What causes destruction in the first few seconds of a bomb explosion? When a bomb explodes (or a grenade, mine, etc.), what is it that causes the most damage in the first 1 or 2 seconds of the explosion and culminates in the destruction of nearby objects? Is it the high amounts of fire coming from the core of the reaction or is it the energy released from the explosion? 
 A: The energy released in an explosion can mainly have two forms : heat, overpressure wave.
The heat comes from the combustion and can be really high but will not be long. On humans, it can cause death, but it will be too short to cause any structure damage (steel, concrete, ...)
Overpressures come from the compression following the combustion and thermal expansion. This can reach several bar. On humans, under 0.7bar, it won't cause any real damage. But on structures, it can be very effective and destroy a lot of things.
A: The most damaging effects on Humans are usually caused by shapnel, bomb fragments or debris which has been accelerated by the explosion. Humans can survive remarkably high transient overpressures of up to 30psi in some cases. Building will sustain major damage above 2psi. However, the duration of the overpressure is a critical factor. A long duration high over pressure from a thermobaric weapon is far more damaging than the same shorter overpressure from a conventional high explosive. Here is a list of overpressure and damage
