Police said the accident happened at 8am on the outbound Rama II highway at the kilometre marker No 18+551 near the bridge over the railway in Tambon Bang Nam Chued. The steel bars blocked two lanes of the road.
Officials used a crane to remove the steel bars from the road and it took about an hour for traffic congestion to ease.
The truck driver, Boonlert Wannakit, 42, told police that he was delivering 32 tonnes of steel bars from Rayong to a buyer in Samut Sakhon. He said he noticed that the part of the road was tilting so he tried to change lanes to the right but a vehicle would not give way.
He said when the truck reached the uneven part of the road, it tilted and some 22 tonne of steel bars rolled off the truck.
No one was injured in the accident.