Question

A single surface dressing job on a 2-lane 1 kilometer road uses 14.4 metric tons of uniformly graded aggregate as shown in Table 1.

TABLE 1: AGGREGATE GRADATION AND SURFACE AREA FACTORS

Sieve Sive (mm)	% retained	Surface area factors , (SAF) (m2/kg)
6.00	0	0.41
4.72	80	0.41
2.36	15	0.82
1.18	5	1.64

Estimate the bitumen film thickness for the surface dressing, given;

1. Weight of bituminous spray = 1.2 kg/m²
2. Width of one lane =3.0 meter
3. Density of bitumen, = 1020 kg/m³
4. Density of aggregate, = 2600 kg/m³

Based on the above answer, give your comments on the durability of single surface dressing compared to dense graded asphalt.

Answer 1

Ans ) We know, bitumen film thickness(T) ,

T = V_b / (S.A x W)

where, V_b = effective volume of bitumen

SA =specific surface area of aggregate

W = total weight of aggregate

Also, SA = 0.01 PP_i xSF_i

Where, PP_i = percent of aggregate passing i^th sieve

SF_i = surface area factor of aggregate passing i^th sieve (m²/kg)

Therefore, SA = 0.01[(1 x 0.41) + (0.2 x0.41) +(0.85x0.82) + (0.95x1.64)]

= 0.0275

Also, Density of bitumen= mass of bitumen/ Volume of bitumen

mass of bitumen = 1.2 kg/m² x area

= 1.2 x 1000 x 6 ( area = length (1000m) x width(2x3m ) for 2 lane)

= 7200m²

Hence, volume of bitumen = 7200/1020

= 7.06 m³

Putting values ,

Therefore, T = 7.06/0.0275 x 14400

= 0.01782 m or 1.782 cm or 17.82 mm