Gradual solar energetic particle (SEP) events are now believed to be associated with CME-driven shocks. As the shock propagates out from the Sun, particles are accelerated diffusively at the shock front and some will escape upstream and downstream into the interplanetary medium. This is in contrast with “impulsive” events, which are believed to be due to solar flares. However, recent observations have found that in some gradual SEP events, the time intensity profile show a two peak feature, suggesting a mixture of particles from solar flares with particles from CME-driven shock. Furthermore, the observed spectra of large SEP events show tremendous variability. The $Fe/C$ ($Fe/O$) ratio behave oppositely in events which have similar solar progenitors. In this work, we use a numerical model to follow particle acceleration and transport at CME-driven shocks. We investigate a possible scenario for the re-acceleration of flare particles by CME-driven shocks and calculate the $Fe/O$ ratio for two exemple shocks. These simulations are helpful in interpreting observations of particle data obtained in situ at 1 AU by spacecraft such as ACE and WIND.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html