9.4 Deflection versus pavement condition,
There is an established relationship between pavement deflection and pavement
condition as expressed by repetitive loading of it by vehicular traffic. As the
cumulative traffic increases with resulting damage to the pavement, higher
deflections will be observed as its structural condition deteriorates. Deflection
remains generally stable until the ‘investigatory condition’ is reached where
deflections will start to increase with applied loading and cracks will begin to
appear. The ‘failed condition’ is arrived at when rapid increases in deflection are
experienced.
In the UK, deflection histories for highways have been derived from the mon-
itoring of deflection for flexible pavements. Research Report 833 (Kennedy
et al., 1978a) developed performance charts relating deflection to cumulative
traffic for three different types of roadbase: granular, cement bound and bitu-
minous. The relationships had been found to be valid for subgrade CBR ranging
from 2.5 to 15. Each illustrates deflection trends from its construction to the
onset of structurally critical conditions, referred to above as the investigatory
phase, and contains four envelope curves relating to different probabilities that
the critical life of the pavement will be attained (the point at which strengthen-
ing will be required to prolong its useful life).
Critical conditions are defined by TRRL Laboratory Report 833 as cracking
confined to a single crack or extending over less than 50% of the wheel track
and/or rutting 19 mm or less. By contrast, failed conditions are defined as inter-
connected multiple cracking extending over the greater part of the width of the
wheel path and/or rutting 20 mm of more. Effective maintenance requires that
remedial work is commenced before the onset of critical conditions. Such an
approach will deliver greater value for money than delaying work until the failed
condition is reached and total reconstruction becomes the only way of prolong-
ing the highway’s useful life.
Figure 9.4 illustrates the deflection/cumulative traffic relationship for bitumi-
nous roadbases as given in TRRL Laboratory Report 833.
(Note: standard deflection relates to the equivalent deflection beam deflection
at a temperature of 20°C. If the deflection measurement is made with a deflec-
tograph it will be necessary to correct this value to a deflection equivalent to
that obtaining at a pavement temperature of 20°C (LR833)).258 Highway Engineering
Example 9.1
A pavement has a current rate of flow of 950 commercial vehicles per day
using the slow lane. It is 10 years old. The damage factor over the time since
opening is set at 0.9. The historic growth rate was estimated at 3%.
The standard deflection was measured as 50 ¥ 10 -^2 mm.
Assume a bituminous roadbase and surfacing:
Contd