What is UHPC? - An attempt at a definition
Definition:
UHPC binder is a combination of cement, nanofiller and plasticiser.
UHPC concrete is a combination of UHPC binder, water and mineral or ceramic aggregates.
UHPC binder is not just any combination of materials - all three materials must be very precisely matched in quantity and quality. If the materials are perfectly matched, you have a very special binder.
If the matching is not perfect, you have a binder of mediocre quality, any standard cement would be better.
It is not easy to take the matched high quality cement out of this UHPC combination and replace it with a simple standard cement. The result would be, at best, a concrete of slightly better quality than the concrete obtained with the standard cement. It is not possible to produce a high performance concrete with a low quality cement. The same statement applies, of course, to the arbitrary substitution of nano-fillers such as microsilica or plasticisers.
There are always statements from companies that claim. Buy a "UHPC concentrate or additive" from us, mix it with a local cement and aggregates of your choice and you have a UHPC. Only what kind of UHPC is it, if it is a UHPC at all?
The question is always: What do we mean by the term UHPC?
Do I mean a concrete that reaches a compressive strength of 100 - 130 MPa?
This compressive strength can also be achieved with a very good cement, quartz sand and fibres.
Do I mean a concrete that reaches a compressive strength of 140 - 250 MPa?
This compressive strength can no longer be achieved with standard raw materials. Here, particularly high qualities of cement, nanofiller, plasticiser and mineral or ceramic aggregates are required.
Do I mean a concrete that reaches a compressive strength of 260 - 400 MPa?
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I can no longer achieve this compressive strength by using particularly high-quality raw materials alone. Here, the UHPC concrete must be subjected to different treatment methods.
So the lower I specify the quality of the UHPC concrete, the closer I come to the standard concrete quality and thus also its mixing and processing methods.
The type of mixing method or mixer types have a big influence on the final quality of the UHPC. I can mix a normal concrete in a simple free-fall mixer. In this mixer, the cement (cement is always ready homogenised) reacts with water during the mixing process, as a two-component mixture.
In a UHPC mixture, the cement reacts with the microsilica, plasticiser and water, thus forming a four-component mixture. This mixture must first be homogenised and then the components need time to react. The higher the UHPC quality is defined as described above, the more decisive is the mixer type. The lower the UHPC quality is defined, the closer one approaches the standard mixer and mixing method of standard concrete.
A horizontal or pot mixer is required for a high-quality, optimum mix quality.
What good is the best binder material if it is poorly processed or mixed?
Then there are the differences in the understanding of UHPC regarding fibre addition.
In some countries it is necessary to improve the comparatively low UHPC binder quality with a fibre addition. This method has been used for many years for standard concrete.
Then there are the high quality UHPC binders that achieve very high technical values, even without the addition of fibres.
In order to achieve extreme strength values, however, fibres of a certain quality, geometry, size and quantity must also be used for the high-quality UHPC binders.
The arguments for the use of fibres are manifold. Here are some examples:
Structural/Civil Engineer, P.Eng., M.A.Sc.
2 年As you mentioned that the is a grey area between HPC and UHPC and that common defined zone to more from HPC to UHPC is the microstructure and mixing energy/technique witch provide proper dispersion for particles. From my little experience that transition point where increasing cement and reducing water would not be make a significant change in concrete properties. For instance, 100 MPa could be reached using 600 kg of cementitious material with low w/c ratio, from 100 to 140 MPa a great amount of cementitious material should be added to reach such strength. I would argue that microstructures and particle shapes and material electrostatic charges could not be always guaranteed for most of the ingredients, what leads mix designer to excessively utilize premanufactured materials (silica sand and 25% silica fume) as most of proprietary mixes focus on silica sand and silica fumes as well as special micro-fibres. In applications regard, I could not agree more that mixing and heat treatment are very effective to reach strength. However, for site applications, which is significantly larger to date, small scale high-shear mixer being utilized what restrict production size and make it very demanding job for contractors. noting that heat treatment is not applicable in site conditions. Whereas majority of precasters to date, define many challenges in applications of UHPC such as; heat curing, unsuitability for existing mixers to handle it, many limitations.
Developer and producer of UHPC binder systems since more than 25 years
2 年So far, more than 3000 members have looked at our article and no one has written down their opinion on this article as a comment. Now we wonder if everyone agrees with our classification/definition of HPC and UHPC and that is the reason why no one writes a comment.