11 August 2021
New Technology Uses Cracked Concrete and Fungus to Preserve the Integrity of Buildings
It sounds like science fiction, but it isn’t. It’s pure science.
Concrete and Cracks 101
Far be it from us to preach about concrete and cracking to a roomful of architects and engineers, but we’d like to take a minute to outline a couple of basics for those readers who are less familiar with the intricacies.
Concrete, the most ubiquitous building material on Earth, has one unavoidable weakness: its tendency to crack. Ironically, cracking is made worse by the very thing that made concrete so popular in the first place: its compatibility with steel, and the ability of reinforced concrete to meet the requirements for an advanced building material. Not all cracks are the same, though, and to the trained eye, they’re as individually identifiable as a fingerprint. Additionally, cracks can be caused by several different factors such as deformation, hydraulic shrinkage, thermal shrinkage, swelling, and corrosion. It’s the latter we’ll be focusing on.
Concrete is highly alkaline, which actually protects steel reinforcement, ensuring that reinforced concrete is an outstandingly durable material. However, under certain conditions, the rebar may corrode, forming rust. If the volume of the rust that forms inside the concrete is greater than the volume of the initial rebar, this causes tensile stress on the concrete, which results, as one would imagine, in the cracking of that concrete. The main causes of corrosion of steel in concrete are:
- Carbonation of the concrete. The presence of moisture may start the corrosion process. Concrete carbonation is the result of an electrochemical reaction between carbon dioxide, moisture and calcium hydroxide that is present in cement, producing calcium carbonate. Calcium carbonate lowers the alkalinity of concrete from a pH of 12 or 13 to a pH of around 9. While this does harden the concrete and increase its compressive strength, at a reduced pH level the protective passivation layer (a chemical treatment creating a coating to resist corrosion) surrounding the reinforcing steel begins to break down, leaving the steel vulnerable to corrosion.
- The presence of marine salts or de-icing salts in the concrete and around the rebar. Once again, the presence of moisture starts the corrosion reaction.
The consequences of structural steel (or rebar) corrosion can be devastating.
An initial failure may have triggered a structural avalanche.
On 24 June 2021 at approximately 1:20 a.m, multiple sections of the twelve-story Champlain Towers South condo building in Surfside, Miami, sheared away from the rest of the building and collapsed within seconds. As of this writing, 95 deaths have been confirmed and approximately 14 residents are still missing. Rescue workers are currently working to recover those who are unaccounted for, hopefully bringing some closure to grieving families.
Naturally, a full-scale multi-authority investigation into the cause of the collapse is underway, but there appears to be a modicum of consensus in the early comments from various contractors, engineers and building inspectors with regards the cause.
Several experts have examined video footage captured by security cameras in the neighborhood and all have focused their attention on an area in the lower part of the condo building — either in or below the underground parking garage — where an initial failure may have triggered a structural avalanche.
Known as “progressive collapse,” the slow spread of multiple failures may have occurred for a number of reasons, such as design flaws or less stringent building codes 40 years ago (when the condo building was built). However, this progression would likely not have occurred without some signs indicating that the building needed structural attention. Close inspection of the security cam footage may be providing the initial clues as to where the trouble-area was located.
There was something very, very wrong with this situation.
According to the website, insider.com, Donald O. Dusenberry, a consulting engineer who has viewed the footage, said, “It does appear to start either at or very near the bottom of the structure. It’s not like there’s a failure high and it pancaked down.”
The structural fiber of the building was largely reinforced concrete slab, which should have provided critical strength when the concrete cured.
Charles W. Burkett, the mayor of Surfside, echoed Dusenberry’s sentiment in a statement. “It’s not at all a common occurrence to have a building fall down in America. There was something very, very wrong with this situation.”
Architects, engineers, building inspectors and permitting authorities all possess the skills to identify structural issues in a building and make professional recommendations (or referrals) for repairs and remediation that ensure the structural integrity, safety and longevity of a building.
With the technological advances available in the AEC industry today, how could this be prevented in the future?
Three years prior to the deadly collapse, a structural engineering consultant had performed a structural survey and discovered what was described as “major structural damage” to the concrete slab below the pool deck, which had erroneously been installed level, instead of sloped away from the building to allow for drainage. There was also evidence of abundant cracking and crumbling of the columns, beams and walls of the parking garage under the building and a report stating all the above was delivered to the residents association. Plans for a $12 million remediation project were underway at the time of the collapse.
As Mr. Dusenberry points out, “The corrosion of reinforcing steel identified in that report could have been a critical issue if it occurred on or near the supporting columns and was pronounced enough.” Corrosion there could have weakened the connection to the columns, potentially leading to a failure, he concluded.
Being so close to the ocean, marine salts can hasten and exacerbate corrosion of the reinforcing steel, which may result in cracking and structural failure. David Peraza, a structural engineer, said, “The same idea holds for the reinforced concrete pilings — deeply buried, vertical supports on which the entire building sat.”
In Florida, foundations for high-rise buildings need to be deep and strong to withstand wild weather and the movement of substrate that tends to be sandy. Below the sand lies the Florida aquifer, a sponge-like layer of limestone, riddled with underground rivers and water-filled caverns, and below that, eventually, you hit bedrock.
So, what could have been done about the reported cracking and the ensuing corrosion? Or, with the technological advances available in the AEC industry today, how could this be prevented in the future?
If one considers the data that is thus far available with regards the collapse of the Champlain Towers building, we know from initial reports that there was cracking; we know that there was standing water in areas below the pool deck; we know that there was exposed and visibly rusted rebar; and we know that the building was subject to the marine salts mentioned in point two above. Sadly, this turned out to be a deadly combination of factors.
One possible solution, deployed timeously, could have been the use of a type of bioconcrete, or, more specifically, self-healing or self-mending concrete
Everything It’s Cracked Up To Be
Imagine if, when you cut your finger, your body miraculously stepped in and produced new tissue and healed the cut. Oh, wait — it does! Imagine if concrete could do the same thing. Believe it or not, it can.
Bioconcrete, sometimes also known as biocement, covers two slightly different types of concrete. One is a self-healing concrete product and the other is a self-replicating concrete. Both are created through the addition of bacteria or fungi that lie dormant within the material until conditions are perfect for their reawakening. What are these perfect conditions?
Water and oxygen find their way in, which causes the fungal spores to germinate.
For self-healing concrete, it’s cracking. At the first sign of micro-cracking, water and oxygen find their way in, which causes the fungal spores to germinate, grow and deposit calcium carbonate in the fissures, which, in turn, seals the cracks. Once the cracks are filled with calcium carbonate, no further moisture or oxygen can enter the crevices, at which point the fungi releases spores and goes dormant once more, silently waiting until future cracking occurs, at which point it can repeat the performance.
The principle is fairly similar in the case of self-replicating concrete, but based on the type of bacteria incorporated in the concrete, the outcome is slightly different. In this material, the concrete acts as a growth medium for the bacteria. The bacteria biomineralize the concrete with calcium carbonate, contributing to the overall strength and durability of the concrete.
In self-replicating concrete, the bacteria react to humidity changes. They are most active and reproduce fastest in an environment with 100% humidity. A drop to only 50% humidity doesn’t actually have a huge impact on the cellular activity, but lower humidity does produce a stronger material than high humidity. As the bacteria reproduce, their biomineralization activity increases. With an annual daily average of around 73% humidity, self-replicating concrete would be kept busy in the Miami area.
The Skin That You’re In
Perhaps, had this technology been more commonly known and utilized back when the first cracks became visible at the Surfside condo building, tragedy could have been averted. A self-healing “skin” may have protected the concrete substrate and prevented the corrosion of the rebar.
As previously stated, the full scope of the concrete work required at Champlain Towers and the results of the investigation into the collapse are, as yet, incomplete, but perhaps the use of bioconcrete should be considered more frequently in the AEC industry, even if only as an outer coating or “skin” that will self-repair (much the way our own skin does), preventing moisture or marine salts from reaching the structural steel encased in the concrete.
Perhaps the use of bioconcrete should be considered for more frequent application in the AEC industry.
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