Page 30 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

surface with shattered slabs is the percentage of slabs or areas with two or more interconnected cracks, with
pumping possible in bad cases, and the area that needs to be repaired.

The performance of CRCPs also depends on crack spacing. If the crack spacing is less than 0.5 m, the pavement
loses stiffness, stress on the subgrade support increases, and punch-outs can develop. Areas of the pavement with
less than 0.5 m crack spacing are a high risk area for punch-outs.

Areas that exhibit faulting in excess of 5 mm are a function of the erosion and pumping of the subbase, as well as
the deflection at the joints or cracks under traffic loading, which is dependent on the effectiveness of load transfer
between the slabs.

The long term behaviour of concrete pavements is different for plain jointed, jointed reinforced or continuously
reinforced concrete pavements. The long term changes in the functional and structural indicators of rigid pavements
are shown schematically in Figure 14.

Riding quality

Legend: 
Plain jointed
Dowel jointed
Shrinkage Continously reinforced
cracking
Punch-outs

Fatigue cracking Shattered slabs


T Time, traffic
Time, trafficime, traffic

Figure 14. Long-Term Changes in Functional and Structural Indicators of Rigid Pavements

3.5.3.3 Concrete Block Pavements
Under traffic, block pavements tend to gradually accumulate rutting. In this respect, the performance of block
paving is similar to that of conventional flexible pavements (refer to rut depth in Figure 11).

The blocks spread concentrated loads over a wide area of earthworks layers. This means that blocks do not merely
act as a wearing course, but also as a load bearing course. The blocks have significant structural capacity when
properly installed. The blocks themselves are generally hardly affected by high surface stresses. However, wear or
abrasion of the blocks has been observed in some applications.

The performance of block pavements is highly dependent on interlocking of blocks, joints between blocks, the
bedding sand and the stabilized subbase. The sand used in the joints should be of high quality, preferably crushed
material, and impermeable. The bedding sand should be continuously graded, but permeable crushed material. A
stabilized subbase is preferred, because of increased support to the bedding sand, its structural stiffness, and its
protection of the sub-layers. All these aspects are discussed in more detail in Chapter 9: 13.

Under traffic, concrete block pavements tend to stiffen, provided the blocks are “locked” in between curbs or beams
on the edges to prevent widening of the joints between the blocks. This leads to the pavements achieving a quasi-
equilibrium or ‘lockup’ condition, beyond which no further deformation occurs. Often the increase in stiffness in the
block layer that accompanies lockup is substantial. After lockup, it may be possible to increase the loads applied to
the pavement without causing damage. The development of lockup is contingent upon careful control of
construction standards and layer works quality. For example, subbase layers of low bearing capacity do not permit
the development of interlock during the early life of the pavement. Where conditions are favourable for achieving
interlock, it can be allowed to develop gradually under traffic or may be more rapidly induced by proof-rolling.

Section 3: Design Considerations
Page 19