Page 24 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

3.4.2 Concrete (Rigid) Pavements

The principle of pavement balance does not apply to concrete pavements. The strength of concrete is significantly
higher than that of asphalt, granular and lightly cemented layers used in flexible pavements. Concrete has a very
high modulus of elasticity, which results in great load spreading in the top layer and hence low stresses in the
underlying substructure, consisting of the subbase and subgrade. The concrete layer thus carries the majority of the
applied load, and the distribution of stresses to the lower layers is low. Concrete pavements can therefore be
constructed on poor subgrades, and generally have fewer pavement layers than flexible pavements.

3.4.3 Concrete Block Pavements
The behaviour of concrete block pavements is somewhere between the behaviour of flexible and concrete (rigid)
pavements. The blocks carry much of the load, but some load is distributed to the underlying layers. Pavement
balance should be considered for the support layers.

3.5 Pavement Behaviour
A proper understanding of basic pavement behaviour is required before attempting a pavement design. This includes
the behaviour under loading, load sensitivity, long-term pavement behaviour and materials.

3.5.1 Behaviour Under Loading

The effect of vehicle loading on a pavement is relatively small, when considering each vehicle or loading individually.
However, the cumulative effect of many such loads causes distress in the pavement. An understanding of the short
term effect of loading on a pavement provides a good background for how the cumulative affects manifest, and are
modelled.

Under the action of a moving vehicle load, the pavement deflects, and rebounds when the load has moved away.
The effect of a heavy vehicle load generally extends over an area of 1 to 2 metres from the point of loading, in all
three directions. This deflected area tends to form a circular, deflected indentation known as a deflection bowl. The
size and shape of deflections bowls vary and depend on the pavement structure, the strength and stiffness of the
materials, pavement balance, temperature and of course, the loading magnitude, duration and contact area. For
flexible pavements in a good condition, the maximum deflection is typically less than 500 microns under a standard
axle load.

The most common method of measuring pavement deflections is with the Falling Weight Deflectometer (FWD),
shown in Figure 7. The FWD measures the deflections with sensors placed on the road surface. As with most
equipment, while the FWD is a valuable tool, it has limitations, which should be considered during any analysis. A
good reference for most aspects of FWD measurements is “Guidelines for Network Level Measurement of Pavement
Deflection” (COTO, 2009, which will be republished as TMH13).

Figure 7. Falling Weight Deflectometer (FWD)

The pavement layers influence the deflection bowl. This is illustrated in Figure 8 for a simple three layer pavement
structure. The stress distribution through the pavement from the FWD loading is represented by the black curve. As
the load is distributed into the pavement, the stress distribution increases, although the intensity of the stress is

Section 3: Design Considerations
Page 13