Screeding or "to screed" is the process of placing a thin layer of material on the top of a concrete poured surface. It is the first process of finishing any flatwork to strike off or level slab concrete after pouring.
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Screeding is performed primarily to prevent bleeding of concrete that is caused shortly after placement. It is also used to provide a surface for proper finishing of the surface. A screed layer helps to make variations to the levelness and flatness of the surface.
Generally, the basic steps involved in finishing and curing concrete are screeding, tamping, finishing, and curing. These methods ensure the provision of a good quality finished product.
Composition of Screed
Conventional screed is made from cement and sharp sand. To get a thicker layer, coarse aggregates can be used for industrial purposes. Compared to screed, concrete is more coarser and stronger, given that they both form a type of cement mixture.
Based on the type of mixture, screeds can be:
1. Dry ScreedA dry screed is a pre-mixed material typically composed of sand and cement, which is applied in a dry form and doesn't require water during installation. It is commonly used as a leveling and smoothing layer for various floor finishes such as tiles, wood, or laminate.
Dry screed is known for its rapid drying time, allowing for quicker installation of subsequent flooring materials.
2. Flowing or Liquid ScreedFlowing screed, on the other hand, is a self-leveling compound that includes gypsum or calcium sulfate binders. This type of screed is mixed with water and poured onto the floor, allowing it to flow and self-level.
A flowing screed is ideal for large areas, providing a smooth and even surface. It is commonly used in projects where a high-quality finish and quick installation are crucial, such as in residential and commercial buildings.
Types of Screed in Construction
The three main types of screed are bonded, unbonded, and floated screeds. Let's discuss one by one.
1. Bonded Screed
Bonded screeds are types of screeds that are bonded to the given slab or substrate below by means of a bonding agent. It is typically used for projects where heavy loading is common like driveways, carways, or in underfloor heating system construction.
The bonded screed fails when the bond between the screed and the substrate fails. This is more likely to happen when the thickness of thickness is high. Hence, the thickness of bonded screed is limited below 50mm i.e. between 25 to 40 mm.
2. Unbonded Screed
The screed layer that is intentionally unbonded from the below substrate layer by means of a membrane is called an unbonded screed. A PVC/damp-proof membrane is used to separate the layers, especially in areas where damp issues exist.
An unbonded screed fails by lifting or curling, which is likely to happen when the screed is thin. Hence, the thickness of the unbonded screed is designed greater than 70 mm.
Advantages of Unbonded Screed
- Flooring is not in contact with the main substrate and thus remains unaffected by any deformations of the screed.
- Thick screeds can be provided without worrying about drying time or the formation of cracks similar to thinner sub-layers
- The PVC or polythene membrane provided sufficient damp proofing to prevent moisture-raising issues from the substrate.
3. Floating Screed
Floating screeds are a type of unbonded screed that is laid over an insulation layer or an underfloor heating system. The intermediate layer can be polystyrene or polyurethane panels or soundproofing material.
The main objective of a floating screed is to provide solutions to meet the minimum performance requirements of soundproofing systems in buildings as per the local set laws.
The minimum thickness of a floating screed is calculated based on the intrinsic characteristics of the material used and the material stresses expected to come over the surface. A thickness of 65mm or greater is applied for heavy-loaded floors.
4. Heated Screed
A heated screed is a floating screed with elements embedded in the screed in spiral or coil pattern. These elements can be pipe-work in plastic or composite material with a metallic core.
The thickness of the screed above the elements must be maintained at least 25 mm with a metallic mesh inserted in the screed as shown in the figure. The size of the mesh is dependent on the total thickness and the design loads.
The heated screed is laid only after checking the pipes for leaks and any other issues.
Miscellaneous Types of ScreedScreed can be created in a bonded, unbonded, or floating manner, depending on the specific construction needs. Within these methods, it can be reinforced or enhanced for increased wear resistance according to specific specifications. Additionally, screed is available in different types based on the chosen materials, which will be elaborated on in the forthcoming article.
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