Page 76 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

In a jointed concrete pavement, erosion of the supporting layer not only results in pumping, but also causes
displacement of subbase fines from the slab downstream of the transverse joint towards the upstream side of the
joint. Accumulation of fines below the upstream slab causes it to rise, whereas the downstream slab goes down due
to the void. This results in a step at the transverse joint, also called faulting. Usually the void that develops under
the downstream side of the transverse joint results in transverse cracking about 2 metres away from the joints.
Faulting affects the riding quality of the road, and is rectified by grouting the voids underneath the slab followed by
grinding down the step at the joints.

Structural failures under traffic loading usually occur at, or close to, transverse joints or shrinkage cracks. This is
because of a break in the uniformly constructed pavement, a decrease in the ability of the pavement to transfer the
load across the joint/crack, and, thus an increase in stress in the concrete slab. The design philosophy therefore is
to ensure proper load transfer at the joint/crack under moving traffic loading. This capability to transfer loads across
the joint/crack is defined in terms of relative vertical movement as the load moves across the joint/crack. This is
measured by moving a wheel load across the joint/crack, or by using the FWD. With the FWD, the difference in
deflection between the loaded slab and the adjoining slab on the other side of the joint/crack is measured. The
calculated relative vertical movement is used in cncPAVE as an input into the calculation of tensile stress. The
measured relative vertical movement on existing pavements can be used to design overlays. As a guide, the relative
vertical movement suggests the following rehabilitation options:
• Relative vertical movement < 0.09 mm: A combination of asphalt layers with modified binders is successful.
• Relative vertical movement > 0.10 mm: Reinforced concrete pavements with or without steel fibres are
recommended to achieve an acceptable life expectancy.
• Relative vertical movement > 0.20 mm: Methods such as crack-and-seat, installing dowel bars, grouting at
joints/cracks, or, slab repairs to improve the relative vertical movement, should be considered before overlaying
the old concrete slab.

Punch-outs, i.e., loose blocks about 300 mm x 300 mm in size, are only found in CRCP. Punch-outs develop where
transverse shrinkage cracks are relatively wide and are closely spaced (load transfer at these cracks is limited),
pumping is occurring and longitudinal cracks occur between these transverse cracks.

Once cracking of the slab becomes excessive, resulting in loose blocks, punch-outs and a loss in riding quality, it is
defined as a shattered slab, which then requires a full depth repair or even slab replacements. Methods for this are
described in the Concrete Road Construction Manual (C & CI, 2008).

Overlays using asphalt or concrete are also considered as a long term rehabilitation option. To avoid reflection
cracking, the type of asphalt mix must be carefully selected to resist the relative vertical movement at joints and
cracks.

Before overlays are placed, and as routine maintenance, severely cracked or shattered slabs should be replaced,
cracks should be cleaned out (routed) and resealed with silicone or bitumen rubber.

5.3.2.3 Appropriate Rehabilitation Options: Block Pavements
Block pavements can exhibit deformation as in flexible pavements. Cracking of the concrete blocks can occur which
generally indicates a strength problem of the concrete used for the manufacturing of the blocks. Should the block
pavement need to be repaired, the blocks can be removed, the support reinstated and then the same blocks relaid,
provided the blocks are undamaged.

5.3.2.4 Appropriate Rehabilitation Options: Composite Pavements
Composite pavements are pavements that are rehabilitated by using concrete and bituminous materials
together. For example, concrete overlays can be constructed on flexible pavements. This is often referred to as
white-topping. A special case is an Ultra-Thin Continuously Reinforced Concrete Pavement (UTCRCP), which is at
least 50 mm thick and constructed on an existing flexible pavement (see Chapter 9: 12.2.2). Full or partial depth
concrete inlays are constructed in the slow lanes of flexible pavements, where the heavy traffic has increased to such
an extent that flexible pavements are no longer cost effective. The fast lanes of heavily trafficked roads can be
constructed as a flexible pavement and the middle and slow lanes, or only the slow lane, in concrete, to cater for the
heavier loads.

Concrete roads with insufficient texture depth, or where the riding quality is unacceptably low, are repaired by
regrinding to restore the skid resistance and riding quality. However, this type of restoration requires expensive
specialised equipment and, if it is not required on large scale, is likely to be unfeasible. Pavements which have been
heavily repaired, or show severe faulting of joints and cracks, are overlaid with asphalt or concrete. Where new

Section 5: Pavement Investigation and Design Process
Page 65