The mishap occurred when the vehicle, weighing about 40 tonnes, came to unload the concrete mix, police said.
The driver was reversing the vehicle and it overturned and fell into the pit.
The 18-year-old assistant suffocated to death as he was caught in the concrete mix while the driver died in the impact of the vehicle crashing into the pit, police said.
The assistant was identified as Dinesh hailing from Tiruchirappalli while the 40-year-old driver, who died, was identified as Chellamuthu from Salem district,they said.
