The increase of the effective concentration of interstitial traps with increasing Frenkel defect concentration has been investigated in pure Al samples irradiated at 55 K with 1 MeV and 3 MeV electrons. The increase of the trap concentration per Frenkel defect produced was found to be larger for the 3 MeV than for the 1 MeV irradiation. The difference is ascribed to di-interstitial formation within the small cascades produced by 3 MeV electron irradiation. For the cascades containing two to five Frenkel defects it was found that in the average about 12% of the interstitials produced within these cascades encounter upon migration and form di-interstitials.