You will want to try Shatter. The timing of the explosion is based on the position and size of Force 1. By default the explosive force fills most of the frame so as soon as you apply shatter the layer starts to break up. Change the position or the size of the force and you can change when the explosion happens. The size and shape of the particles can be changed and you can even use AE''s comp camera to move around through the exploding pieces.
If you type Shatter in the Search Help field at the top right corner of AE you'll find links to some training on the effect that should get you started.
very good thanks a lot
Yes, as Rick suggested, the Shatter effect is what you need.
Here is another video tutorial I did using shatter that does not seem to show up in the search results of Rick's link: