Selecting the right steel fibre alloy for the application is essential to achieve a good aesthetic performance
One of the frequent questions during project negotiations is why it is necessary to use expensive stain inhibiting steel fibres.
To answer this, I will show some examples of how different fibre types look after a few years in actual use.
But first a little about the history: CRC® was originally documented using black steel fibres – which work fine to ensure both strength and durability as Bendt has described previously, but aesthetically lead to visible corrosion at the surface.
The initial projects were sold like this, however years later, that may be hard to accept for the tenant of an apartment, and so gradually it was realised that even if it was agreed with the contractor and the owner, we had to do better, if the end users were to experience Hi-con products as the premium products we intend them to be.
A big issue with black steel fibres is that corrosion appear random – In some cases, corrosion of black steel fibres was limited and did not pose an aesthetic problem like the gallery to the left, but in other places on the same building the corrosion was very visible as the image on the right illustrates.
Consequently, experiments were made with various plastic and glass fibres since they do not corrode. But they also proved to have close to zero effect on the essential ductility parameters. Therefore, they are only suited for elements loaded to significantly less than cracking moment, and are virtually not used in our products.
Instead it was attempted to use stain resistant steel alloys, and the first type used was 1.4016 (AISI 430). This Ferritic alloy was supposed to be corrosion inhibiting, but that proved not to be the case. In fact, it proved quite volatile and unpredictable:
A good example of moderate 1.4016 fibre corrosion. Balcony overview (left) showing the area of the close-up of the edge (right). The 1.4016 fibres are clearly visible in the surface – most perform fine without corroding, but here and there single fibres corrode significantly.
This lead to an intense search for better alloys, and the choice fell on two Austenitic types: 1.4301 (AISI 304) and 1.4401 (AISI 316) which have been used since with good result, e.g. for galleries exposed to wear and de-icing agents and even for a stair to a water slide in an indoor swimming pool.
CRC i2® outdoor galleries in a harbour area exposed to wear and de-icing agents made with 1.4301 fibres after several years of use – no corrosion visible.
1.4401 are – in theory corrosion and acid resistant and are normally used in extreme environments such as this cantilevered CRC i2® stair at the Egmont boarding school.
But even though we now have found fibres that match our ambitions to provide Premium products regardless of the exposure situation, we are continuously looking for alternatives, both because even better types may become available, and because the alloys we use – and consequently the fibres – are quite expensive.
I hope above helps to explain why we insist on using expensive but proven fiber types: A dedication to continuous improvement of the end user experience and a perceived premium quality in the day-to-day use of Hi-Cons UHPC solutions!
Tommy Bæk Hansen
Group Product Development Manager
Read about Tommy’s 20 years of experience with UHPC right here