mixture by ensuring the minimum VMA, selecting the appropriate design void ratio, and fully considering the effects of RAP and RAS in the asphalt mixture.