Engineers warned of "major structural damage" to the concrete of the Champlain Towers South condominium in Miami, which collapsed last week leaving 10 people dead and 150 missing.

A 2018 report on the condition of the 136-unit residential building in the town of Surfside, Florida, noted that deterioration in the concrete would "expand exponentially" unless repairs were made in a "timely fashion".

Part of the 12-storey beachfront block collapsed suddenly at 1:30 am local time on Thursday 24 June.

The Champlain Towers South condominium was built in 1981 and was coming up for its 40-year recertification.

Repair work had just begun prior to collapse

Buildings in the county of Miami-Dade must be inspected 40 years after they were first built and then every 10 years after that. This system was introduced following a building collapse in 1974.

Repair work had just begun on the roof when the disaster occurred.

Surfside's building official James McGuinness was on the roof inspecting it 14 hours before the disaster ABC News reported and said there was "no inordinate amount of equipment" that could have triggered a collapse.

Structural engineers Morabito Consultants submitted a report on a field survey undertaken of Champlain Towers South on 8 October 2018.

Report warned repairs would be disruptive and expensive

"Major structural damage" was documented in the concrete slab below the pool deck.

Morabito Consultants noted in the report that the pool deck's reinforced concrete slab wasn't sloped so any water was pooling and evaporating.

"Failure to replace the waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially," warned the report.

"It is also important to note that the replacing of the existing deck waterproofing will be extremely expensive as removal of the concrete topping slab to gain access to the waterproof membrane will take time, be disruptive and create a major disturbance to the occupants of the condominium building."

Cracks found in concrete of underground garage

In the underground parking garage below there was "abundant cracking" in the columns, beams and walls. Morabito Consultants advised these be repaired in a "timely fashion".

The New York Times reported that experts had examined footage of the collapse and come up with a theory that a failure in the lowest part of the tower could have set off a "structural avalanche" as failures in the concrete spread from one initial failure.

The 2018 report also detailed flooding in some of the units caused by a hurricane and cracking along the edges of the concrete slab balconies and their soffits.

Rescue work is ongoing in Surfside

Cracks were also documented on the parapet walls of the roof along with "minor spalling"  – flaking concrete – on the walls of the stair tower.

Concrete built close to the ocean often sufferers corrosion and saltwater that leeches in can also rust steel rebar if it isn't well enough protected.

"We are deeply troubled by this building collapse," Morabito Consultants said in a statement. "[We]  are working closely with the investigating authorities to understand why the structure failed."

Residents had previously raised concerns about the construction of a Renzo Piano-designed apartment block next to Champlain Towers South. Eight Park Seven is an 18-storey condominium building on the Surfside waterfront that completed in 2019.

Teams of rescuers have been working constantly to carefully pick through the rubble to find survivors, although no one has yet been found alive. A trench 125 feet long (38 metres) and four storeys deep has been dug alongside the collapsed building to allow rescue workers access.

The main image is courtesy of the Miami-Dade Fire Rescue Department.