First we need the volume. Average human height is typically 5'9", or 175.26 cm. Shoulder width is typically is 46.355 cm. Width is typically 58.42 cm for the human head; put this together and you get 474409.664082 cubic cm.
Next the compressive strength of concrete. We'll use 17 MPA as a low end, 28 as a mid end, 70 as a high end. After multiplying the volume by each end, we'll multiply by 8 j/cc since this is fragmentation.
Low end: 64519714.315152 joules, 0.0154205818152848943 tons
Mid end: 106267764.75436 joules, or 0.0253986053428202699 tons
High end: 265669411.88592 joules, or 0.0634965133570554452 tons
All ends are Small Building level. This'll do nicely in the upgrade thread.