In essence, while screed and concrete share similarities, they differ significantly in terms of strength. To comprehend these differences, it is essential to explore what screed is and how it diverges from concrete.
What Is Screed?
Screed is a thin, grey-brown layer, typically composed of cement and sharp sand, applied over a subfloor. It can be installed in various forms, such as solid, in-situ, or pre-cast concrete floor slabs. Screed is primarily laid on subfloors or insulation pipes to provide a smooth, flat finishing surface, serving to even out the surface and cover any cracks or damage before the top flooring layer is applied.
Many use screed as a base for decorative flooring finishes like carpets, tiles, or floorboards, while others may leave it exposed, as seen in warehouses. Residential screed, while not particularly durable, contrasts with industrial screed, which is more robust, featuring a course 6-10mm layer with added aggregates for increased thickness and strength.
Different Types of Screeds
There are three primary types of screeds, each tailored to different applications based on the construction project’s nature and objectives:
- Bonded Screed: As the name suggests, bonded screed is directly bonded to the concrete base using a bonding agent. It is ideal for projects where unbonded screed isn’t feasible, and where the floor demands higher durability to withstand heavy foot traffic or loads. Bonded screed typically has an optimal thickness of 25-40mm.
- Unbonded Screed: In contrast to bonded screed, unbonded screed is not attached to the concrete layer, reducing the chances of shrinkage and settlement. These screeds are laid on a damp-proof membrane or PVC and require a minimum thickness of 50mm. They are preferred for areas prone to moisture.
- Floating Screed: Ideal for pairing with underfloor heating systems, floating screed acts as an insulating material and is applied on top of a damp-proof membrane. Its thickness ranges from 65-75mm, depending on the area’s traffic, making it highly resilient to both light and heavy foot traffic. A notable example is calcium sulphate screed, typically used in 35mm thickness for residential projects and up to 400mm for commercial projects.
Screed v/s Concrete: What’s the Difference?
Let’s explore the various dimensions in which concrete differs from screed:
Composition Proportion
Both concrete and screed are composed of cement, sand, water, and aggregates. However, the key distinction lies in the proportion of these constituent materials. The screed differs in the following aspects:
- Aggregate size i.e. screed uses fine aggregates, while concrete contains coarse aggregates.
- Cement grade
- Consistency of the mixture
- Cement finish
Concrete typically follows a fixed proportion of 1:2:3 (1 part cement, 2 parts sand, and 3 parts coarse aggregates, i.e., ballast/gravel). In contrast, screed follows a fixed proportion of 1:3-5 (1 part cement to 3-5 parts sand, typically 0.4mm sand for levelling screeds). The usual thickness of sand-bonded screed ranges from 25-40mm, while unbonded screed is typically 50mm thick.
Appearance & Texture
The variation in composition results in different consistencies and textures for both materials. Floor screed is a smoother mix, containing small aggregates ranging from 4-5mm. It is typically laid on top of concrete to prepare the base for the final flooring layer, such as tiles, vinyl, wood, or carpet.
Conversely, concrete has a coarser mixture with hard-core aggregates up to 20mm in size, providing a rough and tough texture. This characteristic texture contributes to its high compressive strength, making it highly resilient and durable.
Uses
Concrete is primarily used for structural tasks in the construction industry due to its significant strength. Screed, on the other hand, is applied as a top layer for finishing touches on concrete subfloors. While screed is predominantly used for internal floor finishes, concrete can be utilised both inside and outside the structure for creating walls, floors, landscape structures, and a variety of other applications.
Application
Due to its fine finish and thin consistency, the screed can be easily applied evenly using simple tools like a trowel, requiring less effort. In contrast, the application of concrete, with its thicker consistency and large aggregates, necessitates specialised tools for mixing and application.
By examining these aspects, it is clear that concrete and screed serve distinct purposes and possess unique properties tailored to their specific applications in construction.
Do I Need to Screed a Concrete Floor?
The answer is not definitive, as it depends on the project’s requirements and your budget. However, experts such as K M Flow Screed, a registered and licenced company operating in the construction industry as screed specialists, recommend adhering to a general rule of thumb. If your floor’s surface is bumpy and uneven, it is advisable to screed the floor before laying the top flooring layer to ensure a pristine finish.