Considerations when using High Performance Concrete
Designing feasible structures in High Performance Concrete means designing at the limit of what’s possible. The reason (as always) is money. HPC isn’t cheap so we have to use as little as possible. At Pieters we’ve been designing HPC structures since 2008 and still it sometimes feels like we’re designing with gold. Engineering heroes like Gustave Eiffel and Vladimir Shukhov were very skilled in minimalizing material use, but in modern construction, often labor is more expensive than materials. For normal structures this means fast and simple manufacturing and construction are more important than saving materials. With CRC® however this is not the case and we often design slender structures, which probably Heinz Isler would be proud of.
Using HPC requires different skills of a concrete engineer. An HPC engineer needs to have knowledge of the problems that arise with light-weight structures, such as deflection and vibration stiffness, or even fatigue. To design a structure with a unity check of 1,0 you need sufficient knowledge of the applied material, the mechanics and the codes. In this blog I’ll give you some points of attention for designing with HPC.
Woontoren de Verkenner in Utrecht, Netherlands with extreme cantilevered balconies.
Know your material
There isn’t one type of HPC-mix that works. There is CRC® but there is also e.g. Ductal and BSI. One thing these mixtures have in common is that they have been researched extensively for many years. Because we know these materials inside out it’s possible to design extremely slender structures.
Engineering with these materials means combining various codes with material specific testing. Lately we see new suppliers trying to get a piece of the HPC-market with their own HPC. A problem with these new mixtures is that they have not been researched and although they might be high strength, that doesn’t mean they are also high performance. As an engineer working with new materials I always ask suppliers for their documentation. I’ve found CRC to be very well documented and that’s why we’ve been using it for years now in our designs. We regularly test and measure projects after completion and each time it’s reassuring to see the material behaves as expected.
Which code to use
In the last 10 to 15 years a lot of research into the application of HPC has been performed in countries such as Denmark, Germany, France and the USA. Many projects have been realized, but the codes are trailing behind. The French code is currently probably the most advanced, but difficult to use for designs in other countries and with non-French HPC’s. Therefore we’ve been using the Eurocode for projects in the Netherlands with some adjustments to utilize the advanced properties of HPC. These adjustments have been documented extensively and we’ve found this practical approach to work well in receiving approval from other engineers such as the building authority.
Which code would you use to calculate these slender balconies?
Limitations of the code
The Eurocode gives minimum dimensions for certain structural members. For example NEN-EN1992-1-1 article 9.3.2 states that a plate with shear reinforcement should be at least 200mm thick. With very slender structures this means that not only the material, but the entire structure falls outside the code. For some projects we’ve performed full-scale tests to confirm the extremely slender HPC structure would behave the same as a normal slender concrete structure. We’ve found this was not always the case but that the fibers made up for this effect. Together with the TU Delft we are actually researching this phenomenon at the moment.
Connections such as this patented strandvilla-connection fall outside of the Eurocode because of their dimensions. The calculation rules of normal concrete don’t apply here.
Strength and fire resistance
The Eurocode has a factor for the reduction of strength due to brittleness. If you use this formula for a HPC of 150Mpa this results in a 50% reduction in strength. This reduction doesn’t have to apply to HPC when the right fiber amount prevents brittleness, however this needs to be tested. The same goes for fire resistance. It’s well known that higher strength concretes exhibit explosive spalling. The Eurocode gives measures to prevent this. But when using HPC it is important to test the effectiveness of these measures and the overall behavior under fire conditions.
Usually the most important but also the most difficult aspect of a structure to determine. The right HPC-mixture with the right fiber content can outperform anything. It exhibit’s strain hardening and self-healing behavior. Even under very high strains and with very low covers, structures can last for 100+ years. The key is crack control, which is only achieved with the right combination of material properties. A good sign of a well-documented HPC is when the supplier can show you documentation about crack-control, both short and long term.
Proving the durability - under high strains - for a very slender pedestrian bridge is no simple task.
As you’ve probably concluded yourself by now, designing with high performance concrete relies a lot on documentation and testing. From the outside most concretes look the same. Whether it’s a normal C20/25 or a C90/105 or a high performance concrete of 150MPa. They all look grey. But that doesn’t mean they behave the same. Fiber Reinforced High performance Concrete is a fundamentally different material than normal concrete. On many aspects it behaves completely different, such as the way the forces are distributed through the material. Don’t underestimate this difference and make sure to have proper knowledge and documentation. If you do, then HPC is the best material to work with!
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