Concrete screed is a finely graded, compactable mixture applied as a thin layer over a concrete substrate to create a stable, level surface. Often mistaken for plain concrete, screed contains a higher proportion of fine aggregate and cement, resulting in a smoother finish that acts as a stable base for flooring installations. This specialized layer is critical for ensuring dimensional accuracy and providing the flatness required for tiles, wooden boards, and industrial epoxy systems.
The Role and Function of Screed
The primary function of concrete screed is to level the substrate and absorb point loads from furniture or machinery. It bridges minor imperfections in the base slab, distributing loads evenly to prevent cracking in surface finishes. Furthermore, it provides a clean, dust-proof surface that is chemically resistant and dimensionally stable, which is essential for maintaining the integrity of the final flooring system over the structure’s lifespan.
Types of Screed and Their Applications
Not all screeds are created equal; the selection depends entirely on the project’s requirements for speed, load, and finish. The industry generally categorizes screeds based on the binding agent and the method of installation.
Cementitious Screed
This is the most common type, composed of cement, sand, and water. It is suitable for a wide range of domestic and commercial applications. Depending on the formulation, cementitious screeds can be laid thin for underfloor heating or applied thicker for heavy-duty industrial floors. They are cost-effective and robust, though they require careful curing to prevent shrinkage cracks.
Anhydrite Screed
Also known as gypsum screed, this type utilizes anhydrite cement rather than Portland cement. It offers significantly faster setting times and lower shrinkage, allowing for early foot traffic. This screed is ideal for projects with tight deadlines and is particularly well-suited for underfloor heating due to its excellent thermal conductivity.
Understanding Screed Grades
To ensure the right product is specified, industry standards classify screeds into distinct grades based on their flow and final surface texture. Choosing the correct grade is vital for meeting the functional and aesthetic demands of the finished floor.
SRN 24 Flowing Screed
Flowing screed, often referred to as self-leveling, has a high water-to-binder ratio. It is poured and spread easily, allowing it to self-level and fill the entire profile of the substrate. This grade is used when a smooth, seamless finish is required, such as in high-end retail spaces or where precise leveling is necessary for machinery alignment.
SRN 18 Semi-Dry Screed
With a lower water content, semi-dry screed is compacted by hand or vibration screeders. It offers a thicker build layer and high early strength, making it suitable for areas subject to heavy traffic immediately after installation. This grade provides a balance between durability and installation efficiency.
The Installation Process
Proper installation of concrete screed is a meticulous process that dictates the final performance. It begins with preparing the substrate, ensuring it is clean and structurally sound. Guide rails are then set up to control the thickness and flatness of the screed layer. The mixture is poured and distributed, followed by consolidation with a straightedge or power screed to remove air pockets and achieve the desired level.
Curing and Drying Times
The timeline for screed usage is dictated by the type of binder and environmental conditions. Cementitious screeds generally require a wet curing process and can take up to 28 days to reach full strength. Anhydrite screeds, while faster, still require protection from rapid drying and direct heat during the curing phase. Installing floor coverings before the screed has fully cured is a common cause of flooring failure, so patience is essential to ensure a durable result.
