What is screeding

 

Concrete screeding is the process of leveling and smoothing freshly poured concrete to create a flat and even surface. It is a crucial step in ensuring the surface is ready for finishing, whether for floors, patios, driveways, or any other structure. Here’s how the process works:

Steps in Concrete Screeding:

1. Pouring the Concrete: Fresh concrete is poured into the area where the surface is being created. This is typically done using a concrete mixer or truck.

2. Setting Screed Rails or Forms: To guide the screed and ensure a consistent level, screed rails or edge forms are set up around the perimeter of the pour area. These act as a reference for the desired height and level.

3. Using a Screed: A screed, which can be a straight edge (usually a metal or wooden board), is dragged across the surface of the concrete. The screed is pulled across the screed rails to level the concrete. As the screed is drawn, it pushes excess concrete into lower areas, filling them and leveling the surface.

4. Compacting and Leveling: As the screed is passed over the surface, it both levels the concrete and helps remove air pockets. In some cases, a vibrating screed is used to further compact the concrete and achieve a denser, more uniform surface.

5. Finishing: After screeding, troweling tools may be used to further smooth and finish the surface. For larger surfaces, power trowels may be used.

Tools Used in Screeding:

• Straight edge: A long, straight board or aluminum bar used to level the concrete.

• Vibrating screed: A machine that vibrates to help compact and smooth the concrete as it levels.

• Screed rails: Temporary guide rails or pipes that act as a height reference for the screeding process.

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Screeding is a critical step in creating a durable, level, and aesthetically pleasing concrete surface.

The composition of materials in a concrete screed typically depends on the type of screed being used, but the basic materials are quite similar to standard concrete. Here is a breakdown of the typical components:

1. Cement

• Role: Cement acts as the binder that holds the materials together, giving strength and durability to the screed.

• Common types: Ordinary Portland Cement (OPC) is commonly used.

2. Aggregates

• Fine aggregates: These include sand (coarse or sharp sand is often used), which helps in providing the screed with a smooth finish and proper workability.

• Coarse aggregates: In some screeds, especially thicker layers, small gravel or crushed stone may be included, although for finer screeds (like thin overlays), only fine aggregates are used.

3. Water

• Role: Water is necessary to hydrate the cement, causing it to react chemically and harden. The amount of water used is crucial to achieving the correct consistency and strength.

• Water-cement ratio: A careful balance is needed—too much water weakens the screed, and too little can make it difficult to work with.

4. Additives (Optional)

• Plasticizers: To improve workability and make the screed easier to spread without adding excess water.

• Fibers: These can be added to reduce the risk of cracking and improve tensile strength.

• Accelerators/Retarders: To adjust the setting time of the screed depending on environmental conditions.

Common Mix Ratios

The mix ratio for concrete screed typically ranges between 1:3 to 1:4.5 (cement to sand by volume), depending on the intended use and strength required. For instance, the screed mix ratio for general-purpose floors might be:

• 1 part cement: 3 parts sand (by volume) for stronger screeds.

For thinner layers or more specialized uses, a finer blend with a higher cement content might be used.

Types of Screeds

1. Bonded Screed: Directly applied to a concrete substrate.

2. Unbonded Screed: Separated by a damp-proof membrane.

3. Floating Screed: Laid over insulation or underfloor heating systems.

Each of these types may require slight variations in mix design to suit the application.

Summary

• Cement (acts as the binder)

• Sand (fine aggregate for smoothness)

• Water (to mix and hydrate the cement)

• Optional additives (for improved performance, e.g., plasticizers, fibers)