Page 75 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Measures for rectifying subgrade swell and collapse are not aimed at strengthening the pavement.
Drainage may be the indirect cause of these problems, which then needs to be rectified. A thick asphalt levelling
course can be applied to counteract undulations, after the pavement has settled. For swelling clays, the
migration of moisture into and out of the subgrade needs to be limited and moisture equilibrium maintained. In
the case of existing pavements, special treatments, such as construction of cut-off membranes to sufficient depth
on both sides of the structure, may be required.
• Post-construction compaction actually increases the structural capacity of the pavement. Normally, only a
thin levelling course is required.
(iii) Rectifying Cracking
Different remedial actions are considered, depending on the origin of cracking:
• Reflective cracking from previously stabilized layers is rectified by ripping or milling and recompacting the
treated layer. Granular overlays have also been proven effective in preventing propagation of reflection cracking.
Because reflection cracks are often not associated with structural deficiency, some authorities permit their
occurrence, but require sealing to prevent ingress of moisture as part of the routine maintenance programme.
• Fatigue cracking us rectified with an overlay of sufficient thickness; inlay (asphalt or concrete) to decrease the
deflection; or, partial reconstruction, but be careful of reflective cracking. To mitigate potential reflective
cracking, a stress absorbing membrane interlayer, typically a bitumen rubber seal, may be constructed before
proceeding with the overlay. Where secondary cracking becomes visible, patching or milling of the defected
areas are common pre-treatment requirements, before construction of an overlay for strengthening. See also
crocodile cracking.
• Crocodile cracking caused by an aged surfacing is rectified with a special surface treatment; replacing the aged
material with fresh material; or, where the extent is large, complete removal by milling.
• Surfaces with advanced cracking should be removed before an overlay is placed.

Note that when deep in situ recycling is used, the surfacing layer can be recycled in with the base material to form
the new base.

5.3.2.2 Appropriate Rehabilitation Options: Concrete Pavements

(i) Cause and Mechanism of Distress
The mode and type of distress serves as pointers towards the origin and cause of distress in pavements. The types
of distress that manifest in rigid pavements are illustrated and discussed in Chapter 14: 4.2.

The distress types in concrete pavements are:
• Cracking, including corner and mid-slab cracking, cracking close to a joint and closely spaced cracks.
• Shattered slabs
• Pumping
• Faulting

These distresses are construction, environmental, traffic and load related. Poor or erodible support under concrete
pavements leads to pumping, and later faulting and cracking under traffic loading, especially if water is allowed to
enter through cracks and poorly maintained joints.

Cracking in slabs is initiated by shrinkage of the concrete, either due to the concrete mix properties or low
temperatures. Corner and mid-slab cracking are only found in jointed concrete slabs, with curling at the corners or
between transverse joints, loss of slab support and loading at the corners or joints occurs. These cracks need to be
properly sealed to keep surface water entering the pavement structure. When surface cracking occurs, partial depth
repairs may be considered. However, a low success rate makes this a questionable approach.

Cracking close to joints normally develops as a result of poorly designed or constructed joints. Closely spaced cracks
develop in CRCP pavements where there are variations in the concrete. These cracks do not require maintenance,
but may eventually lead to punch-outs in CRCP.

If cracks are wide enough, and joints are not maintained properly, surface water enters the pavement and
accumulates between the concrete slab and the supporting subbase layer. The deflection of the slab under traffic
loading results in erosion of this supporting layer, manifesting as pumping of fines through joints and cracks.

Section 5: Pavement Investigation and Design Process
Page 64