Concrete building materials are in the news spotlight. In this Formaspace report, we take a look at the role material testing labs play in investigating concrete structural failures and what you can do to keep concrete structures safe.
The Growing Crisis of Reinforced Autoclaved Aerated Concrete (RAAC) Buildings in the UK
UK news in recent weeks has been dominated by the news that over 50 schools won’t be able to reopen in the fall term due to safety concerns that flat roofs and other key structures built from reinforced autoclaved aerated concrete (RAAC) decades ago could suddenly collapse.
RAAC, which also goes by the names “aerobar,” “aircrete,” and “bubbly” concrete, was originally pioneered in Sweden. When it was introduced in the UK in the 1950s, RAAC was considered an innovative, lightweight, and lower-cost alternative to traditional concrete for use in applications such as flat roofs until its use was discontinued in new structures during the mid-1990s.
Why has this become a crisis, critics might ask, given that from the beginning, RAAC was known to have a finite lifespan of circa 30 years?
One possible explanation is there may be confusion between the economic lifespan of a structure and the actual physical lifespan of structurally sound, safe structures.
Many buildings are said to have an economic lifespan of a certain number of years; 40 years is typical, but future retrofits can usually be updated and recertified to provide additional decades of service.
Unfortunately, structures built with RAAC appear to fall outside this common understanding of a building’s lifespan.
Material testing laboratories confirm that as the RAAC material reaches the end of its useful life, it can become inherently dangerous and subject to sudden failure, especially if there has been water intrusion or inadequate reinforcement added during construction. The risk of sudden collapse without warning is the justification for closing schools with roof structures built with RAAC.
NIST investigates the Champlain Towers South Collapse near Miami Beach
Fortunately, there do not appear to be any structures built with RAAC in the United States. However, material testing labs are called in to evaluate the soundness of existing concrete structures, and, sadly, sometimes, they are first called to the scene only after a fatal collapse occurs.
Tragedy did occur two years ago in Surfside, Florida (located just north of the Miami Beach city line), where the Champlain Towers South condominium tower partially collapsed in a matter of minutes, costing the lives of 98 residents and guests.
After the collapse, accident investigators from the National Institute of Standards and Technology at the U.S. Department of Commerce, commonly known as NIST, secured the site as rescue and recovery operations concluded to capture important evidence that could shed light on the underlying causes of the concrete tower’s partial collapse.
(Readers of our articles may recall we also wrote about NIST’s involvement in cyber security protection for businesses.)
The approach that NIST investigators take is very similar to that taken by the NTSA after an airliner crash: every relevant piece of structure that could hold evidence of points of failure was measured and documented on-site, and key items were then transported to a large hanger-like structure where NIST material scientists and accident investigators could piece together what happened.
In June 2023, NIST officials presented their preliminary report on the Champlain Towers South collapse. The report was able to eliminate several potential causes of the collapse (including an explosion, or a vehicle impacting a support column in the parking structure), but a couple dozen hypotheses remain open for investigation.
Among these, NIST investigators are focused on areas of the original design that appear to provide only half of the structural strength required by 1981 building codes, poor placement of concrete rebar in the structure, the extra weight of heavy materials added to the building since it was constructed, and photographic evidence of major cracks and concrete spalling found in the months prior to the collapse.
Material Science Research and Accident Reconstruction Points to 10 Factors that Can Help Extend the Safe, Useful Lifespan of Concrete Structures
Laboratory research by material scientists investigating past failures has revealed some regular patterns of bad design, inferior construction, and inadequate maintenance practices that can effectively shorten the useful lifespan of concrete structures – potentially resulting in sudden failures causing a disastrous loss of life.
Here are ten points to consider:
1) Avoid Construction Drawing Design Shortcomings or Consequential, Unapproved Changes during Construction
One of the concerns that NIST investigators have expressed about the Champlain Towers South collapse was whether the design captured in the architect’s original construction drawings was sufficiently robust to support the structure, as well as whether the then-current 1981 building code requirements were adequate.
Building accident investigators have also found that fatal failures in concrete structures can occur when design changes are made during construction. The concrete walkways traversing the multi-story tall Hyatt Regency in Kansas City are a useful, if tragic, example. Investigators found that a switch during construction to a series of interconnected support rods during construction (rather than the single uninterrupted rods originally specified) caused the walkways to collapse during a gala event in 1981, killing 114 people.
Dangerous design changes can also happen years after initial construction, for example. after renovations or additions to the structure. An example of this is the 1995 collapse of the Sampoong Department Store shopping mall in Seoul, South Korea, which claimed 502 lives. Accident investigators point to actions by the developer, who added multiple stories to the existing structure, which was not capable of supporting existing loads spanning across wide open areas of the department store.
(Of course, structural failures typically have more than one root cause. For example, there were significant changes made in the design of San Francisco’s Millennial Tower’s foundation that we’ll discuss that detail below in point #5.)
2) Stop Poor Construction Workmanship Practices and Use of Inappropriate or Inferior Quality Materials
NIST investigators are concerned that the concrete support piers of the Champlain Towers South building were built with inappropriate rebar connections to the poured concrete floor. In some cases, there may have been too little rebar, in other cases, the rebar may have been bunched too closely together, providing little room for the concrete to affix to the connection.
Inadequate connections can lead to a failure mode known as “punching shear” where the concrete slab held up by a column fails and falls to a lower level – leaving the column in place. (The result looks like the column has “punched” its way through the slab, hence the name.)
Another common root cause of concrete structural collapse is the use of poor materials that are not up to spec. The recent Mexico City Metro transit overpass which collapsed while a train was crossing may be a good example; investigators are concerned that the concrete material used was not strong enough. This incident in 2021 killed 26 people.
Poor design details can also lead to failure. The “Big Dig” tunnel traversing under Boston had a fatal accident when overhead panels fell onto passing vehicles, killing one passenger; researchers believe that the fasteners used were too short and the epoxy glue was insufficiently strong.
3) Increase the Design Safety Margin to Mitigate Environmental Disasters, including Windstorm, Storm Surge, Earthquake, and Fire
Over the decades, building structural engineers and material scientists have discovered new modes of failure due to earthquakes, which can cause serious damage to reinforced concrete structures that lack the necessary elasticity to absorb severe ground shaking.
The Cypress Viaduct collapse in Oakland during the 1989 Loma Prieta earthquake is a prime example of such a failure. The connections between the columns were unable to support the double-stacked roadways, causing separation a pancake-style failure, and the death of 42 people.
In response, Cal-Trans engineers have developed newer, more reinforced designs and/or have retired dangerous structures.
Despite popular assumptions to the contrary, concrete structures can also fail due to prolonged, high-temperature fires. Two recent fuel truck accidents, one in Atlanta and one in Philadelphia, which burned under high-traffic interstate highway overpasses, led to the structures being compromised and taken out of service for weeks.
Climate change is another area of concern for structural engineers who foresee a world with a higher risk of tornadoes, stronger hurricanes, hailstorms, flooding, and firestorms.
Earthquakes notwithstanding, concrete structures are, aside from prolonged fires fed by fuel trucks, relatively better at coping with wind, rain, and fast-moving wildfires than other construction methods. However, as we’ll see below, flooding that undermines foundations and incidents of water intrusion that corrode steel rebar can be very damaging to concrete structures.
4) Add Fail-Safe Measures to Meet Unexpected Overload Factors and/or Accidental Impacts
Structural engineers typically design to a load requirement and the building code standards in effect at the time of construction.
However, changes can happen over the lifetime of the structure that can lead to failure.
For example, NIST researchers investigating the Champlain Towers South building were concerned about how much weight was added to the building since it was built in 1981. A new paver surface to the pool deck was added on top of the original concrete, along with heavy concrete planters containing palms and other large landscaping plants. In individual condominiums, many owners upgraded their kitchens with heavy marble countertops or installed heavy tile floors in their units and/or on their balconies.
Over time, changes like these can go unnoticed, but the cumulative added weight can cut into the safety design margin of a structure.
Added weight may also be one of the root causes of parking garage structures, such as the recent failures in New York City. Today’s large SUVs, pickup trucks, and electric cars are much heavier than the vehicles in use when these structures were designed, and this may have contributed to the recent structural failures.
Accidents can also be responsible for concrete structure failures. In 2001, just four days before 9/11, four runaway barges hit the Queen Isabella Causeway near Corpus Christi, Texas, toppling three bridge sections and killing 8 motorists. The rebuilt bridge was more heavily reinforced, and a warning system was installed to shut the bridge in the event of a collapse.
5) Build Concrete Structures on a Solid Foundation
Concrete structures are heavier than equivalent steel structures and less flexible.
As such, they require a stable, solid foundation that keeps them level and secure.
The optimal foundation for tall concrete structures, such as skyscrapers, would be a series of pilings that go all the way down to bedrock; however, when a deep foundation reaching bedrock is not economically feasible, structural engineers can turn to alternatives, such as a buoyancy raft or mat type foundation that ‘floats’ on the ground below.
There is concern in San Francisco about the Millenium Tower, which is leaning as much as 28 inches (as measured from the top). Critics who have looked at the history of the building point to the foundation design (a concrete slab supported by 60- to 90-foot-deep concrete friction piles) as one of the underlying reasons the building is tilting. They allege this design was originally conceived for a much lighter steel building rather than the final concrete structure as built. Critics are also concerned that the excavation of a nearby subway tunnel for the San Francisco MUNI and BART transit systems may have also compromised the tower’s foundation.
Poor infill can also lead to failures. The 2013 Rana Plaza collapse in Bangladesh, which took over 1,000 lives, was partially attributed to a structurally unsound foundation that had been built over a filled-in pond. (Several stories had also been added to the original structure, which appears to have been another significant factor in the collapse.)
Finally, flash flooding due to climate change is an emerging risk to concrete structure foundations in many areas.
Rapidly moving flood waters can quickly scour out the earth underneath concrete support structures, putting them at risk of failure. This is something that structural engineers will need to address in areas that previously had little record of flooding.
6) Prevent Water Intrusion that Weakens Concrete and Steel Reinforcement
Concrete is strong in compression, but weak in tensile (pulling) strength.
To compensate for this, structural engineers typically add iron rebar to carry the tensile load.
Water intrusion is the enemy of this concrete/iron combination – so if (or more accurately, when) water enters the structure, it can cause the iron reinforcement bars to corrode, which causes them to expand, creating further damage to the surrounding concrete which will begin to crack in response. The result is a vicious circle of more water intrusion, rusting, expansion, and cracking, which results in more water intrusion, etc. The problem can be more pronounced in cold winter climates where water that permeates structures can freeze (expand) and then thaw repeatedly. Exposure to road salt (especially on bridges and other road structures or buildings in marine environments) also accelerates the corrosion process significantly, leading to rapid deterioration of the concrete due to cracking or spalling, in which layers of the concrete “shed” away.
In the case of the Champlain Towers South collapse, NIST researchers saw photographic evidence collected prior to the collapse (including those from a structural engineering company called in to investigate the building prior to its 40-year recertification) that showed significant water intrusion from the pool deck above that had seeped into the concrete support structures in the parking structure below.
7) Perform Regular Detailed Inspections and Material Testing on Existing Structures
Because concrete structures are vulnerable to incidents of shifting foundation, live load overstress, water intrusion, rebar corrosion, etc. they need regular inspection by qualified building engineers, who may elect to take core samples for further strength analysis performed by materials scientists in the laboratory.
Taking a core sample is important if a structural engineer suspects hidden damage. Several core samples were taken from the pool deck area of the Champlain Towers South building in the months before its collapse, and after investigating them in the lab, lab technicians were concerned that they indicated potential weakness in the concrete.
Other tests can be performed without cutting into the structure; these are classified as NDI, or non-destructive inspections. New tech advances, such as high-definition video cameras mounted to sophisticated drones and remote controlled robots are a boon to building inspectors, who can get a bird’s-eye view of areas that are hard to reach using conventional inspection methods.
Structural engineers also need to take into account the need for future inspections when designing structures. An overpass bridge over the Boulevard de la Concorde in Montreal, built in 1970, was considered innovative when new, but inspections were hampered because the design would have required the surface deck to have been removed to inspect the supporting structure. The bridge collapsed in 2006, killing five people.
8) Ensure Timely, Prompt Maintenance of the Structure and Mitigate any Developing Issues
Before the collapse of the Champlain Towers South building, the Florida legislature passed an important new law requiring that condominium complexes in the state be “re-certified” for occupancy forty years after initial construction.
Perhaps because of this law, we know a good deal about the deteriorating conditions of the structure, as the condominium board had engaged a local structural engineering firm to help evaluate the building as part of the recertification process. The investigators were concerned about issues such as concrete spalling and water intrusion in the below-grade parking structure as well as the condition concrete of the pool deck above the parking area.
Unfortunately, critics point out that the board was slow to react to the problems identified in the report, and some owners reportedly complained that the process to mitigate the issues was not timely enough.
Quicker action was needed to save lives.
9) See something? Do Something – Immediately! It Could Save Lives.
There are often obvious signs of impending structural failure that you should be aware of – even if you are not a trained structural engineer.
Concrete structures are not supposed to shift or move, crack open, or make unusual sounds.
If you see something, take immediate action.
In the lead-up to the Champaign Towers South collapse, there were a few precious minutes where residents who were awake in the middle of the night saw abnormal building shifting and heard unusual noises. The guards at the security station had access to a building-wide announcement system to wake residents and tell them to evacuate – but tragically, they were never trained in how to use it and were unaware of its existence.
This is not an isolated occurrence. Engineers had been monitoring the concrete pedestrian overpass at Florida International University for several days after unexpected cracking was discovered, but they did not immediately close the bridge or the highly trafficked road underneath. Five days later, a university employee heard a loud “whip cracking” sound and reported it to the investigators, but no closure was ordered. Just under five hours later, the concrete pedestrian bridge collapsed, killing six people and injuring ten.
10) Create an Orderly Plan for the Structure’s End-of-Life
There can be a lot of management and political inertia when coming to terms with a concrete structure that is near the end of its useful life.
Often, there is budgetary pressure to hold off on investing in a replacement structure – surely another maintenance band-aid can get us by for another few years, right?
There is some good news on this front, at least for public infrastructure. The Bi-Partisan Infrastructure Law passed in 2001 allocates billions of dollars toward repairing or replacing many of the 45,000 bridges rated in “poor condition.”
Replacing concrete dams that have reached the end of their useful life is an especially daunting project. Here we’d like to give a call out the Swiss for tackling the issue head on.
They recognized that the original 1932 hydroelectric Spitallamm dam that impounds Lake Grimsel, high in the Alps of the Canton of Berne, needed to be replaced due a vertical separation crack discovered on the lakeside face of the dam structure. Engineers became concerned this crack could lead to the dam collapsing in the case of a moderate earthquake.
The project takes a unique approach: the new replacement dam is being carefully constructed at the foot of the existing 1932 structure, which will eventually be flooded, causing it to disappear under the water when the new dam structure is completed sometime in 2025.
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