What binds rubber to concrete
Ready-mixed concrete: costs, processing and tips
Mix it yourself or use ready-mixed concrete? Most hobby craftsmen will ask themselves this question at some point. The foundation for the garden shed, fence posts or rotary clothes dryer is built with ready-made concrete from a bag in no time at all - and the costs are manageable.
What is precast concrete?
Ready-mixed concrete is offered as a dry product in sacks and already contains all components such as cement, sand and chippings in the perfect mixing ratio - you just have to mix it with water. A chemical process then ensures that the concrete hardens. In the trade, this ready-mixed concrete is also called dry concrete. The right mixing ratio of water and cement is decisive here. Depending on the manufacturer and product, this water-cement ratio can be different. Ready-mixed concrete is available in a wide variety of recipes. Some products are also offered as "lightning concrete" or "quick and easy concrete" because, thanks to special additives, they harden particularly quickly as soon as they come into contact with water.
If you do not want to mix the precast concrete with water yourself, you can also buy fresh concrete from a specialist dealer and then tap it, for example, at a fresh concrete filling station. If you need larger quantities or have special requirements for the concrete, you can mix it individually and have it delivered as so-called ready-mixed concrete with a special vehicle. However, this is usually only worthwhile for larger construction projects - for example, if you have to pour a floor slab for a larger building.
Which aggregates does precast concrete contain?
Concrete contains various coarse and fine-grained aggregates. These are jointly responsible for its properties. While normal building concrete, which is mostly used by craftsmen, usually only contains sand and gravel in addition to cement, expanded clay and brick chippings are used for lightweight concrete blocks and basalt and barite for the production of heavy concrete. Larger grain proportions are important for hardness. Sand and small grains fit well into cavities. So you need less cement as a binding agent and have a higher compressive strength. There are a total of seven strength classes for concrete, which are precisely defined in the corresponding DIN standards. Ready-mixed concrete can also contain additives such as plasticizers, sealants and setting accelerators.
What does precast concrete cost?
Dry concrete in bags from the hardware store are usually a cheap option if you only need small quantities and have no special requirements for the ready-mixed concrete. Here you can expect around three to five euros per 25 kilogram sack. However, the price varies depending on the brand, composition and strength class. For larger quantities, such as pallets, you usually get discounts. It is important that you estimate the required amount correctly - for this you calculate the requirement in cubic meters, i.e. length times width times height. You get about 13 liters of freshly mixed concrete from a 25 kilo sack. Check the manufacturer's information on the yield in each case. If you have decided on ready-mixed concrete, you pay between 65 and 100 euros per cubic meter, depending on the strength class. In addition, there is a surcharge of around five to ten euros per cubic meter if you need certain additives such as antifreeze, superplasticizers or setting accelerators. In addition to the pure costs for the material, with ready-mixed concrete you of course also pay transport costs, unloading time or disposal of residual concrete. Watch out for hidden costs like weekend surcharges for Friday afternoons.
How do you mix ready-mixed concrete?
If you mix dry concrete with water, you should stick to the manufacturer's mixing ratio as closely as possible. Then you also have the concrete properties specified by the manufacturer, such as the stated strength. When mixing precast concrete, be careful not to inhale the concrete dust. Depending on the product, wear protective goggles and rubber gloves: Concrete has a high pH value in the strongly alkaline range, is correspondingly highly corrosive and dries out the skin.
If you're mixing smaller amounts of precast concrete, you can do so with a trowel or shovel in a bucket, masonry bucket, or wheelbarrow. Alternatively, you can use a drill with a special concrete whisk. Gradually add the required water and mix the concrete very thoroughly - if it still contains dry lumps after mixing, it will not achieve the necessary strength.
For larger quantities, you should rent a concrete mixer at a hardware store - in this case, however, it is considerably cheaper to produce the so-called in-situ concrete yourself from the ingredients cement, sand, gravel and water, because the thorough mixing of the aggregates, which is not required with ready-mixed concrete, is included no problem with the appropriate technology.
How do you process precast concrete?
Once you have mixed the concrete, you should use it quickly as it hardens quickly. To do this, fill the fresh concrete into the prepared formwork and compact it. For example, if you want to pour a point foundation for a post, fill in the concrete in layers and compact each individual layer with a wooden slat or a small tamper. Proceed as follows:
- Mix the ready-mixed concrete "earth-moist" - it should make a very tough paste, but not flow.
- Depending on the depth, gradually fill the foundation hole up to about halfway with concrete and stamp each layer with a wide wooden batten to remove any air pockets.
- Now insert the metal post anchor and push it into the damp concrete until it is the right height. Then align it with the help of a tensioned mason's cord and a post level so that it is exactly vertical and in the correct position.
- Pour the remaining precast concrete in portions into the hole up to the top edge of the foundation and also compact it several times.
- After filling the foundation, check the position of the post anchor again and correct it if necessary. If you have to pull the post anchor up a bit, you should compact the concrete again afterwards.
- Smooth the upper edge of the foundation with a mason's trowel and bevel the upper edge slightly outwards so that no rainwater remains on it later - it would otherwise accelerate the corrosion of the post anchor.
- Allow the concrete foundation to harden for at least a week, depending on the product and the weather, before setting up the post.
The processing of quick-setting concrete is often even easier, depending on the manufacturer: it is poured into the foundation hole in layers and compacted without prior mixing with water. Then pour some water over it with a watering can and pour in the next layer. However, very quick processing is also important here, since depending on the product, quick-setting concrete often sets after a few minutes and then becomes brittle if, for example, the position of the post anchor has to be corrected.
How long does precast concrete take to set?
The setting time of precast concrete is very different depending on the product, temperature and ambient humidity. After about 28 days, every concrete has reached its so-called standard strength and can be fully loaded. The setting time is, however, strongly dependent on temperature - the standard strength is only achieved at an outside temperature of at least 12 degrees in the specified period. At 5 degrees, however, you have to wait about twice as long. By the way: It takes several years for concrete to fully harden - the material has only reached its final strength when the water it contains has completely evaporated.
With the erection of posts or the erection of a wall on a point or strip foundation, however, you usually do not have to wait until full strength has been achieved, as the pressure load is comparatively low. For structures with low loads such as pergolas, fences or garden walls, allow a week to wait before you can continue building. Rapid concrete sets after 5 to 30 minutes, depending on the product. However, you should also give it at least 24 hours to harden before, for example, putting up a post or erecting a garden wall on it. It also only reaches its standard strength after the specified waiting time. Rapid concrete is rarely used in building construction due to the critical processing time. However, it is a good choice for setting fence posts in concrete.
While it is setting, it is important that you protect the concrete from dryness and low temperatures: in case of frost, it is essential to cover it with a foil or - even better - a thick layer of bark mulch so that the water inclusions do not freeze. In hot and dry conditions, you should wet the surface regularly. In the case of foundation slabs, for example, a lawn sprinkler is often set up in the first few days to keep the hardening concrete evenly moist. Simply moisten funky and strip foundations from time to time with a watering can.
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