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

Mixer Types: Making Sense of all the Options

There are a lot of concrete mixers out there in the market today for precasters to choose from. In my experience I have work closely with at least six distinct type of concrete, in passing with few other, from so many companies that it is beyond count. I have worked for three different companies that manufacture concrete mixers and worked for two other that sold concrete mixers as a part of Turn-Key projects (yes, that is five different employers...I know). Since I started in the concrete equipment industry 20 years ago I have learned quite a bit about concrete mixer for a factory setting, and here is what (I think) I know:

Counter Current Mixers (Planetary Mixers):

This is the mixer that I mostly grew-up around Pedershaab (which was the first company I sold equipment for) often paired their plants with Skako and sometimes Haarup, both Danish companies making Planetary mixers. Masa and Wiggert both make planetary mixers, and are companies I sold equipment for. Mixer Systems here is the US, and my current employer, also produces a planetary mixer. I have more experience with these mixers than any other.

In a nutshell, the planetary has the design to be one of the fastest mixing mixers available. they are good and reliable mixers. in some applications they excel beyond the pan and horizontal shafts. Also, the ability to have multiple discharge door as on the pan, can be a huge advantage. However, which today's more complex mixes such as SCC and even a standard zero slump, the mixing speed advantage is reduced. As I grew up, I saw the planetary as a one-size-fits-all, be-all-end-all of mixers. Today I see the planetary as a very useful tool in a belt of very useful tools, it has it applications but there may be better options, depending on what needs to done.

Cyclone /R12 Mixer:

This is an exceptional mixer, especially for Face-Mix on pavers. When I was with Masa, who produces paver plants, they produced a version of this mixer, and I was amazed at its ability to use a single shaft with mixing tines, revolving at an exceptionally high speed to pulverize the materials. this was very important as face mix is often prone cement and color balls because of the fine materials used. This is what high-sheer really is. Price on a mixer of this sort is often the biggest issue, as they often run 2 -3 times the price of another style mixer in the same size.

Another drawback to this mixer is a single discharge door though that can be overcome with proper planning and supplementary equipment.

Horizontal Shaft Mixer (Paddle or Spiral Blade):

Slow and Steady Wins the Race! When I first started with Mixer Systems, I had a bias against this mixer. Now I think it is the best mixer ever made. OK… that may be a little much, but let me explain my enthusiasm on this.

At Mixer Systems we call this mixer the Praschaak. There are other producers with this same design concept including Besser and, I believe, Columbia. They are seen as old technology,  often eliciting jokes such as "This is how cavemen made concrete". Maybe it is true that the technology is old, Praschaak was making this mixer in the 1940's, I have seen many in my travels running daily that were produced in the 50's and 60's. So, yeah they are old. They are simple to. A single horizontal shaft with either paddles or blades mounted to it turning at a pedestrian revolution every 1.5 -2.5 seconds. The body I like a barrel on its side, and yet it's produces just about every kind of concrete on the market and does a good job of it time and again.

I have seen horizontal shat mixers used for producing concrete pavers on various block and paver machines, and making consistent concrete with consistent colors, 4-5 minute cycles. Indeed the US block industry developed using such mixers. However, the real magic of the horizontal shaft mixer is for the producers of SCC that pour with buckets, this mixer's door design allows for the least amount of drop from mixer to buckets and that saves on clean-up. Also, the difference in mixing times of the Horizontal shaft and mixers such as the planetary or twin shaft become far less noticeable, within seconds of each other, once a plasticizer and/or other admixes are properly mixed in. Good concrete takes time regardless of RPMs

Pan / Turbin Mixer:

The pan is an interesting mixer. It is unique among the other mixers listed here as it does not have an open body to mix in. Instead, the center of the mixers body is dedicated to the transmission while a trough is created between the inner and outer wall where the mixing takes place. I used to consider this mix a cheap alternative to the planetary, but that was an incorrect and unfair assumption on my part while there may be several applications in which the planetary is said to be superior, I have on numerous occasions seen pan mixers performing noticeably better. At Mixer Systems we produce Turbin/Pan mixers, as do companies such as Teka and Voeller.

I have seen pans running 90 second cycle time while feeding a concrete pump in a precast operation. I have spoken to Prestress producers running both Pan and Planetary Mixers side by side, who have told me that the Pan is far superior for zero slump concrete, and that the mixers run at nearly the same cycle. Indeed, Pan mixers have been a mainstay in the concrete pipe industry for years.

Pans often consume more wear parts than mixer such as the twin and horizontal shaft mixers. But they also offer additional versatility by allowing for multiple discharge doors

Ready Mix Truck:

I am not a ready mix expert. Though wonderful for delivering concrete far and wide to remote locations, It is my understanding that mixing in a ready-mix truck requires up to 15% more cement than a batch mixer to get the same product strength. If I am wrong on this please correct me.

Twin Shaft Mixers:

The twin shaft mixer is a truly fantastic mixer. It offers mixing times similar to the Planetary, durability similar to the horizontal shaft, and a very dynamic mixing action. It is usually is a pretty expensive mixer and if it is not…Beware! This mixer, unlike a horizontal shaft is designed to have the material being mixed over the height of the shaft. This means that the shaft seals are in constant contact with concrete. Not such a big deal for dry cast concrete, but a very big deal with wet-cast, and especially SCC. Those running concretes are as crafty as water when it comes to finding ways to escape, and standard shaft seals would last forever (and by forever I mean 6 months) for such a mix, an air-purge or similar seal is critical, and they are not free.

Another small and easily overcome drawback to the twin shaft is that there is a single discharge door and it is the length of the mixer usually. A chute is often need to focus the flow of material.

 In Summary:
Each mixer type has its own unique assets and limitations there is no one mixer that is best at all things, though many do all things well. The biggest factors in choosing a mixer often relate to the plant designed around and the company you are working with.