Mixing UHPC: Ultra High Performance Concrete

I was introduced to UHPC, or Ductal as it is often called, nearly ten years ago when visiting a concrete pipe plant in Quebec. They had been running tests with it and when they showed it to me, what I saw was a very thin sheet of very dark concrete. I remember the piece seemed no more than a quarter inch thick. Imagine my amazement when the plant manager casually tossed this material down to the concrete floor, and though the piece banged it did not break. It didn't even crack. I was enamored with this material. How could such a thin piece of concrete withstand that. I was told it was a secret and quite expensive mix that was being developed by one of the big cement companies.

Later that same year a very gifted engineer I know from Italy, told me about a way to make a concrete pipe that would have the tensile strength for it to handle lower head pressures. This was notable because concrete pipe that can handle internal pressures is almost always made with a steel liner to compensate for concrete's lack of natural tensile strength. My friend explained to me that the secret was in the mix design. He said that the mix would be a fine one, with a high quantity of Silica. In all honesty I think he explained the entire mix to me, but I retained only the word Silica. I now know that what he was describing is the mix we today refer to as UHPC or Ultra High Performance Concrete.

Years went by as they always do and other things took my attention (mostly concrete paver equipment), until I was lucky enough to meet a group of people that are a driving force in the development of Ductal / UHPC in North America. They were looking for a concrete mixer that could mix UHPC effectively and a mixer that could be transported to and easily set-up at bridge construction sites. After several discussions about the concrete and the projects, I had a feeling that a Praschaak horizontal shaft mixer would be the right tool for the job. I also quickly realized, as they explained this product, that this concrete may be a magical stuff I had seen and heard of nearly a decade earlier.

Only a couple weeks after our initial conversations about the making of UHPC I met up with a few experts from the Field at the Mixer Systems factory in Pewaukee, Wisconsin to run tests, trying to determine which mixer type would best suit the production of UHPC. As I suspected it was the Praschaak, but not for the reasons I had thought.

It was the horizontal shafts and powerful motor of the Praschaak that had propelled it beyond other mixer types such as the Pan and the Planetary. With the shaft sitting horizontal the arms and paddles sheer through the concrete vertically. This moves the materials from the top to the bottom of the mix, and from the bottom to the top. The paddles are also aligned to creat a flow in the mixer that creates an additional mixing action on the horizontal plain.

UHPC is a very dry concrete to mix, essentially a zero slump. What little water is put in seems to disappear and one would that that I could have no effect. Even after the addition of plasticizer, the mix tumbles around like dirt. But, given a bit of patience, the mix magically transforms to a thick taffy-like substance. It seems mystical to watch this unlikely transformation. Even the seasoned experts gathered to watch it happen.

It was that taffy like nature of the Concrete that made the Praschaak such an ideal solution for mixing it. It could be that all mixers would do an equally good job making UHPC up until this point transformation, but once that point is reached the Shaft and paddle configuration of the Praschaak is the bee's knees. As the paddle traveled through the mix, the mix would resist the inertia, and what came of this was a kin to the pulling of taffy. Indeed the mixed UHPC seemed solid when squeezed in a gloved hand, but it would ooze right though my figures when I relaxed the grip, and the Praschaak mixing method took great advantage of this property of UHPC to produce high quality and high efficiency mixes.

Modifying the mixer to be run on a job site rather than in the factory took some design and planning but had been done by Mixer Systems before, if never quite in this same manner.

Since we initially ran those tests and built those first mixers for UHPC at bridge sites. Mixer Systems has made several for this purpose and we have even built mixers and plants for the making of UHPC in factory setting for the decorative and architectural concrete industries.

There is, of course, Much more to be said of UHPC and how to mix it. This is just a quick tale of my experience.

If you have any thoughts or questions feel welcome to contact me

Steve Nelson
262-893-8554