Concrete is a versatile building material. In a highly solid mix. It gives solid foundations for buildings. In lean mixtures, it can be processed very well into flooring panels. After all, completely unencumbered, it can be used excellently for the preparation of a construction site. You will learn how to mix which concrete correctly in these step-by-step instructions.
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Important questions about the mix
Why mix concrete yourself?
Mixing concrete yourself is actually rather unusual today. As soon as larger quantities are needed, the ready-mixed concrete is the usual format in which this building material is delivered. However, if the construction site is particularly inaccessible or if the budget prohibits renting a concrete pump, then there is no avoiding manual mixing of the concrete. Even if only small quantities are required, there is usually no need to call the concrete plant. You can mix the concrete yourself in a few simple steps.
How much concrete is needed?
Concrete hardly shrinks when it hardens. There are different types of shrinkage, but even if these are added together, the total shrinkage is still in the per mille range. So, you can equate the amount of concrete you need exactly with the volume you need for your project. The advantage of mixing yourself over ready-mixed concrete is that you only mix as much concrete as you need. It is also much cheaper.
What you need for concrete mixing
Concrete consists of aggregate, cement and water. The strength of the concrete depends on the type of aggregate, the amount and class of cement and the ratio between water and cement. The last factor in particular, the so-called “water-cement ratio”, is of decisive importance for the quality of fresh concrete. The relationship between water and cement may only move within a very specific range. “A lot helps a lot” is just as bad for the result as “As dry as possible”.
However, concrete allows many moisture levels in processing, so that a suitable water-cement ratio can be found for every application.
The calculation of the water-cement value is: w / c = water weight / cement weight
The water cement value we have chosen is 0.4. There are 1.6 kg of water for every 4 kg of cement (1 liter of water corresponds to approx. 1 kg). This is because the cement can chemically bind up to 40% of the total weight when it sets. If the water-cement value becomes even smaller because the water content is reduced, then this leads to fine-capillary pores and the cement quality deteriorates. In the case of ultra-high-strength and high-strength concrete, however, water-cement values between 0.2 and 0.4 are required in order to minimize the proportion of water, which also minimizes the distance between the cement grains. Otherwise, the water cement values for concrete are between 0.45 and 0.75. The choice of the correct W / C value depends on the exposure class.
An exposure class indicates the environmental conditions to which the concrete is exposed:
The aggregate is the largest volume fraction in the concrete. It is most important for compressive strength. Blast furnace slag offers the highest strength. However, this is only used for special applications such as vaults. Mixtures of gravel are the aggregate used for normal applications. It is important that not just any gravel is used, but always ready-made concrete gravel. The aggregate must always have the right ratio of small and large stones, otherwise it will not be compressible.
In the do-it-yourself sector, three types of aggregate are normally used. For thin, fine layers, such as screed, a gravel mixture with a grain size of 0-6 millimeters is used. Solid stones, lintels, slabs or formwork are made with a gravel mixture with a grain size of 0-16 millimeters. A gravel mixture of 0-32 millimeters is recommended for foundations. Concrete surcharge costs around 15 euros per ton.
The cement is also available in various compressive strength classes. However, commercially available cement CEM I 42.5 N 25 kg is perfectly adequate for most DIY applications. The 25 kg sack costs around 2.50-2.80 euros
Calculation for one cubic meter
Concrete has a density of 2.4-2.5 kg / dm³. That is 2300-2400 kg per cubic meter.
The mixing ratio between cement and aggregate is 1: 4.
For 1 m³ of concrete you need:
(2300/5) x 1 = 460 kg of cement = 18.4 bags of 25 kg = 52 euros
(2300/5) x 4 = 1840 kg surcharge = approx. 25 euros
The amount of one cubic meter should always be mixed in a mixer. Otherwise, an efficient production is hardly possible. Mixing this amount by hand only means that the concrete is partially already hardened, while the last mixed concrete is still liquid. That makes compacting the concrete almost impossible. The mixing ratio of 1: 4 is always the same. Whether it is mixed with a shovel in a mortar bowl or in a mold with a trowel, the ratio always remains the same.
Concrete must always be compacted after pouring. This is done by knocking on the formwork from the outside or shaking it in the concrete with a stick. The concrete must be completely distributed in the mold and must no longer have any air bubbles.
Mixing it yourself is worth it
The direct comparison with ready-mixed concrete from the sack shows that a lot of money can be saved by mixing it yourself. A 25 kg bag of ready-mixed concrete also costs 2.50 euros. Calculated on the 2300 kg of one cubic meter, this results in a requirement of 92 bags. That corresponds to costs of 230 euros. This means that ready-mixed concrete from the sack is exactly three times more expensive than mixed concrete. It is therefore only recommended for the smallest applications to choose the ready-to-use mixes. Ready-mixed concrete costs around 130 euros per cubic meter. For large applications such as strip foundations, false ceilings or floor slabs, there is no alternative to ready-mixed concrete in most cases.
Tips for readers in a hurry
- If possible, do not use any precast concrete
- make sure you use the right amount of water
- choose the correct surcharge
- Thoroughly clean all tools after concrete production
- Always compact concrete