Page 117 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Although an adjustment for temperature variation is provided, no adjustment guideline is provided for
seasonal variation.
• The method was developed to provide adequate cover to the subgrade and does not allow for the evaluation of
the adequacy of the individual pavement layers.
• The thickness design chart is not sensitive to a RRD below 1 mm.
• The original overlay design chart was only developed for traffic loadings up to 7.3 million equivalent standard
axles and was only later extended to 50 million.
• The mathematical formulation of the RRD and pavement life relationship is provided, but not for the overlay
design chart. Only part of the method is therefore well suited to application in modern software systems.
• The Benkelman beam is becoming obsolete and the current preferred instrument for deflection surveys is the
Falling Weight Deflectometer.

Figure 48. Asphalt Institute Overlay Design Chart for Granular Base Pavements

Advantages of the AI Method Disadvantages of the AI Method
• Very simple and easy to apply • Empirical: derived from data collected in North
• Corrects deflection for temperature variation America
• Incorporates spatial variation of deflection data • Only for granular pavements with thin surfacings
• Only developed for traffic up to 7 million E80
• Insensitive to deflections less than 1 mm, which
is where many South African pavements
operates
• Outdated: based on Benkelman beam deflection

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 106