Highway Engineering

202 Highway Engineering

ment at opening is predicted to be less than 5 million standard axles, a Type 2

material can be utilised. A standard axle is defined as 80 kN with the cumula-

tive number normally expressed in millions of standard axles (or msa). For all

unbound granular subbases, the CBR must be a minimum of 30%.

Type 1 is seen as the most suitable both because of its free draining nature

and the high degree of interlock it helps to develop between the aggregate par-

ticles. It is, however, more expensive. It can be crushed rock or slag or concrete

with up to 12.5% by mass passing the 5 mm sieve and the fraction passing the

425 micron sieve being non-plastic. Type 2 can be sand, gravel, crushed rock or

slag or concrete with the fraction passing the 425 micron sieve having a plastic-

ity index (PI) of less than 6.

Table 7.5 gives the grading for both Type 1 and Type 2 granular materials

(Specification for Highway Works, 1998).

Percentage passing by mass

BS sieve size Type 1 subbase Type 2 subbase

75 mm 100 100

37.5 mm 85–100 85–100

20 mm 60–100 60–100

10 mm 40–70 40–100

5mm 25–45 25–85

0.6 mm 8–22 8–45

0.075 mm 0–10 0–10

Table 7.5Grading

requirements of subbase

materials for use within

flexible pavements in the

UK

For rigid-type pavements, a cemented subbase is required to minimise the risk

of water penetrating the slab joints and cracks and thereby weakening the

subbase itself. An impermeable membrane should be placed over the subbase

prior to the construction of the upper layers of the pavement. Ideally, strong

cement bound material (CBM3) should be used, unless the design traffic loading

at opening is less than 12 msa, where weak cement bound material (CBM2)

becomes permissible.

It should be noted that weak cement bound materials (CBM1 or CBM2) can

be used as subbases for flexible type pavements. CBM3, CBM4 and CBM5 are

high quality materials prepared in most cases at a central plant from batched

quantities of material such as crushed rock or gravel. CBM2 is usually processed

from sand/gravel or crushed rock while CBM1 may include unprocessed gran-

ular soils. Both CBM1 and CBM 2 can be mixed in situ rather than at a central

plant.

Typically, strengths of 7 N/mm^2 are required for cement bound materials.

Lean-mix concrete can also be used as subbase material for concrete pavements.

A dry lean-mix concrete would typically have a strength of 10 N/mm^2.

Table 7.6 gives the grading for cement bound materials CBM1, CBM2 and

CBM3/4/5 as well as for a typical dry lean concrete (Specification for Highway

Works, 1998)