Two thousand pounds, David Hambling explains (in Swarm Troopers), has long been the standard bomb size deemed necessary for destroying structures made of reinforced concrete:
If you lay explosives by hand, about two hundred pounds of well-placed C4 will do the job of a two-thousand pound bomb.
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The M150 Penetrating Augmented Munition (PAM) is a portable demolition device weighing just forty-two pounds. First introduced in 1998, it is highly effective against reinforced concrete structures. When triggered it goes through a complex, four-stage firing process that is like a speeded-up version of the miner’s approach of drilling and blasting.
PAM’s first charge punches a tunnel deep into the target. The subsequent stages cut through any steel reinforcing bars, propel a powerful explosive charge into the tunnel, and detonate it. Concrete is strong in compression, but weak in tension. It is almost impossible to crush a concrete block, but comparatively easy to tear it apart from inside. That’s how PAM can replace a two-hundred pound charge of C-4, or the warhead on a 2,000-pound bomb, and demolish a reinforced concrete structure such as a bridge support measuring fifteen feet by five by six. It would take a team of seven about three hours to rig a target with C4 explosives for demolition, whereas with PAM, the same process takes about two man-minutes.