Pavement Traffic Resistance

Pavement traffic resistance

Two things decide how much traffic a pavement can take: the total ESALs it will see over its life (Equivalent Single Axle Loads of 80 kN) and how strong each layer is, which engineers read off the CBR (California Bearing Ratio). To estimate the cumulative ESALs you use N = AADT · FE · 365 · Y · (1+r)^Y, where AADT is the average annual daily traffic, FE the axle-equivalence factor, Y the design life in years and r the annual traffic-growth rate. Take AADT = 5000, FE = 0.4, Y = 15 and r = 0, and you land at N ≈ 1.1 × 10⁷ ESALs.

DNIT/AASHTO sets a minimum CBR for each layer: CBR > 80% for rigid pavement and the surface course under heavy traffic, CBR > 60% for the base, CBR > 30% for the sub-base, and CBR > 2% for the subgrade. Drop below that last figure and you have to swap out the soil. To work out the total thickness of the structure, the DNIT method leans on the thickness coefficients K_R, K_B, K_sB together with the N × CBR chart.

Applications

You'll see this in federal and state highway design, in airport runways and taxiways (FAA / ICAO), in heavy-duty industrial yards and port container yards, on urban arterial streets and parking lots, and in rehabilitation work involving overlay or recycling. ESALs also feed the fatigue analysis in mechanistic-empirical methods (MEPDG).

FAQ

What is the standard axle of 80 kN? It's the single axle with dual wheels carrying 80 kN (8.2 t), which AASHTO uses as its reference. Every other axle gets converted into ESALs through factors that scale with the load raised to the fourth power.

What is the difference between CBR and resilient modulus? CBR is an empirical penetration index, given as a percentage. The resilient modulus M_R (MPa) measures the elastic response under cyclic loading, and mechanistic methods tend to favour it. A rough correlation: M_R ≈ 10·CBR (in MPa) when CBR < 20%.

Does the climate affect it? It does. A saturated soil shows a much lower CBR, which is why the test is run after four days of soaking. In tropical, rainy regions you'll need either thicker asphalt or chemical stabilisation to compensate.

Rigid or flexible pavement? Rigid pavement, in concrete, runs 30–40 years but costs more upfront. Flexible pavement, in asphalt, is cheaper and quicker to build, with a life of 10–20 years. Which one wins comes down to the traffic, the subgrade and what the whole lifecycle ends up costing.