Frequently asked questions about concrete
Beginner questions | Advanced questions| Expert questions
Is concrete the same as cement?
No, cement is only one of the three main components of concrete. The other two are sand and water. For coarse-grained concrete, gravel is used instead of sand. When the components are mixed together, a reaction begins between the water and the cement, at the end of which the liquid becomes a solid mass. The more cement the mixture contains, the harder the concrete becomes, because the cement is the glue that holds everything together. If you use more water than necessary, the adhesive deteriorates and you end up with an inferior concrete. The sand or gravel has the task of limiting the shrinkage of the concrete, as it is the only component that does not shrink when the concrete hardens. Without it, the concrete would crack badly.
Frost - At what temperature can concreting still be carried out?
Concrete should no longer be poured at ambient temperatures below 5°C. Although frost damage is only to be feared from 0°C, when it is very cold, the clock virtually stops for the concrete and it only sets extremely slowly. If there is a night frost before the concrete has reached sufficient strength, it can be damaged. Of course, small concrete objects are more at risk than large structures, as the heat generated by the setting inside the concrete dissipates more quickly. If you still want to pour concrete at low temperatures, you should use accelerated concrete to keep the concrete clock running. The Moertelshop offers the BUMS and WUTZ mouldable mixes or the VITO, VITO PIANO, MOBY FIX and TURBO 20 pourable mixes for this purpose. Of course, you can also accelerate normal concrete yourself, for example by adding our CSA cements CALUMEX WHITE and NEXT BASE or our CARO WHITE alumina cement.
Which concrete to use for handicrafts?
When making decorative objects, it is best to use a fine-grained concrete with particularly high strength so that the pieces do not break so easily later on. As DIY stores usually only sell poor quality craft concrete at very exorbitant prices, it is well worth taking a look at the Moertelshop. It depends entirely on the planned processing method which mixture is best suited, as concrete can not only be poured, but also modelled, carved, kneaded or spread in thin layers. The perfect craft concrete for casting is VITO, which is available in grey or white. You will have a finished result in your hands after just one hour. VITO PIANO is even more suitable for those who prefer a slower pouring process. However, it is a completely different matter if you want to produce small decorative pieces by modelling. PLASTY FIX is ideal for modelling. For carving, on the other hand, use SKULPTIN, a very special mixture that sets quickly at first, but then remains so soft for two days that you can scrape out any shape with a simple spoon. Finally, there is lamination. LAMBO is available for this. It is similar to modelling clay, but is much easier to spread. If you want to cover a balloon with a thin layer of concrete, e.g. for a lantern, you should therefore use LAMBO, which is also available in grey or white.
How to make lamps or luminaires from concrete?
The modern high-strength concrete types for artists and designers can be used to produce fantastic lamps and lampshades because they can also be processed into very thin walls due to their enormous strength. Depending on the working technique (casting, modelling or laminating), VITO, PLASTY FIX or LAMBO are suitable for this. You can find DIY instructions by designer Torsten Stephan for one of 1000 possible models on video here.
Concrete look - How can I design a concrete-look floor, kitchen, table, bathroom and furniture?
For the reproduction or imitation of concrete surfaces (concrete look), special concrete can be applied in very thin layers to a given substrate, e.g. wood. Our BASIL self-levelling floor concrete is best suited for floors and table tops. This has the particular advantage that it sets without shrinkage and therefore cannot crack. Wooden substrates should first be thinly filled with KLEMO so that the concrete adheres well to the wood. Joints in the wood can be bridged by inserting a mesh fabric. For walls and vertical surfaces, it is best to use MARFINO, a cement-based wall paint that can be painted or levelled.
How to mix concrete correctly - agitator, mixing machine or compulsory mixer?
There are very different types of concrete, which also need to be mixed differently. The simplest is probably ready-mixed concrete, which requires no mixing at all, such as Setz-Fix from Sakret or Ruck-Zuck-Beton from Quick-Mix. You fill it in, pour water over it and that's it. Of course, this results in a very low-quality concrete, but for some tasks this is enough. Conventional construction concrete, on the other hand, is mixed in a vat with an agitator (a powerful mixer) or in a mixing drum. In the vat, you should first premix the dry ingredients and then add the water. In the drum mixer, on the other hand, you usually start with the water and then add sand, gravel and cement. Mixing high-strength concrete such as MOBY DUR, which is required for special purposes such as concrete kitchen worktops, is the most time-consuming process. Because you have to make do with extremely little water here, you need a compulsory mixer and at least 5 minutes mixing time. However, you can also use a normal free-fall mixer if necessary, as shown in this video.
Do you have to vibrate concrete to compact it??
Yes, you can do this, but there are also more elegant methods. In construction, it is common to vibrate the concrete with a vibrator bottle or external vibrator, which causes the concrete to slump and create a denser structure. As soon as the vibrator is switched on, the concrete mass flows into the mould and when it is switched off, the concrete stops again. The grains of sand and gravel in the concrete are packed closer together after compaction, which significantly improves the final strength. However, with the help of modern superplasticisers such as our FLUP-PCE-104 (powder) or our FLUP-PCE-375 (liquid), concrete can also be made to run into the mould and compact all by itself. This is known as self-compacting concrete (SCC), which does not require shaking or vibrating. Whether the concrete actually flows like honey, however, also depends on the fine flour content of the concrete aggregate. This is why a good SCC mix design always contains marble powder, microsilica, metakaolin or something similar as a filler.
Which concrete for table tops?
You can't use just any concrete for a concrete table or a concrete table tennis table because it would be far too heavy and thick. It is best to make something like this from high-strength concrete, because then you can manage with very thin walls. This saves weight, looks more elegant and also has the advantage that high-strength concrete also enables very good, smooth and non-porous surfaces. Our MOBY DUR, which is available as MOBY DUR® white or MOBY DUR® grey, is a casting concrete specially developed for furniture and accessories made of concrete and based on the special Dyckerhoff FLOWSTONE® cement. To ensure that everything goes smoothly during processing, it is essential to read the Flowstone instructions beforehand, which are also available as a free download in the Moertelshop. It is also very helpful to watch our Flowstone mixing instructions video.
Why and for how long does concrete need to be cured?
After concreting, you should cover your work with foil to keep it moist. This is called curing. If it is forgotten, the concrete surface may dry out too quickly and then crack or chalk. How long the film has to stay on depends on the environment. Normally, the concrete is out of the woods after just one day and the film can be removed again. However, if the sun or wind causes it to dry excessively, it is better to leave the film on for a day longer. If you pour concrete in a draught-free and shady room, you have less to worry about than outside. When it is very cold, concrete dries much more slowly and the film should be left on for longer in winter. Of course, you can also keep concrete moist by watering it with a spray mist, e.g. for large areas such as a freshly concreted screed, for which you may not have a sufficiently large film.
When is concrete loadable / walkable?
Normal concrete at room temperature can usually be loaded or walked on after just one day. Its strength naturally continues to increase over the next few days, which is why it is better to wait a little longer before exposing it to greater loads. However, you don't have to wait 28 days, as is sometimes wrongly assumed. This is only the time that a test laboratory waits before it can determine the so-called final strength, because after that the strength in the concrete hardly increases any further. As a precaution, a fragile workpiece such as a kitchen worktop with a recess should only be removed from the mould or turned over two or three days after casting. However, with a more compact piece, such as a bust, where nothing can break off so easily, you do not need to wait so long. If you are working with an accelerator or at high temperatures, the waiting time is shorter, and at very cold temperatures it can take a very long time before the concrete can be loaded.
Concrete too fast - what to do?
Some ready-mixed concrete mixtures are adjusted so that they set very quickly, such as STEINGUSS or VITO Blitzbeton. If you don't do everything right, the mix may react too quickly and harden in the mixing vessel. It is important to realise that such mixes react even faster when it is very warm or if too little water is used. You can easily help yourself against the heat by using particularly cold water in summer and keeping the dry mortar in a cool place, in the fridge if necessary. A sometimes difficult to recognise cause of too rapid setting is too little water. In this case, you should not rely on your own judgement, but use a scale or measuring jug to measure the portions correctly according to the instructions on the packaging before mixing. Then combine the water and dry mix in a suitable mixing vessel and mix with a suitable mixing tool. That sounds simple, and it is. Nevertheless, here are a few examples of possible errors:
Case 1) Peter only ever needs small amounts, which he mixes in a drinking cup. To do this, he pours the dry mixture into the cup, goes to the tap and adds water in small portions, stirs and adds more water until the mixture is as it should be. He returns to his workstation and before he can pour, the mixture has already set. ERROR here: Peter started the mixing process with too little water and only gradually added more water. As a result, the concrete sets faster than usual at the beginning. In addition, it takes more time than necessary to readjust the amount of water, and with each new stirring, heat is added to the concrete, which further accelerates the setting process.
Case 2) Hannah has misplaced her mixing tool and has to improvise. She takes a tablespoon and a salad bowl from the kitchen and starts to work through the mixture. Before she has finished mixing, the concrete has already set. ERROR here: Hannah's tools are so unsuitable that she needs a much longer time to mix than usual. During this time, the cement begins to hydrate, and as there are areas in the bowl that are not yet homogeneously mixed with water that are even drier than others, the setting process progresses particularly quickly. The setting then spreads from there to the entire mass.
Case 3) Willi has a small mixer with a timer that he uses to mix his concrete. His mixes are usually perfect after 3 minutes of mixing. That's why he always leaves his timer set to 3 minutes. With his new accelerated mix, however, he finds that the concrete sets in the mixing vessel shortly after switching off the mixer. ERROR here: The mixing time is too long. Willi has therefore mixed more than necessary, which has artificially fuelled the setting process, because mixing generates frictional heat, and heat has an accelerating effect.
Case 4) Susanne produces small cubes of flash concrete that sell well. She therefore invests in a larger mixer so that in future she can pour 100 cubes at once instead of the previous 3. Normally, in her experience, she has 5 to 8 minutes to pour, which would be sufficient for 100 cubes. But the larger portion of now 25 kg, which she has mixed for this purpose, solidifies after just one minute, so she doesn't even get round to pouring it. ERROR here: Large portions behave differently to small ones because the onset of hydration in the concrete generates heat. The heat in large portions cannot escape to the outside as easily as in small ones. As a result, the temperature inside rises, which further accelerates the setting process.
What does it mean when you say: The concrete is cupping?
Particularly when casting thin slabs, the phenomenon occurs that the concrete warps during setting. The edge of the slab moves upwards by one or even several millimetres, and only the centre of the slab still rests on the casting mould. Typically, a shiny spot surrounded by a duller surface then appears on the later visible side, i.e. the side that is at the bottom during casting. The reason for the curvature of the slab is that the concrete shrinks more on the upper side than on the underside during setting due to uneven moisture distribution. This causes an air gap to form between the casting and the mould. Calcite crystals then grow out of the fresh concrete in this gap, making the surface appear dull, whereas where the slab is still in contact, the surface remains smooth or shiny. The generally undesirable calcite crystals are also known as efflorescence. You can counteract this by carefully covering the part after casting and using a shrinkage reducer if necessary. In contrast to normal concrete, accelerated or shrinkage-free concretes have little or no tendency to mould.