What do you get when lightning strikes a corn-filled, concrete silo? What a Cresco, Iowa, farmer got after his silo was struck by lightning was a silo full of spoiled corn. The spoilage was actually caused by air penetrating the silo, through cracks in the concrete caused, the farmer believed, by a lightning strike. That's where Iowa State University civil engineering professors came in. The structural engineers studied the cracks in the silo and then, back on campus, searched for information about the effects of lightning on concrete. They found very little so they conducted their own research. A graduate assistant set up slabs of concrete and "zapped" then with discharges from a generator ranging from 13,000 to 60,000 amps. Unreinforced specimens 1 1/4 inch and 3 inches thick were used for the first tests. Two types of damage were noted: inverted cones of concrete were ejected from top and bottom slab surfaces, and cracks were visible starting at the point of simulated lightning discharge. In a second test series, larger slabs of plain concrete and column-like samples with reinforcement were used. Concrete was explosively ejected from the reinforced specimens where there were longitudinal gaps between rebars.