The 1970s structure at the Queens Medical Centre in Nottingham was closed on Monday following an inspection by structural engineers Arup and the Health & Safety Executive (HSE).
"The multi-storey car park has been weakened by deterioration in the concrete’s strength," said Nottingham University Hospitals NHS Trust health and safety lead Dr Stephen Fowlie.
"After detailed specialist work over the past 48 hours which the Trust commissioned we have agreed with the structural engineers and with the HSE that the car park should close with immediate effect."
The HSE said it was copied in on a report about the condition of the car park the previous week and had agreed to attend a meeting at the hospital on 29 September to discuss the findings and proposals for action.
"However, further technical investigations carried out by the hospital meant that there was no other option but for the hospital car park to be closed with immediate effect to protect the safety
of the public and staff," an HSE spokesman said.
The four storey, 750 space structure was already scheduled to be demolished within the next year. It is unclear what has caused the deterioration in the concrete strength. Local media reports cite "concrete rot", while one expert said it was likely that the report was referring either to reinforcement corrosion caused by chlorides and salts or to alkali silica reaction which causes cracking.
He added that it was more likely to be corrosion as alkalai silica reaction is more common in Devon or Cornwall, where locally sourced aggregates are most closely associated with the problem.
Chloride attack was behind the failure of the 1960s concrete carpark at Pipers Row in Wolverhampton in 1997.
Research by the HSE found that if properly maintained, Pipers Row car park should have had a reasonable margin of strength in relation to the actual loads imposed in use (NCE 27 March/ 3 April 1997).
However, deterioration of the concrete over and around a column/slab zone led to punching shear failure of the concrete slab.
"One of the most common faults is chloride attack from deicing salts," said engineering consultant and former NCE technical editor Dave Parker.
"The concrete quality was also poor in those days. Cement strength got stronger, but to make it workable they used more water, which led to a high water to cement ratio which made it more
porous and vulnerable to voids."