How much cement and gravel / sand / grit are required for 1 m³ of concrete?

• Important questions about the mix
• Tips for quick readers

Concrete is a versatile building material. In a high-strength mixture, it enables particularly safe spaces. For buildings he gives solid foundations. In lean mixtures, it can be very well processed into flooring panels. After all, it can be perfectly used to prepare a construction site, completely unloaded. How to mix which concrete correctly, you will learn in this step by step guide.

Important questions about the mix

Why mix concrete yourself ">

Concrete hardly fades during curing. Although there are various types of shrinkage, even if these are calculated together, the total loss is still in the per thousand range. So you can set the amount of concrete required exactly equal to the volume, what you need for his project. The advantage of self-mixing compared to ready-mixed concrete in this point is that you only mix in 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 relationship between water and cement. Especially the last factor, the so-called "water cement value" is of crucial importance for the quality of a fresh concrete. The relationship between water and cement may only move in a very specific area. "Much helps a lot" is just as bad for the result as "As dry as possible".

However, concrete allows many moisture levels to be processed so that a suitable water-cement ratio can be found for each application.

The calculation of the water cement value is: w / z = water weight / cement weight

The water cement value chosen by us is 0.4. On 4 kg of cement come 1, 6 kg of water (1 liter of water corresponds to about 1 kg). This is because the cement can bind chemically up to 40% of the total weight during setting. If the water cement value becomes even smaller because the proportion of water decreases, it results in fine-pore pores and the cement quality deteriorates. For ultra-high strength and high strength concrete, however, water cement values ​​between 0.2 and 0.4 are required to minimize the amount 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. Choosing the right W / Z value depends on the exposure class.

An exposure class indicates to which environmental and environmental conditions the concrete is exposed:

environmental impact	exposure classes		stress intensity	max W / Z value
none		X0	none	no requirement
carbonation	XC	XC 1	dry	0.75
		XC 2	wet	0.75
		XC 3	damp	0.65
		XC 4	Wet and dry	0.60
chloride	XD / XS	XD / XS 1	damp	0.55
		XD / XS 2	wet	0.50
		XD / XS 3	Wet and dry	0.45
frost	XF	XF 1	Moderate water saturation	0.60
		XF 2		0.50
		XF 3	High water saturation	0.50
		XF 4		0.50
Chem. Actions	XA	XA 1	weak	0.60
		XA 2	moderate	0.50
		XA 3	strongly	0.45
wear	XM	XM 1	moderate	0.55
		XM 2	strongly	0.45
		XM 3	very strong	0.45

The surcharge is the largest volume fraction in concrete. It is most important for the compressive strength. The highest strength is blast furnace slag. However, this is only used in special applications, such as vaults. For normal applications, mixtures of gravel are the aggregate used. It is important that not any gravel is used, but always ready-made concrete gravel. The surcharge must always have the right ratio of small and large stones, otherwise its compressive strength is not given.

In the home improvement sector usually three supplement varieties are used. For thin, fine layers, such as screed, a gravel mixture with a grain size of 0-6 millimeters is processed. Solid stones, lintels, slabs or formwork are produced with a gravel mixture that has a grain size of 0-16 millimeters. For foundations, a gravel mixture of 0-32 millimeters is recommended. Concrete surcharge costs about 15 euros per ton.

The cement is also available in different compressive strength classes. Commercially available cement CEM I 42.5 N 25 kg is perfectly adequate for most home improvement applications. The bag to 25 kg costs about 2.50-2.80 euros

Calculation for one cubic meter

Concrete has a density of 2.4-2.5 kg / dm³. That's 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 sacks to 25 kg = 52 euros
(2300/5) x 4 = 1840 kg surcharge = about 25 euros

The amount of one cubic meter should always be mixed in a blender. Otherwise hardly any efficient production is possible. Mixing this amount by hand will only result in some of the concrete having already hardened while the last touched concrete is still liquid. This makes compacting the concrete almost impossible. The mixing ratio of 1: 4 is always the same. Whether mixed with a scoop in a mortar bowl or with a trowel in a mold, the relationship is always the same.

Concrete always has to be compacted after casting. To do this, tap the formwork from the outside or shake it with a rod in the concrete. The concrete must be completely distributed in the form and must no longer have any air bubbles.

Mixing yourself is worthwhile

The direct comparison to the finished concrete out of the bag shows that self-mixing can save a lot of money. A 25 kg bag ready-mixed concrete also costs 2.50 euros. Calculated on the 2300 kg of one cubic meter, this results in a requirement of 92 sack. That corresponds to costs of 230 euros. Thus, ready-mixed concrete from the bag is exactly three times as expensive as even mixed concrete. It is therefore recommended only for the smallest applications to choose the ready mixes. Ready-mixed concrete costs about 130 euros per cubic meter. For large applications such as strip foundations, false ceilings or floor slabs, however, ready-mix concrete is in most cases no alternative.

Tips for quick readers

• if possible do not use precast concrete
• Pay attention to the right amount of water
• select the correct supplement
• Thoroughly clean all tools after concrete production
• Always condense concrete