Concrete Strength: Bond and Particle Behavior in UHPC

What makes a good building material? In our opinion, it must be strong and durable; light, with minimum design constraints; easily installed and compatible with common techniques; and long-lasting, created from the start to protect against damage. Cor-Tuf Ultra High-Performance Concrete (UHPC) has all of these qualities, by design.

These high-strength and contractor-friendly properties of our UHPC have people taking notice. But perhaps a less-understood phenomenon is the ability of the material’s matrix to essentially level itself during construction. Bond behavior, particle interactions, and void space (or lack thereof) enable a process by which the UHPC cures in a practically perfect form. Chemical and water intrusion are halted, and common concrete problems are deterred altogether. The ability of UHPC to adapt to itself promotes long-term reliability.

Self-Leveling the Perfect Set

When contractors pour concrete, they prefer a flowing, easily directed mixture that will enter and fill forms with relative ease. After pouring the mix, it is spread and leveled, sometimes using screeds to promote the final set. Vibration might be needed for conventional concrete as well. If the mix is clumpy, slow, or too dry, the process may need a reboot or quality will be compromised. Workable material without the need to compromise is what really gets the job done.

Cor-Tuf UHPC is exceptionally friendly to contractors in this respect. It is workable and form-fitting, thanks to the particle size of its constituents. Small components in the mix promote tight packing, which in turn promotes structural benefits, and can be optimized for the desired flowability. With conventional concrete, there may often be small inconsistencies in the mix that manifest in surface inconsistencies, leading to problems with cracking, substrate contamination, or being prone to easy abrasive damage. Even the best leveling approach will result in imperfections and the concrete will have to be repaired later in life. Not with Cor-Tuf UHPC.

The fill rate on our pours is virtually 100 percent. The particles flow into the smallest nooks and crannies, filling voids of the form with a level of air-tightness that doesn’t even let in a water molecule. This rate of efficiency is a result of the high density of our UHPC’s microstructure— airtight bonds of cement, fibers, and other particles. These bonds preemptively exclude common challenges with bad pours, because they promote even consistency instead of those small but important imperfections that occur on the surface of traditional concrete.

Theoretical research backs up our observations in the field. Principles of UHPC design dictate that fine-filling materials and sufficient grain dispersion promote better rheological properties. This, in turn, results in better mechanical performance. UHPC binders are governed by an efficient particle-packing process, resulting in better flow. Its properties are superior as a result.

Ultimately, a pour that is both level and consistently dense is the goal. By creating a cohesive, tightly packed matrix, endlessly layered upon itself, our UHPC is essentially immune to the ailments of concrete stress. The material levels itself in a process by which the constructed overlay, deck, beam, or other 100-year-strong structures being built will resist the spalling and surface issues that plague other concrete installations. Triage and regular repairs will be needed on common projects, but not by self-leveling Cor-Tuf UHPC.

The Miracles of Bonding

Classic concrete cracks, deteriorates, and gives way to impact stress. Abrasions and cracking expose its inner matrix to the elements. Erosion, freeze-thaw, and scaling further imperil concrete’s ability to maintain form and protect itself. Despite its reputation as a strong material, concrete is not inherently blast-resistant. A stronger bonding capability would address these concerns by keeping particles together and resisting the separation and movement at the root of these failures.

Bonding also enables damaged concrete repair and extends the useful life of deteriorating structures. A variety of techniques can be applied to repair the damage, but only certain solutions will truly address the deficiency. The same properties of UHPC that help level itself and avoid imperfections also come in handy for repairs. Instead of a deep wound being further weakened by invasive contaminants, a concrete crack patched with UHPC will become a hyper-bonded shield, impervious to new damage and ready to live out its purpose.

The secret here is the bonding behavior among and between the UHPC particles and the traditional concrete substrate. A concrete crack must first be prepped and cleaned, but once it’s ready, UHPC completely saturates the volume of the void and adheres to its surface. Test results showed that patching this way results in a superior microstructure at the transition zone between the old concrete and its UHPC repair. High bonding strength and an impermeable interface at the former crack lining extend performance. Bridge beams and other members can add decades to their useful life as a result.

Bond tests of UHPC prove this theory as well. US government studies have demonstrated that UHPC is a high performer when repairing underwater canal linings. Compared to alternatives such as epoxy coatings, UHPC overlays were superior. Abrasion testing was successful because of impermeable bonding behavior, and underwater success shows its versatility in different environments and use cases.

Concrete exposed to harsh environments and years of seasonal weather often succumb to breakdowns in durability. Sulfate, frost, and corrosion issues may decompose concrete surfaces and attack the microstructure. Repair or replacement may be required, or a complete structural failure may occur. Couple these concerns with the growing demand for sustainable alternatives, and the solution, yet again, is UHPC.

Research shows that the interfacial bonding between a damaged normal concrete and an applied thin layer of UHPC exhibited a significant improvement in mechanical properties. Full-scale ultrasonic pulses and bond pull-off testing demonstrated that UHPC can be used as an external protective coating that reduces or eliminates common environmental concerns. The bonding properties proved reliable and effective because the normal concrete had rough, uneven surfaces, while the fine-dense and flowable UHPC was able to establish an aggregate interlocking mechanism. Couple this with the low impermeability of the UHPC coating itself, and environmental contaminants have no way to damage the underlying structure.

Blast resistance is another benefit of strong bonding. High impacts such as military explosions, natural disasters, and vehicle incidents all provide use cases for blast-resistant materials. UHPC holds tight bonds together where traditional concrete will shatter and give way. Strong bonds equal safer structures and a lower risk of financial ruin during an extreme situation. Safety and the need to replace conventional concrete point to UHPC once again. 

Bonding is Easy with Cor-Tuf UHPC 

Prevention and repair are achieved by UHPC bond behavior. The science behind it involves complex microstructure interactions and chemical layering, but installing UHPC is not complicated. We’ve tailored our products to work with traditional concrete techniques so that no fancy laboratory is needed to get real benefits to the jobsite. Our mix and fibers blend with local materials and still achieve the flowability and bonding capability needed for outstanding results.

Form-fitting, workable, and strong for a century. Achieve long-lasting quality with your sand, your cement, and our Cor-Tuf UHPC.

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