What is dry lean concrete
Buy ready-made lean concrete - strength & prices - instructions for mixing yourself
Lean concrete is a mixture of cement and concrete aggregate. This surcharge is the same gravel-sand mixture that is used in normal concrete. Normal concrete for the construction of pressure-resistant structures is mixed in a ratio of one part of cement to four parts of aggregate. Lean concrete, on the other hand, only receives half of the cement - it is mixed in a ratio of one to eight. This makes this building material very inexpensive, but it does so at the expense of its strength.
Lean concrete - what you should know
What can normal concrete do?Normally mixed concrete made from aggregate and cement, mixed in a ratio of 1: 4, is sufficiently suitable for all tasks involved in building your own home. It has a compressive strength of 12-25 N / mm². There are still concretes with significantly higher strengths. These are not used in home construction. For example, vaulted concrete has a strength of 45 N / mm².
The compressive strength of concrete depends on its water content, its cement, its aggregate and, of course, its mixing ratio. The cement acts as an adhesive, which connects the aggregate of the concrete with each other and leads to a permanent and solid form. The leaner the concrete, the lower its compressive strength - but the lower the prices per cubic meter.
Use of lean concrete
Lean concrete is often used in horticulture. It is well suited for setting garden stones or as a foundation for fence posts. If it is only a matter of giving a structure that is only slightly stressed, then lean concrete is usually completely sufficient. Even simple wooden huts such as carports, garden sheds or tool sheds can be well equipped with a foundation made of lean concrete. That keeps the prices for these lightweight constructions low. However, a brick garage can no longer be built with lean concrete. A pressure-resistant concrete must be used for the foundation.
A particularly popular application for lean concrete is the so-called "blinding layer". This is an approx. 2-5 cm thick layer that is put in immediately after a construction pit has been excavated. A blinding layer is a quick and easy measure to prepare the excavation for all further work. No slush forms in the pit after rainfall. The standing water no longer seeps in through the blinding layer. But it can now be pumped out very easily. Immediately after pumping out, the construction pit is accessible again. This would not be possible without a blinding layer.
Lean concrete can also be used to secure slopes. For this purpose, the concrete is poured out at the edge of the pit and reinforced with a reinforced steel mat. This prevents the slope from continuously softening when it rains and slipping into the construction pit. However, this does not work for deep pits with basements. Sheet piling remains the best way of securing slopes. Lean concrete slope protection should not be used on slopes over one meter in height.
After all, lean concrete is also very popular for backfilling well foundations. In contrast to unconnected gravel, lean concrete does not wash out.
Prices for ready-mixed concrete
Concrete that is ordered from the factory and delivered to the construction site is called “ready-mixed concrete”. He is driven into the mixer truck. The concrete transporters with two axles can deliver four cubic meters of concrete, the trucks with three axles have a capacity of six cubic meters. The largest concrete trucks are the four-axle vehicles. They deliver eight cubic meters of concrete. It is therefore worthwhile to calculate exactly how much concrete is needed at what point in time. A blinding layer of 100 square meters and a thickness of 5 cm (i.e. 0.05 meters) only has a volume of 5 cubic meters. That makes ordering a truck hardly worthwhile and the use of the mixer makes more sense.
The prices are essentially based on the compressive strength class. The more pressure-resistant a concrete is, the more expensive it is. The most important factor for the compressive strength is the surcharge. Immediately afterwards come the type of cement and the amount of cement added. Accordingly, lean concrete consists of a slight aggregate and only a small amount of cement. Amazingly, the cheapest and most expensive types of concrete are only about 40-50% apart. However, significant amounts of concrete are consumed on a normal construction site. Typical prices for transported lean concrete are:
- Lean concrete with a compressive strength class of c 8 - c 10: 89 - 91 euros per cubic meter
- for drainage and filtering tasks (pressure resistance up to c 30): 96 - 108 euros per cubic meter
- Lean concrete for blinding layers (compressive strength c 12): 99 - 101 euros per cubic meter
There are also travel costs. These are divided into zones. These encompass a radius of 5 - 30 km and are charged at an additional 56 to 70 euros.
You can see that ordering lean ready-mix concrete is generally not useful. Manual labor is much cheaper here and is also easier to implement.
Mix lean concrete
Mixing lean concrete is pretty easy. However, it is highly recommended that you use a free fall blender. Mixing by hand is very exhausting and beaters on drills are quickly overwhelmed by the gravel.
Basics of concrete mixers
Before the instructions for lean concrete are delivered, a few words about concrete mixers. A distinction is made between free-fall mixers and compulsory mixers. No matter how small, a construction site always has a circular saw and a mixer. Both devices are available today at such low prices that it's not worth going without. The rental solutions do not make sense either, as both devices are used for the entire duration of the construction site. Here we recommend using a cheap but new machine from the hardware store if necessary. The prices start today at just under 200 euros per machine. Even if they fail after the construction project, they have still done their job.
Used machines always run the risk of breaking while working. Nothing is more important in a construction project than a continuous flow of work. So don't take any risks and buy new devices from specialist retailers. With many devices and other places there are opportunities to save money on a construction site. However, you should always play it safe with mixers and circular saws.
For lean concrete you need:
- Free-fall mixer: From 200 euros
- Wheelbarrow: From 30 euros - but we recommend fully galvanized wheelbarrows with solid rubber tires. These start at around 70 euros.
- 2 shovels: used from 5 euros or new from 15 euros.
- Gravel From approx. 10 euros / 1000 kg
- Portland cement: From approx. 3 euros / 25 kg sack.
- 10 liter bucket: From 1 euro (mayonnaise bucket from the chip shop)
- Eye protection
- Concrete puller
- Garden rake with handle: From 10 euros
The drum of the mixer is rotated at an angle of 30-45 °. Then the machine is turned on.
Danger: Never reach into the drum when it is running! Wear eye protection when mixing concrete! If cement splashes in the eye, rinse immediately and consult a doctor! There is a risk of loss of vision!
1. Put a bucket full of water in the rotating drum of the mixer.
2. Then add a scoop of cement and wait until a cement slurry has formed without lumps or dry pockets.
3. Then add four shovels of gravel.
4. Now check the humidity. If the concrete is to be poured precisely (e.g. to support low boards), the concrete must be drier. If the concrete is to be spread over a large area (e.g. for a blinding layer), the concrete should be more fluid.
5. Top up with water as required.
6. Then add another scoop of cement.
7. Then add four more shovels of gravel. That should be enough for now. It doesn't make sense to mix too much lean concrete at once. The fuller the drum gets, the harder it becomes to handle.
8. Now pour the concrete into the wheelbarrow and drive it to the place of application.
Drainage concrete is open-pored concrete. It is also made from lean concrete. The instructions provide for making it as dry as possible. The gravel only sticks to one another in certain spots. Drainage concrete is used to make the concrete layer permeable to seep. It is very suitable as a "load distribution layer" under open terraces. The strength of the drainage concrete is completely sufficient for this. The advantage of drainage concrete is that it does not seal the floor. Rainwater can seep in and does not puddle. It is also fed back into the water table. Drainage concrete is very popular with building authorities, as they usually want to prevent surface sealing.
Apply blinding layer
Since the strength plays a less important role in lean concrete, the water can be used quite generously. The more fluid the concrete, the easier it is to spread it. If there is any reinforcement mesh left over, it is definitely worth placing it in the thin blinding layer. The concrete puller is suitable for the precise application of the blinding layer. This can easily be made by yourself. For that you need
- approx. 10-15 cm wide and 1.50 meters long board
- 2 battens
The board is nailed to the end of the broomstick in a T-shape. An angle is chosen so that the flat side of the board rests flat on the floor when the operator holds the puller in his hand. Then the transverse board is fixed in a triangular shape to the broomstick with the two slats. This prevents wobbling during removal.
The concrete is poured as wide as possible. Then it is pulled apart with the rake and smoothed with the puller - done.
Buy ready-made lean concrete
Lean concrete cannot be bought in bags. Ready-mixed concrete as bagged goods is already very uneconomical in itself. It does guarantee strength, but you can achieve this yourself by following the instructions for mixing. The prices for the precast concrete are far too high. If you don't need very small amounts, you should always use the self-mixes.
Too Much Concrete?
When ordering ready-mixed concrete it can happen that a small amount is left over. In this case, follow these instructions:
Throw leftover ready-mixed concrete for which there is no longer any use in a heap that is as compact as possible. Then bend a hook made of structural steel and stick it in the pile. Bend the ends of the hook into an S-shape. Dimension the hook so that it goes as deep as possible through the pile. Wiggle the hook a few times until it is completely covered. Then let the lump harden. Now the lump can be conveniently attached to the crane and transported away. After a week, the lump will have sufficient strength to be attached.
Tips for fast readers
- Schedule clean shifts for Friday afternoons. Here, high-quality concrete can be ordered inexpensively as excess quantities.
- If possible, always insert reinforcement meshes in the case of clean layers and slope supports.
- Line the terraces with drainage concrete.
- Calculate all quantities very precisely for ready-mixed concrete. The more precise the amount of concrete, the more precisely the concrete can be ordered. Pay attention to the sizes of the transporters. always order a full 4, 6 or 8 cubic meters. If necessary, ask the concrete plant which transport sizes are available.
- Do not use lean concrete where high strength is required!
Do you need more information about concrete, fine concrete or craft concrete? You can find more informative instructions and tips here:
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