Soviet WWII 25kg concrete Avia bomb
I'm not sure that this isn't some elaborate hoax, but there seems to be enough history from different eras that it must be true.
Concrete bombs were made between November 1941 and August 1942 in Novorossiisk, USSR, until the German Army approached and the concrete plants were moved away from the front to Georgia. Concrete casings were made for bombs up to five tons, stuffed with either explosives or chemicals.
Slate mines, also Soviet WWII weapons, cast asbestos concrete into slabs (or slates) which then were assembled into boxes and stuffed with explosives. Only the fuses were metal, so escaped mine-detectors. Slate mines were very inexpensive, but quite fragile.
Solid low-collateral damage small-dimension concrete bombs were used by the US Army in the late 1990s and again in the Iraq War, laser-guided for direct hits on specific targets. In theory, there is less collateral damage in civilian areas because there isn't the wide spread of shrapnel. Some concrete bombs are loaded with explosives; many are concrete alone, relying on speed and weight to knock out a narrow target.
A 300kg concrete bomb was dropped by a French Mirage on a Libyan tank in 2010.
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Iran's ultra-high performance concrete, UHPC, is made of sand, cement, powdered quartz and, variously, polypropylene fibres, long steel fibres, plus various metal-oxide nanoparticles. The stronger the concrete bunkers, and UHPC is seven times stronger, the larger and more penetrating must the missiles be. The larger the missiles and bombs, the larger and more reinforced the bombers must be. Right now, according to this 2012 piece in the Economist, 'Smart concrete', there are 'bombs which can tunnel through hundred of metres of rock and concrete'.
On one hand we have great chunks of concrete dropping from the sky onto tanks, on the other we have nanotechnology escalating bombing and bunkering to a scale unimaginable to civilians. The US Air Force has acquired the Guided Bomb Unit-57A/B Massive Ordnance Penetrator which weighs 15 tons and can penetrate 200' of hardened concrete. There is more semi-technical stuff here.
Someone on one of the military forums, which one is inevitably drawn into when tracking down anything at all to do with war things, commented, 'it is the 1st century meets the 21st', by which I think he meant laser-GPS-guided boulders.