Pavement performance criteria

The pavement performance criterion, which is taken into account in this methodology, is the pavement serviceability level. 

Pavement serviceability is defined as the ability of the pavement to serve the user/traffic. This is expressed with the PSI, which obtains values ranging from 0 (impossible road) to 5 (perfect road). Since an impossible road cannot be travelled on by a user and a perfect road is almost impossible to be constructed, the terms terminal serviceability index (pj and initial serviceability index (p ) have been introduced. 

The terminal serviceability index (pj expresses the lowest tolerable serviceability level before pavement resurfacing, rehabilitation or even reconstruction becomes necessary. The terminal index value is related to everything that is characterised as tolerable level by the user, depending on the road significance. AASHTO suggests a value equal to 2.5 or higher for highways or main road arteries and 2.0 for all other cases. 

The initial serviceability index (po) expresses the initial serviceability index when pavement opens to traffic. The maximum value of this index is 5.0, but given that is impossible to have an absolutely perfect construction, a lower value should always be used. The values observed at the AASHTO road test were 4.2 for flexible pavements and 4.5 for rigid pavements. 

Having established the p and p values, the change in PSI (APSI) is determined by the following equation  

---

Pavement performance criteria related to the user belong to the second category. These criteria are expressed in terms of initial PSI, p and terminal PSI, p. ... 

In contrast to all other design methodologies for flexible pavements, the AASHTO methodology uses a pavement performance criterion, which, apart from cracking and subgrade deformation, includes other parameters affecting the performance of the pavement. This criterion is called serviceability, expressed as present serviceability index (PSI). 

No precise correlation is available between the immersion-compression test and results to be expected from pavements in the field. The test provides comparative guidance on water damage susceptibility. For the diabase aggregate, the Sulphlex-233 mixtures measured dry, and after immersign in the case of wet specimens, exceed the 300 psi (2.07 x 10 pascal) minimum compressive strength usually considered necessary for acceptable mixture performance in the field. Retained strengths generally fall below the 70 percent criterion considered acceptable for asphaltic mixtures. 

---