A medium crack traversing the slab either transversely or longitudinally
Medium width transverse/longitudinal intersecting cracks originating at a
slab edge.
For reinforced concrete pavements, the relevant formula is:
(8.6)
Ris the quantity of high yield reinforcement within the reinforced concrete pave-
ment slab, in millimetres per metre run. Normally, British Standard mesh is
used, with Rhaving the following three standard sizes:
R=385 mm^2 /m
R=503 mm^2 /m
R=636 mm^2 /m.
Estimates of the equivalent foundation modulus derived by Mayhew and
Harding are detailed in Table 8.2 for different subbase materials and subgrade
CBRs.
The slab thicknesses derived from Equations 8.5 and 8.6 are for concrete
pavements that, under fully trafficked conditions, have a 50% probability of
achieving the service life specified. A highway engineer may require a design
incorporating a higher probability of survival; 85% is considered generally
acceptable.
In order to increase the probability of survival, the slab thickness derived in
either of the above equations should be increased by the value indicated by the
graph in Fig. 8.10.
Ln Ln Ln Ln
LnLHS M
R
()= ()+ ()+ ()
+ ()-
4 786 3 171 0 3255
1 418 45 15
...
..
Structural Design of Pavement Thickness 243Subbase Subgrade
Upper layer Lower layer
Type Depth Mod Type Depth Mod CBR Mod Equiv.
(mm) (MPa) (mm) (mm) (%) (MPa) found
mod
Granular Capping 600 70 1.5 23 68
Type 1 150 150 350 70 2 27 65
225 150 None 5 50 89
Lean 600 70 1.5 23 261
Concrete Capping 350 70 2 27 268
(C10) 150 28 000 150 70 5 50 358
None 15 100 683
Lean 600 70 1.5 23 277
Concrete Capping 350 70 2 27 285
(C15) 150 35 000 150 70 5 50 383
None 15 100 732Table 8.2Typical equivalent moduli for foundation (Crown copyright 1987)