SURFACE AESTHETICS

Surface finishes you can expect from Ultra High Performance Concrete elements

DEVELOPMENT OVER TIME

We are often asked what surface finishes can be expected from Ultra High Performance Concrete (UHPC) CRC elements. This is a very simple question – but not so easy to answer.

CRC i2® is a unique cement-based material, and like conventional concrete and other natural materials, one should expect variations in color and texture that develop over time. This is a significant part of concrete’s visual appeal, as it provides a lively and varied expression.

If a uniform, unchanging surface finish and color are desired, I suggest using a painted surface or another material. Otherwise, neither the user nor the supplier will be satisfied with the project’s outcome.

This sometimes comes as a surprise because concrete is often considered an unchanging homogeneous solid. The truth is far from this.

A HETEROGENEOUS POROUS MATERIAL

Like other cement-based materials, UHPC CRC is a heterogeneous porous material, with less variation and very low porosity due to the special composition of the CRC mix design. This means that when CRC elements are delivered, they will vary in color and texture, such as the number and distribution of air holes, smoothness of edges, etc. - and there may be varying degrees of efflorescence, dirt from transport, etc.

In short: CRC elements visually vary in many ways similar to "normal" concrete elements.

So, what can be expected, and why can't we at Hi-Con just control it and make all elements identical, uniform, and unchanging?

MANUFACTURING ELEMENTS IN ULTRA HIGH PERFORMANCE CONCRETE

Casting with UHPC CRC is not easy.

The material is very viscous - like syrup, and filled with small steel fibers. To this is typically added very dense reinforcement with conventional reinforcement bars and mesh, tightly packed into the mold. This makes placement difficult and has a natural tendency to trap air at the surfaces, making some air holes on vertical surfaces unavoidable, even with heavy vibration during casting. The mix design is fixed due to the need for documentation of multiple properties, so we cannot simply make it more fluid and easy to cast - CRC is actually an improved version.

When demolded, the surfaces are as susceptible to efflorescence and dirt as any other concrete surface and remain so for years - just like normal concrete. Furthermore, due to the large volume of steel fibers in the mix, the cast surface (the free side from which the concrete is poured) is very difficult to smooth and level, often resulting in a rather rough finish. The steel fibers also prevent the use of finishing options like exposed aggregate surfaces, polishing, acid treatment, or similar methods to even out surface variations.

In short, CRC elements, despite their slender design and extensive details, have the same strengths and weaknesses visually as conventional concrete when it comes to the initial surface finish.

AESTHETIC AGING OF UHPC CRC

Once the elements are in place, they are subjected to the same degradation mechanisms as any other surface on Earth: erosion, dirt, and organic growth.

Erosion: Surfaces exposed to wear from wind-driven particles, chemical degradation, and freeze/thaw cycles slowly degrade and are washed away by rain. This mechanism can smooth out mountains.

Dirt: Airborne particles such as soot, soil, sand, etc., are deposited on the surface. Dirt can also be of chemical origin (e.g., efflorescence or graffiti).

Organic growth: Algae, moss, lichen, or even more developed plants attach to the surface and grow to cover it.

CRC elements are much more wear-resistant than conventional concrete, and therefore the erosion process is very slow. This also means that organic growth takes hold more slowly on vertical surfaces because the low porosity of the surface reduces access to moisture. Dirt is not reduced by the use of CRC because it mainly consists of particles that accumulate on surfaces.

If you clean the surfaces, use water, soap, and a soft brush or sponge. The same applies to indoor surfaces, where saturation (to the extent possible) with a natural soap before use is the recommended treatment. Acidic cleaners, high-pressure washing, and mechanical treatments such as grinding or polishing should never be used, as they can damage the surface and the fibers.

So basically, the answer to the question of what to expect is: Expect UHPC CRC surfaces to vary and change over time, just like conventional structural concrete elements, and that the variation and change depend on the specific project (element geometry, environmental exposure, degree of cleaning, etc.).

Therefore, we recommend that you request a comparable reference project and inspect it on-site. A range of reference projects can be found on our website.

A reference is the best way to get an idea of what to expect and to form your own opinion on whether a CRC surface is the right choice for your project.

You can explore our reference projects here