Determination of the penetration value of bitumens by near infrared spectroscopy

The penetration test is widely used to characterize bitumens and also to define bitumen grade in many countries. The combined use of near-infrared (NIR) spectroscopy and multivariate calibration allows penetration in bitumens to be estimated. A direct correlation between the penetration value and the NIR spectrum, however, does not afford the construction of a proper model and exposes a non-linearity in the system. In order to correct such non-linearity, the penetration value must be transformed using a non-linear function (the natural logarithm). The final penetration value is obtained by inverting the non-linear transformation. In order to improve the results, samples from different geographical origins also require different calibration models, according to the spectral similarities. The consistency between the calculated and reference values testifies to the goodness of the proposed method.