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The structural performance of the pavement can be defined in terms of the percentage of the road with

 

•shattered slabs, SH

•faulting, FA (more than 5 mm)

•pumping, PA

•crack spacing, X (in the case of continuously reinforced pavements)

 

The percentage of the road surface with shattered slabs is the percentage of the slab area that has cracked and is pumping. The percentage of shattered slabs depends on the damage constant a and the damage exponent b (8) .

 

Researchers and practitioners have reported values for the damage exponent that vary from 3.2 (11) to 8.4 (12). These different performance curves are discussed in a paper (8). A limited evaluation of South African pavements has shown that the damage exponent probably varies between 4.1 and 5.0 with a value of 4.52 being the best estimate. The user should please realize that the value of damage exponent he or she chooses is also used in the calculation of E80s, which then appear on the Facts and Axle Load pages. Also, a change in the value of damage exponent b requires a change in the value of the damage constant a.

 

The area of the pavement that suffers from faulting in excess of 5 mm is a function of erosion and pumping of the subbase as well as the deflection at the joints or cracks under traffic loading. If the user wants to ensure that deflection at a joint or crack (as indicator of a risk of pumping and also faulting) is within acceptable limits, the calculated deflection value can be compared with the following suggested limits.

These suggested limits have also been incorporated into the program to calculate the potential area of pumping. The area with pumping therefore is a function of the deflection and drainage of the pavement, the erosion potential of the subbase and the moisture environment in which the pavement has to perform.

 

If crack spacing is greater than 3 m, cracks become wide enough to allow water through and load transfer at cracks also decreases. Again this leads to damage of the slab support system and failure. The optimum crack spacing is between 1.5 m and 2.0 m for continuously reinforced concrete (CR), and less than 0.5 m for ultra-thin continuously reinforced concrete (UT).

 

An important note: Please remember that the deflection reported by cncPAVE results from the applied distribution of axle loads - see the Axle Loads Page. Should you wish to simulate the deflection under a single load, e.g. the 40 kN usually applied by the FWD, do the following:

 

Go to the Axle Loads Page. Set the k-factor to 1.0. Click the Other traffic situation radio button. Enter 0 into the field for all axle load frequencies. Enter 34 into the field for 7.5000 t and enter 66 into the field for 8.5000 t. Save all changes. Check that the value of E80/ax printed in the graph's footnote is 1. After this, click the tab of the Control Page and run the process.