Highway Engineering

244 Highway Engineering

In certain situations the pavement may not be fully trafficked as a margin of

approximately 1 m on each side of the highway may be included to enable vehi-

cles to pull over and stop. Such a margin, termed a tied shoulder, may also be

used as a cycle track. If this section of the road is built as part of the overall

slab, it will act to increase the overall effective strength of the concrete pave-

ment, for practical purposes increasing its design life. On the other hand, if the

designer has decided on a set life for the pavement, the thickness of the pave-

ment can be decreased if the tied shoulders are allowed for.

Figure 8.11 gives the amount by which the slab thickness calculated in Equa-

tions 8.5/8.6 should be lessened if tied shoulders are to be taken into account

within the overall design.

Figures 8.12 and 8.13 give final slab thickness values for jointed concrete pave-

ments using the baseline Equations 8.5 and 8.6 along with the probability and

tied shoulder allowances. The following specific assumptions have been made:

 The probability of survival for the slab over its full design life is set at 85%


 The proportion of unreinforced slabs failing,F, is set at 30%


 Two different reinforcement percentages,R,are referred to:

 zero steel (unreinforced)

 636 mm^2 /m


 Two equivalent foundation moduli, M, equal to 68 and 680 MPa, are

assumed. (The family of curves for other values of equivalent foundation

modulus can be found in Mayhew and Harding (1987).)


 The 28-day compressive strength,S, is set at a mean value of 48.2 MPa, the

target value for the C40 mix (characteristic strength value of 40 N/mm^2 ).

40

35

30

25

20

15

100 200 300

Basic slab thickness (50%

probability) in mm

Correction to basic slab

thickness in mm (plus)

Figure 8.10Positive

correction to slab

thickness to increase

probability of

survival from 50%

to 85%. (Crown

copyright 1987)