09.06.2020 - Knowledge Center
Do we really need another guideline for UHPC?
A number of guidelines on UHPC are already available and more are on the way. As it takes a lot of effort – and time – to develop these guidelines it makes you wonder why all this effort is necessary. Why would Germany – or Spain - not just adopt the guidelines from France or Switzerland or Japan? Of course I know there are reasons for this – and as a UHPC nerd it is quite interesting to follow all these different documents and note the differences from one guideline to the other. In the following I will comment on some of the new guidelines – and on the differences between them. And I warn you that this post might be a liiiiittle long (of course just right in my own opinion) so I have added a few pictures to give you a chance to catch your breath.
Since the first applications of UHPC in the late nineties, there has been a need for guidelines to help building authorities relate to the new applications and to help ensure that structural elements are produced in a safe – yet effective - manner. A wealth of technical reports, white papers and state-of-the-art reports have been issued over the years and it is really a matter of definition whether we are talking about a technical report or a guideline One of the first “guidelines”, however, was issued by VSL in Australia (in 2000) and was aimed specifically at design of Ductal prestressed beams. Since then a number of other guidelines have been issued dealing with UHPC with probably the most comprehensive one being the AFREM guideline from France. A large group of companies and technical institutions in France developed the guideline with the first version available in 2002 and an updated version available in 2013. The guideline has now been replaced by two national annexes related to EN 1992-1-1 and EN 206 – both on UHPC. One annex deals with materials (NF P18-470), while the other deals with design (NF P18-710). It is the intention that a third national annex – NF P18-451 dealing with execution – will be issued in 2019.
Comprehensive technical reports – such as this one from the FHWA in the US - or guidelines are already available in a number of countries – including Japan (2004) and Switzerland (2016) – while they are under preparation in other countries. Just a few of these are mentioned in the following:
Canada has been active in UHPC since the nineties and have a number of applications. They are working on a system of 2 guidelines for UHPC – similar to the French – where one guideline will deal with design and serve as an annex to the standard on bridge design, while the other guideline will deal with materials. The materials guideline will be included as an annex to the 2019 edition of the Concrete Standard from the Canadian Standards Association. It is the intention that this annex will eventually be incorporated in the standard itself – and it is open for comments until April 18 so you may still have time to influence it if you think it needs improvement.
Germany has been developing their guidelines for a long time and they are expected to be issued in 2018 – again one part dealing with design and another dealing with materials and methods. It is expected that the guidelines will be largely based on the results of a large coordinated research program on UHPC that was carried out between 2008 and 2014. The drivers behind the German guidelines are the universities so the guidelines will be based on extensive research and documentation and relatively few applications – in contrast to France where the drivers were companies in the building industry. A lot of applications have been carried out and some of the documentation is considered confidential. This is also the background for another big difference. While the French standard is based on a number of proprietary mixes with a demand for an “identity card” for each mix, the German guideline will contain a section on developing mixes. This section should enable concrete producers to use cost efficient UHPC mixtures based on available raw materials in different regions and adapted for different applications. It will be very interesting to follow how this turns out and how this will be handled in practice in a country that is normally considered as a bit conservative and has been reliant on special testing in the form of zulassungs when something is a bit unusual.
The Free Harbor Tower in Copenhagen with balconies, columns, C-frames and L-frames in CRC i2®.
Spain should be mentioned as they have taken a rather admirable approach to developing new guidelines – or at least they have been better in communicating this than others. One of the focus points of their UHPC guidelines is to make them simple and fast to use and UHPC characterization should be easy to perform. With this in mind they intend to copy as much as possible from other guidelines, while adapting as necessary. Just deciding on which guidelines to copy will be quite a task – and there is a risk that fulfilling the requirements of “fast and simple” will lead to many compromises – but it will be very interesting to follow their attempt. I don’t know if the progress of the task group will be commented on the link below – but it is certainly a blog that tries to cover a lot of aspects of UHPC in quite an informal way:
The Nordic countries are also worth a comment. Norway was one of the early leaders in UHPC research based on their huge concrete offshore structures – although things have been very quiet for the last 20 years. Denmark has been active in UHPC applications since Hans Henrik Bache developed DSP in 1978 and CRC in 1987 (and it is perhaps also worth mentioning that Hi-Con has now produced more than 80,000 tons of structural elements in UHPC over the years). No guidelines on UHPC are expected from the Nordic countries, however. The main producer of UHPC in Scandinavia is Hi-Con and we have decided to operate within the system of the Eurocodes, which means we have produced an ETA (European Technical Assessment) for balconies produced in UHPC. This makes it possible to obtain a CE marking for our balconies and the next step will be to obtain a similar assessment for other products. The ETA can be considered as a very specific guideline – for producing a specific type of product in UHPC – that is valid in the European Union and not just in one country. The pictures in this post are of very different balconies that are all covered by the ETA.
Examples of recent balcony projects covered by the new ETA. Fælledudsigten (L) and Islands Brygge (R).
With the wealth of national guidelines developing it is clear that international guidelines would be preferable to make things simpler, but as researchers and practitioners with very different cultural backgrounds have to agree on something, this will go very slowly. ACI has established a committee on UHPC (in 2013) with the aim of establishing guidelines and fib has done the same. In 2004 fib task group 8.6 was formed to write guidelines for UHPFRC – with Joost Walraven as chairman. I joined the group at their 4th meeting in Leipzig in 2005 – and while the meetings were very interesting and a lot of significant work was presented – it was clear that reaching a consensus would be quite difficult. A lot of the material that was presented at the meetings have since been used in either the French national annex on UHPC or in the draft for the German guidelines and while the task group in fib is still active (it has now changed to task group 4.2) I would not expect the guidelines to be ready soon. Another alternative would be to produce standards under the Eurocode system, but this is also unlikely to happen within the next 10 years. The use of UHPC will be limited compared to the use of conventional concrete already covered by the Eurocode. France has developed national standards while Germany is producing Technical Guidelines for UHPC rather than standards, as this covers building materials and constructions that are “already tested but not frequently used”. While I may seem a bit pessimistic with regard to international guidelines I would be very pleased to see something being available within the next few years as this would make it much easier to work across national borders – but I don’t have high hopes.
Sometimes I think that we at Hi-Con would have been much better off trying for our first applications today – when there is so much information available with relation to application of UHPC – rather than in 1995 when we were producing the first drain covers for the Great Belt Link. At the time, we had to provide extensive documentation – and there was very little information available on UHPC except the documents we had produced on our own based on experience with CRC. On the other hand I sometimes consider ourselves lucky that we have been able to try so many different things without having guidelines for everything – guidelines that are of course set up to protect the public and provide the necessary level of safety, but that can sometimes be a bit too restrictive.
I mentioned an example of this in my post on the UHPC workshop in Montpellier. With a mean compressive strength of 165 MPa measured on cubes in our regular production we would not be able to comply with the French or German standards for structural UHPC (but we would have no problems with the Canadian, Swiss or Spanish guidelines). We could easily achieve the required characteristic strength of 165 MPa on cubes by using heat curing, but it would not improve our product with regard to our specific application – and it would make our product more expensive and less able to compete against steel or conventional concrete.
Another example is with regard to effect of fibres. While we do require ductility and crack control in the fibre reinforced matrix we always use CRC in combination with conventional reinforcement. This means we rely less on fibre distribution than what is described in the French standard, where the concrete is typically made with no conventional rebars.
145 organic shaped balconies on Ørestadshuset in Copenhagen and C shaped balconies on Ceres Corner in Aarhus
But to answer my opening question (you are probably thinking- finally!!!): Do we really need another guideline for UHPC? I think that when the German, Canadian and Spanish guidelines are ready (and any others that are on the way) we have probably covered the most important points, so any new guidelines could be made as combinations of the guidelines already available. It will probably also be necessary to do a bit of editing as we gain experience with using the guidelines, but hopefully this will be minor. It would be great if a consensus was reached and an international guideline was agreed upon, but as I already mentioned I am not too optimistic about this (and I hope I will be pleasantly surprised).
Finally I apologize for the long post (another one!), but this is actually something I am quite passionate about (on a level suitable for an elderly engineer). One of our engineers (and yes Birgitte – I am referring to you) caught a sight of the draft – before I added another page – and told me that only a very few would finish reading this. So if you got through the whole thing, please drop me a line at our Wordpress Blog and let me know what you think – should we intensify efforts to combine the different guidelines, should we just praise the diversity – or does it matter?