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Mix concrete yourself: This is how it's done
Mix concrete yourself: This is how it's done

Video: Mix concrete yourself: This is how it's done

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Three ingredients and their correct mixing ratio determine the quality, durability and possible uses of concrete. In our instructions you will find out how to mix the building material yourself for your needs.

Table of contents Table of contents Mix concrete yourself: This is how it's done

  • What is concrete made of?
  • Cement as a binder
  • Grit
  • It all depends on the water
  • Mixing concrete: how it works
  • How to fill a foundation with concrete
  • Let the concrete dry

Table of contents Table of contents Mix concrete yourself: This is how it's done

  • What is concrete made of?
  • Cement as a binder
  • Grit
  • It all depends on the water
  • Mixing concrete: how it works
  • How to fill a foundation with concrete
  • Let the concrete dry

Cement, mineral aggregates such as sand and gravel and some water - there are only three ingredients from which a high-strength, artificial rock is created after setting: concrete. The respective mixing ratio of the ingredients determines the properties of the finished concrete, its durability, resistance or even water resistance. And that has been the case since antiquity, because the Pantheon in Rome, which was built around AD 126, has a concrete dome - the secret of its durability lies in the special cement.

Concrete is divided into normal concrete, light concrete or heavy concrete according to its density. Depending on how resilient the concrete is. In addition, there is reinforced concrete, i.e. concrete provided with gussets as a support corset for high tensile forces and so-called unreinforced concrete.

The home window usually uses ready-mixed concrete from the hardware store or mixes large quantities itself using tried-and-tested mixes. Static calculations are necessary for foundations or to concrete a floor slab. The structural engineer then also determines the correct concrete. You can mix it yourself, as it must be pressure-resistant, but is not necessarily exposed to static loads and special tensile forces. The situation is different with building work with concrete ceilings and other elements. You shouldn't mix the concrete yourself.

What is concrete made of?

Modern concrete has become a kind of high-tech multi-component adhesive with up to six additives for numerous applications, even if nothing has changed in its basic composition. Therefore, if you want to lay concrete foundations, you usually still take the classic.

You can add other substances to the classic ingredients, such as glass or plastic fibers, so that fiber concrete is created. In turn, other materials result in acid-proof concrete, aerated concrete or self-cleaning concrete for facades. The additives generally influence the properties of the concrete, for example it can flow better, it becomes smoother, more frost-resistant or it can be processed longer after mixing.

Mixing concrete: cement as a binder

Cement serves as a binder or adhesive for the aggregates and consists of finely ground and burned limestone, marl and clay. Pure, gray Portland cement or Portland cement with various additives such as silicate dust is common. In the case of Roman concrete, it was Pozzolan, a mineral-rich volcanic ash that made the mixture so extraordinarily solid.

DIN EN 197-1 differentiates the so-called normal cements into five categories and three strength classes - class 32.5; Class 42.5 and Class 52.5. These numerical values ​​indicate the strength after 28 days in millimeters per square millimeter. Blast furnace cement is used, for example, for concrete that has to deal with sea water. Cement hardens after a reaction with water and a subsequent drying time and then remains structurally stable.

Tip: Do not use masonry cement when mixing for concrete as it contains setting retarders that are undesirable in concrete.

Beton mischen
Beton mischen

If you only need small amounts of concrete, you can mix them in a tub.

Photo: iStock / photovs

Which grain size to choose for concrete?

The mineral aggregate influences the properties of concrete due to its grain sizes and accounts for a good 70 percent of the total volume of the finished mixture. The grain sizes determine the flowability and compressive strength, but also ensure that the concrete does not shrink and crack when hardened. Pure cement gets very hard, but it also cracks.

The chemical, geometric properties are so important that a separate standard is dedicated to them, namely the "DIN EN 12620 aggregates for concrete". The largest proportion are stones with a grain size of more than 2 millimeters, everything below is sand.

Fine concrete has a maximum grain size of 16 millimeters, and concrete for do-it-yourselfers often contains 32 millimeter stones. Of course, there are not only such chunks in the mixture, but rather a mixture of all grain sizes from 0 to 16 or 32 millimeters. If you leave out the gravel and only mix water, sand and cement, you get mortar.

The size of the gravel also determines how and where you can use the concrete, how well you mix it into a homogeneous mass and how long you can process it. The more uniformly you can mix the ingredients, the fewer possible voids the concrete will have later and it will become firmer. The largest grain also largely determines the later surface of the concrete, which is particularly important with exposed concrete. If reinforcing bars are close together in the formwork, the concrete must flow and must not be too coarse-grained - the grains must predominantly be smaller than the space between the reinforcement and the formwork. In general, the largest grain in the concrete should not be larger than a third of the smallest component dimension, otherwise the corresponding part will not be strong enough. Use cleaned masonry sand as sand.

It all depends on the water

Water turns the cement into a kind of glue. The water is not only responsible for the consistency of the fresh concrete, it also initiates a chemical reaction in the cement and thus starts the hardening process, which ultimately turns the viscous mass into solid concrete. The ratio of water and especially cement influences the consistency during processing. The more water, the more liquid the fresh concrete becomes - clear. But you should definitely stick to the recommended amount, because the higher the water content, the worse the quality of the concrete. Only part of the water is chemically combined with the cement, the rest evaporates when hardening and can cause fine cracks - too much water is harmful.

Good to know: If you have fresh concrete delivered to the construction site, you must not add any additional water according to German regulations. This falsifies the ratio of cement and water and the concrete can lose its properties.

Mixing concrete: how it works

To put it straight away: Mixing concrete is hard work. You can mix small quantities for the point foundation of a carport well by hand, but larger quantities - such as those required for the slab foundation of a garden house - cannot avoid a concrete mixer. That also makes it cheaper than buying numerous bags. The devices can be borrowed from the hardware store.

A common mix for a universal concrete, such as you can use, for example if you want to lay terrace slabs, build steps or garden walls yourself, consists of a ratio of gravel to cement of 4: 1. For example, from 12 parts of gravel (for home improvement mostly 0 up to 16 millimeters), 3 parts cement and 1 part water. You can count on 4 liters of water per 10 kg of cement, making 10 kg of cement, 40 kg of gravel and 3.5 to 4 liters of water for this mixture - depending on the desired consistency. For foundation concrete, put a little more gravel in the concrete, i.e. 10 kg of cement, 50 kg of gravel and then 4 liters of water.

When mixing, you should have the later application and therefore the necessary amount of water in mind. If the concrete is to be poured into reinforcements, formwork or formwork blocks, for example, it needs a different consistency than lean concrete for edging blocks in gardening. Be careful not to inhale the cement dust, as it irritates the mucous membranes. Therefore, be sure to pay attention to the wind direction or even better wear a face mask. Observe the weather forecast, because to mix and pour concrete, it should be dry for several days, ideally cloudy and warmer than 10 degrees Celsius.

Betonfundament im Garten anlegen
Betonfundament im Garten anlegen

Smaller concrete projects in the garden, such as laying the foundation for a new garden shed or a garden wall, are no problem for DIY enthusiasts.

Photo: iStock / Gajus

Mix concrete by hand

Put the powder in a container, add the amount of water according to the manufacturer's instructions and mix everything well: it is very easy with ready-mixed concrete mixes, but the concrete is only suitable for small projects.

If you need larger quantities, first mix the dry ingredients on a blueprint, in a vat or in a wheelbarrow. Then gradually add the water and mix everything with a trowel to a homogeneous mass, larger quantities with an agitator. If the mixture is too liquid, you only have the option of making up for it with more cement and gravel.

Concrete mixer

The concrete mixer - usually a drum mixer - ensures the best mixing, consistent quality and you can produce the concrete directly on the construction site. However, a water connection is necessary. With the mixer, first add a third of the amount of water to the drum and then always add, for example, 4 scoops of the gravel-sand mixture and 1 scoop of cement. Add the rest of the water and let the drum run for 3 to 4 minutes until the concrete has a slightly shiny surface. Process the ready-mixed concrete within half an hour, or in cool and damp weather within one hour, as the hardening begins immediately. Therefore, also clean the equipment immediately - it is difficult to get rid of solid concrete.

Betonmischer
Betonmischer

If you want to mix a large amount of concrete, borrow a concrete mixer from the hardware store. It makes the job so much easier.

Photo: iStock / EvgeniiAnd

How to fill a foundation with concrete

In order to pour the fresh concrete into a foundation, it must be flowable, or at least sufficiently viscous, so that it can also be completely distributed in the formwork and, in the case of reinforced concrete, between the reinforcing bars. With smaller foundations, pour the concrete from buckets into the formwork, with larger ones directly from a wheelbarrow. The concrete will spread itself, but you can still help with a rake. For large foundations, the concrete is usually delivered and poured directly into the foundation from a thick hose using a concrete pump.

Reading tip: Instead of wooden formwork, you can also use formwork blocks - this is easier and faster.

Important: Then compact the concrete so that air pockets disappear. With small foundations and stronger concrete, it is sufficient to tap the concrete with the shovel. If the concrete is more liquid, poke the concrete with a squared timber and thereby drive air bubbles out of the concrete. For large foundations, there are of course corresponding machines for compacting.

Let the concrete dry

For ready-mixed concrete, the exact drying time is on the packaging. Strictly speaking, concrete does not dry, but hardens in a chemical reaction. Dryness is even an obstacle to hardening, because if too much water evaporates before the concrete is hard, it will crack.

It is best to cover the concrete and the formwork with a film and leave the formwork to stand for a good two weeks. In hot weather, you should water the concrete every two days to prevent cracks. Concrete can be walked on after a few days and has its minimum pressure capability after 28 days when it is warmer than 10 degrees Celsius. It can take years before it hardens completely, but the concrete is of course fully resilient.

Thomas Hess

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