What is Perpetual Pavement?
Perpetual Pavement is an asphalt concrete pavement that is designed to last 50 years or longer without any repairs below the wearing course (surface level). It typically has a multi-layered structure which resists fatigue over time. The triple-layered structure resists structural damage for as long as the top surface is maintained and replaced. While the outer surface is cared for, the rest of the pavement will not to be fully repaired or replaced for many years.
Resistance to Pavement Fatigue
The goal of perpetual pavement is to be a deep-strength asphalt paving that resists fatigue. It does so by reducing cracking from the bottom-up. When pavements undergo road traffic, fatigue develops from the cyclical nature and weight of the traffic. The fatigue begins as cracks at the pavement foundation. These foundational cracks then work their way up to the higher layers of pavement. As they do so, the cracks spread out and multiply. Finally, over time, the cracks show themselves on the surface of the asphalt pavement.
Perpetual pavement is designed specifically to resist bottom-up fatigue cracking and structural rutting. These two types of damage are some of the worst that pavement can receive. It is superior to low-grade pavement that has rutting and cracking before their design-life is achieved. It is also superior to high-grade traditional pavement because it uses materials more efficiently.
Differences from Traditional Pavement
Empirical Pavement Design
Traditional pavement is made based on empirical designs. High-quality pavements are typically made using empirical observations. Afterwards, the pavement is designed to meet the observations made. These observations rely on the following set of factors.
1) Traffic levels.
2) An indicator of material quality,
3) Pavement material layers.
Numbers 1 and 2 interact with each other in order to predict what the appropriate levels for number 3 should be. This method is called empirical pavement design.
Essentially, it works like a math formula. For a given material design (number 2) and traffic levels (1), the pavement material layers (3) should increase or decrease.
The flaw in empirical pavement design is that eventually the pavement thickness is more than enough for the load of traffic. Pavement will be thicker and thicker, but at no real benefit. What this means to builders is that costs will go up but not affect quality. Pavements over-designed by adding 1.5-4.5 inches of thickness added 600-1,800 tons of material per mile of single-lane road. All of this extra weight was beyond necessary levels for the function and lifespan of the road.
Mechanistic-Empirical Method (M-E Pavement Design)
Perpetual pavements use a different method of design than traditional pavements. This method uses more factors to create a model. The model is then used with testing and more experimentation to figure out allowable loads and damage. Once the damage level is acceptable, it is ready to go to the final design stages.
There are 3 inputs.
3) Pavement layer thickness.
These three inputs are combined into an analytical model, which gets pavement values. The pavement values determine the allowable load (according to the model)
Actual loads are compared to allowable loads on the model to compute damage.
If the computed damage level is not too high, the pavement can go into the final design.
Optional Step 4
If the computed damage level is too high, the pavement layer thickness is increased, and then steps 1-3 are repeated.
This methodology was originally used to test metal fatigue.
For a detailed, long-form article about perpetual pavement, as well as good flow charts showing the design process, look at this article from the Asphalt Pavement Alliance.
Pavement Design Life
Regardless of the method of pavement design, a pavement’s life is based on several other factors. In general, for any type of asphalt, the design life can be seen as a function of several factors.
1) Design requirements (method used for design, M-E, or empirical pavement design).
2) Material characteristics (quality of construction materials).
3) Construction practices (quality of build/structure).
4) Layer thickness.
5) Maintenance Activities.
6) Failure Criteria (how failure is defined according to the designer).
- Perpetual pavements must have enough structural integrity and thickness to stop cracking, fatigue, rutting, and deforming.
- Perpetual pavements must be durable to resist damage from traffic and the environment.
1.5 To 3 inches of HMA (hot mix asphalt) or OGFC (open grade friction course). HMA is flexible and deflects, or flexes, under loading. The load is dispersed throughout the pavement so each sub layer carries less weight. OGFC is a porous asphalt allowing water to drain through it. It also reduces wet weather crashes.
There are 4 to 7 inches of rut resistant material in layer 2. This is high-strength material that is flexible enough to expand and contract and resist cracking.
Layer 3 is composed of 3 to 4 inches of durable material that resists fatigue. This is the part of the pavement that will receive the most tensile strain (horizontal push/pull). Layer 3 must be resistant to fatigue cracks over time.
The final layer of pavement is the foundation. This can be compacted subgrade, stabilized subgrade, stabilized aggregate, or unstabilized aggregate like gravel.
Why are Perpetual Pavements Beneficial?
- Cyclical costs are reduced on the pavement. As long-term damage is avoided, yearly or multi-yearly-accrued damage doesn’t need to be repaired.
- More efficient design. They eliminate overly-thick sections of asphalt.
- Reconstruction costs are eliminated.
- Delays due to maintenance are fewer and less costly.
- They reduce use of non renewable asphalts and aggregates.
Considering Perpetual Pavement?
If you are thinking about getting your next paving job done, and perpetual pavement seems right for you, let us know. Reliable Paving is a paving contractor with over 35 years of experience across the South and Southwest. We are happy to assist you with any questions you may have about pavement, long life pavement, and pavement maintenance.