Control of Production Variability
The standard EN 13108-21 contains a second element of FPC—namely, the control of production variability. This control is applied through the determination of the running mean deviation from the target and is described in Item A.5 of the standard. It involves monitoring systematic trends at the production stages of asphalt mixtures to prevent permanent one-sided deviations. The following parameters of asphalt mixtures are under such supervision:
• Mass percentage of material passing through the sieve D
• Mass percentage of material passing through the sieve D/2 or the characteristic coarse sieve
• Mass percentage of material passing through the 2 mm sieve
• Mass percentage of material passing through the 0.063 mm sieve
• Soluble binder content
Calculations of the mean value of deviations from the target should be conducted for each of these properties. The mean deviations calculated on an ongoing basis should be compared with admissible values given in Table A.5 of the standard (Table 14.2.).
Running mean analyses are undertaken for the following groups of mixes (clause A.2 of standard):
• Fine grained (D less than 16 mm)
• Coarse-graded (D greater than or equal to 16 mm)
• Mastic asphalt (gussasphalt) and hot rolled asphalt
The mean of the latest 32 analyses should be calculated for each of these groups. By and large, mean deviations exceeding the appropriate values in Table 14.2 indicate a group of nonconforming products (Item 7.4 of the standard applies to them). In these circumstances, suitable corrective measures should be taken; the OCL should be reduced by one level as long as the mean deviation remains outside the permissible range.