Page 17 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Provide a subgrade that adheres to the minimum subgrade strength requirement, to act as a proper support
for the structural layers. If the in situ subgrade material is of an insufficient quality, material should be imported.
• The transition from material with high shear strength and stiffness at the top of the pavement structure to
relatively lower shear strength and stiffness in the subgrade should be a gradual transition, resulting in a
balanced pavement structure (see Section 3.4.1).
• The strength potential of all layers should be maximised through proper compaction to achieve a high density,
and by keeping unbound granular layers as dry as possible. This is achieved with proper surface and subsurface
drainage design and maintenance as well as maintaining the integrity of the surfacing layer. Drainage is not
discussed in SAPEM, and is adequately covered in SANRAL’s Drainage Manual (SANRAL, 2006).

Although the subgrade cannot be improved at reasonable cost for rehabilitation projects, the principle that the
subgrade (and other weak layers in the pavement) should be outside the region of high shear stress applies equally
to new and rehabilitation design. Additional design principles apply to rehabilitation design to eliminate obvious
problems. For example:
• Repair badly cracked areas before overlay, to prevent the existing cracks reflecting through the overlay.
• Select appropriate remedial measures for areas not requiring structural strengthening.
• Maximise the use of the existing pavement structure to provide the required structural capacity.

2.2 Rigid Pavements

The principles for concrete or rigid pavement design are similar to those for flexible pavements, except that the
transition from material with high shear strength and stiffness at the top to lower shear strength and stiffness
material in the subgrade is rapid, not gradual. The primary load supporting element of a concrete pavement is the
rigid layer or concrete slab. The shear strength and stiffness of concrete is high in relation to asphalt or crushed
stone road bases, and the imposed stresses are dissipated quickly in the rigid layer. A thin layer of concrete thus
protects the subgrade in a similar way as thicker layers and combinations of asphalt, crushed stone and gravel
materials. This is illustrated in Figure 4.

Half-axle tyre loadsHalf-axle tyre loads

Concrete layer
Concrete layer Structural layersStructural layers
•High shear stresses•High shear stresses
•Medium strains•Medium strains
Subbase layerSubbase layer
Subgrade
In situ subgradeIn situ subgrade Subgrade
•Low shear stresses
•Low shear stresses
•Small strains•Small strains

Figure 4. Typical Stress Distribution in a Concrete Pavement

The essential elements of concrete pavement design are to design
the slab length, slab thickness, and subbase support type. The slab
length is important to mitigate shrinkage cracking. In Plain Jointed
Concrete Pavements (PJCP), shrinkage cracking is controlled by Subbase in Concrete Pavements
providing joints at regular and relatively short intervals. Moisture enters concrete pavements
through joints and cracks. This causes
Failures in concrete pavement generally occur at joints and cracks. erosion of the subbase, causing more
Design, therefore, focusses on joints and cracks, with the aim of vertical movement of the slab. The
ensuring proper load transfer. Dowels are often installed at joints to design of the subbase is essential to
improve load transfer across the joints and the concrete pavement is mitigate erodibility. This is generally
then referred to as a dowel jointed plain concrete pavement. See achieved by selecting good material and
Chapter 9: 12.2.2 for advice on installing dowels. The width of by stabilizing the subbase.

Section 2: Design Principles
Page 6