When discussing potential projects, I am often asked what surface expression to expect of CRC i2® elements.
It is a very simple question – but not as simply answered.
CRC i2® is a unique cement-based material, and as conventional concrete and other natural materials, variations in colour and texture should be expected, and should be expected to develop over time. This is a large part of the visual appeal of concrete, because it offers a living and varied expression.
If a uniform, non-changing surface expression and colour is what is actually desired I suggest using a painted surface or a different material. Otherwise, neither user nor supplier will be happy with the outcome of the project.
This sometimes comes as a surprise, because concrete is considered an unchanging homogeneous solid. The truth is very far from that.
As other cement-based materials, CRC i2® is a heterogeneous porous material, only with less variation and very low porosity, due to the special composition of the CRC mix design. This means, that when supplied, CRC i2® elements will vary in colour and texture, such as number and distribution of blowholes, smoothness of edges etc. – and there might be lime efflorescence to varying degree, dirt from transport etc.
In short: CRC i2® elements, in many respects, varies visually much like “normal” concrete elements.
Again, this sometimes is a bit of a surprise, because the CRC i2® concrete is so very different in many other respects, such as strength and durability.
So what is to be expected, and why can’t we at Hi-Con just simply get our act together and make all the elements identical, uniform and unchanging?
Giving birth to CRC i2® elements
Casting with CRC i2® is not easy. I know this just sounds like a bad excuse, but it really is!
The material is very viscous – like syrup, and full of small steel fibres (like small needles). Added to this is typically a very dense reinforcement with conventional rebars and meshes tightly packed in the mould.
It makes it difficult to place and with a natural tendency to catch air at the surfaces, making some blowholes in vertical surfaces inevitable, even though heavy vibration is used during casting. This is also known from other UHPC types:
The mix design is fixed because of the need for documentation of multiple properties, so we cannot in a simple way make it more flowable and easy to cast – the CRC i2® is actually an improved version.
Once demoulded, the surfaces are as susceptible to lime efflorescence and soiling as any other concrete surface, and remains so for years – as does normal concrete.
Also, because of the large volume of steel fibres in the mix, the cast surface (the free side from which the concrete is poured) is very difficult to trowel and smoothen, often resulting in a rather rough expression (can be almost like porridge).
The steel fibres also prevent the use of post treatment options like exposed aggregate surface, polishing, acid edging or similar to even out surface variations.
In short, CRC i2® elements, despite their slim design and elaborate detailing, has the same strengths and weaknesses visually as conventional concrete when it comes to initial surface expression – and then some.
Aesthetic ageing of CRC i2®
Once the elements are in place, they are subjected to the same degradation mechanisms as every other surface on Earth: Erosion, soiling and organic growth:
Erosion: Surfaces exposed to wear by wind driven particles, chemical degradation and freeze/thaw action are slowly broken down and washed away by rain. This mechanism level mountains
Soiling: Airborne particles such as sooth, soil, sand etc. are deposited on the surface. Soiling can also be of chemical origin (e.g. lime efflorescence or graffiti)
Organic growth: Algae, moss, lichen or even more developed plants adhere to the surface and grow to cover it.
CRC i2® elements are much more wear resistant than conventional concrete and therefore the erosion process is very slow. This also means that organic growths is slower in taking hold on vertical surfaces, because the low porosity of the surface reduce access to moisture.
Soiling is not reduced by using CRC i2® however, because it is mostly particles accumulating on the surfaces – just as windows, it requires regular cleaning if you want the surface to stay clean.
If you do clean the surfaces, use water, soap and a soft brush or sponge. The same for indoor surfaces, where saturation (to the extent possible) with a natural soap prior to use is the recommended treatment. Acidic cleaning agents, high pressure cleaning, mechanical treatments like sanding or polishing should never be used as they can damage the surface and the fibres (the risk is that the fibre alloy may be degraded, causing fibre corrosion at the surface).
So basically, the answer to the question of what to expect is: Expect that CRC i2® surfaces vary and change over time similar to conventional structural concrete elements, and, that the variation and change depends on the specific project (element geometry, environmental load, degree of cleaning etc.).
Therefore, I recommend you ask for a comparable reference project and inspect it on site. A selection of reference projects may be found at the web page.
A reference is the best way to get an idea of what to expect, and to form your own opinion of whether or not a CRC i2® surface is the right choice for your project or not. Otherwise just ask!
Tommy Bæk Hansen
Group Product Development Manager
Read about Tommy’s 20 years of experience with UHPC right here