High strength concrete not only allows to build filigree, slender constructions, but also to save cement and reduce carbon dioxide emissions.
What is “high strength concrete” and how to use it?
The difference between standard concrete and high strength concrete lies in their compressive strength. High strength concrete HPC, usually has a compressive strength of 50-120 Mpa, while standard concrete has less than 50 Mpa. Ultra-high strength concrete UHPC, has a compressive strength of more than 120 Mpa. These values however vary a little from country to country.
High strength concrete can be used for frame structures of a large range of different buildings. With high rise buildings or bridges, columns, beams and other structures often are very massive and impractical when standard concrete is used. By using high strength concrete it is possible to have more filigree, slender constructions, which leads to more usable space inside buildings. It also gives these buildings a better optical appearance from the architectural point of view. Not to forget that structures made of high strength concrete have a higher longevity.
One huge benefit of high strength concrete is the possible savings. Savings, by example, can be generated, from the reduced amount of raw materials used. If you save 25% of the concrete required, you may save approximately 20% of raw materials. The cement saving is a key factor, because the production of cement is generating massive carbon dioxide emissions. If 20% of cement can be saved, carbon dioxide emissions can be reduced respectively.
Materials and mix design
For high strength concrete, materials with a good and solid quality are an absolute must. Dense, crushed gravel and coarse sand as well as different binders are important ingredients. HPC requires quite a lot of binder material, in some cases up to 650 kg/m3. Most usual binders besides cement are silica, fly ash, limestone powder and GGBFS (Ground Granulated Blast Furnace Slag). Some of the cement usually gets replaced by fly ash, limestone powder or GGBF to improve the workability and reduce the amount of water required as well as avoid high concrete temperature. Silica is used to achieve an even higher compressive strength.
W/C-ratio of high strength concrete is generally very low, usually only between 0.2 and 0.3. The low water content in the mix leads to reduced workability, a challenge that can only be met with special concrete admixtures. If superplasticizer it is possible to gain easy mixing, good workability and pumpability.
Keeping the temperature below a certain level is another important matter, especially when concrete is poured for massive constructions. This is possible only if the temperatures of all materials (aggregates, cement and water) are controlled, the poured concrete, if necessary, gets protected, properly cured and the concrete hardening process is under supervision.
With higher concrete temperatures, hardening of HPC will be fast, which might lead to problems, for example caused by cracks. By using retardants it’s possible to slow down the hardening process and achieve a better concrete quality.
High strength concrete - requirements for the concrete mixing plant
There are two different methods of producing concrete: with a wet mix or dry mix plant.
A dry mix plant only doses the components while the actual mixing process takes place afterwards in a truck mixer in free fall mixing. That’s the reason why dry mix plants generally are not suitable for the production of HPC.
The solely possibility to produce HPC is with a wet mix plant. In a wet mix plant the concrete gets mixed with the compulsory mixer installed inside the plant. This is the best way to properly and reliably mix all materials and achieve a perfect homogeneity of the concrete.
When producing HPC it is very important to know what the actual aggregate moistures are, because the aggregate moisture massively influences the water content in a mix. Measurement of the aggregate moisture and correction of the additional water to be added to the mix must be done automatically. Due to the low W/C-ratio, even a small mistake in the calculation of the amount of water brought along with the aggregates might ruin the whole batch.
Accurately working dosing devices and weighing systems all play an essential role in the manufacturing process. It also is very important to have the standard deviation in dosing from batch to batch as small as possible. If there is a high variation in aggregate dosing between two batches, this will cause a mistake not only in the amount of aggregates dosed, but also in the total amount of water in the mix.
Not only the water content is an important matter, so are the temperatures of all the raw materials being used for a mix - especially the temperature of the aggregates. In winter time there can be a quite high variation in the aggregate temperatures, if the plant does not have a proper heating system and a control system, which takes care of material heating and adjusting of the concrete temperature. On the other hand, under warm conditions, the temperature of concrete might sometimes rise too high and by this also cause some problems. If too warm, HPC probably can’t be used for the desired purpose anymore. Under certain circumstances it might be necessary, to cool the aggregates and the cement or to add chilled water or even flaked ice, to be able to keep the concrete temperature low enough.
For mixing high strength concrete mix the concrete mixing plant must have a good quality compulsory concrete mixer, preferably either a planetary countercurrent mixer or a twin-shaft mixer. With these, the best possible mixing of high strength concrete can be guaranteed. Planetary countercurrent mixers usually are the best choice, because with these the unfavorable side effect of an increase of the air content in the concrete can be avoided.
Another relevant factor of course is the concrete plant’s control system. When producing HPC, as mentioned earlier, a fully automatic temperature and moisture control is required, as well as precise dosing of all materials with only a very small standard deviation between the batches.
Summary
Producing HPC means, to use appropriate materials, suitable mix designs and a wet mix concrete plant.
Anyway, producing high strength concrete must not necessarily be more difficult than producing regular concrete if you are using a good quality wet mix concrete plant. Small standard deviation in dosing between the batches, exact measuring of the aggregate moisture and automatic correction of the amount of water and aggregates, an effective mixer, a modern control system and an automatic temperature control, are all fundamental necessities. Under certain circumstances a proper heating or cooling system might be required to be able to produce temperature controlled concrete.
Tecwill concrete plants and high strength concrete
All Tecwill concrete plant models are appropriate to produce high strength concrete. Especially our extremely precise measuring of the aggregate moisture, and accurate dosing with very low standard deviation, guarantee the reliable production of high-quality HPC. Our aggregate and water heating systems with automatic mixing of temperature controlled concrete are worldwide successfully operating under the most challenging conditions.
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