Flexible pavement layers and the CBR test

A flexible pavement spreads wheel loads through layers of decreasing quality from top to bottom. Each layer only has to be strong enough for the stress that reaches it.

The layers, top down

Layer	Typical material	Job
Surface course	Bituminous concrete (BC)	Waterproof, skid-resistant, takes tyre wear
Binder course	Dense bituminous macadam (DBM)	Main bituminous structural layer
Base	Wet mix macadam (WMM)	Spreads load
Sub-base	Granular sub-base (GSB)	Drainage and further spreading
Subgrade	Compacted soil, top 500 mm	Foundation of the road

California Bearing Ratio (CBR)

The CBR test compares how the subgrade soil resists a plunger with how a standard crushed stone does.

A 50 mm plunger is pushed into a soaked sample at 1.25 mm per minute. The load at 2.5 mm and 5.0 mm penetration is compared with the standard loads:

CBR = test loadstandard load × 100

Penetration	Standard load
2.5 mm	1370 kg
5.0 mm	2055 kg

Normally the 2.5 mm value is reported. If the 5.0 mm value is higher, the test is repeated, and if it stays higher, the 5.0 mm value is used. Samples are soaked for 4 days to model the worst moisture condition.

Traffic in msa

IRC:37 counts traffic as cumulative million standard axles (msa) over the design life. A standard axle is an 80 kN single axle with dual wheels. Heavier axles do far more damage, roughly by the fourth power of the load ratio:

Damage factor ≈ ( axle load80 kN )⁴

So one 160 kN axle does about 16 times the damage of an 80 kN axle.

Putting it together

The designer finds the effective subgrade CBR (or resilient modulus) and the design traffic in msa, then picks layer thicknesses from IRC:37 so that both fatigue cracking at the bottom of the bituminous layer and rutting at the top of the subgrade stay within limits.