highway engineering

2. The Purpose of the Road
The main purpose of the road structure is to provide a means of
reducing the stress or pressure due to a wheel load to a value
which the ground under that structure can support.

3. The Structure of the Pavement
This usually consists of four layers of road construction material,
these being built up on the formation

4. The four layers of the road structure have been re-named since 1 January
2004 and are:
1 The sub-base, (a) assists in load-spreading, (b) assists in subsoil drainage
and (c) acts as a temporary road for construction traffic.
2 The base/road base (formerly road base), is the main load-spreading layer
of the structure.
3 The binder course (formerly base course), which supports the wearing
course and also assists in protecting the road.
4 The surface course (formerly wearing course), which (a) provides a skid-resistant
surface, (b) waterproofs the pavement and (c) withstands the direct
loading of the traffic.
The term ‘sub-grade’ is used to define the natural foundation or fill which
directly receives the loads from the pavement.

5. COMMON
DISTRESSES
ON PAVEMENTS

6. Factors affecting pavement performance
Subgrade soil
Pavement materials characteristics
Asphalt cement
Aggregate
Modifiers for asphalt cement (e.g., rubber and polymers)
Additives or stabilizing agents for aggregates (e.g., lime
and cement)
Traffic loading
Environment
Moisture; and Temperature

7. Common Pavement Distresses

8. LONGITUDINAL CRACKING
Cracks that are approximately parallel to pavement centerline and are not in the
wheel path. Longitudinal cracks are non-load associated cracks..

9. FATIGUE CRACKING
Cracks in asphalt layers that are caused by repeated traffic loadings.
The cracks indicate fatigue failure of the asphalt layer.

10. TRANSVERSE CRACKING
Cracks that are predominately perpendicular to pavement centerline .Thermal
cracking is typically in this category.

11. RUTTING
Longitudinal surface depression that develops in the wheel paths of flexible
pavement under traffic.
It may have associated transverse displacement

12. CORRUGATION
Transverse waves appear at regular intervals due to the unstable surface
course caused by stop-and-go traffic

13. SHOVING
It is generally caused by braking or accelerating vehicles, and is usually located
on hills or curves, or at intersections.

14. OVERLAY BUMPS
Cracks in old pavements were recently filled.
This problem is most prevalent on thin overlays.

15. DELAMINATION
Loss of a large area of pavement surface. Usually there is a clear separation of
the pavement surface from the layer below.
Slippage cracking may often occur as a result of poor bonding or adhesion
between layers.

16. STRIPPING
The loss of the adhesive bond between asphalt cement and aggregate, most often
caused by the presence of water in asphalt concrete, which may result in raveling,
loss of stability, and load carrying capacity of the pavement or treated base

17. PUMPING
Leaking or ejection of water and fines from beneath the pavement through cracks.

18. SEGREGATION
Separation of coarse aggregate from fine aggregate as a result of mishandling of
the mix at several points during mix production.

19. BLEEDING/FLUSHING
Excess bituminous binder occurring on the pavement surface.
May create a shiny, glass-like, reflective surface that may be tacky to the touch.
Usually found in the wheel paths.

20. ROCK LOSS
Wearing away of the pavement surface in seal coats.

21. BLEEDING / FAT SPOTS
Excess binder occurring on the surface treated pavements.
May create a shiny, glass-like, reflective appearance. Fat spots are contained
bleeding.