## 4 Discussion  
At the end of HWD tests, the ACN/PCN method was used to evaluate the load bearing capacity of the tested materials [43; 44]. It is a system of rating airport pavements designated by the International Civil Aviation Organization [45] to compare the airport pavement strength (Pavement Classification Number, PCN) to the operation conditions of the traffic mix considering its gross weight and the subgrade bearing capacity (Aircraft Classification Number, ACN). The ACN value is twice the derived single-wheel load expressed in thousands of kilograms, with single-wheel tire pressure standardized at 1.25 MPa, which requires the same pavement thickness required by the examined aircraft. The calculation methods of ACN consider for flexible pavements 10,000 coverages and for rigid pavements 2.75 MPa concrete working stress [44]. Both values are representative of typical airport pavements.  
Two criteria allow the evaluation of PCN: the \"Using\" aircraft or the \"Technical\" evaluation methods [44]. The first one is based on the experience, the second one on analytical procedures. In this study, the technical method proposed by the software Elmod(r) 6.1.75 has been used to calculate the PCN value. It considers the elastic modulus of layers (evaluated by mean back calculation), the configuration of the main gear of the design aircraft, and the number of coverages during the service life.  
An airport pavement is verified when Equation (1) is satisfied:  
\[\text{PCN}\geq\text{ACN}. \tag{1}\]  
Data obtained in the HWD tests allowed the calculation of the PCN, using the software Dynatest Elmod(r) 6.1.75 [40]: different values were calculated for B3, M1, and M4 test fields at different maturity ages and for three traffic levels. 100, 1000 and 10,000 coverages during the service life were considered to simulate expedient, temporary, and permanent repairs according to the standard STANAG 7208 [36].  
In the study, the subgrade under the test fields (Figure 11) was classified as \"C\" (low strength), therefore the ACN of the reference aircraft is 35.  
Tables 16-18 list PCN values obtained for test field respectively with B3, M1 and M4 materials: the red cells indicate PCN\(<\)ACN, while green ones PCN \(\geq\) ACN.  
The results reveal M4 has the most rapid PCN evolution: after 6 h maturity time, the verification ACN/PCN is satisfied. B3 and M1 ensure comparable results, but their reopening time (when Equation (1) is satisfied) is longer (7.5 days) than that of M4.  
At the end of the study, an economic analysis has been carried out to estimate the unit costs for repairing a runway discontinuity [46; 47]. Unit costs derive from the official price list used by the Italian Air Force (internal document) (Rome, Italy). Table 19 lists the results of the economic analysis.  
According to the results listed in Table 19, M4 is the material that best suits needs for RRR: its technical and economic characteristics balance the conflicting objectives of budget and performance. Using M4, the airport could be reopened 3 h after the conclusion of the emergency repair (100 coverages during the service life), while 6 h after the RRR works it is possible to achieve a permanent repair, which allow 10,000 coverages during the service life.