Page 41 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.1.3 Standard Axles and Equivalent Standard Axles

The standard axle load concept originated from the AASHO
road test (Highway Research Board, 1961 and discussed in
Chapter 1: 2.2). It is based on the principle that any load may Standard Axle Load
be converted to an equivalent number of standard axle loads,
based on the damage done by the load in relation to the The standard axle load for South Africa is an
damage done by a standard load. Results from the AASHO test 80 kN dual-wheel, single axle load. Any other
indicated that the amount of pavement damage is not load P may be converted to its equivalent
proportional to the axle load, but increases according to a number of standard axles (E80s) based on the
power law. In South Africa the standard axle load is an 80 kN damage done by load P in relation to the
single axle load with a dual wheel configuration. The centres of damage done by the standard load.
the wheels are 350 mm apart. The standard axle load bears no Equation (1) is used to calculate the relative
relation to the permissible axle loads, but is a design standard. damage, with a value of 4 or 4.2 for n.

Load sensitivity is generally expressed by the damage law, often called the fourth power law. The load equivalency
th
factor (LEF) of load P is calculated using Equation (1), which is known as the power damage law, or the 4 power
law. The unit of the LEF is equivalent standard axles (ESA) or E80s. The abbreviation MESA is often used for million
equivalent standard axles. A load equivalency factor of 10 ESAs for load P therefore indicates that load P does
damage equivalent to the damage done by 10 80 kN standard axles.

Structural capacity for reference load P n
Structural capacity for load P = LEF = � 80 � (1)

where LEF = Load Equivalency Factor (E80)
P = Any axle load for which the load equivalency is required (kN)
80 = Reference axle load, typically 80 kN (standard axle)
n = damage exponent

The damage exponent n was originally determined as 4.2 from the AASHO Road Test, although a value of 4 is often
used. Heavy Vehicle Simulator testing in South Africa has shown that depending on the pavement type, pavement
balance and distress mechanism, n may vary from 2 to 6, as shown in Table 10.

Table 10. Suggested Values for the Relative Damage
Exponent (TRH4)
Base/Subbase Combination Range of Values
(Recommended Value)
Granular/granular 3 – 6 (4)
Granular/cemented 2 – 4 (3)
Cemented/granular
pre-cracked 4 – 10 (5)
post-cracked 3 – 6 (5)
Cemented/cemented
pre-cracked 3 – 6 (4 –5)
post-cracked 2 – 5 (4 – 5)
BSM/granular 2 – 6 (4)
Hot mix asphalt/cemented 2 – 5 (4)
Concrete (4.5)

By applying the power damage law, the axle load histogram for mixed traffic can be converted to an axle load
histogram consisting of only 80 kN axle loads with a frequency equal to the equivalent number of standard axles, as
illustrated in Figure 18. This value for the equivalent standard axles is now used
to calculate the daily equivalent traffic parameters (Section 4.2).
Standard Axle &
Legal Axle Loads
In South Africa, an 80 kN
dual wheel axle configuration
constitutes a standard axle.
This is below the legal axle
load limit of 88 kN.

Section 4: Design Traffic Estimation
Page 30