15.06.2020 - Knowledge
World leaders of UHPC
When you have worked in this field for many years – and I have personally worked with Ultra High performance Concrete (UHPC) since 1987 – you see a lot of claims to being world leaders in High Performance Concrete (HPC). It is a claim that we have made ourselves, but since we produce only Ultra High Performance Fibre Reinforced Concrete (UHPFRC) at Hi-Con and have never worked with conventional concrete we believe that our claim has some merit, especially as more than 70,000 tons of structural elements have been produced in CRC over the years.
Plenty of acronyms
You may also have noticed already that we use a lot of acronyms when working with this type of concrete and I will briefly try to explain a few of these:
CRC – short for Compact reinforced Composite – is the special type of UHPFRC used at Hi-Con. It was developed (and patented) at Aalborg Portland by Hans Henrik Bache in 1986 as the first UHPC, combining a compressive strength of more than 150 MPa with high tensile strength and ductility – due to a high content of steel fibres – and closely spaced rebars to achieve a behaviour similar to that of steel, but with the advantages of concrete. At the time terms like UHPC were not really used, but rather we were talking about High Strength Concrete (HSC). Compressive strength had been one of the main parametres being developed, but it was soon realised that a number of other properties such as durability or ductility were at least as important and the term High Performance Concrete (HPC) was coined. The idea behind this was that the concrete was designed specifically for a particular application and this did not necessarily include a property such as high strength. It could be high tensile capacity, high durability, extreme workability or something similar. In the mid to late nineties, a number of other types of commercially available HPC’s had been developed such as Ductal (Lafarge), BCV (Vicat) and BSI/Ceracem (Eiffage and Sika). In an attempt to put these different concretes into classes the term Ultra High Performance Concrete (UHPC) was put into play to characterize concretes having a compressive strength higher than about 140-150 MPa. As most of these concretes also included fibres to add some ductility the next step was UHPFRC. When combining this type of concrete with closely spaced reinforcement an HR for Heavily Reinforced is sometimes added and as we always use conventional rebars in CRC the proper term for our concrete would be HRUHPFRC – but then it just sounds ridiculous, so we usually stop at either UHPC or UHPFRC.
Composition of UHPFRC – and CRC
UHPFRC is generally characterized by having high compressive strength, low permeability, a large ductility and typically a bending strength higher than 25 MPa. This is achieved by having a high binder content (900-1500 kg per m3), often with a large content of micro silica and glass powder, and a matrix where the largest grains are often smaller than 0.25 mm. As water/powder ratio is typically below 0.2 it is necessary to use highly effective super plasticizers, and in order to ensure the necessary ductility for structural applications fibres (most often steel fibres) are used in quantities between 100 and 300 kg per m3. Conventional mixing and placing equipment can be used, but as the water content is very low it is necessary to use long mixing times to achieve a homogeneous mix. As many of the UHPFRC's are used with a combination of fibres and prestressing, curing is often at high temperatures, which enhances the mechanical properties and ensures that most of the shrinkage has occurred before prestressing.
CRC – the concrete used at Hi-Con - is not a typical UHPFRC. With local aggregates the mean compressive strength measured on cubes is around 150 MPa, but as the Eurocode is based on 150x300 mm cylinders for measurement of compressive strength we use a characteristic compressive strength of only 110 MPa in our design. The matrix has a high binder content, a large content of microsilica and water/binder ratios of typically 0.15 to 0.18. We use sand up to 4 mm, which means that the binder content in CRC can be reduced compared to UHPFRC with only very fine sand, and this gives slightly lower strength, but a more economical mix and lower shrinkage.
The CRC matrix is self compacting, but with the high fibre contents (typically from 120 to 230 kg/m3) and the high viscosity of the matrix needed to ensure good fibre distribution, CRC is best suited for precast production, where vibrations can be applied to improve compaction.
30 years of experience?
30 years of experience in the field of UHPC sounds good, but if you are not careful, it may actually turn out to be 5 years of experience that has been reused for the last 25 years. You have to keep adding to your level of knowledge. We pride ourselves of being up-to-date in the new developments in the field of UHPC, but paradoxically we are still using basically the same formulation of CRC that was developed in 1986. Over the years we have made a few improvements – with regard to increased workability, reduced cost and a reduced need for finish – but if we change the composition of CRC dramatically it would be difficult to use the large amount of documentation that we have developed over the years. While our mix is not very different from what was used 20 years ago, we have gotten to know our concrete extremely well – both its strengths and limitations – and that allows us to design and produce very complicated elements and to provide the necessary documentation. For some of our products we will produce mixes that are a little different from the original CRC-formulations, but at the moment we are using the basic mix design for 95% of what we produce. This allows us to very quickly see where we can add something to a building project – or whether our product would not really be suitable for a particular project. And it is safe to say that there are still plenty of projects that are well suited for CRC!
And finally, to answer the headline question: Yes, we do consider ourselves among the world leaders in UHPC. This comes from being a commercial company designing and producing CRC every day for almost 20 years – competing against mostly steel and conventional concrete – and if we were not good enough at that, we would not exist today.
This is my – admittedly biased – introduction to CRC and how it fits into the world of UHPC. If you disagree, have questions or just want to comment please do so. Unfortunately we cannot do this directly on the main website, but please do so using the link below. Tommy and I will try to cover various topics in this blog – and your comments could give us ideas for new entries – or if it is necessary to get outside help to answer your questions we will try to do this. The floor is open.