Officers are treating the death of the man on the M11, south of Bishop's Stortford and near Goose Lane bridge, Essex, as suspicious.
It took place between junctions seven and eight heading northbound, at about 16:50 BST on Wednesday.
The driver, who was in his 60s and from Sible Hedingham, died at the scene.
His Vauxhall Corsa van hit the central reservation barrier before leaving the road and crashing into a tree.
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Essex Police said several lines of inquiry had been ruled out following an investigation, including that the concrete had been thrown from a bridge or the roadside, or that it was falling bridge masonry.
Officers closed the bridge at Goose Lane, which crosses the motorway, to carry out forensic work.
"We continue to keep an open mind as to how it came to strike the windscreen and ended up inside the van, and whether this was the result of a deliberate act or accidental," said Det Ch Insp Martin Pasmore.
The force is still appealing for dashcam footage from motorists in the area at the time.
Police say a "brick-sized" piece of concrete which hit a van driver's windscreen before he crashed and died was not thrown from a motorway bridge.