What is a Traditional Screed?
Traditional screed is also known as sand and cement screed. An aqueous mixture of sand and cement has been traditionally used and is still used for creating smooth and levelled surfaces on concrete flooring. It is applied before installing vinyl, carpets, wood, tiles, or marble flooring. The purpose is to create a smooth surface so the final flooring is levelled.
Well-graded sand and Portland cement are used to minimise shrinkage and cracks. When mixed with water, the cement acts as a binder and holds the sand particles in place. Both are mixed in specified proportions to achieve a homogenous and effective mixture.
Once the mixing has been done, the screed is applied to the surface. Rails or wooden battens are used to achieve the desired thickness and smooth the layer. After the application, it is compacted with trowels, rakes, and shovels to remove air pockets. It is then left to dry and cure. Drying and curing time may vary depending on various factors, such as the thickness of the layer, environmental conditions, and the type of cement.
What is a Self-Levelling Screed?
The purpose of the self-levelling and traditional screed is the same, i.e., to create a smooth and levelled surface for the flooring installation. However, self-levelling screeds automatically spread to form a smooth, flat layer without intensive labour. They have a liquid consistency that allows them to flow easily and spread uniformly over the prepared surface.
Their fluid nature allows them to fill in depressions, low spots, and uneven areas, thus creating a levelled surface. As their application is easy and requires less labour, they take less time to apply. They are also relatively quick to cure and dry, making them more suitable than dry screeds. Self-levelling screeds can be a valuable tool in flooring installation.
Especially for large or complex areas where achieving a level surface with a traditional screed would be time-consuming and labour-intensive. However, proper preparation, application, and adherence to manufacturer guidelines are essential for a successful outcome.
Differences Between Semi-Dry and Self-Levelling Screeds
Consistency and Application:
- Semi-Dry Screeds: Semi-dry screeds are a mixture of sand, cement, and water, often with an added admixture to improve workability. They have a relatively dry consistency compared to self-levelling screeds. Semi-dry screeds are manually spread and compacted using tools like shovels, rakes, and trowels. They require more manual labour to achieve a level surface.
- Self-Levelling Screeds: As the name suggests, they have a highly fluid consistency that allows them to spread and level themselves across a surface. They are poured onto the subfloor and naturally flow to create an even layer without extensive manual intervention. This property makes them suitable for larger areas and situations where precision and speed are crucial.
Ease of Installation:
- Semi-Dry Screeds: Installing semi-dry screeds requires more labour-intensive work, as they need to be manually distributed, compacted, and levelled. The process can be time-consuming, especially for larger areas, and achieving an altogether level surface may be more challenging.
- Self-Levelling Screeds: Self-levelling screeds are easier and faster to install than semi-dry screeds. Their self-spreading nature reduces the need for manual labour and minimises the chances of human error in achieving a level surface.
Speed of Application:
- Semi-Dry Screeds: The application of semi-dry screeds is generally slower due to the need for manual spreading and levelling. The process involves multiple steps and often requires more time for curing.
- Self-Levelling Screeds: Self-levelling screeds can be applied much more quickly due to their self-spreading properties. This can significantly reduce installation time, making them advantageous for projects with tight schedules.
Finish and Precision:
- Semi-Dry Screeds: Achieving a perfectly level and smooth finish with semi-dry screeds might require more skill and expertise, as well as additional surface preparation and finishing work.
- Self-Levelling Screeds: Self-levelling screeds are designed to automatically create a very level and smooth finish, reducing the likelihood of imperfections or unevenness.
Thickness and Application Range:
- Semi-Dry Screeds: Semi-dry screeds can be applied at varying thicknesses to accommodate different requirements, from thin layers to more substantial floor elevation changes.
- Self-Levelling Screeds: Self-levelling screeds are typically poured at a consistent thickness, making them suitable for achieving an even surface but less suitable for significant elevation changes.
Mixing or Batching of Screed
Traditional Sand and Cement Screed:
- Ingredients: Sand, cement, water, and sometimes additives.
- Mixing: The sand and cement are mixed in a ratio specified by the manufacturer or project requirements. The mixture is dry-mixed thoroughly to ensure uniform distribution of the materials.
- Water and Additives: Water is added gradually while mixing to achieve the desired workability and consistency. Additives like plasticisers or accelerators can be included to improve the screed’s properties.
- Mixing Equipment: Mixing can be done using a forced-action mixer, pan mixer, or similar equipment. Hand mixing is also an option for smaller areas.
- Application: The screed is manually placed and levelled on the prepared substrate once mixed.
- Ingredients: Self-levelling compound, water, and sometimes primers or additives.
- Mixing: The self-levelling compound is typically supplied as a pre-packaged dry mix. The process involves adding water to the mixture according to the manufacturer’s instructions.
- Mixing Equipment: A low-speed drill with a mixing paddle is commonly used for mixing self-levelling compounds.
- Application: The mixed self-levelling compound is poured onto the subfloor and then naturally spreads and levels itself. Minimal manual intervention is required.
Placement and Compaction
The placement and compaction of the screed are critical steps in creating a level and smooth surface for flooring applications. Proper placement and compaction ensure the screed adheres well to the substrate, eliminates voids, and achieves the desired finish. The methods for placement and compaction vary depending on the type of screed being used. Here’s a general overview:
Traditional Sand and Cement Screed:
- Once the screed mixture is mixed correctly, it is spread and placed onto the prepared substrate in manageable sections.
- Wooden battens or “screed rails” are often used along the edges of the area to act as guides, ensuring the screed is at the desired thickness.
- After placing the screed, it must be compacted to remove air voids and achieve a dense surface.
- A straight-edged tool, such as a longboard or a screed bar, is used to level the screed and remove excess material. It is moved in a sawing motion to distribute the screed evenly.
- A tamper or vibrating screed can further compact the screed and ensure a uniform surface.
- Self-levelling screed is poured directly onto the substrate in the area to be covered. It naturally flows and spreads to create a level surface.
- Self-levelling screed is designed to eliminate the need for manual compaction. It levels itself out and doesn’t require additional tools for compaction.
- Anhydrite screed is poured onto the prepared substrate and spread to achieve the desired thickness.
- Like traditional screeds, anhydrite screed may require compaction to remove air voids and ensure a dense surface.
- A straight-edged tool or a trowel can spread and level the anhydrite screed.
- Vibrating equipment can help with compaction while ensuring an even distribution.
Polymer-Modified or Fibre-Reinforced Screed:
- Polymer-modified and fibre-reinforced screeds are placed and spread onto the substrate, similar to traditional screeds.
- The same compaction techniques used for traditional screeds can be applied to polymer-modified and fibre-reinforced screeds to ensure proper consolidation.
Drying Time of Self-levelling Screed
The drying time of a self-levelling screed can vary depending on several factors, including the specific product used, environmental conditions, thickness of the application, and the manufacturer’s recommendations. Generally, self-levelling screeds are designed to dry relatively quickly compared to traditional sand and cement screeds. However, it’s important to note that “drying time” and “curing time” are two different concepts:
- Drying time refers to the period during which the surface of the screed becomes dry to the touch and suitable for light foot traffic or further work.
- Self-levelling screeds can typically dry to the touch within a few hours to a day, depending on the conditions.
- Curing time refers to the time required for the screed to achieve its full strength and durability. It’s the time needed for the chemical reactions within the screed to complete, leading to the development of its properties.
- Complete curing of self-levelling screeds can take several days to weeks, depending on the product and conditions.
Factors Affecting Drying and Curing Time:
- Environmental Conditions: Humidity and temperature play a significant role in drying and curing. Higher temperatures and lower humidity levels generally expedite drying and curing.
- Ventilation: Adequate ventilation helps in promoting drying by allowing moisture to evaporate from the screed.
- Thickness: Thicker applications of self-levelling screed might take longer to dry and cure compared to thinner applications.
- Type of Screed: Different self-levelling screeds have varying compositions and additives that can influence drying and curing times.
- Product Specifications: Always follow the manufacturer’s guidelines and specifications for drying and curing times. Each product may have specific recommendations.
- Surface Preparation: Proper surface preparation, including priming the substrate, can impact the self-levelling screed’s adhesion and drying/curing performance.