How Compaction Impacts AC Pavement Performance

The construction of high-quality roads plays a crucial role in mitigating pavement distresses such as rutting, cracking, and other forms of damage, thereby enhancing the long-term performance of the pavement. Among the various factors influencing the performance of asphalt concrete (AC) pavements, compaction stands out as one of the most significant ones. The air volume within a pavement holds considerable importance as it profoundly impacts the pavement's performance life.

Compaction refers to the process of compressing the asphalt and aggregate materials to reduce their volume. It is widely acknowledged that achieving proper compaction of asphalt concrete is critically important for ensuring optimal performance of flexible pavements.

This blog discusses the factors that impact air voids and compaction levels have on asphalt properties.

1. Stability & Stiffness of the pavement

Stability refers to an AC pavement’s ability to endure deformation when exposed to different traffic loads and environmental circumstances. A resilient pavement maintains its form and evenness despite repeated loads. Usually, stability improves as density increases and air voids decrease. In this context, a decrease of one percent in air voids results in a decrease in stability by around five or more units.

Moreover, the density of an asphalt mixture affects its stiffness. Studies have shown that as density increases, the mixture becomes stiffer, indicating better ability to bear loads. The strength of the asphalt mix, measured by its stiffness or modulus, is also connected to the level of compaction. In Figure 1, it is illustrated that when the air void content increases from 5% to 8%, the stiffness or load-carrying capacity decreases by 20%.

Figure 1: Relative Modulus vs. Air Voids

 2. Durability

The durability of an AC pavement refers to its ability to withstand weathering and the wearing effects of traffic. Factors like weather conditions and traffic loads can affect its durability. A good level of durability means that the pavement can last a long time without developing cracks or wearing out too quickly. The durability of asphalt concrete depends on the quality of the asphalt cement used. Over time, the asphalt may become more permeable. Once the water is able to seep into the pavement through the voids and reach the base course, it can weaken the base material and cause the pavement to fail.

Research has shown that the rate of asphalt hardening is associated with the total air voids in the asphalt concrete. In other words, compacting a well-designed paving mixture with minimal air voids will slow down the rate of hardening of the asphalt binder. This leads to increased longevity of the pavement, reduced maintenance needs and improved overall performance of the road surface.

3. Ravelling

Ravelling refers to the erosion or loss of aggregate from the surface of an asphalt mixture. This can occur from both the natural aging of the materials and insufficient interlocking of aggregates caused by inadequate compaction. In either scenario, the presence of high air voids and inadequate compaction significantly diminishes the overall durability and service life of the asphalt.

4. Rutting

When the air voids in an asphalt mixture are low (less than 2%), the binder almost completely fills the void spaces between the aggregate particles. This causes the mixture to behave more like a fluid and makes it less resistant to rutting when exposed to heavy traffic. Similarly, inadequately compacted mixtures also exhibit lower resistance to rutting due to a weaker structure under traffic loads. Figure 2 shows the relative rutting rate of a mix designed with 5% air voids and compacted to different void levels.

Figure 2: Relative Rutting Rate vs. Air Voids

5. Fatigue Life

The fatigue life of a pavement refers to the pavement’s ability to resist cracking under repeated loads. The fatigue behaviour of an AC pavement is significantly influenced by the presence of air voids as fatigue life is directly correlated with the level of compaction. Previous research has revealed that mixtures with higher air void contents tend to have shorter fatigue lives. These findings suggest that changes in air void content have a substantial impact on the fatigue life of the material. Laboratory studies have demonstrated that for each one percent increase in air voids, the fatigue life of the AC pavement can decrease by 30% or more. Figure 3 illustrates the outcomes of fatigue testing, highlighting that an increase in air voids from 5% to 8% leads to a significant 50% reduction in fatigue life.

Figure 3: Relative fatigue life vs. air voids

6. Pavement Flexibility and the link to Moisture and Permeability

Flexibility refers to the ability of an asphalt pavement to adjust or conform to long-term variations in base and subgrade elevations. Mixtures with acceptable stabilities, higher asphalt contents, and increased air voids tend to demonstrate strong flexibility without developing cracks. However, there can be instances where the requirement for flexibility conflicts with stability considerations. For example, an open-graded mixture, designed to provide higher flexibility and permeability, is of course more permeable to water than a HMA pavement. On the other hand, a dense graded mix is relatively impermeable but less flexible.

Conclusion

Compaction plays a critical role in determining the performance and longevity of AC pavements. Adequate compaction ensures the achievement of a dense and uniform pavement structure, which enhances its strength, durability, and resistance to deformation under traffic loads.

We can use the Ground Penetrating Radar (GPR) to conduct rapid sampling to assess the strength and the scatter of signals, often making it possible to find voids and changes in compaction, bond and moisture content. Click here to download the technical brochure.  

References

Chu, T. H. V. (n.d.). A Self-Learning Manual - Mastering Different Fields of Civil Engineering Works (VC-Q&A Method). Available at: https://www.academia.edu/17537735/A_Self_Learning_Manual_Mastering_Different_Fields_of_Civil_Engineering_Works_VC_Q_and_A_Method

‌AFPA. (2017). Work tips: Air Voids in Asphalt.  Available at: https://www.afpa.asn.au/wp-content/uploads/2017/03/Worktips-17-Air-voids-in-asphalt.pdf  

F.  N.  Finn  and  J.  A.  Epps. (1980). Compaction  of  Hot  Mix  Asphalt  Concrete.  Research  Report  214-21,  Texas Transportation Institute, The Texas A&M University System College Station. Available at: https://static.tti.tamu.edu/tti.tamu.edu/documents/214-21.pdf

L. N. Robert, M. P. Joe, and J. C. Newton. (1990). Effect of compaction on asphalt concrete performance. Transportation Research  Record,  no.  1217,  pp.  20-28.  Available at: http://onlinepubs.trb.org/Onlinepubs/trr/1989/1217/1217-003.pdf

Pavement Interactive. (2023). Compaction Importance. Available at: https://pavementinteractive.org/reference-desk/construction/compaction/compaction-importance/