[leonpoi]

what might be useful is (from a source book that I can't remember, plus some of my own interpreation) the following roughly applies:

Penetration needed to take down bridges
Wood: 22-28
Stone: 45-55
Metal: 55-70

the higher range will take down the bridge on a success, the lower range will on an outstanding success.

Now I'm rusty on DP and Pen. but taking reo as metal we might decide we need 70 Pen (strictly according to the rulebook this would give 7x250mm reinforced concrete penetration). if the charge was somehow tamped then the Pen is doubled -> Pen 35 worth of explosives -> 96 DP. A 1 kg block of plastic is 6 DP -> 16 kg of plastic. Who knows if this is realistic, but ignorance is bliss?? using the min amount if 55/2 -> about 60 DP -> 10 kg. So 10kg to damage and maybe destroy, 16kg to destroy. The sourcebook stated that a damaged bridge will collapse if a vehicle >10 ton (or maybe it was 8, I can't remember) is driven over it (high chance of collapse, about 80%. A construction/CVE test to determine how much it can hold), or if damaged again. If you can't tamp then the DP to destroy is 392 -> 65kg, so I hope that you can rationalise a method for doing so.

I'm guessing that this amount would be needed on each pylon if you really wanted to make a mess.