Smoothing concrete – when delivered, concrete is a liquid mixture of cement, water, sand, and gravel. So that it can be used sensibly, it must be poured into stable molds. Concrete primarily has technical tasks. Visually, it is not very appealing due to its gray color. Nevertheless, it makes sense to let it solidify with a surface that is as smooth as possible.
A smooth surface on the sides of a concrete body provides a good basis for further processing. A smooth top, for example with a concrete ceiling, makes the further construction of the floor coverings much easier. Concrete is a very flexible material that can be processed with the appropriate equipment in any hard state.
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Concrete maturation process
Fresh concrete hardens in three stages:
Concrete is poured into the formwork in a liquid or plastic state. It is completely soft and only follows the laws of gravity. Concrete can neither be kneaded nor piled up, but always flows and sags along the path of least resistance. It retains this state for approx. 1 hour after mixing.
When the concrete “matures”, it no longer flows, but it still has a moist and detachable surface. The cement slurry can still be washed out. This is how exposed aggregate concrete is produced.
The concrete is completely hardened or set after approx. 24 hours. Its surface can now only be processed with heavy equipment and a lot of force. But the industry also offers the right tool for this level of hardness.
Smooth and compact fresh concrete
The easiest way to smooth concrete is to pour it. A distinction must be made here as to whether the concrete body should only have a surface that is as smooth as possible on its upper side or on the reverse side. With wall elements, a smooth surface on the side makes sense, with beams and concrete ceilings also on the underside of the concrete. For this purpose, the inside of the formwork is lined with panels that have the smoothest possible surface.
The standard material for this purpose, which has been tried and tested for decades, is “Betoplan”. These are easy-to-work multiplex panels with a, particularly smooth surface. These panels are simply cut to size with a circular saw and attached to the inside of the formwork with as few nails as possible. When calculating the formwork, the wall thickness of the concrete plan must be taken into account. Betoplan is 21 to 27 mm thick and costs around 28 euros per square meter. If a structured surface is desired instead of a smooth one, very interesting effects can be achieved with the appropriate facing formwork.
Before pouring the concrete into the formwork, it must be generously sprayed with formwork oil. Dry areas can tear out when removing the formwork. To get the concrete body smooth again at this point, reworking by filling and smoothing is necessary. You can save yourself this if the formwork is oiled.
Wherever concrete and external formwork meets, the escaping cement slurry automatically creates a smooth surface. The greatest enemy of the smooth structure is trapped air. This is removed during concreting through consistent compaction. Vibrators or vibrating bottles are used for this purpose.
Vibrating bottles, also known as “compressors”, are construction equipment that is particularly important for placing concrete in wall formwork. They ensure that the concrete is really completely distributed within the formwork and also runs into the furthest corners. When concreting concrete ceilings, the vibrator ensures that the surface is leveled, which makes the subsequent smoothing particularly easy.
Concrete vibrators are highly effective devices that make the work of concreting considerably easier and can significantly improve the results. To use the vibrator correctly, a few basic knowledge are important.
The vibrator consists of a long and heavy hose that is connected to an electric motor on one side and a bottle-shaped tip on the other. An unbalance rotates in this tip, which generates the strong vibrations of the shaker.
In the case of wall formwork, it makes sense to lower the vibrator to the bottom of the formwork before pouring the concrete. Once the formwork is filled, the vibrator will no longer reach the ground. It is ideal if concrete can be poured in layers. Before each filling step, the concrete is thoroughly “shaken off” until no more air bubbles rise. But be careful: if you vibrate too much and for too long, the concrete separates and loses its strength.
Consistent compaction is particularly important for heavily reinforced formwork. Only structural steel completely enclosed with concrete is protected against corrosion and can thus ensure permanent stability. When vibrating reinforced concrete, you have to be careful that the vibrating bottle does not get stuck. This happens relatively easily. In the worst case, the bottle has to be cut off and left in the formwork.
In the case of ceiling formwork, the vibrator is immersed into the fresh concrete every 30-50 cm. The concrete levels itself immediately and creates an already very even surface. A well-shaken and leveled concrete ceiling is much easier to smooth and coat.
After concreting, the vibrator must be cleaned thoroughly. Caked concrete is difficult to remove. That is why it makes sense to also spray the vibrator thoroughly with formwork oil before use. A vibrator costs around 36 euros in the daily rent.
After shaking off a concrete ceiling or a floor slab, smoothing takes place. Various methods are available for this. It is enough to nail a piece of betoplan to a broomstick to make functional concrete smoother. On the other hand, more professional handheld devices promise better results. These are sold under the name “concrete smoother”, “concrete puller” or “concrete distributor” and cost between 40 and 180 euros. They consist of sheet steel to which a holder for a handle is welded. These hand tools are pulled directly over the freshly compacted concrete. They only serve to push floating gravel back into the fresh concrete mass. The concrete can be leveled a little with light shaking movements.
The next evolutionary stage of the hand-operated concrete trowels is pullers with a motor. A small petrol engine sits on the puller, causing the transverse blade to vibrate slightly. This results in a particularly smooth surface.
These devices are a real recommendation, as they make work much easier. Unfortunately, they are rarely found at rental stations. The daily rent for a gasoline-powered concrete trowel is around 65 euros.
Smooth maturing concrete
The most widely used machine for smoothing concrete is the power trowel. It consists of a powered propeller that slides over the freshly hardened concrete. In order to be able to use the power trowel, the concrete must have set so far that you can step on it without sinking into it.
A hand-operated power trowel is sufficient for small concrete surfaces. It is driven by an electric or gasoline engine and simply swiveled back and forth on the handle across the concrete surface. The wings, which constantly stroke the fresh concrete, not only smooth the surface but also stimulate the capillary action of the fresh concrete. This causes the cement slurry to float up and also ensures an absolutely flat surface.
Ride-on trowels are available for large concrete surfaces. They are particularly efficient. They also require some practice to operate. Ride-on trowels are equipped with two propellers. Hand-operated trowels cost between 65 and 120 euros a day. Ride-on trowels, also known as twin trowels, cost around 250 euros a day. However, they usually also cost a large deposit, which is only partially refunded if the device has not been cleaned. Thoroughly spraying with formwork oil and just as careful cleaning can help save costs here.
The wing propeller creates a concrete surface with a defined evenness. It can again be used as a starting point for structuring that is also defined.
For some applications, a roughened concrete floor makes sense, for example, if a flowing screed is to be applied. A roughened floor strengthens the connection between the individual layers. To roughen and structure the freshly smoothed concrete, it is sufficient to carefully pull a plastic street broom over it.
Smoothing hard concrete: blasting, grinding and polishing
Modern construction technology also offers suitable smoothing devices for renovation projects with which concrete surfaces can be straightened. What was a very dirty and exhausting task just a few years ago can now be done quickly and cleanly with modern machines.
The breakthrough in the smoothing technology of old concrete was the shot peening machine. This shoots small steel balls at high pressure onto the concrete floor and sucks them up again immediately. These machines are therefore extremely dust-free and very easy to operate. With working widths of 30-50 cm, even large concrete surfaces can be quickly leveled. A shot peening machine costs between 100 and 250 euros a day to rent. However, they are far too expensive for the do-it-yourselfer to buy. A purchase price of 6,000 to 22,000 euros must be expected for a new shot peening machine.
The shot peening machine levels out the greatest irregularities in an old concrete surface, but it only creates a very rough surface. Concrete grinding machines are used to smooth this roughened concrete. In terms of construction, these are very similar to wing trowels or polishing machines: A motor with a sanding pad is attached to a long handle, which drives the sanding pad. Elements with a diamond coating or tungsten carbide are used as abrasives. These are very durable and only wear out slowly. Modern concrete grinding machines have an integrated extraction system so that working with these devices is very clean. A concrete grinder is also ideal for removing old residues of glue or filler.
Hand grinders for kneeling corner processing cost around 30 euros a day. Disc grinding machines, Hand-Held or walk-behind are available at a rental price of around 90 euros per day.
In some cases, sanding a surface is not enough to smooth the concrete. Then, in the final step, the concrete surface can be polished. Industry and trade also offer suitable devices for this purpose. First of all, every commercially available concrete grinding machine can also be converted into a polishing machine with the right abrasive. However, the machines specially designed for concrete polishing promise a particularly fine and fast sanding. These are available in many variants on the market. Two-plate machines, which are technically closely related to the ride-on concrete trowels for fresh concrete, are particularly efficient for this. These machines work very quickly and, thanks to their wet grinding, produce a particularly smooth surface.
Freshly smoothed and polished concrete surfaces are very sensitive. After the top layer of cement has been removed, the capillaries in the concrete are open and therefore very absorbent. Any spilled liquid now penetrates deep into the concrete and creates unsightly stains. It is therefore very important to seal the surface immediately after sanding and polishing the concrete. Concrete sealing based on epoxy resin, therefore, offers a permanent and scratch-resistant solution. These agents, also sold as “liquid plastic”, can be applied with a roller or simply poured out. They level themselves and therefore create an absolutely smooth surface. Concrete sealing is commercially available as a clear or colored medium. The price per liter is around 48 euros. However, the installation height is only approx. 0.5 millimeters, so that even small containers are very economical. With clear concrete seals, the ground and polished concrete darken and creates a very appealing, high-contrast pattern.
Tips for readers in a hurry
- Always shake off fresh concrete thoroughly
- When concreting columns or walls, insert the vibrating bottle beforehand
- do not jiggle for too long
- Machine trowels are very effective on floor slabs and ceilings
- Do not step on smoothed concrete
- cover the fresh concrete with foil at hot temperatures
- Grinding and polishing concrete creates interesting optical effects
- Always seal the ground and polished concrete