The heavy block smashed into the roof of the HGV on the M4 near Bridgend in South Wales.
Debris from the masonry and lorry damaged four other vehicles on the road.
South Wales Police said it was "extremely fortunate" no one was hurt.
The block was dropped from the bridge that links the A4063 Sarn bypass and Penycae Lane shortly before 8pm on Wednesday.
Officers are appealing for anyone with information to contact them on 101, or call Crimestoppers anonymously on 0800 555 111.
In December last year, police warned roads in Essex were being deliberately trapped with concrete blocks, wood and barbed wire with the intention of causing danger to motorists.
Nine incidents were reported in the area of Tiptree, southwest of Colchester, in just over a week.
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