Mix the concrete yourself – knowing the correct mixing ratio is crucial for this. This determines the later properties of the material and thus the strength, durability and resistance. Whether as a foundation for a garden house, as a foundation for the wall or the construction of paths, the building project will only succeed with the right instructions. You can mix smaller quantities by hand; for larger quantities, we recommend using a concrete mixer.
If you are using instructions for mixing concrete, then you need to pay attention to the areas in which the concrete is to be used. The consistency and the mixing ratio not only influence the ease of processing but also the possible applications. Depending on the amount required, you can also calculate the amount of ingredients up or down using the rule of three. It applies here that the instructions are only a guideline and form the basis for the finished mixture. The concrete mix can only be used when the correct consistency has been achieved. You have to decide on a certain density of your concrete (lightweight concrete, normal concrete or heavy concrete).
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Manufacture normal concrete
As a rule, normal concrete is selected for private construction projects, which has strength class C20. This corresponds to a mixing ratio of 4: 1, i.e., one part cement and four parts gravel. The grain size should be a maximum of 32 millimeters. Water is added as a third component. The amount of water here should be half the amount of cement, but this can be varied in practice in order to achieve the right consistency. Depending on the amount required, this means:
- 25 kg of cement
- 100 kg of gravel
- 12.5 liters of water
Different mixing ratios
A universal mix for fresh concrete is:
- 1 kg of cement
- 4 kg of gravel
- 0.5 liters of water
The universal mixture is particularly suitable for do-it-yourself work such as steps and garden slabs.
The foundation concrete mix:
- 1 kg of cement
- 5 kg of gravel
- 0.5 liters of water
The foundation concrete mix is used for concrete elements in the ground. This includes, for example, the anchoring of fence posts.
A particularly robust mixture:
- 1 kg of cement
- 3 kg of gravel
- 0.5 liters of water
If the components are particularly affected by environmental influences and the concrete mix must be resistant and resilient, then this mix is the right choice. One possible area of application is street pavement.
Tip: The more fluid the fresh concrete, the easier it is to pour the concrete afterwards. A higher proportion of water also means a longer drying time.
Use rule of thumb as a mixing ratio
Do-it-yourselfers often use certain rules of thumb for mixing concrete. One possible variant and widely used variant is:
- 300 kg of cement
- 180 liters of water
- 1900 kg surcharges
= 1 cubic meter of concrete with strength class C25 / 30
However, you must note that the properties of the concrete depend heavily on the cement and aggregate selected. Therefore, rigid formulas can only be used to a limited extent and can only represent the basis for the mixing ratio. Adjustments must be made in individual cases.
Check the consistency of concrete
The consistency is an indication of whether you have chosen the right mixing ratio. It depends on the chosen application and must be determined before construction begins. In some cases, a tough consistency can be an advantage, in other cases a liquid fresh concrete is required. The quality of the concrete determines its workability, which is crucial for further construction work. In order to create a uniform definition, the different consistency areas are standardized and range from very stiff to very fluid. To check the consistency, various methods are available that can be used with simple means. These include, among others:
- Setting attempt
- Attempt to spread
- Compaction attempt
The setting attempt
Material and tools:
- frustoconical shape of standardized size
- Tape measure
- Place the frustoconical shape on a base plate on the floor.
- Bring the fresh concrete into the mold in three layers of the same thickness. Compress each of the layers with 25 strokes.
- Peel off a supernatant flush.
- Now pull the shape upwards without shifting it sideways or rotating it. The movement should be completed in around 5 to 10 seconds.
- The concrete stump collapses. Now measure the height of the stump at the highest point immediately after removing the form. The difference between the original and the new height is the slump. This now gives conclusions about the consistency:
- 10 mm – 40 mm: plastic
- 50 mm – 90 mm: soft
- 100 mm – 150 mm: (very) soft
- 160 mm – 210 mm very soft
- greater than 220: flowable
The spreading test is suitable for aggregate grain sizes of up to 63 millimeters. Self-compacting concretes cannot be checked using this method.
- Smoothing trowel
- Truncated cone shape
- Tape measure
- Moisten a table top with water and remove the resulting film of water with a smoothing trowel. The goal is a matt, damp surface. It is used to ensure that the consistency is not changed by the table.
- Place the truncated cone shape in the middle and fill it with fresh concrete. The protruding concrete is smoothed off at the opening.
- Now pull the shape up. The fresh concrete will spread out towards this.
- Now lift one side of the table without jerking 15 times about 4 centimeters and let them fall. There should be a period of 1 to 3 seconds between the individual runs. The fresh concrete spreads further due to the vibrations.
- Now measure the diameter of the mass. To achieve the most accurate value possible, measure crosswise twice and calculate the mean value in millimeters. Now round the value to 10 millimeters.
- The following table now shows the consistency of the mass:
- rigid (F1): 340 mm
- plastic (F2): 350 mm to 410 mm
- soft (F3): 420 mm to 480 mm
- very soft (F4): 490 mm to 550 mm
- flowable (F5): 560 mm to 620 mm
- very flowable (F6): 630 mm to 700 mm
The compaction attempt
With the compaction test you check stiff, plastic and soft concrete mixes. Use a rectangular container with a base of around 20 cm x 20 cm and a height of 40 cm. You will also need a tape measure.
- Now fill the bucket with fresh concrete. Pull the concrete smooth on the surface.
- Compact the concrete mix on a vibrating table. Air escapes and the surface sinks to the bottom.
- Now measure the distance s between the concrete surface and the top edge of the container. Since this does not have to be uniform on the four sides, calculate the mean value.
- Now calculate the degree of compaction v. Use the following formula for this:
v = 40 / (40 – s)
Tip: The 40 is created by the height of the bucket, which is 40 centimeters. According to the formula, divide 40 by the height of the concrete column after shaking it.
Now read the consistency from the following table:
- v is greater than 1.20: stiff
- v is between 1.19 and 1.08: plastic
- v is between 1.07 and 1.02: soft
When do I need which consistency?
If you work with components that have a high degree of reinforcement or a complicated geometry, then flowable concrete is necessary. Another factor for the choice of the mixing ratio is the delivery of fresh concrete. If this is to be conveyed with a concrete pump, the consistency must be plastic. Water can be added to liquefy the fresh concrete.
The German DIN standard defines various consistency areas. These are called very stiff, stiff, plastic, soft, very soft, flowable, and very flowable. The abbreviations for this range from F1 to F6, whereby F6 is considered to be very fluid.
The production of smaller quantities
Tools and equipment:
- Protective clothing
- Wheelbarrow or bucket
- Trowel or shovel
- Mortar mixer
Since your clothes can easily become soiled when mixing concrete and, in return, all the items you need must be at hand quickly, the utensils should be close at hand. To avoid skin irritation, you should also wear gloves and long-sleeved clothing. In addition, there are protective goggles that protect against splashes in the eyes.
Step 2: the mixing
If the work is done by hand, then put the gravel and cement in a wheelbarrow or a bucket. Mixing in the wheelbarrow is often chosen so that the fresh concrete can be easily transported.
Step 3: the stirring
Blend the mixture with a trowel or shovel.
Step 4: the water
Now carefully add the water. You have to be careful not to make the mixture too runny. Then stir the fresh concrete, a mortar stirrer will help.
Tip: Smaller quantities of fresh concrete are ready to mix in around 5 minutes. If fresh concrete is used in the garden, a paste-like consistency is recommended.
The production of larger quantities
Step 1: turn on the concrete mixer. Put around two-thirds of the water you need in the concrete mixer.
Tip: Dosing is particularly easy with a bucket.
Step 2: Now put the gravel in the mixer. Then the cement is added. The remaining water is now gradually added to the mass.
Step 3: Let the cement mixer run for a few more minutes so that the water, cement, and gravel can mix well together. The surface should have a slight sheen. If the consistency is still too firm, carefully add a little water.
Step 4: When mixing fresh concrete, the tools must be cleaned as quickly as possible so that the mass can be removed easily. Every tool and all surfaces that have come into contact with the concrete must be rinsed off immediately. You need to clean the concrete mixer and the shovels particularly thoroughly. Dried concrete residues are almost insoluble and therefore usually remain permanently on the floor or the tool. If concrete that is not required is left in the mixer, it can become unusable.
Tip: Caution is also required if the fresh concrete ends up in the sewer system. To prevent the pipes from clogging, you need to add enough water to dilute them. This applies even to smaller quantities of fresh concrete. Larger concrete residues must be properly disposed of in accordance with the guidelines on building rubble.
Tips for fast readers:
- Concrete consists of water, gravel, and cement
- the consistency determines the behavior
- liquefy by adding water
- can be mixed by hand or a cement mixer
- Check the consistency by attempting to set, attempt to spread, or attempt to compact
- Normal concrete: 25 kg cement; 100 kg of gravel; 12.5 liters of water
- Universal mixture: 1 kg of cement; 4 kg of gravel; 0.5 liters of water
- Foundation concrete mix: 1 kg of cement; 5 kg of gravel; 0.5 liters of water
- robust mix: 1 kg of cement; 3 kg of gravel; 0.5 liters of water