One of the main barriers for an increasingly use of UHPC in concrete precast industry is the difficulty of finding a realiable UHPC mixture at an affordable price. If you want to start producing UHPC elements you have two choices: (i) ask specialised companies, commonly cement companies, to provide you with some of their already developed premixes, or (ii) develop one of your own.
If you have already asked these specialised companies for a high-quality and reliable UHPC supply and you think what they offer to you is far from making your products competitive (which may be the most likely scenario) and you are afraid you can’t do it on your own, in this post you can find some basics you need to follow. Why not give it a try?
First thing you have to know is that despite sometimes we talk about UHPC as if it were kind of almighty material whose development is only for the few, UHPC is just concrete. A special one. That’s true. One that requires a special care. We can’t deny that. One in which a mistake may lead to a substantial loss of money. Don’t forget it anytime, but don’t despair.
UHPC is a special type of concrete you can reliably produce if your are already producing concrete
As a conventional concrete, UHPC is made of cement, aggregates, superplasticiser and water. In case of UHPC, maximum aggregate size is commonly lower than 2 mm. UHPC is a mortar !! Most typical aggregate used is quartz sand. Regarding cement, good news! UHPC has been made with a wide variety of cements. However, we strongly recommend low heat hydration rate cements. The use of one or another cement depends on (i) local cement availability ,(ii) its compatibility with superplasticiser, (iii) your needs of higher or lower initial strengths, and, sometimes, (iv) the colour desired.
Unlike conventional concrete, UHPC contains pozzolanic additions, such as silica fume, fly ash, gound granulated blast-furnace slag (GGBS), metakaolin, nanosililca, fluid catalytic cracking residue, etc.; and fibres, commonly short straight steel fibres. We suggest using silica fume as a compromised solution between prices, strength and stability of the mixture. Regarding fibres, we strongly advice you to look for the best price in the market. Fibres represent between 60-80% of the overall price of UHPC mixture. That’s quite a lot !! If you are dealing with short straight steel fibres (most typical ones), only start negociating below 2.5 €/kg. Don’t be ripped-off !!
If you want a cost-effective UHPC, make your own !! Use available local materials, whenever is possible. If you have problems to get it, look for the support of specialised consulting companies on UHPC. There is no doubth that it is going to be both quicker and cheaper.
Is it require a huge investment in equipment to manufacture UHPC in your own precast factory?
You don’t need special or high technology equipment to produce it. You just need a mixer and a more or less advanced system to weight material with little deviation.
It is fine if you can weight all components with a margin of error of 2%. In case of water, you need to guarantee a margin of error below 1%. Accomplishing that is not a tragedy in most of existing precast factories. Don’t never forget that water is the most important thing you need to control. A low deviation in the water content may lead to a high deviation in strength, stability and flowability of the mixture. So, follow these two advices:
- Margin of error in the added water must be below 1%
- If you can’t control moisture content of your materials preciseley, buy them dry
Regarding the mixer, there is only one thing you need to know! UHPC can be made in all existing mixers. A man of science would tell you that an intensive mixer is required (i) to a better dispersion of fine and ultra-fine particles, (ii) to achieve higher strength by reducing water content; (iii) to reduce mixing times. Even though that is absolutely true, it is also true that we can achieve all of that in a common industrial planetary mixer.
UHPC can be made in a common industrial planetary mixer, taking between 15 and 20 minutes
Following video shows the process of manufacturing 15 litres UHPC in laboratory conditions using a very low intense mixer. It is quite impressive to see all changes in the manufacturing process of UHPC.
During the first 40 seconds the dry mixing of cement, sand and additions takes place.
At 40 seconds all the water required is added. However, for the next 4 minutes UHPC remains with the same aspect!! And it would never change if not for the superplasticiser.
Superplasticiser is introduced at minute 4, and it takes 20 minutes to react with cement particles to achieve flowability required. The higher the mixing energy, the less this process takes. Here lies the importance of an intesive energy mixer.
Steel fibres are added at 25 minutes, once mixture is flowable. If you are to produce UHPC, it is a must that you keep always the following sentence in your mind: “fibres must be added once UHPC is flowable to reduce the amount of mixing energy required or to produce a higher volume of UHPC with a given mixing energy”. Believe me, your mixer will hate you forever if you put fibres at the beginning of the mixing process.
Are you ready to have your own UHPC and start playing with it?