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The Limits of High Compressive PSI  

Compressive strength is the primary property of concrete, indicating its overall ability to withstand structural loading. A higher PSI rating (or MPa, in metric) means a concrete member can withstand a higher direct, static load without failing. Any force that squeezes the concrete matrix meets the resistance of interlocked cement, sand, and other bonded components that maintains form instead of collapsing or breaking apart. Buildings and bridges need concrete with higher compressive strength than sidewalks, to resist larger weight bearing down on them.

Because of this property’s importance and almost linear correlation to stronger structures, intuition leads us to believe that a higher PSI of UHPC (or traditional concrete) is always better.

At small scales, this is true. Contractors would rather err on the side of mixing concrete that is too strong (i.e., with a higher compressive strength result) rather than too weak. A test result exceeding the design PSI is typically accepted by the engineer. A mix that is too weak, on the other hand, might have to be re-poured due to the risk of premature structural failure.

The misconception here is that a higher PSI has no other tradeoffs and that a mix can never be too strong. Within reasonable bounds, a higher PSI indicates better performance without compromising other behaviors of the structure. 

However, surpassing certain thresholds can actually be detrimental. American Concrete Institute (ACI) codes typically require 1.33 times the specified compressive strength as the maximum. Beyond this, other properties of the mix might be compromised. 

In the pursuit of stronger materials, Ultra-high Performance Concrete (UHPC) was developed to provide higher values using less material in total. UHPC is an ideal material for large structural components, repairs, and other uses. But UHPC, like all cementitious products, has a sweet spot in regard to its compressive values. Beyond this point the sacrifices made to the other attributes of the material become so severe as to hold very little practical value from a construction material perspective.

Tradeoffs in Concrete Properties

Structural design requires engineers to consider a material’s response to various scenarios. Applications of stress, strain, and differential loading cause potential modes of failure that the concrete should survive over an extended period of time. 

Increasing compressive strength helps with direct loads, but it might diminish performance in certain circumstances. It also might cause more difficult installation procedures, adding further costs and potential errors.

Ductility

In response to stress, on a small scale, concrete or UHPC will temporarily deform. Its ability to bend slightly, absorb localized forces, and subtly contract and expand results in a small but important level of flexibility. This property promotes longevity because the material adapts to different load schedules and environmental factors.

Ductility measures the ability to resist permanent deformation from loading. If the structural member becomes warped or disfigured due to low ductility, it will fail to resist certain loads over time and require replacement. Optimal materials exhibit an ability to absorb differential loading without being broken or warped.

Materials that are too brittle or stiff may not have enough ductility to withstand initial localized stresses. They will fracture easily. Adding compressive strength properties may result in lower ductility and higher brittleness, because the mix ratio has been shifted from promoting elasticity and ductility to providing only direct strength.

Tensile and Flexural Strength

Other strength properties of concrete are also measured in pounds per square inch (PSI) or megapascal (rMPa), which apply to other types of loading. Tensile strength, and its related property of flexural strength, are important for beams and other members supporting loads pulling on the concrete, instead of pressing in on it like compressive loads (read more about strength properties here). 

With traditional concrete, the tensile design relies on reinforced members consisting of steel rebar and the concrete surrounding it. The concrete should not be cracking on its own before the reinforcement properties kick in. Engineers must consider the dimensions of the rebar in relation to the properties of the concrete matrix. A matrix with much higher compressive strength from trade-offs in water/cement ratios, aggregate selections, or other factors might not be appropriate for certain members. The rebar will still resist tensile loading, but the cracked concrete will chip away, exposing the steel to rust and corrosion. This will eventually require repair and replacement. 

UHPC enables engineers to skip reinforcement altogether thanks to internal steel fibers. Break tests on UHPC, which measure its load-bearing capacity, exhibit multiple inflection points. The first break test represents an initial maximum internal capacity, followed by a steep drop in pressure, after which comes a gradual rise as the tensile strength of the fiber begins to take over. After these phases, the final break occurs when a significant force has been applied. It is important to consider the codependent properties within the matrix to optimize the material’s performance and avoid negating the benefits of the fibers. 

Extremely high compressive strength PSI ratings can be achieved in UHPC by decreasing the already-low water-cement ratio or by adjusting the sand content. However, altering these properties can sacrifice flexural performance. A more brittle matrix around the steel fibers will become the limiting factor in a shear-loading scenario, and the material will be unable to leverage the full phase of resistance observed in an optimal testing regime.  

Contractor Friendliness

Cor-Tuf UHPC is straightforward to mix and install. Its ready-mix capabilities and consistency make it easy to mix and pour to use in a variety of projects. The flowability of UHPC is dependent on its mix parameters, and that includes ratios that may impact compressive strength.

Specifying high PSI might be warranted for certain installations, but an excessively high-PSI UHPC mix will require more sensitivity in the field and is likely to drive up costs. Additional admixtures, sand, fibers, or external heating may be needed. Errors in mixing and forming are more likely to occur. Quality assurance/quality control plans are more likely to be bogged down with specific caveats and additional documentation.

As with the properties above, there is a limit to the benefits of higher compressive strength with installation procedures. The mixture needs to be applied in a reasonable time using reasonable methods and manpower. A mix that emphasizes high PSI without considering how it will be installed is putting the quality, cost, and schedule of the project at risk.

The UHPC Sweet Spot

Considering performance properties, installation methods, and costs, we have arrived at a good range of compressive strength for our products. UHPC is intended to be rated at least 17,000 PSI in compression, but experimental mixes can now exceed 60,000 PSI or more. Always consider the diminishing returns of higher PSI in compression due to fewer use case applications. In other words, specialized preparation of these higher-end mixes may not be warranted.

Our mixes generally range from 25,000 to 30,000 PSI. These levels of compressive strength enable optimum bridge and building designs. They resist enormous compressive loads, in addition to tensile and flexural stresses, without compromising contractor-friendly implementation techniques. They allow us to provide a basic package of mix components while allowing contractors to use their own sand, cement, and machinery.

The compressive strength sweet spot might be unique for different projects, but in each case, engineers and contractors must understand what they are installing. Higher PSI ratings are generally better for concrete and UHPC structures, but the optimal solution may not be one of excess. Achieve better ductility, tensile performance, and construction usability with an appropriate product mix. Your sand, your cement, our Cor-Tuf UHPC. 

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