Page 74 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

under a single contract. Consideration should therefore be given to combine
uniform sections in the design and documentation stage to ensure
constructability. Short Uniform Sections

5.3.2.1 Appropriate Rehabilitation Options: Flexible Pavements From a practical point of view,
it is not advisable to have
Before suggesting an appropriate rehabilitation option, typical causes and uniform sections with short
mechanisms of distress are discussed. lengths of slightly different
designs to be constructed
(i) Cause and Mechanism of Distress under a single contract.
The mode of distress, cracking or deformation, serves as pointers towards the
origin and cause of distress in pavements. The types of distress that manifest in flexible pavements are illustrated
and discussed in Chapter 14: 4.1.

Deformation in flexible pavements results from the following:
• Deformation caused by surfacing inadequacies may be identified by a poor rut-deflection correlation, and the
DCP indicating a sound base/subbase combination.
• Deformation caused by a poor base and/or subbase normally exhibits a narrow, deep rut with heaving next
to the rut. The DCP should also indicate a poor base and/or subbase quality.
• Deformation of the subgrade is normally characterised by a wide rut and a better rut-deflection correlation.
Although deformation occurs in the subgrade, it may not necessarily indicate a poor subgrade quality and may be
the result of inadequate protection or cover by the base/subbase layers.
• Post-construction compaction is normally characterised by the absence of heaving next to the rut. The width
of the rut may depend on the depth at which the post-construction compaction occurred. DCP tests done in and
between the wheel-paths should indicate an increased strength in the wheel-paths.
• Subgrade swell and collapse normally take the form of transverse undulations spaced along the length of the
road, and are uniform across the width of the road. Knowledge on the geology of the subgrade also assists in
determining the cause of distress as either swell of a clay subgrade or collapse of a sandy subgrade.

Cracking in flexible pavements typically results from the following:
• Crocodile cracking. May be confined to a region close to the wheel-tracks with an associated rut. High
deflections with a small radius of curvature (from deflection measurements) may point towards fatigue cracking.
A soft base layer may also cause fatigue cracking, which may be identified with DCP testing or radius of curvature
measurements.
• Surface cracking from aged, brittle asphalt surfacing. In this case, the cracking is distributed over the full
width of the pavement and is not traffic associated. Rutting is not a prerequisite. The closing of these cracks in
the wheel-tracks is an indication that the crack may be limited to the surfacing only.
• Block cracking associated with shrinkage of treated/stabilized materials has a characteristic pattern, which is
not confined to the wheel-tracks.
• Longitudinal cracking that is not confined to the wheel-tracks is indicative of construction joints or
embankment instability.
• Transverse cracking is symptomatic of temperature associated distress, and can initiate structural problems if
left unsealed.

(ii) Rectifying Deformation
Different remedial actions need to be considered, depending on the origin of deformation. Surfacing deformation is
rectified by recycling or replacing the surfacing.
• A thick overlay may not be the most appropriate rehabilitation option for rectifying deformation originating from
a poor base or subbase. The recycling of the base layer material, with the
addition of low percentages of bituminous or cementitious binder should be
considered. The deep, in situ recycling of base and subbase layers with the
addition of low percentages of cement, emulsion or foamed bitumen is Measurement Types
popular in South Africa. Concrete inlays and overlays can also be and Associated Criteria
considered. Chapter 6: Road Prism and
• Subgrade deformation due to insufficient cover is rectified by Pavement Investigation,
reconstructing the base and subbase combination to form a reworked Section 7, contains discussion
subbase, followed by the addition of a new base or a thick overlay. on all measurement types and
the associated criteria.

Section 5: Pavement Investigation and Design Process
Page 63