Page 113 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Advantages of the FWD SN Method Disadvantages of the FWD SN Method
• Models riding quality deterioration • Empirical: derived from data collected at one
• Models available for flexible and rigid pavement site in the USA
design • Outdated: derived from data collected almost 50
• Relatively simple to apply years ago
• Principles used in the HDM IV economic analysis • Developed for foreign conditions and materials
software • For rehabilitation design only
• Uses FWD data, which is quick and relatively • For initial assessment purposes only
inexpensive to obtain

7.7 The TRRL Surface Deflection Design Method

This discussion of the TRRL surface deflection method is taken largely from Jordaan (1989). The TRRL method was
developed by the Transport and Roads Research Laboratory in England and is applicable to rehabilitation design.
The primary pavement input to the method is the peak Benkelman beam surface deflection under a dual wheel load
of
31 kN (single axle load of 62 kN) moving at creep speed, at a temperature of 20 °C 40 mm below the pavement
surface. The design standard in South Africa is, however, an 80 kN axle load. Jordaan adjusted the design charts
linearly to accommodate deflections at 80 kN. Deflections should be recorded at intervals of less than 12 metres.

The TRRL surface deflection method provides design charts to estimate remaining life and determine overlay
thickness. Although the method is based on Benkelman beam deflections, it also allows for the use of Deflectograph
deflections by converting using a single conversion relationship. FWD deflections also need to be converted to a
Benkelman beam deflections before the method can be used. It should also be noted that the conversion of
deflections measured from one deflection device to another depends on the pavement structure, materials and
moisture conditions, and therefore conversions should always be used with great care.

Charts for adjusting the deflection for variation in the pavement temperature at the time of the deflection survey to
the reference temperature of 20 °C for the “standard deflection” are provided for different pavement types. Jordaan
warns that the temperature corrections are inaccurate for pavements with weak or strong subgrades at the extremes
of the conversion ranges.

(i) Estimate Remaining Life
Charts to estimate the remaining life of an existing pavement to reach a critical (terminal) condition are available for
the following bases types:
• Granular bases
• Granular bases exhibiting self-cementing
• Bituminous bases (HMA)
• Cement-bound bases

Figure 45 shows an example of the charts, this one for a self-cementing granular base.

The charts for estimating remaining life should be used with caution on pavements with extensive surface distress.
These pavements may have low deflections, but the
surface distress leads to rapid deterioration and early
failure.
Converting Deflections from Different
The following cautions apply to the use of the chart for Devices
pavements with cement-bound bases: Converting deflections measured from one
• The performance of pavements with cemented layers deflection device to another depends on the
depends on deterioration associated with pavement structure, materials and moisture
shrinkage cracking of the cemented layer. conditions. It is advised not to convert deflections.
• The chart for estimating the remaining life of cement- However, if conversion is done, it must be done
bound pavements is only valid above 10 million with great care.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 102