“One of the worst parts of the Miami condo collapse tragedy is that it didn’t have to happen. People didn’t need to die,” says Douglas Darling, founder and chairman of Cor-Tuf. “New, available concrete technology is impervious to water and chemicals, and can be used to repair failing, traditional concrete, and to build new structures that will not fail.”
By now we all know that traditional concrete doesn’t last. The recent Miami condo collapse is terrible proof that a building, a bridge—or any other structure made out of traditional concrete—can and will fail.
As our chairman points out, though, the hard truth is that what happened in Miami didn’t have to happen. There is a better way. Here at Cor-Tuf, we are all duty-bound to make sure everyone understands why and how concrete fails, and why, if construction teams take advantage of newer concrete technologies, this kind of disaster never has to happen again.
Why and How Concrete Fails
First, let’s look at the fundamental problem. Traditional concrete is porous. Concrete absorbs the fluids, chemicals, and gases in its environment. For example, salted roads and seaside structures are an obvious danger to concrete. The chemicals leach into the road surface, reach the steel rebar inside the concrete, and the deterioration process begins. At first it is not visible, but as the rebar rusts and expands, the problems finally reach the surface and are obvious even to the naked eye.
“Concrete spalling” is the term used to describe what we see when the concrete starts to flake and chip. When the spalling is this evident, the integrity of the structure has already been compromised from within.
Engineers know this, so they tend to over-build concrete structures, with the goal of extending the life of the concrete structure despite the known deterioration properties of concrete. But this is just a band-aid; the inherent porous nature of the concrete guarantees that damage will occur. That’s the bad news.
The good news is, already-damaged concrete can be repaired with a more advanced concrete, protecting it from further damage. Even better, structures built from the start with a proven, non-porous concrete will not have these deterioration problems.
Cor-Tuf UHPC: Totally Impervious
Unlike traditional concrete, Cor-Tuf UHPC is not porous. (UHPC stands for ultra-high performance concrete.) Chemicals, fluids, and gasses cannot permeate UHPC. All the problems associated with traditional concrete simply don’t exist.
UHPC is immune to spalling, scaling, discoloration, crazing, bugholes, honeycombing, buckling, curling, efflorescence (crystallized salts that remain on the surface of the concrete after water evaporates), and pop outs (sections that pop out of a surface, usually due to freeze/thaw cycles).
Cor-Tuf UHPC was developed originally by the US Army Corps of Engineers; Cor-Tuf is the exclusive licensee of the material in the United States. Both the Corps and Cor-Tuf have conducted numerous tests on the strength and longevity of the material. One long-term test conducted by the Army Corps of Engineers involves blocks of UHPC concrete that have been sitting in the surf of the North Atlantic for 25 years.
“Samples taken of this concrete have shown no deterioration so far,” says Darling. “Chloride ion salt has not penetrated more than 5 millimeters into the surface of the material, in spite of the constantly crashing waves and somewhere in the neighborhood of 3,000 freeze/thaw cycles.” Traditional concrete starts to deteriorate after 28 freeze/thaw cycles.
Cor-Tuf UHPC doesn’t require rebar; the structural materials are steel fibers that are .008 inches in diameter and .5 inches long. Cor-Tuf UHPC uses 80 percent of the same materials as traditional concrete. But the other 20 percent includes the addition of nanomaterials, which eliminate the holes that exist in traditional concrete. These nanomaterials are so small that molecules can’t permeate them.
These nanomaterials are also not contiguous within the concrete. “In fact, they prevent continuous pathways or pores by chemically binding with their fellows, and physically blocking any ingress,” says Darling. “This is indeed one of the most beautiful aspects of the material.” Any fluid that did get in would not follow lines of weakness. If you look at traditional concrete under a microscope, you can see contiguous lines running through the concrete, which makes it easier for water and chemicals to course through the concrete and start to destroy it from within.
These differences eliminate the problems that so many concrete structures are now experiencing. The Miami condo, the Washington, DC pedestrian bridge that collapsed, and the closed I-40 Mississippi River bridge, are all recent examples. Unfortunately, they are the tip of the iceberg.
Cor-Tuf UHPC can be used to repair traditional concrete. This is done by first removing the bulk of the damaged concrete, then filling in the area with Cor-Tuf UHPC. “Fresh, uncured Cor-Tuf UHPC is extremely adhesive,” says Darling. “It bonds with incredible tenacity to the remainder of the structure made with traditional concrete. No additional adhesives or processes are necessary. All that is needed is a clean, dust-free, moistened surface.”
Cor-Tuf UHPC can also be used as a binder between panels of traditional concrete, and as a thin coating over a traditional concrete surface, which will protect the underlying concrete while making the entire structure stronger. And, of course, it can be used to create a completely new structure that is non-porous, lighter, and free from the flaws that have plagued traditional concrete for years.
Here is how Cor-Tuf UHPC stacks up against traditional concrete:
|Traditional Concrete||Cor-Tuf UHPC|
|Permeability||800 – 1300 coulombs*||44 coulombs|
|Freeze/Thaw Resistance||28 cycles||3,000+ cycles|
|Useful Life||15 – 25 years||100+ years|
|Compressive Strength||2,500 – 5,000 psi||25,000 – 50,000 psi|
|Flexural Strength||400 – 700 psi||2,000+ psi|
|Tensile Strength||300 – 700 psi||1,700 psi|
|Weight||20 – 30% lighter|
*Using ASTM C1202 and AASHTO T277 tests
Conventional Processing Methods for a Non-Conventional Concrete
UHPC has been around—and used—for decades. But until we brought the Cor-Tuf version to market, it was lacking in one important area: contractor convenience. Special methods and processes were required to use it, limiting its appeal.
“Knowing what we know about concrete and what a difference UHPC can make to all concrete structures, we’ve done everything we can to make it easy for contractors and construction workers to use Cor-Tuf UHPC,” says Darling.
Our efforts have resulted in a product that can be mixed, transported, and poured using all of the traditional concrete-processing methods. We’ve even created a mobile batch plant that prints out industry-standard “tickets” that describe how and when the concrete was made.
“Cor-Tuf UHPC uses about 80% of the materials used in traditional concrete,” says Darling. “The processes required to work with it are familiar to construction workers. Contractors can use the same construction workers and equipment they use for traditional concrete.”
We are well past the time for the construction industry to adapt to a more superior material. Forty years ago, when the Miami condo was built, UHPC technology was not available. But now it is.
It is now possible for contractors, architects, and engineers to specify a concrete that will last for more than 100 years. They can design and build concrete structures with a clear conscience. And, best of all, terrible disasters like the Miami condo collapse will never happen again.
If you’re ready to pour with a product that is superior to traditional concrete, contact Cor-Tuf today at (540) 270-9239.