From a pavement wear perspective vehicle mass is often expressed in terms of the number of ESA loads.

ESAs are a measure of the relative pavement wear of the load of individual axles or axle groups compared to a standard reference load. An example of a standard axle load in Australia is the reference load of 8.15 tonnes on a single axle using dual tyres (see table below).

The NESA for a particular load and axle configuration is defined as the number of passes of the standard axle load which could cause the same wear on a pavement as a single pass of the load and axle configuration in question (Austroads 2013).

To calculate the NESA, first assume that vertical deflection of the pavement is used as the criterion of damage. It is common to employ a fourth power law to relate the deflection and hence damage to the fourth power of the applied load. Let Pa be the load on the axle configuration in question, and PESA be the standard equivalent axle load (= 8.15 tonne for a single axle, dual tyre combination). Then one pass of a load of Pa on the axle configuration in question produces the same deflection as NESA passes of the standard axle load.

The equation is therefore:

N_{ESA}X (P_{ESA})^{4}= 1 x (P_{a})^{4}

or

N_{ESA }= (P_{a}/P_{ESA})^{4}

For example, the number of ESAs due to a 10 tonne load on a single axle, dual tyre combination is (10/8.15)4 or 2.27 for that axle group.

Other reference loads for different tyre-axle combinations are shown in the table below to achieve the equivalent damage as the standard axle.

##### Table: Reference loads for different tyre-axle combinations

No. of axles | Single | Single | Twin | Tandem | Tri |

No. of tyres | Single | Dual | Single | Dual | Dual |

*Reference load kN | 53 | 80 | 90 | 135 | 181 |

(tonne) | (5.40) | (8.15) | (9.17) | (13.8) | (18.5) |

* Reference load on a single axle or an axle group (Austroads 2006).

**Further reading**

Austroads 2013, Guide to Traffic Management Part 3: Traffic Studies and Analysis, Appendix G, AGTM03-13, Austroads, Sydney, NSW.

Austroads 2006, Correlation of WIM data with classified counts, an initial analysis, AP-T61-06, Austroads, Sydney, NSW.