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Almost all modern floor coverings require an even base, which is easiest to achieve by making a floor screed. The process will take a long time. Before laying the coating, most likely, you will have to wait about a month, but the floor will be reliable. The cheapest way to make a screed mortar is from a mixture of sand and cement. Although there are other options.
The composition of the mortar for floor screed: DSP or concrete with gravel
The floor screed is most often made from a cement-sand mixture. That is, the solution contains only cement and sand, sometimes with additional additives. In the classic version, the screed is poured only by mixing sand and cement in a certain proportion, the mixture is diluted with water. Such a solution is also called sand concrete. In the sense that only sand was used as a filler. This is the cheapest option, but not the only one.
With a screed thickness of more than 5 cm, concrete with fine gravel aggregate can be used. Classical concrete: some amount of crushed stone is added to sand and cement. Its dimensions are no more than half the thickness of the screed. Since the minimum dimensions of crushed stone are 20-25 mm, the minimum thickness of the concrete screed is looming – 50 mm.
But concrete with gravel is heavier and more expensive. Its plus is that it is less prone to cracking during shrinkage and therefore warm floors are poured with concrete. Here a small number of cracks is critical. For a conventional leveling screed, the presence of shrinkage cracks is not such a problem. The budget is usually limited and therefore the cement-sand mixture is more often chosen.
Sometimes gypsum-based compounds are used for screeding. But they are afraid of moisture, the solution has a shorter life, the surface strength is lower. All this makes them unpopular. Gypsum-based screed mortar has been a rarity in recent years.
Brand of cement-sand mortar for screed: choice of strength
What brand of cement-sand mortar is used for screed? Previously, they could put M50 or M75. Now the minimum is M150. Why? Because, above all, the finishing requirements were much lower. What was previously considered normal – small holes, cavities, cracks – is now unacceptable. And it’s not just aesthetics. To a greater extent, these are the requirements of manufacturers of finishing coatings. They require an almost perfect surface that does not dust, and only a solution with a strength of at least M100 can give such a surface.
There are other reasons why a higher brand of solution is used. First. No one is sure about the quality of the cement, so they prefer to play it safe than redo it. The second is that modern coatings require an even, solid base and the screed mortar must be strong. And thirdly, under self-leveling compounds or under modern tile adhesive with polymer additives, you simply cannot put a low grade. So that the two parts of the coating do not exfoliate, the difference in strength should be no more than 50 units. That is, if the leveling compound has a strength of M250, the floor screed mortar must have a strength of M200 and not lower. The same goes for tile adhesive. So pay attention to it.
The proportions of the classic screed from DSP
The classic floor screed mortar, as already mentioned, is cement with sand diluted with water. The proportion (amount of sand per unit of cement) depends on the required strength of the screed and the brand of cement used. To make the floor surface durable, use expensive Portland cement grade M400 and higher.
For floor screed in utility rooms, you can also use the cheaper M300. It will go a little more, but there will be savings. For a foundation in a house or apartment under modern coatings, it is better not to take such cement. Alteration will require significantly more savings on cement.
It seems to beginners in the construction business that if you take more cement, there will be a stronger screed. And here it is not. For strength, the correct ratio of all components is important, and an excess amount of cement can cause a decrease in strength. If you want a stronger screed, use high-quality cement and accurately measure the proportions. Water, by the way, is also not worth taking more. This will increase the fluidity of the solution, but increase the number of shrinkage cracks. So once again: to get durable and reliable concrete, you must strictly observe the proportions.
What kind of sand to take
It is better to take river sand, and – washed, at least two fractions: large and medium. Why river? Because it has sharp edges, and this reduces the likelihood that it will settle into the lower layers. This is understandable. Why washed? It has a minimum of dust. The less dust, the higher the strength of the solution. Sand is also needed in different sizes so that the strength of the solution is normal.
If you are going to lay an expensive coating on the floor with high requirements for the strength of the base (parquet, parquet or engineering board, vinyl tile), it is better to take just such sand. There will be fewer problems.
Mixing sequence
When a floor screed mortar is made, the dry ingredients are first mixed – cement and sand. With manual mixing (in a trough), what to throw in the first place – cement or sand, there is not much difference. If a concrete mixer is used, sand is immediately thrown in and twisted for a couple of minutes without cement. Then, gradually, usually with shovels, cement is added. After each serving, wait until it is more or less evenly distributed, then throw the next one. After adding the entire amount of binder, mix until a uniformly colored mixture is obtained.
When the dry ingredients are mixed until a homogeneous gray mass is obtained, water is gradually introduced. It is calculated from the amount of cement. Usually, 1-0,45 parts of water are taken for 0,55 part of cement. Why don’t they specify exactly? Because the amount of water depends on the moisture content of the sand. And it is desirable to pour a minimum of water: so there will be fewer cracks during drying.
Ready mortar or sand-cement mixture
Those who have at least once kneaded DSP or concrete on their own are more likely to buy ready-made concrete. And not a mixture of sand and cement in bags, but concrete from a concrete mixer. Yes, the money is more expensive, but it takes much less time and effort. Another plus of this solution: pouring without cold seams. And this means less cracks and problems in the future. The next plus is that concrete mixers can deliver the solution to the desired floor. Imagine that you need to drag a couple of tons of sand and cement. Even if there is a freight elevator, it is not easy. It can be costly if you pay helpers. Climbing stairs “on your shoulders” is generally a problem.
What are the advantages of buying a ready-made sand-cement mixture in bags? In that the proportion is maintained exactly, sand is used in several fractions and in the right quantities. That is, the screed is guaranteed to have the desired strength. Minus – the price. You can buy the same amount of cement and sand for a much smaller amount. This is if you do not bother with sand fractions. If you take care of this, then the savings will be less: not all factions are cheap.
Additives: necessary or not?
In a classic floor screed mortar, it may be recommended to add plasticizers and fiberglass or other micro-reinforcing substances. Are they needed or not? First you need to understand what it is and why.
plasticizing additives
Plasticizers are substances that increase the plasticity of DSP. It is easier to work with such solutions. Concrete with a plasticizer lays down better, is easier to level, gives a smoother surface. In general, if all the components are of normal quality, well mixed, then it is not difficult to work with them with closed water. With additives, of course, it’s easier. But factory plasticizers cost a lot of money, and this increases the cost of the screed. They need to be added in small quantities, but the bill for filling the floor in the house goes to cubic meters, so the costs will be tangible.
As usual, the craftsmen found a replacement for factory plasticizers. Add ordinary soap to the solution. Its consumption is quite small – a glass or so for one concrete mixer. The plasticity of the solution increases, so many people use this type of additive. For beginners, it is worth saying: do not exceed the recommended dose. The solution will not get better, but it may well be worse. Soap increases plasticity by “lubricating” the sand, reducing its “adhesion” to the cement slurry. Exceeding the dosage can lead to a decrease in the strength of the screed. So be specific.
Micro-reinforcement
As you know, when drying, the cement-sand mortar shrinks. The shrinkage value is from 1,5% to 3% of the volume. Specifically, the percentage of shrinkage depends on the amount of impurities (if the sand is washed, the shrinkage will be less), the correctly selected aggregate composition (in this case, sand), exactly the proportions observed, and a number of other conditions and factors.
Everything would be fine, but during shrinkage, cracks form in the solution. They are always there, only larger or smaller, in larger or smaller quantities. To reduce the number of cracks, micro-reinforcement materials are added to the solution. Most often, fiber is used in everyday life. It happens:
- fiberglass;
- basalt;
- metallic;
- polypropylene.
The most popular for domestic purposes is polypropylene fiber. It is the most inexpensive and gives a good result. How does it work? 100 grams of this supplement contains a huge amount of synthetic fibers. They are very thin, but synthetics are highly durable. These fibers are randomly but evenly distributed over the entire thickness of the solution. In concrete, they form a semblance of a lattice in space. When stresses occur during the drying of the screed, they bind the parts of the solution together, reducing the number and size of cracks.
The second effect of fiberglass is a smoother and more durable surface. So this additive to the floor screed solution is more useful and definitely worth using. But again, strictly according to the recommendations. It seems that if you add more fiber, then there will be less cracks, but no. The strength of the tie will decrease.
Calculation of the volume of mortar for screed
To determine the volume of materials, you need to know how much solution is required. Then, using the necessary proportions for the screed, it will be possible to calculate the approximate amount of sand and cement. To calculate the solution, you need to know the area on which we will fill the solution and the thickness of the layer.
It is easy to calculate the fill area: multiply the length of the room in meters by its width. We get the area. You should already know the maximum and minimum screed layer. According to the degree of evenness of the base, it is possible to determine approximately the average thickness. If the found area is multiplied by the thickness of the screed and we get the required volume of the solution.
Let’s look at an example. The room is 2,8 m by 3,4 m, the thickness of the screed is 6 cm. We find the fill area – 2,8 * 3,4 = 9,52 m². To get the cubic meters of concrete that we need, we need to convert 6 cm to meters. To do this, divide 6 cm by 100. We get 0,06 m. Now we multiply the pouring area by this figure: 9,52 * 0,06 = 0,5712 m3. That is, for a room area of 9,5 squares with a screed thickness of 6 cm, approximately 0,6 cubic meters of solution will be required. With such a volume, the floor screed mortar will definitely have to be kneaded on its own. No concrete plant will deliver less than a cubic meter of mortar.
If it will be necessary to fill the screed in several rooms at once, you can first calculate the area of u6bu9ball rooms for pouring, then multiply by the thickness of the screed. This option is possible if there are no large differences in height between different rooms. If in one room the screed is XNUMX cm, in another XNUMX cm, it is better to calculate the volume for each room separately, and then add up the results.
Consumption of cement for screed
If you decide to mix the screed mortar yourself, you need to decide on the amount of cement that you need. It can be calculated based on the found volume of the solution. There are tables that show the consumption of cement for a screed, depending on the brand of mortar and binder.
Calculate the amount of cement for one cube of sand concrete screed grade M150. If we use cement M400, it will take 400 kilograms of cement per cube (according to the table). To find how much cement will be needed for the example described above, it is necessary to multiply the found volume of the solution by the norm: 0,6 m³ * 400 kg = 240 kg. That is, this room will need 240 kilograms of cement. To determine the number of bags, divide this figure by the mass of cement in the bag.
- If there are 50 kg of cement in a bag, it will be necessary: 240 kg / 50 kg = 4,8 bags.
- When packing 25 kg: 240 kg / 25 kg = 9,6 bags.
Other packaging also happens, but is rare. When you decide on the brand and manufacturer, you can accurately calculate the number of bags of cement per floor screed.
Another cement consumption can be calculated based on the amount of sand available. You never know. Maybe someone will buy sand and so that there are no residues, it must be all used up.