CALCULATIONS WITH CRC I2®

How do you make your own calculations for elements in UHPC?

CALCULATIONS OF UHPC ELEMENTS

How to make your own calculations for elements based on a material not covered by most codes. The short answer is, you don't...

Most codes, including EuroCode, allow you to deviate from standard materials and calculation principles, provided you can document that performance in terms of safety and overall behavior aligns with the principles outlined in the codes.

This is the case for Ultra High Performance Concrete (UHPC) CRC i2®, which is why we at Hi-Con are able to calculate load-bearing structures using it.

Through extensive and thorough documentation, including accelerated and full-scale testing, both static and dynamic behavior, as well as durability and fire behavior, have been determined for CRC, leading to the development of a design guide for CRC i2® used internally at Hi-Con.

However, knowing the limits - what will work or not work, from production to installation, and finally to service level - requires more than "just" the design guide, and that's why all engineers at Hi-Con are thoroughly trained in the complexities of pioneering UHPC applications. This is one of the main reasons Hi-Con performs all calculations internally.

But aside from the necessity of practical experience, just focusing on static calculations according to EuroCode, there are three areas where the extensive documentation of CRC i2® is particularly important.

You can read more about Ultra High Performance Concrete in the links below:

What is Ultra High Performance Concrete?

COMPRESSIVE STRENGTH

EN 1992-1-1 allows for compressive strengths up to 90 MPa (based on 150x300 mm cylinders). This is exceeded by several so-called UHPFRC materials, including UHPC CRC, and naturally requires extensive documentation to verify the level and consistency of strength.

Furthermore, extraordinary testing and documentation are required to deviate from the reduction factor η described in EN 1992-1-1, section 3.1.7. The factor accounts for increased brittleness as a function of higher strength, meaning, for example, a 90 MPa characteristic compressive strength is reduced by 20% to a characteristic design value of 72 MPa. The design value—depending on NAD—is typically reduced from 60 (90) to 48 (72) MPa.

CRC i2® is documented to exhibit robust and ductile behavior and is therefore designed with η = 1.0.

DURABILITY

UHPC is typically used to create slender structural designs with limited cross-sectional dimensions. To do this effectively, it is necessary to reduce the required cover thickness compared to the values specified in EN 1992-1-1, section 4.4.1. To prevent reinforcement corrosion, the concrete must be extremely dense, and it must be documented that both carbonation and chloride ingress under loaded conditions are appropriately slow.

Documentation under loaded conditions is crucial because slender structures are subject to relatively high stress loads, necessitating significant bending stress. It is essential to document effective crack control and how microcracks impact carbonation and chloride ingress.

FIRE RESISTANCE

In several aspects, UHPC can exhibit poorer performance under fire exposure compared to regular concrete. One of these aspects is explosive spalling (see EN 1992-1-2, section 6.2), observed in examples like the Great Belt Link tunnel concrete and the Channel Tunnel. When very dense concrete is heated quickly, steam forms inside, which cannot escape fast enough, resulting in very high internal pressure that can lead to explosive spalling.

If the material's tensile strength is high, the pressure build-up before release can generate relatively powerful explosions. Therefore, it is crucial to have sufficient knowledge of parameters such as critical moisture content, permeability, and tensile strength for a specific concrete under particular fire exposure conditions (e.g., stress levels).

Also, very dense concrete with a low water/powder ratio, and steel fibers like UHPC CRC i2®, conducts heat more easily than regular concrete and has a lower heat capacity. On the other hand, they often exhibit better residual tensile and compressive strengths at elevated temperatures.

This balance between opposing properties underscores the importance of documenting material properties through actual fire testing before using UHPC materials in a structural fire design.

UHPC CRC i2® has been tested in numerous full-scale fire tests under various conditions, as well as exposed to a few actual fires (even a bad situation can provide valuable information). Unlike most conventional concrete types, we know how CRC i2® behaves in a fire situation.

SO HOW DO I CALCULATE UHPC ELEMENTS ON MY OWN?

To summarize, using a material not covered by codes is not easy—it requires a lot of documentation, knowledge, and experience. You need to know what is acceptable and what is not. Especially because the actual dimensioning factor when using UHPC involves eigen-frequency, deflection—both short and long term—and crack risk and reduction. These are some of the hardest parameters to calculate and model accurately. At Hi-Con, we heavily rely on our extensive track record and the accumulated empirical data for a wide range of element and load cases to ensure the elements perform as desired.

As a consequence, UHPC CRC i2® elements are calculated and dimensioned exclusively by Hi-Con's own engineers and selected trained partners.

So if you have any questions or inquiries about a potential project—don't hesitate to contact us.