The chemical evolution of the Universe is largely guided by the life of massive stars. These stars are born with a convective core on the main-sequence, and are heavily influenced by additional mixing occurring at both the convective core boundary and in the radiative envelope. Such mixing transports additional hydrogen fuel from the envelope to the convective core, allowing the stars to live longer and to enhance their final helium core mass at the end of the main-sequence evolution. As a consequence, chemical mixing has a high impact on the stellar evolution of both intermediate- and high-mass stars, and is the dominant uncertainty in their stellar structure and evolution theory. Asteroseismology is a powerful tool for probing stellar interiors through the detection and interpretation of stellar oscillations. Gravity modes in particular are highly sensitive to the near-core properties of the stars. These modes form period spacing patterns, the morphology of which depends on both the amount and shape of the internal mixing as well as the near-core rotation rate of the star. We present the results of the first asteroseismic modelling of an ensemble of 26 Slowly Pulsating B-type stars with detected period spacing patterns. Their Kepler space photometry, ground-based spectroscopy, and luminosities derived from Gaia distances allowed us to size, weigh, and age the stars, as well as to derive their internal element mixing profiles. We confront our modelling results with predictions from the theory of stellar evolution for the appropriate mass range.