Pavement Materials: Soil, Aggregates and Bitumen

A pavement is only as good as its materials. GATE asks what each test measures, typical limits, and the Marshall mix design calculations.

Subgrade soil

• California Bearing Ratio (CBR): load needed to penetrate the soil with a standard plunger, as a percentage of the load on standard crushed stone (1370 kgf at 2.5 mm and 2055 kgf at 5.0 mm). The higher of the two values is used (re-test if the 5 mm value is higher).
• Plate bearing test: modulus of subgrade reaction k = p/0.125 cm (pressure at 1.25 mm settlement), for rigid pavement design. Correct for plate size: k₁a₁ = k₂a₂.

Aggregate tests

Bitumen tests

• Penetration test: depth (in 0.1 mm) a standard needle sinks in 5 s at 25 °C under 100 g. 80/100 bitumen is softer than 30/40.
• Ductility: elongation before breaking (≥ 75 cm for many grades).
• Softening point (ring and ball): temperature at which bitumen softens.
• Flash and fire point: safety during heating.
• Indian bitumen is now specified by viscosity grade (VG-10, VG-20, VG-30, VG-40; higher = stiffer).
• Emulsions (rapid, medium, slow setting) and cutbacks (bitumen with solvent).

Marshall mix design

Specimens at different bitumen contents are tested for stability and flow.

• Theoretical specific gravity G_t = 100/Σ(Wᵢ/Gᵢ).
• Air voids V_v = (G_t − G_m)/G_t × 100 (G_m = bulk specific gravity of the mix). Target about 3–5%.
• Voids in bitumen V_b = G_m × W_b/G_b.
• VMA = V_v + V_b; VFB = V_b/VMA × 100.
• Optimum bitumen content: average of the contents at maximum stability, maximum unit weight and target air voids.

Worked example

A mix has G_t = 2.50 and G_m = 2.40 with 5% bitumen (G_b = 1.02) by weight of mix.

• V_v = (2.50 − 2.40)/2.50 × 100 = 4%.
• V_b = 2.40 × 5/1.02 = 11.76%; VMA = 15.76%; VFB = 74.6%.

Practice