Calculating with CRC®
As an engineer, you might ask yourself how to make your own calculations of elements based on a material not covered by most codes.
The short answer is, you don’t. Sorry.
Most codes, including the EuroCode, allow you to deviate from the standard materials and calculation principles provided you can document that the performance in relation to safety and over-all behaviour is in accordance with the principles stated in the codes.
This is the case for CRC i2® and why we at Hi-Con are able to calculate load carrying structures using it.
Through extensive and thorough documentation, including accelerated and full scale testing, both static and dynamic behaviour, as well as durability and fire behaviour has been determined for CRC i2®, leading to development of a design guide for CRC i2® used internally at Hi-Con – see Bendts blogpost, World leaders in UHPC? on the origin of CRC.
Even so, knowing the boundaries - what will work or not from production ov er installation to service level state - requires more than “just” the design guide, and hence all engineers at Hi-Con are thoroughly trained on the intricacies of cutting edge UHPC application – this is one of the main reasons why Hi-Con does all calculations internally.
But apart from the necessity of practical experience, just focussing on static calculations according to the EuroCode (still geographically the area where most Hi-Con elements are used), three areas should be mentioned, where the extensive documentation of CRC i2® is especially important:
EN 1992-1-1 allows compressive strength up to 90 MPa (based on 150x300 mm cylinders). This is exceeded by several so-called UHPFRC materials, including CRC i2®, and of course requires comprehensive documentation to document the level and consistency of strength.
Furthermore, additional extraordinary testing and documentation is required to deviate from the reduction factor ƞ described in EN 1992-1-1 section 3.1.7. The factor takes increased brittleness as a function of higher strength into account, and means that e.g. a 90 MPa characteristic compressive strength is reduced by 20% to a characteristic calculation 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 a robust and ductile behaviour, and consequently is designed with ƞ 1,0.
UHPFRC including CRC i2® is typically used to create slender structural designs with limited cross section dimensions. To do so effectively, it is necessary to reduce the cover layer thickness needs compared to the values stated in EN 1992-1-1, section 4.4.1.
To avoid reinforcement corrosion the concrete must be extremely dense, and it must be documented that both carbonation and chloride intrusion in the loaded state is suitably slow.
Documentation in a loaded state is important, because with slender structures exposed to comparatively high live loads and hence significant bending tension, it is necessary to document effective crack control and to document how micro cracks affect carbonation and chloride intrusion.
For CRC i2® the extensive documentation means that we can guarantee 200+ years durability in seawater with very small cover layers – as low as 10 mm to black steel reinforcement.
In several aspects, UHPFRC may exhibit inferior performance to fire exposure compared to ordinary concrete. One of these aspects is explosive spalling (see EN 1992-1-2, section 6.2), as it was observed on examples like the Great Belt Link tunnel concrete and the English Channel tunnel. When very dense concrete is heated rapidly, steam is generated inside that cannot escape fast enough, and consequently very high internal pressure builds up, that can lead to explosive spalling.
If the tensile strength of the material is high, the pressures reached before release may generate relatively powerful explosions. Consequentially it is very important to have sufficient knowledge about parameters such as critical moisture content, permeability and tensile strength for a particular concrete, under particular fire exposure conditions (e.g. stress levels).
Also, very dense concrete with low water/powder ratio and steel fibres such as CRC i2® conduct heat more easily than ordinary concrete and have a lower heat capacity. On the other hand, they often exhibit better residual tensile and compressive strengths at elevated temperatures.
This balance of opposing properties underlines how vital it is to document material properties through actual fire testing before using UHPFRC materials in a structural fire design.
CRC i2® has been tested in several full-scale fire tests under various conditions, as well as being exposed to a couple of actual fires (Even a bad situation can provide valuable information). In contrast to most conventional concrete types, we know how CRC i2® behave in a fire situation.
So how do I calculate CRC i2® elements on my own
To sum it up, it is not easy to use a material not covered in the codes – it requires a lot of documentation and knowledge, as well as experience – you need to know what is OK and what is not.
Particularly because the actual dimensioning factor when using UHPFRC is Eigen frequency, deflection – both short and long term – and cracking risk and mitigation. These are some of the most difficult parameters to accurately calculate and model, and to a large extent we at Hi-Con rely on the very long track record (see Bendts blogpost) and the accumulated empirical data for a large variety of element and load cases to make sure the elements perform as desired.
As a consequence, CRC i2® elements are exclusively calculated and dimensioned by Hi-Cons own engineers and select trained co-operation partners.
So if you have a query for a potential project – do not hesitate to contact us.
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