# What is the “allowable number of axle-load to failure equation” used in Pavement ME Design?

How is the allowable number of axle-load to failure equation actually used in the software? Is there any documentation regarding the calculation of the tensile strains?

## 1 Answer

The allowable number of axle-load repetitions to failure, $$N_f$$ is used to calculate the incremental fatigue damage using Miner's law for each analysis interval (which is equal to one month, or two 15-day intervals if there is a freezing cycle in the month). ME pavement design procedure considers the following inputs in the damage calculation process:

• Truck traffic: Different FHWA truck classes, different axle loads, tire configurations (single, tandem, tridem and quad axles), hourly and monthly adjustment factors, etc.
• Temperature variation within the pavement structure as a function of time. This is used to calculate asphalt mixture stiffness $$E_{HMA}$$ which is used to calculate $$N_f$$.

Pavement ME calculates the horizontal tensile strain at the bottom of AC layer (for bottom-up fatigue cracking cracking) and at the AC surface (for top-down fatigue or longitudinal cracking) using JULEA (linear elastic analysis procedure). Within each damage computation interval, the strains are calculated for each $$i^{th}$$ group of axle loads (axle passes, $$n_i$$) at critical locations that are defined separately for different axle types. These strains are then used to calculate allowable load repetitions $$N_{f,i}$$ for the $$i^{th}$$ load group, and damage $$D_i = n_i / N_{f,i}$$. Cumulative damage due to all traffic is the sum of incremental damage caused by each load group in all analysis intervals.

References

1. National Cooperative Highway Research Program, “Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Part 3, Chapter 4 - Design of New & Reconstructed Flexible Pavements.” NCHRP Project 1-37A Final Report, Washington, D.C. (2004). [Contains definition of critical strain computation locations]
2. American Association of State Highway and Transportation Officials, “Mechanistic-Empirical Pavement Design Guide - A Manual of Practice”, 2nd edition, Washington D.C. (2015)