What Is a Semi Dry Screed, Its Advantages and Weaknesses

Semi-dry screed, as usual, is used for leveling rough floors for various coatings or for use in industrial floors after sanding and painting with special concrete paints. The changes relate only to the installation technology, which is caused by some innovations introduced into the traditional solution formula.

Compared to the usual method of pouring concrete or a mixture of cement and sand, the semi-dry method has several advantages, but it does not lack disadvantages. You will learn about all the advantages and disadvantages of semi-dry floor screed by reading the article below. In the analysis, it may seem that the disadvantages of semi-dry floor screed are very significant, but if so, it would not become so widespread, especially in industrial construction.

Advantages and disadvantages of semi-dry screeds

What is a semi-dry liner and what materials are used for it

Among all types of screed, semi-dry occupies a middle position between normal (wet) and less ordinary dry. Compared to conventional screed, it is characterized by a lower water content in the finished solution. The remaining components – cement and sand, are used in normal proportions, and the strength of the semi-dry mixture after hardening is practically indistinguishable.

Standard types of semi-dry screeds include:

k1

Cement brand M500.

k2

Coarse sand with fractions not less than 0.25 mm.

к3

Mineral fiber 12 mm long.

k4

Plasticizers.

k5

Pure water.

Water is added in such an amount that the dry mixture has plasticity and it was possible to make a bra from the material. In general, the amount of water is determined by the recipe, but you can use this simple method of determining the readiness of the mass.

Another component that is not used in other types of liners is microfibers – special fibers made of propylene mixed with most solutions. Although they are small in size (thickness and length), they reliably strengthen the solution and do not allow cracks and chips to appear after drying. With mechanical tensile strength, bending and impact of the screed becomes similar to reinforced concrete.

Semi-dry coat in microfiber

In the presence of a series of sleeves of pipelines, cables and other communications and structural elements, the thickness should be such that the solution of the covered structure is not less than 1 cm. This gives sufficient communication protection and good structure resistance to screed deformation stress, eg after expansion of the pipe underfloor heating or thermal movement of electrical cables due to heating.

In the classic case, the semi-essential liner is a mixture of factory preparation, which is fed by special pumps through the place of the pipeline. The plasticity of the mass enables its lifting to a height of 30 floors, which practically removes the restrictions of use in high-rise construction. This is one of the main advantages – the mass is fed to the whole area at the same time, except for manual movement with rudders or carriers.

The plasticity of the composition facilitates smoothing and smoothing of the surface, and the presence of polymer fibers does not allow cracking. Accelerated drying, thanks to the reduced amount of water, allows you to work quickly and move the surface much faster than is the case with conventional methods of laying the floor under the floor.

When preparing mortar for screed, it must be taken into account that the presence of chlorine and fluorine in the water significantly worsens the strength of the finished layer. Water should not be taken from the city water pipe, but from natural reservoirs or deep wells, as, indeed, for the preparation of any other solution, on the strength of which the result of construction depends.

Advantages of semi-dry screed

The main positive sides of the semi-dry screed are manifested by the industrial installation using special equipment that allows a mixture of up to 200 m horizontally and a height of up to 100 m. Calming and smoothing of the mixture is done immediately after filling the floor surfaces and does not require a time interval for adjustment.

If we consider the obvious advantages of a semi-dry liner, we can distinguish several main ones:

  • Adjustable stands;
  • Surface quality;
  • Absence of leaks;
  • Possibility of arranging different levels;
  • High performance;
  • High thermal and mechanical characteristics;
  • Ecological properties;
  • Costs.

Adjustable speed adjustment stand

The physico-chemical processes that occur inside the fittings are the same as in the conventional solution. But there is little moisture here – it is enough that all cement molecules hydrate and participate in understanding the solution. Water practically does not evaporate from the surface – everyone is involved in chemical bonds. This reduces the drying time of the screed – the surface can be used after 12 hours after installation and leveling.

But under the canopy it follows with some precautions. Although the humidity is low and the cow, it contains enough water to create unfavorable conditions for wooden constructions. Therefore, ceramic concrete, tile, linoleum can be laid after 2 – 3 days after the floor leveling semi-dry screed, but wooden floors, parquet and laminate can be laid after 10 – 14 days, waiting for all the moisture to evaporate, not only from the surface but also liner volumes.

Take into account the fact that the thickness of the layer should be at least 5 cm. With an area of ​​10 to 75 m 2, moisture is contained in it, although little, but to remove it to a level safe for wood, it takes at least 10 – 14 days.

Surface quality

The consistency of the solution and the presence of fiberoptic eliminates the loose surface during drying, the formation of cavities and cavities after water inside the evaporation (nothing simply evaporates), the appearance of cracks due to uneven drying. sanders Application screed surface when the mortar additionally closes the outer layer and they turn practically monolithic waterproof concrete.

With the subsequent laying of solid floors of epoxy or polymeric materials, their consumption is significantly reduced – the porosity of the base is very low and the material largely does not penetrate into the screed. However, this does not affect the level of adhesion at all – the coating sticks to the semi-dry screed, which is no worse than normal.

Material properties enable high precision – after grinding, deviations in height do not exceed 2 mm per longer meter. If you take into account the high cost of the floor, the reduction of material consumption leads to significant savings, which will exceed the cost of using special equipment.

Absence of leaks and cleanliness of work

Another advantage of using a semi-dry screed is the cleanliness of the work. In addition to the absence of contaminated splashes and cracks on the surface, a small amount of water in the mass excludes the flow through the joints of the floor panels to the lower floors. This allows the use of this version of the screed in case of repair, when the finishing is already completed on the lower floors.

Compared to completely dry esters, the danger of water penetrating into the lower floors is somewhat higher. However, in the classic form, dry screed is practically not used when performing industrial construction. The sphere of its application are dachas and frame houses, civil engineering, cottages.

Possibility of arranging different levels

Due to the plasticity and density of the solution, as well as the possibility of keeping the damped shape, the semi-dry method for laying screed allows the process to be performed continuously when preparing substrates for different coatings. In the case of liquid cement-sand mortar, it is much more difficult to perform such work in one operation – it will require a significant time interval or payment, which in turn requires a time interval and additional costs for materials and payment for work.

High performance

Compared to other methods of leveling bases and preparing industrial floors, semi-dry screed provides the highest productivity – up to 400 m 2 of finished layer per shift. The quality is especially valuable when installing screed in large areas:

  • hangars;
  • trade halls;
  • industrial shops;
  • garage;
  • sports facilities;
  • warehouses.

The mechanical supply of ready-made solution of the same consistency and chemical composition allows without limiting the maximum installation area of ​​the semi-dry screed. This method was appreciated by major construction organizations, as it allows for a reduction in construction time.

High thermal and mechanical characteristics

The composition of the components of the semi-dry screed and the monolithic structure significantly increases the surface resistance to abrasion. Even when used as industrial floor coatings, the surface is not dusty and is not damaged when using warehouses or workshop equipment on pneumatic or plastic wheels.

When exposed to the surface of the screed, the load is evenly distributed and the entire structure of the room is less deformed. Individual porosity of the carrier layer also has lower loads on the base or overlap layers.

The porosity of the carrier layer and the presence of mineral or polymer fibers increase the thermal and sound properties of the material. This allows you to save on the use of additional dampers and heat protection materials on the ceilings – their layer can be thinner or even eliminated. But the thermal properties do not prevent the use of semi-dry screed as a basis for the installation of warm floors. A small layer of fibrous concrete on top of pipes or cables does not reduce the efficiency of the heating system.

Ecological properties

In terms of ecology, consider a semi-dry screed both from a work point of view and in the context of the construction process.

As already mentioned, the finished semi-dry liner:

  • practically not dust,
  • does not absorb moisture,
  • don’t make noise,
  • it is very clean and tidy, even without the use of even moderately active detergents and light abrasives.

When heated by direct sunlight, it does not deform, crack or fall. It is also indifferent to the semi-dry coat to low and high air temperatures. When heated, the material behaves similarly to concrete, not by releasing volatile compounds and allergens. Polymer and mineral fibers are firmly bonded with cement and do not come out in the form of dust even under strong mechanical influences on the surface.

Work on the construction site is also safe – the whole phase of the mixture is made in a special closed container – a mixer. The supply is made by hermetic pipelines, which have no impact on the environment. When smooth surfaces and ironing a wet surface, they do not form dust and sprays that can irritate a person’s respiratory tract or mucous membranes.

The cost of laying a semi-dry screed

The cost of semi-dry screed is lower than wet, and even more dry. The most sensitive economic effect is manifested when it is installed on surfaces over 100 m 2 . Such quantities enable maximum use of equipment and reduction of labor costs for the production of 1 m 2 of finished surface. When you prepare the solution by hand and pack it without the use of special machines, the cost is almost the same as the usual, wet one, but all the main advantages remain.

Semi-dry screed mass

In addition to the obvious merits of semi-dry esters, there are a number of shortcomings, which are mostly objective in nature and have been observed in all conditions of its application. These qualities:

  • High price for small quantities of use;
  • Difficulty working at low temperatures;
  • Poor flow;
  • Fear of water;
  • Thickness;
  • Insufficient seal.

High price for small amounts of use

One of the disadvantages is the relatively high cost, if the work takes place in limited areas. For example, the cost of installing such a joint in a private house can be several times higher than wet, even with the preparation of the plaster.

For the preparation of the solution, a special sand is needed, which is very rare. Delivery of sand over long distances significantly increases the cost of the mixture, especially if a small amount is needed, about 1.5-2 m 3 . However, if the screed is installed in a large area, the sand is delivered by heavy vehicles, which somewhat reduces costs.

Same picture with cement – buying a few bags of cement will cost more than wholesale with delivery. It is also difficult to prepare the necessary solution on the spot, for that it is necessary to deliver the necessary equipment to the facility. Rental equipment costs a certain amount of money.

Equipment for semi-dry ties

When laying the wet layer, the costs are significantly lower – sand is suitable for everyone, for mixing a fairly simple concrete mixer, there is no need to order pneumatic equipment. But again, this minus is felt when you work in small areas or facilities, far from each other. With the industrial construction of a multi-storey house or a shared house, these costs are leveled off and the cost per square meter of screed is significantly reduced.

Low temperatures

Another disadvantage when working with semi-dry esters is related to the air temperature. Installation can be only at a positive temperature or in heated rooms, and the base for screeds is also quite warm. But even here, “traps” are waiting. All equipment for the preparation of mortar, as a rule, is located on the street.

At low temperatures, the sand freezes into blocks and stones, which are difficult to grind by hand. There is a difficulty with water delivery. Although not too much is needed, it is still impossible to store in tanks, and hoses and pipes freeze quickly.

In the conditions of large construction, the way out is quite simple – install the equipment in a heated temporary room, heated by air cannons. It is usually enough to heat the air to +5 – 7 0 C, so you can prepare a solution without obstacles. It will not freeze in motion in the pipelines – the heat capacity of the cement-sand mixture is high enough and until it reaches its destination, its temperature will have time to fall only 1 to 3 degrees, which is not critical.

In large cities, when building in a densely built environment, there is a new complexity – the lack of space to install equipment and store a large number of shares of raw materials. But with the right organization of the construction site, this shortcoming is simply eliminated. Perhaps a large number of options, it all depends on the specific conditions on the construction site and the excellence of engineers and supervisors.

A small liquid solution

One of the main disadvantages, determined by the characteristics of the mixture, is insufficient fluid. If in the pipeline, delivered under pressure, the mass flows quite tolerantly and even behaves like a liquid, then in the free state it spreads very weakly. This leads to difficulties when filling corners, lines of contact with columns, walls and shelves.

Eliminating this shortcoming is also quite simple, but like all additional actions and materials, it costs a certain amount of money and increases costs by one square meter, however, it is not critical. In order to increase the liquid, plasticizers, special chemical reagents are added to the formulation, which practically does not affect the hydration process and does not reduce the strength of the finished coating. Plasticizers only slightly increase fluids, which is usually enough to make the solution easier.

If that is not enough, in the initial phase of delivery of the solution, you can manually massage the mass in the crosses. As a rule, the length of the contact lines with the wall is not too long, so the task is quite feasible, and the cost of the screed and the speed of the work performance do not affect much.

Sealing the joints

The only thing that needs to be done is not recommended – add water to the solution. In this case, the mass becomes too liquid, and the liner in the wetting areas will have low strength.

Hydrophobicity

The next drawback is hydrophobia. Before complete drying, contact with water should be completely eliminated and the effect of the liquid should be limited during further use. The fact is that the hydration process with a lack of water is slow and incomplete. There is always a certain percentage of unreacted cement molecules that are ready to interact with the water in contact. As a result, the screed may crack and swell, which is undesirable before applying the paint or in bulk.

Thickness

Semi-dry screed smaller than 5 cm is not recommended. This is also related to the processes that occur within the mass. If the layer is too thin. That water evaporates faster than setting solutions and there is no time to stick to the basics. Therefore, detachments are observed, often quite large in the area.

However, a layer thickness of more than 7 cm is inappropriate due to the large weight of the material – a square screed 7 cm thick weighs more than 120 kg. Such overlapping loads are undesirable, a thicker layer can be made, except on the ground or loose substrate. Frequent filling of expanded clay or filling with expanded clay concrete under semi-dry esters is also used on floors, if it is necessary to raise the floor.

Shirt thickness

Insufficient seal

Often the screed is not compact for various reasons, which leads to partial destruction after the installation of the floor covering, the appearance of guns and noise. To prevent this, it is absolutely necessary to start smoothing and polishing the surface. Timely application of grinding machines will enable the compactness of the material to the required limit.

In order to exclude a larger number of unwanted phenomena, screed installation is strictly according to the technology. Minor deviations lead to cracking or peeling. Carefully observe the proportions of the components, especially the amount of water. Above humidity, as well as its lack, are equally undesirable. In case of excessive amount, the screed loses its density, becomes less durable and shrinks and cracks. If the water is low, then the hydration does not take place completely, which leads to the appearance of pores and caves.

Part of the shortcomings, often cited by opponents of semi-dry screed, are not actually shortcomings. As an example, you can give an inhomogeneous density – the base of the screed is much more fragile than the upper part. The density gradually increases from the bottom up due to the low flow of material. The average density of dried screed is up to 1800 kg / m 3 , which is slightly lower than for liquid concrete (sand / cement, without mineral fillers).

During the test, the breaking strength of the control cube of the semi-dry screed is up to 150 kg / cm 2 , which corresponds to M150 concrete, it is recommended for the equipment of bases under floors.

As for the reading that the semi-essential joint forms an uneven surface, which must be leveled from the floor to be filled, then this is not entirely true. If you use lighthouses and served grinding machines, then any floor covering can be installed without leveling in bulk. In the general case, after the installation of the semi-dry ester, only its positive qualities are manifested, high-quality materials and strictly respected technological requirements should be used.

It should also be noted that each of the construction technologies, including the most innovative ones, does not contain some advantages. You can always find something that doesn’t suit you and isn’t suitable for a particular application. It is the knowledge of all the positive and negative qualities that allow you to choose variants of this or that way of construction, in this case the arrangement of the liner.

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